Isn’t the engine the big issue at ISA+15, and the airfoil works roughly the same at same IAS?
Peter wrote:
My Q is whether some aerofoils are better than others.
As long as the IAS is kept the same, the temperature doesn’t affect the airfoil at all.
So I guess the question you’re asking is — are there airfoils where drag doesn’t change much with angle of attack.
Interesting.
The AoA will be greater if the air is hotter because it is less dense so more AoA is needed to support the given weight.
So you are probably right.
The reason I ask is that the post-TKS TB20 seems more affected by ISA+15 conditions (in terms of a reduced operating ceiling) than it used to be. But that is just a very vague recollection – researching this properly would be damn hard. It could equally be a slightly differently set ignition timing (my mag drop on a single mag is 150-200F which is more than I recall it from the past) but the general performance is identical and the low level IAS is exactly as before; 138 IAS at 11.7 GPH.
Assuming sub-sonic, in less dense air you still have to fly “faster” TAS to get the same lift for a given AoA, but this will give you exactly the same IAS (air density in Lift vs AoA and TAS will cancel out with the one from IAS vs TAS), so as long as you think of IAS as input you shoud not care about density for aerodynamics but it still matter for your engine power (sucks oxygene, propeller has no notion of constant IAS) and flight enveloppe (coffin corner, VNE…)
In super-sonic, density changes (or temperature) at the shockwave layer will matter a lot for aircraft aerdynamics and besides you include another domensions TAS vs IAS and TAS vs Sound speed (mach number)
Or maybe the metal of the wings did grow few at ISA+15 but I dont think you will notice it that much?
Peter wrote:
The AoA will be greater if the air is hotter because it is less dense so more AoA is needed to support the given weight.
I can’t imagine that the temperature in isolation would have any significant change to the lift, unless it changed the reynolds number significantly. i.e. if you flew at the same density altitude you should get the same performance.
However what may be happening given the change in airfoil shape is the lift to drag/ratio is now different for a given angle of attack. i.e. more drag at or less lift at say 12degrees AoA than you had previously, where as at 4degrees there may not be any significant change. I recall you said the stall characteristics were slightly different?
A little like the effects you might see from vortex generators.
My two pennies.
Isn’t the change of Reynolds number is already reflected in change of Vstall? which is less sensitive to temperature as far as measurement goes (but very sensitive to aerfoil shapes), also turbulence considerations will matter less for cruise as it tends to be done in laminar flow?
I would rather blame instrumentation precision as function of temperature than any second order effects that goes beyond what is encapsulated in density and IAS speeds?
