How Strong is the RV-8 tail?

[BillL]

Originally Posted by sblack

I will add to the confusion It is not a question of the strength of a particular member. Putting on a "stronger" stab or rudder is not going to necessarily allow you to go faster. The issue is: what is the resonant frequency of the structure (which varies as a function of stiffness and inertia), vs the resonant frequency of the moving control surface (which varies as a function of true airspeed and its intertia [mass balance comes in to play here]). When these 2 frequencies, or their harmonics, intersect, the structural oscillation diverges, rather than converges, and the structure will tear itself apart, sometimes in milliseconds. This is an extremely simplistic explanation of a complex subject of which I only understand the most basic principles.


At the company where I work, we do ground vibration testing to figure out the structural frequencies, then based on that they do predictions of structural damping vs TAS and Mach and then we go fly with lots of accelerometers on the airplane and flutter exciters, trained test pilots with parachutes and a high speed drag chute on the airplane, and we inch out to the edge of the envelope, often with days between test points for analysis, to ensure that the trends we are seeing with increases in speed are acceptable.

So you can assemble an army of engineers and specialized test equipment and do the testing in a similar fashion, or you can just go out and put it in a dive and see how it goes and hope for the best. You can see how one approach might a whole lot more risky than the other one. It is really hard to do this safely without a lot of knowledgeable people supporting you. Or you can respect Vne and you will know that you have a margin of safety.

Nice summary, Scott. Here is a GVT on a small plane.
https://www.youtube.com/watch?v=0VMMVuVrweM

Would they then add some exciters when they fly to ensure the excitation inputs are not random? Oh - what would a basic test like this cost for an RV? (ball park)

[HackerF15E]

Originally Posted by F1R

He quite openly claimed long cross county flights in oxygen required altitudes at many knots above VNE- with flight aware records to verify.

Does FlightAware show TAS/CAS/IAS, or just GS?

[F1R]

Good catch. I have only ever seen GS The altitudes claimed matched and I did not try to correlate the upper winds vs claimed TAS to the GS.

[David Paule]

Originally Posted by DanH

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Many years ago, at a school that taught flutter testing, they taught me that that the typical case was a curve like (3) in the picture, but without the return to stability - because the aircraft won't survive long enough to get there. They emphasized that when the curve starts heading towards instability, things get very rapidly worse, sometimes within a couple of knots. And that once you're in the unstable area, you're coming home under canopy.

Scott pretty much nailed it, I think.

As a structural engineer in aerospace, I learned that often the aft fuselage stiffness affects the aircraft's flutter characteristics. How much? That depends, and without analysis or testing, it's impossible to estimate. Also, since flutter depends on the natural frequency of part of the plane, it's worth knowing that there are several different modes of vibration that all might have relatively low natural frequencies. Some of them couple one surface to another or to the fuselage.

It can get really complicated. And please note this: I've spent my career doing static and dynamic structural analysis for aircraft and spacecraft, and don't consider myself anywhere near qualified to do flutter analysis. That takes training I don't have. On my RV-3B, I intend to stay within the lines.

Dave
RV-3B, now building the fuselage

[flynwest]

That is an interesting Video. Amazing how much goes into the testing.

[Toobuilder]

I've had some discussions about flutter with some of the aerodynamics and dynamics guys at work and their advice: short of a full GVT series, go light, stiff, and 100% balanced - and you have a very high probability of success.

It seems that where light aircraft have failed is predominently a failure of one or more of those criteria. We are aware of the Rocket that lost a vertical because of fasteners not installed (stiffness), we've seen tails come off because of (apparently) too much paint applied (heavy, under balanced).

The hard part for us rank and file types is understanding exactly what constitutes "light and stiff". Many builders do the balancing of control surfaces before paint and never go back and check afterwards. And paint is heavy! Especially these Osh wining, multi graphic, multi stage schemes that are becoming more prevelant. And lets also not forget that the tail is often the first time a builder has ever shot a rivet. For some, the criticality of edge distance and tolerance stack up in holes is completely lost on them. As a result, there is going to be a huge disparity in strutural integrity between builders.

Van has to account for that variation and that means the really well built airplanes with minimal paint and filler are probably going to be well inside the flutter margin and the poorer examples are going to consume some of that margin.

So it comes down to the same thing that weve heard all along: Ignore Van's guidance and you are on your own. Just because one RV-8 tail stays on at xx knots over Vne does not mean yours will.

[flynwest]

Good info thank you all for the thoughts.

[jdmunzell]

If you will recall, Jimmy Leeward's highly modified P-51 crashed at a Reno a few years back. The cause was catastrophic failure in the tail section due to flutter. I believe he experienced a sudden high positive "G" event, causing his seat to collapse on the floor of the aircraft. Photos show failure in the horizontal stabilizer and the elevator. Other factors there reflected some loose fittings possibly contributing to the flutter. He may have either passed out from that, or even died long before hitting the ground.

My point here is that with tail flutter failure and loss of control of the aircraft, one might not be conscious enough to even be able to bail out. Especially if "g" forces have you pinned in the aircraft! Scary stuff!

No thanks for me... I'll build my aircraft according to the engineer who designed it and I will stay within the design limitations published when flying it!

There are old pilots and there are bold pilots, but there are generally no old, bold pilots!

My 2 cents...

[HackerF15E]

Originally Posted by jdmunzell

If you will recall, Jimmy Leeward's highly modified P-51 crashed at a Reno a few years back. The cause was catastrophic failure in the tail section due to flutter.

FWIW, the actual "cause" was more complicated than simply "flutter". There were not-completely-engineered design changes and poor maintenance of the trim tab system that were the actual root issues which allowed flutter to occur.

Especially egregious, in my opinion, were the attachment lock nuts that were supposed to be replaced every annual, but were still painted yellow (from when the whole airplane had been painted yellow in the late 1980s!).

Also consider just how much above the published P-51 Vne Leeward was going when this happened.

To wit:

The accident airplane had undergone many structural and flight control modifications that were undocumented and for which no flight testing or analysis had been performed to assess their effects on the airplane's structural strength, performance, or flight characteristics. The investigation determined that some of these modifications had undesirable effects. For example, the use of a single, controllable elevator trim tab (installed on the left elevator) increased the aerodynamic load on the left trim tab (compared to a stock airplane, which has a controllable tab on each elevator). Also, filler material on the elevator trim tabs (both the controllable left tab and the fixed right tab) increased the potential for flutter because in increased the weight of the tabs and moved their center of gravity aft, and modifications to the elevator counterweights and inertia weight made the airplane more sensitive in pitch control. It is likely that, had engineering evaluations and diligent flight testing for the modifications been performed, many of the airplane's undesirable structural and control characteristics could have been identified and corrected.

The investigation determined that the looseness of the elevator trim tab attachment screws (for both the controllable left tab and the fixed right tab) and a fatigue crack in one of the screws caused a decrease in the structural stiffness of the elevator trim system. At racing speeds, the decreased stiffness was sufficient to allow aerodynamic flutter of the elevator trim tabs. Excitation of the flutter resulted in dynamic compressive loads in the left elevator trim tab's link assembly that increased beyond its buckling strength, causing a bending fracture. The flutter and the failure of the left elevator trim tab's link assembly excited the flutter of the right elevator trim tab, increasing the dynamic compressive loads in the right elevator trim tab's fixed link assembly beyond its buckling strength, causing a bending fracture. The investigation found that the condition of the trim tab attachment screws' locknut inserts, which showed evidence of age and reuse, rendered them ineffective at providing sufficient clamping pressure on the trim tab attachment screws to keep the hinge surfaces tight.

FWIW, Trim tab flutter on Mustangs at high speeds is not a unique problem. Another racer, Voodoo, had the same problem in 1998 but with a different outcome:

http://www.warbird.com/voodoo.html

Also recall that the prototype Piper PE-2 Enforcer (a modified turboprop-powered Mustang) crashed because of trim tab flutter (with a modified trim tab system, too!) back in the early 1970s.

http://www.ntsb.gov/about/employment...id=61033&key=0

[sf3543]

Since we are talking about RV8 tails, I'll relate an issue I came across recently. It may actually belong in the thread about RV tail horror stories, but here goes.
I was asked to look at some cracks noticed on an RV8 rudder. The cracks were at the very top of the spar where the spar and counter weight arm come together.
It turned out that the two rivets that hold the spar to the top rib were not installed and only the skin was holding the top rib and counter weight to the rudder. Over the about 60 hours on the plane the skin started to tear.
I suppose the top of the rudder would have eventually torn off if left alone, but would have probably remained controllable for landing.
This plane was a second builder plane where the first builder built the empennage.
If nothing else, it emphasises the need for good pre-flight inspections and thorough condition inspections each year. Also,if you are building, don't under estimate the value of having other people look over your project. Anybody can miss something in their build and never notice it.
Sorry for thread drift.