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  #21  
Old 06-30-2011, 11:50 AM
jrs14855 jrs14855 is offline
 
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Default vne testing

In the early EAA era some builders were required to test to 110% of VNE. The wording " controllable throughout its normal range of speeds" tells me that the FAA expects a test to VNE. Very simple solution for this-if you are afraid to test to the recommended VNE, make the VNE on YOUR individual aircraft a number that you are comfortable with.
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  #22  
Old 06-30-2011, 01:40 PM
RV8R999 RV8R999 is offline
 
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yet they conspicuously leave out the requirement to document Vne. Why? Because they really do not want to tell anyone they have to test to Vne. The word "normal" is not a legal term I assure you.

Only the builder decides what "normal" is but is not required to be documented anywhere.

The Op-Lims for my BD-4 dated 1976 do not require Vne testing or documentation so when and why do you think it was ever REQUIRED to go to 110%?

Going beyond ANY established design limit is simply irresponsible unless you are backed by analysis, and have the requisite skill and experience as well as a genuinely VALID reason to do so. If you disagree then why stop at 110%? why not 120%, 130%. Certainly someone can rationalize this....

I watched a flutter test in the wind tunnel at NASA AMES and what I heard and saw was BRRRAAAAAP, CRACK, BOOM in about 2 microseconds the entire structure destroyed. One instant it was flying nomally in less than a second later it was nothing but shards. No warning, no opportunity to slow down, just BOOM! No thanks.

Do what you will...but don't pretend it is a requirement.
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  #23  
Old 07-05-2011, 04:39 PM
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ChiefPilot ChiefPilot is offline
 
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Based on comments here and reading done elsewhere, I plan to test to 110% of Vne by slowly working up to it in increments of 5kts to published Vne and 2kts to 110%. Others have been past that (reference the turbo RV6) so I don't expect an issue but will have full gear (chute, etc.) anyway.

Some of the comments in this thread are interesting when juxtaposed with those from the turbo'ed RV6 thread showing operation past the published Vne...
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Last edited by ChiefPilot : 07-05-2011 at 09:38 PM.
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  #24  
Old 07-11-2012, 12:35 AM
mike in phx mike in phx is offline
 
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Quote:
Originally Posted by ChiefPilot View Post
Based on comments here and reading done elsewhere, I plan to test to 110% of Vne by slowly working up to it in increments of 5kts to published Vne and 2kts to 110%. Others have been past that (reference the turbo RV6) so I don't expect an issue but will have full gear (chute, etc.) anyway.

Some of the comments in this thread are interesting when juxtaposed with those from the turbo'ed RV6 thread showing operation past the published Vne...
Can you please tell me what you have going on here mr chiefpilot
I would like to hear your explanation
Why all the "reference"
What is your obsession with vne and my airplane ????
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  #25  
Old 07-11-2012, 06:58 AM
LotusSeven LotusSeven is offline
 
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Default Not for the un-instrumented

In the spectrum of flight testing we do at the Navy, flutter testing remains some of the riskiest. Before an aircraft even flies we do structural modeling and physical shake tests to determine where the resonance peaks are. When it comes time to do Vne/flutter testing, it is done with a fully instrumented aircraft and in most cases the data is telemetered to the ground so lots of engineers who know what they are doing can take a look at the data real time (not to say we don't know what we are doing).

For Vne testing, the control surface in question would be instrumented for strain and position and the aircraft will be trimmed to a slower speed than target so if the stick is released it will pitch up and slow down on its own. Then, at incrementally increasing airspeeds, the control surface is "rung", often by the pilot simply slapping the stick. Engineers will analyze the response of the control surface to determine the damping ratio of the system. As the speeds increases the damping ratio will become smaller. The trending of the damping ratios will be tracked and extrapolated to determine the Vne and the testing will be called off some margin before that speed is reached. This obviously is a simplification, but the process is basicly correct.

Flutter will build energy quickly, so if you go out looking for the actual Vne and it happens to be dictated by flutter and not something else there is a good chance that you won't be able to recover regardless of piloting ability. Personnally, I would make sure my hinges and control runs were as free of slop as possible, verify the balance points of the control surfaces are as recommended, and test it out to Van's suggested Vne (because you know someone has gone there at least once).
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  #26  
Old 07-11-2012, 07:25 AM
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pierre smith pierre smith is offline
 
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Default Good advice.

Quote:
Originally Posted by LotusSeven View Post
. Personnally, I would make sure my hinges and control runs were as free of slop as possible, verify the balance points of the control surfaces are as recommended, and test it out to Van's suggested Vne (because you know someone has gone there at least once).
For some of the newer guys on here, I have a friend, now 82 years old, named Nick Jones, who had his Cassutt F-1 develop aileron flutter at 5,000' in Texas, just before an air race.

The wing self-destructed instantly and he was gyrating to earth at near 300 MPH and couldn't get his three over-the-center latches to release the canopy, so he body-slammed upwards, pulled around 22 rivets and bailed out. His feet were around telephone pole height when the canopy filled!

Flutter can instantly be disastrous and he still tells this story.

Best,
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  #27  
Old 07-11-2012, 07:59 AM
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LettersFromFlyoverCountry LettersFromFlyoverCountry is offline
 
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I'm in the middle of phase one testing and I don't have a cool helmet and parachute like Brad so I'm not testing to Vne, but seriously, I have a question about this. It's not rhetorical.

Let's suppose you take it right to the limit and go to or exceed Vne and your plane doesn't come apart.

What exactly have you just learned?

That you can exceed Vne? How do you know that? Does exceeding it once mean the next time you exceed it you'll have the same outcome? (I'm not an engineer so I can't answer these questions).

And if the value is that it's easy to find yourself in an RV exceeding Vne, what does THAT tell you? Your next encounter with Vne might be 1 knot over, 5 knots over, 10 knots over. Since you don't know you're going to hit Vne accidentally, you don't know how much MORE you're going to exceed Vne by so you don't KNOW what the design is going to allow, do you?

I'll hang up and listen.
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  #28  
Old 07-11-2012, 08:12 AM
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Ironflight Ironflight is offline
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The truth, from an engineering standpoint , Bob, is that if you exceed the expected or published Vne and nothing bad happens, you have only proved that nothing bad happened THAT time...and not much else. Without additional analysis and examination, you really don't know anything knew except that either the airplane is well and over-designed, or you were lucky. If you flip a coin once, and it comes up heads, does that mean that it will ALWAYS come up heads? No way to know - not enough data.

Engineers build in margin,and often-times, you don't know really know what that margin is. they might guarantee a certain percent - but might secretly build in a little more for their own peace of mind (I have almost made a living trying to guess how much secret margin there is in some aerospace equipment).

To directly address at least one of your good questions, just because you exceeded Vne once without trouble does not mean it will do the same thing next time. You just don't know. Would I worry about a small exceededance in smooth air if you got slowed back down? Probably not. Find yourself 20% over red line in the bumps? I would do a VERY thorough inspection for wrinkles and signs of things like cracked paint.
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  #29  
Old 07-11-2012, 09:01 AM
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Are we sure that flutter is the primary concern for Vne? Could it be the windscreen? Some other structure limit? Granted, flutter is the scariest but a colapsed windscreen could also ruin your day.
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  #30  
Old 07-11-2012, 09:04 AM
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DanH DanH is offline
 
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Please note Matt's test procedure description in post #25.

Exceeding published VNE while firmly gripping the stick doesn't tell you much. You are an excellent damper. Releasing the stick might show you a surprise. Bumping to excite oscillation is a test.

There is a popular brand of ultralight and light plane kits whose design originally did not include aileron counterbalance. The wing was built around a single large tubular spar, which is not very stiff in torsion. The example I flew would enter a classic torsional flutter mode at 43 MPH if I merely released the stick. Grabbing it for damping and pitching to lose 2 MPH would stop it.

Here's the thing....my buddy the airplane owner had never seen this in 100 hours of flight. His style included a firm stick grip.
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