Vne, IAS, TAS and BVDs

Never Exceed Speed (Vne), Indicated Air Speed (IAS) and True Air Speed (TAS) are closely linked, and if you don't understand how and why, they could have an effect on your BVDs.

Studying the Pilot Operating Handbook of a Grob 103 I was surprised to see a chart that showed never-exceed speed decreased with altitude. The prop powered aircraft I've flown for over 40 years all have a redline at Vne and none varied with altitude. Vne was Vne. Don't exceed that one speed or bad things will happen. The Navy jets I flew didn't have a never exceed speed, they could take whatever speed you could get out of them.

But now, the Grob manual told me, Vne was not a fixed speed on the airspeed indicator. Vne, it insisted, was related to true air speed (TAS) not indicated air speed (IAS). To keep from exceeding the 135 knot TAS Vne, the never exceed indicated airspeed had to go down as you climbed higher.

Now hold on, either all those powered aircraft were exposing me to some risk by not cautioning me against a problem the sailplane folks understood, or the glider manufacturers were overly cautious, or I simply didn't understand. I knew the latter was the case, of course, but common sense said IAS should be the limiting factor not TAS. What was I missing?

How could the aircraft be exceeding Vne if the needle was way down in the green arc, which the POH and a placard demanded? After all, the air speed indicator is measuring how hard the air is hitting the pitot tube (and the airspeed indicator diaphragm) which is a measure of how hard the air is hitting the wing, so what's TAS got to do with it?

TAS, it seemed to me, was the true speed the aircraft was moving through an air mass, and the lying air speed indicator wouldn't show me that, at altitude, because it was confused by the lower air density. To get the true story you had to pull out your prayer wheel and correct IAS for altitude and temperature--in other words correct IAS for air density--to get TAS.

In fact, the air speed indicator doesn't measure speed at all, it measures dynamic pressure caused by the air molecules packed into the pitot tube. But, I reasoned, those molecules are buddies of the ones that are banging into the wings, so why does the speed through the air mass make any difference?

The answer has do with something that almost sounds sweet and friendly: flutter. Ribbons and butterflies flutter in a pretty way, but airfoil flutter can ruin your whole day. Here's why, and here's why it's TAS that matters.

Flutter is related to the velocity of the air, regardless of density, moving past the wing. Why? Because flutter is a so-called aeroelastic effect that results from the springiness (elasticity of the airfoil and surrounding air), aerodynamic forces, and inertia. And inertia is a component that's related to speed, in fact it's the time derivative of speed.

Remember from high school physics? The change of speed over time is acceleration [ a(t)=dv/dt ]? Force equals mass times acceleration [F=ma)?]*

A wing has mass and when it's tweaked--when a force is applied, say by turbulence or a control input--it overcomes the inertia of the mass, and the wing responds by flexing. Usually, that acceleration is absorbed or damped by the wing and the shock-absorbing effect of the air around it and it returns to its original position. But if that tweak is suddenly imposed or is more energetic you can make the structure resonate like a tuning fork.

Add a bit more energy and the structure will destroy itself. Quickly.

So the velocity of the air, not the mass of air flowing by it, is the issue. And at altitude there's less air to act as a shock absorber. The only way to avoid flutter  (without redesigning the wing) is to go slower.

So how does an aircraft manufacturer determine Vne? Numerical methods can be used to computed Vd, design diving speed. (FAR 23, by the way, requires that Vd be not less than 1.4 design cruise speed, so the days of aircraft that climb, cruise and glide at 80 are over.) Flight testing is used to establish Vdf, flight test dive speed, which must be lower than or equal to Vd. Vne is set at 90% of the Vd or Vdf, whichever is lower.

Why do small prop powered aircraft have one fixed indicated airspeed for Vne? Because their structures are more rigid than gliders and less prone to flutter, and few fly higher than 10,000 feet in any event.

*The first derivative of position (x) with respect to time (t) is velocity (v), the second--the rate of change of velocity--is acceleration (a). The rate of change of acceleration, the third derivative, is known as jerk (j). Instrument specs often require a specified limit for jerk and acceleration to avoid damage. A forth derivative was specified for the Hubble telescope, the rate of change of jerk. There's no universally accepted term or symbol for it, but some have suggested snap (s), crackle ( c) and pop ( p) for the 4th, 5th, and 6th derivatives. I can picture a rate of change of acceleration. But the rate of change of the rate of change of acceleration? Not so much.