Scraps (building a dream)

16-17 December 2024

If it has not been realised, it is still a dream.

Oz : Scraps plan - free download

I tried to build 'Scraps' over the weekend. The motor stick (fuselage), wings, tails are all from 1/16" balsa sheet. The only exception on the plan is the propeller hub and blades, calling for 1/8"x1/8" and 1/32" balsa sheet. The longest wood is only 8" (200mm).

Some things I've discovered: 1) my smallest plier is too big, 2) I don't have a fine-toothed saw, 3) the motor stick is only 1/16"x3/16" cross-section, 4) annealing paper clip helps to make it softer and it can be easily work hardened, no need quenching, 5) 1/16" balsa appears to be a limit, any thinner and it is too flexible. 

Wing Rib

The 7 wing ribs are identical. The wing will only be covered on the top side, the ribs are curved for better airflow.

1. Cut 2x1"x1/8"x1/16" balsa pieces (guide rail).
2. Superglue one guide rail on packing taped jig board.
3. Cut a balsa sheet to the exact length, place it against the first guide rail.
4. Superglue the second guide rail against the opposing edge of the balsa sheet. Be careful not to glue the balsa sheet. Remove the balsa sheet. 
5. Cut a strip of card, about 1" deep, longer than the combined rails and balsa sheet.
6. Superglue the card on the guide rails, this will become the rib template.
7. Remove from jig board the rib template, reinsert the balsa sheet between the guide rails. Remember to stripe the balsa sheet beforehand so we will know which edge is against the leading/trailing.
8. Cut and sand rib template and balsa sheet to curvature.
9. Slice off the 7 required pieces of curved ribs by pulling down the rib template from the top edge of the balsa sheet 1/16" at a time. I didn't measure, I think my ribs are only slightly deeper than 1/16". I will sand the whole wing to finish it up, but I can't add depth. A cross section of 1/16" square appears to be a limit.

Nose Bearing

My thinnest paper clip is still too thick, I annealed it to close the nose bearing as much as possible, it has much free play.

On the next iteration, I will try to make a nose bearing from aluminum can. The aluminum wall is very thin, it is very easily pierced with a pin. I will cut strips with a pair of scissors, pierce them one at a time, assemble 2 or more of this together, bend to shape and drip superglue into the joints. This ought to result in a light easy nose bearing with perfect play.  

And yet another design which I think will be lighter is to cut a right angled triangular piece with rounded corners from the same aluminum wall. Pierce near the apex, then with the temporary shaft in placed fold the triangle over the 1/16" thick balsa motor stick. While it is only a single skin, the fold, the short moment, and the ample gluing area makes it strong and rigid for the pull of 1/16" rubber loop.

All these designs do not allow thrustline by itself, the model is too compact, but if the motor stick can be longer and we have a longer propeller shaft, then it is possible to by making a second pivot point on the propeller shaft, such as a small looped wire or strip. Get the propeller shaft into the loop and then superglue the end/s of the wire or strip.

Propeller Hub and Propeller blades

The plan calls for a 1/8" square balsa hub with diagonal cuts to receive 1/32" balsa blades. I don't think I can make small saw cuts on 1/8"x1/8" and thought out an alternative.

The Hub

My propeller hub is a 3cm length of lollypop stick (the plastic stick, not paper). Start by using a hot needle to pierce the plastic tube 1.5cm from a squared end. The plastic tube is small, so eye-ball the entry as near centre as possible. Re-heat and re-enter the pierced hole to heat pierce the other side of the tubular wall. Hopefully, the exit point is accurate, usually it is off, but sometimes it is still usable as is by enlarging he hole slightly. The propeller shaft is supposed to enter and exit the tube hub perfectly (perpendicularly and in the centre), but if it does not, make the offending hole that bit bigger until it does. The propeller shaft at the final stage will be binded with thread and held in placed with superglue, so it is still usable.

Now temporary insert the propeller shaft and use the grids on the cutting mat to help with scoring the plastic tubular hub at 1.5cm from the propeller shaft. Score both sides and snap off. Sand the tubular hub square, scrap the inner edges with a NT cutter to remove burrs.

This hub accepts blades with cylindrical blade spars, the spar will be held by friction with the hub, so the angle of attack of each blade can be adjusted. 

The Blades (adjustable angle of incidence)

I have a 6" plastic propeller for rubber power. I think I will make the propeller blades this way:

1. Trace and cut out multiple copies of the propeller blade's shape from 70gsm photocopy paper. 
2. Apply glue stick to the first paper blank, place it over the plastic propeller (either the top side or the under side), glue side up. Place the second paper blank over it.
3. Apply glue stick to the third paper blank, place it over the second paper blank, glue side down.
4. 3 ply of 70gsm paper ought to be sufficient, if it is later discovered to be insufficient, repeat the 3rd step for each layer. 
5. Cut out identical lengths of the blade spars, they have to fit the hub snugly later. It is ok to have loose fitting spars because they can be tissued over later.
6. With the paper blade still on the plastic propeller (serves as a jig/alignment tool in order that subsequent blades are made identical), superglue on a blade spar. 
7. Remove from plastic propeller, soak in superglue (strategically along the leading edge) to lock in the twisted curvature of the propeller blades. If the glue stick came undone later, wet it and bring it back to the plastic propeller jig to dry. You can also consider soaking in a bit of superglue along the delaminated area.

Gluing the Propeller Spars to Blades

This section is on how to do up the previous step 6, gluing spar to blade.

Obviously, a flat spar offers a larger contact area to the blade, however, for simplicity, I'd use round sectioned spars. How to hold the blades securely and accurately is the focus of this section.

• With the laminated paper blade still on the plastic propeller, draw a straight line, tip to tip on the single paper blade. Mark where the line intersects the hub. On the blade, the line marks the spar's position.
• Glue the spar with an end against the hub and following the marked line on the blade.
• Prepare a few tissue patches and glue it on the blade, round it over the spar. This should add sufficient strength and security.