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  #11  
Old 01-26-2021, 03:28 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,170
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You may recall a fatal P51 accident at Reno a few years back. It was blamed on flutter in the trim tab (to destruction), which in turn was blamed on loose attach bolts, which were blamed on worn nut locking (nylon) mechanisms.
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  #12  
Old 01-26-2021, 09:33 PM
iaw4 iaw4 is offline
 
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Location: Los Angeles, ca
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I apologize, I should have been more precise in my question---even though I see that most of you are having fun discussing where flutter starts. I had read Vans very informative flutter document for the RV9A...because I used to own one. now I have an RV6A, presumably capable of withstanding higher speeds due to its shorter wings.

[1] when Vans tells you TAS in their marketing materials, how do they measure this and what would it show for IAS? (useful to compare max speed IAS to max speed Vno)

[2] when Vans tells us a safe TAS speed (Vne), at which one is safe from flutter when not applying much control inputs, how much safety margin is built in? [what I meant by flying for a long time, I meant "so that probabilistically would encounter a situation where flutter would reasonably likely kill me"]. if I were to fly TAS of 300mph (presumably direction earth), would I likely encounter flutter that would take my airplane apart?

regards,

/iaw
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  #13  
Old 01-26-2021, 10:49 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Quote:
Originally Posted by iaw4 View Post
[1] when Vans tells you TAS in their marketing materials, how do they measure this and what would it show for IAS? (useful to compare max speed IAS to max speed Vno)

[2] when Vans tells us a safe TAS speed (Vne), at which one is safe from flutter when not applying much control inputs, how much safety margin is built in? [what I meant by flying for a long time, I meant "so that probabilistically would encounter a situation where flutter would reasonably likely kill me"]. if I were to fly TAS of 300mph (presumably direction earth), would I likely encounter flutter that would take my airplane apart?

regards,

/iaw
Vne is most likely calculated from various measurements, including vibration frequencies of various airframe parts.
Standard ‘safety margin’ is 15%. But not every airplane is exactly like Vans’s prototype, so the margin in yours is unknown.
I feel like I’m being suckered in here, but if you really don’t know, the relationship between IAS and TAS is IAS=TAS*sqrt(actual air density/sea level density).
Flutter is not dependent on control inputs. Exceeding flutter speed one time is very likely to be fatal, which is why Vans does not send test pilots up to ‘test it’.
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  #14  
Old 01-27-2021, 04:25 PM
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swisseagle swisseagle is offline
 
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Iaw, have a look at this video:

https://www.youtube.com/watch?v=KbFMogBNUa0

This is how Vans is working!
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  #15  
Old 01-27-2021, 04:44 PM
Taltruda Taltruda is offline
 
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Ride em cowboy!
https://youtu.be/TfL6iyeH8OA
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  #16  
Old 01-28-2021, 12:51 AM
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Gash Gash is offline
 
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Location: Goodyear, Arizona
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Quote:
Originally Posted by scsmith View Post
I do not believe this is true. Loading a surface does not change its stiffness. Elastic materials are linear. The natural frequencies of the modes won’t change appreciably. AND actually if you load a surface enough to get local skin buckling (oil canning) on the compression side the stiffness goes down, not up. Also loading does not change the lift curve slope or the damping unless there is onset of flow separation. Since the first flutter mode is on the rudder and fin this is kind of moot anyway. The one thing that loading would do is take all the play out of hinges and control linkages, although once flutter starts the load reversals would still be fed by the play.

The reason to put on two g’s if you get flutter is that it is the safest way to slow down.
^^^^
This is why I love VAF. Dumb political science majors like me get to rub shoulders with brilliant aeronautical minds. I learned something new. Thank you!
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  #17  
Old 01-28-2021, 10:46 AM
iaw4 iaw4 is offline
 
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Location: Los Angeles, ca
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Quote:
Originally Posted by BobTurner View Post
Vne is most likely calculated from various measurements, including vibration frequencies of various airframe parts.
Standard ‘safety margin’ is 15%. But not every airplane is exactly like Vans’s prototype, so the margin in yours is unknown.
I feel like I’m being suckered in here, but if you really don’t know, the relationship between IAS and TAS is IAS=TAS*sqrt(actual air density/sea level density).
Flutter is not dependent on control inputs. Exceeding flutter speed one time is very likely to be fatal, which is why Vans does not send test pilots up to ‘test it’.
From another thread

Quote:
Van's uses 75% at 8000' density altitude. That will take 2700 rpm and ROP mixture.
this should allow me to calculate IAS :-). density coefficient at 8000' is rho=0.5258 (https://www.engineeringtoolbox.com/s...ere-d_604.html). sqrt is 0.72. so, IAS should be about 145mph or 127 knots. hmmm...this seems too low to me.

---

as to flutter, unless they have a robot pilot, indeed not a good idea to try out. but there is always computer modeling analysis. if Vans uses a safety margin to avoid flutter for its own plane and then uses another safety margin to avoid poor builders, it would be nice to know (but not test!) the expected flutter speed, off of which they presumably calculate quoted speed by substract 5 or more standard deviations....
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Last edited by iaw4 : 01-28-2021 at 06:36 PM. Reason: IAS
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  #18  
Old 01-28-2021, 08:15 PM
calpilot calpilot is offline
 
Join Date: Aug 2018
Location: Independence
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It is IAS!
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  #19  
Old 01-28-2021, 09:14 PM
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aturner aturner is offline
 
Join Date: Jan 2005
Location: Clarion, Pennsylvania
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Quote:
Originally Posted by iaw4 View Post
From another thread
this should allow me to calculate IAS :-). density coefficient at 8000' is rho=0.5258 (https://www.engineeringtoolbox.com/s...ere-d_604.html). sqrt is 0.72. so, IAS should be about 145mph or 127 knots. hmmm...this seems too low to me.
...
Units, units, units .....the "density coefficient" you used is actual atmospheric density (Kg/m3) at 8000 meters. You need the ratio to sea level density. At 8000 feet. Probably best to whip out the E6B and put it to work. 205 mph TAS is about 177 IAS at 8000'.
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