Stealth and Supersonic Aircraft - Buckling????

[astro808]

I was wondering if anyone could tell me for sure if skin buckling is permitted to on stealth or supersonic aircraft?

I assume it is not allowed on a stealth plane because the skin buckling would cause a change in the skin shape which is important to the stealth characteristics of the plane.

For a supersonic aircraft I think it could go either way. Depending on the speed it may or may not be desirable to change the loft of the wings.

 

[SWComposites]

Buckling at what load level? Might be acceptable above limit load, since loads above limit should only occur ~ once in the life of the aircraft. Also depends on whether the critical limit load can occur when supersonic.

 

[astro808]

I was thinking about limit loads. You make a good point by asking under what condition the critical load occurs.
I was just looking for a general answer. I am trying to justify if the panels are designed to account for load carrying capabilities beyond initial bucking (ie post-buckling) under a standard, non-event, operational flight profile. Say during a long-range cruise.

 

[IRstuff]

Why would it be allowed? Buckling is a sign that a load limit somewhere in the system was exceeded.

If you saw a car that had buckled skin, would you not assume that it had been in a crash, and therefore is possible compromised?

TTFN
faq731-376

 

[ESPcomposites]

I don't know that there is a fixed rule. From what I have observed, those scenarios have not allowed buckling to ultimate load.

IRstuff, the commerical aircraft industry often takes advantage of elastic buckling of the skin below ultimate load (see Bruhn). Buckling may even occur for benign load cases, provided it is elastic and does pose a fatigue problem. Clearly, the redistribution of load must be accounted for.

Brian

http://www.espcomposites.com

 

[astro808]

@IRstuff

Wing panels can carry additional load beyond the initial buckling load. It is common practice to utilize this additional load bearing capability, even for below limit load cases. I included one quick ref from an online search.

"In a typical wing structure the skin panels are subjected to in-plane stresses under various load conditions. Under compressive and shear loads, the buckling behaviour of the panels becomes a driving factor for design. Often the panels have high reserve strength beyond buckling, which can be exploited to achieve efficient structures. Hence, buckling and postbuckling studies of composite panels have received much attention over the years." - Varughese, B. "Buckling and Post-buckling Studies on Stiffened Composite Panels with Circular Cutouts"

To your point, designers typically take into account the perception of the passengers in the design, avoiding any possible concern from passenger visible buckling.

 

[IRstuff]

I wasn't arguing that buckling was a catastrophic fail, just that visible buckling is an indication of out of norm history. Nonetheless, if something is buckled you have no idea what its condition is anymore, and therefore, if it were up to me, visible buckling would not be acceptable.

TTFN
faq731-376

 

[rb1957]

the onset of buckling is a company dependent decision ... some say never, some say limit, some say something else. the balance is between (as IR posts) aesthetics, (as the OP posts) performance, and weight.

permanent buckling would be limited to ultimate.

personally i find the initial post too vague ... we start talking about stealth or supersonic a/c. then its "I was just looking for a general answer. I am trying to justify if the panels are designed to account for load carrying capabilities beyond initial bucking (ie post-buckling) under a standard, non-event, operational flight profile. Say during a long-range cruise."

is that rodent i smell ?

 

[btrueblood]

Sit over the wing of a 737 at takeoff, and sight down the wing. You should see the upper skin develop a series of wrinkles, indicating they have elastically buckled. Fairly common for commercial a/c.

Agree with rb, initial post is vague. With a stealth fighter one would expect that analyses of radar XC for various loadings and deflections of surfaces would need to be analyzed.

FWIW, several years ago I had an opportunity to get up close and personal with F16 fighters on the factory floor, and a year or so later to a veteran Mig 21, both times in the company of some very bright engineers and pilots. For all the benefits of smooth skins for supersonic fighters, the Mig got by just fine with fairly rough construction details downstream of say 10% of chord. Flush, filled rivets and smooth construction on leading edges, but very rough (quite evidently hand-beaten to shape) surfaces, unfilled and domed rivets elsewhere. Contrast to a typical Western fighter with very smooth surfaces and attention to contour on every surface. Of course, the Mig's had very powerful engines for their time, and endurance was not as critical to their mission as for Western fighters.

 

[IRstuff]

MiGs had very design and environmental different requirements; they supposedly designed them to take off from muddy fields and whatnot, not a typical F16 design requirement.

TTFN
faq731-376

 

[KENAT]

Plus btrue, if you look at the kill/loss ratio...

Even for stealth aircraft it could vary based on if they are optimized to stealth from ground based or also airborne radar.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[btrueblood]

Not sure rough fields had much to do with the poor surface finish, as much as limited skilled labor pool for the construction of the plane.

The 21 was one of the first supersonic fighters, and was designed for area defense/interception, not air superiority/loiter. My point was that if clean, smooth surfaces were so critical to M>1 performance, one would expect time and attention to be paid to it. The 21's wing surfaces give proof that with enough horsepower and a fairly easy endurance requirement, you don't need to pay much attention to the surface details, just get the shape pretty close and smooth out the leading edges.

 

[IRstuff]

Rough fields mean lots of FOD, and if the fuselage couldn't tolerate that, then it would fail its requirements. Conversely, since it's supposed to be able to fly with attached FOD and potential damage from FOD, there's no reason to spiff up the surfaces, which drives up the power requirements.

This is in contrast to US planes that need to be stored in hangars, have flat runways, and an army of guys sweeping FOD off the runway.

TTFN
faq731-376

 

[btrueblood]

Fair 'nuff. But FOD on upper wing surfaces? The Mig we looked over had wing root fairings and similar surfaces that clearly showed the original manufacturer's hammer marks from forming. I think we are saying the same thing regarding final results, though, and to the OP - go do the analysis, but also read a bit about b.l. thickness and its effects on supersonic drag.

 

[rb1957]

.... and another thread gets hi-jacked ...

 

[astro808]

Sorry the post seems to vague. Let me pose the question this way.

When designing a commercial aircraft, do companies ever decide that it is acceptable to allow for buckling below limit? Ans: YES, effect on aero performance can be minimal.

When designing a stealth aircraft, do companies ever decide that it is acceptable to allow for buckling below limit?

When designing a supersonic aircraft, do companies ever decide that it is acceptable to allow for buckling below limit?

 

[btrueblood]

"When designing a stealth aircraft, do companies ever decide that it is acceptable to allow for buckling below limit?"

Dunno, suggest you ask them. Prepare for silence, as that is probably very highly classified information.

"When designing a supersonic aircraft, do companies ever decide that it is acceptable to allow for buckling below limit?"

Likely yes, but that's just an opinion. Inlets and leading edges need to be clean, and any buckling in those locations is likely designed out. But on wing surfaces past the 10% chord location or further, it likely is less of a concern, and the weight savings in structure that can be gained by thinning the skin and allowing elastic buckling will likely be important for the performance to the same or greater degree than the hit on aero drag.

I know that the P-38 maintenance crews were given an allowable amount of dimpling on the upper wing surface, i.e. some amount of permanent set due to buckling between ribs was deemed acceptable for a front-line fighter. Ok, it's not supersonic, but did operate in the transonic regime and suffered from compressibility issues. This is, however, hearsay from an old pilot I spoke with many years ago.

 

[btrueblood]

Rb,

If you are suggesting I hijacked the thread, I apologize. Will stay out of yer sandbox.

 

[rb1957]

astro, this is your first post here ...
it sounds to me like you're trying to check up on something you've been told.

your areas of interest (SS/stealth) are way to restricted for you to get a sensible answer on an open forum. for all we know you could be a north korean trying to find out secret information !? at the same time, for all you know we could be a bunch of lunes.

 

[astro808]

rb - lol... I was leaning towards the later, self included

BTW this is my second thread, first one was over the summer is a different forum. You are somewhat correct about my reasons for looking although the person I am trying to check is myself. I believe that this is true, just curious if anyone could validate this for me. As btrueblood points out it is hard to get a valid answer. You are probably correct though even if I got an answer here how much credit could I actually give it???

Thanks everyone for the help!