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Plane on a treadmill

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Comments

  • #62
    Biro
    Closed Accounts Posts: 4,091 ✭✭✭Biro


    Celtic Mech wrote: »
    Here's my 2 cents...now back to Basic Aerodynamics that was studied a few years back!
    i think firstly we have to question how any aircraft produces Lift. The shape of the airfoil is designed in such a way that the air passing over the wing causes a drop in pressure. The pressure below the wing is Higher than that of the pressure Above the Wing...it is this pressure difference that is the fundamental in causing Lift. It causes an upward force from the High Pressure below to the Low Pressure above.
    What is essential in causing this Upward force is the relative Airflow to the Airfoil...i.e the Speed of the Airflow.
    Now take our treadmill query. (In this example we will ignore friction). Lets say we have an Airbus A320 with a Take off speed of 140 Kts. Treadmill Spins rearwards to a speed of 140Kts. In return to keep the aircraft Stationary on the treadmill, we must have the engines produce Thrust, thus allowing us to combat this rearward 140Kts. Ok, engines up to take off power, aircraft is now stationary. rearward force of treadmill is equaled to be the Thrust of engine in Fwd Direction. We MUST now look at the Lift needed. As we go back to basic Aerodynamics, for an aircraft to Take Off (i.e. Climb) Thrust Must overcome Drag AND Lift MUST Overcome Weight! As i have shown above, we know how to produce Lift..its the airfoil shape and the Airflow in relation to this...i.e. the relative wind. basically, with the aircraft on the treadmill, we have a Stationary Aircraft..i.e The speed of the air over the upper surface is ZERO. The pressure above and below the Wing is the same. no lift is created. The Aircraft Cannot Take off!!!
    Looking back thru the posts, a lot of people are getting hung up on the speed of the wheels. they have nothing to do with it. Wheels are being spun up to 140kts but the actual airframe is going nowhere as its stationary. I think some are confusing the situation with Momentum. if, all of a sudden we stopped the treadmill, the aircraft would immediatly be propelled forward and may even take off but it will NOT take off from a stationary position (Before i get lept on...Im taking an example of a passenger jet...i know there are military jets that can take off from stationary etc).
    What we must remember is that when an A320 or B737 is tearing down the runway at V1...the wheels are spinning at a speed of 140Kts BUT the Airflow against the Wings is ALSO going at 140Kts due to the Fwd motion of the Aircraft. The Fwd momentum of the aircraft...Trust has overcome Drag i.e the Engines accelerating us to our rotation Speed, but more importantly the 140kts of the airflow is causing the Low Pressure area Above the Wing Causing Lift. it will be the Positive Forces of Lift and Thrust that will overcome the negative Forces of Drag and weight allowing us to Take off.

    Read the whole thread.
    Your post is wrong.


  • #63
    Celtic Mech
    Registered Users, Registered Users 2 Posts: 246 ✭✭Celtic Mech


    Sean_K wrote: »
    No-one's saying it will take off stationary. What people are saying is that the motion of the treadmill will not significantly affect the ability of the plane to gain speed. It will move at the same speed relative to a stationary point and exactly twice that speed relative to the treadmill.

    The wheels serve only to remove friction between the plane and the runway. If I hold a toy car on a treadmill, it will not move. If I push it against the motion of the treadmill, it will move forward despite the motion of the treadmill, I do not have to overcome the speed of the treadmill.

    Ok...Many Thanks Sean_K for this. Unfortunately i read the post when half asleep and missed out on a few things/too tired to fully read it...please accept my apologies. I was taking up the initial situation wrong and took it as a stationary a/c...Now Going by Sean_K and what is explained i would agree to his explanation fully. The A/c will take off. The A/c will gain speed thus produce lift required and be able to take off.

    Apologies again for coming on here and not reading the inital post when fully awake!


  • #64
    Delta Kilo
    Registered Users, Registered Users 2 Posts: 1,211 ✭✭✭Delta Kilo


    Celtic Mech wrote: »
    Ok, engines up to take off power, aircraft is now stationary. rearward force of treadmill is equaled to be the Thrust of engine in Fwd Direction. We MUST now look at the Lift needed. we know how to produce Lift..its the airfoil shape and the Airflow in relation to this...i.e. the relative wind. basically, with the aircraft on the treadmill, we have a Stationary Aircraft..
    What we must remember is that when an A320 or B737 is tearing down the runway at V1...the wheels are spinning at a speed of 140Kts BUT the Airflow against the Wings is ALSO going at 140Kts due to the Fwd motion of the Aircraft.

    Wrong. You are actually contradicting your own theory here. The forward motion of the aircraft has nothing to do with the ground, as you said it is:
    the airfoil shape and the Airflow in relation to this

    The treadmill can go as fast as it likes but the thrust of the engines pushing it forward will always combat this because there are 2 different mediums, namely air and the ground. The wheels are free to move. In theory, if the air was blowing at 140 kts in the same direction of take off, the plane would not take off. It has nothing to do with the ground because the power is not at the wheels.

    Think long and hard about it!

    EDIT: I forgot to read page 5 of this thread before I posted the reply to Celtic Mech


  • #65
    tracker-man
    Closed Accounts Posts: 706 ✭✭✭tracker-man


    There are two different ways of thinking about this, some people assume the aircraft is stationary on the treadmill, others assume it is able to move forward on the treadmill.

    So lets put this thing to bed:

    If the treadmill is running, and the aircraft is moving at sufficient speed to match the speed of the treadmill, and thus be stationary on the treadmill but the wheels still spinning, the aircraft will NOT take off.
    If the treadmill is running and the aircraft powers enough speed to move forward on the treadmill ( creating airflow on the wings ) then yes ot will take off.


  • #66
    asdasd
    Closed Accounts Posts: 3,185 ✭✭✭asdasd


    If the treadmill is running, and the aircraft is moving at sufficient speed to match the speed of the treadmill, and thus be stationary on the treadmill but the wheels still spinning, the aircraft will NOT take off.
    If the treadmill is running and the aircraft powers enough speed to move forward on the treadmill ( creating airflow on the wings ) then yes ot will take off.

    Jesus wept, get off the fence. And read the original post.

    Obviously if it moves it flys. The question is does it move if the treadmill accelerates to the takeoff speed of the airplane.

    And as we have worked out. It does.


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  • #67
    Celtic Mech
    Registered Users, Registered Users 2 Posts: 246 ✭✭Celtic Mech


    Delta Kilo wrote: »
    EDIT: I forgot to read page 5 of this thread before I posted the reply to Celtic Mech

    I kinda didnt read the thread properly the first time, so you forgot to read mine on page 5...call it even so?:D

    asdasd wrote: »
    The question is does it move if the treadmill accelerates to the takeoff speed of the airplane.

    And as we have worked out. It does.

    Well said asdasd...it will take off. The aircraft will be able to overcome the threadmills force and move fwd on it to its own take off speed.

    Now...anyone care to try work this out via a formula?? If we say there is no wind on the day, standard day..i.e. 15 degrees C, Pressure of the day is standard 29.92 inches /1013.25 mb and we are at sea level, gravity as 9.8 m/s/s, the mass of the aircraft for take off is 50,000kg and we have a take off speed of 130kts.(lets say, from a standing start, engines at full power, it will take from 0kts to 130 kts 25 seconds) As in the Mythbusters experiment on page 4 posted by ian m, can we work out at what distance from our start point will we reach our V1 ?


  • #68
    Manic Moran
    Moderators, Society & Culture Moderators Posts: 17,031 Mod ✭✭✭✭Manic Moran


    It seems that you would also need to know the thrust of the engines (to determine acceleration), the co-efficients of static friction and ordinary friction for the wheels, treadmill speed, and the drag co-efficient (and frontal surface area?) of the aircraft to have any hope of achieving a reasonably accurate answer.

    NTM


  • #69
    *Kol*
    Registered Users, Registered Users 2 Posts: 2,549 ✭✭✭*Kol*


    Would the distance to reach V1 be the same as always due to the fact that you will start from basically a standstill? i.e. once the aircraft has balanced out the effect of the treadmill by advancing the throttles to a certain point it should then be able to accelarate away as usual?


  • #70
    willit
    Registered Users, Registered Users 2 Posts: 108 ✭✭willit


    I am a member of many forums and this has been the single most infuriating thread I've ever encountered. Don't get me wrong, I think it was a great post, fair play to the OP, but for anyone that still doesn't get it I can't put it any better than Delta Kilo's post so please go to page four and read that post. Basically, the treadmill stops having an effect on the movement of the aircraft as soon as you give power to the engines because, very basically, the thrust created by the engines comes from the air (see the rope in DK's post) and therefore the treadmill velocity has absolutely no bearing on proceedings as no forward momentum comes from the aircraft's wheels. The engine thrust does not have to match the treadmill speed to bring the aircraft to a halt, the wheels will spin at the speed of the treadmill plus whatever extra thrust is delivered via the engines and the aircraft will move forward. The plane will take off as soon as it reaches the required velocity just like a conventional take off.

    Again, excellent question. ;)


  • #71
    Manic Moran
    Moderators, Society & Culture Moderators Posts: 17,031 Mod ✭✭✭✭Manic Moran


    Yes, but what if the treadmill goes backwards...?

    :P

    NTM


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