Originally Posted by
jsfBoat
Can someone explain how the angle of attack changes with altitude...
Speaking in generalities, higher AoA is associated with higher altitude.
... and if trying to hold a constant Mach, True or Indicated airspeed...
For an airplane in steady flight, TAS and Mach go up with altitude while IAS goes down. TAS is actual speed in the air, as well as over the ground assuming still air. As the air thins out at altitude the airplane can go faster for a given thrust. This is one of the advantages of higher cruising altitudes- drag is lower and forward movement is easier. The flip side is that lift is also not as great for a given forward speed. This is because thinner air can't do as much work creating lift as dense air. The airplane compensates by flying at a higher AoA. Depending on the design criteria of the wing it may be a nice bargain in terms of energy use but both effects are there, lower drag and lower lift at higher altitude.
...If trying to maintain a constant mach on a climb, TAS would increase along with Mach...
At constant Mach you would not have changing mach, so maybe you mean "if trying to maintain constant mach in a climb, TAS would have to decrease". Remember, mach is the ratio of aircraft speed to the speed of sound. Both items change in a climb- TAS tends to rise while speed of sound comes down. For this mach ratio to remain the same in a climb, only TAS can be altered because diminishing speed of sound is a physical fact given a climb. So if you want as a thought experiment to try for a constant mach ratio in the climb, you will need to slow the airplane down as you go. This is not something most aircraft have an interest in doing of course, they want to go fast, so it is only done in thought experiment.
...but IAS would decrease...
Correct. Lower density and thus lower dynamic pressure is associated with thinner air.
...not sure if the angle of attack would increase or decrease. I think it would decrease...
No, increase. You need more AoA as the air gets thinner, because density is part of the equation for dynamic pressure (rho). To simplify it a little, thinner air does not "blow as hard" as thicker-denser air. AoA makes up for the loss.
...What about for a constant TAS in a decent?
If you want to maintain constant TAS as the air gets thicker and denser, you are going to need a lot more thrust to maintain the same forward speed. Drag is greater and more difficult to overcome in denser air. However, AoA will go down as well per the tradeoff, so the net result is still in question, it may be better or worse depending on the particular aircraft wing. The net plus or minus for the lift-drag-speed plot depends on the design of the airplane, which is why it is important to cruise at the correct speed and why aerodynamics takes a bunch of college courses to really understand! It's too complex to go through all the variables here, but the formal subject of aircraft performance examines these relationships at length. You can get the basics of performance easier than anything else, especially if you pass over the math and stick to generalities. The math is actually not that hard either.
The main thing to grasp here is dynamic pressure (q) -it goes down with altitude increase and goes up with altitude decrease. The pieces of "q" are density and the square of airspeed (.5*rho*V^2). So for a given speed, density drops with altitude and consequently "q" drops as well. Thinking of various other relationships is easier with this as your fundamental clue.
dynamic pressure (wikipedia)