Pour Foam Mod: Strengthening wings & elevators
 

There are easier ways to fix weak wings/elevators that could potentially fail without having to cut them open and replacing/reinforcing ribs. This will involve some weight gain but the results are worth it,

https://www.youtube.com/watch?v=woqKQX5MMCs&list=UUFDWEFOpiZexe_HXXJ5XIsg Here I fixed 1/6 F-18 elevators.

So you did a whole video on the theory of it and not actually showing the process?

I showed the results, mixing foam isn't something I needed to waste time on:

https://www.youtube.com/watch?v=KMAL3yF_Z_E&list=UUgaj9Fl59v7yd-DEnqQP-Yg make a opening, mix equal parts foam, pour in the foam.

I dont really see the foam adding any strength, its failure rating is most suredly lower than the composite layup that is already there. I dont doubt that it does add some rigidty (and weight) to the part though.

The only way to really say it adds strength would be by doing a test to failure of the part.

I have used the 2 part foam before, to make my boat float better. I would like to see this done without splitting your stab open, realtime Seems awfully risky to me..

If this was a good idea would not many of the manufacturers be doing this?

Absolutely no structural value at all. My company uses this stuff to make custom foam for shipping customer products. NOT everything on the internet is true.

It makes very good heat insulation for pipes and tanks. Plus it can be used to prevent drafts.

I recall that BVM used to use it in the manufacture of foam core wings, (for the old Agressors and Mavericks.) It was used as a bonding agent between the polystyrene and balsa skin. I think these wings were produced in a steel mould. Totally different application to that described above.

I guess if the idea is to create a foam core wing, it might have some merit as a concept. Although I doubt that simply injecting foam into an already finished built up wing would work.. The material itself is not very strong, and I would be very worried that it would “burst” the wing open as it expands. These structures are not designed for internal pressures.

Well, I think it would help to improve the rigidity ot flying surfaces, As we know this was a issue in many, if not most of the FEJ crashes. By the way, this is the solution a friend of mine used in a early J10 with very flimsy wings, before its maiden flight.. I think he did a few flights with this plane so far.

However there are other issues, like elevons rotation pins screwed in honeycomb or subdimentioned wing spars wich this mod doesn´t fix,.. anyway, in my opinion any measure that improves the rigidity of any airframe part is welcome regarding jets.

Rigidity doesnt mean strong... Take a un-cooked spaghetti noodle for instance.. Rigid and weak

Your plane wants to act as spaghetti at warp speeds.

http://www.youtube.com/watch?v=1ijkixREibs

And uncooked noodles fly better, faster and further than wet ones.:D

Foam core wings are exactly what this is, 2 sheets of balsa between foam, in fact yellow aircraft wings are the same way. I know of composite kayaks built exactly this way using 4lb polyureathane foam.

Their are varying densities, 2 lb being the weakest and 16lb being as hard as bass wood. 4lb density sandwiched between the skin on a FEJ makes it Solid.

you are probably using 2 lb density that expands to 32x original volume

4lb expands 16x its volume

and 8 lb expands to 8x its volume

the 4lb absolutely adds strength and the 8 lb is overkill.

the higher densities also expand slower so exploding wing/stab is less likely.

Try some 4lb on some scrap as a test.

There is a misconception regarding PU foams. Take your fridge or freezer. They are made out of .40 mm formed and stamped steel with a 4 mm plastic tank that was vacuum formed in the fridges or steel tanks in the freezers.
If you can imagine the assembled unit is like cooked spaghetti, once foamed UNDER PRESSURE it is an extremely ridged Cabinet / Box.
But as I say to get the full strengths of the foam it must be ' over packed ' and held in shape by a jig or all you will do is blow everything apart, that is why some boat owners / companies land up popping their decks off.

Any other questions please feel free to ask.
Thanks Wayne.

Perhaps there is another way to view this.. supposing a hollow wing, without ribs or any kind of structure inside, but just a thin monocoque of fiberglass or even honeycomb.. filling it with expanded foam will increase the strenght and rigidity of it by a factor, as unlike air, expanded foam has a tensile strength of 45-60 MPa and a Young modulus of 3000-3500 MPa depending on its density..

Sorry, I forgot to mention, the ratio is 1.25 Iso / Poly. regardless of what the instructions say, this is the best formulation for flexibility and strength.

1 or 2 pound density foam with composite skins on both sides is very rigid and will carry a load, this is basic 101 sandwich construction in composites but the application you speak of has a very high potential of just blowing the entire flight surface apart as the two part foam builds pressure inside with no encasement on the outer mold line to contain the pressure as it cures. So I see this as a lesson in what not to do.

Bob

Disco, where can I get this part A and part b foam?

Thanks

100 % correct Bob, the foam can exert up to 10 Nm. per cm. so as I said before if you do not have a correct mould to put the part in for added support you are in trouble, a friend built the moulds etc. and tried it, unfortunatly there was a slight miscalculation regarding the volume and the foam actually crushed the honey comb skin.

Follow this rout at your own peril. I don't mean to step on anyones toes. but please be aware of the dangers involved it could get expensive, but done right could have lots of benefits.

Thanks Wayne.

I found this to be a very effective solution, I have used this before jets with great success. Some of the strongest wings I have seen are foam core, granted this is not as good but it will prevent the collapsing from being flimsy. I would caution folks who try this, dont rush the application. Just like was stated in the video if you happen to add to much in a confined space it will cause the skin to deform, crack or even bust it.

dude that is a bad idea! you have jest made man hole covers! I have the mold for them stabs the easiest and best way is to make new stabs!

I dont' think making new stabs is the easiest of most cost effective way, you need to see my stabs before and after I'll post another video showing the F-15 wings before and after and the process.

I poured 1 OZ mixed foam at a time 4lb density, absolutely NO issues, the PU pour foam is stackable, pour 1 oz mix at a time you wont blow up your wings/stabs.

Yeah if you just dumped 8 oz then it will probably be a big problem.

ALSO 4LB density DOES NOT expand as fast a 1lb density or 2 lb density, what you see in packing material is usually 1lb or .5lb density that stuff expands 64x, 128x original volume. 4LB only does 16x is very rigid in a sandwich.

ALSO you don't need to have the hole nearly as big it can be a 1/4 diameter and be fine. Just pour slowly.

I dont' think making new stabs is the easiest of most cost effective way, you need to see my stabs before and after I'll post another video showing the F-15 wings before and after and the process.

I poured 1 OZ mixed foam at a time 4lb density, absolutely NO issues, the PU pour foam is stackable, pour 1 oz mix at a time you wont blow up your wings/stabs.

Yeah if you just dumped 8 oz then it will probably be a big problem.

ALSO 4LB density DOES NOT expand as fast a 1lb density or 2 lb density, what you see in packing material is usually 1lb or .5lb density that stuff expands 64x, 128x original volume. 4LB only does 16x is very rigid in a sandwich.

ALSO you don't need to have the hole nearly as big it can be a 1/4 diameter and be fine. Just pour slowly.

I think a lot of you guys are ignoring the obvious benefits here because of the person who posted them.

supporting the structure of a poorly designed airframe with anything is a benefit, the fiberglass is very unlikely to tear compared to the likelihood of the internal structure failing and allowing the wing to fold and eventually snap.

your going to have to be quite overzealous in your application of this method to be able to blow the wings apart, all you need is an escape route for the foam, the foam is done expanding long before it sets so as long as the excess has somewhere to go your good.


I have seen this done a few times and I have owned jets with wings that have been modified this way and they are extremely strong, much stronger than pre modification however I cant handle the added weight, if it was mine I would open the surface and make new ribs but this doesn't invalidate this method for guys who might not have that ability

1/2 which jets do you have stab molds for? I was going to ask you but forgot about it.