RV-7A Flutter & In-Flight Breakup

[RV7Ron]

There was a time when new pilots were specially cautioned about the first 300 hours after training.

Excellent point, there is a great book called "The Killing Zone", which talks specifically about the first 300 hrs in a pilot's career, and how dangerous it can be to the unaware. I am in this category, at least regarding flight time, much like the pilot in this incident. Its a great book and I recommend it to all new pilots...or even experience pilots.

 

[Sig600]

No offense Reiley but this is opinion presented as fact. Obviously the Canadian transportation board found this as one link in the sequence of "events" or "findings" that led to an in-flight breakup. Discarding this as "BS" (when you are an EAA Tech Counsellor) could lead someone to believe it is not important. The conclusion didn't state that paint or balance was a single causative factor but that it likely played a role. I do agree with your advice to fly by the numbers.



Ditto this

I think his point was not that, you should ignore balancing the controls, but that if this individual had stayed within the performance margins of his a/c (and his capability) he would have stayed safe. The point the safety report pulled out was that IF the controls were balanced... he may have survived as the flutter speed would have been pushed further outside of Vne.

This didn't fully understand exactly the performance demands he was asking of his a/c to do what he was trying to do with the camera. Pulling lead or pure pursuit while keeping his speed up, whatever the lead was doing, would require more from him. I can explain those details in another thread if need be. I won't say that his inexperience was a contributing factor, but a lack of understanding... which I know for a fact a lot of high time pilots don't understand either.

 

[Bob Axsom]

Consider strength & counter balance rudder design

The counterbalance was never found but it is known to have separated from the rest of the rudder. The top of the rudder is not supported at the extreme upper end and the counterbalance horn is cantilevered off of the top of it. It is possible that it is more vulnerable to being ripped off with no flutter when subjected to excessive velocity than the well fared rudder with no cantilevered balance horn (air load and strength issue rather than flutter).

Bob Axsom

 

[chepburn]

The counterbalance was never found but it is known to have separated from the rest of the rudder. The top of the rudder is not supported at the extreme upper end and the counterbalance horn is cantilevered off of the top of it. It is possible that it is more vulnerable to being ripped off with no flutter when subjected to excessive velocity than the well fared rudder with no cantilevered balance horn (air load and strength issue rather than flutter).

Bob Axsom

Hmm.
The aerodynamic load of the horn, even if we give it a Cl of 1.0 when deflected fully to the stop (which is very generous for that shape) will only generate about 30kg of force ahead of its attach point (at 125 m/s).... The structure there would support that load.
That being said, the mass of the counterbalance, accelerated and decelerated in a flutter mode will generate considerably more load about that attach point, which is supported by the tearing of the top half of the rudder.

I dont think we need to be concerned about the horn design.

 

[LettersFromFlyoverCountry]

I would suggest to consult with VANs and other knowlagble people in this area and not leave it for chance. You also may want to wait and balance it after paint, if you are going to paint the plane. When I finished mine, after painting my elevators were on not balanced any more and after talking to Vans, they recommend to balance them as closely as possible. I end up adding about 5.5 OZ of weight to my elevator to get them fully balanced.

I believe I did consult Van's at one point and if I recall correctly the answer was something along the lines of, "Are you ever going to fly that thing?" Bet nobody can figure out who said that.

I would be curious how people balance these things after painting. The emp tip attachment precludes getting access to the lead weight.

 

[rocketbob]

The emp tip attachment precludes getting access to the lead weight.

Exactly why I use #4 screws/nutplates for tip attachment. Over the years I got some hangar rash that would have been easy to fix had I had used screws on my -6 elevator tips. Balance them slightly heavy during construction, then once its painted pop the tip off and drill a few holes in the counterweight to get them to balance.

 

[Bavafa]

I would be curious how people balance these things after painting. The emp tip attachment precludes getting access to the lead weight.

There are many ways of doing it, one easy way is to proactively, during construction, install one or two nut plate in the tooling hole of the elevator, next to the original weight. If any weight needs to be added then it can be bolted on there.

Balance them slightly heavy during construction, then once its painted pop the tip off and drill a few holes in the counterweight to get them to balance.

I did just that, only to find out that after painting my trailing edge was heavy. I had to add about 5.5 oz to make it balanced. A removable tip is another good way to go about doing it.

 

[Bruce]

Guys,

Borrowed this from Rockets web site.

Know your speeds. RV-7A

VNE--230 MPH------Exceeded by 32KTs or 37 MPH

VA----Exceeded 140 MPH

Great article. Great read.



On November 12, 2001, American Airlines Flight 587, crashed shortly after takeoff from New York?s John F. Kennedy International Airport. The crash killed all 260 people aboard and 5 people on the ground. The National Transportation Safety Board (NTSB) determined ?the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design loads that were created by the first officer?s unnecessary and excessive rudder pedal inputs.? As a result of this accident and subsequent investigation, it was revealed that many pilots have a misunderstanding of what the design maneuvering velocity (speed), VA, represents. Many pilots believe that as long as the airplane is at or below this maneuvering speed, they can make any control inputs they desire without any risk of harm to the airplane. This is not true.
The design maneuvering speed (VA) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane. Even though the accident discussed above is a part 25 airplane, VA is applicable to part 23, CAR 3, and LSA airplanes. Also, even though experimental airplanes may not have a published VA , they will still have some maximum maneuvering speed associated with the maximum structural design loads. Therefore, the pilot should be aware of what speed this is, and adhere to the guidance herein. The regulations governing the design strength requirements for airplane structure require adequate strength for full control deflection (below VA). However, they do not require the manufacturer to make the airplane strong enough to withstand full control input followed by a full control input in the opposite direction, even below VA. Neither do they require the manufacturer to design the airplane for more than one simultaneous full control input such as full ailerons with full elevator and/or rudder.
VA, as published in the airplane flight manual (AFM) or pilot?s operating handbook (POH), is valid for operation at the gross weight stated, which is typically at max gross weight. It is especially important to note that VA decreases as the airplane weight decreases. At first, this may seem counter intuitive. All pilots understand that when the airplane is subjected to an external force, such as the 2 aerodynamic force from a control surface, the airplane responds by accelerating (rotational acceleration) about one of the airplane?s axes. This was stated many years ago in Newton?s Second Law of Motion. The law states that when an object of mass ?m? is acted upon by a force ?F?, it will undergo acceleration ?a? in the same direction as the force. More simply stated in the widely known equation ?F = ma?, which can be rewritten as ?a = F/m?. Rewritten this way, it is clear for a given control force ?F?, as the airplane weight ?m? decrezses then the acceleration ?a? will increase. This higher acceleration gives rise to higher loads on the airplane structure. Therefore, as the airplane weight decreases, the allowable maneuvering speed must also decrease, to ensure that the airframe is not damaged. Pilots may remember from their written exam that VA-NEW = VA √ (WNEW/WMAX-GROSS) as the way to calculate the corrected (new) maneuvering speed due to operating at a weight less than the maximum gross weight. NOTE: This formula is for calculating the VA change about the pitch axis; however, it can be used for all axes.
Recommendations
The FAA wants to clarify that operators should know what the maneuvering speed is and to caution pilots on what to avoid by adhering to the information described above and contained in the regulations. We recommend the following for maneuvering at, or even below, VA:
DO NOT apply a full deflection of a control, followed immediately by a full deflection in the
opposite direction.
DO NOT apply full multiple control inputs simultaneously; i.e., pitch, roll and yaw
simultaneously, or in any combination thereof, even if you are below VA.
Reduce VA when operating below gross weight, using the following formula:
VA-NEW = VA √ (WNEW/WMAX-GROSS)
For Further Information Contact
Mark James, Aerospace Engineer, 901 Locust, Room 301, Kansas City, MO 64106;
phone: (816) 329-4137; fax: (816) 329-4090; email: [email protected].

 

[bignose]

Guys and Gals,

This paint... balance is BS !!! Fly by Van's numbers and everything will be fine balanced/painted or NOT.

Just a question of good, healty sense! The Yellow arc on analog Airspeed Indicators was an eye opener, red arc was reason for panic! NEVER Exceed Velocity - VNE.

Maibe this pilot wasn't quite adaptet to the EFIS.

Also I think that 234 Knots on a little RV 7A isn't quite reasonable!

There where a lot of slick retractables coming apart and loosing their (V)tails in inadvertent dives before.

Just respect your envelope and nothing will hurt you. As simple as that.

 

[Brantel]

My EFIS also has color coded strips beside the ASI...

Just a question of good, healty sense! The Yellow arc on analog Airspeed Indicators was an eye opener, red arc was reason for panic! NEVER Exceed Velocity - VNE.

Maibe this pilot wasn't quite adaptet to the EFIS.

Also I think that 234 Knots on a little RV 7A isn't quite reasonable!

There where a lot of slick retractables coming apart and loosing their (V)tails in inadvertent dives before.

Just respect your envelope and nothing will hurt you. As simple as that.

 

[Bob Axsom]

Not according to the earlier post

Just a question of good, healty sense! The Yellow arc on analog Airspeed Indicators was an eye opener, red arc was reason for panic! NEVER Exceed Velocity - VNE.

Maibe this pilot wasn't quite adaptet to the EFIS.

Also I think that 234 Knots on a little RV 7A isn't quite reasonable!

There where a lot of slick retractables coming apart and loosing their (V)tails in inadvertent dives before.

Just respect your envelope and nothing will hurt you. As simple as that.

Not according to the earlier post featuring the airliner that lost its tail due to pilot rudder actuations on departure well below VNE.

Bob Axsom

 

[Bob Kuykendall]

...Flutter is a product of TRUE AIRSPEED, not dynamic pressure or IAS...

No, that is not necessarily true. I believe that that's already been addressed on this forum. There is good evidence that flutter is directly relative to neither IAS nor TAS, and that a reasonable rule of thumb is that it tracks a speed that splits the difference between the two.

Thanks, Bob K.

 

[dick seiders]

I have just digested all the facts presented in this sad and sobering account of the unfortunate loss of a fellow RV'er. I wouldn't hazard a guess as to what may have been the cause and the many inputs here have it well covered. For my part I believe in staying w/i the limitations outlined in the flight manual. I am probably not as exciting a flier as many who push the envelope hard, but I am OK with that. One reason is while flying to Destin, FL in my 6A a few years back and following a lead plane's course/alt. I became aware he had descended from 10K level we were at as I spotted him some 7000 or so ft below the thin cloud cover. This sudden awareness can be a distraction and I simply nosed over to a reasonable descent angle to get into approach altitude and very quickly found my AS at about 220kts. I made a timely, but gentle correction and slowed my AS to about 120. This experience was very enlightening as to how quickly limits can be exceeded. I have not forgotten it and use the event to avoid angering the physical lawmakers. I am certain many have experienced the same or similar events. Just wanted to share this one as it may be a helpful reminder to others.
Dick Seiders RV12 120093

 

[Bubblehead]

Weight on the control surface is a factor even if balance in the case of the rudder is not. I'm not an expert on flutter but have done a lot of engineering and testing in mechanical systems that exhibit harmonics. In my case it is long freight trains with varying amounts of dampening between the cars. (insert laugh track here)

(Opinion starts here) How does that relate to flutter? You have a mass (the rudder) being excited by an outside force (aerodynamic lift). It can be compared to a mass-spring system bouncing. The moving mass has momentum which is removed by the spring, however the spring just stores the energy, it does not absorb or dissipate it. At the point the mass stops the spring is at max energy, which is then returned to the mass causing acceleration (F-ma).

If there is zero dampening (and zero frictional or aerodynamic losses) the mass will continue to bounce up and down forever. If you add a little dampening it will bounce a long time. If you put lots of dampening in it will bounce some.

Our rudders are masses and unless we have our feet firmly on the pedals they have no dampening. If we stomp on one pedal and then let the rudder freely swing it will generate a lot of aero force that moves the tail but also tries to return the rudder to neutral. Now, if the rudder is heavy and has a lot of energy it may swing past neutral and generate aero force on the other side. Given the right combination of rudder mass and speed the rudder could start oscillating.

Back to the railroad train. A loaded coal train has up to 120 cars and weighs about 16,000 tons and moves at speeds up to 65 mph. Each car is a mass, and the masses are joined together by couplers and dampening systems called "draft gears". These draft gears runt he gamut from high dampening to low dampening and 7 or so years ago we did a lot of modeling and testing and demonstrated braking events that would excite the train in a way to generate huge forces (>1,000,000 lbs) throughout the train. It was a harmonic for the train based on mass-spring-dampener.

Bottom line - Bondo and paint on a rudder probably has an undesirable affect on the speed (combined with an excitation event), and the rudder is undamped if our feet are on the floor. Perhaps in turbulence or at high speed we should have our feet on the pedals and try to dampen out any wiggle.

 

[RV7AV8R]

Perhaps in turbulence or at high speed we should have our feet on the pedals and try to dampen out any wiggle.

Thanks, this is a great suggestion we can easily do to help reduce any risk, however small.

 

[Jamie]

Thanks, this is a great suggestion we can easily do to help reduce any risk, however small.

As long as the rudder is not inadvertently kicked. Flutter requires some sort of upset of the control surface.

 

[frazitl]

For an in depth description of flutter

consult Barnaby Wainfan's excellent articles in the March 2010, April 2010 and later issues of KitPlanes : http://www.kitplanes.com/issues/27_3/designers_notebook/Structural_origins_of_flutter_9130-1.phtml

I'm not an aero engineer, but the conclusion I draw is that underbalanced ailerons or elevators (trailing edge low) are most subject to catastrophic flutter events. The rudder is less clear to me (torsion effects etc.), but probably the same. The safest bet is perfect balance which will UNCOUPLE the airframe flexibility and the control surface movements.

I chose to balance my elevators separately (despite the Van's instructions to balance as a group), because I didn't want to rely on the stiffness of the interconnecting weldment to dampen both sides when one "wanted" to flutter. I did not, however balance the rudder. It was constructed to plan.

This thread has me thinking differently even though I have tested the "system" to Vne+11% based on CAS , and Vne +22% based on TAS .

After painting, I will be certain to re-balance both elevators (independently) and rudder. I will then retest (parachute on) to Vne=((TAS+CAS)/2)+10%.

My plan only. YMMV of course...

It's a shame that a precious life was lost here, but we can at least honor his passing by learning all we can from this unfortunate event...

 

[chepburn]

The rudder is operating in a different axis.

Hi all,
With the talk of mass balancing the rudder, I took advantage of the fact that I am currently painting my tail group on my RV-8. First, the RV-8 rudder is different from the 7 (but only slightly), so this comment may not apply.

It takes approximately 2.5 kg of additional weight on the horn to balance the rudder in the HORIZONTAL plane. However, the rudder operates primarily in the vertical plane where gravity is not helping us. (My rudder has no filler aft of the hinge line, and only the first coat of primer at this point)

In short, adding more mass to the rudder to balance is not the answer to improving the flutter margin. A 2.5 kg difference is definitely outside the designer's intention.

I think if any of you are planning to add weight to your rudder to ward off the onset of flutter, you might want to give the mothership a call first.

The rudder does not flutter as designed, when operated within the design limits.

Chris

 

[Bob Kuykendall]

...It takes approximately 2.5 kg of additional weight on the horn to balance the rudder in the HORIZONTAL plane. However, the rudder operates primarily in the vertical plane where gravity is not helping us...

First, a disclaimer: I am not an engineer, aero or otherwise.

The intent of mass balancing is to reduce or eliminate the amount of surface deflection you get when you shake or wiggle the hinge axis in a variety of ways. The orientation of the axis of rotation is immaterial, and gravity is immaterial.

In a 100% mass-balanced surface, wiggling or shaking the hinge axis results in no rotation of the control surface around the hinge axis. That is what you want for flutter resistance. It prevents feedback reactions in which shaking the hinge axis results in control surface deflection, and the deflection results in an amplification of the shaking.

It is important also to recognize that flutter is a dynamic concern; and in order to understand it you have to think about all the ways airplane structures bend, twist, shear, wiggle, and vibrate. For small, stiff, low aspect ratio airplanes like the RVs, it is usually less of a concern than for large, limber, high-aspect ratio aircraft like sailplanes.

I believe that the energy that flutter can bring to bear to tear things up scales with the square of the speed, so when you get some speed behind these little airplanes, things can go sour pretty fast.

...The rudder does not flutter as designed, when operated within the design limits.

Yes, that seems to be the case.

Thanks, Bob K.

 

[DanH]

consult Barnaby Wainfan's excellent articles in the March 2010, April 2010 and later issues of KitPlanes : http://www.kitplanes.com/issues/27_3/designers_notebook/Structural_origins_of_flutter_9130-1.phtml

Some others:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA955270&Location=U2&doc=GetTRDoc.pdf

http://ntrs.nasa.gov/archive/nasa/ca...2007008628.pdf

http://www.oshkosh365.org/saarchive/eaa_articles/2003_12_09.pdf

http://www.eaa.org/sportaviationmag/2003/s312.xls


I'd like to offer a reminder for builder's and TCs....remember this?

I've seen this error during inspections of quite a few RVs. The plans are not real clear. Builders seem to get led astray by looking at the rivet lines on the VS spar reinforcement. They drill the 3/16 holes in that line, which puts them far too close to the edge in the spar's most highly stressed area.

Let's be clear; I am not suggesting the above as the cause of this particular accident. I will argue that if flutter of the VS/rudder was the root cause, the above build error would reduce the stress necessary to break the spar.

 

[chepburn]

The intent of mass balancing is to reduce or eliminate the amount of surface deflection you get when you shake or wiggle the hinge axis in a variety of ways. The orientation of the axis of rotation is immaterial, and gravity is immaterial.

In a 100% mass-balanced surface, wiggling or shaking the hinge axis results in no rotation of the control surface around the hinge axis. That is what you want for flutter resistance. It prevents feedback reactions in which shaking the hinge axis results in control surface deflection, and the deflection results in an amplification of the shaking.


Thanks, Bob K.

Bob,
I agree with you...I was trying to make the point (badly) that the usual method we use to balance our control surfaces (in the horizontal plane) would not be correct for the vertical stab. Vans has designed the rudder mass balance to be different by 2.5 kg. So, I was trying to say that they did that for a reason...mainly the ones you describe.
Thanks for the clarification.
Chris

 

[frazitl]

Rudder balance weight.

Wow!, Chris. If it takes 2.5KG to balance the rudder (or thereabouts), then balancing is not a good option. The structure needs to be strong enough and stiff enough to handle any weight added.

Rereading the Kitplanes articles, it is apparent the partial balancing might actually make a flutter situation worse! Partial balancing apparently can change the natural frequency of the rudder moving it into a dangerous range. Just as adding some weight to partially balance a control surface can make things worse, adding additional weight to the unbalanced (aft) side (extra heavy paint, rudder trim servo systems, etc.) could make the system more prone or less prone to flutter over the normal operating speed range. What sounds like unusually heavy paint and filler in the current situation might be better, and might be worse.

So we are back to Vne it seems. Our Vans aircraft are proven quantities when flown within the proven flight envelope. Outside that envelope we are conducting an entirely new set of (potentially very dangerous) experiments with MANY interrelated variables. Not for me...

 

[johnny stick]

Mass balancing

So we are all talking about mass balancing the control surfaces to prolong the speed at which flutter occurs. A long time ago, i heard a rumor that the some air force bomber's control surfaces where over balanced; seems that the CG was ahead of the hinge line for better resistance to flutter. Does any of the experts out there have a sense for this, and will over balancing help a RV type of aircraft ?

 

[chepburn]

So we are all talking about mass balancing the control surfaces to prolong the speed at which flutter occurs. A long time ago, i heard a rumor that the some air force bomber's control surfaces where over balanced; seems that the CG was ahead of the hinge line for better resistance to flutter. Does any of the experts out there have a sense for this, and will over balancing help a RV type of aircraft ?

Please dont interpret the discussion so far as an intention to change anything with any control surface mass balance.

It was suggested here that we try to balance the rudder after paint, but, the rudder is NOT mass balanced in the same way as the aileron and elevator....so that would be a bad idea.

We are NOT suggesting changing anything with ANY control surface. DO NOT change any mass balance outside of what is recommended in the plans on any control without contacting Vans.

The current design works as intended through the designed operating speed envelope.

 

[SvingenB]

So we are back to Vne it seems. Our Vans aircraft are proven quantities when flown within the proven flight envelope. Outside that envelope we are conducting an entirely new set of (potentially very dangerous) experiments with MANY interrelated variables. Not for me...

Exactly my thoughts. My condolences to his family. Just read through this thread and the report. The structure and strength of the airplane is relative to the airspeed (the forces are a direct result of the airspeed). The faster you fly, the weaker the airplane gets in relative terms. Past Vne the airplane is not an airplane any more, it's just an assembly of flimsy aluminium sheets with no structural rigidity, stability or strength. Exactly what happens beyond Vne is therefore only of academic interest and any analysis will be speculative at best.

It is therefore rather puzzling reading the report and the "analysis" of the structural failures. The pilot conducted aerobatic while exceeding the max weight limit at a speed way past Vne. That is all there is to it.

 

[DanH]

Curiosity questions please.....

The RV-7 design originally used same VS and rudder as the RV-8?

Later the rudder was changed for one with more area and a riveted wedge trailing edge?

Are the current RV8 and RV7 rudder balance weights the same lead block?

 

[LifeofReiley]

Here's what...

Van's Website states:

I have not had time to research plans...

Counterweights for optional 0.020 Skins, RV-6/6A, Early RV-7/7A,
RV-8/8A, All Rudders E-614-020 more info $15.40

Counterweights for 0.020 Skins, Late Model RV-7/7A and RV-8/8A.
E-714 more info $19.00

Many have read WAY to deep into my comments on this thread and I guess thats OK. Balancing FLYING surfaces is pretty clear in the plans. Many aircraft I've looked at plan for a little heavy on the down side of the elevator to allow for paint and balancing. IMHO, the down side method works pretty darn well.

Curiosity questions please.....

The RV-7 design originally used same VS and rudder as the RV-8?

Later the rudder was changed for one with more area and a riveted wedge trailing edge?

Are the current RV8 and RV7 rudder balance weights the same lead block?

 

[jrs14855]

balance

Apples to oranges BUT-I fly Twin and single engine Cessnas-NONE have the elevators balanced 100%. They are all quite tail heavy. I never investigated rudder balance. I will try to remember to look at maintenence manuals. I have looked at maintenence manuals for A36 Bonanza and Baron 58. The Baron has a trim tab on one aileron and the balance tolerance for the aileron with the tab is slightly different than the balance for the opposite aileron. Years ago a friend with a Midget Mustang I had to overbalance the ailerons. I don't think he had a fully developed flutter but the ailerons were moving around and the fix was 125% balance. Having survived aileron flutter on an early homebuilt I can say without question flutter can be induced by wake turbulence in close formation.

 

[Jamie]

Curiosity questions please.....

The RV-7 design originally used same VS and rudder as the RV-8?

Later the rudder was changed for one with more area and a riveted wedge trailing edge?

Are the current RV8 and RV7 rudder balance weights the same lead block?

Yes. The "new rudder kits" that Van shipped to customers did not contain the lead weight (and several other parts). Those parts had to be scavenged from the old rudder.

 

[DanH]

Exactly what happens beyond Vne is therefore only of academic interest and any analysis will be speculative at best.

Dr. Svingen, I'm shocked...."only of academic interest"?

Kidding aside, interest in some quarters is more than academic. The racers are an obvious example, as are those who routinely fly high (the long fast descent) and those working on efficiency. It is useful to know what physical factors lessen the flutter margin so they can be avoided, even for those who are diligent about observing the manufacturer's VNE. Pilots make mistakes. Airframes with more margin (flutter, structural strength, etc) bring them home.

 

[mv031161]

Interesting reading. I just read all 14 pages and i would like to congratulate all of you. Very nice posts.

I read somewhere: the tail looked like it had a stick of dynamite explode inside: look at the picture. The rudder movable prtion is destroyed. The vertical stabilizer paint job does a visual diservice and it looks like the skin is all peeled. The v.stab is complete!

Something that comes to mind: i did not read anywhere if the pilot was using a parachute. It is always a good idea to wear one regardless if the regulations allows you to fly aerobatics without one when flying solo. I dont know about canadian regs, but i will assume the same or harder reg,s. I dont fly without one ever!.

The Lord be with you Andrew!

 

[rjcthree]

Failure progression

I would offer another hypothesis: the structural failure preceded the flutter onset. The discussion here has focussed on flutter causing structural failure.

1. Significant mass was added in paint and filler. Resultant structural stresses are higher.
2. Once plastic deformation (which for this dicussion is my definition of failure) begins, you're in a place where all your equations/calculations/ simulations are ONLY directionally suggestive - MAYBE.
2.1 You are outside all previous flutter analysis of the airframe following plastic deformation/structural failure of the airframe. What Van's may have tested Re: flutter/Vne, etc. is irrelavant.
3. A reasonable metalurigical analysis may determine the progression in this case, I'm not seeing that type of analysis in the summary report. It would be time consuming and destructive, and would take a fairly talented metalurgist. The sad outcome wouldn't change.

Regardless, the aircraft was in a situation that it could not sustain; we should individually be mindful of the risks we take.

Rick 90432

 

[RV7Ron]

1. Significant mass was added in paint and filler. Resultant structural stresses are higher.

Nothing personal, but this comment is speculation. The report only does best and worst case estimates, and only estimates. The larger 1/8" rivets added approx. 0.05 lbs. The filler, as measured at its thickest AND if uniformly covered the rudder would have weighed approx 5 lbs. That is an absolute worst case scenario. I cant imagine any builder coating the entire part with filler so I think its unrealistic to say that he had 5 lbs of filler on that rudder. So, to my mind, this isnt a significant mass increase.

I agree that added weight will affect flutter characteristics and should be considered carefully, but I'm not convinced this incident had a lot to do with the added rudder weight or the so-called 'balancing' of the rudder. There are just too many of these planes flying with exactly the same type of filler/paint as this gentleman had with no tails involuntarily coming off. Although, my gut feeling is that 234 kts, although far exceeding Vne, shouldnt cause an in-flight breakup. There may be more to this puzzle than the report shows.

 

[mv031161]

Nothing personal, but this comment is speculation. The report only does best and worst case estimates, and only estimates. The larger 1/8" rivets added approx. 0.05 lbs. The filler, as measured at its thickest AND if uniformly covered the rudder would have weighed approx 5 lbs. That is an absolute worst case scenario. I cant imagine any builder coating the entire part with filler so I think its unrealistic to say that he had 5 lbs of filler on that rudder. So, to my mind, this isnt a significant mass increase.

I agree that added weight will affect flutter characteristics and should be considered carefully, but I'm not convinced this incident had a lot to do with the added rudder weight or the so-called 'balancing' of the rudder. There are just too many of these planes flying with exactly the same type of filler/paint as this gentleman had with no tails involuntarily coming off. Although, my gut feeling is that 234 kts, although far exceeding Vne, shouldnt cause an in-flight breakup. There may be more to this puzzle than the report shows.

I personally tend to agree with you. Exceeding aircraft designed parameters, and Speeds bit this poor low time pilot and had to pay the ultimate price!

 

[Flyfalcons]

Although, my gut feeling is that 234 kts, although far exceeding Vne, shouldnt cause an in-flight breakup. There may be more to this puzzle than the report shows.

I have to disagree. When Van's came out with the Vne on the airframe, it wasn't a "Gee guys, this is a speed designed for the lowest common denominator pilot in the group, nudge nudge wink wink", it was a "This is never exceed speed. Never exceed this speed under any circumstances. If you have any questions, refer to the previous sentence" type of deal. Not only was the accident pilot exceeding Vne (by about 15%), it appears he was doing a bit more than straight and level (or at the very least, unaccelerated) flight.

I do agree with the rest of your post though, in that the RV-7 airframe has been flying long enough to be considered fairly well proven if built as designed.

 

[RV7Ron]

I have to disagree. When Van's came out with the Vne on the airframe, it wasn't a "Gee guys, this is a speed designed for the lowest common denominator pilot in the group, nudge nudge wink wink", it was a "This is never exceed speed. Never exceed this speed under any circumstances. If you have any questions, refer to the previous sentence" type of deal. Not only was the accident pilot exceeding Vne (by about 15%), it appears he was doing a bit more than straight and level (or at the very least, unaccelerated) flight.

I do agree with the rest of your post though, in that the RV-7 airframe has been flying long enough to be considered fairly well proven if built as designed.

My counterpoint to this would be that aircraft design usually follows a factor of safety of 1.5...its been done this way since the beginning of time it seems. The Vne is NOT when an aircraft is designed to fail, just a recommended velocity with adequate safety margin built in. I am in no way advocating using those numbers as a 'nice to follow' rule, they should be strictly adhered to whenever possible. But we all make mistakes, lets not pretend we are immune to doing something "this foolish"..its possible. And if it does, I'd like to feel like I could trust the airframe to handle a slight overshoot...even 234kts, which is still well within the theoretical failure envelope according to Van's.

If you read the report, you will see that Van's flight tested to 217kts and theoretically to 300kts. Where does the 300kts figure come from?...1.5 x 200kts (Vne), there's your standard factor of safety built into the Vne. So, the point to my post was, that 234kts doesnt automatically imply that the aircraft will come apart because it is exceeding the Vne. It certainly may have been a contributing factor, but I personally just somehow think there is more to the story than that.

In addition, I'm not sure the 'overgross for aerobatics' nor the 'exceeding Va' argument holds much weight either. He didnt overstress the airframe according to the EFIS...only maxing out at 3.5g's during the incident...within design limits even for that weight. The airplane structure could care less how much it weighs...its the g-loads that will cause failure during manuevering flight. Va just lets you know how fast you can go and still make a full deflection control movement without ripping the wings or tail off. Doesnt sound like he did that or he would have registered a higher g-load....but maybe not? A full rudder application probably wouldnt register a high g-load? Some of this will remain a mystery.

The disclaimer to all of my above ramblings is, these design limit discussions assume a quality, to specification, build. If not, that is another variable to contend with and likely would erode the factor of safety margin that I refer to.

Just my 2 cents to add to what has been a great discussion on this topic...I can bet that everybody that has read this thread will pay more attention to their Vne in a shallow dive in the future. We are all more aware and thats always a good thing, even if we still dont know all the answers.

 

[flyeyes]

My counterpoint to this would be that aircraft design usually follows a factor of safety of 1.5...its been done this way since the beginning of time it seems. (snip)
(snip)
Va just lets you know how fast you can go and still make a full deflection control movement without ripping the wings or tail off.(snip)

I can bet that everybody that has read this post will pay more attention to their Vne in a shallow dive in the future. We are all more aware and thats always a good thing, even if we still dont know all the answers.

Hi Ron.

I'm not an engineer of any kind (and don't even play one on tv) but Im pretty sure the 1.5 safety factor is for mechanical loads in things like wing spars in wood or metal aircraft(and a 2.0 safety factor in composite aircraft FWIW).

The 300 knot figure from GVT just means they couldn't find any resonant frequencies likely to be excited and cause flutter below 300 knots. It doesn't guarantee the airframe won't fail from overloads (off the top of my head the BD-10 is an example).

For years, because of what I was taught as a student pilot in the 70s, I took it as an article of faith that you couldn't break an airplane with any control application below Va. I didn't realize I was wrong until I read articles in the aftermath of the Airbus breakup overNew York resulting from vigorous rudder inputs in the climb (presumably below Va)

Va only guarantees that a single, smooth application of up elevator will result in a stall before overloading the wing spar. Nothing else.

There are many examples of numbers without a 50% safety factor in airplanes, and Vne is one. Certified airplanes do promise an 11% safety factor.

Please don't take this as picking at you--I don't have your educational background, but I've been fooling around with airplanes long enough to realize some of these things aren't as simple as we think..

 

[Flyfalcons]

So, the point to my post was, that 234kts doesnt automatically imply that the aircraft will come apart because it is exceeding the Vne. It certainly may have been a contributing factor, but I personally just somehow think there is more to the story than that.

There is more to the story than that. 243 knots by itself? Maybe, maybe not a huge deal, but that's not the only thing that was going on. Maneuvering in an aerobatic fashion (3.5 G's), plus who knows what kind of aileron or rudder input was involved, added to the 234kts makes the RV-7 go boom boom.

 

[RV7Ron]

Hi Ron.

I'm not an engineer of any kind (and don't even play one on tv) but Im pretty sure the 1.5 safety factor is for mechanical loads in things like wing spars in wood or metal aircraft(and a 2.0 safety factor in composite aircraft FWIW).

The 300 knot figure from GVT just means they couldn't find any resonant frequencies likely to be excited and cause flutter below 300 knots. It doesn't guarantee the airframe won't fail from overloads (off the top of my head the BD-10 is an example).

For years, because of what I was taught as a student pilot in the 70s, I took it as an article of faith that you couldn't break an airplane with any control application below Va. I didn't realize I was wrong until I read articles in the aftermath of the Airbus breakup overNew York resulting from vigorous rudder inputs in the climb (presumably below Va)

Va only guarantees that a single, smooth application of up elevator will result in a stall before overloading the wing spar. Nothing else.

There are many examples of numbers without a 50% safety factor in airplanes, and Vne is one. Certified airplanes do promise an 11% safety factor.

Please don't take this as picking at you--I don't have your educational background, but I've been fooling around with airplanes long enough to realize some of these things aren't as simple as we think..

Very good points and some information I didnt know, thanks. I am an Engineer, but lets be clear I dont design aircraft, I am learning just like most of us on here.

There is more to the story than that. 243 knots by itself? Maybe, maybe not a huge deal, but that's not the only thing that was going on. Maneuvering in an aerobatic fashion (3.5 G's), plus who knows what kind of aileron or rudder input was involved, added to the 234kts makes the RV-7 go boom boom.

With all my babbling, that is what I was kind of trying to get at. 234kts in straight and level flight without any abrupt control inputs shouldnt make it go boom...and that is just my gut feel, nothing more. I think there are missing pieces to the puzzle so we're in agreement on that.

 

[N8RV]

Parachute?

Because this discussion drifted momentarily into a discussion of the importance of wearing a parachute, I did a little archive search (to make Paul Dye happy ) to see if ANYONE has ever successfully bailed out of an RV of any model. Other than poor Von Alexander a long time ago, I came up with nothing.

I suppose that there's no way to really ever know how many RV accidents would have been survivable IF the occupants had used parachutes. We probably all think of the unimaginable being a manageable event, where time slows and we calm down, plan our egress and successfully leave our disabled aircraft, D-ring in hand. My guess is that many catastrophic failures in flight result in high-g contortions that would render the argument of whether a canopy can be jettisoned in flight moot.

Given the difficulty of getting into and out of an RV-8 while on solid ground makes me wonder if we are only kidding ourselves by wearing parachutes in flight for any reason other than to comply with FARs?

 

[mv031161]

Because this discussion drifted momentarily into a discussion of the importance of wearing a parachute, I did a little archive search (to make Paul Dye happy ) to see if ANYONE has ever successfully bailed out of an RV of any model. Other than poor Von Alexander a long time ago, I came up with nothing.

I suppose that there's no way to really ever know how many RV accidents would have been survivable IF the occupants had used parachutes. We probably all think of the unimaginable being a manageable event, where time slows and we calm down, plan our egress and successfully leave our disabled aircraft, D-ring in hand. My guess is that many catastrophic failures in flight result in high-g contortions that would render the argument of whether a canopy can be jettisoned in flight moot.

Given the difficulty of getting into and out of an RV-8 while on solid ground makes me wonder if we are only kidding ourselves by wearing parachutes in flight for any reason other than to comply with FARs?[/QUOTE)


Well...to have one and to try to use it, it might have beaten the final outcome...I used one on my Pitts and thats a lot harder to egress than any RV..I now use one in my giles and it is a lot harder than the pitts....

 

[DanH]

Parachutes are not relevant to this accident. Given the velocity, pitch and altitude noted in the report, the pilot had about 6 seconds from failure to impact.

Question for Dynon users....does the unit record yaw angle?

 

[N8RV]

Parachutes are not relevant to this accident. Given the velocity, pitch and altitude noted in the report, the pilot had about 6 seconds from failure to impact ...

Exactly my point, Dan.

 

[jrs14855]

chutes

Many years ago Nick Jones was testing a modified Cassutt at an air race at Ft Worth. At a speed in excess of 300 statute, the wings left the airplane. The fuselage pitched straight down and presumably continued to accelerate. Nick was able to leave the fuselage with great difficulty and survived with only minor injuries. Never say never. The standard protocol for flight testing, at least for homebuilts in the US, is a dive test to 110% of Vne.

 

[whittfic]

RV6 rudder

Have caught onto this thread a little late in the piece so have only recently read the accident report. Sobering reading indeed and has given me pause for thought re my own RV6 build. I will probably go to Vans for their view on this but thought I would also seek the input of the forum as well.

My kit is one of the earlier ones with the smaller rudder without any balance weight. With all the discussion about excess paint, filler etc possibly compromising the flutter characteristics, I am wondering if I should re-think my intention to fit a strobe light to the bottom rudder fairing. Am I tempting fate by locating the extra weight of the light to the rearmost point, particularly on this type of rudder?

While I have no intention of even attempting to operate the aircraft beyond the design limits, I'm guessing the pilot of the accident aircraft didn't either. As we all know mistakes can be made and I'm no steely eyed Top Gun test pilot, so don't want to set myself up for bad things to happens.

Any ideas people??

Clive Whittfield
RV6 - wiring etc

 

[Neal@F14]

Plenty of RV-6's out there flying safely for many years with their pilots operating them within the prescribed airspeed limits... and even a bunch of them past their published Vne in air races with no reports of flutter so far. In fact I don't think there has been a single RV-6 ever broken up the airframe in flight... some think it's the strongest airframe Van has designed of the whole RV series. I don't think there is anything to worry about the RV-6 about as long as you don't ever blast one way past Vne, or get one into a fully developed spin at too low of an altitude, or try to do anything really stupid in one. It's a very sound design.

EDIT: Lot's of them out there flying with a small strobe/taillight combo on the bottom trailing edge of the rudder too, with no ill effects.

 

[Bob Axsom]

Put it on

Rest easy, it has been tested. I have the combination strobe and nav light on the same RV-6A short tail rudder design you are talking about and it has seen the far side with no problem. If you stay within Van's Vne envelope it is a non-issue.

Bob Axsom

 

[B25Flyer]

Just because an airplane has been to a given IAS once, does not mean it safe from flutter at that speed...

Flutter is a function of TAS not IAS and it is possible to exceed the flutter speed significantly as long as nothing energizes the flutter.

Test pilots use a little mallet and tap on the stick or tap the rudder pedal to generate an oscillation to see if it will damp or increase.

Here is some food for thought....

200 KTAS at 10K ft is 172 KIAS....

Tailwinds,
Doug Rozendaal
F1 EVO

 

[Bob Axsom]

Does that imply anything about the tail light question?

Just because an airplane has been to a given IAS once, does not mean it safe from flutter at that speed...

Flutter is a function of TAS not IAS and it is possible to exceed the flutter speed significantly as long as nothing energizes the flutter.

Test pilots use a little mallet and tap on the stick or tap the rudder pedal to generate an oscillation to see if it will damp or increase.

Here is some food for thought....

200 KTAS at 10K ft is 172 KIAS....

Tailwinds,
Doug Rozendaal
F1 EVO

Doug, does that imply in any way that the short tail 6 is has a safety of flight issue if the combination strobe and white nav light are installed on the premolded lower rudder cap and it is never flown above Van's specified Vne? Or, is it a wise observation on things involving flutter in general?

Bob Axsom

 

[sailvi767]

Doug, does that imply in any way that the short tail 6 is has a safety of flight issue if the combination strobe and white nav light are installed on the premolded lower rudder cap and it is never flown above Van's specified Vne? Or, is it a wise observation on things involving flutter in general?

Bob Axsom

One thing I have wondered about in the flutter discussion is the Harmon Rocket. It is my understanding that many are built with a stock RV4 tail. Is this correct or is the tail modified for a higher flutter margin?

George