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  #1  
Old 01-25-2021, 11:46 AM
iaw4 iaw4 is offline
 
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Default Vne and Vno

Vans' RV-14 top speed in its ad flyer is 205 mph. I am guessing this is TAS, because TAS looks higher to potential buyers than IAS. (what altitude and conditions do they measure their TAS in and what would be the shown IAS?)

now I am looking at the Vans' extended airspeeds for the RV14

https://www.vansaircraft.com/wp-cont...4_V_speeds.pdf

Vno (max structural cruising speed) is 180 mph, IAS. This is below its top speed. what is Vans TAS equivalent? (or equivalently, what is Vans' top speed IAS?)

Vne TAS is just 25mph above top speed. Like many other airplanes, this seems like a very low safety margin---except that I would hope that most of the safety margin is above Vne.

I wonder at what TAS one would truly expect an RV14 to disintegrate if flown for a very long time. I am all for scaring pilots into not exceeding the Vne, but I would also like to know what the real number without safety margin above should be.

also---how do they test these speed? do they have an airplane that they test out to breakage? or do they just stop at some point and go "good enough"?

/iaw
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  #2  
Old 01-25-2021, 12:38 PM
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DeeCee 57 DeeCee 57 is offline
 
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... and here we go... again
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  #3  
Old 01-25-2021, 01:12 PM
scsmith scsmith is offline
 
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V_ne is set by demonstrating a maximum speed that is flutter-free and then reducing that by 15%. By test or analysis they may have a good idea of the true flutter speed and then demonstrate a speed “close” to that, or they may just demonstrate a speed that is “god enough” for the intended mission. External to the company process you have know way to know, but you can feel safe that the V_ne has a 15% margin on max demonstrated flutter-free speed. Just like structural margins, that margin is owned by the designer to cover a variety of variables. You are not to eat into that margin just cuz it is there.
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  #4  
Old 01-25-2021, 01:17 PM
Desert Rat Desert Rat is offline
 
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Van wrote a couple of articles about this that will go a long way toward answering your questions. Long story short; Vne is a function of TAS, Stall speed, maneuverings speed & whatnot are functions of IAS.

here's a link to the articles that are in the support tab on Vans web site under the Tech Q&A section.

https://www.vansaircraft.com/faq/hor...epower-engine/
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  #5  
Old 01-25-2021, 01:43 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by iaw4 View Post
I wonder at what TAS one would truly expect an RV14 to disintegrate if flown for a very long time. I am all for scaring pilots into not exceeding the Vne, but I would also like to know what the real number without safety margin above should be.
/iaw
As Steve and others have indicated, the main thing limiting Vne is flutter considerations. It has nothing to do with flying an airframe for a very long time. And, flutter can be sensitive to some small things, ranging from the calibration accuracy of your torque wrench to the weight of the paint on the rudder, so Vans cannot give you ‘the real number’ for your exact airplane. Part of the safety margin involves estimates of likely builder variability.
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  #6  
Old 01-25-2021, 01:52 PM
terrye terrye is offline
 
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Default Vne and Vno

Quote:
Originally Posted by scsmith View Post
V_ne is set by demonstrating a maximum speed that is flutter-free and then reducing that by 15%. By test or analysis they may have a good idea of the true flutter speed and then demonstrate a speed “close” to that, or they may just demonstrate a speed that is “god enough” for the intended mission. External to the company process you have know way to know, but you can feel safe that the V_ne has a 15% margin on max demonstrated flutter-free speed. Just like structural margins, that margin is owned by the designer to cover a variety of variables. You are not to eat into that margin just cuz it is there.
I've noticed that the Reno racers, particularly the sport class with turbocharged engines, nitrous oxide injection, etc. regularly exceed the published Vne speeds for their particular airframes (composite and aluminum). In addition they are flying over 5000', in hot turbulent air and pulling 3+ Gs. How is this possible? Do the pilots have to demonstrate flutter free behavior at these speeds prior to racing them? Or are they relying on the margin that the designer owns?
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  #7  
Old 01-26-2021, 11:20 AM
agent4573 agent4573 is offline
 
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Quote:
Originally Posted by terrye View Post
I've noticed that the Reno racers, particularly the sport class with turbocharged engines, nitrous oxide injection, etc. regularly exceed the published Vne speeds for their particular airframes (composite and aluminum). In addition they are flying over 5000', in hot turbulent air and pulling 3+ Gs. How is this possible? Do the pilots have to demonstrate flutter free behavior at these speeds prior to racing them? Or are they relying on the margin that the designer owns?
Pulling Gs reduces flutter, so flying beyond Vne at 3g's is likely safer than flying it at 1g. Obviously this doesn't hold true for all elevated g (so don't go pull 6g while indicating 250 knots thinking its safe) and it can vary widely based on the design. Steps 1 and 2 for recovery from flutter testing was always: 1. Reduce throttle 2. Stabilize between 2 and 3g. Flutter is based on stiffness, and loading a surface increases its stiffness, which increases the airspeed required for flutter. This basically holds true until the surface fails from being overloaded.
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  #8  
Old 01-26-2021, 11:57 AM
scsmith scsmith is offline
 
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Quote:
Originally Posted by agent4573 View Post
Pulling Gs reduces flutter, so flying beyond Vne at 3g's is likely safer than flying it at 1g. Obviously this doesn't hold true for all elevated g (so don't go pull 6g while indicating 250 knots thinking its safe) and it can vary widely based on the design. Steps 1 and 2 for recovery from flutter testing was always: 1. Reduce throttle 2. Stabilize between 2 and 3g. Flutter is based on stiffness, and loading a surface increases its stiffness, which increases the airspeed required for flutter. This basically holds true until the surface fails from being overloaded.
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.
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Last edited by scsmith : 01-26-2021 at 12:10 PM.
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  #9  
Old 01-26-2021, 12:04 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by terrye View Post
I've noticed that the Reno racers, particularly the sport class with turbocharged engines, nitrous oxide injection, etc. regularly exceed the published Vne speeds for their particular airframes (composite and aluminum). In addition they are flying over 5000', in hot turbulent air and pulling 3+ Gs. How is this possible? Do the pilots have to demonstrate flutter free behavior at these speeds prior to racing them? Or are they relying on the margin that the designer owns?
They are exploiting the margins, but many of them have also had structural mods - but probably not all. Most common are added stiffness in the fin and rudder. They are the test pilot for their airplane. Racing rules require you to demonstrate flight safety before racing. Both speed and g margin.
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  #10  
Old 01-26-2021, 01:52 PM
agent4573 agent4573 is offline
 
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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.
I agree that static stiffness is should be linear until you reach deformation, but I think thats over-simplifying it. Most wings behave non-linearly once you look at them in a dynamic setting. I have never seen a linear stiffness matrix in any of my textbooks or analysis. You're absolutely right though, that since the first flutter mode is in the rudder, not the elevator, that g'ing up is kinda useless for RVs except as a way to slow down quickly.

I'm gonna dig back into the books and see if I can't find a reference to higher wing loading and stiffness being coupled. I may be totally wrong, but I'm curious now.
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Last edited by agent4573 : 01-26-2021 at 02:14 PM.
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