TAS and the Yellow Arc

George Foster

Filing Flight Plan
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Bluesideup
Hi, I fly northern NH in the winter. It's not uncommon to fly on a very cold winter day where the density altitude is minus 2000 feet. The plane's AS indicator yellow arc starts at 126 knots. When calculating TAS using the AS indicator, the plane TAS can be 133+ knots. Yet AS needle is pointing to the TAS card in the white arc. Why doesn't the TAS card have a yellow arc starting at 126 knots? Am I in the caution 'zone' or not? Thanks
 
All that matters is the number of air molecules zooming past the wing. That’s what your airspeed indicator is telling you. Any more or less than the special markings on your ASI, and your wing does what it’s supposed to do or not do. TAS does not matter to the wing.
 
Hi, I fly northern NH in the winter. It's not uncommon to fly on a very cold winter day where the density altitude is minus 2000 feet. The plane's AS indicator yellow arc starts at 126 knots. When calculating TAS using the AS indicator, the plane TAS can be 133+ knots. Yet AS needle is pointing to the TAS card in the white arc. Why doesn't the TAS card have a yellow arc starting at 126 knots? Am I in the caution 'zone' or not? Thanks

Indicated air speed is all that matters for most small planes. TAS is irrelevant, until you get to higher speeds and altitudes and start running into Mach issues. The only time TAS matters in a small airplane is when you are trying to calculate winds and ground speeds.
 
Indicated air speed is all that matters for most small planes. TAS is irrelevant, until you get to higher speeds and altitudes and start running into Mach issues. The only time TAS matters in a small airplane is when you are trying to calculate winds and ground speeds.

There can be issues with ‘flutter’ below the Mach range. Some homebuilts have experienced it from putting to big an engine on a plane not designed to go as fast as that engine would take it. How thick or thin the air is does not affect the vibration frequency that will induce flutter.
 
Hi, I fly northern NH in the winter. It's not uncommon to fly on a very cold winter day where the density altitude is minus 2000 feet. The plane's AS indicator yellow arc starts at 126 knots. When calculating TAS using the AS indicator, the plane TAS can be 133+ knots. Yet AS needle is pointing to the TAS card in the white arc. Why doesn't the TAS card have a yellow arc starting at 126 knots? Am I in the caution 'zone' or not? Thanks

Your plane doesn't care about TAS, it cares about IAS. I only care about IAS when staying away from the barber pole on a descent (rare that I worry about that) or when I'm in the terminal environment getting set up and flying the pattern. Most times, TAS is what matters, combined with winds to get GS.
 
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:D

It absolutely does. Air density affects the structural ability of the system to reach that frequency (based on the stiffness modulus of the component, plus the mass balancing of the attached control surface) in the first place. That's why Vne is a true airspeed.

Aeroelastic behavior is affected by the medium because it [the medium] acts as a damper, with the flight structural component in question acting as a spring. As the dampening coefficient changes (reduced) with density loss of the medium, the aircraft attains the first flutter point at a lower dynamic pressure (aka higher TAS at altitude) than if it were flying in higher density and higher parasitic drag for the same powerplant (lower altitude). For small conventional subsonic spam cans this flutter point is often not possible at low altitude due to said parasitic drag and high dampening coefficient, even at high dynamic pressure (high IAS). Which is a technical way of saying you'll reach other failure modes first (likely 1st order divergence).

But at high altitude and lower damping coefficient, it becomes easily attainable to reach flutter on the same structure and powerplant, with much lower dynamic pressure required. That's why you generally can't up-power (or even significantly turbonormalize) kites like the RV case study you mentioned in your post. Not because they can't handle it down low, but because they can't up high due to air density effects on structural dynamics aka aeroelastics.

Also recognize the calculation of Vne in many small aircraft does not correspond to flutter as the failure mode, which is why they get away with spoon feeding you with a Vne posted as an IAS/CAS. This kind of "paternalistic lying" is done often for the benefit of knuckle dragging pilots, just like they simplify G-load limit ratings as a fixed range (+3.8, +4.4 et al) regardless of weight, when if one ever cracked open a structures book it would be immediately apparent the structure's G-rating changes with carried weight. Hell, Piper at one point straight up published non-altitude compensated power tables for their turbo installations (turbo arrow with fixed WG). On a Conti no less, which is borderline attempted murder in my eyes :D

Depending on the airframe, Vne could be reflective of simple 1st order structural divergence (aka the Acme cartoon wings flying off the Wyle Coyote rocket), or sometimes mere control reversal speed, which could occur before flutter failure itself. Other times, flutter could indeed be the first mode of failure, but that speed would be published as a true airspeed.

In fairness I did not stay at a holiday inn, but aeroelastics was my sub-concentration at Purdue for my M.S.A.E. That degree and a buck twenty gets me a cup of coffee, and I'm still the idiot in the room for having done that instead of going to Arizona State for basket weaving, where the girls are twice as hot, half as smart and thus twice as likely to ------- me. :D

It was the RV article that I was referring to when I said “some homebuilts.” For anyone interested, here it is https://www.vansaircraft.com/wp-content/uploads/2019/01/hp_limts.pdf
I think I was right in what I said “...How thick or thin the air is does not affect the vibration frequency that will induce flutter...” But yeah, that doesn’t seem to be the whole story.
 
Most of the above statements are incorrect. Flutter is a TAS issue. Most GA aircraft use IAS for limits however it’s possible if you climb high and then descend on the limit to put the aircraft into a flutter regime. Better modern EFIS setups allow you to set TAS as your VNE limit.
 
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