Help me design a new plane - RC Groups
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Jul 21, 2005, 02:57 PM
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Help me design a new plane

I started a thread a little while back saying that I thought kits and arf's should make better use of available materials to make planes that are light, fly well, aren’t expensive AND are durable. All the balsa kits out there fit the bill for all but the durable part.

My situation is that I basically want a high wing sport plane like a mini-telemaster or clipped wing cub but I don't want an all balsa plane. I want to be able to fly it without worrying that a minor crash will crack a major part of the plane. I only keep one plane flying at a time so when it goes, no more flying.

Currently I fly a T-52 with added ailerons but being a trainer it doesn't quite have the performance i'd like. I also flew an Acrophat for a while but broke the fuse in half a few times on bad landings and never really enjoyed it out of fear of breaking it. It’s a lot more fun to have a plane you can fly low, and fast and know that it its pretty tough. I'm not looking for indestrucable but I think that with smart material choice balsa planes can be improved on a lot in the area of durability without making many sacrafices.

Unable to find anything in the 40"-48" 18-30oz range to take my MP Jet AC 25/35-20 I think I’m going to try to build my own. I've never built a kit but what the hell i'm a recent college grad with no job and plenty of time. The wing is going to come first, I'd like to make a fuse later but if I never get to that I can use this wing on the T-52 fuse. I love foam but I have nothing to work foam with. A foam wing with a cf spar would be great but I don't know how to cut a good airfoil out of a block of foam. So I think I’m going with spar-rib-covering and here is what I’m thinking for it.

WING DESIGN---------------------

--SPAN: ~44". I want something smaller than the 48" T-52 wing and longer than the 40" acrophat wing I have. Being able to slow down well is important and I also want some performance. I think 44" should be good for a 25oz plane.

--CORD: 9". Both the t-52 and the acrophat have 9" chords. I don't see reason to change this.

--AIRFOIL. The t-52 has a thick airfoil of a trainer which is one thing I don't like about it. I want something symmetrical or near symmetrical. Naca 2412 seems pretty popular as a semi-symmetrical so I might go with that if it would let the plane slow down better than a comparable completely symmetrical. Again I want good slow flying but with decent performance. Maybe some inverted flight, and a plane that doesn't need a huge trim adjustment to keep it level when you hit the gas.

--LEADING EDGE/SPAR: .254 OD 48" cf tubes
I already purchased these to force me into this project. The acrophat has 0.31 OD tubes but they are rock solid so I thought there was room to go down. I found these at a decent price so I went with them.

--TRAILING EDGE: Undecided.
Balsa might work fine here as its unlikely to take a hit, so might corrugated plastic. Some other kind of stiffer plastic might also be good. I'm trying to think of some other long narrow plastic that could work here.

--RIBS: Corrugated plastic/Corrugated plastic-balsa sandwich?
A nearby place (charrette) sells corrugated plastic in several thickness. I'm confident it can take the force of compression from the covering. It should also be able to take most of the twisting forces the wing will see. I'm a little worried about the ease at which it bends though. If the wing hits something hard all the ribs could bend between the leading edge and the middle spar completely screwing it up. I'm thinking that sandwiching balsa between corrugated plastic and using it for a couple of the ribs might work well. Even if the balsa cracks it might maintain its form and most of its strength. I'll test this. Other options would be thicker corrugated plastic, glued together layers of corrugated plastic, or something else sandwiched between corrugated plastic.

WING CENTER: Coro/balsa/plastic?
The wing is probably going to be held on with rubber bands so the center needs to be strong and stiff. This means strong ribs close together and a solid covering. Whatever I decide for reinforcing the ribs should work here too. Then the covering needs to be strong and stiff. I'm not really against balsa here because its unlikely to get cracked but coro might do the job too. Also there are those real cheap 3 ring binders that have flexible single layer plastic coverings. I wonder how that would do. Whatever goes here needs to be bent to match the airfoil.

So I’m wondering whether people think these choices are good. Are there any good materials I’m not considering? It seems like there should be places to find more plastics but my binder idea is about all I can think of right now.
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Jul 24, 2005, 05:18 AM
It's a good day to fly.
What about the Ultrafly Cessna? That foam is pretty tough and can be glued back together many times if necessary until that becomes less of a concern. You could cover it for even more strength. It's an excellent flying airplane.

If you do want to build you can get a set of laser cut balsa wings from Nasty Toes Aviation. Requires a spar and very little other bracing. Once covered it's very durable. Of course nothing is going survive a serious crash unless you build it out of rubber.
Jul 24, 2005, 05:41 AM
RIP "Long Shadow Flyer"
crvogt's Avatar
You might try looking at the Foamie Group, one material you did not mention is Bluecor FFF, makes great, extremely durable wings. Other foams can be used for the fuselage to make very crashable planes... I have made bluecor planes with wingspans from 32" to 72". you could scale a plane to any size you need.

Hope this helps
Last edited by crvogt; Jul 24, 2005 at 05:48 AM.
Jul 24, 2005, 10:09 AM
Registered User
podavis's Avatar
My planes are balsa strategically reforced with CF rod, ribbon and laminate. This keeps them light. I agree with the build to fly, not to crash philosophy. Whenever I can I design for both if it's only a small weight increase.

I think I posted to the earlier thread. I have a plane built to survive and it has survived some pretty severe crashes with relatively minor damage.

If there's one thing I've learned from this plane it's; the wing has to have the freedom to rotate, it's not enough to rubberband the wing on. I have found two ways to do this, one is a saddle it can ramp up out of, the other is to butt the leading edge against a straight surface and leave a gap behind the wing. Allowing the wing to rotate once a threshold force has been exceeded absorbs a lot of energy and cartwheel crashes will slow down much faster and the forces of second and third plane-to-ground impacts will be less.

Dealing with nose first crashes is a lot fuzzier, there are a lot of options. The only certainty here is that once again, rubberbands can work as crash energy absorbers. The difference from the wing is that the rubberbands can't prevent all damage because they can't protect the motor. The basic idea I use is to lash the motor mount to the fuselage with 2 rubberbands wrapped around the mating parts tight enough to take thrust force and torque but not too tight so it can move if it has to. It's easy to wrap it too tight.

Landing gear can also benefit from the rubberband treatment. The best solution I've found so far is to leave the gear off completely and belly land in tall weeds. This plane has a light wire and CF rod gear that is held on by rubberbands to the same CF rod posts that are used to tie-down the wing that I use when I want to do take-off's & landings.


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