I have done some thinking (and Googling) about this
controversial question. My understanding is the following. If I make any mistakes or cut any corners in my reasoning (which is quite possible), please let me know!
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In theory:
Flutter is caused by aerodynamic forces, which are a function of the dynamic pressure, Q. By definition of
Equivalent Airspeed EAS, it is tightly connected with Q:
Q=½×ρ×(EAS)² where ρ is the density of air at sea level on a standard day, i.e. a known constant.
Q is also equal to ½×ρ×(TAS)² where ρ is the actual density of the air at your altitude, i.e. not a constant, more complicated to figure out. So Q is numerically "tied" more directly with EAS than with TAS. That's because the relationship between Q and TAS also depends on the density around the airplane, but EAS takes that variable into account. So Q is related directly to EAS but only indirectly to TAS (i.e. the relationship between Q and TAS varies as a function of the local density).
So, assuming that indicated airspeed and equivalent airspeed are very close (or at least roughly proportional, if you have a pitot tube in a good spot and stay well below the speed of sound), then the speed at which you might hit flutter (i.e. VNE times some safety margin, typically ~15%) is a value of IAS, not of TAS. This means that, as you fly higher, you can fly at faster true speeds before you have to worry about flutter.
Most aerodynamicists will agree with this intuitively: The thinner the air is (i.e. the lower ρ is at your altitude), the faster the air needs to go in order to have enough energy to induce the forces that are induced by more dense air at lower true speeds. Nearly all analysis of aerodynamic forces is based on only the Q of the air (i.e. only on EAS, or on TAS and density) as long as the airflow is well away from Mach 1 everywhere, and the Reynolds number (i.e. speeds and sizes of components) does not change much. (Either of those things - shockwaves, or changes in Re - can alter the shape of the airflow and the pressure distribution around the component of the airplane). Some people, e.g.
here and
here, have tried to argue why flutter-based VNE should be a TAS rather than an IAS, but I do not find these arguments convincing. These arguments seem to come from people who have never done an aerodynamics analysis using Q, i.e. from people who do not appreciate that aerodynamic forces are determined directly by EAS (which is very close to IAS) but only indirectly by TAS.
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Conservatively:
A lot of
flight-training material, and a lot of
manufacturers' documentation (such as
the text from Van's mentioned earlier in this thread and in other discussions about this topic) recommend that you treat VNE as a value of TAS, rather than as a value of IAS or CAS.
Basically, this is conservative: You find what TAS might give you flutter at low-ish altitudes, you subtract ~15% to give you VNE, and you stay below that many KTAS. The higher you go, that same TAS corresponds to lower IASs i.e. to lower dynamic pressures. So the higher you fly, the lower your IAS must be (in order to stay below that VNE TAS), and the bigger that "~15%" margin gets.
From the point of view of minimizing the risk of accidents (i.e. safety and liability), I can see why this approach a good idea. It is what I personally do in practice, just to be safe. But it does force the pilot to fly at speeds that are quite a bit slower than what are actually the fastest safe speeds.
(It also forces the pilot to be able to calculate TAS from IAS. If you have an external temperature sensor, then your EFIS probably does this automatically based on the local air pressure and temperature, but many airplanes do not have this capability and would thus require the use of tables, calculators, or other computing aids so that the pilot can determine TAS).
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In reality:
The best discussion I have found on this topic is
here. It is the only text I have seen that acknowledges that... (1) Aerodynamic forces are a function of Q, not just of TAS... (2) Flutter behavior is also dependent on the damping effects of the airflow, which are smaller when the air is less dense... (3) Despite fact "1", a lot of the literature for pilots recommends that VNE be treated as a TAS, i.e. the VNE IAS goes down as you fly higher... (4) The only way for sure to settle the issue is with data.
The bottom line:
As you may know, most aerodynamic forces are scaled with the dynamic pressure, Q ...
But flutter is different, because of the inertial coupling and the damping effect of the air. So flutter speed does not remain a constant indicated airspeed as you increase altitude. The flutter speed (expressed as an IAS) decreases slowly as you increase altitude. But nowhere near as fast as the indicated airspeed would decrease if you kept the true airspeed constant and increased altitude. A really good rule of thumb turns out to be half way between the two.
There are two sources that you can refer to as documentation for this ... A fellow at McDonnell Douglas Helicopter in Tempe, AZ ... had some helicopter flight test data showing the effect of altitude on blade flutter ... What he said was true of the true airspeed for flutter. It does increase with altitude, but not as fast as the airspeed at a constant indicated airspeed, which also increases with altitude. So, I took his data from his letter to the editor and converted it to indicated airspeed, and sure enough, it matched my "half-way" rule of thumb very nicely. The second source is the new book out of Germany called Fundamentals of Sailplane Design, by Fred somebody, sorry I can't remember his last name... He has a plot of sailplane flutter boundary measured at one of the Akafliegs, plotted on a flight envelop, and it shows the same "half-way" rule that I have been suggesting. So I was happy to see that he found that independently.
I would treat it as a rule of thumb, and remember there is nominally a 15% margin for flutter on top of that, which can make up for many sins like instrument error, wear in control surfaces ninges, incomplete mass balance, etc.
For example: Say an RV-6 has a VNE of 185 knots. At sea level, that's 185 KIAS and 185 KTAS. But then say you climb to an altitude where, around that speed, the IAS is about 24 knots below the TAS. (That's
roughly in the ballpark for RV speeds at 8000-ish feet on a standard day). An aerodynamicist might say you can fly at up to
185 KIAS, i.e. 209 KTAS. The manufacturer might say you can fly at up to
185 KTAS, i.e. 161 KIAS. In reality, apparently you can probably
split the difference and fly at up to about 173 KIAS, 197 KTAS.
(But, again, for the record, I personally follow the manufacturer's recommendation anyways, to be safe).
Edits: Formatting and links.