RV-7A Flutter & In-Flight Breakup

[Bob Axsom]

Originally Posted by bignose

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]

Originally Posted by RV10inOz

...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]

Originally Posted by Bubblehead

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]

Originally Posted by RV7AV8R

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]

consult Barnaby Wainfan's excellent articles in the March 2010, April 2010 and later issues of KitPlanes : http://www.kitplanes.com/issues/27_3...r_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]

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]

Originally Posted by chepburn

...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]

Originally Posted by frazitl

consult Barnaby Wainfan's excellent articles in the March 2010, April 2010 and later issues of KitPlanes : http://www.kitplanes.com/issues/27_3...r_9130-1.phtml

Some others:

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf

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

http://www.oshkosh365.org/saarchive/...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.