And, please keep in mind my native language is Portuguese, and all the english I learned comes from high school time, more than 31 yrs ago :-/

 

I'll also keep in mind that your English is waaay better than my Portuguese! :-)  Sorry, as I freely admit, I overreact to that word.

 

Scott

I don't see why the controls should behave very differently for a given IAS, at different altitudes. Keep in mind true airspeed is higher at high altitude, and compensates for the lack of static pressure. Aviation uses IAS as primary speed indication because the aerodynamic effects are more or less constant for a given indicated airspeed. But I'm sure you know that.

 

You're almost right...  regarding static pressure, but the problem has to do with dynamic pressure, as in q = 1/2 p V² which is really the one playing the role when it comes to the lift generated by the control surfaces... and, since given a constant TAS ( the V in the expression for q ), and assuming an ISA atmosphere, the dynamic pressure will be greatest at sea level... you can easily see why flying higher, even if you keep your TAS (instead of IAS) constant, will turn your control surfaces less responsive ;-)

@Pascal_LSGC:

 

Pascal, of course if the IAS / CAS remain constant, dynamic pressure will too.... Now I understand your point ;-)

 

And... it it remains constant, because TAS is actually increasing, and thus compensating for the decrease in density, then there should be no mushiness...

I don't see why the controls should behave very differently for a given IAS, at different altitudes. Keep in mind true airspeed is higher at high altitude, and compensates for the lack of static pressure. Aviation uses IAS as primary speed indication because the aerodynamic effects are more or less constant for a given indicated airspeed. But I'm sure you know that.

 

Pascal

 

Actually the behaviour of the aircraft changes with density (and hence with altitude), even if you keep a given IAS. At first this may seem counter-intuitive, but let's consider, as an example, roll dynamics.

 

Suppose you're keeping a steady IAS and a steady roll rate, at either Sea Level or 40.000 ft. Now suppose you instantaneously center the ailerons. What happens?

 

 

 

As you can see from the image, the AoA the wing is seeing when you center the ailerons, depends on the ratio between the horizontal velocity of the aircraft and the relative velocity induced by the roll rate.

 

At 40.000 ft, for a given IAS, the actual horizontal velocity is doubled compared to sea level, so the AoA the wing is seeing is approximately (for low AoA's) halved.

 

So, when you center the ailerons at 40.000 ft, the wing is seeing the same IAS, but (about) half AoA. It's this AoA that dampens the roll rate, hence the damping effect on the roll rate is approximately half at 40.000 ft than at sea level. This explain why the aircraft feels more sluggish the higher the altitude.

 

I didn't focus on the effects of altitude on other aspects of flight dynamics (pitch, yaw, control response, etc.) but I anticipate the effect should be similar.

 

Marco

Superb description Murmur!

 

The closest I found to this was the excellent explanation (also geometrical) of how sideslip forces tend to bring an aircraft to equilibrium, in my many times used "Principles of Flight" JAR-FCL manual :-)

 

Thank you very very much for your excellent explanation!!!!! Austin needs to contract you ;-)