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Aerospace The stronger the headwind the faster the plane flies?

  1. Aug 28, 2015 #1
    About a plane that increases its ground speed as the headwind intensifies

    Assuming a plane hovers (ground speed Vg=0) against a headwind with a velocity Vw1, is it possible for this plane to start moving (Vg > 0) just because the wind intensifies, increasing its speed from Vw1 to Vw2 > Vw1?

    It is known that once the plane, which can be in the reverse command regime, hovers against the Vw1 wind all the control surfaces and the power delivered to the propeller remain locked.

     
  2. jcsd
  3. Aug 28, 2015 #2

    mfb

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    What would you expect?
     
  4. Aug 28, 2015 #3
    Honestly, in the direct command regime I would not expect the plane to start moving forward as the headwind intensifies. However, if the plane is in reverse command regime (see definition: http://aviationglossary.com/region-of-reverse-command/) it is not clear at all what will happen.
     
  5. Aug 28, 2015 #4

    berkeman

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    You're asking kind of a convoluted question. From your link:

    So the higher power setting at low speeds is necessary because you have to use a higher angle of attack in order to fly that slowly.

    In your question, the headwind would help to reduce the angle of attack for low-speed flight.
     
  6. Aug 28, 2015 #5
    In both cases the angle of attack is the same. The plane is not allowed to move its control surfaces or change somehow its attack angle.
     
  7. Aug 28, 2015 #6

    russ_watters

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    If a plane that is hovering above the ground due to a headwind receives a stronger headwind, it will move backwards over the ground if nothing about the plane changes. Note though that because initially the airspeed over the wings will increase, it will rise as well.
     
  8. Aug 28, 2015 #7

    billy_joule

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    If all else is equal the planes airspeed remains unchanged. So the plane begins to move backwards to maintain the constant air speed:
    Vg new=Vw1 - Vw2
    Where
    Vg new = Planes velocity relative to the ground
    Vw1 = Original air velocity relative relative to the ground, the air speed at which the Vg was zero.
    Vw2 = New air velocity.

    Ever been on a treadmill? Imagine maintaining a constant walking pace and increasing the treadmill pace, the outcome should be obvious.
     
  9. Aug 28, 2015 #8
    Accordin to the definition: "Flight in the region of reversed command means flight in which a higher airspeed requires a lower power setting and a lower airspeed requires a higher power setting to hold altitude. It does not imply that a decrease in power will produce lower airspeed. The region of reversed command is encountered in the low speed phases of flight"

    In consequence, keeping the power constant and increasing the airspeed (headwind intensity) would determine the plane to rise, move forward or both. It seems illogical but this is my interpretation of the reverse command regime in the particular case of the hovering plane.
     
  10. Aug 29, 2015 #9

    mfb

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    It will not move forwards. Neglecting an initial transition period, it will behave exactly in the same way relative to the air as before - it will move with the initial slow speed. Add the stronger headwind and it will move backwards with a speed equal to the increase in headwind speed.
     
  11. Aug 29, 2015 #10
    The plane is in the reverse command regime. How many times do I have to repeat it.
    From the definition of reverse command it would appear that Drag decreases as the airspeed (headwind intensity) goes up and in consequence the plane will start to move forward. Also the propeller thrust might strengthen once the wind intensifies.
     
  12. Aug 29, 2015 #11

    mfb

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    It does not matter.
    The airplane does not care about the ground at all. "Relative to the ground" is completely irrelevant in this problem, it is sufficient to analze it relative to the air. With the result described above.

    If your airplane is in an unstable equilibrium (or not in equilibrium at all), then it can change its speed even without change in wind speed, but that is an ill-defined problem then.
     
  13. Aug 29, 2015 #12

    russ_watters

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    Being in the reverse command regime does not matter because you have stated that the commands are not changing. So there are no commands for the plane to react reverse to.
    No, that's not what the definition you quoted said. It says drag decreases in response to a throttle increase because the pilot drops the angle of attack: the drag is a function of the angle of attack (and airspeed). You're trying to flip the cause and effect in the definition over and apply it to a different situation where it does not apply.

    The reverse command regime is referring to a control scenario where when trying to maintain steady flight, the controls seem to have the opposite of the intended effect. That's why it is said that for pilots, when landing, they use the stick to control airspeed and the throttle to control altitude (sink rate). The force of drag will go up or down based on the plane's angle of attack. In the reverse command regime, increasing the angle of attack increases drag a lot, making the plane slow down and sink.

    The wind can have an impact on this, but only if you are changing the commands. If the wind gusts and you do nothing to compensate, both drag and lift increase and you slow down with respect to the ground (or move backwards in your example) and go up. But if you get a gust and lower the plane's nose to compensate, it will speed up (and may travel forward, depending on how much).

    Please tell me that this isn't an anti-Wright Brothers conspiracy you're trying to find ammunition for, because we had a thread like this a few months ago and that's what it turned-out to be. [edit] Oh - that was you. This thread is on a very short leash.
     
  14. Aug 29, 2015 #13
    The flight in the region of reverse command has nothing to do with changing the position of the control surfaces or the attack angle. It is all about power and airspeed.
    Definition: "Flight in the region of reversed command means flight in which a higher airspeed requires a lower power setting and a lower airspeed requires a higher power setting to hold altitude."

    Playing with the flaps you can easily have a plane that flies faster if a headwind exists than if the wind does not blow at all.
    For example, in the situation of a plane that has two flaps on each wing and flies in the absence of any wind, the pilot rises a bit the inner flaps and lowers the outer flaps breaking the plane and decreasing its speed. This moment a headwind starts to blow. The pilot aligns the flaps to the wings and put them in the neutral position making the plane more aerodynamic. Without changing the power delivered to the propeller the aircraft will accelerate, relative to the ground, despite the fact it has to fight a wind, this time.

    To simplify further, a plane towing a small parachute will fly in perfectly calm weather slower than the same machine keeping the power constant and flying against a headwind but without the parachute.

    So forget about maneuvering control surfaces and attack angles because by changing them you get what can be called a plane with different drag and lift characteristics that, of course, behaves differently.
     
    Last edited: Aug 29, 2015
  15. Aug 29, 2015 #14

    mfb

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    This violates your previous requirement that the airplane does not change anything.

    The airplane stays in the same mode, therefore its airspeed stays the same. It is as easy as that.
     
  16. Aug 29, 2015 #15
    Assuming nothing changes about the plane (power, angle of attack, the positions of the control surfaces) if Thrust = Drag and then somehow an increase in the airspeed lowers the Drag, instead of making it higher, the plane starts to move forward because Thrust becomes greater than Drag and so the airplane gets a positive acceleration relative to the ground (Thrust - Drag = m * a).
     
    Last edited: Aug 29, 2015
  17. Aug 29, 2015 #16

    russ_watters

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    Repeating it over and over again will not make it true. It is false. You are confusing cause and effect. At this point, you basically have two choices: listen to people who know what they are talking about, or don't. Either way, there is nothing more to discuss. Thread locked.
     
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