Why is VNE TAS in the RV-12

[RV8JD]

To reinforce what Ron just posted above, here is a post from Greg Hughes of Van's replying to my earlier query about the somewhat ambiguous way Van's stated Vne in the RV-12 POH:

http://www.vansairforce.com/communit...86&postcount=2

[grubbat]

Decending out of 17,000 ft in the RV-9 which I rarely do but just a case in point, I?ve got to hold Indicated to around 130mph which trues out to around 180 mph. The modern EFIS makes this a cakewalk.

My legacy old inefficient airplanes have the old antique steam gauges but wider margins and a bit simpler. Decending through same altitude, I hold Comanche at 180mph indicated (243mph TAS) and the Aerostar (Deathstar to the newbies) to around 250mph indicated (337mph TAS). 🙃

I LOVE the efficiency of the -9 and I bet the -12 is even better but those old engineers from yesterday sure knew a thing or two about stuff.

[RVDan]

Without going into the reasons why, Some of the factors that relate to establishing Vne are directly related to TAS. In older part 23 airplanes, where all that we had available was a simple mechanical airspeed indicator, a red line was drawn and that was it. By the CAR?s and part 23, Vne was determined for standard day sea level and marked on the IAS indicator.

Airplanes that go to higher altitudes under Part 25 and now Part 23, have always had Vmo in place of Vne. This is a value based on TAS. A sliding scale or flag moves to indicate Vmo based on the current pressure altitude. Even for older mechanical instruments, the Vmo value changed with altitude on the IAS indicator. They also have Mach limitations but that is an aside.

Because gliders can go to high altitudes (25000 ft and above), the Europeans required the Vne to be indicated for the altitude conditions, typically with fixed markings that showed the Vne for various altitudes. It is interesting to note that the US never required this until recently. As a glider pilot we just used the rule of thumb of 2% Vne reduction per thousand feet, or at least some of us did.

With electronic displays we have the ability to see Vne in terms of TAS. Now for even small airplanes we can easily abide by the Vne as it varies with pressure altitude, and that?s a good thing.

I?d like to note that Vans isn?t deficient on this. He followed the standard and continues to follow the standard for Vne that was established by the FAA over the years. There is lots of published info for pilots on the need to reduce the fixed Vne in small airplanes as altitude increases, hence the 2% rule of thumb.

[sailvi767]

I think what is lost in the IAS verses TAS debate for setting VNE is every airframe is different. You really have two curves. You have a limit based on dynamic pressure or IAS that is fixed or valid at any altitude. You then have a flutter limit based on where and when you can expect some portion of the airframe to flutter which is TAS dependent. You need to overlay those two curves to see the big picture but sadly that information is rarely available to us.
As a example some manufactures set VNE as a IAS up to the certified altitude of the aircraft. This is because they know the onset of flutter is above the highest true airspeed the aircraft can obtain at that altitude using the dynamic presssure limit (IAS). A different airframe May have the exact same dynamic pressure limit (IAS) but a much lower flutter limit (TAS). In that case the VNE will need to be set lower on aircraft two. A glider is a excellent example of the 2nd aircraft. They often have low flutter limits and fly very high. Most high performance gliders use TAS exclusively for VNE for that reason.
It’s possible because there are two different curves to set VNE as IAS below a certain altitude and TAS above that altitude. That changeover point could be the same for IAS or dynamic pressure for two different aircraft but very different for TAS or flutter margin leading to two different changeover points. That point could be sea level for some aircraft or 16,000 feet for others as Vans seems to have set at one point for the 12. To complicate all this the TAS is of course pressure not actual altitude.
I think what I have posted above is the reason there is so much confusion on if VNE is IAS or TAS based. The answer is it could be either or both. If both each aircraft type will have different crossover points where one becomes limiting. The above is from my aero courses years ago and there are other factors. It’s a simple explanation.
G

[pjc]

I know this is a very complicated subject, but can any of our experts explain in simple terms why flutter is excited as a function of TAS (velocity of the flow field) rather than IAS (dynamic pressure) ?

Most of what (little) I know about the physics suggest aero phenomena depend fundamentally on fluid density. Seems not to be the case here.

Pointers to readable references welcome.

Thanks, and apologies if this takes us too far off topic ...

Peter

[rvbuilder2002]

Originally Posted by pjc

Most of what (little) I know about the physics suggest aero phenomena depend fundamentally on fluid density. Seems not to be the case here.

In simplest terms, air is a fluid.

In simplest terms of an explanation....
Think of a fixed wing airplane orbiting the earth in the vacuum of space.
There would be zero air molecules to exert any force on the control surfaces or dynamic pressure in the pitot tube so the control surfaces would be free to move with zero (aerodynamic) resistance and the airspeed indicator would read zero (but you would still be moving quite fast).

Climbing towards space the air density would be getting lower and the indicated airspeed would be reducing.
It is the reduction in air density working against (or resisting the movement of) the control surfaces, that begins to make them more susceptible to flutter.

Said in a different way.... the denser the air is flowing past a control surface, the more it works against the movement of the control surface, which has a positive effect on working against the development of flutter.

So by using TAS, instead of IAS as the limit, it automatically adds in a correction for the reduction in air density working against the control surface movement.

This is a greatly oversimplified explanation that doesn't expand on all of the different influences related to the occurrence of flutter but hopefully gets the idea across.

[BobTurner]

I?ll take a crack at this.
The problem is, there are many kinds of flutter. Ailerons oscillating up and down. Wings flapping. Wings bending about their lateral axis (torsion mode). Etc. Now throw in the complication that damping (think of air resistance) is, at best, approximated. Often as rho v squared. But for some low speed stuff, like wings flapping, rho v is a better damping model. So you end up with a bunch of possible modes. For some, the parameters group together as rho v squared, and so IAS is the appropriate single parameter. But for other modes the equation may work out rho V. Then, neither IAS nor TAS is a correct, single parameter characterization. Generally speaking, using TAS is the most conservative, IAS the least, and the ?truth? is usually in-between.

[pjc]

Originally Posted by BobTurner

I?ll take a crack at this.
The problem is, there are many kinds of flutter. Ailerons oscillating up and down. Wings flapping. Wings bending about their lateral axis (torsion mode). Etc.

Good point. For further simplicity let?s limit to the Van?s designs where (I believe) control surface flutter is the first concern. (I wouldn?t expect the short thick wings of the RVs to be susceptible to aero-elastic twisting at anything close to the published Vne).

Originally Posted by rvbuilder2002

Climbing towards space the air density would be getting lower and the indicated airspeed would be reducing.
It is the reduction in air density working against (or resisting the movement of) the control surfaces, that begins to make them more susceptible to flutter.

Said in a different way.... the denser the air is flowing past a control surface, the more it works against the movement of the control surface, which has a positive effect on working against the development of flutter.

I think you are saying that as air density is reduced, the damping forces that inhibit (stabilize) flutter are reduced. Makes sense.

But aren?t the aerodynamic forces that excite flutter also reduced in an equal way ? If so it would seem that IAS (sensitive to density) would be more relevant that TAS.

Still curious ....

[Art_N412SB]

So, the lack of air resistance over the airfoil can lead to flutter. And that is why we may have a lower TAS VNE at altitude. Did I get that right? Thanks Scott.

[Reformed SeaSnake]

Originally Posted by pjc

I think you are saying that as air density is reduced, the damping forces that inhibit (stabilize) flutter are reduced. Makes sense.

But aren?t the aerodynamic forces that excite flutter also reduced in an equal way ? If so it would seem that IAS (sensitive to density) would be more relevant that TAS.


Still curious ....

There are a number of forces at play in flutter, but lets say there are only two to start off with; aerodynamic excitation (density times velocity squared - which is what our airspeed indicator measures) and aerodynamic damping. The damping is proportional to air density but not forward airspeed. Now lets say you are flying at exactly the critical flutter airspeed at sea-level. The excitatory force is exactly in balance with the damping force. If you suddenly increase altitude but maintain velocity, TAS stays the same, aerodynamic excitation (IAS) is reduced and aerodynamic damping is reduced by the same amount. However, you are still at the critical flutter airspeed because damping still exactly balances excitation.

That is a drastically over-simplified case. Critically, in real airfoils there are internal damping forces due to the construction materials and assembly methods (riveted aluminum) which are effectively constant.

Bob hit the nail on the head in saying critical flutter airspeed is neither TAS or IAS but somewhere in between. The conservative approach is to use TAS.