Back to modelling youth: Druine Turbulent

5-7 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.  

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. Each tip is a single balsa strip with a few slits. The strip was wetted and superglued on the jig board. Nudging the strip with my fingers and a 10 cents coin (the 5 cents coin is a bit too thin), I spot glue 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.

  

  

Jig for Curved laminated Balsa Strips

1 November 2024

This is for making 1/8" wide lamination from 1/16" balsa sheet.

1. Square up the 1/16" balsa sheet.
2. Slice thinly through but leave one end intact, repeat 1 more time.
3. Now slice the 1/8" strip. 

You now have a 1/8"x1/16" balsa strip that has 2 or 3 slits through most of the length.

Wet the slitted length.

Curl the slitted length with your fingers.

Superglue the intact end of the curled strip on to the packing tape protected jig board.

Nudge the curve over the plan and drop a bit of superglue intermittently on the slits to hold the curve.

16 April 2014

For a jig to form curved laminated balsa strips:

1. Prepare 2 rectangular pieces of compressed foam. Draw the inner curved line onto one piece cut along this line to separate it to form two pieces of formers, one being the inner former and the other, the outer former.  
2. Glue the smaller piece of inner former onto the second rectangular piece of compressed foam, this is the base board.
3. Select the thickness of the balsa strip. A thickness is suitable if it can be formed readily against the inner former without cracking and the finished product is functional, i.e., 1) not fragile, 2) sufficient strength for intended use, and 3) minimum of 3mm for overlapping of covering. If the strip shall crack while bending free hand, then it is too thick and the strip has to be made from thinner balsa. If the strip is too thin and fragile to be functional, additional strip to be added until the overall desired thickness is achieved.
4.  Prepare the balsa strip/s by soaking them for a good 15 minutes. Time depends on cross sectional area and hardness/water permeability of individual strip, even the temperature of water. 15 minutes would be sufficient for upto 1/8" balsa strips.
5. If using multiple plies of strip, wipe dry the strips, apply white glue to the mating areas and assemble into a single laminated strip.
6. Single strip only needs to be wiped dry.
7. Place strip/laminated strip flat on the base board and against the inner former. Starting at the end where the curvature is the least, temporary secure strip in position with tape.
8. Push the length of strip against the convex curve of the inner former starting at the taped end and lock in position with the outer former.
9. There will be a gap between the outer former and the balsa strip except at the two ends of the outer former. This is ok, because only the two ends are required to hold the strip against the inner former. Glue/pin/clamp the outer former to the base board. The force is at the two ends, these are where to put the pins/clamps.
10. Leave the strip in the jig for a day to dry out.
11. If using super glue for laminated strip, steps 4 and 5 may be omitted, thin super glue is used to tack the laminations after step 9 and the strip may be removed immediately after the super glue is set. After 15 minutes, more of the thin super glue is introduced to the laminating joints to complete the gluing.
12. Remove the curved laminated balsa strip (or formed single balsa strip) from jig and trim it to length.
13. White glue doesn't bond well to compressed foam, it is easy to prise the strip off, bending the jig slightly may help in stubborn areas. Super glue will bond or may erode compressed foam, but since they are used sparingly to tack glue the laminations, it is ok.
14. If jig is still usable, and is should be, repeat steps 4 to 12 for additional laminated balsa strips (or formed single balsa strip).  

Notes

Formed single strip will have spring back, laminated strips retain shape much better. The first choice, therefore, is to use laminated strips.

Curved line may be drawn tighter to accommodate the spring back of single strip but the amount of spring back, in the case of balsa, will not likely be consistent.

Laminations may be made from balsa, plywood, bamboo, paper, plastic, compressed foam, carbon fibre and other similar material.

White glue between compressed foam is not suitable, choose another adhesive that doesn't involve evaporation of solvent.

 

Cardboard Jigs

1 November 2024

Yesterday was Deepavali, a public holiday, and the family gathered at my place for lunch and mahjong. After lunch, I had a few hours free since I don't play mahjong. So I made a few photocopies of the plan  Oz : Druine Turbulent plan - free download and pasted them on some corrugated cardboard. The jig board is then taped with clear packing tape that was around the house. In less than 4 hours, I completed the horizontal stabiliser, the vertical fin and the two fuselage sides. It was actually quite easy, here are some key points that were different from when I was younger:

• Proper chair and table provides a comfortable work station. Compared to crouching on the floor or working on the coffee table, this is more comfortable. 
• The 1/16" balsa sheet was cut to a length that accommodates the longest member with one inch to spare, in this model, about 11". 
• With a Pilot G-2 07 pen, cross the balsa sheet on one side, and near both ends. This tells me which side I should rotate the 1/16" balsa strip to.
• Cut the required balsa strips of 1/8" and 1/16" width with an NT cutter against a ruler in smooth continuous strokes. The result is much better than making strips from 36" long balsa strip.
• Superglue was used for the joints. The capillary action works the glue into the joints. Balsa parts can be held over the jig board with your fingers before superglue is applied to the joints.
• Pressing an NT cutter onto the balsa strip, the balsa strip can be picked up and transferred to a cutting mat. This makes cutting the strips to exact length and angle easy.
• After one member is cut, align that member atop the balsa stick and cut the second member to exact length and angle by first cutting the angle of one end, and then using your thumbnail to align the ends of the member and balsa stick, before cutting the other end. 
• Fuselage spacers are duplicates. Make all the members in duplicates and set out accordingly at the top of your work space. 
• Dot the packing taped jig board with superglue to temporarily secure balsa pieces, away from the joints. No pins, weights, clamps are necessary. When frame glued, slip in the NT cutter between the balsa structure and jig board to pry off easily (#11 scalpel won't work as well as they are too short and stiff). I didn't need to bother cleaning up the jig board by swiping with solvent to remove the superglue, the jig board can still be re-used without this extra step. I did not use hot glue. If the frame was removed too early and there are still some superglue that is still wet, this will smear the NT cutter blade. The NT cutter blade is cheap and can be replaced, but it appears that it still works well for cutting, slicing and prying.
• If you press on 1/16" square balsa strip while it is on the jig board to transfer the balsa strip to the cutting mat, sometimes it will cut through and your task is done. The jig board seems to be tough enough and the cutter did not harm the jig board; I think this is because there is enough resistance from the packing tape and enough give from the corrugated board.
• Pre-curving of lower longerons was completed without template. Dip in hot water and bend by stroking the balsa strip. 

Next session, I'll have to do up the pair of wings, assemble the fuselage sides into a box structure, add formers to the top of the fuselage box, make the removable noseblock and undercarriage.

The fuselage sides will be temporarily glued upside down to the planform jig board with superglue. Start at the point of maximum curvature. Starting with the fore and aft spacer location, mark/cut the spacers in duplicates. Cut the next pair of narrower spacer and gradually bring in the fuselage sides. To handle the spacers that will be glued "in-air" (i.e. the bottom spacers) and also to help position the top spacers that is on the jig board, make a pair of tweezers or a pin prick. This is because there will not be enough space to put in your fingers. So you can start by making your tool. A pin prick is just a thin pin that is superglued to a 3" length of 1/16" square balsa, and it works by piercing it into the spacer. A tweezer is just 2 pieces of 3" lengths of 1/16"x1/8" balsa, spaced one end with a 1/4"x1/16"x1/8" balsa, assembled and glued with superglue.

30 October 2024

Added how to curve 1/16" square balsa strips using jig board and hot glue in posting of 25 October 2024.

25 October 2024

I often read of using a building board and from what I read, they are meant to be re-usable, needs maintenance and proper storage, relatively big and has a high initial cost outlay. 

The function of these yester-years building boards can be replicated cheaply. You can make it re-usable or disposable, very economically. The only requirement therefore is that it has to be flat. To differentiate from the traditional building boards, I refer them as jig boards.

• Make jig boards from flattish double walled cardboard, steel/plastic panels, wooden boards, glass/mirror by gluing on the section of plan.
• Pre curve the balsa strips over simple templates. Use tap water to wet the strips and hold in position until thoroughly dry.
• Use hot glue to temporary secure balsa strips, wet or dry.

On Hot Glue and Curving balsa strips.

1. On cardboard jig board (or those that are not meant to be used with pins), I think it is feasible to use dabs of hot glue to temporarily secure the longerons. Upon completion, it ought to be possible to pry the completed fuselage sides from the jig board, pieces of hot glue that are still on the jig board or balsa longerons can be peeled clean. Wet balsa can be hot glued to a dry jig board.
2. Spring back of balsa. Accept the fact that after removal from template, the pre-curved balsa strip will spring back a bit. You could make the template with tighter curvature to accomodate a bit of this spring back, but I wouldn't bother too much. How much spring back depends on the size and hardness of balsa, the tightness of curvature and whether we are using a single piece, double ply or triple ply. Held to the correct longeron curvature, wet the balsa again. When dried, the slight stress in the fuselage side will be present, but it has been reduced. This is the case also with tighter curved wing tips and stabilisers, the stress is inevitable and sufficient tension braces are needed to counter those stress and maintain the shape. 
3. Here's an idea for curving the 1/16" square balsa strips. Wet the over long balsa strip, position over the jig board and dab hot glue at the peak of curvature. When the hot glue has cooled, bend the wet balsa strip to conform to the curve line and dab another spot to secure. Alternating between the 2 ends of the first dab, work towards each end. Leave the balsa strip to dry thoroughly before removing from the jig board.

23 October 2024

I bought my wife a new ladies golf set, in came in a stout double walled cardboard box. We could keep the box and next time we want to travel (by air) with her golf bag, we could simply re-used the box. Just seal it up and it will be perfect golf case. She decided she does not want that, so the cardboard box has been cut up and I have a good supply of double walled cardboard pieces that I can use as jig boards, templates and carry boxes for my future builds.

Since it is 'free', my imagination will not be hindered. I can use pins, tapes or hot glue.

Building board = jig board = disposable jig board

Big heavy expensive = small light cheap = cheap and disposable

The jig board can be made of any material, so long as its function is served. 

We want the fuselage side frames be assembled squarely and symmetrically. I don't think assemblying in mid air will be suitable, we need another jig board.

On a piece of cardboard, corrugated cardboard works as well, paste the plan view of the fuselage, cut around leaving 1.5"-2" all round and I'll explain why later. This piece of cardboard will be the other "jig board".

Again, we will need jig pieces, but this time the jig pieces need to be tall enough to accommodate the fuselage sides. You could fabricate the bigger jig pieces, here's how to use the jig board to provide the jig pieces: you simply cut slits at the nose, the mid section and tail and then bend it up perpendicularly. The bent up tab pieces will serve as built in jig pieces for the fuselage sides. Offer the fuselage side frames to this jig board, usually, this means top side down because the top longerons are usually the straightest. Similar jig pieces that were used in the assembly of the fuselage sides can be re-used to 'lock' the position of the fuselage side frames.

Proceed then with cutting the horizontal spacers (in pairs if appropriate), using tweezers if necessary and place them between the 2 side frames. The tail of the fuselage ends as a tip, so proceed from the tail end and finish at the nose.

The assembled fuselage frame can now be removed, but there's an optional step you can take. This involves wetting the fuselage longerons while it is still in the jig, it may reduce the internal stress due to the planform shape. You can repeat this process but do allow a day or so before removing the fuselage frame.  

If your jig board is not destroyed by now, you can re-use it. Place the assembled fuselage frame, top side up, on the jig board. Use the jig board to hold the frame stable while you glue the top formers and stringers.

21 October 2024

Jigs may seem like a tedious step to undertake but it has obvious advantages.

Building equal fuselage sides

On a piece of cardboard, corrugated cardboard works as well, paste the side view of the fuselage, cut around leaving 1" all round for easy handling. This piece of cardboard will be the "jig board".

Jig pieces will be useful for positioning. They are just 1/16" or 1/8" scrap pieces of balsa, something like 1/2"x1/4" size. The jig pieces will be located at the nose, the tail and at each vertical spacer of the fuselage side. Some special jig pieces may be necessary, and these are usually just triangular pieces. It is very easy to make the special jig pieces because all you need to do is the extend the 2 lines, position a scrap piece over it, sight and cut along the lines.

Yes, you can just glue the perimeter's jig pieces onto the jig board. It is advantageous too, because you must not let these jig pieces move.

Once the jig pieces are in placed, the real construction starts. Longerons (the upper longerons are usually straight) are drop in position on the jig board, they can be longer than necessary, because they can be trimmed after the sides are complete. Do two sets of vertical spacers, because we are building a left and a right fuselage sides. These vertical spacers will be glued between the upper and lower longerons, so they have to be accurate. Lay all vertical spacers on to the jig board and glue to the top longeron. Soak the lower longeron pieces in water for 5 minutes, because they usually are curved and we need to soften the longeron. Drop the "softened" lower longeron on to the jib base, against the spacers. Using more jig pieces on the lower edge of the lower longerons to avoid crushing the lower longerons. Do not glue at this stage because the wet balsa has not set yet. Wait until the wet longeron is dried, about 2 hours, and then proceed with gluing it to the spacers.

Do not build the second fuselage side over the first, in case they bond together. Remove the first fuselage and start by placing the longeron, follow by the spacers you already cut, and finally the wet lower longeron on the jig board. Similarly, wait until the lower longeron is dry before gluing them together. 

Sand the 2 fuselage sides so they are identical.

Building the Wing

Similarly, on another piece of jig board, paste the planform of the wing and do up a jig board. This time, use 1/4"x1/2"x1/8" jig pieces for the perimeter. The reason why 1/8" balsa is preferred because the intended leading edge is 1/8"x1/16" on-edge. Start by positioning the leading edge and trailing edge on the jig board. The trailing edge may already be pre-sanded. If the model has tip pieces, these are also positioned and "protected" by the jig pieces. 

You can adapt the wing's plan for the jig board (most ribs are evenly spaced) so that you can use the same jig board for left-hand panel and right-hand panel. In tapered or curved planform, this would not be possible and it is faster just to make handed jig board.

With the perimeter wing frame more or less completed (less the base rib), the next step is to enable placing of the ribs vertically and accurately. Pre-cut ribs are offered first to the inside of the leading edge and then trim to fit the trailing edge The ribs' jig pieces can comprise of the same 1/4"x1/2"1/16" jig pieces previously used for the fuselage's spacers (but not glued on) with additional sticks glued on to the flat edge so that it can provide some kind of a wall to help with the vertical alignment of the ribs. When the ribs are all lined up with straight spar slot, insert the top spar and finally glue them together.

The wing panel is ready for follow up work.

Technology

Thank goodness for photocopier and printer!