Stretchy plastic film for very small canopies

20 February 2025

Back in 2018, I saw MaxFliart youtube on using transparent heat shrink sleeve to make his canopies. A great solution that I have forgotten.

6 February 2025

I guess shallow compound curves will be possible for very small pieces that form the segments of canopies. Relatively deep or bigger pieces would not be suitable. white glue or dope won't work, cling film is not receptive, maybe contact glue or other plastic glue such as UHU POR. 

I posted to Gemini:

"Make a male mould slightly smaller than the desired canopy. Pull Saran-wrap over this mould. Apply some elmer's clear or thinned EZE-Dope, leave to dry until tacky. Pull another saran-wrap. "

Gemini 2.0 Flash, replied:

"That's an interesting approach to creating a canopy, using Saran Wrap and adhesive! It's a clever way to potentially achieve a lightweight and somewhat transparent covering, especially for smaller models. Let's break down the process and consider some potential advantages and disadvantages:

The Process (as you described):

1. Male Mold: Creating a male mold slightly smaller than the desired canopy is crucial. This allows for the thickness of the Saran Wrap and adhesive layers.
2. Saran Wrap Layer 1: Pulling Saran Wrap tightly over the mold creates the initial layer of the canopy. The tightness is key to avoiding wrinkles.
3. Adhesive Application: Applying Elmer's Clear glue or thinned EZE-Dope (which is a cellulose-based dope often used in model aircraft) and letting it dry until tacky provides a bonding surface for the next layer. The tackiness is important for the second layer to adhere well.
4. Saran Wrap Layer 2: Applying a second layer of Saran Wrap adds strength and potentially improves the surface finish.

Potential Advantages:

• Lightweight: Saran Wrap is very thin and light, making it suitable for small rubber-powered models where weight is a critical factor.
• Low Cost: Saran Wrap and Elmer's glue are readily available and inexpensive.
• Simple Technique: The process is relatively simple and doesn't require specialized equipment.
• Some Transparency: While not perfectly clear, Saran Wrap does offer some level of transparency, which might be sufficient for small models.

Potential Disadvantages:

• Durability: Saran Wrap is not very strong or durable. It can tear or puncture easily. The adhesive adds some strength, but it's still a relatively fragile canopy.
• Wrinkles: Getting a completely smooth, wrinkle-free surface can be challenging, especially with complex mold shapes.
• Clarity: The transparency is likely to be somewhat hazy, and multiple layers might not significantly improve clarity. The adhesive can also affect the clarity.
• UV Resistance: Saran Wrap is not UV resistant and can degrade and become brittle over time when exposed to sunlight.
• Shape Retention: The canopy might not hold its shape well, especially in warm conditions, as Saran Wrap can stretch or deform.
• Adhesive Issues: Getting an even, thin coat of adhesive can be tricky. Too much adhesive can make the canopy heavy and affect clarity. Uneven drying can also lead to distortions.

Tips and Considerations:

• Mold Preparation: A smooth mold surface is essential. Consider using a mold release agent to prevent the Saran Wrap from sticking.
• Tight Wrapping: Pull the Saran Wrap as taut as possible to minimize wrinkles.
• Adhesive Application: Apply the adhesive in a thin, even coat. Multiple thin coats might be better than one thick coat. Test different dilution ratios for your adhesive.
• Drying Time: Allow the adhesive to dry thoroughly before applying the second layer of Saran Wrap.
• Alternative Materials: Consider experimenting with other thin, clear films, like those used for food packaging or clear acetate sheets, which might offer better durability and clarity.

Conclusion:

While using Saran Wrap and adhesive might not result in a perfectly transparent and durable canopy, it's a creative and accessible method for small, lightweight models. The key is careful preparation, tight wrapping, and even adhesive application. It's definitely worth experimenting with, especially if you're working on a small scale and on a budget. Just be aware of the limitations of the materials."

Scheibe Falke, a motor glider

19 February 2025

A month or two ago, I bought a free flight electric motor kit comprises of a 7mm coreless motor with plastic propeller, a unstated sized 1s lipo (I suspect it is 100mah) and controlled by a small electronic unit. The control unit has a small button, press it and the motor spins for 15 seconds. I think it is ideal for small free flight model, but did nothing further. As usual, keeping a good thing is not a good thing, I've got to use it and then it is a good thing.

I think it is ideal for many models, not ideal where a larger diameter propeller is required to clear the cowling unless another geared motor and propeller replaces the original provided. The weight is condensed and can be kept compact, making short nosed models a possibility. Anyway, it would be convenient to be able to use the flight kit as is. 

There are many possibilities, I think the motor glider is a good choice, such as the Scheibe Falke.

I'll design the model by designing the arrangement of the flight kit. I think a power pod would be great, if the model lasts long, and I got bored, I could swap out the power pod into another model.

Design a one piece power pod using the flight kit, allow for the button to be pressed, allow for the charging of the lipo through the unit, devise locating and securing system, thrustline.

Design the model around the finalised pod. Some parts of the model will be removable, the pair of wings come to mind.

Simplest Pod, Simplest Model

A rectangular piece of 1/16" balsa sheet with flight kit glued on. Then glue or band (or double sided tape?) this power pod to a profile fuselage model.

Why bother? The simplest way

Just glue the motor to a 1/16" balsa sheet, cross grain, and use double sided tape to stick the unit to the profile fuselage. For the control unit, a piece of double sided tape to the fuselage is enough, for the lipo, do the same and the lipo may be repositioned to balance the CG of the model.

Braiding rubber motors

19 February 2025

(Revised)

Making the simplest braided rubber of 2 strands for peanut scale.

1. Cut a strand of 1/16" rubber to a length approximately 2.5 times the distance between the nose block to motor peg. Make a simple loop at each end of the strand, the loops are temporary and have to be released later. An overhand knot would work.
2. Place one loop end in a secured hook, stretch the rubber strand by pulling from the other loop end and wind 4 turns per inch. Let's say the nose-peg distance is 8": the strand is 20" and we wind 80 turns.
3. Now bring the other loop end to the same secured hook, the rubber strand will braid on itself, tug along the braided length to even out the braiding. 
4. Pinch the 2 loop ends together and remove from secured hook, release the loops and tie a square knot with the 2 free ends.
5. Tug and pull the square knot to tighten and add a small bit of superglue to better secure the knot.
6. The basic braided rubber is now completed, and we turn our attention to how we will be able to fit the knotted rubber to the motor peg.
7. We could just insert the motor peg through the knotted end and end it here. It is not as if the braided rubber will be replaced that often.
8. Alternately, we could also use a loose peg which is a cylinder (sort of a bobbin) that can fit loosely over the motor peg. We insert this loose peg through braided rubber at the knotted end by twisting to open up the end. This adds a bit of weight at the tail, but the main advantages are: 1) allows the turns to be freed up at the rear end thus less weight shift as the motor unwinds; 2) easier insertion of motor peg which allows easier motor replacement.
9. With the knotted end in the tail and secured by the motor peg, pull the braided but unwound rubber out with a wire hook. 
10. Twist to open up the end to hook it to a motor winder, when the braided rubber is wounded, remove the motor winder and hook the end to the propeller shaft's hook, then insert the nose block to the fuselage.

Note that if the braided rubber (unwounded) is short of the nose opening, you can pull it out with a wire hook.

If it is too long, it may not be a problem, but it is a problem if the rubber will thrash against the bottom of the fuselage.

Therefore, we would like a braid (unwounded) that hangs on the propeller shaft's hook and motor peg, and clears the bottom of the fuselage. 

17 February 2025

For the simplest rubber loop of 2 strands for peanut scale:

• know the distance between motor hook and peg.
• this can only be an estimate: Take twice plus half the distance, this is the length of a single rubber strand that, when braided, will result in about 110% length of motorhook to peg.
• Mark the centre of the single rubber strand with a permanent marker, slip in a small O-ring (can be a short section of any tube shaped soft plastic), place it near the centre, using sewing thread to wrap the rubber around the O-ring, tie off loosely.
• Hold one end in a binder clip or something that can hold the end securely temporarily.
• Twist the single strand from the other end, the O-ring is still in the centre.
• Bring the end to the secured end, now both ends are together, tug on the o-ring occasionally so that the rubber will wind itself evenly.
• Slip in another o-ring to the end, unsecure the first end, tie in a figure-8 knot, bit of superglue on the rubber.
• Now I have a braided loop of rubber, 2 strands, and about 10% longer than the distance between hook and peg. How much braiding and how big the o-rings will also affect the length. The two o-rings allow easy engagement of hook and peg.

2nd method?:

• know the distance between motor hook and peg.
• this can only be an estimate: Take twice plus half the distance, this is the length of a single rubber strand that, when braided, will result in about 110% length of motorhook to peg, how much braiding and size of loops will also affect the length.
• Tie a loop in the centre of the single rubber strand, you could use an overhand knot, or just directly tie the rubber with a few windings of button sewing thread and superglue. The second method is preferred for simplicity and lightness because you don't carry the extra weight of rubber knot. If you have a suitable o-ring, you can also tie it to the centre, just that o-ring is weight and the real usable length is shortened.
• Loop the loop over a dowel, and twist from each end, if the right strand is twisted towards you, then the left strand is also twisted towards you. The key is to twist both strands in equal number of turns.
• Tie the 2 ends together with an overhand knot but leaving a loop. If there is not going to be a loop, a S-hook can be used later, tie 2 ends together with figure - 8 knots.
• Let free both ends and the twisted rubber will braid itself. Tug the loops a few times and the braiding will be even.
• Unloop the loop from the dowel. Now you have 2 loops, one to the hook (the single loop first created) and the second to the peg (the double loop just created).

Gemini says:

Think of it this way:

• First method: Like building a finely crafted, hand-stitched garment. Higher quality, but more work.
• Revised second method: Like a well-made, machine-sewn garment. Good quality, faster and easier to produce.

AutoGyro

12 February 2025

I think the following is better than the previous slotted foam hub idea for an autogyro powered by a single rubber band measuring 160mm x 1.4mm x 1.4mm.

The hub is an inverted thrust button with a large flange. The large flange can be a stiff disc, glued on an end of the balsa dowel. The dowel has a hole in the center for the wire shaft. The 3 o 4 rotor blades are 10"x1"x1/32" balsa, each has a 4" length of 1mm CF rod glued at 25% chord (1/4"), exposed 2" from the root, the rotor diameter is approximately 24". By providing identical spacer at the rim of the card disc, each blade can be held to the exact conical angle (dihedral). Glue in scraps to lock in the end of the CF rod. Do not glue the CF rod to the stiff disc. Make small holes between each CF rod in the stiff disc, thread continuously with button sewing thread, looping around all the CF rods. This allows pitch adjustment to each blade while balancing and trimming the autogyro. 

When lift equals mass, a fixed wing flies at the model's level flying speed. For an autogyro, the same lift is supported by the rotor. The rotating blades have to fly much faster than the apparent flight speed of the whole autogyro. An autogyro has much greater drag and is therefore much less efficient than an airplane, it needs greater thrust to overcome the drag. An autogyro can be built lighter than an airplane, and it has to be, in order to reduce the thrust requirement. Longer blades and placing the blades further out will allow the blade to spin slower but with the same flying speed needed to maintain level flight; the autogyro's speed then appears to be slower than an airplane. The blades of an autogyro is spun due to autorotation (driven by the relative upwards moving air due to forward flight), the outer portion, i.e. the tip end, powers the lifting portion of the blade, being at the tip it travels faster than the inner portion, the centre of lift of a blade may vary, but for simple autogyro models, I fail to see the necessity to determine that. Just use efficient lifting airfoils, make it long and mount it further away from the rotor hub. I doubt if there will be any noticeable performance difference if the blade's planform is changed from a simple rectangle to tapered/double tapered/elliptical/parabolic. Maybe trim the root and/or tip ends for look. RC helicopter's blades are basic rectangles with the root end trimmed. So do not even consider planform and twists. A slower rotating rotor means lesser friction loss at the hub, a change from 2-blade to 3-blade or even 4-blade will allow the rotor to spin slower because additional "wing" is at work, making the rotor larger will also achieve the same effect. Do not use Da-Vinci's helicopter drawing or the paper windmill as inspiration. Make the rotor bigger, if it is too small, it's like a small wing that has a high wing loading, too difficult to trim and fly.

If the propeller spins anti-clockwise (when viewed from front), the rotor should spin clockwise (when viewed from above), so that the advancing blade can create more lift to 'roll' the model and oppose the torque of the propeller.

21 March 2024

I watched on Youtube a video of indoor autogyro. They are hard to set right and even when they are trimmed, the performance is mediocre at best. When some of them tripped against the wall or ceiling, most does not recover. Of those that flew without accidents, their ascent is steep with the propeller thrust and their glide descent is also steep, looks like they are using relatively torquey rubber motors.  They mostly have 3 to 4 blades of narrow chord on a rotor that is balanced and spins well with just a wave of the model. In comparison, normal airplane models seem so much easier.

Here's an idea for the design of a 4 bladed autogyro rotor for rubber-powered indoor flights.

It appears that the autogyro can be trimmed by increasing or decreasing the angle of attack.

On a clock-wise spinning autogyro, when viewed from top, twisting the blades to increase or decrease will result in variation to the unequal lift, thus changing the direction of the flight like a flat wing's ailerons. Most autogyros have the rotor at an angle of attack to the flight path, some dihedral also for slight stability, largish horizontal stabiliser and fins but the overall length of the fuselage is short, thus it isn't too effective, especially for directional stability, thus requiring fine adjustment of the CG. The 'roll' though can depend on the previously mentioned blade adjustment, but if the model has horizontal stub wings, that can increase the adjustment ease and aid in roll and pitch stability.

To get a rotor that I can adjust the angle of the blades, the fixation to the hub need to be adjustable, here's how:

The hub can be a piece of firm foam, either a circular disc or a square disc. Cut slots to the disc, dissecting at 90degrees for a 4-bladed rotor or at 120 degrees for a 3-bladed rotor.

The slots will accept the blades bamboo rods and by friction alone, allow adjustability. Glue the rods to the hub only when satisfied with the dihedral and blade's angle of attack.

The top and bottom hub surfaces can be reinforced with 1/32" ply pieces or just plastic pieces. Insert a plastic tubing through the centre of the hub.

As re-written by Gemini:

Adjustable Rotor Design for Indoor Rubber-Powered Autogyro

This design proposes a simple and adjustable rotor system for a lightweight, indoor rubber-powered autogyro. The key feature is the ability to fine-tune the angle of attack for each rotor blade, allowing for precise trimming of the aircraft.

Core Components:

• Hub: Constructed from lightweight but firm foam (disc shape - circular or square). Slots are cut into the disc at 90-degree intervals for a four-bladed rotor (or 120 degrees for three blades).
• Blades: Bamboo rods are inserted into the slots, allowing for adjustments in blade angle. Friction holds the blades in place initially.
• Reinforcement: Thin ply or plastic pieces are glued to the top and bottom of the hub for added strength.
• Drive Mechanism: A plastic tube is inserted through the center of the hub to accommodate the rubber band drive mechanism (not described here).

Adjustability and Trimming:

• The friction-based slot system allows for easy rotation of each blade, enabling adjustments to the angle of attack. This directly affects the lift generated by each blade and is crucial for trimming the autogyro for stable flight.
• Once the desired blade angles and dihedral (optional) are achieved, a small screw or locking pin can be added at the base of each slot to secure the blades in place.

Considerations:

• Balancing: Adding or removing material from individual blades for angle adjustments can disrupt the rotor's balance. To maintain balance, consider marking or numbering the blades for re-installation in their original positions.
• Dihedral Angle: While 90-degree slots allow individual adjustments, achieving a true dihedral effect (where opposing blades have a slight upward angle) might be challenging. Pre-cutting the bamboo blades with a slight dihedral angle before insertion could be an option.

This design offers a simple and effective method for achieving adjustable blade pitch in a lightweight indoor autogyro. With some refinements for securing the blades and potentially achieving a dihedral angle, this concept has the potential to improve the flight stability and controllability of your rubber-powered aircraft.

5 March 2024

Designing an Autogyro Rotor

Here's an outline for the design of a simple autogyro rotor for a rubber-powered model plane.

Balancing Lift and Area:

• The autogyro needs less wing area compared to a fixed-wing aircraft for similar lift due to its spinning rotor.
• While the target wing area for the model plane is 24 square inches (12" span, AR 6), the rotor blades can have a combined area of around 16 square inches.

Rotor Design:

• The rotor will have two 8" x 1" balsa wood blades, totaling 16 square inches, which is 2/3 the wing area and has a span of 167%. The rotor disc area is however, much larger.
• A 4" "zero lift" hub will connect the blades, resulting in a total rotor span of 20 inches.

Hub Construction:

• The 1/2" wide hub will be made of three parts: a 4" horizontal balsa beam, a flexible plastic strip 6" long, and a short 1" length of plastic tube.
• The plastic tube will be glued perpendicularly to the horizontal beam to serve as the central pivot point.
• Stoppers, which can be strips of 1/16" balsa, will be attached to the plastic strip to create a slight upward angle for the blades (static coning) and enhance stability.

Blade Construction:

• Two airfoil blades will be made from 1/16" balsa wood.
• The trailing edge of the blades will be thinned to 1/32" to 1/64" for the rear half to improve airflow.
• The leading edge will be sanded to a slight taper for the front quarter with a slight radius underside.

Assembly:

• Chamfer the ends of the horizontal beam and glue the plastic tube perpendicularly.
• Glue stoppers to the flexible plastic strip at a slight angle.
• Punch a hole in the center of the plastic strip and insert it through the bottom of the horizontal beam.
• Apply glue and secure the stoppers against the chamfered edges, then let dry.

Building the Rest of the Autogyro:

Tail Assembly:

1. Motor Stick: Cut a 10" x 1/4" x 1/8" balsa motorstick.
2. Motor Bearing: From an aluminum can, cut a strip and bend one end to form a bracket, use a pin so that the propeller shaft can pass through. Bind and glue this to the motorstick as a motor bearing.
3. Tail Hook: Bend a Z-shape from a paperclip and glue it to the motorstick.
4. Tail Feathers:
   • Fabricate the horizontal stabilizer and vertical fin from 1/16" balsa square strip.
   • Cover one side of each with tissue paper.
   • Glue them to a 4" long, 1/16" balsa square tail boom.

Rotor Mount:

1. Mast and Support: Cut a mast and overlapping angle support from 1/8" balsa square strip.
2. Shaft: Straighten a paperclip and cut a 1.75" length.
3. Attaching the Shaft: Glue 1/2" of the shaft to the top of the mast, leaving 1.25" protruding.

Final Assembly:

1. Glue the tail boom to the motorstick's rear end.
2. Assemble the propeller and rubber band on the motorstick.
3. Find the balance point of the motorstick.
4. Glue the rotor mount to the motorstick, aligning the mast top with the previously identified balance point.
5. Insert the rotor onto the protruding wire shaft, add a glass bead, and glue another bead on top to secure it.

19 February 2024

Teetering Offset Hinge for 2-bladed Indoor Autogyro

Have the 2 bladed rotor prepared.

They are two identical airfoil blades connected to a rectangular strip of plastic. 

Mark out the centre of the rotor and draw a slanted line at 30-45degrees on the plastic strip.

Crease on the slanted line and bend up 5-10degrees on each blade, this will the 2 blades to form a cone angle, works like dihedral and decalage and each blade will have a negative incidence of 5-10 degrees.

Fold a rectangular strip of aluminum (from aluminum can) into a 'M' shape.

The spin shaft pass through the 'V' in 'M' and close the 'V' until it is back to back, glue the shaft in position.

Bend the two 'I' in the outerflaps so that it flares upwards.

Glue the outer flaps to the centre of the rotor along the slanted line.

Now we have a rotor with a perpendicular shaft that will spin freely in the autogyro model plane.

5 February 2024

This is an idea of construction for a small hub for 2 bladed rotors that may be suitable for small and very light autogyros. The rotor is teetered and hinged so that there is some form of correction for asymmetrical lift.

Material list:

• Paper cut to hub-shape
• Right angle balsa/foam spacer pieces 
• small plastic/paper tube
• thin pvc sheet that is flexible and springy for 
• shaft either of CF or paper clip
• bearing made from beads and or thin plastic/metal washer 
• Disc stoppers made with paper/paper card/plastic/wire/rubber etc.

Draw a big T on the plan. Cut 2 pieces of paper to the shape of the hub. Lay 1 piece of hub-shaped paper on the plan. Cut a piece of tube to the horizontal hinge length. Insert a temporary shaft into the tube and lay it over the hub-shaped paper. Cut a piece of tube to the vertical tube length. Insert another temporary shaft into the tube and lay it over the hub-shaped paper. Cut 2 spacer pieces to shape and glue to 1)hub-shaped paper, 2)horizontal and vertical tubes. Sand spacers flush to top of the 2 tubes and glue another hub-shaped paper over it all. Remove the temporary shafts. Cut the horizontal tube and remove the centre portion which will be the pivot tube. Cut the thin pvc shape and glue the pivot tube at the offset angle in the centre of the thin pvc shape. Glue the blades on the thin pvc strip. 

On the actual shaft, glue a stopper, insert the vertical tube, bearing and stopper. Glue the blades to the thin pvc strip. thread short shaft through the horizontal tube and secure the ends from falling out.

Now, this is building up the rotor hub, if we used paper clip as the shaft, we could bend to shape and if it retains under stress, it would be simplified.

Material list:

• small plastic/paper tube
• thin pvc sheet that is flexible and springy for 
• shaft from paper clip
• bearing made from beads and or thin plastic/metal washer 
• Disc stoppers made with paper/paper card/plastic/wire/rubber etc.

Cut the thin pvc shape and glue the pivot tube at the offset angle in the centre of the thin pvc shape. Glue the blades on the thin pvc strip. This is the rotor.

Bend the paper clip; first a 90 degrees bend, another 90 degrees bend so that it runs parallel, and another 90 degrees bend so that it is perpendicular. Maybe the bent shape will open up, maybe it will remain. Insert stopper, bearing, horizontal pivot tube, bearing and another stopper. Then the assembly is inserted to the vertical pivot of the model and add bearing and stopper. 

Another method

Material list:

• Balsa block/laminate
• Aluminum brackets from aluminum can
• small plastic/paper tube
• thin pvc sheet that is flexible and springy for 
• shaft from paper clip
• bearing made from beads and or thin plastic/metal washer 
• Disc stoppers made with paper/paper card/plastic/wire/rubber etc.

Make a rectangular hub base from balsa block or laminates of thinner balsa. Insert and glue a straight length of paper clip, perpendicular to the hub. Fabricate 2 identical brackets from aluminum can or pivot tube from plastic tube with spacer, glue to ends of hub. Measure the distance between the brackets and cut the pivot tube. Cut the thin pvc shape and glue the pivot tube at the offset angle in the centre of the thin pvc shape. Glue the blades on the thin pvc strip. This is the rotor. Offer the rotor to the brackets and insert a paperclip shaft.

Insert vertical shaft to the vertical pivot of the model and add bearing and stopper.

Another method, using paper

Material list:

• Paper
• small plastic/paper tube
• thin pvc sheet that is flexible and springy for 
• shaft from bamboo/toothpick/carbon-fibre rod
• bearing made from beads and or thin plastic/metal washer 
• Disc stoppers made with paper/paper card/plastic/wire/rubber etc.

Cut the paper to hub template. Fold in half. Glue the shaft and pivot tube. Cut to release the central pivot tube. Cut the thin pvc shape, glue the blades. Glue the central pivot tube at the offset angle in the centre of the thin pvc shape. This is the rotor. Offer the rotor to the pivot tubes in the paper hub and insert another shaft.

Insert vertical shaft to the vertical pivot of the model and add bearing and stopper.

29 December 2023

Spinning propeller. spinning rotor

Keep the disc of propeller or rotor spin true, or just less wobbly. That means the bearing holes must have some play but not too much and be spaced a good distance apart.

Bearing structures could be made from thin aluminium sheet with the holes pierced then folded to the hub.

The hub may be 2 identical sheet balsa pieces to anchor and spaced apart the folded bearing structures.

Since the hub is sheet balsa, it will provide good gluing surfaces to the 2 blades.

Distance between bearings, the "height" or "depth" of sheet hub, may be 1/4" for 3-7" prop and 1/2" for 20" rotor. 

Propeller shaft is perpendicular to sheet hub. Rotor shaft can be angled to create a conical disc.

Propeller and rotor shafts may be flat to glue directly onto the sheet hub or they may be thin carbon fibre rods so that the angle of attack may be adjusted. If adjustable rods are used, the holding structure may be tissue/paper, pressed over the rod and glued with superglue on to the sheet hub.

To fold the bearing structure, a former can be used. A former can be 2 pieces of hardwood (ice cream stick), with one edge square and a pin perpendicular to the edge to locate the hole in the bearing structure. Then a plier to fold the aluminium over the same edge.

The blades may be like a paddle where there is no twist between root and tip. Or, the blades may be twisted to form something of a helical pitch.

There is no perceivable benefit to have tip weight to the propeller blades but it may be beneficial to have tip weight to the rotor blades to introduce some momentum stability while the rotor spins.

Rotor constructed above does not have flapping hinge.

Thin rotor blades may bend under flying load and cause something like flapping hinge but there's hardly any control of the hinge axis.

Flapping hinge is beneficial to rotor blades to avoid roll-over.

The flapping hinge may be set at an angle to introduce negative angle as the blade hinges up.

The limit of hinge may be a pair of simple thin aluminium bracket at the flapping point.

6 July 2023

Foam blades with CF rod as spars

1. Blades from foam dinner plates. 
2. Glue CF rod to blade.
3. Balance.

Cardboard hub

1. Draw and mark hub strip on cardboard. 
2. Punch holes according to mark.
3. Fold ends to form isoceles triangle.
4. Make aluminium bearings from drink cans. 
5. Cap the top of the isoceles triangle cardboard hub with the top folded aluminium bearing and add the bottom bearing to the base of the isoceles triangle.

Assembly

1. Insert CF ends of the blades through the isoceles triangle hub.
2. Insert Shaft through the bearings.
3. Adjust and balance.

If the isoceles triangle is used with the base as the top, we have a flat top bearing surface which is better since the lifting/dragging force pushes against the top stop of the shaft. The sides are used as tensile members. The ends of the sides can be lapped to the counterpart side.  

3 July 2023

Paper blades

1. Rectangle blanks, equal lengths, width at 150% of chord. 
2. On underside, mark at 50% chord along span.
3. Crease fold the longer 100% of chord width, the crease line will be 50% and the long edge meets the marked 50% chordline.
4. Slit along marked chordline, glue paper tabs to secure, top of chord and bottom of chord. 
5. Cut 1/32" balsa spars to width of 40% chord.
6. Insert balsa spars to paper blades' pockets, ensure they are identically placed in each blade.
7. Spot glue the balsa spars to the tip and root of the paper blades.
8. Join the balsa spars to the hub and a rotor disc is formed. Could use the balsa spars itself to make the bottom hub.

22 March 2023

Today I am thinking about leverages for pitch roll and yaw stability.

If the rotor is flat, it can't provide any leverage.

Pitch stability

It could be an elevator at the back or a lifting plane at the front. Which is more effective? In the front lifting plane, it would be at the verge of stalling. Front fin would destroy yaw stability, so my choice would be a rear elevator.

A wing can be pitch stable itself, this involves having lifting portion and stabilising portion of the wing front and rear respectively.

A rotor with a dihedral angle will have this form of pitch stability as well.

Yaw stability

It would be a fin at the back. 

Roll stability

It could be by the center of gravity, but more likely some dihedral is required so that together with the fin, there will be less slip.

A dihedral rotor ought to have roll stability too, but I am thinking it is not constant and need supplementation.

To supplement, I think the pitch and roll stability can be enhanced with a lifting plane just in front of the rotor.

So actually, the rotor could be flat without any dihedral if the pitch and roll stability are mainly by a lifting plane. Using the spinning rotor as a additional lifting/dragging device. 

In a beautiful world, a conical rotor only need a fin (for yaw stability).

Differential thrusts changes the pitch, the yaw and affects the roll slightly.  

3 March 2023

Paper blades and ribs.

Ribs: Glue halved paper ribbons to spar.

Blade area: Cut blanks, do a fold, glue to spar and ribs.

That way the ribs can be adjusted for incidence and curled for airfoil.

1 March 2023

Conventional: Tube bearing, bent up tabs, strings and disc

As many rotor blades as needed is made identical with longish spars. 

At root end of each spar is secured an aluminum tab from aluminum can.

Have the central plastic tube surrounded by the aluminum tabs and secured the exposed aluminum tabs against the tube and open up the spars like petals around the pistil. Then glue the blades on each spar, secure with a central disc and adjust.

If the blades are already on the spars, just arrange them with a jig on the tip of the blades.

A disc of card, foam or anything like that with a central hole, is used to lock the tube's perpendicular position, the position of each blade around the tube, and the dihedral angle.

Spars can be made from balsa strips, or carbon rods.

Tabs or joiners to the tube, can be aluminum sheet as mentioned above, or cotton thread.

28 February 2023

How about a cone as the rotor?  It wouldn't spin. What makes it spin? Oblique airflow pushing the blades forward. So what is needed? Some openings in the cone for the air to rush by obliquely? Or just some negative incidence? But there appears to be zero incidence blades and why not?

Try a paper cone with 50% cutouts, the root and the tip will be solid rings. At the top is paper cross pattern to locate the central hub which is a paper tube. Acting like flat blades and adding area to rotor surfaces.

An inverted cone = some camber, more chord = more camber. Very draggy.  But autogyro is draggy and drag contributes lift?

Will location of opening affects anything? It's a 360 conical disc, why should it? Shape of openings? Is circular holes, oval holes, triangular holes, half circles one way or the other way or root or tip? 

27 February 2023

Hub for 3 or 4 bladed rotors, with a different assembly

1. Cut out the top and bottom aluminium shape, punch hole in the centre. In case it is a 4 bladed rotor, the hub piece is a cross-shaped sheet. If it is to be a 3 bladed rotor, then there is 3 spokes.
2. Make a work base that will support the spindle, glue the work plan over the work base.
3. Lay the bottom hub piece through the spindle, over the plan and line it up to the work plan.
4. Glue the arms over the hub piece (don't need chamfering, think of it as adjustability).
5. Bend up the arms to dihedral height over the work base, use a dihedral jig. Start with one arm, then turn and do the bend on the next arm until all arms are bent to the dihedral jig.
6. Remove the arms and bottom hub.
7. Lay the top hub upside down through the spindle, over the plan and line it up to the work plan.
8. Bend up each spoke of the top hub to a jig, fold down the tab and remove.
9. Re-insert the arms and bottom hub to the spindle.
10. Drop the top hub to the spindle and glue the tabs to the arms.
11. Strip identical spacer strips (same height as the jig) and glue to top of the arms and the bottom of the top hub. Alternatively, a tube can be used but it shall be inserted between steps 9 and 10.
12. If the top spokes are too weak, glue balsa strips to reinforce the top diagonal spokes.
13. Remove assembled hub from work base.

Hub for 3 or 4 bladed rotors, with a round tube as upright

1. Cut out the top and bottom aluminium shape, punch hole in the centre. In case it is a 4 bladed rotor, the hub piece is a cross-shaped sheet. If it is to be a 3 bladed rotor, then there is 3 spokes.
2. Make a work base that will support the spindle, glue the work plan over the work base.
3. Lay the bottom hub piece through the spindle, over the plan and line it up to the work plan.
4. Glue the plastic hollow tube to the hub piece, vertically.
5. Glue the arms over the hub piece (don't need chamfering, think of it as adjustability).
6. Bend up the arms to dihedral height over the work base, use a dihedral jig. Start with one arm, then turn and do the bend on the next arm until all arms are bent to the dihedral jig.
7. Chamfer a piece of balsa sheet to meet the vertical tube and the arm, strip to make the corresponding number of diagonal struts.
8. Glue the diagonals to the work piece.
9. Lay the top hub piece through the spindle, over the tube and bend to meet the diagonals and glue in place, 
10. Remove assembled hub from work base.

Hub for 3 or 4 bladed rotors, built up 

1. Cut out the top and bottom aluminium shape, punch hole in the centre. In case it is a 4 bladed rotor, the hub piece is a cross-shaped sheet. 
2. Make a work base that will support the spindle, glue the work plan over the work base.
3. Lay the bottom hub piece through the spindle, over the plan and line it up to the work plan.
4. Glue uprights to the hub piece.
5. Lay the top hub piece through the spindle and over the uprights. Bend to meet the uprights and glue in place, 
6. Glue the arms over the hub piece (chamfered for dihedral).
7. Bend up the arms to dihedral height over the work base, use a dihedral jig. Start with one arm, then turn and do the bend on the next arm until all arms are bent to the dihedral jig.
8. Chamfer a piece of balsa sheet to meet the upright and the arm, strip to make the corresponding number of diagonal struts.
9. Glue the diagonals to the work piece.
10. Remove assembled hub from work base.

10 January 2023

hmm.... just a criss and have the blades as the cross. Reinforce with aluminium strips for the bearing points.

6 January 2023

Foam blank with 45degrees bent to provide negative incidence and dihedral

Tin can aluminium strips with double 45 degrees bend to support the blades at top and bottom.

The bottom strip may be bent at the axle hole or bent into a curve.

The axle passes through the bottom strip, foam blank, and top strip, anchored at the bottom, guided by a wire loop, through the rotor assembly, a single bead and secured at the top with a 90degrees bent or stopper.

Foam blank may be extended with paper sails supported by strips which are glued to foam blank, treat the foam blank as a spar.

6 December 2022

No suitable nozzles? How about carbon rods, paper clips and some ready at hand stuff? 

• A short piece of carbon for the blade shaft and at both ends, tie a Z bent paper clip
• Glue blades on the Z bent wire flat over the table
• Bend for dihedral and twist for incidence
• Glue the spindle rod perpendicular to the center of rotor, reinforce with thin card patch or other means.

5 December 2022

Re-using Covid ART nozzles for spinning Hub and dihedral blades. 2 blades rotor.

If the chosen carbon fibre rod fits tightly in the nozzle hole:

• ice cream stick with hole in middle
• thin aluminum strips to ends of ice cream stick and blades
• Bend the aluminum strips to give equal dihedral and angle of attack 
• Covid ART nozzle hotglued to ice cream stick, over the hole, I prefer bottom.
• insert carbon fibre rod through the nozzle and the ice cream stick

If the rod spins freely (but not too wobbly) in the hole in the Covid ART nozzle kit, insert the rod through one nozzle, sandwich the icecream stick and then another nozzle. Once satisfied, glue the nozzles to the ice cream stick.

11 November 2022

Spinning Hub

• A piece of round or square cardboard with a centre pivot hole. Or a pyramid formed from 4 triangles or ring of 4 triangles, so that this pivot hole is set above the cross-piece pivot hole. (can also have a square cardboard supported by 4 perimeter cardboard spacers or a central single piece spacer with a larger hole in the centre) 
• A piece of cardboard cut to a cross shape with a centre pivot hole.
• The 4 arms are bent to 15 degrees angle, superglue applied to lock the 15 degrees dihedral and decalage.
• The ends of the 4 arms are cut to 15 degrees angle.
• The pivot holes are cored to fit the shaft and reinforced with superglue, since they are made from cardboard.

Flapping Blades

• Root of each blade (4 in total) is a small triangle with a 15 degrees cut to match cross piece.
• Leading edge is a piece of balsa or foam, glued to the root piece.
• Root chord and tip chord is balsa or bamboo.
• Covered on top with 70 g/sm photocopier paper
• Flapping hinge is a trapezoidal piece of flexible foam.

Assembly:

1. Pin the hub in the centre pivot hole, use the same 15 degrees jig to raise and superglue the 4 arms so that the arms is raised to 15 degrees. When one arm is set, rotate the hub and do the next arm.
2. Remove pin, place scrap pieces of flexible foam under the cross piece and re-pin. This raises the entire cross piece, letting the blade to slid under,  
3. Glue flapping hinge to underside of each blade, slip and glue the flapping hinge under the cross piece, make sure a gap of 3mm is between the cross piece and blade root.
4. The 15 degrees jig will support the blade while the glue set.

23 September 2021

Sure Fire Autogiro

Rake: 3/8" over 6"

Slant: nil

Dihedral (each blade): 2.3/4" over 13"

Angle of Attack: 0

Blade x2 clockwise: 11" x 2.3/8" paper

Downthrust: similar to Rake

With horizontal stabiliser

Cierva Autogiro

Rake: nil

Slant: nil

Dihedral (each blade): nil

Angle of Attack: negative 1/8" over chord

Blade x4 anti-clockwise: built up

Downthrust: nil

With roll paddle and elevator

Can't find more.

If blades can be set zero, one less to get wrong.

Rotor hub can be made from a piece of flat wood, paper clip bent to give dihedral where blades are patched.

Blades can be balsa built up with paper

22 September 2021

How about plastic tubes for the hub? If doing a very light version, bent plastic tubes from cotton bud may be enough. 

Bend 2 lengths of plastic tubes in their centre by crimping. (the dihedral effect)

Glue bent plastic tubes at right angle to each other. (4 rotor blades)

Prepare jig to hold the crossed plastic tubes evenly.

Example of a jig: one centre pin, 4 spacers to support the crossed tubes.

Pierce centre, enlarge for shaft.

Use a temporary shaft, tie the crossed tubes with thread and glue.

Prepare 4 rotor blades.

Using previous example of jig: add a triangle to one of the spacer. The triangle set the angle of attack of each blade. Can add blocks so that each blade will be located at the same place.

Glue each blade in turn.

Or, to make the angle of attack adjustable by gluing a bamboo stick to each blade and insert the bamboo into the tubes. Doesn't fit? Sand down before gluing to blade or split the tubes' ends.

A 2 blade rotor with doweled blades would be:

1. Pierce a hole in the centre of a plastic tube, enlarge hole to shaft diameter.

2. Bend up for dihedral, not in the centre of the tube, just a short distance away to avoid skewed hole.

3. Insert blade, adjust to angle. Just eyeball and spin the rotor.

Shaft shall be from a paper clip? It is easy to bend.

9 April 2018

On Sunday, I tried Version 2. Version 1 had a bamboo boom and vertical tail. Version 2 had foam fuselage, horizontal stabilizer and 2 tip fins and  affixed to the other components of Version 1.

I tried running start to spin the main rotor fast at full throttle. The initial launches rolled right and crashed within a foot away. I finally got a left arc about 2 m away after having adjusted the model to have left rudder, left weight, and left rotor. The 8.5mm motor and 56mm propeller seems incapable of overcoming the drag.

The fooling around stopped after I noticed my sewing pin is bent. Maybe next time, I can try rotors with flapping tips. From the anticlockwise prop (from front), anticlockwise rotor (from top), I can have left thrust, left weight, left rudder and left rotor. Or I am also thinking of normal elevator and rudder and if one rotor does not balance, twin rotors. I need also better pushrods then the soft wire can afford.



Version 2

These did not grip the thin soft wires I am using and the nuts get loose easily.



Version 1, as the motor stick is screwed, thrust direction is adjustable. The hotglued boom came loose easily and the bamboo size lacks torsion strength.




While making Version 1, I originally thought of making it a pusher.

5 October 2017

Previous was using plastic tubing, how about just beads?

This will limit the contact of the axle to the two points on the hub.







The ice cream stick option has the blades at 0 degrees. Pack with beveled pieces at the blade holding area, or instead of beveled pieces with balsa blocks that has angled saw cuts (similar to sketch on right), the blades will be held at an angle. Or, maybe we can leave it at 0 degrees, and use flat blades that have toe-in/toe-out near the tips by angling it upwards, which ought to propel the blades.

Or how about a spinning disc, spun by peripheral winglets? The centre area is slow anyway and can't possibly have much difference whether it is spinning slow, fast or not at all. When the disc stop spinning, it's still a larger flying surface than is possible with the normal blades' area.

Change the kite to an autogyro? I think a tractor propeller and a large tail is good.

1 February 2016

Even-Bladed Hubs

Too much theory only ends in theory and I get confused. Maybe just try the following. If I set the blades carefully, I could have some coning too.

2mm dowels from Daiso (Basswood?) for hub and blades leading edges.
2mm foam blades (from Daiso) are glued to the 2mm dowel leading edges and wrapped in "Scotch" tape.
Large heat-shrink sleeves to set the angles and dose of superglue when set is as desired.

How about a Cootie but with this blade hub?

16 September 2015

On the following chain of thought, of which all hypotheses are without any substantiation or even personal experience:

1. Negative angle of attack is necessary to get the rotor spinning in the correct direction against the model's direction.
2. Lift or drag of the rotor amounts to the same thing, it lifts the model so long as there is some element of vertical vector.
3. Negative angle of the rotor blades is not too critical, even if it is at -45 degrees, there would still be 'lift' (point 2 above), except that we shan't go there because everyone will tell you it's wrong and I won't be needing that! (And also for point 12 below.)
4. The angle the entire rotor is presented to the wind has to be positive. I think this is the most important aspect. If it were negative, the model sinks instead of rising.
5. Combining point 3 and point 4, if there is not enough negative angle of the blades, at some point it could become a positive blade angle to the wind if the positive rotor angle is excessive. If a blade angle is positive to the wind, then it would retard the spinning rotor because it would attempt to spin in the opposite direction. The rotor would then stall, and hence a faster spinning rotor is safer than a slower one.
6. Carrying on from point 5, this mean that the negative angle of blade must be greater than the positive angle of rotor.
7. The torque of a free spinning rotor is unlike a motorised rotor. The torque is in the same direction as the rotor direction. Hence, if a tractor propeller has left torque, the rotor should spin clockwise when viewed from top because the blade going against the direction of the wind will create more lift, in this case, the left half of the rotor has more lift then the receding right half of the rotor.
8. Since the rotor is functioning as a wing, dihedral would be desirable if we are using rudder to turn the model. Air flowing through the rotor will tend to cone the rotor in the correct direction, yet the faster it spins, the more centrifugal force of the blades will flatten the cone.
9. Should the blades be like a normal flatbottomed airfoil or should it be mounted inverted instead? If it is mounted like a normal airfoil then the rotor would have more lift because of the downwash. If it is mounted inverted, we are having air directed upwards and therefore there would be less lift and more drag.
10. Carrying on from point 9, the main objective should be to have the blades advancing with the least drag and therefore, a thin non-cambered foil is desirable. Something that has a smooth entry and exit.
11. When the model flies faster, the rotor spins faster and the model rises, so downthrust is required to 'lift' the tail.
12. If there is no elevator to lift the tail, the flyable CG range is very limited and the downthrust has to do the 'lifting' of the tail. There is no lift when downthrust equals negative angle of blade and hence the tractor propeller should be set at a greater negative angle.
13. For a simple rotor, the negative angle of the blades shall be fixed. If the positive rotor angle is 8 degrees and we desire a negative effective angle of 3 degrees, this means the blades are set at 11 degrees and the effective downthrust angle is 3 degrees.
14. When the propeller is lower than the rotor, and assuming it is not a very long way infront of the rotor, there is a upwards turning moment. Therefore the downthrust angle has to be set greater than 3 degrees. However, if we have a stabiliser set at a positive angle leewards of the propeller, the stabiliser would have some downthrust to pitch the model down when forward speed is increased.
15. A simple autogyro with the following parameters: A coned rotor with blades set at 11 degrees negative is set 8 degrees positive (round disc don't really stall) to the datum line. Motor is set at 3 degrees negative and stabiliser at zero degrees.

Glue Comparison

11 February 2025

Property/Glue Type	Superglue (CA)	PVA (White/Clear)	Aliphatic Resin	UHU POR
Description	Cyanoacrylate, fast-setting, strong bond	Polyvinyl Acetate, water-based, general purpose	Similar to PVA, but stronger and more water-resistant	Contact adhesive, flexible bond, foam-safe
Dries	Very Fast	Slow	Medium	Medium
Strength	Very Strong	Medium	Strong	Medium-Strong
Flexibility	Low	Medium	Medium	High
Sandability	Tricky (can be brittle, clogs sandpaper)	Good (but can be a little gummy)	Very Good	Poor (tends to gum up sandpaper)
Gap Filling	Low	Medium	Medium	Medium
Thinning Solvent(s)	Acetone (but often not recommended)	Water	Water (small amounts) or denatured alcohol/ethanol	Heptane, Lighter Fluid (Naphtha), White Spirit
Notes	Excellent for quick repairs, brittle	Easy to use, good for wood and paper	Good for wood-to-wood bonds, sands well	Good for foam, rubber, requires contact, not ideal for sanding

5 February 2025 

Discard old glues, use fresh glues. 

Improve water-resistance by applying thinned EZE Dope.

Minimise sanding by using solvent to wipe excess glue.

Porous filler, such as baking soda, wood dust, cotton wool, reduces the gaps caused by imprecise joints. Baking soda works best for superglue.

I read that UHU Hart can be used on Depron.

I read that Aliphatic glue is stronger, less flexible, better water resistant (almost but not fully) and easier to sand.

Deluxe Material's SuperPhatic is more penetrating but too expensive.

Feature	Super Glue (Cyanoacrylate)	UHU POR	White Glue/Elmer's Clear Glue (PVA)
Strength	Very High	High	Moderate
Drying Time	Extremely Fast	Fast	Moderate
Water Resistance	Poor	Moderate	Poor
Flexibility	Low (Brittle)	Moderate	Moderate
Sandability	Very Difficult	Difficult	Moderate
Versatility	Limited (Best for small, precise bonds)	Primarily for polystyrene	High (Various materials)
Toxicity	Can be irritating to skin and eyes	Generally safe	Non-toxic
Typical Uses	Small repairs, jewelry making	Bonding polystyrene, model building	School projects, crafts
Gap-Filling	Poor	Moderate	Poor
Notes	Cures extremely fast	Designed for polystyrene	Common school glue, many brands available

Simple propeller for indoor rubber powered models (living room flyer)

11 February 2025

Propeller Blades

Discardable Plastic Cups. I don't know what plastic is used for the clear discardable plastic cups. It is transparent, it could be polystyrene or propylene. It's not important, I just know it would make good propeller blades.

Propeller Spars

Don't need spars if the blades are glued to the propeller hubs.

If propeller hubs are made from thin tubes, with the idea of being able to adjust the blades' angle, then thin carbon fibre rods 1mm in diameter is suitable.

Wire shafts and propeller Hubs

Making propellers with these as motor shafts:

 

DIY Propeller Shafts for Indoor Model Aircraft

These pre-shaped steel wires, originally intended for fishing lures, offer a surprisingly effective solution for creating robust propeller shafts for small rubber-powered model aircraft, including indoor flyers like Living-Room-Flyers (LRFs). Their strength and pre-formed hook make them ideal for this purpose.

Features:

• Pre-formed Motor Hook: The integrated hook simplifies motor attachment or winding.
• High-Strength Steel: The 0.7mm diameter steel wire provides excellent strength and resists bending, even in collisions.
• Ideal for LRFs: The lightweight yet strong wire is perfect for the small, light propellers used in indoor models.
• Cost-Effective: A pack of 20 wires provides ample material for multiple propellers.

Specifications:

• Material: Metal steel wire
• Wire Diameter: 0.7mm
• Length: 65-75mm (Suitable for small propellers)
• Loop Diameter: 4mm
• Quantity: 20 pcs

Construction Notes:

These wires are exceptionally strong and require heavy-duty cutting tools. Standard side cutters may be damaged by the 0.7mm wire. A rotary tool with a cutting disc is recommended but they are noisy.

Propeller Hub Options:

Several hub designs can be used with these wires:

1. Balsa Strip Hub: A traditional method using a 1/8" x 1/8" (or larger) balsa strip (or bamboo or other wood) with chamfered sides for the propeller blades and a central hole for the wire. Secure the wire with a 90-degree bend, thread wrapping, and superglue. For added strength, consider using a thicker balsa strip (e.g., 1/4") to increase the gluing surface for the blades.

2. Balsa Dowel Hub: A more robust option using a 6mm diameter balsa dowel with a hole drilled for the wire. Propeller blades are attached to diametrically opposing saw cuts in the dowel. This design offers superior strength, is easier to drill straight holes in, and readily accommodates three or more blades.

3. Integrated Winding Hook: The pre-formed hook can also be used as a winding hook. Thread the wire through the hub components from the propeller side towards the tail, and then bend the motor hook to the desired shape (e.g., reverse-S, diamond, or round). For this method, create a 0.7mm slot in the balsa strip or dowel to securely position the wire and expose the winding hook.

Important Considerations:

• Cutting the Wire: Standard side cutters are not recommended. Use a rotary tool with a cutting disc for clean cuts. It will be noisy!
• Propeller Material: Lightweight materials are essential for LRF propellers. Avoid using heavy plastic propellers. 
• Hub Strength: Ensure the hub is strong enough to withstand the forces of the rubber motor. The balsa dowel method offers increased durability.

Conclusion:

These pre-shaped wires offer a simple and effective way to create durable propeller shafts for small rubber-powered model aircraft. Their strength, pre-formed hook, and compatibility with various hub designs make them a valuable resource for model builders. Just remember to use appropriate cutting tools and consider the strength requirements of your specific model.

Plastic Cups

11 February 2025

I don't know what plastic is used for the clear discardable plastic cups. It is transparent, it could be polystyrene or propylene. It's not important, I just know it would make good propeller blades.

DHC Chipmunk

11 February 2025

If I were to aim for lightest construction, for this DHC Chipmunk or Airacobra (which incidentally has what I think as the perfect proportion, especially the longer nose, I would construct it solely from EPS using hot wire cutter and lots of sanding, the fuselage, the wings, the tails.

Concerns such as stress areas can be strengthened easily let in either internally or on the outside. If built up Chipmunk weighs 8-15grams AUW, EPS Chipmunk could weigh 10-30% lesser.

EPS being spongey means some flexibility and won't explode on impact.

Hate the sanding and there're lots to sand.

11 February 2025

This is an adaptation of "Chipmunk. Rubber scale model. Wingspan 9 in. by Howard McEntee, from Aeromodeller Annual, 1953". The plan is enlarged to about 13" wingspan.

  

For the first version of the 13" span DHC Chipmunk, the simplest (i.e. the easiest) construction is  chosen. It shall be powered by a single 160mm x 1.4mm x 1.4mm rubber band.

Fuselage: There is no removable noseblock, instead, a large thrust button (or call it the nose plug) is removable. The thrust button comprises a 7/16" diameter fibre washer glued to 1/4" diameter hard balsa dowel, for ease, let it be 1/2" diameter on 1/4-3/8" hardwood dowel (weight at nose is usually not a concern). The motor shaft wire will run centrally through this removable nose button. It is important that the nose plug be at an angle for right thrust and is securely seated in the nose of the fuselage, add scraps of balsa to ensure that, you can be generous, because weight near the nose is a good thing. The sides are from 1/32" balsa sheet, from the nose ring to the end of the tail. Formers 1 and 2 are from 1/16" balsa sheet (the 3/8" hole in the formers, is already larger than the 1/4" nose plug clear the winded up rubber band comfortably, I expect there may be occasional slapping noise, I can increase the diameter within reasons). (Note: It is possible to make lighter formers by using frame construction for this small formers, but the weight savings from the reduction of balsa will be offset to a large extent by the increase in glue, in the end, there is no appreciable reduction and the additional rigidity is unnecessary). If the 3/8" hole cutouts from the 1/16" formers are intact, they will be the motor peg reinforcement glued cross-grained to the insides of the 1/32" sides, if not, square pieces of 3/8"x3/8"x1/16" balsa is equally fine to reinforce for the motor peg. The motor peg be sympathetic to the right thrust nose plug, i.e. the right side will be slightly further back from the left side, something like 1/32" only. The front and back, top and bottom deckings are of 1/32" balsa sheet (yes, it may be unnecessary heavier than it need to be, but it is easier to work with). An access hole will be cut at the back bottom decking, under the motor peg location.

The original plan calls for the completed fuselage to be cut to accept the wings. First we make the wings, obtain the top line and draw it on the fuselage sides, so when the fuselage is tissued over, the line is there to cut loose the portion below the wing. The little shell is then trimmed by 1/32" and glue back after the wing is glued.

Wing: The wing is from 1/16"x1/16" balsa strips and 1/16" balsa sheet tips.It is constructed flat, then the ribs are cracked to be cranked airfoil, joined at dihedral angle, add centre rib and landing gear ribs. It will be covered with tissue on the top surface only. 

Tail feathers are from 1/32" balsa sheet. It is tempting to make it lighter (keep tail light mantra!), but remember, it is about the easiest construction.

The AUW target is 8-15 grams.

Notable considerations from Techniques for making your peanuts lighter:

• flight time is inversely proportional to weight to the 3/2 power, decreasing the weight by 25% should increase the flight time by 40%. 
• at lower speed, the effects of small warps, drag, and other imperfections will be less. ... will also recover from stalls and other disturbances more quickly because of momentum considerations. The model will be less sensitive to thrust and other trim adjustments.
• Aerodynamic considerations show that lift is proportional to velocity squared. To generate twice the lift, the model must fly 1.41 times the original velocity. ... therefore doubling the weight means the model must fly 1.41 times as fast!! ...it possesses twice as much kinetic energy!  ,,, fly at 71% the (original) velocity and will possess 25% the kinetic energy when it hits the wall.

10 February 2025

Design ideas for Chipmunk, summarizing 7 February 2025's ideas.

Requirements: a single rubber band 160mm x 1.4mm x 1.4mm.

Design ideas for this fuselage.

1. Draw a symmetrical top view outline of the fuselage with a centerline. Add in 2 degrees right thrust line, bisecting the nose insert. Draw internal lines following the top view outline, 1/8" inside the outline (this is the horizontal keel which will be cut from balsa sheet). Measure off 150mm from motor hook on this thrust line, mark perpendicular line to indicate the motor peg; motor peg is perpendicular to thrust line, not the center line of the fuselage. Draw out the details of nose's, motor peg's and other braces.  
2. Make the nose insert from 1/8" balsa sheet.
3. Construct the horizontal keel of the fuselage from 1/16" thick balsa sheet. Use the prepared nose insert as a temporary former. 
4. The horizontal keel will be glued to the fuselage shells, and it depends on the fuselage shell for torque resistance to the twisted rubber band.
5. The horizontal keel does not have to be along the fuselage's datum line. It can be angled for down thrust. Alternatively, if practical considerations due to space constraints do not allow this, the wing and tail can be later glued to the fuselage at a positive angle (while maintaining their decalage). 
6. The portion of fuselage above and below this horizontal keel can be made non-load bearing, thereby improving lightness. Material such as EPS and later cover them with light tissue will provide sufficient torque resistance to the horizontal keel. Thin or very small areas can even be paper/tissue or tissue/tissue.

I posted this to Microsoft 365 Copilot and it gave me this:

7 February 2025 

Spotted a DHC Chipmunk From Oz : Chipmunk plan - free download

Chipmunk. Rubber scale model. Wingspan 9 in. by Howard McEntee, from Aeromodeller Annual, 1953.

Scale, F/F, LowWing, Trainer

It is an all-sheet balsa construction. I like it for its simple lines, caged-canopy.

The plan prints on A4 paper. I enlarged it by printing on A3 and it gives me a wingspan of just under 13" and a fuselage length that appears appropriate for a single rubber band 160mm x 1.4mm x 1.4mm.

Thoughts on the Construction of the Fuselage

Instead of using 1.4 times the thickness of 1/32" balsa sheet, I could maintain 1/32" thickness, it would be proportionally thinner and therefore lighter. But why stop there? A frame structure with upper and lower longerons of 1/16" square would be lighter. Also, the moment of inertia would be much better than a simple 1/32" sheet. I used Gemini and it calculated a saving of about 1 gram. 

One deviation from traditional frame construction which is by cutting spacers accurately to fit between the longerons, I would use 1/8"x1/32" balsa strips on the inside of the longerons. This would be easier because excess can be trimmed against the longerons. It will also be structurally stronger as the joints are lapped. For the lower longeron, it has to accept a one piece wing. I will construct it as segments, comprising of appropriately pre-bent 1/16" square balsa strips and a laminated wing saddle piece (2 pieces of 1/32x1/16" balsa strips glued together over a rib template). The previously mentioned spacers will overlap the segment joints, unifying the fuselage's frame structure.

I learned this while building the similar sized Druine Turbulent: the rubber motor would interfere with any top formers or spacers. In the Druine Turbulent, I simply lowered the thrust button so that the entire rubber motor will be below the top spacers, but this mean it is not in its scale position. The Chipmunk isn't much better as it has a high thrust line. I would avoid any top former/spacer, but then, what would maintain the tissued paper decking in its position and what if the fuselage opened up under the tensive strain of the rubber motor?

2 possible solutions:

• Stiffen the upper longerons by gluing another strip of 1/16" square balsa.
• Laminate the upper formers  (2 pieces of 1/32x1/16" balsa strips glued together over a former template). The laminated former will be glued to the 1/8"x1/32" inner vertical spacers.

I will choose laminated upper formers, hopefully, they will provide sufficient tying in force on the upper longerons, and they also support the tissued paper decking. To improve the tying in force, I can improve the rigidity of the laminated formers as follow. 

1. Choose the widest former, as it is usually somewhere in the middle of the fuselage where the tying force is most needed. In the Chipmunk's case, that will be the location at the wing's leading edge.
2. Glue 1/32" balsa sheet with the grain diagonally across one edge and glue another 1/32" balsa sheet with the grain diagrammatically opposite across the other edge and side. The inverted V void would clear the top of the rubber motor. 

The diagonal braces to the top former can be 1/8"x1/32" balsa strips and performed on the building board so that it can be clamped or weighted down, ensuring a clean and strong construction.

For tying in the lower longerons, they may be spaced out with 1/8"x1/32" balsa strips to unify the left and right sides of the fuselage by overlapping the top of the lower longerons. Lower gussets is a possibility to resist torque, but do it only on the widest former, i.e. front of wing saddle. Again, think of ease and neatness of construction, it can be 1/8"x1/32" balsa strips like the top formers, one strip on one side, or alternatively, both strips on the same side. Lower formers to complete the bottom of the fuselage are easy. Cut the formers from 1/32" balsa sheet and glue a 1/8"x1/32" strip across the grain. This ensures maximum strength where it is needed. If a centre keel/stringer is required, the reinforced formers can be slotted while on the building board.

Thoughts on the Construction of the Wing

The tapered wing panels and small curved tips (unlike the Turbulent's nearly elliptical tip) are easy to replicate with 1/16"x1/16" balsa strips and 1/16" balsa sheet tips. This will be a one piece wing, as I do no want to glue root ribs to fuselage. It will be covered with tissue on the top surface only. The ribs will be sliced from 1/16" balsa sheet and their primarily purpose is to support the tissue.

The landing gear will be glued to ribs, this pair of ribs will be full ribs and be triple thickness, i.e. 3/16" thick, to accept the pushed-in landing gear later. Cut the top line with the same universal rib template and then a straight cut 1/16" down, so that it will seat flush like the rest of the ribs. Similarly, do a centre rib of 1/8" thick. These 3 ribs will connect to one another through 1/16" balsa spar and dihedral brace after the wing panels are at the desired dihedral angle.

1. Construct the wing panels by laying out the leading and trailing edges, the tips and all the ribs (i.e. the landing gear ribs, the secondary ribs, but exclude the centre rib for now).
2. Crack the leading and trailing edges, raise one panel's tip to the desired dihedral angle.
3. Position the centre rib, straddling across the cracked leading and trailing edges and glue.
4. Add spars and dihedral brace.
5. Finally add gussets and any reinforcements. 
6. Sand round the edges.

Using 1/16" square strips is convenient but it won't be as durable as using 1/8"x1/16" similar to that of the Turbulent's construction. This time round, it will still be a one piece wing, but it will be covered with tissue on both upper and lower surfaces. The ribs are full depth, the landing gear ribs the same 3/16" thick, but employ two root ribs, one for each panel.

1. Construct individual wing panels but using the 1/8"x1/16" balsa strips for the leading and trailing edges. Lay out all the ribs perpendicularly on each panel but remember to position the root rib at the dihedral angle.
2. Add spars and gussets. Remove from building board and trim the protruding spars, leading and trailing edges to match the angled root rib.
3. Glue the left and right wing panels together, insert the dihedral brace and any reinforcements.
4. Sand the completed wing.

I asked Gemini and it prefers the 2nd approach, quoting an increase of perhaps 1.5 grams, accounting for the additional balsa in the leading and trailing, full depth 1/16" thick ribs and additional EZE-Dope coated bottom tissue covering. The added durability and strength is worth the 1.5 grams penalty, it's a good trade-off for a more robust and better-flying model.

Thoughts on the Construction of the Tail Feathers

Here I will go for 1/32" balsa sheet covered both sides with EZE-Doped tissue for ease of production. Cut and sand the balsa sheets, round the edges, draw on the hinge lines and other details. Cover one side and then the other side with tissue. Apply EZE-Dope. This ensures stiff and flat tail feathers that will allow adjustments easier.

The fin has anti-spin strake, remember that the balsa grain would be different from the vertical fin. The finished product will have the drawn lines showing through slightly on the translucent tissue. 

Thoughts on Construction of other Details

The cowling and noseblock can be one, after the nose block is shaped, gluing tissued paper will form the cowl. The noseblock can either be glued to the fuselage, or, as I'd prefer, removable, so that I can change the propeller, shaft, rubber. The nose of the fuselage's frame structure can be reinforced by adding balsa, paper or card. All it needs is a piece of 1/8" balsa that matches the nose to be glued to the nose block.

I could follow the plan and make the main landing gear. Or, I could avoid damage by removing them when flying and re-inserted when on display. I'd simply choose the latter. Still, I'd like it to be light, so I wouldn't use the suggested thin wires and spinning wheels. Instead, I'll drill the landing gear ribs to accept thin carbon fibre legs. At the wheel end of the legs, a disc of cardboard and foam laminate is glued directly to the carbon fibre rod. The wheels won't spin naturally, but it'll probably still ROG. 

The framed canopy would be easy to do by using clear packing tape over nicely cut paper frame.

The roundish tail cone can just be tissued covered paper. So can the wing tips.

The motor peg will obviously be at 150mm from the motor hook. Place the two side frames together, place the 2 pieces of 1/16" thick balsa reinforcement, drill the hole together.

Fuselage decking is basically tissued paper.

Tissued paper is simply doping tissue on a flat piece of paper. Then it is prettied and cut to shape, rolled or bent, and finally glued to the structure at the edges.

No thrust button, no tubing and the whole propeller, motor shaft and noseblock is one unit. Here's why: Will I do that little thrust button on my removable nose piece? Well, only if the thrust button, together with the motor hook and propeller can be removed from the noseblock. This will probably necessitate making not just the hole for the thrust button, but also a slot that allows the motor hook to enter and exit. The simplest form I can think of is to make a slot in the removable nose block that can pass through the motor hook, a hole that fits a plastic tube but is bigger than the slot to prevent it sliding around, the plastic tube with a card flange and secured with windings of cotton thread. I intend to use paperclip wire for the shaft, so it will wobble in the tubing. If I don't do that little thrust button, I can simply glue the tubing on the removable nose piece, in this case, the propeller, shaft, tube, noseblock is one unit. When I need to replace the motor shaft or propeller, I will need to make another removable nose block. Not a biggie, I only need to face the nose block with some flat and smooth enough surface for the rubber motor to press against. There is no need for tubing or thrust button. In this respect, I think it is a more practical approach. A largish hole through the noseblock, a front and rear metal/plastic piece that has a hole slightly bigger than the wire shaft will be glued over the largish hole of the noseblock so that the wire shaft will be guided to spin in position. To make the precise holes in plastic pieces, heat the wire and pierce it. If it is metal, then sharpen a wire and pierce the hole before cutting to shape. I will use 2 or 3 lamination of thick card.

Make the removable nose block by first making the insert that fits into the fuselage. Then glue this insert on to the rear of the nose block and only then, shape the nose block to match the fuselage. To make sure that the insert will fit neatly into the fuselage, you can start by sizing the insert and make multiple copies especially when you may want to make multiple removable noseblocks. Take one insert and use it first as a former to bring the fuselage frame together. This ensures perfect fitting inserts. Or to put it another way, perfecting fitting fuselage nose to the inserts.