• 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. Never exceed that speed or bad things will happen.

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, 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 sailplane's POH and a placard demanded?

TAS is the true speed an aircraft moves through an air mass, and the lying air speed indicator doesn't show that, at altitude, because it's confused by the lower air density. To get the true story we have to pull out our prayer wheel and correct IAS for altitude and temperature--in other words correct IAS for air density--to get TAS. No new news there, every student pilot learns how and knows why.


So what's the problem, why does Vne go down as you go higher? The answer has do with something that almost sounds sweet and friendly but is actually very nasty: flutter. Ribbons and butterflies flutter in a pretty way, but airfoil flutter can ruin your whole day life.

Flutter is a so-called aeroelastic effect that results from the springiness (elasticity of the airfoil and the surrounding air), aerodynamic forces, and inertia.

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 (even a metal wing). Usually, that acceleration is absorbed or damped by the wing, the structure it's attached too, and the surrounding air and it returns to its original position. But if that tweak is suddenly imposed or is energetic enough as the result of high speed, you can make the structure resonate like a tuning fork.





It doesn't have to be a plastic ship, either. Here's a NASA film of a Twin Comanche's empennage (note how the fuselage skin flexes just ahead of the horizontal stab).



And a Lockheed C-141 (note the tail is resonating too)



What happens if you go too fast, push the aircraft past the speed where it flutters? Here's one of Grumman Iron Work's A6 Intruders.




Why does Vne go down with increasing altitude? Because there's less air to act as a shock absorber. Because there's less resistance to the wing (or other structure) flexing, you have to reduce the forces on that structure. The only way to avoid flutter (without redesigning the wing) is to go slower.

Aircraft manufacturers determine Vne by using numerical methods to compute 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. Then Vne is set at 90% of the Vd or Vdf, whichever is lower, to add a margin for error--theirs and yours.

What does all this mean to you as a glider (or power) pilot? The faster your True Air Speed the more likely you are to encounter flutter. Push your slick glass ship up to redline and you're flirting with danger.

Remember, your airspeed indicator lies. It won't tell you what your TAS or never exceed speed is. In fact, it may be a compulsive liar and even lie to you down low (click to enlarge) or visit https://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=25093&key=0








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5 comments:

  1. Had a good chuckle reading your post. Thanks and keep safe.

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  2. Really positive updates! Thanks for sharing

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  3. Brilliant real world examples especially the A-6

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  4. Nice article and site. Keep up the good work.

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  5. man flying looks harder than I thought, props to you and many safety blessings!

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