Back to modelling youth: Druine Turbulent

5 December 2024 (conical sanding tool)

In order to sand the 1" diameter foam wheel, a flat sanding stick can be used to sand the rounded edges, it will just take a bit longer. Alternatively, a strip of sandpaper can be glued to form a conical loop, this conical sanding tool's advantage is that more of the wheel's edge can be sanded uniformly in one rotating swipe. The disadvantages is how one can hold the wheel and rotate the conical sanding tool. 

4 December 2024 (wheels)

An alternate to Method C for the wheels is to forego the 2 circular cardboard discs. Hold the skewer with the master hand vertically with the pointy bit down against the worksurface. By holding the hotwire cutter vertically in the non-master hand, rotate the skewer. 

Method D for making 1" wheels? Glue a thumbtack on a steel rule, thumb side against the steel ruler. Pierce the foam on the sharp end of the thumbtack. The steel edge serves as a guide for the hot wire cutter ("HWC"). Rotate foam, slice with HWC and repeat until the wheel appears round.

No skewer, no cardboard templates. Just a steel ruler and a thumbtack.

In the end, this was how I did it:

1. I used a circle template and drew 2 discs on a thin piece of card.
2. Cut the card discs with a pair of scissors. It's ok to be a bit off because it will be sanded round after the basic wheel is completed.
3. I pushed a thumbtack through the centres, locate a disc to the top right edge of a wooden block and pin it. Remove the disc and thumbtack, reinsert through the foam sheet back onto the wooden block. The key is getting a right angled block of wood with the radius pinned accurately. 
4. Slice the foam using the edges of the wooden block as a cutter guide.
5. Glue with foam safe UHU POR with the card disc sandwiched between the foam pieces.

To complete, insert bushing, sand and paint (future).

25 November 2024 (rubber bands, propeller shaft, tissuing)

I have bought something called "Rotating Sequin Steel Wire", perhaps it can be used as propeller shafts because it seems that I only need to bend and trim the ends. This product is for making your own lure for fishing, it appears to be a stiff length of wire with a small eye that is not closed, much like the rubber hook. I bought it because the hook is pre-formed and it is only a dollar plus for 50 pieces. If I have to buy piano wire to make it myself, I would probably have to pay much more at a hobby store for the wire alone.

I have also bought a pack of rubber bands and am waiting for them to arrive. It was described as 102mm diameter, 1.4mm thick. Gathered to a loop without stretching, the loop is 160mm. The motor hook to peg distance will be about 180mm. I bought it because it is promising and it is cheap, only a dollar plus for a bag of perhaps 100 rubber bands. Paying $20 for 10m of 'proper' rubber strip is comparatively expensive. I'll first try with just a single band, it will be a stretch, but let's see if it can fly? After that I'll probably need longer rubber bands/loop and if possible, to have the proper slack so that the propeller can free wheel. So how can I make up the length of the rubber motor, do I conjoin with a wire extension or another rubber band? Looping another rubber band will be 160+150=310mm long (assuming 10mm for the loop around itself), a bit too long in total. And then as I type out this problem and thought about it, I have another idea. I could use a smaller rubber band instead. I can insert a smaller rubber band through the longer rubber band and use the ends of the smaller rubber band to hook onto the propeller shaft. The smaller rubber band will have double the cross-section of the 160mm rubber band, it will be more durable at the rubber hook position and provide a good grip to the 160mm rubber band. The combined length will be longer than the 180mm and it can allow the free-wheeling ramp on the plastic propeller to function. I reckon that the power of this hybrid loop of rubber is still 2 strands of 1.4mm square section rubber, but the energy is increased because the length is increased and more winds can be turned in. For small adjustments, I could twist the smaller rubber band before looping each end over the rubber hook. Anyway, if the propeller does not free-wheel, it is ok, because I doubt it matters much. 

To cover the model with tissue, understand that the tissue panel is flat, so I will start with the smaller and trickier surfaces. For the wings and horizontal stabilizer, I think generally it should be the wing tips, then the bottom and finally the top. For the fin, I'd do the starboard side follow by the port side because I like to hold it in my right hand and look at the left of the model. For the fuselage, I'd do the small bits first, follow by the top, then the bottom and finally the sides. This is because the top longeron is straight, so it is a simple task to cut rectangle strips of tissue, overlap the top and bottom tissue edges and then trimming to suit the curved lower longerons.

Now I am thinking that the propeller shaft will be running in plastic tube that is in turn glued to the removable nose block with some down thrust and right thrust but much lower than the 'scale' position as shown on the plan. The scale position is too high near the top longerons, I fear that at the designed position of the plan, the rubber hook and rubber motor will rub against the top spacers of the fuselage, therefore I should lower it to clear the top spacers.

20 November 2024 (I don't like the wings)

The wings are completed, but I don't like them because they are not identical (hmm, because I didn't line the root ribs accurately enough?). Also, while sanding, one of the spar broke, yes it has been replaced but such is the inherent weakness of 1/16" square balsa. It was relatively easy to sand the 1/8"x1/16" trailing edge and the ribs, I just glued it to the edge of my packing tape jig board, whittle and sand away. within my ability. I was surprised that the spar broke so easily, perhaps the sheet wood was heavy enough, making me think it is strong enough, but I must be missing something, perhaps the grain of the cut, or it's just my luck to have picked a "bad" balsa sheet.

Maybe the pair of wings would still work, I can try but I think it is inherently weak and I still have doubts as to aligning the wings to the fuselage.

In the early years of aeromodelling, there's a method called the Ritz method that I think I can adapt. It would use wider leading and trailing sheets, about 1/2" wide, root and tip sheets, all with the grain running spanwise. Assemble 2 panels. Super glue the tip rib, and all the intermediate ribs on the jig board, (no root rib yet). Lay and superglue the panel over the ribs. Trim the excess wing panel at the root to allow for the dihedral angle and then glue the curved root ribs to the roots. Round the edges of the wing panel and cover with tissue, upper surface only.

It is still a hassle to make matching root ribs that has an airfoil and is curved on the planform. An eyeball approximation is probably the easiest after the root blanks are curved.

Here's how to make the ribs:

1. Measure the chord of the wing panel, Druine Turbulent is a basic rectangle planform, so the ribs are identical.
2. Cut a sheet balsa to the chord of the wing. The grain runs along the wing chord.
3. Draw on a piece of card, the wing chord. Draw a triangle that has the apex at around 40% of the wing chord, the apex is 3/16" above the first chord line. The 1/2" wide x 1/16" thick leading and trailing sheets will be glued to the slopes of this triangle.
4. Cut the card but extend the slopes outside the chord and round off the apex. Glue guiding strips of 1/16" square balsa to both the leading and trailing points. This will give the rib template that can be slide down the previously cut balsa sheet.
5. Draw on the sheet balsa from Step 2, parallel lines 1/4" apart. This lines are the base of the intended ribs.
6. Align the rib template over the drawn sheet balsa and cut the top slope and curve.
7. Using a steel ruler over the drawn lines cut the bottom of the airfoil.
8. Repeat Steps 6 and 7 until the desired number of ribs is cut. 

To ensure that both wing panels are glued according to plan:

1. Think of the stress points: these are the root leading, root trailing (both tension) and a point near the top of the airfoil (compression). 
2. Dot this points on the completed fuselage. These dots indicate the bottom of the wing panels. The centre dot is for the spar stubs, position it over the vertical spacer of the fuselage. 
3. Draw out the geometry of dihedral and cut 2 spar stubs, 1" long, 1/4" deep x 1/16" thick. The spar stubs function as a locator and dihedral keeper. We don't need it to handle the minute compression force while in flight, that can be handled by the wing panel itself. So, once the wing panels are glued to the fuselage, the spar stubs can be broken off. 
4. Superglue the apex spars over the apex dots on the fuselage.
5. Now it is a simple matter of supergluing the wing panels to the fuselage, using the previously marked dots and spar stubs as locators.
6. Any gaps? Cover with tissue.

11 November 2024 (Overthinking?)

A simple compass cutter for paper/card/balsa sheet upto 1/16" thick)

Just buy one, but I prefer to improvise. Buy one, keep one, lose one, buy another one.

Here's one way using the few wooden squared up blocks that I already have.

• Mark the wooden block, a location away from the edge the radius of the intended circle.
• Insert a small pin at this location, this will be the centre pin.
• Snap off a blade from NT cutter, superglue it to the perpendicular edge, this is your cutting tip, the tip should be on the same level as the centre pin. The cutting edge should be near perpendicular for tight circles. 
• Hold this make shift compass cutter with your masterhand over the paper/card/balsa.
• Do not twirl your compass cutter, you just hold it in position, making sure the pin won't move and apply light pressure on the cutting tip.
• Instead, keep rotate the paper/card/balsa around the centre pin.
• Do this over your cutting mat and you did prepare the blanks of the paper/card/balsa, right?
• When you have no use for the compass cutter, use a plier and pry off the centre pin and cutting tip. Discard the pin and tip.

9-10 November 2024 (5 hours?)

I completed the wing over the weekend. The first step was making the curved and slanted root ribs. I made a blank, drew the parallel lines (with oil based pen) and wet curled it by hand until it looks like the side curvature on plan. When dried, there was no spring back. Placing the curled blank on a steel rule, the edges were sliced with the NT cutter held at the approximate dihedral angle.

This was how the curled blank look after slicing off the edges for the base of the root ribs.

Now this was how the ribs were cut out. I figured that cutting by NT cutter will be good enough, leaving the final sanding if any to the later stage.

The rib template print out was glued to card stock. 

The leading and trailing lines were extended beyond the rib to simplify the cutting. 

The cardboard rib template was cut with a pair of scissors.

A piece of 1/16" balsa scrap piece was glued to the bottom of the cardboard, aligning a straight balsa edge to the bottom of the printed rib's base line.

With this template and a NT cutter, straight ribs were cut. I cut an extra one piece incase I need to make another rib.

With the same template, but edged up to the curled blank, the rib profile was drawn.

This photo shows a few developments.

The rib template was modified so that I can trim the ribs to accept the leading edge and also notch them to accept the spar.

The notching tool is also a sanding tool. It is a popsicle stick with superglued with 240 grit sandpaper.

The wing panels was constructed on the jig board with the root ribs (curled), laminated tips, multiple standard ribs, 1/8"x1/16" balsa leading and trailing edges. Start by supergluing the laminated tip, leading edge, curled rib and standard ribs. Trim the end of the 1/16" square spar so that it matches (roughly) the wing tip, superglue at the wing tip and the adjacent standard rib, wet at the rib and bend down the spar piece to fit in the spar slots of the other ribs. Add superglue to all the other ribs and hold down the spar until it is cured.

A steel ruler was used to guide the NT cutter along the trailing edge. Each rib is individually cut. When all is cut, superglue the 1/8"x1/16" trailing edge.

At the end of the week end session, I now have 2 wing panels. The wing panels fit the fuselage. Now, the magnitude of the flying dihedral as indicated on plan is realised. I was surprised, but it's ok, so long as it can fly in a stable manner. Not forgetting that this is my first low-wing free flight model plane.

The jig to assemble the model will come later, after the individual major components are covered and painted, so I will have a lot of time to think about it. The major components so far: fuselage, wing panels, fin, elevator.

The next stage is to sand the major components, tissue them, shrink the tissue and paint them.

5-8 November 2024 (Overthinking?)

Overthink then Simplify. The key to successful model building is often in simplicity, especially in unexperienced areas. Overcomplication can lead to frustration and delays. I can achieve good results by focusing on the core principles and using practical techniques.

The curved slanted root

The next step is to make the pair of wing panels. The plan of the left and right wing panels show the root ribs are to be slanted (for dihedral) but are otherwise straight. The top view plan of the fuselage is clearly curve at the junction and the fuselage box structure has been completed. 

1. These curved root ribs are to be installed without extending the leading and trailing edges.
2. Two oversized piece of balsa are wetted and curved to match the fuselage curve.
3. Tape these curved root ribs to the fuselage.
4. Jig the fuselage and wing panel so that they have the correct dihedral angle relationship. The incidence is only tweaked when gluing the wing panels to the fuselage. I am probably overthinking if it is possible, to make a single re-usable jig that can be used for both panels for this temporary set up process and the final gluing process.  
5. Superglue the wing panel to the curved root rib, add gussets.
6. Remove the wing panel by unpeeling the tapes and sand the bottom of the curved root rib first, follow by the top of the curved root rib.

Point 4, a beam type of jig for dihedral and a cradle type for root incidence may be easiest.

I am probably overthinking about the curved root ribs. In order to get the oversized curved root ribs to have the airfoil shape in Step 6:

1. Superglue 2 pieces of balsa template that mimics the dihedral on the curved blanks.
2. Sand the bottom of the curved blanks until the base is flush with the balsa templates.
3. Measure the height of the normal straight ribs at the points where the balsa templates are and transfer this to the curved blanks.
4. Cut roughly to shape and proceed to Step 4, the markings will guide the final sanding to airfoil shape matching the normal rib airfoil in Step 6. 

The simplest method being to draw and cut a rectangular piece of 1/16" balsa sheet. The rectangle is longer than the path of the curved rib and wider than double the height of the intended airfoil height with a centre line drawn in. On the other side, parallel lines are drawn across the width of the rectangle piece, these lines mark the positions along the airfoil which will be used to determine the top curvature later. Wet and curl this piece until it matches visually with the curve on the fuselage top view. After the blank is dry, check against the top view again. Re-wet and tweak the curvature as necessary and let dry again. Repeat the tweak until the dried blank matches the curvature, or, when I get bored and decide it is good enough. The high line of this curvature will need trimming since there is the dihedral angle to consider. Sand to match the dihedral angle. Do the same for the other long edge. Cut along the centreline and we have a pair of root ribs. Mark the height at the vertical lines previously drawn on the inside of the curved blanks and cut approximately. The 2 root ribs can now be used for the wing panels. They cap the root end of the wing panels.

Propeller shaft

The following instruction is when viewed downwards. For the simple hook, 1) use a vice and grip the short end of the wire across the shank of a vertically placed drill bit, the wire being near side 2) pull the free end around the shank clockwise beyond 180 degrees, 3) relax the vice and remove drill bit and the single coiled wire, 4) adjust the coiled wire so that the coiled wire appears to have a straight entry and straight exit, 5) grip with round nose plier and bend the longer end, adjust until it appears to bisect the loop, 6) cut off the other end of the wire leaving a gap for the rubber to pass through. 

Over thinking? This is almost as complicated as doing the reverse S hook.

Maybe a pair of round nose pliers bend in the air is enough.

For diamond hook, similarly, a pair of round nose plier to get the 2 critical points correct, the lowest point and the highest bend that is the shaft. Whether the other bends are perfectly right angled or round loop shouldn't matter.

I think I'll do the propeller shaft after the full model is trimmed as a glider. I'll have enough time to think if I should use a plastic tube as a bearing or I can use the aluminum from a beverage can to face the removable nose block as a bearing surface.

Modelling weight

I discovered that in the past, modelers used plasticene to balance the model. Plasticene has inherent disadvantage which I dislike. It appears oil is present in the product, I had to wash my hand with soap or it won't be cleaned. I noticed also that it stains balsa wood, and dropped off easily on my chuck gliders. Sand and small particles are attracted to plasticene. Therefore, it does not appeal to me.

Blu-Tack is a relatively modern product that is cleaner and stickier, although it is more expensive.

I'll use Blu-Tack when balancing my model. I can use it on tissued surfaces, I can use it to attach small metal objects like nuts, bolts, coins.

Balsa Cement

I read that balsa cement was a favorite with modelers when gluing on the horizontal stabilisers for free flight model. When more or less tail incidence is required, the modelers dissolved the joints with solvent and re-glue. So smart, I thought, I didn't know that.

Disappointingly, balsa cement is not readily available anymore. Superglue, a relatively new product, is readily available. I think I can use superglue in lieu of balsa cement and use acetone/alcohol/nail polish remover to dissolve the joints.  

 

Wheels

This is overthinking. To make 1" wheels from extruded foam.

Yes, I can make true spinning wheels using a drill and sandpaper, but this method does not appeal to me.

Method A

1. Make a 90 degrees perpendicular cut on the extruded foam sheet.
2. Using double side tape, stick two Singapore $1 coins to both sides of the foam sheet, the coins being directly on the 90 degrees perpendicular cut.
3. Cut or sand away the rest of the foam around the coin.
4. One advantage of using coins is that it is metallic and you can hot-wire cut perpendicularly the foam.
5. To save effort, you can stack 2 pieces of extruded foam sheet after step 1, aligning the foam sheet 90 degrees perpendicular edges.
6. Remove both coins and use a circle template to locate the centre of the foam discs.
7. Pierce the foam discs with thin wire, ream with drill bit, glue the axle bearing. 
8. Axle bearings may be short sections of plastic tubing or rolled paper tubing.
9. The coin's diameter is less than 25mm, glue paper strip around the discs until it is 1" in diameter.
10. Sand the wheels to shape.

Method B

1. Rough cut 1.5" square 6mm foam pieces. Make more, if you need 2 make 4.
2. Pierce the foam pieces with bamboo skewers.
3. Make a jig from corrugated cardboard, it has a 1" slot that is about 30mm wide.
4. Bend 2 spindle saddles to allow the bamboo spindle to rotate freely within.
5. Position the saddles so that they are 13mm from the edge of the jig slot. 
6. Place the hot wire bow cutter at the entrance of the slot and by rotating the skewer, the hot wire will cut the 4 discs of foam pieces.
7. Cut a pair of disc from thin aluminum or transparency sheet for each wheel.
8. Pierce each disc with a sharp pin for the axle.
9. Remove the foam discs from skewer and glue the thin axle bearing discs against each face.
10. Sand the wheels to shape.

I think I will choose method B because I have a small bow hot wire cutter and I can use the jig for many wheel diameters by adapting the jig. This method is also suitable for straight simple cones and an adaptation of this method can hot wire cut straight transitory cones using base and tip templates.  

Method C

This is the simplified method B and is without the jig board, steps 1, 2, 7-10 are identical.

1. Rough cut 1.5" square 6mm foam pieces. If you need 2, make 2.
2. Pierce the foam pieces with bamboo skewers.
3. Insert a circular cardboard template to each end of the foam. Simply draw the circles. pierce the centres and cut with a pair of scissors. 
4. Hold the skewer in non-master-hand and the hot-wire bow cutter with the master hand, slice against the templates.
5. Cut a pair of disc from thin aluminum or transparency sheet for each wheel.
6. Pierce each disc with a sharp pin for the axle.
7. Remove the foam discs from skewer and glue the thin axle bearing discs against each face.
8. Sand the wheels to shape.

I think I will choose method C now because I have a small bow hot wire cutter and everything is literary done by hand.  

Trimming and Power Pod

The model is designed for rubber power.

I have the removable nose-block prepared but without the propeller.

When first trimming the model, I will not install the propeller and rubber. 

I will start by trimming it as a glider, using Blu-Tack as nose weight and it will take awhile because the flying surfaces may have to be adjusted or re-located.

Without the propeller and rubber, the model will be lighter and lighter models fly slower and slower models live longer.

Having a propeller in placed at this stage can mean broken propeller or bent shaft.

After the CG is determined, the next stage would be the power phase.

The propeller, shaft etc are installed on the removable noseblock and fixed weights will be used to balance the model until it reaches the same CG location previously identified in the gliding phase.

The rubber motor will be incrementally and the noseblock will adjust the thrust angle during the power phase.

This led me thinking that I could convert the model to a capacitor powered electric motor plane.

People can say that rubber strip is easily available, but that is not my experience, so it will be good to have an alternative to fly the model rather than leaving it 'broken' as a static model.

I can always make another removable noseblock without the propeller, shaft etc. A simple tray can be glued to this new noseblock, and inserted to the fuselage, like a power pod. The tray will mainly house the motor and the capacitor. This electric power pod will be lighter than the previous propeller, rubber combination, and because the weight concentrates near the nose, there is a high chance that the entire model is much lighter than before. Too bad about the high pitch whine of the tiny electric motor, but it is a viable option. No rubber or propeller breakages or bent shafts and no need to wind up the rubber motor, just switch on and fly.

Am I simplifying, am I over thinking or am I adapting what modelling resources we can enjoy now?

4 November 2024 (12 hours?)

Decades later, the wizened youth rediscovered his creative spark. Now that I know what I know now and have what I did not then, it becomes easy.

I made a few photocopies of the plan  Oz : Druine Turbulent plan - free download and pasted them on some corrugated cardboard. Covered with transparent packing tape, these are my small, light, cheap and fast building boards. The packing tape and corrugated cardboard provides a surface where I can superglue balsa components and if necessary, to cut, for these reasons, I call them my jig boards.

Here, the fuselage side frames is superglued on the jig board and the spacers added, start with the longest and end at the tail and nose.

I deviated from the plan and tried laminated tips. Information from the internet will be to make strips from 1/32" balsa sheet and glued at one end. My method is a single balsa strip with a few slits, from 1/16" sheet. The strip was wetted and superglued on the jig board. Nudged with my fingers and a 10 cents coin (the 5 cents coin is a bit too thin), I spot glued the slitted slip to hold the curvature. Progress until the other end is reached.

The glass of water was not for drinking. It's for wetting balsa strips.

Here, the left handed wing tip will be discarded, it is not long enough.

This jig board is to build the right wing, I pasted the wing tip of the left wing over it. This way, the same jig board can be used for both wing panels.

The slightly rusty carbon steel guitar string is 0.38mm diameter (PL015).

A masking tape was laid on the cutting mat and drawn as shown. 

Lift one edge of the tape to slip in the guitar string, then fold over, and the small marks show where to bend. 

Here, the string is bent but not cut, and the masking tape has to be removed. 

Try to bend it symmetrically. It probably won't happen, but you can tweak it after the tape is removed.

Tweak the wire against the plan. The wire is so small that I can tweak it without pliers.

When satisfied, or in my case, when I gave up, I taped the wheel axles with masking tape onto the plan. The balsa piece below the wire has been test fitted to the fuselage. 

  

Later, I tissued over the wire and balsa and soaked it in superglue. I didn't feel like searching for cotton strings.