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I Lift vs Thrust force

  1. Mar 14, 2016 #1
    The thrust of an airplane changes the speed of the airplane, or you can look at it as it is increasing the airspeed relative to the airplane. The airspeed across the wings is only as much as the thrust allows it to be.

    The airspeed across the wing is what produces thrust, and I realize how it does so, via either pressure difference, camber, angle of attack, or a vortex lift system.

    What my question is, how can it be possible that a plane can produce more force in lift, then it has in thrust. I ask because the lift is directly dependent upon the thrust, so where does the extra force come from?

    Can someone break it down, via both calculations, an example, and a conceptualization?

    Thank You All Very Much!
     
  2. jcsd
  3. Mar 14, 2016 #2

    SteamKing

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    It does? Then why do they put those engine things on planes?
    Lift is a force which usually acts perpendicular to thrust.

    For an aircraft in level flight cruising at constant velocity, the thrust provided by the engines balances the drag of the aircraft at that cruising speed, while the lift produced by the wings and horizontal tail surfaces equals the weight of the aircraft and its contents.
     
  4. Mar 14, 2016 #3

    russ_watters

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    I can't imagine why they should be related. Are you aware that gliders have glide ratios of 20 or 30 to 1? Drag is due to friction and a pressure unbalance need not be significantly related to lift.
     
  5. Mar 15, 2016 #4
    I realize that they have the high drag to lift ratio, but I am asking how it is physically possible for it to produce more force from lift than thrust.

    The lift comes from the thrust of the airplain, or glider, so how is it that the force can come out of no where?

    I am just trying to understand, thank you all for your help
     
  6. Mar 15, 2016 #5
    I realize that, what I do not understand is how it is possible to have more lift than thrust. Doesn't energy have to be consetved? Where does the extra energy come from?
     
  7. Mar 15, 2016 #6

    SteamKing

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    Lift and thrust are properly forces, not energy. The lift of a wing is determined by among other things, its shape and the speed of the air flowing over it. Thrust is created by some sort of engine, and the power output of this engine or engines is not dependent on the amount of lift being generated by a wing.

    If a plane is not moving, or a wind is not blowing, its wings cannot generate lift. However, it can still generate thrust by having its engines operating and the pilot standing on the wheel brakes.

    When aircraft are launched from an aircraft carrier, the engines are running at full power before the catapult is engaged to fling the plane forward and off the ship.
     
  8. Mar 15, 2016 #7

    A.T.

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    Do you understand the relationship between energy and force?

    https://en.wikipedia.org/wiki/Work_(physics)#Mathematical_calculation

    What is the work done by the lift force if that force is perpendicular to the planes velocity?
     
  9. Mar 15, 2016 #8
    Yes, but the lift generated by a wing is from the airflow, which is produced by the thrust.
    So how can their be more force in thrust than the force of the airspeed, or the force of the thrust?
     
  10. Mar 15, 2016 #9

    SteamKing

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    Wings can generate lift if there is no thrust being applied to the aircraft. All it takes is air flowing over the wing from the wind to generate lift. This may not produce enough lift to make the plane fly, but it can reduce the takeoff speed or distance required to become airborne. This is why when carriers launch aircraft, the ship is pointed into the wind and is traveling at full speed (usually around 30 knots) before launching aircraft with catapults. The extra wind speed over the deck (30 knots + ambient wind) helps to get the plane airborne after launch.

    After a plane is flying at altitude, sometimes its engines lose power and there is no more thrust available. The plane does not immediately drop out of the sky, but it can glide for a certain time, trading altitude for speed to keep air flowing over its wings. When an aircraft is gliding, by definition, there is no thrust being applied, although lift is generated by air flowing over its wings and drag is created from the passage of the plane thru the air.
     
  11. Mar 15, 2016 #10
    Engine provides the power and we can fix arbitrarily powerful engine which can provide forward thrust and hence accelerate the plane consequently due to Bernoulli effect net upward pressure can be increased.So all depends on Engine.
     
  12. Mar 15, 2016 #11

    A.T.

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    Have you heard of levers?

    https://en.wikipedia.org/wiki/Lever

    You can put in a small force, and get out a greater force. There is no force conservation.
     
  13. Mar 15, 2016 #12

    boneh3ad

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    There is no "conservation of force" rule in physics. Lift is, in a certain sense, independent of thrust. Lift depends on forward motion of the plane. Thrust must only overcome drag in order to provide that forward motion. It's also perfectly feasible to get forward motion without thrust, in which case you have zero thrust and positive lift.
     
  14. Mar 15, 2016 #13
    So the extra force comes from the additional wind that is their? How then is it possible for it to take off on a non-windy day?
     
  15. Mar 15, 2016 #14

    rcgldr

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    I think the intended question here is how can lift / drag ratio be greater than 1.0? Consider how much smaller the profile of a aircraft as seen from the front (drag), is compared to the profile as seen from above (lift). The wings are relatively thin, but long (wing span). Lift is generated by a wing diverting the relative airflow downwards. Wings operating in an efficient mode divert a relatively large amount of air downwards by a relatively small angle.
     
  16. Mar 15, 2016 #15

    A.T.

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  17. Mar 15, 2016 #16

    boneh3ad

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    The aircraft carrier sailing into the wind at 30 knots creates at least a 30-knot headwind that is helpful for takeoff. Any additional wind speed on top of that is just bonus and is not necessary for takeoff.

    No, his question is how can lift/thrust be greater than 1 and the issue is that these two quantities are not really related in any sense other than thrust is what generally accelerates a plane up to a speed where lift is significant.
     
  18. Mar 15, 2016 #17

    SteamKing

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    I believe naval catapults are designed to launch aircraft with weight restrictions even if the carrier is not sailing into the wind, creating additional wind speed.

    Some carriers do not use catapult-assisted launching at all. The forward end of the aircraft deck has a special "ski jump" ramp built on it. The aircraft takes off like it would on land, but when it reaches the "ski jump", it is given a momentary upward boost as it runs up the ramp, which boost is sufficient to keep the aircraft aloft as it begins flying.

    Aircraft can take off from land runways without any wind blowing. It just takes an aircraft a longer distance to build up sufficient speed to allow the wings to generate enough lift so that the plane can fly. This is not to say that any aircraft of any weight can take off from a given airfield on a calm day, only that it is possible to do so under certain conditions. Whatever wind is present just makes it easier for takeoff to occur.
     
  19. Mar 15, 2016 #18
    They are completely related, because the headwinds, or airspeed is directly determined by the thrust. So how can it have a higher ratio than 1?
     
  20. Mar 15, 2016 #19
    Ohh, but levers sacrifice speed for force, what do planes sacrifice?
     
  21. Mar 15, 2016 #20

    SteamKing

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    You put a bigger engine on the plane, or even a rocket like the X-15.

    https://en.wikipedia.org/wiki/North_American_X-15

    The X-15 had a max. loaded weight of 34,000 pounds, which is the amount of lift its wings had to generate to keep the craft flying. On the other hand, the rocket engine which powered the X-15 could develop max. thrust of over 70,000 pounds.
     
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