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Vne and Vno

iaw4

Well Known Member
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-content/uploads/2019/01/RV-14_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
 
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 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.
 
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?
 
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.
 
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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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.
 
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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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.
 
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
 
[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’.
 
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!
 
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

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/standard-atmosphere-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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From another thread
this should allow me to calculate IAS :). density coefficient at 8000' is rho=0.5258 (https://www.engineeringtoolbox.com/standard-atmosphere-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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