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4 Answers

Why doesn’t the aircraft stall while performing aerobatics?

Asked by: 3621 views Aerodynamics, General Aviation, Student Pilot

hello We know stall is a phenomenon independant of airspeed. it is dependant on angle of attack.But when we are performing aerobatics, donot we cross the critical angle of attack? Just take example of simple LOOP, we pitch up the aircraft and obviously it crosses the critical angle of attack. But the aircraft does not stall.what is the phenomena behind this that the aircraft doesnot stall in aerobatics. Your guidance will be highly appreciated

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4 Answers

  1. Best Answer


    Steve Pomroy on Jul 25, 2012

    Hi Khoda.
     
    You’ve hit on a key point of understanding in aircraft control, and a common point of mis-understanding.
     
    Angle of attack (AOA) is the angle between your wing’s chord and the relative airflow.  Pitch angle is the angle between your longitudinal axis (as viewed from the side) and the horizon.  The two are not the same.  During flight, our visual reference is the horizon, we can’t actually see angle of attack.  Many larger aircraft have AOA indicators, but in small airplane, we use airspeed as a (not very good) proxy for AOA.
     
    In cruise flight (i.e. – straight and level), our pitch angle is quite a good indicator of our AOA.  However, during climbs the relative airflow is from above, so our AOA is reduced as compared to our pitch angle.  Likewise, during a descent, the relative airflow is from below, so our AOA is increased as compared to our pitch angle.
     
    With regard to your specific note about loops:  During a loop, we pitch through 360 degrees, but our flight path also changes through 360 degrees.  So our AOA may be quite high (but not stalled) on the entry, but is generally much lower over the top of the loop, and then high again on the exit.
     
    Cheers,
    Steve
    http://www.flightwriter.com
    http://www.skywriters.aero

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  2. Bob Watson on Jul 25, 2012

    That said, sometimes they do stall, and it’s generally not a pretty sight. As Steve said, in a loop, the AOA is higher at the bottom of the loop. If you try to pull out too quickly, the plane can enter an accelerated stall (like you might encounter if you stall from a steep turn).
     
    The key point is that an airplane can stall at any attitude and any airspeed, but always at the critical angle of attack. Unfortunately, as Steve said, you can’t see the AOA. They make an instrument for that, but it’s not found in most light planes. Having one would make some aspects of maneuvering so much easier.

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  3. Nathan Parker on Jul 25, 2012

    “we pitch up the aircraft and obviously it crosses the critical angle of attack.”
     
    No, it doesn’t.  The stall only happens if your pitch rate exceeds the rate at which your flight path increases.

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  4. Sam Dawson on Sep 25, 2012

    It was always interesting taking CFI applicants for spin training. One of the maneuvers I would have us do was a loop as seeing the loop, seeing the airspeed and listening to the stall horn was a good demonstration of angle of attack and energy. We would enter the loop at 120 MPH and pull aft about 5 Gs. Pull too little and the airplane will run out of energy before the top; pull too much and you get an accelerated stall. Ideally I would just hear the stall horn as I would be just a little prior to critical angle of attack.
    At the top of the loop we would unload the airplane and float over the top. Airspeed would be about 30 MPH- yet no stall horn.
    As we start pulling again on the second half of the loop the stall horn comes back on as we would load the airplane. At the bottom of the loop as pointed out the nose is pointed down, we are approaching 120 MPH again and the stall horn is on as we are approaching critical AOA (but hopefully not exceeding it).

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