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Question regarding AOA, Lift, and Airspeed

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Question regarding AOA, Lift, and Airspeed

Old 10-09-2013, 10:38 PM
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Default Question regarding AOA, Lift, and Airspeed

Hello everyone. My apologies if this ends up being a silly question without even knowing it. Thank you in advance for any and all answers to this question which I am greatly pondering about. It's my understanding that both airspeed and AOA directly affect lift. As airspeed increases, so does lift. Likewise, as AOA increases, so does lift (up to the critical angle of attack of course). Now, my question is this. Say an aircraft was traveling at 140 knots in level flight. If power was reduced but pitch was not changed, airspeed would of course be reduced, thereby reducing lift and causing the aircraft to now descend. From what I have researched, it is my understanding that at this point the pilot would need to pitch up to increase the angle of attack and thereby increase the lift to counteract the lift lost by the decrease in airspeed in order to maintain level flight. It's here where I have an issue. Airspeed is decreased so lift is decreased. Pitch is now increased to increase lift and compensate. However, doesn't airspeed decrease further as an aircraft pitches up?? Therefore it would go in this order. A decrease in lift due to decreased airspeed. An increase in lift due to increasing pitch and angle of attack to compensate. A decrease in lift once again due to a decrease in airspeed caused by pitching up. So basically you would be right back where you began when you first decreased power and thereby decreased lift. However, it's my understanding that this is not what happens. Decreasing airspeed in level flight and then pitching up to compensate will maintain level flight. How does that work based on what I explained?? Very confused.
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Old 10-09-2013, 11:12 PM
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Its not a never ending cycle. As you pitch up the you will get increased induced drag but the airplane will eventually reach a steady state condition where the drag will equal the thrust. Remember as you slow down you shed a lot of parasitic drag also, but gain only a small amount of induced drag.

This all assumes you are on the front side of the power curve though. On the backside, it takes more power to go slower because of the large amount of induced drag due to lift.
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Old 10-10-2013, 02:12 AM
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Thanks for your reply, Knotcher. Very much appreciated. Haha that's exactly how I was thinking of it! As a never-ending cycle. Funny that you put it that way. I was thinking of it in a "linear" fashion. Meaning if I were to put lift as a numerical value, what was happening in my mind was basically the following. As an airplane in level flight slowed down, lift decreased by a value of say "10". To compensate and maintain level flight, the pilot pitches up and increases lift by "10", balancing out the previous decrease of the same value. However, because airspeed was reduced further in the act of pitching up, lift was again decreased by "10", so you're back where you started when the airplane first slowed. There's the "never-ending cycle" haha. I see that it doesn't work like that when taking into account the effects of parasitic and induced drag. Basically what I'm getting from what you're saying is that while lift is decreased as an airplane slows, its parasitic drag is greatly reduced. When the pilot pitches up to compensate for the decreased lift, lift is increased due to an increased AOA, however induced drag is only increased by a small amount. So in the end lift is re-established to equilibrium while you still basically had a net loss in drag since parasitic drag was greatly reduced in the slowdown while induced drag was only increased by a little while pitching up. Therefore while lift is re-established to where it was before the airplane slowed down, the airspeed was not greatly affected by pitching up. Am I getting that right? Anyone else who reads this thread and would like to chime in, please feel free to do so. Any input is greatly appreciated.
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Old 10-10-2013, 08:30 AM
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That is exactly what I am saying. Look at the speed versus power curve. On the front side of the curve the slower you go the less power is required since you get a net loss of drag (large drop in parasitic, small gain in induced).

For example, lets assume you are at 150 knots and your drag values are:
Parasitic drag = 100
Induced drag = 10
Total drag = 110

Now if you slow to 130 knots:
Parasitic drag = 60
Induced drag = 15
Total drag = 75

Thus you have a net loss in drag when you slow.

Parasitic drag increases with the square of velocity, so just a small increase in speed means a big increase in drag and vice versa.

And on the backside of the curve it is opposite, induced drag becomes exponentially bigger the slower you go.
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Old 10-10-2013, 01:18 PM
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Here Aston, watch this video, it will show you how increasing angle of attack and changing power settings will affect lift. During some of it, he's got smoke on, so you can follow along more easily.

At one point he actually stops the airplane in mid air and just sits there for about 5 seconds, hardly moving.

Rob Holland's 4-Minute Free at IAC US Nationals 2013 - YouTube
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Old 10-10-2013, 07:39 PM
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Phenomenal performance. Thanks for posting this.
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Old 10-11-2013, 03:27 AM
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My favorite move was his double hammerhead.

I've done lots of hammerheads, in lots of different airplanes, but I never thought to try to keep it going all the way over and around again! I had to watch that a couple times and the whole time I'm thinking, how does he do that??!
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Old 10-13-2013, 02:13 AM
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Can someone explain how the angle of attack changes with altitude and if trying to hold a constant Mach, True or Indicated airspeed. If trying to maintain a constant mach on a climb, TAS would increase along with Mach (but only because of cooler temp), but IAS would decrease, not sure if the angle of attack would increase or decrease. I think it would decrease. What about for a constant TAS in a decent?
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Old 10-13-2013, 11:25 AM
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Originally Posted by jsfBoat View Post
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)

Last edited by Cubdriver; 10-13-2013 at 06:27 PM. Reason: clarity
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Old 10-15-2013, 11:12 AM
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There was a statement earlier in this thread that with a constant Mach, TAS would go up with decreasing temperature. This is backwards. Holding a constant Mach (without regard to altitude) TAS varies directly with temperature. Higher temperature equals higher TAS.

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