Downward air resistance on helicopter

  1. Hello,

    Why are helicopter blades put on top of the aircraft? Doesnt the air pushed down from the blades push down on the helicopter? Wouldnt it make more sense to have the blades on the bottom?
     
    Last edited: Jan 20, 2009
  2. jcsd
  3. DaveC426913

    DaveC426913 16,245
    Gold Member

    Bottom or top, the body of the craft is still in the way.

    Think about a fan. Cover the front of it, you stop the flow. Cover the back of it ... and you stop the flow.
     
  4. So the air above of the aircraft is sucked downward into the blades, and around the helicopter as resistance, and is equal to the force of the air being pushed downward below the blades?
     
  5. minger

    minger 1,498
    Science Advisor

    There is also the stability factor. If the upward force is applied below the center of gravity, then the system is inherently unstable and requires constant adjustments to be made. However, if the force is above, then any disruptions in body roll will be self-adjusted.
     
  6. DaveC426913

    DaveC426913 16,245
    Gold Member

    There are certainly many other reasons why one might not want whirling blades of death on the underside of a vehicle, but in the spirit of the OP's question, he's interested primarily in the efficiency aspect.

    Consider: if he were right, and it were more efficient that way, it would be worth trying to solve these other problems for the sake of efficiency. I'm trying to show him that, in principle, there's no reason to put them on the bottom.

    Yes.
     
  7. It's more for safety than anything else.

    Blades on the bottom are just 100% IMPRACTICAL.
     
  8. FredGarvin

    FredGarvin 5,087
    Science Advisor

    I'd like to know if I had to go in hard, momentum wouldn't be making me go into the rotor disc.
     
  9. Not only that, rotors on the bottom are just FUNDAMENTALLY wrong on almost every level.

    Q: What is the POINT of a helicopter?

    A: To do what an airplane cannot do.

    Are you going to rescue someone by winch by lowering a basket *through* a rotor disk?

    How will you get *into or *out of the helicopter? Wait for a three minutes until the blades stop spinning?

    How will you put wheels or landing skids on it?

    The list goes on..................




    ...........and on.....






    ........and on.
     
  10. DaveC426913

    DaveC426913 16,245
    Gold Member

    Guys, there are lots of solutions to rotors on aircraft, some have them on top, some have them in the fuselage, some have them on the wings, or even in the wings.

    If one wanted to design a better rotor craft, one would look at each of these challenges you mention (rotors being in in the way) and determine whether they were something that could be worked around or not with different configurations.


    The OP is proposing a modification that when isolated as a design factor i.e. all other factors being equal or ignored he thinks should make a more efficient craft.

    I'm trying to show that in principle this not going to achieve the desired improvement.
     
  11. We'll, we are answering his question. In principle, it would be better to have to rotors on the bottom. This is exactly why the Chinook has the front rotor at a lower elevation that the aft rotor. This way, the air entering the rear rotor is realtively unaffected by the downwash of the front rotor.

    Also, downwash of the rotor on the fuselauge increases the power requirements by approx. 5-7%. Putting the rotors below the fuselauge would theoretically make things better.

    But then you would have a useless helicopter with no utility. And a helicopter is a utility vehicle. Not a point A to point B vehicle. That's the job of an airplane.
     
  12. DaveC426913

    DaveC426913 16,245
    Gold Member

    Really? Oh. Then I am wrong, and the OP is correct in his assertion.
     
  13. It makes sense when you think about it. The rotor is entraining air from above the rotor and imparting momentum on the air down below it. When its being accelerated downwards, it literally hits the fuselauge, creating a downforce on it. This is exactly the design tradeoff you pay when using a compound helicopter (one with wings on it to increase lift). If the rotor is at the bottom of the helicopter, there is nothing for the air to "run into".

    But then again, such a helicopter is purely an academic exercise and of no value.

    Example of a compound helicopter:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Jan 22, 2009
  14. DaveC426913

    DaveC426913 16,245
    Gold Member

    Right, but that should have the same effect if the fuselage and winglets are above the rotor; it just reduces the inflow rather than the outflow.


    No, I guess not... It's much harder on a vacuum or fan's efficiency if you block the outflow rather than blocking the inflow...
     
  15. FredGarvin

    FredGarvin 5,087
    Science Advisor

    You would, undoubtedly, have to increase the rotor disc size because you would block a larger area towards the center of the hub. That in itself would be a design killer since tip speed for rotors dominates a design.
     
  16. Those are both true. The question is which would increase power more. Since no one has ever made a rotor below the fuselauge, my opinion* is that it would be slightly better for the same reason you put a pusher prop behind an airplane to avoid downwash onto the fuselauge.
     
  17. So from what I gather it might be slightly more efficient?
     
  18. But who cares, the center of the hub contributes very little to the overall lift! You care about the outter 1/3 of the blade.

    To the OP: I'm going to send an email posing this question to my professor of helicopter aerodynamics. He is a leading world expert.
     
  19. The outer half of the blade produces _____% of the lift. (?)
     
  20. Right. The air entering the blades draws on a larger area. Therefore a slower velocity. Therefore less drag on an element above than below the rotor. The airststream below the rotor tends to draw inward, to something less than the diameter of the rotors--the opposite of the airstream above. Air drag tends to run proportional to velocity squared.
     
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