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This may be a stupid question

  • 28-03-2013 2:24am
    #1
    RealExpert
    Registered Users, Registered Users 2 Posts: 125 ✭✭


    If an aeroplanes required takeoff speed is 120mph and if its taking off into a headwind of 60mph does this mean that the actual takeoff speed is 60mph or will its takeoff speed always be 120mph regardless of the headwind speed?


Comments

  • #2
    McNulty737
    Registered Users, Registered Users 2 Posts: 226 ✭✭McNulty737


    What counts is the speed of the airflow over the wing (dynamic pressure, or 'indicated airspeed'), not the speed of the aircraft over the ground (ground speed). Speed in aircraft is measured very differantly than how its measured in cars.

    So with a 60mph head wind you would have a shorter take off roll and a lower groundspeed when the required indicated airspeed (120mph or kts as its measured in aviation) is reached.


  • #3
    amen
    Registered Users, Registered Users 2 Posts: 2,781 ✭✭✭amen


    (120mph or kts as its measured in aviation)

    1 km though is not equal 1 kts.

    A good rule is 1kt = 1.85 km/h


  • #4
    coylemj
    Registered Users, Registered Users 2 Posts: 25,624 ✭✭✭✭coylemj


    RealExpert wrote: »
    If an aeroplanes required takeoff speed is 120mph and if its taking off into a headwind of 60mph does this mean that the actual takeoff speed is 60mph

    You could have phrased the question better, I think this is what you meant to ask....

    If an aeroplanes required takeoff airspeed is 120mph and if its taking off into a headwind of 60mph does this mean that the actual take-off ground speed is 60mph[/QUOTE]

    The answer is 'yes'.


  • #5
    LeftBase
    Registered Users, Registered Users 2 Posts: 743 ✭✭✭LeftBase


    RealExpert wrote: »
    If an aeroplanes required takeoff speed is 120mph and if its taking off into a headwind of 60mph does this mean that the actual takeoff speed is 60mph or will its takeoff speed always be 120mph regardless of the headwind speed?

    The speed shown on the "speedometer" in an aircraft is Indicated Airspeed. This is the Dynamic Pressure or the speed of the air over the wings. To take off an aircraft needs the air to be moving over the wings at a certain speed. Lets say for example that speed needed for take off is 100 Knots Indicated Airspeed. Lets also say that we have a 10 knot wind blowing straight down the runway.
    When you line up on the runway and hold in position the Airspeed Indicator will show 10 knots as the 10 knot wind is blowing over the wings even though you are not moving. You power up the engines and accelerate down the runway until the air is moving over the wings at 100 knots. You can then lift off the runway safely and climb.

    Groundspeed is the speed the aircraft moves over the ground. Indicated Airspeed is the speed of the air moving over the wings. If there was no wind in our above example the aircraft would be moving at 100 knots(nautical miles per hour) when it lifted off. However because of the 10 knot headwind it is moving at 90 knots when it lifts off.

    This principle explains why often you travel quicker going West - East while flying transatlantic. Generally speaking the winds blow from the West and so the aircraft's speed over the ground is higher than on the East - West crossing.

    Example: Airspeed of 320 knots with a 50 knots headwind in flight means that you cover 270 nautical miles in an hour.
    However if you were flying the opposite way and had a 50 knot tailwind you would cover 370 miles in an hour.

    The above is the basic principle laid out. There are more in depth issues to look at when you talk about airspeed at high altitudes etc. However they do not change the above all that much and are beyond the scope of what was asked.


  • #6
    bombs away
    Closed Accounts Posts: 291 ✭✭bombs away


    Dont mind any of that nonsense.

    This is all you really need to know


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  • #7
    eatmyshorts
    Closed Accounts Posts: 821 ✭✭✭eatmyshorts


    But what if there was a conveyor belt?....


  • #8
    sunnysoutheast
    Registered Users, Registered Users 2 Posts: 2,072 ✭✭✭sunnysoutheast


    eatmyshorts wrote: »
    But what if there was a conveyor belt?....

    Ha ha do you remember that legendary thread from the football365 forum about whether a plane could take off from a massive treadmill, it went on for ages

    I'd love to know if it was archived somewhere, it was a sad day when that forum went members-only


  • #9
    LeftBase
    Registered Users, Registered Users 2 Posts: 743 ✭✭✭LeftBase


    sunnysoutheast wrote: »
    Ha ha do you remember that legendary thread from the football365 forum about whether a plane could take off from a massive treadmill, it went on for ages

    I'd love to know if it was archived somewhere, it was a sad day when that forum went members-only

    They did it on mythbusters

    http://www.youtube.com/watch?v=YORCk1BN7QY


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