Vne and flutter to be respected
I just received my empennage kit this week and thought I'd get some in put on what I could do while I'm building it to help reduce flutter and increase VNE. I know the Rockets fly faster than Van's published VNE numbers and Dave Anders is well above Van's numbers for the -4.
So what keeps these guys from shaking their tails off? Is it careful balancing of the control surfaces? Building the surfaces stiffer (bonding in addition to riveting)?
Thanks for any input
Well our fellow VAF members kind of gave you an answer,
, sarcastic, but no offense intended. They are right.
It's a good question Kelly. It comes up time to time. My question back to you is WHY?
I see your logic, Dave Anders RV-4 does it, F-1 Rocket's do it (with basically similar empannage), why not me. My Mom told me if all the kids are doing it, does that mean I should.
Seriously, in engineering there are "margins of safety". Clearly Dave, who I've had the pleasure of talking to, is a smart & careful guy, respects Vne. Mark at Team Rocket is smart as well.
First Dave is doing 250 mph (his approx top-speed, level flight) in only smooth air. The Rocket's I can't speak to, but think they limit their Vne around 250 mph and Mark no doubt has done some test. There is one Rocket that I know of, which may have had a fatal in-flight break-up, under suspicious conditions. The conditions that day where turbulent. Not sure if it was a HRII or F1? Bottom line, flying slower or to a lower Vne, gives more margin to flutter and structural failure. That is a fact. But what is safe?
If you want to expand your Vne there are flight test methods, test plot school stuff and regulation criteria (for certified planes) as a guide to verify Vne. As you can imagine it involves diving to Vne + the factor of safety over target (giving a margin of safety). If the plane does not disintegrate, you win. The speed you demonstrated, minus the margin, is Vne. Rinse & repeat for all CG's, weights and many altitudes!
If you want to be a test pilot, you follow progressive dive test. Obviously a helmet, chute, change of clean underware and bowl of wheaties in the morning would be in order.
Van
does do a nominal and responsible amount of Vne testing to verify Vne for RV's, which gives a reasonable, practical and conservative limits. Van's Aircraft could put a higher Vne on their planes. However pilots mess up, and the current limit is reasonable for the expected design cruise speed. Of course Vne is not only flutter, it can be an ultimate structural load.
Van apparently did a good job because there have been few flutter incidents but NOT ZERO. Of all the ones I've heard of, most pilots lived to talk about it, and in most cases (but not all) the plane did not suffer permanent damage. There are no guarantees with flutter and happy endings.
Most RV'ers get in trouble at altitude. I'm not going to pretend to explain aero-elasticity, but as you climb your Vne (indicated) goes down. Most GA single engine planes don't really mention it, and give a one size fits all indicated Vne for all altitudes. The lower performance Pipers/Cessna's pretty much keeps you out of trouble. However RV's fly high and fast, and if you get too enthusiastic in descend at Vne indicated, your true airspeed can be spectacular. Many have felt the buzz of the elevator in descent. It scared them and it should.
Not even Van knew this or worried about the high altitude affect. It was not until the last 4 or 5 years has Van wrote articles about this. Most RV'ers flying in or near the teens start descents gently, for comfort (ROD) and keeping the engine warm. Thus the airspeed does not build. However as RV folks have put in IO-390's, lower drag cowls and other low drag mods, Vne is easier to exceed or push, even at higher altitudes in level flight. This is Van's main reason for caution against turbo charged engines, high TAS at altitude.
With that said, I think RV's first sign most of flutter shows up in the elevator. Usually its dampened enough to notice with out damage, allowing time to slow down. However
flutter can happen instantaneously, spontaneously, with devastating affect in a heart beat. Your elevator may have a mild buzz, but who knows if the aileron is about to rip the stick out of your hand & rip wing off.
Don't accuse me of being dramatic, that is what happens when flutter goes "divergent" with out notice. The great Steve Whittman and his wife where lost to flutter in cruise flight.
I know enough to know flutter is a bit of a mystery. You can do computer models (FEM, CFD) and all kinds of wind tunnel, but until a test pilot dives the thing its an unknown. WE KNOW THE KNOWN Vne NOW. We know following Van's numbers, we are safe, but we now know the margin decays as we climb. If you want to fly at 250 mph, you may get a way with it down low on a smooth day. You don't want to even push Van's recommended Vne on a day that's not smooth. Vno, normal operation and the lower Va, maneuver speeds are there for a reason. You can break a plane from a structural overload due to gust in level flight if going too fast. Gust's on a bad day have even destroyed large airliner or military aircraft.
Last story. When I got my multi-rating, the examiner was a flight test pilot once in his career. He broke a plane in half, literally, half the fuselage behind the wing departed the front half & wing. He unbuckled and exited the big hole that was behind him, which was once the aft cabin of this once one piece twin engine plane. What happened? During Vne test, after diving to target plus, he started a shallow climb (always done so your are slowing for safety during the test). He than "rapped" the yoke and let go. Normally the plane porpoised once or twice, than stabilize with out touching the yoke. This time he rapped the yoke and it went FULL BACK to the STOP and slammed back full forward in a fraction of a second! Plane broke in half. He and his chute than bailed out. The aircraft had gone through some small mods that needed re-certification and the CG was max aft. Those small mods made a huge difference on Vne. That is why flight test are done. I flew this same plane flying freight. There was a in flight break up of a simular plane. A freight pilot flying the last leg back to base, started a high speed controlled decent. It broke up in flight
LINK. The FAA determined high speed (may be near over Vne?) caused (flutter?) structural failure. This is a certified plane that has been tested. The CG and weight where out of limits.
Every RV builder is suppose to verify Vne during Phase I before taking passengers. If testing a RV, stick loose, not held, is different than one that is held. Holding the stick on a RV, with a very tight rod/bearing control system like the RV has, affects flutter with no doubt. I am not recommending you rap your stick like a Pro test pilot, only you be aware that letting the stick go can cause a flutter to start or get worse.
It's a very complicated topic. I go back to my question to you. WHY? If you want a jet go buy a jet.
Keep in mind I'm conservative & believe in limits and flying away from the extreme edge of the envelope. It's our safety net, or margin. However if I do intentionally fly near the edge, I like to know what I'm doing. In the case of a RV, wearing a chute is a good idea when playing test pilot or Bob Hoover.
