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Homework Help: Thrust 'vs' Gravity

  1. Aug 31, 2006 #1
    The F22 raptor has an approximate 2:1 thrust to weight ratio. It can climb vertically and stand on its tail in a hover.

    Vertical acceleration is given by: Thrust - Drag - Weight = mass * acceleration (aka gravity). The opposing force to thrust involves an acceleration due to the force of gravity which is roughly equal to 9.8 m/s/s. So a 2:1 thrust ratio, though fun, cannot overcome the force of gravity i.e. accelerate continually -- in which case the craft would reach escape velocity which is in excess of 11 km/s or in the vicinity of mach10. ​

    Please advise about any misinformation here. As well as the mechanics that keep this craft from accelerating beyond 70k feet. How does one conceptualize per second per second acceleration? Distance per second is conceivable. Acceleration is observable. But the designation escapes me.

    Thanks for any advice.
    Last edited: Aug 31, 2006
  2. jcsd
  3. Aug 31, 2006 #2


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    Staff: Mentor

    That paragraph doesn't make a whole lot of sense. At a 2:1 thrust ratio, a plane will accelerate vertically until drag slows the acceleration and then will decelerate as the engines lose thrust at high altitude.

    I'm not sure I understand your question in the last part. m/s/s is the units of acceleration.
  4. Aug 31, 2006 #3
    At the very least, it shouldn't really be too hard to concieve of the units of acceleration. Meters per second per second means how many (meters per second) it gains every second.
  5. Nov 5, 2006 #4
    Acceleration is the change of velocity per second. For example in the case of gravity, acceleration indicates a change in velocity of 32 ft/sec each second.

    For constant vertical acceleration, the jet aircraft would require constant atmosphere. As the aircraft climbs, the presence of oxygen diminishes and dramatically affects engine output. Combustion efficiency depends on cool dense atmosphere. High altitude flight affects turbine engines and decreases thrust, altering the thrust to weight ratio and acceleration.
  6. Nov 5, 2006 #5
    Conceptionalise acceleration in terms of gravity. For example, you might say the raptor has 2 g's of acceleration. It can overcome the force of gravity (which means to hover), it just can't overcome the force of air resistance pressing against it at a few times the speed of sound or beyond. Also, it can't fly without the atmosphere, which (ignoring limited fuel supplies) is the only reason helicopters can't just slowly inch their way to outer space.
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