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How Strong is the RV-8 tail?

F

flynwest

Well Known Member
Just a question for the brain trust here on VAF. There are some really smart people here and I am going real fast with my 8 and wondering how fast is too fast. Is the tail the most likely problem area? Is there something that should be beefed up before I go any faster? I know the rockets go real fast with a 4 tail. But how fast is too fast for an RV8? Any body have an answer Steve Smith? Dan Horton?
 
tail strength

Compared to a Cessna the 8 tail is very strong. I'd be more concerned about balance and flutter not strength.

Steve
 
Flutter is the key. If you haven't done a comprehensive formal flutter evaluation, don't even think of going faster than Vne.

As for static strength, the maneuvering and gust penetration speeds define those limits.

Dave
 
rocket review...

perhaps a detailed review of the changes that are incorporated in the harmon rocket from the -4 as a starting point.
 
Fast

I have a really fast RV6 and have ask questions such as yours before and the real answer is your in uncharted territory and a test pilot.No two planes are built the same I have a friend with an RV8 and the counter weights are just bolted on and not balanced mine are but just to the best of my ability which I'm sure is not perfect who has a chance of flutter?
Bob
 
My Rocket (like many others) is equipped with a bone stock RV-8 tail. I have been to 250 KIAS before and routinely see 235KTAS on descent on a cross country. I am far from the fastest Rocket out there.

Take it all with a grain of salt, however. The Rocket Vne issue has come up before and there really is no good answer... People follow the Nike slogan and "just do it".
 
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You may be interested to search for video of aerodynamic flutter. It is amazing how quickly aircraft catastrophically fail in flutter.
 
Nordo,

(1) I can't offer a figure greater than the official VNE, even if I knew one, which I don't.

(2) It is clear to any diligent observer that the airframe doesn't explode at 201 KTAS.

(3) Speaking without formal analysis, for which I am unqualified, I believe the RV-8 tail has the most flutter margin of any RV model, largely due to the rudder design. Please note the difference between a belief and a fact.

(4) I am not aware of an RV-8 tail loss due to flutter.

(5) That said, clearly there is a limit, and the evidence suggests that if you find it for us, your contribution could be posthumous.

(6) The illustration below is from AC 23.629-1A "Means of Compliance With (FAR) Section 23.629, Flutter". You can download a more recent version (1B) here:

https://www.faa.gov/regulations_pol....cfm/go/document.information/documentID/22454

It may not answer your specific question, but it may help with general understanding of the problem.

 
Toobuilder said:
My Rocket (like many others) is equipped with a bone stock RV-8 tail. I have been to 250 KIAS before and routinely see 235KTAS on descent on a cross country. I am far from the fastest Rocket out there.

Take it all with a grain of salt, however. The Rocket Vne issue has come up before and there really is no good answer... People follow the Nike slogan and "just do it".
Click to expand...

This is important, so just to verify . . you built this?

But aren't there important changes in the aft fuse too? Like the notch deck plate thickness? More?

Do you still have your plans? Original HR?
 
I have built two HRIIs using stock RV8 tails. They pretty much bolt into place and I used them rather then the RV4 simply because they were easier to build. The counterbalanced rudder was a bonus and solved the tag wags the dog syndrome that I had with my previous RV4.
It has been 15 years since I completed these aircraft and I can not remember if the forward joint between the HS and VS was strengthened.
The RV8 elevators work on the rocket but they are borderline small and I have often wondered if the 7 elevators are larger and would bolt in place?
I never flew those aircraft in excess of 225? knotsTAS.
 
Good question Dan, but you won't find the answer here.

No one has the nerve to go out and fly faster and faster until the tail fails.

I took the RV-7 through 200 knots IAS descending from 8000' before fully appreciating the TAS limit. TAS may gave been 230 or so.

Some 55 years ago had a training flight in F-86L to earn the Mach Buster pin...have not been that fast since. Now at my age if I want to go really fast I buy a ticket on SWA. :)
 
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Rocket tail mods

What did the rocket plans call for to beef up the tail. Thicker skin? More Ribs? Stronger spar? My stock 8 tail has been to 270 mph. But I did not slap the stick. Sounds like a lot of Rocket builder went with the 8 tail. Is the back of the fuse similar to the stock 4? Thanks for all the info.
 
BillL said:
This is important, so just to verify . . you built this?

But aren't there important changes in the aft fuse too? Like the notch deck plate thickness? More?

Do you still have your plans? Original HR?
Click to expand...

I did not build the airplane, but the guy who did told me the -8 tail was a "bolt on" deal and right to Vans plans. He went that way because the -8 tail is easier than the -4 tail. "Parts bin engineering" at it's best.

I'm now sorry I offered my very general example to answer a very general question concerning a very complex subject. Flutter is not to be taken lightly, and just because some of us get away with it, does not indicate an endorsement. Van clearly has some margin built in to the design, but how much remains in any given tail- who knows?
 
How fast

OP wanted to know how fast was too fast. The right answer is Vne. Inquiring minds want to know how far the devil lives from the envelope edge. Almost reliving the Right Stuff. Where is that edge? Who knows. Lots of variables from plane to plane may not get you a precise number. NTSB reports indicate that the tail 'may' be the weak link if there is one. If the RV-8 and RV-7 share the same tail design, then there should be some data out there already but not sure if we know how fast they were traveling when the tail departed. If you decide you want to go there, please wear a chute and report back when you find the edge.
 
Toobuilder said:
I did not build the airplane, but the guy who did told me the -8 tail was a "bolt on" deal and right to Vans plans. He went that way because the -8 tail is easier than the -4 tail. "Parts bin engineering" at it's best.

I'm now sorry I offered my very general example to answer a very general question concerning a very complex subject. Flutter is not to be taken lightly, and just because some of us get away with it, does not indicate an endorsement. Van clearly has some margin built in to the design, but how much remains in any given tail- who knows?
Click to expand...

No worries, Tom cleared that up. Thanks for the lead in anyway - I have pursued an understanding of the 8 rudder for use on my 7. Just stared at it yesterday wondering if the trouble to change it before first flight was worth it. Maybe a spin evaluation with the 7 (rudder) then with the 8 (rudder) will occur before final selection.
 
I will add to the confusion :rolleyes: 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.
 
grubbat said:
OP wanted to know how fast was too fast. The right answer is Vne. Inquiring minds want to know how far the devil lives from the envelope edge. Almost reliving the Right Stuff. Where is that edge? Who knows. Lots of variables from plane to plane may not get you a precise number. NTSB reports indicate that the tail 'may' be the weak link if there is one. If the RV-8 and RV-7 share the same tail design, then there should be some data out there already but not sure if we know how fast they were traveling when the tail departed. If you decide you want to go there, please wear a chute and report back when you find the edge.
Click to expand...

Craig, I've built the 7 and 8, the tails are not the same.
 
RV8A STUD where are you?

About a year ago there was a poster with an ID something close to RV8A STUD

He quite openly claimed long cross county flights in oxygen required altitudes at many knots above VNE- with flight aware records to verify. Soon after questioned about the speeds and (flutter) safety margins, he deleted his posts and went underground. He claimed to have engineering support and hoped to commercialize his developments. This might be a good time to come out of the closet.
 
Nasa flutter test

I watched a NASA video of a piper fluttering. I think the fluttering was in the tail cone not the tail itself like the patch area of a V tail B just in front of the Horizontal. Looks scary.
 
sblack said:
I will add to the confusion :rolleyes: 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.
Click to expand...

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)
 
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.
 
DanH said:
FlutterCurves_zps6a72e3e9.jpg
[/URL]
Click to expand...

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
 
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.
 
Remember Jimmy Leeward...

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...
 
jdmunzell said:
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.
Click to expand...

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.
Click to expand...

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/_layouts/ntsb.aviation/brief.aspx?ev_id=61033&key=0
 
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.
 
sf3543 said:
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.
Click to expand...

Good post Steve. Thanks.
 
Harmon Rocket Tail Experience

Dan, I may have shared my personal experiences with you in the run up to Reno but also happy to share with everyone.

My Harmon Rocket II has a standard RV 4 tail with angle of incidence optimized for higher speed. I have been studying the tail section as speeds have been pushed up over the last three years with a supercharged SEO-540 on the nose. Confidence to push up the speeds depends on the design and build quality.

The designer published Vne on the Rocket is 240 KTAS (say 275 mph). In preparation for the Reno Air Races, I have completed power on descents in 2015 and 2016 for flutter testing (more than a few body parts were puckered up). Since I am here to write this note is a good thing. I took my 1998 Rocket (built in Bakersfield by three time RV builder) to 270 Kts true airspeed (310 mph) without adverse results. Stick forces get quite heavy but otherwise well mannered. Not recommended for RVs.

The conditions at Reno (density altitude, dirty air off racers and surface mechanical turbulence) still make me nervous on the flutter side, especially in the chute. Racing on Sunday with the Lancers and Glasairs, the true air speed for release in the chute got me out of my comfort zone, so pulled back the throttle until on the course.

Throttle UP! BB
 
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