Stunt Foam Wing Construction

LAST EDITED ON Jan-23-04 AT 09:19 PM (CDT)
 
I have a foam core for a Ted Fancher Imatation model. The foam core is not cored out, so I would like to save as much weight as possible.

I notice on the plans that there is not center spar. Should there be a center spar like on the Genesis plan?

Tom Dixon's quick build method does not mention anything about how to construct and install the center spar. Is the outside sheeting the only thing holding the wing together? This does not seem right?

Just from reading threads here in the past, I gather that the sheeting alone is enough to hold the wing together. Mind you, Stunt wings are really thick. I've read that some people put glass cloth over the root area, and most people apply silkspan over the entire wing (mainly for finish, but it also adds to the strength).

I have built several foam wings, some withLite ply spars some without. In years past I never used a spar and the only trouble I had was when I did not put a lite ply spar in when the gear was in the wing. I know all you experts are saying, why in the world wouldn't you use a spar, but I guess I thought I could get away with it. In any event, the first bouncy landing ended in the sheeting getting cracked and breaking the gear out. Cure, put the gear in the fuse. I had my Profile ST 60 powered BANDIT at the 99 Nationals in intermediate. I stuck a 1/8 X 1/4 piece of balsa the full length of the wing which appeared to be a built up wing, but was actually a foam wing. That wing did not use a lite ply spar. That plane never ever showed a stress crack. All of the foam core wings I am talking about were cored in three sections. The last 2 nights I have been playing around with a set of cores I have had laying around my house now for several years. I am using the Tom Dixon method. I have to say that this has been very easy to do. I have all the sheeting on with the exception of the cap strips. I can assure you that won't take long. I did hot wire a 1/8" groove in the wing for a lite ply spar.

I guess as I look back on this response, I should answer your question. In reference to the Imitation wing. I would core the wing out without question. If you core it in three sections, you will have two foam spars running the length of the wing. In the front foam spar, take your hotwire and carefully slice out an 1/8 slot. I use 1/8 X 1/4 spruce as a guide for my hotwire. Make a lite-ply spar and epoxy it in. Once that is completed, you can run your leading edge sheeting to the back edge of that lite ply spar or in my case, I sheeted past it about 3/8 of an inch. Moving on, once you glue your wing together, always, always fiberglass the middle section.

>I guess as I look back on this response, I should answer
>your question. In reference to the Imitation wing. I would
>core the wing out without question. If you core it in three
>sections, you will have two foam spars running the length of
>the wing. In the front foam spar, take your hotwire and
>carefully slice out an 1/8 slot. I use 1/8 X 1/4 spruce as a
>guide for my hotwire. Make a lite-ply spar and epoxy it in.
>Once that is completed, you can run your leading edge
>sheeting to the back edge of that lite ply spar or in my
>case, I sheeted past it about 3/8 of an inch. Moving on,
>once you glue your wing together, always, always fiberglass
>the middle section.

I would core it, yes. You could core it in two places instead of three, but I would definitely leave a foam spar in the middle/front area. Adding a part way 1/8 inch lite ply spar inside this foam spar greatly strengthens the center area, and this spar (along with the foam spar) should be in the area of the belcrank with an appropriate notch for clearance.

Now, when you glass the center, do it in several layers using lighter materail. Instead of a 6 pound...er, ounce strip that is only an inch and a half wide and doesn't even reach the fuselage sides, use three layers of three quarter ounce fabric, one 6 inches or so wide, one 4 1/2, one 3, so each one is safely outside the fuselage. Then make rounded or jagged edges so you don't have a hard breaking surface at the end of each layer. This will bring the stress out beyond the fuselage, is quite easy to blend in, and will keep that embarrassing shear of the wing from happening right at the fuselage.

Personally, I am not a fan of the Dixon method of building unless you are purposely simulating a built up wing. If you are simply trhying to save weight, use 1/20 or 1/32 balsa over the whole wing. It will be lighter and stronger than the Dixon method and much easier to finish (I hate sanding between ribs, simulated or not.)

It will be lighter and stronger than
>the Dixon method and much easier to finish (I hate sanding
>between ribs, simulated or not.)

Me too. I like surfboard wings...

>

>Personally, I am not a fan of the Dixon method of building
>unless you are purposely simulating a built up wing. If you
>are simply trhying to save weight, use 1/20 or 1/32 balsa
>over the whole wing. It will be lighter and stronger than
>the Dixon method and much easier to finish (I hate sanding
>between ribs, simulated or not.)

speaking of sanding between ribs... how DO you do it??? I always manage to sand through the silkspan somewhere on edges and not sand enough in the open bay areas... Would a flexable cloth backed sand paper help? or, is this one of those "practice, practice, practice" things?

Ron B.
F4Fguy

Iskander:
You're right about the skins being the primary stress areas.1/32" 4-6# balsa + silkspan +dope,PC,etc.,is easily up to the task,if properly supported.The problem is to maintain the integrity of the skin as related to the section modulus.As you said,the separation of the skins is the main concern.
The liteply spar has little to do with being a reinforcing "beam".It's function is to act as a shear web at the point of maximum bending,to keep the skins from crippling.This is a far more effective structural solution than extension of the"glassing"on the center seam.If you do even a cursory evaluation of the imposed loads and their distribution,there's no need for any more reinforcement of the center seam than is needed to compensate for the butt joint in the skins.
Another function of the liteply spar is sort of a "gimme",that is for torsion loads from wing mounted gear.It does need help here in the form of half ribs to transfer the loads to the skins.
A couple of other points;foam,by itself is a terrible structural material,it needs a lot of help;NEVER try to skin a foam wing AFTER it's been cored,you'll get marginal(if any)adhesion over the cored areas.Shoving the cores back in doesn't solve this as the angle of the cores will not allow them to take up all the space left by the cutting wire.

Only built two foam wings covered with balsa and finishing stuff. One had a full-length spar. The other, a Sig Mustang Stunter, didn't. Been having gear problems every flight. If you have a wing-mounted gear, at least put a spar between the gears to glue the gear blocks to. Otherwise, probably a strong enough wing, but there was a post here recently from someone with a plane having problems with cracking of the inboard wing along the fuse area.

What direction should grain be, parallel to the leading edge or trailing edge or both.

Can I do something like this:

I always make mine parallel with the leading edge. I am sure there are many ways of doing this, but to keep it simple, I make mine go with the leading edge of the wing. I have been working on a wing for the past couple of days and I have just finished up with the capstrips, I will post a picture tomorrow.

LAST EDITED ON Jan-26-04 AT 06:05 PM (CDT)
 
>What direction should grain be, parallel to the leading edge
>or trailing edge or both.
>
>Can I do something like this:

Ron B.
F4Fguy

The easy way is parallel to the leading edge all the way.The better way is as you've shown.This gives the best compromise between beam and torsional rigidity.In practice either way works just fine.The big problem with any method is the same,getting "0"-"0" fits between the edges of the joints.By this I mean,NO visible gap,un-glued and un-stressed(not pulled or taped together).Any gapping that's pulled together by force causes the assembled skin to go out of flat putting localized stresses on the skin/foam bond and making the whole wing subject to crippling under beam loading,which is the primary load on the wing.In this system,the skins carry the stresses in tension and compression.the foam acts as a shear web.The usual cause of failure in a foam wing is in the surface of the foam in the bond line or poor or non-bonding.
The adhesive is relatively unimportant as long as its compatible.The contacts have the advantage of some measure of flexibility,tranfering loads to the foam a little more"gently".The epoxies are marginally lighter when properly used,but are so much stronger than the foam that fatigue failures in the foam at the foam/epoxy interface are more likely.Urethanes offer a little of both worlds both good and bad.In the end it comes down to the integrity of the skins and the ability to support them and keep them from buckling under compression(tip to root).Tension loads in the skin are seldom a problem,unless you get over enthusiastic with the sanding bar on a 1/32"skin and end up with 1/64".
I've been using foam wings almost exclusively since the early '60s and I've used just about every "flavor of the month"method.The above comments represent that experience.My present methods are somewhat different than any of the usual ones and would take another whole post to describe.However,they are very specific to my needs and probably would be considered overkill by most people.

Tom's quick build method uses 2-inch fiberglass tape wrapped around the center joint. This should be more than adequate to handle the G-loads you will see in CLPA.

I would core the wing if possible and would suggest a lower spar to tie the wing gear blocks together. If you land like I do, that's where the shock loads will occur.

A foam wing Imitation is a very nice machine.

Ron King

Ron, etc,

Tom's quick build method works, but proper wood selection is important, especially if you use film to cover the wings. As I posted in an earlier thread, my foam winged Nobler had a failure in the wing right at the edge of the fillet on the inboard wing. This is beyond the 2" glassed area. My error was in using contest wood for the 1/16th centre section sheeting. Subsequent models I have built this way have harder sheeting, and should avoid this issue. The nobler in question here had only 100 flights to visible cracking in the sheeting and 130 flights until the repairs to said crack failed. When I built the Nobler three years ago, I thought:"How much G could a lightly built Nobler see?, contest wood should be fine." I know better about wood selection now......(and how many Gs a small light model will see!)

Steve, You make a very good point here. As I was working on my wing i used contest balsa all over with the exception of the center sheeting. I used some much stiffer balsa. Good point and great suggestion!

This is the wing I have been working on for the past couple of nights. As you can see, the wing has yet to be cored as I don't like to do that until after the sheeting is in place. Whether you like or dislike the way Dixon does his foam wings, it sure is easy!

Keith,
As always..Nice work!

Keith,
Looks great!
Question: Do you split it at C/L after coring for Bellcrank and/or installation or ????
Roger

>Keith,
>Looks great!
>Question: Do you split it at C/L after coring for Bellcrank
>and/or installation or ????
>Roger

Sorry, above should read:
Do you split it at C/L after coring for Bellcrank and/or spar installation, or ?????? Inquisitive mind wants to know.

Roger

I am not for sure what you are referring to when you say split it. The photo is a bit deceiving in that it is currently 2 seperate wing panels. I pushed them together for the photo. I will now put my templates back on that I used to cut the wings and core the wing. Once that is completed, the controls will go in and it will be time to button things up.

When do you attach the LE and TE. After the sheeting or before?

I typically sheet the wing and then add the Leadeing and Trailing edge. On this particular wing, I glued my trailing edge on and then sanded it down to the foam core and then placed the 1/16th sheeting over top of that. I just wanted to try something different and I saw where Tom Dixon was doing his that way so I thought what the heck! I do like the fact that you do not have a joint visible at the trailing edge. I think the technique would work with the leading edge as well, I just need to give it a try. I have another foam wing sitting around downstairs so I may give it a whirl with that wing.

>I typically sheet the wing and then add the Leadeing and
>Trailing edge. On this particular wing, I glued my trailing
>edge on and then sanded it down to the foam core and then
>placed the 1/16th sheeting over top of that. I just wanted
>to try something different and I saw where Tom Dixon was
>doing his that way so I thought what the heck! I do like the
>fact that you do not have a joint visible at the trailing
>edge. I think the technique would work with the leading edge
>as well, I just need to give it a try. I have another foam
>wing sitting around downstairs so I may give it a whirl with
>that wing.

Keith,
I usually do the TE before sheeting too. I think you might run into some problems with doing the LE first though. I would think that after sheeting, then sanding the LE to shape you'll get a long "wide" glue joint along the LE. I can't remember if I ever tried to do the LE first then the sheeting. Let us know how it turned out.

Thanks.
The photo gave me the impression that the two panels "were" glued together.
Roger