Has ANYONE tried to SIMULATE the RV's flutter?

[quypfiev]

Hi everyone,
Flutter is one of the most important phenomena in aeronautics. Has anyone tried to simulate the flutter of RV-10???I'm trying to study on this phenomena but it's quiet difficult. If anyone interest in this phenomena, we can discuss together.

[gmcjetpilot]

Originally Posted by quypfiev

Hi everyone,
Flutter is one of the most important phenomena in aeronautics. Has anyone tried to simulate the flutter of RV-10???I'm trying to study on this phenomena but it's quiet difficult. If anyone interest in this phenomena, we can discuss together.

Hi Quy: You are from Vietnam, interesting. I gather you are a student or interested in aeronautics.

No one should try to intentionally try to cause flutter in an aircraft flying. It can cause serious or catastrophic damage. Van's RV-10 is flight tested to assure they will not flutter under normal flight.

You would NEVER simulate flutter intentionally in flight, because it could KILL YOU. There have been some accidental cases where a pilot went too fast in a dive and felt a mild flutter or "Buzz". They where lucky and where not hurt, and they where able to slow down before damage was done.

Full flutter does happen and in the past on other planes it caused the whole plane to be destroyed in a matter of seconds. Flutter is NOT something you intentionally do. The RV's have good to excellent control surface flutter resistance.

All planes have max speed and are tested to show flutter will not happen, as long as that speed is not exceeded. This is the Vne speed, or NEVER exceed speed. That Vne speed however goes down at higher altitudes.

All the RV's, including the RV-10 have been tested a Vne speed. Vne is marked on the airspeed indicator with a red line. As you know flutter usually is of most concern at high speed.

Van's aircraft thru design, analysis, experience and flight testing has established the Vne speed. Van's aircraft flight tests for flutter at Vne speed and past it by about 10% or 15%. This gives an extra margin of safety for all expect flight conditons.

This does not mean a RV-10 can not fly faster than Vne, only that it has been safely tested to a speed well above normal cruise or descent speed, so its not a concern in normal operations.

Yes flutter is an interesting and difficult subject to understand. Flutter can be vary dangerous and can happen with no warning. That's why its important for the aircraft designer to consider this and the pilot to obey Vne.

You may not know, but all RV's have 100% mass balanced elevators and ailerons. Some models also have a mass balanced rudder as well. 100% means the control is evenly balanced on the hinge line. Balanced controls increase resistance to flutter. Flutter is a diverging (increasing) in oscillation response to a force. Balancing dampens or reduces this response.

Most builders of RV aircraft will flight test their plane to Vne but not beyond.


There are other ways to test for flutter: wind tunnel and computer. A scale model and wind tunnel is a method designers use to learn more about an airplane. A computer and a mathematical model of an airplanes geometry and structure can also be used to check for many things, including flutter. These programs are called FEM (Finite Element Modeling) and CFD (Computational Fluid Dynamics). The programs are very expensive. Here is an example:

http://www.aircraftdesigns.com/flutter-analysis.html

In some test the full size plane is tested on the ground with "shakers". These devices move the plane at exact frequencies and amplitudes. Measurements are taken with accelerometers to see what the response is to the excitation. This helps the designer to determine if the airplane is safe or subject to flutter. On some planes the whole wing can flutter. On the RV we know that the control surfaces are the critical item for flutter.

Small Plane like the RV-10 usually do not use advanced methods like FEM, CFD and ground test. With extensive experience and basic hand calculations a good designer can make safe plane. Flight test is used to verify that the plane is safe after building and flying. This is what Van's aircraft does. There is little or no wind tunnel or computer analysis. However they do use CAD or computer aided design programs to make drawings and parts.

With a big plane like a Boeing 777 they use all the advance methods. They want to know the plane will fly properly even before flying, because changing the design would be too hard and expensive.

Flight test is a separate and interesting subject. There is a safe and sure way of learning about an airplane in flight, while remaining safe. This takes a pilot with special skill, knowlege and experience to test the limits of a new design.

Van's aircraft does such a good job, every plane that we build from the kit performs very well and is very safe. That is from the tallent of the designer. So if you are going to study planes the RV-10 is a good place to start.

Good Luck on you studies. Are you going to build a RV-10? Do you want to design your own plane? Sounds like you are trying to learn how to design or analyze aircraft structure, all the best. That is all I know. You may want to get some books on the subject of "aeroelasticity".

[airguy]

Originally Posted by gmcjetpilot

All planes have max speed and are tested to show flutter will not happen, as long as that speed is not exceeded. This is the Vne speed, or NEVER exceed speed. That Vne speed however goes down at higher altitudes.

Careful - not technically accurate as you have stated it.

Flutter is entirely dependent upon the airstream's true speed. Vne is usually referred to in INDICATED airspeed, not true airspeed. The TRUE airspeed version of Vne will never change, but of course indicated Vne will decrease as you increase your density altitude. To induce flutter, the air flowing over the surface must exceed a specific TRUE velocity, regardless of the indicated velocity which is dependent upon ram air pressure, and thus density. You can induce flutter at ridiculously low indicated airspeeds if your altitude is sufficient and you have enough power. This was one of the limiting factors for the U-2 aircraft, at operational altitude they would have only a few knots of envelope left between stall and Vne.

[DanH]

<<All planes have max speed and are tested to show flutter will not happen, as long as that speed is not exceeded. This is the Vne speed, or NEVER exceed speed.>>

Just polishing the pins, old bean; Vne should mean "We did a full flutter test series at all speeds below this speed, plus 10% or so above it, and found no evidence of flutter". I say "should" because from time to time a design gets out there that will flutter at some speed less than the published Vne. Sometimes the cause was a slight variation in construction as compared to the prototype, and sometimes it was just poor test proceedure with the prototype.

Because of references to Vne, many folks have the idea that flutter only happens at the high end of (or above) the aircraft's practical speed range. Nothing is further from the truth.

Dan Horton

[mlw450802]

Originally Posted by quypfiev

Hi everyone,
Flutter is one of the most important phenomena in aeronautics. Has anyone tried to simulate the flutter of RV-10???I'm trying to study on this phenomena but it's quiet difficult. If anyone interest in this phenomena, we can discuss together.

guypfiev,
I would be interested in seeing the models and the software that you are using.

-mike

[airguy]

See this, from Vans website, which talks about horsepower limitations based on flutter margins and explains flutter somewhat.

http://www.vansaircraft.com/pdf/hp_limts.pdf

[quypfiev]

Of course, it's dangerous to simulate flutter by flying. I mean simulate flutter by computation(element finit method, structure analyse....).
How can we find Vne speed???
Has anyone simulated the flutter of RV-10? How is the result? That means What's the RV's Vne speed, How the wing deform when the flutter happen?
I'm studying this problem and I want to know the precedent results in order to compare with those of mine.
Thanks.

[Kahuna]

Have taken my Super 8 to an 225KIAS=258mph @3k' with no ill effects.
Over the 10% margin of Vne discussed here.
Best,

[gmcjetpilot]

Originally Posted by airguy

Careful - not technically accurate as you have stated it.

Flutter is entirely dependent upon the airstream's true speed. Vne is usually referred to in INDICATED airspeed, not true airspeed. The TRUE airspeed version of Vne will never change, but of course indicated Vne will decrease as you increase your density altitude. To induce flutter, the air flowing over the surface must exceed a specific TRUE velocity, regardless of the indicated velocity which is dependent upon ram air pressure, and thus density. You can induce flutter at ridiculously low indicated air speeds if your altitude is sufficient and you have enough power. This was one of the limiting factors for the U-2 aircraft, at operational altitude they would have only a few knots of envelope left between stall and Vne.

If you read I state indicated flutter speed decrease with altitude. Be careful here not to out smart yourself, there are many factors that affect flutter, not just altitude. However traditionally Vne is based on indicated speed for low speed GA planes. RV's being fast do push the envelope a little. Thats why we have mass balanced control surfaces.


GA planes, normally aspirated generally have ONE Vne and do not make altitude corrections (like high flying aircraft like Jets and gliders, yes gliders).

Vne covers several airframe limitations, like strength and flutter. You really don't know what is the limiting factor is unless you have the all data.

I can say RV's have shown to have very high flutter speed, where RV/Rockets are going 250 mph. Also RV's tend to protest with a buzz before going divergent and destroying the aircraft instantaneously. To be clear my opinon is flutter is not something to be take lightly. My motto is follow Van's limitations, weight, speed and so on.

RV's are faster than most GA planes. RV's can build more speed in descent than a C-172, so be careful. In level flight its unlikely Vne will be exceeded or flutter margins reduced at any altitude, with a non-turbo-ed engine.

Van knows a normally aspirated RV goes slower (true and indicated) as it climbs due to loss of HP, but if you climb into the teens and dive to Vne indicated, you are reducing your flutter margins. What is the flutter speed? I don't know? You would have to test your RV until it flutters! NO THANKS. All we know is Vne is OK below say 10 or 12 thousand. If you do the full flutter test, let me know, I want to get an insurance policy out on you.

One RV-4 was "tested". Being dove at altitude the TAS was in the 240's or 250 mph range. He got the "Buzz". He reduced power and raised the nose. No damage. Lesson learned:

We all should reduce Vne (mentally) as you climb and be careful with high speed high, altitude descents. To start a descent just lower the nose for a +5 or +10 mph increase. That will give plenty of descent. As you descend start pulling power back and maintain speed or increase slightly (2 mph per 1000 ft).

Consider Vne as a moving target or as a TAS speed limit, at least in respect to flutter. This only comes into play at high altitude and in a descent. For most RV'ers who fly at 8,000 to 10,000 ft operations, Vne indicated is acceptable in smooth air, but don't forget about about Vno.

EXAMPLE:

RV-8 w/ 200 HP, Vne 230 mph indicated; max level flight speed at 10,000 feet (std day) is 210 mph true (182 mph indicated).

Assume the Vne = 230 mph TRUE, at 10,000 feet its only 200 mph indicated. With a 182 mph IAS cruise, 200 - 182 = 18 mph margin to Vne in cruise. To start a descent out of 10,000 you have 18 mph to play with. Another way to look at it is reduce Vne by 2 mph for every 1,000 ft

For normal operations Vne indicated covers most RV's very well as long as you fly around 10,000 ft or less. Super high flying RV'ers in the teen's, with big HP engines do need to take extra care. RV's are fast. Equipped with big engines you're closer to flutter, that's a given. This is why VAN does not like bigger engines than recommended in his planes.

To avoid the HIGH DIVE descent that scare's (Boo) your passenger and pops ears takes descent planning. Start down at 3 to 1, allow 3 miles for every 1,000 feet. At 10,000 ft above field elevation start down 30 miles out, PLUS add 5 miles to slow down for pattern and 1 mile for every 5 mph tail wind.

I think we agree, but my point is NORMAL operations and Vne give protection, not withstanding your U2 dreams.

BTW: when jets fly at max altitude as you point out, stall and high speed buffet (like flutter) are a few kts apart. This is called the "coffin corner". Boo happy Halloween.

[gmcjetpilot]

Originally Posted by quypfiev

Of course, it's dangerous to simulate flutter by flying. I mean simulate flutter by computation(element finite method, structure analyse....).
How can we find Vne speed???
Has anyone simulated the flutter of RV-10? How is the result? That means What's the RV's Vne speed, How the wing deform when the flutter happen?
I'm studying this problem and I want to know the precedent results in order to compare with those of mine.
Thanks.

I think we are running into a language barrier.

You want data that does not exist, NO DATA, NO RESULTS, NOT AVAILABLE.

You want DATA to verify your test or analytical results. Right?


The result was the WING DID BREAK OFF!


SORRY you will not get any detailed data here. We are just builders and pilots, not the manufacture of the kit, Van's Aircraft.

Quy, it is unlikely Van's Aircraft has detailed flutter analysis or wing deflection data for the RV-10.

Vans DOES static ultimate strength test or "sand bag" wing test to destruction as well as flight test. RV wings are so stiff total deflection is small.

The RV-10 was flown to Vne + 10% at various altitudes. No doubt they tested for flutter with "control wrap" or "free control test".

The result, wing and controls stayed on in flight test. Mission completed.

This is NOT difficult structure to analyze. Only paper and pencil analysis is required. Wings and control surfaces have been made the same way for years.

Quy this is a light plane, build with traditional structure and control surfaces. With over 50 years of experience, each plane designed following the same configuration, does not need detailed analysis or need to "reinvent the wheel".

"RULE OF THUMB" or standard design practices allows Van's aircraft to make a Kit plane without extensive detailed analysis, like you want.

RV's also have very STIFF structure, so wing deflection is small. From a flutter standpoint, the control surfaces can be assumed to be mounted rigidly. The control linkage is also very tight, with NO SLACK. All controls (except rudder) are controlled with push-pull control rods on bearing and use hemispherical rod ends! This control system and 100% mass balanced gives good flutter charecteristics. RV's are only 200-230 mph planes so it is not like a jet going +500 mph.

This is KEY Quy, stiff wing, mass balance controls and tight control linkage. Forget wing deflection. No FEM has been done to my knowlege, just hand analysis and static ultimate strength test.

Quy, before computers, designers built many planes like the P-51, F-86 and SR-71 with out computers! If I may suggest, you need to learn how design and analyze without computers first. It will give you insight to how aircraft structure and flight controls work. Some books I recommend are: Aircraft Structures by Brun & Aerodynamic by Horner (also vibration and performance).

Good luck with your search. Call or write Van's with you questions. Here is their web site: http://www.vansaircraft.com/public/contact.htm