Pull-Pull for weak structure and hinges, but it will be heavier

27 Aug 2018

Here is an idea of a form of indirect Pull-Pull control system for weak structures and hinges. It is more complicated and it will be heavier, but it minimizes the stress forces to structures and hinges. It uses pushrods from the servo arms to operate pulleys (or pulley strings) and at the rear part of the pulley system, it can use pull strings or pushrods to move the control horns. It is always a balancing act, depending on what I need and what I am willing to forego. Afterall, a simple wire supported in a few spots with tubes might already be the control solution if the slight weight gain is acceptable.

I have done a couple of unsatisfactory pull-pull for 1s RC (WLToys' receiver servos board). Pull-Spring is not optimal because there was a constant drain due to the servos having to resist the spring. Pull-Pull was tried on that large winged stick motor glider and I had to brace it with additional tie strings because the pull strings exerted enough force to bend the long and slender fuselage boom stick. There was also that idea of converting a single horn into a pull-pull by gluing on a pulley to the front of the horns which seems pretty neat and light, except that it could still bend the fuselage and it requires better hinges than tape as all the forces are exerted on the hingelines.

At the core of the proposed indirect Pull-Pull system is a central core which supports the load of a series of pulleys. In this way, the stress is isolated and the aircraft structure's slenderness is reduced. Servo arms' pushrod could be heat-shrunk to the pulley lines, and the output could either be control pushrods similarly heat shrunk to pulley lines, or pairs of pull strings tied to pulley lines.

Small Gliders: HLG; Whitehead 21; F15SG

10 August 2018: F15SG in RSAF 50

Print out the F15SG, paste on foam board.

Cut out the pieces, insert icecream stick to fuselage, cut and cover the other sides.

Glue the fuselage, plan form, vertical tails together.

Far end is one without foam. All has notches on ice-cream stick.

Prior to this I made some HLG fuselages from ice-cream sticks and bamboo dowel. I did a 3rd HLG and found that the nose is much lighter than the tongue-depressor' type and had to add blob of hot-glue at the nose of the ice-cream stick for balance. This 3rd one was given away before test flights.

For the two previous HLGs, I managed hard launches and catapult launches and they behaved. I was a bit concerned about the flexing bamboo boom (i.e. the one with the curved dihedral) , but doubted if I can detect a difference between that and the stiffer boom. Both gliders' left wings tore up on launching; the first was the stiffer boom straight dihedral model on hand launching, the second followed when I catapulted it. Both wing panels have been hot-glued back. Ready for more flying. Flight pattern is rolling left on launch and left glide at top.

25 July 2018

A few weeks ago, I was thinking of making simple the wings for a Whitehead No. 21 inspired model.

The top wing is compound curved by slitting and the bottom wing is by arching.

Then I played with the idea of using 1.8mm bamboo dowel, bit of foam and plastic bag for the bottom wing. Many thoughts were entertained, on the possible failure and fixes that may be necessary. Then I thought that perhaps I should just make the wing from 2mm foam sheet as that should exclude a lot of vagaries. Forget about compound curve or slitting.

Then the shape reminds me of a chuck glider and I set about doing HLG.

Considering my current physical status, This is what I came up with.

I folded a piece of A4 paper (to get the centre line without measuring) and drew free-hand the wing and tail elements.

I traced the cut patterns on a 2mm foam sheet and cut out the wings and tail elements.

I found that the 2mm foam sheet has 'grain' due to the manufacturing process. The first wing was cut with the grain running chord-wise. This resulted in a floppy wing. Never mind though, since I am going to make a light glider, I just curled the wing in my hands for a curved dihedral. The second wing was cut with the grain running span-wise and this resulted in a stiffer wing.

For the straight dihedral wing, I used a piece of 9mm x 3mm wooden section (from Daiso), snapped it approximately in half, tapered the rear end and left a long nose because the CG of the fuselage is not forward-centric, not enough anyway. I will need some noseweight. 

For the curved dihedral wing, I hotglued a whole length of 1.8mm bamboo onto a tongue depressor that I got from my doctor. The resulting CG is much more forward. In fact, it seems to require no additional nose weight as the model glides well. A shark tooth was cut so that it can be a catapult glider. I figured I don't need a finger piece because I can grip/rest/push on and against the tongue depressor.

Maybe the straight dihedral wing's fuselage is a failure, it is however, conventional and stiffer. As for the curved dihedral wing's tongue depressor bamboo combination fuselage, it is lighter, easier and faster to make, the CG is there so it will result in a lighter model because nil or less nose weight will be needed. but, it seems flexible. Tossing it against the bed's head board demonstrated a very slow floaty glide with good glide gradient. Perhaps flutter wouldn't be an issue, given my physical status.

Looking forward to their test flights.

Construction notes:

1. Foam pieces cut with a NT cutter, no sanding, leading edges are 'rounded' with thumbnail both sides and trailing edges are 'tapered' with curved end of steel ruler.
2. Straight dihedral is cut from bottom partway, cracked, and then glued to fuselage.
3. Wing is glued to fuselage first, then the horizontal tail is glued slanted to encourage left turn, and vertical tail is glued straight.
4. Hotglue used throughout. The nose blob of the straight dihedral wing's model is an attempt to use hotglue as safety noseweight; not effective because hotglue is too light, maybe I can glue a paper clip for catapult launching.