Wing Structure - RC Groups
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Feb 24, 2013, 03:30 PM
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Wing Structure

Hey everyone, so just a little background information: I'm working on a senior design project at my school and I am in charge of designing the wing of our airplane. I have a nifty code for calculating the lift distribution along the wing, which will in turn help me to determine the deflection I can expect for the wingtip.

I was thinking about a structure with 2 sparsne at the leading edge and one midspar. I was hoping that I could get some thoughts and opinions from experienced builders on what they thought of this idea for the structure. I would really appreciate any advice I could get. Thanks everybody!
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Feb 24, 2013, 03:40 PM
Hobby King Hater
Kimber's Avatar
We need a lot more info.

Wing span, material, weight, purpose.

A very light plane needs only a leading and trailing edge to support it.

As the weight increases it needs more. The main spar is normally
at the CoG of the wing load. The leading and trailing edge are
not load bearing to much extent.
Feb 24, 2013, 03:51 PM
Registered User
Its has to carry some golf balls. The total weight of the aircraft will be somehwere in the vacinity of 3 lbs. It will weigh a maximum of 4 lbs with cargo. I figured that the leading edge spar would be a good idea since my team members and I do not have much experience flying. It needs to be durable enough to withstand a little abuse. I'll be running some materials testing tonight. The likely candidates are some sturdier types of balsa wood. Also, there will be no ailerons on the wing. Span of 6 feet. Does that help?
Feb 24, 2013, 04:24 PM
Hobby King Hater
Kimber's Avatar
A 72" wing at three pounds will have to be lightly build and not too

You might think about a foam wing with CF spars.

And you need someone with RC experience.
Feb 24, 2013, 04:33 PM
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I thought about foam, but the teacher wants us to build an actual structure. because of our lack of experience, it will have to be fairly durable. Able to withstand a crash or two before it busts. We are competing against numerous teams, none of which has any prior experience either, so we are looking into software to simulate flying an R/C plane.
Feb 24, 2013, 07:12 PM
Hatters gonna hat.
madbomber's Avatar
If you are able to purchase a copy of the RealFlight simulator, I am fairly certain you can model your airplane in some type of cad program and import it into the simulator. That way you can get some practice on the sticks AND you could have a decent idea of how your design will work. There may be other, less expensive simulators that offer this feature, but I dont know of any off the top of my head.
Feb 24, 2013, 08:40 PM
Registered User

It sounds like you are building a glider given the weight and wingspan. It has been done before, so it can be done. Given the wingspan you are hoping to achieve, the best structure will probably be one where the first 1/3 or so of the cord is sheeted using thin balsa. In essence, the balsa skin covers the top and bottom surfaces of the wing between the LE and the main spar. This creates a box structure which is quite strong in bending as well as in torsion. An open structure using two spars will be relatively weak in torsion unless you use some diagonal bracing in between the ribs or you provide supporting struts. It would probably also be heavier.

The main spar can be a simple I-beam where the spar caps are made from spruce. The shear web between the spar caps can be balsa with the grain oriented vertically. At 4lb total weight, the spar caps can probably be made from 1/4x1/8 spruce. If need be, you could increase the depth of the spar cap near the wing root to 1/4x1/4 to provide additional strength.

Finally, I would try to enlist the help of someone who has RC experience. After the plane is built, you will have too much time and effort invested on it to use to teach yourself how to fly. It will probably not end well.

If you are looking for technical assitance, take a look at The have a very good techincal forum. Also see if you can get ahold of a copy of ANC-18, the bible for wood aircraft structures. The file is a little too big to post here.

Good luck,

Last edited by viva_peru; Feb 24, 2013 at 08:47 PM.
Feb 24, 2013, 09:08 PM
Registered User
Originally Posted by Kimber
A 72" wing at three pounds will have to be lightly build and not too tough.
I've got a 84" Quaker that's a bit over 4 lbs. all-up. It's a 1937 design, and was originally designed for free flight.

The fuselage is made almost entirely of 3/8" balsa sticks. The wings have a pair of 3/8" hardwood spars. I could have made it lighter using film covering, but I used SIG Koverall and dope for ruggedness. It can be flown for several minutes on a 3S/2200 battery, but I usually fly it with 4S/2600.

It is not a fast plane by any means, and won't tolerate much wind. But I'd say it's quite robust for its intended flight envelope.
Feb 25, 2013, 09:24 AM
Hope to get out of life alive
kenh3497's Avatar
IMO you MUST get some help when it comes time to fly the plane. Some people can pick up the transmitter and fly but they are far and few between. The flight simulators help a lot, but they are not like the real thing.

If you Google "model airplane plans" you will find many examples to give you and idea of what you need to do to building a strong wing. No use reinventing the wheel here. You may and I would expect you would find examples of the very plane type you are attempting to model.

Feb 25, 2013, 09:14 PM
Registered User
I flew a kit today that I was provided for practice. My flying skills aren't too shabby, except for a couple hard landings. At this point, I've done some material testing to get the modulii of different materials. Based on the material properties, I just need to design a midspar that will only deflect a certain amount based on the lift being generated.

Does anyone have any ideas what my max deflection should be? And thank you to everyone who has already given me advice so far! It has been really helpful!
Feb 26, 2013, 07:44 AM
Registered User
I have read somewhere that for full scale light aircraft, the rule of thumb is about 1" per G. If we assume a 26 ft wing span, the 1" deflection happens over 13 ft or so. To make the math easy, assume 12 ft. Your plane will be 6 ft, so 3 ft per panel, or about 1/4 of full scale. With that in mind, I would say 0.25" per G. If you design it to take 4 G's. then 1".

Are you in engineering school or is it high school. You never really said. As for getting data on wood properties, take a look here:

and here,

Good luck,

Feb 27, 2013, 09:15 PM
Registered User
I've determined a deflection through some tests. I need to pick material for the wing spars. The main load carrying beam can't have dimensions larger than 1 inch (I am going with a rectangular shaped beam). Any thoughts on some popular woods used that I could look into?
Feb 27, 2013, 09:53 PM
Hobby King Hater
Kimber's Avatar
For the weight you are dealing with balsa is fine. A boxed
spar is the strongest. A top and bottom spar with a plate
between them, called a shear. It is important that the
shears grain in 90` to the spar.

Like this:

Feb 28, 2013, 07:34 AM
Registered User
Wow, thank you for that, Kimber! I see that there is balsa sheet on the leading edge and trailing edge. Its placed there more for keeping the aerodynamic shape, right? It must help support the structure to some extent, or is that typically too minimal and I'm just over thinking?
Feb 28, 2013, 08:16 AM
Registered User

The built up trailing edge provide the area needed to adhere the covering, but since it is in the shape of a C-channel it also provides some torsional stiffness to the wing. Since most joints on an R/C plane are butt joints, adding the sheeting over the trailing edge piece adds quite a bit of stregth. If you are not going to have ailerons on the wing, the trailing edge could come to a point and this could be done by having the TE top and botton sheeting meet at the tip of the rib.


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