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13 Answers

Pitch effect on attitude, speed and altitude

Asked by: 6698 views Aerodynamics, Flight Instructor

Hey Guys!

been struggling with the following question for a while, would greatly appreciate some help and explanation *why*.

scenario:

cruising in my cessna- cruise attitude 2300 rpm trimmed for 95.

then I Pitch up say 1 inch of back pressure and hold it.

so initially plane starts climbing and slows down,  but what next?

how will the plane look in say 5 mins, with me still holding that 1 inch pitch up, what attitude speed and altitude will it have?

 

Thanks!

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13 Answers



  1. RobA61 on Jul 22, 2020

    To be exact, it depends on what your “1 inch of back pressure“ is referring to. One-inch of yoke deflection is difficult to translate to degrees of pitch. If you mean one-inch of pitch angle on the attitude indicator, that seems like quite a bit (15-20 degrees perhaps?). In either case it seems like a lot of pitch-up attitude.

    If the aircraft has enough power, airspeed will stabilize somewhere below your 95 cruise number and the aircraft will continue to climb (climb performance decreasing the higher you go). If the aircraft does not have enough power to sustain that climb, and you do not relieve the “one-inch of back pressure” at all, the aircraft will cease to climb and would potentially stall (assuming you maintained this back pressure indefinitely). Note that if you are referring to the attitude indicator with your one-inch description… you would need to increase back pressure as the aircraft slowed down, thus further aggravating the stall situation.

    If you can clarify your “one-inch” description I can better clarify the answer.

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  2. leha007 on Jul 22, 2020

    Hey Rob,
    Thanks for responding.
    I think he just means to pull the control column 1 inch back relative to its trimmed position at cruise attitude, or just apply a slight back pressure and keep holding it there without decreasing or increasing it.
    I guess the question refers to how pitch changes the forces acting on the plane and what would eventually happen when I apply any back pressure (pitch up) and the elevators are deflected upwards relative to cruise.

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  3. RobA61 on Jul 22, 2020

    In a scenario where the control column is pulled back to initiate and maintain a climb, the aircraft will continue to climb so long as there is excess thrust (power) available to do so. This available power decreases with altitude (i.e. you won’t have the same climb performance uniformly as you go up).

    Concerning forces on the aircraft, during the transition from straight/level flight to climb, angle of attack momentarily increases and thus lift momentarily increases. At that moment, lift is greater than it’s opposing force (weight) and the aircraft climbs. The simultaneous increase in thrust/power (as you throttle-up to climb power) gives the aircraft excess power to continue that pitch attitude and continue the climb, until it can no longer provide that excess power (maximum service ceiling due to atmospheric conditions).

    There is a good visual on the steady-state forces in a climb here: https://www.flightliteracy.com/wp-content/uploads/2017/11/5-2.jpg

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  4. leha007 on Jul 22, 2020

    Hi Rob!
    Yes in that case the outcome is obvious to me 🙂
    The problem is- his question is, I apply that slight back pressure to deflect the elevators up, hold id like that and do NOTHING else. No throttle, no trim, just slight up deflection relative to what it has been in cruise.
    What happens to the plane then?
    Initially yes, begins climbing and speed decrease, but how will it look in 5 mins?
    Thanks!

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  5. RobA61 on Jul 22, 2020

    Well the exact answer to your question, “what happens after 5 minutes”, requires a lot of specific known parameters (i.e. atmospheric conditions, what type of Cessna you’re flying, how much payload you have… the list goes on!). But the basic answer to your question (assuming as you say, no power increase), is that the aircraft will eventually not be able to sustain the pitch attitude at some point. The less power you give it, the sooner that happens. The aircraft will want to decrease angle of attack as your speed decreases towards stall speed. If you want to maintain the same pitch attitude regardless of the impending stall, you will need to continue applying more back pressure. At some point you reach the critical angle of attack and a stall will occur. How quickly that happens depends on the assumptions/parameters I mentioned earlier.

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  6. leha007 on Jul 22, 2020

    I feel like we are getting there 🙂
    So you are saying that if I want to continue the same pitch attitude I”ll need to pitch up more, and then we will stall.
    My question is- say i pitch up slightly, and “lock the elevators”. no more pitch up or down.no more power change. no more trim. just put the control lock in the control column with the elevators slightly deflected upwards, and don”t touch anything else?
    will the speed slowly decrease until I don”t have any more excess thrust to climb?
    and what will happen then since the elevators are still locked in that “slight up position”. will the plane just increase it’s angle of attack, stall and pitch down even with the up deflection of the elevators?

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  7. Richard Eastman on Jul 22, 2020

    This is, in fact, a pretty simple answer. If the elevator is pulled back one inch … and held there — then it will have the same effect as trimming the nose up 1 to 1-1/2 strokes of the trim tab. The airplane nose will come up … the airplane will slow down … the airplane will start to sink and gain speed … as it gains speed the nose will come up … i.e. — the airplane will oscillate up-and-down for a period of time and then stabilize at a somewhat slower airspeed than the originally identified 95 mph. It will not stall unless additional pull-back on the yoke is added. When it stabilizes at the new slower airspeed, the airplane would normally also start a slow descent … although if this is a Cessna with a constant speed propeller, you might just end up going slower (a function of how much nose-up pitch “one inch” give you).

    The fundamentals of flight prevail. Pitch controls airspeed … power controls altitude. If you pitch up and hold that new pitch steady — then airspeed will decrease. If you do NOT add incremental power at that point, then in most cases, the airplane will descend.

    The discussion points above distort the reality of the question asked.

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  8. Richard Eastman on Jul 22, 2020

    This is, in fact, a pretty simple answer. If the elevator is pulled back one inch … and held there — then it will have the same effect as trimming the nose up 1 to 1-1/2 strokes of the trim tab. The airplane nose will come up … the airplane will slow down … the airplane will start to sink and gain speed … as it gains speed the nose will come up … i.e. — the airplane will oscillate up-and-down for a period of time and then stabilize at a somewhat slower airspeed than the originally identified 95 mph. It will not stall unless additional pull-back on the yoke is added. When it stabilizes at the new slower airspeed, the airplane would normally also start a slow descent … although if this is a Cessna with a constant speed propeller, you might just end up going slower (a function of how much nose-up pitch \”one inch\” give you).

    The fundamentals of flight prevail. Pitch controls airspeed … power controls altitude. If you pitch up and hold that new pitch steady — then airspeed will decrease. If you do NOT add incremental power at that point, then in most cases, the airplane will descend.

    The discussion points above distort the reality of the question asked.

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  9. Richard Eastman on Jul 22, 2020

    This is, in fact, a pretty simple answer. If the elevator is pulled back one inch … and held there — then it will have the same effect as trimming the nose up 1 to 1-1/2 strokes of the trim tab. The airplane nose will come up … the airplane will slow down … the airplane will start to sink and gain speed … as it gains speed the nose will come up … i.e. — the airplane will oscillate up-and-down for a period of time and then stabilize at a somewhat slower airspeed than the originally identified 95 mph. It will not stall unless additional pull-back on the yoke is added. When it stabilizes at the new slower airspeed, the airplane would normally also start a slow descent … although if this is a Cessna with a constant speed propeller, you might just end up going slower (a function of how much nose-up pitch \\\”one inch\\\” give you).

    The fundamentals of flight prevail. Pitch controls airspeed … power controls altitude. If you pitch up and hold that new pitch steady — then airspeed will decrease. If you do NOT add incremental power at that point, then in most cases, the airplane will descend.

    The discussion points above distort the reality of the question asked.

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  10. leha007 on Jul 22, 2020

    So he gave me a clue that it has something to do with the trim system.
    again- imaginary scenario- we cruise 2300 rpm 95 kts TRIMMED for cruise.
    apply 1 inch of yoke back pressure, put the control lock in so it can’t move anymore and let go, don’t touch power don’t touch trim.
    What Happens and more importantly WHY?
    Thanks!

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  11. Mark Kolber on Jul 23, 2020

    I’m still not sure what heis trying to get at.

    You are trimmed for level flight at 2300 RPM and pull the yoke back one inch and lock it in place.

    The airplane is going to slow and *initiate* a climb at whatever airspeed and vertical speed that power setting allows for at that particular pitch attitude at the altitude where you were when you pulled the yoke back.

    As you continue to climb both the power you have available and the efficiency of your wings and propeller will decrease due to the increased density altitude, so both your indicated airspeed and your vertical speed will go down.

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  12. Mackey on Sep 06, 2020

    Why don’t you go do it?

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  13. Brian on Sep 09, 2020

    This stability demo will be stick fixed.
    Throttle and prop position fixed.

    From Trimmed and level pitch back some small amount and lock controls.

    For simplicity, gyros will be assumed to control roll and yaw allowing us to analyze stick fixed pitch stability by itself.

    We expect the aircraft to pitch up initially and exchange speed for altitude, then pitching down (positive static) to pick up speed, repeating these oscillations a few times till stabilizing at some slower speed in a climb. Exhibiting stick fixed static and dynamic longitudinal stability. This climb will be maintained till the ceiling is reached for this particular configuration.

    Now we get in and test this theory out!

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