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Jet vs. Hovercraft - the momentum dillemna

  1. Apr 25, 2009 #1
    Well I was wonderin about this for quite a while. Why does a jet airplane not move about like a hovercraft? You know how your sittin in that seat, of a hovercraft, and once your goin, you have to wait several seconds for the momentum to get changed by the force - ie your movin 90 degrees one way and you turn the hovercraft to go the opposite way and you have to wait a while for the momentum to get changed by the engine force - when your in a jet plane its totally different. When you turn, you turn instantly theres no momentum getting in the way.
  2. jcsd
  3. Apr 25, 2009 #2
    This is because the Jet doesn't hover. The only force keeping the plane in the air is the drag on the wings which provides lift. With the hovercraft you have the force pushing up directly and fans that pull the nose to the left or right. Once you get the nose moving one way you have to slow it down by using a force in the opposite direction then that force will turn the nose back the other way. The jet is moving so fast that when you change direction the result is almost instant. Therefore you do not notice the change in direction of the Jet.

    Sort of drawn out and wordy but I hope it helps.
  4. Apr 25, 2009 #3


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    Hi drdede welcome to pf.

    Are you asking why the 'skid' in a hovercraft and not a jet craft when performing a turn?

    Well a jet is designed to have "yaw" stability. If this were not so, then pilots would not have the ability to control any turn.

    When an aircraft performs a turn, the reason why it does not skid is because of the rudder. The rudder prevents "adverse yaw" from occurring. If you analyze the forces in an aircraft turn, there is an imbalance of forces on the aircraft due to differences in lift, therefore the rudder creates a counter-moment stabilizing the aircraft.
  5. Apr 25, 2009 #4
    Hi, and thanks for the replies. But I do not think this is the case.

    The Jet's travelling speed should make no difference. Either way the force needed would be proportional to the speed, so if jets work with the same principles as hovercrafts you would see planes with rotation independant of their motion, which is not the case. Eg, you would see a plane rotated 90 degrees and the plane travelling 90 degrees the opposite way. It doesnt matter how fast the principles would still apply. The only way the principle would not apply is if the plane's acceleration time was extremely fast, ie. it took the plane a millisecond to reach maximum speed, which is not the case either.

    I don't think that is the case either. All the rudder does is rotate the aircraft. I don't fully see how it could change the actual momentum of the aircraft, now the wings themselves may play a part though. But I don't understand during yaw manuevers how the wings could do anything, since when yawing, the wings dont provide any extra friction or stopping force against the wind. I dunno, maybe I am at a misunderstanding. I just dont see the tail fin as being big enough to slow the planes momentum that dramatically.
    Last edited: Apr 25, 2009
  6. Apr 25, 2009 #5


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    You are indeed misunderstanding. Aircraft's do not only require yaw to turn, they require roll also. Both getphysical's and my descriptions imply the same occurrence. Think in terms of forces, not in terms of momentum and our descriptions will become clear
    Again, aircrafts bank for a turn, not only yaw. Banking creates differences in lift and drag on the wing. The wing higher will introduce a greater lift, thus a greater drag. The extra drag on the wing will create a moment about the CM of the aircraft. To counter this moment, a rudder is required to create an opposing force. Without this counter moment, the aircraft's nose would spin the opposite direction of the turn, thus the "skid" or spinning out of control.
  7. Apr 25, 2009 #6
    Can't an aircraft which is perpindicular to the ground use only yaw and still turn?
  8. Apr 25, 2009 #7


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    By perpendicular I hope you mean parallel. Using only yaw to turn would send the aircraft spiraling out of control because of the imbalance of forces. You should read up on stability and control of an aircraft.

    The rudder is there to prevent any yaw movement that would disrupt stability such as a gust of wind.
  9. Apr 25, 2009 #8


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    A Harrier in hover mode moves like a hovercraft.
    The engine force is a fraction of the weight of the hovercraft, say thrust is 1/4 of the weight, maximum acceleration is 1/4g. The wings on a jet can handle over 7 g's or in the case of a F16, over 9g's, so the acceleration rait is much higher.

    Yes, because the lift (force perpendicular to the direction of travel) is generated by the sides of the fuselage. Radio control models can do knife edge loops.

    The "skid" is there in both cases, depending on the cornering acceleration. At an airshow I watched an F16 fly overhead while making a 9g turn (afterburners required). Angle of attack is over 15 degrees, and it was obvious the aircraft was pointed inwards over 15 degrees from the actual path traveled, "skidding" in the air.
  10. Apr 25, 2009 #9


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    note you said a harrier jet in 'hover mode' not all jets have thrust vectoring.

    An RC model doing a loop is not a jet doing a turn.

    note this is not the case of a jet doing a simple turn.
  11. Apr 25, 2009 #10


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    I mentioned this example to explain it was wing generated thrust and not engine generated thrust that allow jets to have higher centripetal acceleration.

    No, but a jet and most aircraft can peform a low g turn using just yaw movement (rudder plus any required aileron input to oppose yaw induced roll).

    All wings require an effective angle of attack (perpendicular deflection of air) in order to generate lift, so at least the wing is "skidding" (angled inwards from direction of path) when producing lift.
  12. Apr 25, 2009 #11
    Yes parrallel is what I meant. I think I am somewhat clearer but do not understand it fully. I found about 10 links from google, half of them were expensive books you had to pay for, the other 10 links were either in gibberish (math-speak), or only covered stability, ignoring talk about momentum.
    I'm going to draw a diagram. The question mark is the mystery force I need to identify: http://xs138.xs.to/xs138/09176/planephysics151.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  13. Apr 25, 2009 #12
    A quick way to visualize this might be to consider the difference between performing "maneuvers" on ice wearing boots or skates. Which would you use?

    When talking about forces, it would be much easier not to get KE and such mixed in.
  14. Apr 25, 2009 #13


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    For starters, the aircraft cannot turn itself 30 degrees in one second. It is more gradual than that - time enough to convert momentum to a new direction.

    That aside, at 300mph, in that one second that the craft has begun its turn, it has moved 500 feet down-range in the direction of travel. What makes you think that forward motion disappeared?

    One of the factors that you're not considering that differentiates a jet from a hovercraft is that the hovercraft is not aerodynamically-controlled. It must redirect its thrust in the desired direction of the turn in order to begin moving that way. i.e there is a distinct period where the hovercraft must has its thrust oriented in the new direction of travel while its residual motion is still in the original direction of travel.

    A jet does not have to use its thrust to change its direction of movement; it uses flight control surfaces. There is a distinct period of time where the flight control surfaces are set to turn the cart yet the craft is not yet turned. However, once the craft starts to turn, its new direction is in the desired direction. In this case, the thrust actually follows the turn, rather than preceding it, as above.
    Last edited: Apr 25, 2009
  15. Apr 25, 2009 #14
    Okay, it makes more sense now. Thanks.
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