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  #51  
Old 01-16-2020, 06:56 AM
sailvi767 sailvi767 is offline
 
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Location: Charlotte NC
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The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G
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  #52  
Old 01-16-2020, 08:38 AM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 338
Default Vne Test Format

I'm not familiar with other threads on this subject, but no mention was made of the suggested flight profile while conducting flutter tests.

John Thorp (no stranger to flutter) recommended (of course) that you start slow,

As test speeds increase beyond the aircraft's straight and level speed a dive is employed to reach about 5 mph IAS above the "test" speed.

The technique is to enter a slight climb until the speed drops to the "test" speed, when the excitation pulse is initiated.

The aircraft has already flown at the next "test speed" and is slowing through the test speed, providing some limitation to the frequency the aircraft is exposed to.

One T-18 flutter test left the pilot with a disturbing sensation, until he realized the frequency back through the stick caused a blister to his hand.

The second point, this test protocol only tests one control surface at a time. A thorough test program would witness combined tests of two or three surfaces at a time (combination of simultaneous aileron-elevator pulsing for example)

Third point, in the above example, the undetectable flutter (until the blister) was at a higher speed, the pilot, feeling he was being careful, experienced another flutter event at about 180 MPH because of structural damage caused in the first incident.

Fourth point, reflected in the FARs is if you have a trim tab on a control surface, the Vne series should be repeated with the control arm (actuator) to the tab disconnected to determine if a broken connection will not contribute to flutter.

Be careful!

Last edited by Marc Bourget : 01-16-2020 at 08:41 AM. Reason: Corrected spelling and adding the disconnected TAB point
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  #53  
Old 01-16-2020, 08:55 AM
Tom Martin Tom Martin is offline
 
Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,591
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There are continuing debates regarding IAS and TAS when determining Vne.
At ground level the two speeds are very close. As you climb indicated airspeed drops off. This was never really a factor with Cessna 172, for example, as they seldom went high. With the RV and Rocket type aircraft many of us regularly fly over 10,000 feet and there can be a huge difference between indicated and true airspeed. The actual number is somewhere between but as we can not easily determine what that real Vne number is, then take the safe route and use TAS. Most of the modern screens offer that on the EFFIS screen. If not it is easy to use one of the aviation apps and make yourself a little chart for the dash.
For example IAS for 5000, 10,000, and 15,000 feet vs. TAS at using standard lapse rate charts for temperature.
At the very least work it out for yourself and you can see the huge difference in IAS vs. TAS at higher altitudes.
Watch out during descents with power the TAS number will go increase at very quickly.
Use TAS, it may save your life.
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  #54  
Old 01-16-2020, 09:10 AM
rocketman1988 rocketman1988 is online now
 
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Default Danger!

"...Fourth point, reflected in the FARs is if you have a trim tab on a control surface, the Vne series should be repeated with the control arm (actuator) to the tab disconnected to determine if a broken connection will not contribute to flutter..."

I would HIGHLY discourage this kind of test...from personal experience.

I had a C-421 that had a "failure" of the bolt connecting the right elevator trim tab to it's linkage. (The failure was due to negligence of the company that did the annul inspection.) The failure occurred at 16,000 feet climbing to a cruise altitude, at a speed SIGNIFICANTLY less than Vne.

When the bolt came out of the linkage, the right elevator began to flutter in a divergent mode, resulting in an airplane that was nearly uncontrollable. Luckily, the two elevator torque tubes are joined in the middle with bolts through a flange. These bolts sheared, separating the elevators. It also stopped the flutter.

This happened in the space of about 5 seconds.

The aircraft was flown to a successful landing with one elevator in control and the other in trail...lucky to say the least.

Flutter is nothing to mess around with, and it doesn't matter if you THINK you are the ace of the base...
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  #55  
Old 01-16-2020, 09:19 AM
jask jask is online now
 
Join Date: Feb 2018
Location: Ramona, CA
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Default Vne

Quote:
Originally Posted by sailvi767 View Post
The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G
The 6 above has the short tail. If you have a 6 or 7 with the tall tail, I would be very respectful of the stated Vne.
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  #56  
Old 01-16-2020, 11:09 AM
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skylor skylor is offline
 
Join Date: Aug 2009
Location: Southern California
Posts: 1,022
Default -6 VNE + 20%

Quote:
Originally Posted by sailvi767 View Post
The Canadian aircraft probably fluttered at 269 MPH. The recent poster claims 20% over VNE or 276 MPH.
G
The -6/-6A has a VNE of 210 MPH. 20% over 210 MPH is 252 MPH.
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  #57  
Old 01-16-2020, 11:38 AM
jrs14855 jrs14855 is offline
 
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Location: Lake Havasu City AZ
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Default Trim Tab

Any one who intentionally attempts to fly with a trim tab disconnected is nuts. Twin Cessnas have history of trim tab issues. The Cessna 441 Conquest would up with a dual actuator on the elevator tab. This evolved from an inflight break up caused by the elevator trim tab problem.
Some of the model airplane inspired tabs and actuators found on EAB aircraft are scary just to look at.
I sincerely doubt that most pilots understand what a violent event flutter is and how little if any time there is to do anything about it.
Still total silence on the EAB airplanes that race at Reno way beyond Vne.
Glasair III Vne 335 statute, fastest Reno Glasair is doing well over 400 lap speeds.
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  #58  
Old 01-16-2020, 11:42 AM
z987k z987k is offline
 
Join Date: Mar 2017
Location: Alaska
Posts: 39
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Quote:
Originally Posted by David Z View Post
Commercial transport category airplanes have the airspeed limit set in IAS, but it's a moving target, not painted on the dial. There's two needles, one indicates the current airspeed. The other one looks like a "barber pole" and slowly decreases with altitude. My work plane (big twin turboprop) is 242kias from sea level to 14000', then slowly decreases to 207kias by 25,000ft. I am certain the engineers determined the limiting factors and set the airspeed limit safely and appropriately.

Once I finish my plane and get flying, I certantly won't be flying faster than the engineers intended.
It's almost like that barber pole is converting IAS into TAS for your redline...
hmmmmm.

The only time Vne is not in TAS is when the engineers thought you couldn't get it fast enough and/or high enough for there to be an appreciable difference to matter. In some really old twin turbo props I flew, they just listed Vne limits in relation to Density altitude, but that was just a placarded conversion of an IAS to TAS. In other words Vne/Mmo is ALWAYS in TAS whether you know it or not because physics is a thing.
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  #59  
Old 01-16-2020, 12:50 PM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
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jrs14855 said: "Any one who intentionally attempts to fly with a trim tab disconnected is nuts."

Well . . . . From 23.629:

[snip]

(f) Freedom from flutter, control reversal, and divergence up to VD/MD must be shown as follows:
(1) For airplanes that meet the criteria of paragraphs (d)(1) through (d)(3) of this section, after the failure, malfunction, or disconnection of any single element in any tab control system.

and ...

John Thorp used this reference to dissuade builders from adding trim tabs to the ailerons.

FWIW

Last edited by Marc Bourget : 01-16-2020 at 12:53 PM. Reason: attempt to highlight didn't work
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  #60  
Old 01-16-2020, 02:14 PM
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RV7A Flyer RV7A Flyer is online now
 
Join Date: Jul 2013
Location: US
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Quote:
Originally Posted by Marc Bourget View Post

(f) Freedom from flutter, control reversal, and divergence up to VD/MD must be shown...
Could be "shown" through any of analysis, test or demonstration, I'd hazard.
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