Paper and tape (M8 Condor)

[soundnfury]

For several years I've had a goal of building an RC plane out of paper and Scotch tape, making use of rolled paper tubes for main structural elements. This goal got a lot closer on July 20th with the first test flights of the M8 Condor. This is a high-wing monoplane with straight wing and conventional tail (leading-edge sweepback ~30° on fin) and an open-work triangular fuselage. In a departure from 'paper purism', the mainwheels and tailskid are made of foam. The wing has two spars, one at the leading edge and one at about ⅔ chord.

Flying controls consist of a single central elevator (with inset-hinge balance) and ailerons at the wing-tips with horn balances. (No rudder.) All are actuated by opposed pairs of darning thread in tension.

Main dimensions (approximate): length 3ft, span 4½ft, chord 6”. All-up weight is about 315g (just over 11oz).

First up were some taxiing tests. These are the only bit I have a photographic record of.
Photos of preparing for taxi test: #1 #2.
Video of taxi test.

Apart from some trouble with torque reaction (and concern about the low ground clearance of the aileron control posts), this all looked good enough, so we then took the 'plane to a nearby park to attempt to fly it.

First up was a chuck glide test. It was immediately clear that the aircraft was tail-heavy as it rapidly stalled, so we added about 25g (1oz) of ballast to the nose. On the second try it made a fairly controllable glide and landed fairly smoothly, so we then went ahead with a powered test.

Launching at about 50% throttle, the 'plane started losing height straight after the throw; lacking experience I pulled the stick back and stalled it. Trying again with full throttle, the 'plane seemed to fly (just about) but started pitching up too much (there isn't much elevator authority). Stupidly I reduced throttle, and it stalled again, having spent only a few seconds in the air. The repeated crashes were taking their toll and on the next launch it turned out the main spar had cracked at the left-hand fuselage joint, causing the 'plane to roll violently to the left despite full opposed aileron. So that was the end of "flying" for the day, and the wing is currently sitting in my room waiting for repair. I also plan to increase the elevator chord and possibly aileron throw, as well as lengthening the motor mount to bring the CG forward a bit.

 

[Chuppster]

This is quite interesting. I would like to see videos of test flights! From what I can tell it looks like you would do well to double or even triple the size of your elevator, especially if you plan to slow-fly it. Also, are you concerned about the wired for the ailerons slipping off their posts? You may want to shorten them a bit and add some kind of end stop so that they can maintain tension and keep it from sliding off. I really like the mechanism though! Keep it up!

 

[soundnfury]

This is quite interesting. I would like to see videos of test flights!

Yeah, sadly with one person on the sticks and one person launching, there was no-one left to hold a camera. Next time I think I might try runway-launch rather than throw.

From what I can tell it looks like you would do well to double or even triple the size of your elevator, especially if you plan to slow-fly it.

Do you think it needs more span, or will just increasing its chord suffice? (The latter is much easier, so that's what I'm planning.)

Also, are you concerned about the wired for the ailerons slipping off their posts? You may want to shorten them a bit and add some kind of end stop so that they can maintain tension and keep it from sliding off. I really like the mechanism though! Keep it up!

Yeah, still experimenting with those — I'm worried that if the wire's up against an end stop, it'll increase the friction (which is already a bit much). What I'd really like to do is have L-shaped rocker arms on the posts, but I can't figure out at all how to build that.

 

[Chuppster]

Do you think it needs more span, or will just increasing its chord suffice? (The latter is much easier, so that's what I'm planning.)

My intuition says that increasing the chord and the span each would yield a linear gain in control. I don't know if there are large benefits to one over the other, but I haven't seen many airplanes that don't have full-span elevators.

 

[Crow929]

The overall construction looks great, given the materials used were limited to paper and tape. Well done!
Do you happen to have video of the flight surfaces moving? That is a very unique way of hooking up the ailerons, I'd love to see how they moved.

 

[soundnfury]

Do you happen to have video of the flight surfaces moving? That is a very unique way of hooking up the ailerons, I'd love to see how they moved.

Not yet, but I'll remember to get some next time.
I believe a similar arrangement of aileron cables was fairly standard in the 'stringbag' era (pre about 1930), only replaced by rigid rods when airspeeds over ~200mph necessitated it.

 

[soundnfury]

So I took it out today for another taxi-test after finishing the repairs and modifications, and the motor pulled the firewall out of the engine mount. (It was only at about half throttle, too!)
So I redesigned the firewall to make it more rigid, and tried again. This time it tore its way out of the engine mount and broke the prop on the ground.
Back to the drawing board...
(Fortunately, no damage to anything behind the front 4” or so. And the engine mount is a detachable unit, so the new one will just slot right in.)

 

[Chuppster]

So I took it out today for another taxi-test after finishing the repairs and modifications, and the motor pulled the firewall out of the engine mount. (It was only at about half throttle, too!)
So I redesigned the firewall to make it more rigid, and tried again. This time it tore its way out of the engine mount and broke the prop on the ground.
Back to the drawing board...
(Fortunately, no damage to anything behind the front 4” or so. And the engine mount is a detachable unit, so the new one will just slot right in.)

Is the motor mount made of paper like the rest of the airframe?

 

[soundnfury]

Is the motor mount made of paper like the rest of the airframe?

Mostly, but there're a couple of bits of cardboard in the firewall, mainly to provide a plate for the bolts to go through. The next version might use some more cardboard to strengthen the various joints; I'm still mulling over various designs.

 

[Chuppster]

Mostly, but there're a couple of bits of cardboard in the firewall, mainly to provide a plate for the bolts to go through. The next version might use some more cardboard to strengthen the various joints; I'm still mulling over various designs.

I can respect the desire to keep things all-paper or all paper-based, but I can't recommend enough that you use some kind of plywood for your firewall. Nothing stinks more than getting 60 feet in the air then the motor tearing itself from the firewall and slicing its way through your fuselage, destroying the airplane before it can even hit the ground (do I make it sound as if I've done this?).

 

[kpixels]

I think 60 feet in the air would be a win, great idea for a challenge!

 

[soundnfury]

Progress over the last month-and-a-half: built the new motor mount (with cardboard-reinforced joints), doubled-up the wheels (to stop them slanting on the axle so much), and reinforced the inner wings and forward fuselage. This last involved a new constructional innovation: after making a paper tube, it can be stove-in on one side to yield a crescent-shaped cross section. This can then be taped onto an existing tube (for example, the leading-edge spar) to increase its rigidity. I reckon I should be able (weather permitting) to make the next series of test flights some time this week. (I am also switching to a three-bladed propeller, chiefly because that's what my friend had in his second-hand parts bin.)

And yes, if it ever gets 60ft up I'll consider that a success no matter how many pieces it comes down in

 

[soundnfury]

Well, I took it down to the field again today, and things didn't go so well, mainly because the structure was warped from the previous "landings". On the first attempt it nosed down, probably because the thrust line was pointing down a bit due to a slightly droopy nose. After tweaking the thrust line, on the second attempt it rolled left despite full right stick; the cause is probably either unequal wing incidences or just too much prop torque.

Video to follow (it's currently on my friend's phone).

So the M8 is going on the boneyard, but fear not, this project is not dead, for I have already started building the M9. The plan is for a biplane with pronounced dihedral on the lower wing, and a 60cm span (hopefully it will be much stronger than the long cantilever monoplane). I've also come up with a new way to construct the wing ribs, which should give a more consistent aerofoil profile, and I'm considering possibly using single-acting ailerons (because those hinges never worked properly on the M8. Unlike the one on the elevator, which was always fine).

 

[soundnfury]

Video to follow (it's currently on my friend's phone).

Taxi test (also shows control surface actuation).
First attempt (nosed down).
Sadly the footage of the second attempt was lost. (You're not missing much, though.)

As for the M9, have a front elevation I did to work out some dimensions and (grainy) photos of the upper wing (with chordwise struts but not yet built-up ribs) and port lower wing (with built-up ribs and lower skin). Note that there will be more wing surface behind the rear spar, but I need to experiment a bit more with trailing-edge rib designs first.