USF2000 rear wing spar cracks

[Speed Sport Engineering]

Has anyone else experienced a failure of the end ribs inside the Van Diemen / Elan USF2000 rear wings?

See attached photos. Both ends are cracking around the nut inserts.

This was a new rear wing purchased in 2020, not used until 2021. About 5 race weekends, no impacts.

Edit - to clarify, the issue is on the lower element where the end plates attach. The rib and the wrapped wing material are sanded flush. Although in the photos it looks like the rib is recessed slightly from the wing surface, it is not.

[DaveW]

The ribs' fatigue cracks are from vibration/flexing of the endplates - in front view they are pivoting laterally centered on the cracked attachment points. Considering the ribs are likely 1/16" thick aluminum, even 16 gauge (0.059") thick would not withstand that sort of stress, even if they were 6061T6. Of course, if they were a soft, untempered, alloy, the situation would be worse.

So one question is: why are the endplates able to move L & R at the top enough to cause this? Are the top attachments too flexible? Or are the ribs slightly convex on the ends, allowing all this flex at the attachments? Ideally you would want the ribs to be a tiny bit inboard of the ends of the wing skins so the skin ends would be in compression (keeping the endplates from rocking) and the ribs reinforced internally with a doubler surrounding the mounting points.

All of my conjecture may not be the main cause - the 1st thing I'd check is what alloy the ribs are made from. If they are very soft and not reinforced, they'd have VERY poor fatigue resistance.

[Speed Sport Engineering]

Thanks Dave -

You're right - those are the ribs and not the spar. I should have known better. I updated the original thread.

I understand all of the reasons why it could be failing, but this was a standard VD element purchased from a large supplier of which I assume there are lots of them in use. I'm using all standard VD wing components purchased from the same supplier except the end plates. So....

1) Either the end plates are the issue, if so why? They are replicas of standard VD end plates.
2) I received a bad lower element
3) Others are going to soon have the same issue.

[DaveW]

Thanks Dave -

You're right - those are the ribs and not the spar. I should have known better. I updated the original thread.

I understand all of the reasons why it could be failing, but this was a standard VD element purchased from a large supplier of which I assume there are lots of them in use. I'm using all standard VD wing components purchased from the same supplier except the end plates. So....

1) Either the end plates are the issue, if so why? They are replicas of standard VD end plates.
2) I received a bad lower element
3) Others are going to soon have the same issue.

My guess then is that the end ribs are not strong enough (your # 2) - either lacking reinforcement, too thin, or soft alloy. I doubt that the endplates are the issue unless they are too thin (resonant frequency lower) or too heavy, but I doubt you'd put on endplates that were heavier.

If that's the case, I'd remove the end ribs and add a 1/8" 6061T6 reinforcement sheet (doubler) in the attachment areas and then rivet the nut plates through it all.

[problemchild]

Five events may have exceeded the expected lifespan of a USF wing.

I have had the lower rear element "delaminate" from normal use. I think the harmonics of some Zetecs create some extra stress on some components. Those harmonics may be creating some excessive sideplate vibrations that would not happen with the MZR, or even other Zetec cars.

I have an exhaust extension we want to run for sound control reasons. Clamps, rivets, sheet metal screws won't keep it attached for even a few laps, so we eventually went to street car exhaust clamps with 5/16 hardware and a 1/8" thick steel hanger.

Being a regular at Road America with that curbing may not be helpful either. If it will shake out camber shims and break steering arms, it cannot be easy on wing components either.

[Phil Creighton]

We have been selling an aluminum version of this wing with no issues since they first appeared.
phil

[jtaphorn]

Yes, I have experienced the problem on two aluminum lower wing elements that I have used on my FC. As has another FC racer, who lives adjacent to me.

[Rick Kirchner]

My lower element is probably similar (metal structure wrapped in fiberglas) and a few years back it broke internally at Charlotte. Kevin Kloepfer saw it flopping about on track, almost like a variable incidence device! Having no spare I had to string it up like a Sopwith camel with wire rope.

My internal structure was steel (the attachments to the car are aluminum). The welds had broken between the stringers and the spars. No cracks at the sideplate attachments though.

The whole thing had to be cut apart, re-welded, and then the shell bonded back on.

It had been on the car for at least 13 years though (although I'm probably not as hard on the equipment as some)

[tlracer]

The element of the wing assembly ( 2 sideplates, upper & lower wing elements) forms a rectangle which, as any structural engineer will tell you, has little resistance to parallelogram deformation.

In the early days of sideplate-mounted wings these tended to be fixed via a lower tube (think Ralt RT1) which had a significant depth and gave resistance to this deformation. Added to this sideplates were either plain aluminium sheet, which could flex and temporarily take on a flattened 'S' shape or else they were thin ply, which is relatively 'dead' to vibration/harmonics.

In the quest for less aero drag wings have become thinner; increasing stiffness means sideplates have become more rigid. Taken in combination what these have done is to create a wing assembly that is not much more than 4 thin-section, stiff elements that are, virtually, hinged together: Thin-section wings give little room for spread on the endplate fixings, so they're basically in a straight and near-horizontal line.

Increasing the material strength and/or gauge of the end ribs may help to stiffen the areas and reduce cracking.

Alternatively - and especially if the wing is rigidly fixed to the car - incorporating a degree of compliance here, such that the whole wing structure can move slightly ( = rotate axially) at this point, may help to reduce the amount of parallelogram deformation and so unload the wing/endplate joints.

[Chris Livengood]

What we have here is a case of things doing things and that results in other things.

You could always beef up that thing and hope that things stop doing things that results in other things, but you'd probably just cause things to happen to other things.

I hope I wasn't too technical.

[Speed Sport Engineering]

Thank you for the responses.

Given the responses it doesn't sound like a wide spread problem, therefore I'm going to assume the problem is related to the side plates I hade made as they were the only non-VanDiemen part.

I am getting another set made and going from .100 thick to .125 thick and switching from 6061 to 2024 to try to reduce any occurring oscillations that are fatiguing the outer ribs.

[DaveW]

...I am getting another set made and going from .100 thick to .125 thick and switching from 6061 to 2024 to try to reduce any occurring oscillations that are fatiguing the outer ribs.

Stiffness (modulus) won't change significantly between 6061T6 to 2024, but 2024 will have a higher yield stress.

And I'm guessing that changing the stiffness of the end plates may not solve the rib cracking. In fact, making them thicker/heavier "could" make it worse. It depends on why the flexing was occurring. If the lack of rib strength/stiffness was the predominant cause, you may not solve it w/o reinforcing the end ribs.

In any case, if I had that issue I'd reinforce the end ribs no matter what else I might change.

[Speed Sport Engineering]

Stiffness (modulus) won't change significantly between 6061T6 to 2024, but 2024 will have a higher yield stress.

And I'm guessing that changing the stiffness of the end plates may not solve the rib cracking. In fact, making them thicker/heavier "could" make it worse. It depends on why the flexing was occurring. If the lack of rib strength/stiffness was the predominant cause, you may not solve it w/o reinforcing the end ribs.

In any case, if I had that issue I'd reinforce the end ribs no matter what else I might change.

I agree on all points. Ideally strengthening the ribs is the best option which i an looking at doing as well, but to do so i need to successfully disassemble the VanDiemen wings, get new ribs made that are thicker, then reassemble the wing. I'm going to try the easier path first of the stronger end plates.

It may make it worse, but considering it's the only difference between all the vandiemn wings in use, seems like the likely source.

[Lyngengr]

Thank you for the responses.

Given the responses it doesn't sound like a wide spread problem, therefore I'm going to assume the problem is related to the side plates I hade made as they were the only non-VanDiemen part.

I am getting another set made and going from .100 thick to .125 thick and switching from 6061 to 2024 to try to reduce any occurring oscillations that are fatiguing the outer ribs.

I had some end plates made out of 0.125 thick 6061-T6 with the trailing edge turned outward to increase the rigidity and possibly influence the wing aero. See the picture. You need to be careful of width, but I had room to add the turn-out and still stay within the FC wing dimension limit.

[EricP]

Couple thoughts:

1) could over torquing this make it worse?
2) is there a gap between the nut plate and wing plate? Seems that would create tension that might magnify this issue.

[DaveW]

If the wing endplates had some space between themselves and the ribs, the ribs would have been drawn into contact with the endplates forming a relatively rigid assembly, which, if it stayed tight, would not have been subject to fatigue. What caused the rib cracking was repeated flexing of the rib at the nutplates.

So, IMO, it was not possible to over-tighten that assembly unless it stripped out the threads. Also, IMO, a slight gap drawn and staying closed would have made flexing and fatigue less likely than having everything flush with the end or the ribs extending beyond of the end of the wing skin.

So, considering that the ends of the wing assembly were sanded flush, it goes back to my original opinion that the ribs were not strong enough to keep the assembly tight, thus allowing fatigue around the nutplates.

[jtaphorn]

IMO, the problem stems from the rear wing's side plate attachment bolts being in a relatively straight line to one another. The narrow chord of the lower wing doesn't permit meaningful offset of the flange rivnuts/nutserts mounted in the lower wing's outer ribs. As a consequence, it is relatively easy for the sideplates, with the top weight of the upper airfoil(s), to rock side to side under cornering loads and eventually work harden the lower wing's aluminum endplate above and below the rivnut/nutserts. This leads to the endplates eventual cracking and failure.

I have used larger outside washers in an effort to help but the small distance between the nutserts/rivnuts limit the size of the fender type washer.

Last weekend while racing a COTA, I noticed an FC with a modification I hadn't seen before and his wing was extremely rigid with no discernable side to side play. Inside each rear wing sideplate and mounted between the sideplate and the lower wing's endplate was an ~1/8" thick triangular piece of aluminum. The aluminum addition was as wide as the width of the endplate and extending up about 8" in the shape of a triangle against the inside facing side of the sideplate.

Frankly, I could understand how this added piece would reduce potential flex in the sideplate itself and was less certain as to how it would help the endplate attachment cracking problem. I can only attest to being impressed as to the rigidity of his assembly.

My thought is this piece on the outside of the sideplate may work better acting as a large washer.

Perhaps his wing assembly was simply new. They all start out nice and rigid.

[DaveW]

...As a consequence, it is relatively easy for the sideplates, with the top weight of the upper airfoil(s), to rock side to side under cornering loads and eventually work harden the lower wing's aluminum endplate above and below the rivnut/nutserts. This leads to the endplates eventual cracking and failure....

Except for the fact that the endplates didn't crack and it was the end ribs that fatigued, I agree with the reasoning behind your comments.

[jtaphorn]

Dave, I meant exactly what you said. Thanks for clarifying.