How pitching up when decreasing airspeed maintains level flight if pitching up causes further decreased airspeed??

12 Answers

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   Mark Kolber on Oct 10, 2013

   I think you are mentally separating events that go together and adding something that isn’t there.

   Power is reduced. With the reduced power, the airplane will need to either go down or slow down. If you choose slow down, you will increase pitch to compensate. Once you do, you are simply creating a new steady-state flight equilibrium. The same condition that existed before you reduce power, although at a lower airspeed.

   Your extra step is postulating that additional changes take place all by themselves beyond that point, in which case any hands-off trimmed level flight would be impossible

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   AviatorAston on Oct 10, 2013

   Thanks for the answer, Mark. I understand that you are creating a new steady-state flight equilibrium by pitching up, I just don’t understand how that equilibrium is achieved. When you reduce power but don’t change your pitch angle, the plane will slow down and therefore lift will decrease and weight will be in excess of lift and the plane will start to descend. To counteract this you can pitch up thereby increasing the angle of attack and increasing lift, thus creating a new steady-state flight equilibrium as you mentioned. However, when pitching up, doesn’t that cause airspeed to decrease even further? Meaning it essentially becomes a never-ending cycle. You decrease power, airspeed decreases so lift decreases. You pitch up which increases AOA and increases lift thereby canceling out the previous reduction of lift, however, the act of pitching up causes airspeed to decrease even further and now lift went down again. So basically it’s lift goes down (due to decreased power and airspeed, lift goes up (due to increased pitch), but then lift goes right back down (due to airspeed decreasing from pitching up). Basically the pitching up action in my mind seems like a double-edged sword. You increase lift by increasing AOA, but at the same time decrease lift by causing airspeed to decrease by pitching up (unless you add some power of course, which is not the case here), so you get a zero net effect on lift. I know that’s not what happens in reality, I just don’t WHY that’s not what happens, since what I’m thinking seems how it would work logically. Unless I’m completely missing something here.

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   Mark Kolber on Oct 10, 2013

   >>However, when pitching up, doesn’t that cause airspeed to decrease even further?

   Only if you pitch up more than necessary to create the new level flight equilibrium.

   I’m not sure what you are doing in analyzing it. It seems as though you are separating pitch, power, lift, drag, and AoA rather than seeing how they combine to produce a given performance profile.

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   Mark Kolber on Oct 10, 2013

   I really can’t identify the fallacy you are working from but I’m reminded of the logical analysis that says you can’t ever cross a room from one side to another since you have to cross half the room first, then half of the remainder, then have of the new remainder, then half…. ad infinitum.

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   AviatorAston on Oct 10, 2013

   Thanks. I might be looking at it too literally. I guess I was just trying to view it in regard to the physics statements behind flight. “Increasing airspeed increases lift”, “Increasing AOA increases lift (up to critical AOA of course)”. So I just did a sort of mathematical equation-type problem in my head. I said ok, but how does increasing pitch and AOA increase lift if pitching up causes airspeed to decrease which decreases lift (given that you didn’t increase thrust)? Basically you have one action (pitching up) which has 2 net effects. Increasing AOA thereby increasing lift, and causing airspeed to drop thereby decreasing lift. So in my mind the simultaneous increase and decrease from that one action of pitching up cancel each other out. Thanks for taking the time to help me out. I really appreciate it. Maybe I need to fly a REAL plane and see this in action lol

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   Bob Watson on Oct 10, 2013

   I think I see what your analysis is missing…

   Assume you’re in level flight. You reduce power a little. The plane descends in an attempt to maintain the previous airspeed. Without changing anything, this becomes the new equilibrium: a constant descent. (The reason being it takes both lift and thrust to maintain level flight. You altered that by reducing thrust. So the plane descends, but at the level-flight airspeed. The “Go down” option.)

   Now, you want to level out with the reduced power setting. So you pull back to increase the angle of attack. This new AOA increases lift as you say, however it also increases drag, so the airplane slows down. Now this also reduces lift, so you pull back a little more to compensate again. What you’re analysis is missing is that each subsequent adjustment to your pitch will be smaller and smaller and reduce in smaller and smaller changes to the airspeed. After a few corrections (the final ones being too small to notice, perhaps), you’ll reach a new equilibrium with the reduced power, a reduced airspeed, and level flight (The “slow down” option).

   The caveat to this is that this only works on the front side of the power curve. On the back side, you can still reach equilibrium, but control movements are different.

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   AviatorAston on Oct 10, 2013

   Lmao that’s probably what I’m doing. Taking it out of context and making it way more complicated than it is. I guess I’m just a step-by-step mathematically thinking type of guy, ya know? 4 + 4 – 4 = 0. That’s how I’m looking at the actions taking place in the plane. Lift goes down by “10” (decreasing airspeed), lift goes back up by “10” (pitching up), lift goes back down by “10” (airspeed decreased by pitching up. Net effect = same point you were when you first decreased power and therefore speed. The plane is now descending. That’s not what happens in real life, but when I think of it in that mathematical sense in my mind, that’s how it should work to me.

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   AviatorAston on Oct 10, 2013

   Bingo Bob! Thanks so much. I think I understand what you’re saying. I was thinking of each action having the same “nominal value”. Like lift drops by “10”, you pitch up and lift now increases by “10”, pitching up causes increased drag so lift goes back down by “10”, and you get stuck in this never-ending cycle thinking of it like that. With the subsequent adjustments getting smaller and smaller, the numbers aren’t always going up and down by “10” (a made-up number that I created to represent what’s happening with lift haha).

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   Physicist on Oct 10, 2013

   Now Try to Answer this question:

   When Airspeed is decreased in a turn, what must be done to maintain level flight?
   A. Decrease the angle of bank and/or increase the angle of attack.
   B. Increase the angle of bank and/or decrease the angle of attack
   C. Increase the angle of attack.

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    Brian on Oct 10, 2013

    “If power was reduced but pitch was not changed, airspeed would of course be reduced”

    No. Instructors and pilots argue to the ends of the earth over whether to teach pitch for airspeed power for altitude or power for airspeed pitch for altitude. If I had to bet on it, you were taught the later. For practical applications there may be an argument for either or.

    However, if you care to analyze what is physically happening you need to understand that a change in power, all else being equal, does one thing and one thing only: it controls your rate of climb. Or descent, as it were. Rate of climb is mathematically equal to power minus drag divided by weight. Increase power, increase climb, and vice versa.

    The change in airspeed, in your example, exists because you change the airplanes angle of attack so as to hold altitude. Remember that equation from last paragraph? Rate of climb equals power minus drag divided by weight? You’re weight can be considered constant for this analysis, so ignore it. Now the question becomes, what happens to drag as your airspeed is reduced?

    “Airspeed is decreased so lift is decreased.”

    You’re skipping a key point in your analysis. You are analyzing an aircraft in level flight that reduces power and subsequently slows down by increasing angle of attack to maintain level flight. (You lowered power so you lowered drag..remember that rate of climb formula!)

    Back on track, what you’ve missed is the basic definition that answers the question: What is level flight? Level flight is a condition where what equals what?

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    Bob Watson on Oct 11, 2013

    One rule to remember here, climb is the result of excess power (more than you need for a given flight condition) and descent is the result of insufficient power.

    If you want to climb (or stop descending) and you don’t (or can’t) change the power available (or delivered), you need to alter the flight requirements to need less power. The L/D curve shows how much power is required for level flight in a give plane (and weight and configuration) at a different airspeeds. On the front-side of the curve that means a lower airspeed, on the back-side, it means a higher airspeed.

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    ccwebb on Oct 14, 2013

    Too many times us humans forget about the 3rd dimension. It can either be this or that… up or down, forward or backwards. Then we try to explain what lift is!

    Bob – Nicely worded on Oct 10th.

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