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Airplanes and Windspeed

  1. Jul 10, 2004 #1
    Hey Everyone,
    I found myself in this scientific debate with my father on our way to the airport about the effect of wind speed on the airplane's velocity. He claimed that if the wind was in the exact direction of the plane, the plane's new speed would be the sum of both speeds. I didnt really accept that, as I think of the wind a force that affects the plane, and does not necessarily induce the same speed. Though he claims it's similar to a boat in a river, where we add the speed of the current to the boat. That does make sense, but I'd like to hear it from your prespective.
    Thanks,
    A.Z. Husseini (aka Xeno)
     
  2. jcsd
  3. Jul 10, 2004 #2

    Doc Al

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    I'd say your father is correct. The analogy with a boat in the river is a good one.

    Think like this: The plane moves at a certain speed with respect to the air. And if the air is moving at some speed with respect to the ground, then the speed of the plane with respect to the ground is [itex]V_{plane/ground} = V_{plane/air} + V_{air/ground}[/itex].

    If the air moves in the same direction as the plane, the two speeds add; if the air moves against the plane, the speeds subtract.

    Let me know if that makes sense.
     
  4. Jul 10, 2004 #3
    Hey Doc Al,
    Thanks a lot for your quick reply, now it really makes sense..
    Turns out I was confused because I thought of the case in which wind comes in a direction not parallel to the plane. We'd have to use vectors in this case, and we get different speeds. :biggrin:
    Thanks again for your explanation, now I can sleep in peace. :zzz:
    Xeno
     
  5. Jul 10, 2004 #4

    chroot

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    Yes, the wind definitely affects airplane velocities. In fact, learning to understand and deal with winds is one of the biggest components of flight school and becoming a pilot. When you're in a headwind, the wind is blowing right at you, and your ground speed is your airspeed minus the windspeed. When you're in a tailwind, your ground speed is your airspeed plus the windspeed. When the wind is blowing at an arbitrary angle, it pushes you off the course you intend to hold -- even if the nose of your plane is pointed due west, for example, the wind can be blowing you northwest. Pilots have to deal with all of this every day.

    - Warren
     
  6. Jul 10, 2004 #5

    Clausius2

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    I'm rather in disagree with your father. It would be a great theme to thread in the Aerospace forum. In my point of view, your father would be right if there is no slipping between nearest molecules of the air and aeroplane external structure. Turbulence makes impossible a perfect velocity transmission, also with the boat example. But we are suppossing that it is not a jet engined aeroplane, aren't we?.

    MMM... Let me see..
     
  7. Jul 11, 2004 #6
    would the same thing happen with a person running in the wind....i definitly think not...but that is different then thinking about an airplane or a boat right.....unless the earth's surface were moving...then you could add the speeds, am i correct
     
  8. Jul 11, 2004 #7

    chroot

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    Well, a person running on the ground has physical contact with the ground! An airplane only pushes against the air.

    - Warren
     
  9. Jul 13, 2004 #8

    LURCH

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    Yes, you are correct; for a person running on the ground to be a good annalogy, the ground would have to be moving, and then we could add the volocities. A good example would be someone running 5kph on a treadmill; the treadmill is going -5khp relative to the "stationary" observer, while the runner runs +5khp. Net result: nuttin'!
     
  10. Jul 14, 2004 #9
    I disagree. The moving air column becomes the new frame of reference for the airplane. Essentially, once an airplane enters a new moving air column it eventually loses all information about the previous air column. It's speed adjusts so that eventually it thinks that the surrounding air is moving as normal. So the speeds are truly additive.

    Another way of thinking of it is to ask the following question.

    Suppose an airplane flies at 300 mph in non-moving air. Now it enters a tail wind of 40 mph. If the airplane is now only flying at 320 mph, what is keeping it from flying the additional 20 mph? It can't be drag, because the airplane is in a tail wind.
     
  11. Aug 7, 2004 #10
    The airplane flies at 300 mph in the "tail wind" as well. To an observer on the ground the machine would be flying at 340 mph.
     
  12. Aug 7, 2004 #11

    Janus

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    The thing to keep in mind is what one means by the "plane's velocity". If you are talking about the reading on the airspeed indicator, you are talking about the plane's speed with respect to the column of air that in is presentally in.

    The airspeed indicator works like this:
    There is a pitot tube that sticks out from the plane, pointing forward . The forward movement of the plane forces air into this pitot and into a pressure chamber. the faster the air speed of the the plane, the greater the pressure in the chamber. The pressure difference between the this chamber and the static air pressure is used to determine the air pressure. Note that if the plane is stationary with respect to the air, these two pressures are equal and the air speed indicator reads zero.

    If the column of air the plane is traveling in is moving at some speed with respect to the ground, then the plane's ground speed will be a vector addition of the planes airspeed and wind speed. At tail wind will give the plane a ground speed of its air speed and the tail wind, and a head wind will give it a ground speed of its airspeed minus the head wind.

    One caveat:
    The indicated reading on your airspeed indicator is not usually your true airspeed. this is because that it is calibrated for standard temp and pressure (29.92 in of barametric pressure & 15°c) when ambient conditions vary from these, the accuracy of the airspeed indicator suffers. For instance, at high altitudes, where the air is thinner, it takes a greater airspeed to create the same pressure difference in the pitot tube than it would at sea level. Thus at high altitudes, your indicated airspeed (IAS) would be less than your actual true airspeed (TAS). For instance, if you were flying at 5000 ft on a day when the barometer read 29.92 and the temp was 15°c, and your airspeed indicator said you were doing 100 knots(IAS), your actual true airspeed(TAS) would be 110 knots.

    This doesn't change anything about the addition of airspeed and windspeed. It just means that you have to make an adjustment from what you read on your airspeed gauge to get your actual speed with respect to the air.
     
  13. Aug 9, 2004 #12
    That's what they teach in flight schools, that's what they use for navigation instruments and charts, and that's what pilots use to decide whether to take off or not in heavy winds.
     
  14. Aug 10, 2004 #13

    russ_watters

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    Yet another caveat - lower indicated airspeed means lower lift (as if the plane were actually flying slower), so indicated airspeed is actually generally more useful than true airspeed.
     
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