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F-35 uses lift fan which can create 20,000 lbs (9 tonnes) of lift in a small compact platform. I can see three or four of such lift fans that could lift a heavier plane and is seems better than a helicopter because it can travel at higher speeds.

  1. So what are the pros and cons of each design?
  2. Which one is more efficient in lifting, lift fan or rotate wing?
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1 Answer 1

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To answer your second question first:

Generally, it is always better to accelerate a big amount of air a little than a small amount a lot. Therefore, the helicopter rotor is much more efficient when creating lift, similar to a propeller which is more efficient than a jet. In the same way, more wing span reduces induced drag since more air can be used for lift creation.

The advantages of a lift fan are mainly due to its compact size:

  • Low mass, low inertias
  • Compact size, therefore easy to tuck away for fast flight
  • Low torque for a given amount of lift

and the disavantages stem from the same source:

  • Low efficiency
  • High exit speed, so beware of the blast.
  • No autorotation possible when the engine fails.
  • Lift acts at the center of the fan, so you need additional jets for pitch and roll control. A helicopter can shift the center of lift away from the rotor axis (within limits) by using cyclic changes to the angle of attack of the single rotor blades.
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  • $\begingroup$ You can't autorotate with a lifting fan $\endgroup$
    – rbp
    Commented Jan 14, 2015 at 13:33
  • $\begingroup$ You typically need several lift fans and they need to be redundant if one fails. $\endgroup$
    – tssch
    Commented Jan 14, 2015 at 16:15
  • $\begingroup$ @rbp: Good point! Thank you, I added it. $\endgroup$ Commented Jan 14, 2015 at 16:28
  • $\begingroup$ How about using a ducted fan for pitch and roll? $\endgroup$
    – rbp
    Commented Jan 14, 2015 at 21:24
  • $\begingroup$ @rbp: The usual way is to have two main jets, one from the engine exhaust and one from a lift fan or dedicated lift engines. Pitch and roll control is done by simple downward-pointing tubes with rotary valves at the extremes of the airframe which are fed by bleed air. Replacing the lift jet by a moveable ducted fan would be mechanically very complex. $\endgroup$ Commented Jan 14, 2015 at 21:31

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