Climb at constant pitch attitude

[manucordier]

Hello,

If an aircraft is climbing to cruising level at constant pitch attitude (= constant body attitude ??) with max climb thrust (no supercharger) the following parameters change that way :

* Flight path decreases,
* angle of attack increases,
* IAS decreases

* IAS :
I understand that for a constant body attitude ant full power, at a higher altitude we'll have a lower IAS because the thrust is decreasing as altitude increases however the Drag may be assumed constant (which I believe is not 100% correct because Rho decreases thus D too). Thus if you want to maintain IAS you must lower the nose. And if you don't, IAS decreases.

* Flight path :
The flight path will decrease because (Thrust - Drag)/Weight decreases as altitude increases and thus the angle of climb (= flight path ??) decreases.

But what about the Angle of attack ? Why does it increase ?

Could someone confirm that my understanding is correct for IAS and Flight path. And could someone explain the change in angle of attack ?

Thank you,
Emmanuel Cordier.

[Ayotteted]

Aerodynamic stalls depend exclusively on angle of attack. As altitude increases, air density drops, which causes the wings to generate less lift. To maintain level flight, either angle of attack or speed must be increased with increasing altitude. If speed is increased, AoA remains constant, and so no stall occurs. If AoA is increased, speed remains constant, but there is a risk of stalling.

Speed in these cases means true airspeed, not indicated airspeed. Because indicated airspeed closely tracks air density, the indicated airspeed at which an aircraft will stall, all else being equal, remains fairly constant at all altitudes. The true airspeed increases.

So if you're talking about indicated airspeed, the answer to your question is generally no. If you are talking about true airspeed, the answer is yes.

Things become more complicated at transonic and especially supersonic speeds.

[FakePilot]

I'll guess AoA is increased because the vertical component of airflow is reducing because of climbing slower at higher altitude?

[manucordier]

FakePilot, yes but the thing is that IAS and maybe TAS (I guess) decrease too because of engine power decreasing. Thus the horizontal component of the relative wind must decrease.

Thus if the vertical component of the relative wind decrease and the horizontal component as well, what will be of the Angle off attack ?

Emmanuel.

[UncleNobby]

If you take off and assume constant pitch attitude eventually you will reach an altitude where that pitch attitude will only yield level flight. Therefore the AOA is greater at that altitude than it was at take off (nose was more aligned with flighpath on T/O, nose high to flight path as altitude increases).
You will also need to increase elevator pressure throughout the climb to maintain the constant pitch since velocity is decreasing.

[manucordier]

Thanks UncleNobby ! Its clear and concise !

Emmanuel Cordier.