Increasing VNE

[1:1 Scale]

I just recieved 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

[mgomez]

Meaning no disrespect whatsoever, this is a bit like asking "Can anybody tell me how to design an airplane."

Don't do it, don't even think of doing it, without reading a lot of books, taking a lot of engineering courses, and becoming familiar with how Vne is determined. I've done all of that, and I still won't do it to someone else's design.

Live long and prosper and don't dig craters with your airplane,
Martin

B.Sc. Aeronautical Engineering
M.Eng. Electrical Engineering
M.S. Applied Physics

[f1rocket]

The Rockets use a larger empennage, thicker skins, more stiffners, and riveted trailing edges. (Those are just the changes that are apparent to my untrained eye).

I'm no aeronautical engineer, so tread carefully. You only get a chance to experience high speed flutter once (unless you are wearing a parachute).

[mburch]

I've decided to install an airspeed indicator calibrated in km/h. With this setup I'm hoping for a Vne of around 370.

mcb

[gmcjetpilot]

Originally Posted by 1:1 Scale

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.

[1:1 Scale]

Thanks for the responses, although according to Martin, it seems there should be no experimentation in experimental aviation I know the realities of aviation- I was reminded of them yet again a little over a week ago when a Caravan owned by my aunt and uncle "dug a crater" right about the same time I was filling out the order form for my empennage.

The reason I'm inquiring is mainly to see if there is anything simple that I can do to increase the speed envelope (thereby increasing safety) while I'm building the tail. I'm not talking about fabbing thicker skins or reshaping the airfoil, or any other engineering project. But if bonding the trailing edge wedge and stiffeners in addition to riveting them (or something similar) could add a couple of knots to the Vne, I see no reason not to do it. I'm not planning to build a 300 knot RV or strap JATO bottles under the wings

So does anyone have any input for building the tail "better"?

[flytoboat]

I figure that building as close to the plans as I can will make it "better".
Don't forget, you're starting with the best kit available...

[Andy_RR]

Kelly,

I often wonder why people who build experimental aircraft get on their soapbox when someone asks a question like this. To me, the of building an experimental aircraft is a journey for the curious and inquisitive. So why take the wind out of these sails with such authoritarian stance?

Van, Bob Hoover and all the others are mere mortals like the rest of us. It is possible for us less celebrated folk to understand aerodynamics better and to learn the whys and wherefores. No need for 'people-in-the-know' to shut the book of learning in quite such a determined fashion.

FWIW, (and I'm no aerodynamics expert) I understand that flutter originates by vortex shedding. Every shape will shed vorticies at some frequency relationship with true airspeed. We can use principle this as an airflow measurement device in engine management systems and other process control applications, by measuring the frequency of the sound this generates - go visit a marina on a windy day and listen to the wind amongst all the masts.

Well, when this vortex shedding reaches the natural frequency of part of the system, it will begin to vibrate. I guess this is where the complexity comes in, because there can also be some positive feedback mechanisms in terms of aerodynamic lift-v-angle of attack if the vibration in the wing generates a torsional mode, which it almost certainly has.

Like all dynamic systems, stiffness, mass, moments of inertia, backlash in linkages and inherent or external damping all play their part. I would say the whole concept could be sensitive enough that even two examples of the same type could experience flutter onset and widely varying TAS.

Anyway, that's my understanding badly explained for you! Keep searching and asking questions, though. It can bring on interesting discussion and learning which we can all benefit from.

A

[penguin]

Kelly,

Your question assumes that Vne is set by a property of the empennage. I would be surprised if that were the case, so the short answer is that there is probably little you can do to the emp to gain a small increase in Vne.

As you know Vne is set as 0.9*Vd, to give some margin (for many things including contruction error & wear in service). Vd is set at initial design time as a function of several factors using a formula given in FAR 23. The aerodynamics and structure are then designed to meet the chosen speeds, and testing performed to ensure the design predictions are met, or - more accurately - that no flutter is evident at the tested flight conditions.

The maximum speed that can be flown free from flutter is a function of the natural frequency of the airframe. If you had more time & money than sense you could probably predict that speed through out the flight envelope, and then run tests to validate your predictions, then modify the design to increase the speed and rebuild yor emp - if indeed the emp was the limiting factor, it may be the wing.

We do know that the basic design is flutter free throughout the declared envelope. A few people in the World (who have experience of flutter analysis) may be able to tell you off hand what modifications would be required to increase that speed, but I'm not one of them. My advice would be to build to print. I'm not against experimenting, but the stakes are high in this area and you need a lot of expertise to be reasonably sure of being successful.

Pete
(BSc in Aero Eng & 25 years in the aerospace industry)

[mgomez]

I get on my soapbox because I don't want someone to get killed due to my inaction. I didn't say "Don't." I said "Don't until you've educated yourself."

That's what experimental aviation is all about. Educate yourself. Then experiment. Be the experimenter, not the lab rat.

You're curious and inquisitive? That's great! It's a prerequisite to learning. You want to improve or modify an existing airplane design? You want to be a test pilot? More power to you! Building a homebuilt is a great opportunity to learn about aeronautical engineering and test piloting. But the emphasis is on "learn."

"Why discourage curiosity?" Quite the reverse. I encourage you, indeed beg you, to be curious. I just implore you to use your curiosity to get you informed, not killed.