Is Vne a true airspeed or a indicated airspeed?

[Rally]

Was having a conversation with a RV owner after reading a recent article about Vne. Is Vne a function of indicated or true airspeed? The article was in reference to putting too big of a engine on aircraft (experimental) that say were turbo'd or turbo normilized and being at altitude say 25,000 and having issues with flutter and other aerodynamics even though they were well bellow the 230 mph never exceed indicated but were in fact way over 230 mph true. Any thoughts? I always thought Vne was indicated. But know I am going hmmmm.

Thanks

 

[aldassy]

Vne is indicated

 

[avbug]

Indicated airspeed is a function of air density. Fluter is a function of balance, airspeed, and air density.

Vne is an indicated airspeed. The wing and the airplane doesn't know how fast it's really going, only the force of the atmosphere acting upon it. While for a given indicated airspeed, true airspeed increases with altitude, the airplane doesn't know this.

 

[Rally]

That it is exactly what I thought. However mach tuck is the difference between stall speed and the speed of sound? What.....am I missing here.

 

[ackattacker]

Indicated airspeed is a function of air density. Fluter is a function of balance, airspeed, and air density.

Vne is an indicated airspeed. The wing and the airplane doesn't know how fast it's really going, only the force of the atmosphere acting upon it. While for a given indicated airspeed, true airspeed increases with altitude, the airplane doesn't know this.

Avbug's post is correct... Vne is indicated airspeed. But some aircraft have reduced Vne's at altitude, for various reasons.

To expand a little bit on avbugs statement that flutter is a function of balance, airspeed, and air density.. some aerodynamic qualities such as oscillatory stability and flutter do have a "true airspeed" effect. The reason is complex, for a good explanation read "Handling the Big Jets" by Davies. Cliff notes version: aerodynamic damping is reduced at high altitude and high true airspeeds. Consider that for a given magnitude of disturbance, there is a certain momentum that needs to be damped. The restorative force is a function of the angle of the disturbed surface to the airflow. At first glance we would say that if the surface is at neutral then the restorative force is zero, and this is true in the static condition. We can say that in a static conditoin that restorative force is a simple function of indicated airspeed and angle of attack. But in a dynamic condition such as flutter the motion of the surface is adding it's own velocity component to the angle of attack of the airstream. As the surface passes through neutral it has an effective sideways airspeed, and a resultant angle of attack and restorative force. This "sideways velocity" makes a vector diagram with the true airspeed to determine the true angle of attack. The net effect is that the "restorative" damping force in a oscillatory or vibratory mode is reduced at high altitudes...

Sorry if this is hard to follow, it would be easier to draw a picture.

But the point is that flutter may be a risk at high altitudes.

 

[ackattacker]

That it is exactly what I thought. However mach tuck is the difference between stall speed and the speed of sound? What.....am I missing here.

No... the "coffin corner" refers to the "corner" of the operating envelope where the airspeed range between stall buffet and mach buffet is narrow.

Mach tuck is an expression refererring to the tendency of most jet aircraft to develop a nose-down tendency as they near Mne, due to:

a) Shock wave on the wing causes the center of lift to shift aft
b) Swept wings tend to experience shock waves first at the thicker root, also causing the center of lift to shift aft.
c) The shock waves cause a reduction in downwash over the tail.

 

[mzaharis]

There was an interesting article in Flying, written by Garrison, about why the Columbia 400's Vne decreased with altitude. In the POH, it decreases 4.4 knots for each 1,000 feet above 12,000 ft. It was basically to keep it WAAAY out of the range of any compressibility effects.

 

[Rally]

There was an interesting article in Flying, written by Garrison, about why the Columbia 400's Vne decreased with altitude. In the POH, it decreases 4.4 knots for each 1,000 feet above 12,000 ft. It was basically to keep it WAAAY out of the range of any compressibility effects.

I think this is what I was getting at.

 

[mzaharis]

I think this is what I was getting at.

Compressibility effects are a function of mach, which does not go down with density altitude. So, for a constant TAS, your mach number actually increases as you go up in altitude, due to the temperature lapse rate (lower temperature means speed of sound is slower; speed of sound is purely a function of temperature). So, if you have a compressibility-based Vne, your Vne (IAS) will decrease at altitude, to maintain a constant "Mne", in effect.

Columbia hit this problem with the 400 when they wanted to leave the possibility of certifying it for speeds potentially above mach .4 (not that it flies that fast, but they needed to leave a certain margin for future growth, if I recall). This gets you into the range where the FAA forces you to take compressibility effects into consideration in your certification.

Like I said, if you can find the Peter Garrison article from Flying a few months back, he has a pretty complete discussion of how compressibility effects impacted the Vno and Vne for the Columbia 400. Most of what I'm trying to pass off as knowledge on the subject (other than generic knowledge about mach number) is regurgitated from my recollection of that article.

 

[avbug]

Compressibility effects are a function of mach

Compressibility effects are a function of air density (temperature and altitude), airspeed, shape, and angle of attack.

speed of sound is purely a function of temperature

and altitude and pressure...density altitude, for which temperature is part of the equation.

So, if you have a compressibility-based Vne, your Vne (IAS) will decrease at altitude, to maintain a constant "Mne", in effect.

Vne, Mmo. Never Exceed Speed, Maximum Mach Operating limit. Mne is sort of like crossing a bulldog with a schitzu...