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The Principle of the Helicopter

  1. Oct 24, 2011 #1
    The first heavier-than-air flying machines had wings whose airfoil effect provided the force of lift, due to their curvature (convex in the downward direction) which allowed air to flow more slowly along the lower surface of the wing than along the upper surface of the wing, whose curvature forced the air to traverse a longer distance, creating a partial vacuum at the trailing edge of the wing which sucked the air on the top of the wing into itself at a speed greater than that of the air at the bottom of the wing. This made the air on the top of the wing less dense than the air at the bottom of the wing, which gives the lower face of the wing a platform of dense air to rest upon, as well as a partial vacuum applying to the upper surface of the wing sucking it upwards, as long as forward motion is maintained sufficient to maintain the necessary airflow.

    The wings were fixed to the fuselage, such that the only way to make the thing fly was to pull the whole aircraft, fuselage and all, through the air at a certain minimum speed.

    What pulled the aircraft through the air? A propeller, which was, itself, an airfoil, curved so as to attack the air at the proper angle for lift (or, in this case, due to the axis of application of it's force, propulsion) at each point along its length (recognizing the increasing velocity of a rigid revolving body as one moves further away from the axis of rotation).

    This gave some people the idea that one might liberate the fuselage from the wings, and use the propeller as the wing, by altering its axis of rotation to one perpendicular, rather than parallel, to the surface of the earth. This way, the wings move through the air without necessarily pulling the entire aircraft along with it, allowing the possibility of hovering flight.

    Of course, the rotary wings create tremendous mechanical torque, such that the rotor can choose to plow through the air, or to allow the air to resist it and expend the resultant built-up energy by spinning the helicopter fuselage beneath it in the direction opposite that of the rotor's rotation. This is why you need a tail stabilizer rotor:


    or twin tandem contra-rotating rotors:


    or twin side-by-side contra-rotating rotors:


    or twin coaxial contra-rotating rotors:



    I hope I helped!
  2. jcsd
  3. Oct 24, 2011 #2


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    Interesting subject. I happen to be a RC-helicopter pilot. Helicopters have a main rotor which rotates at the same speed all the time - at least very close to it. The reason is that a possible change in RPM will provide a great torque on the helicopter body - more than the tail rotor can counteract. So pitch adjustment is used to rise and descent. The blades on most helicopters have an asymetrical cross section where there is a convex curve at the top, and either flat, or concave underneath. In the pro-RC world, most helicopters have a symetrical cross section of the blades, so the only thing that provide lift is a positive pitch. Positive pitch means that the leading edge of the blade is slightly higher than the other edge. In order to fly up-side-down, we must apply negative pitch. Negative pitch is absolutely not common on commercial helicopters.

  4. Oct 24, 2011 #3


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    Can someone explain to me how this works:


    My problem is that I can't see how the rotors don't chew each other up.
  5. Oct 24, 2011 #4

    Obviously, the pitch of a rotor blade affects its angle of attack, which, in turn, affects it's ability to transmit air along its chord (i.e., a symmetrical blade's attitude vis-a-vis pitch controls the velocity of the airflow around it just as does the asymmetrical shape of a traditional airfoil).

    Great observation!
  6. Oct 24, 2011 #5
    They're synchronized so that the miss each other.
  7. Oct 24, 2011 #6
    Last edited by a moderator: Apr 26, 2017
  8. Oct 24, 2011 #7


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    Staff: Mentor

    Just like a baking beater.
  9. Oct 24, 2011 #8
  10. Oct 24, 2011 #9


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    I think I get it now. I was assuming that one of the beaters would HAVE to hit the vertical portion of the other rotor, but I see now that they each pass OVER the other's center. Very clever.

  11. Oct 24, 2011 #10
    You're most welcome.

    And one of the great things about that rotor arrangement is that it allows the rear of the fuselage to have clamshell doors which are safe from tail-rotor strike (as there is no tail rotor) for loading of stretcher cases for medevac, or offloading of rescue personnel.

    Pretty neat for such a small aircraft!
  12. Oct 24, 2011 #11


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    I've never bought this description of how a wing works. I argued against it with one of my flying instructors with the following argument:

    Take a long square tube with a venturi in the middle of it. The venturi restriction is asymmetric in that there is only a lower surface that curves in towards the middle of the box section. A flow of air that passes through this square tube will [by the classic 'wing' argument] now experience a greater flow rate along the lower surface of the tube (as it is a larger surface), thus creating a vacuum on the lower surface. The box section will then experience a lift upwards as a result (even though the air coming out of the other end does so in the same direction as when it entered the tube).

    He reckoned that this 'boxed wing' would, indeed, produce lift.

    It is, of course, nonsense. Lift is produced by the deflection of air downwards. Without a mass [of air] being thrusted downwards, a wing will produce no lift.

    The 'vacuum on top of the wing' argument has its uses in wing design because there is a relationship between the vacuum generated and the acceleration of air downwards, according to the wing profile, but is, ultimately, bogus because this is cause and effect the wrong way around.
  13. Oct 24, 2011 #12


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    Not entirely sure we needed to wait until fixed-wing airplanes were in the air before this was realised:


    "Codex on the Flight of Birds", Leonardo Da Vinci, c. 1505
    Last edited by a moderator: Apr 26, 2017
  14. Oct 24, 2011 #13


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    A completely flat or rectangular wing profile will provide lift, but the wing must be pitched a bit upwards in the leading edge to do so. Profile RC planes are good examples of flying airplanes without actual lift in the wings. Any surface can fly straight through the air just by distributing 1/4 of the wing area in front of the CG, and 3/4 behind it. Try a single sheet of paper with a couple of paper clips at the leading edge. It flies perfectly.

    Edit: Sorry. I misunderstood your reply. Well, a venturi should be shaped and aligned correctly in order to provide lift of that square tube...

  15. Oct 24, 2011 #14


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    Exactly. It is the displacement of air that provides lift.

    In fact, it was following experiments on lift and wings that lead Sir George Cayley to state "lift shall be achieved by the application of power against the resistance of air" in 1799 (but who also went on to publish that lift was generated by a region of low pressure above the wing, in 1809). I guess I am having my cake and eating it there, but you don't need an aerofoil section to generate lift, therefore it would be wrong to claim it is due to low pressure arising from the shape of the wing. Aerofoil section makes the wing more efficient, and more capable in a wider range of angles [of attack] to the flow of air.
  16. Oct 24, 2011 #15


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    It cannot produce lift, if the air coming out of the tube is flowing in the same direction as the air entering it. It doesn't matter at all how fast the air is travelling over any surfaces during its transit between entering and exiting the tube. This is exactly the same for a wing; all that matters is the flow of air ahead of and behind the wing. If your wing generates a net motion downwards, you get lift. Doesn't matter what the wing does, or how it does it, so long as more air is moving downwards behind it than ahead of it.
  17. Oct 27, 2011 #16
    One further means of counteracting the aerodynamic torque generated by the helicopter blades spinning through the air:

    a contrarotating coaxial counterweight, which has the advantage of making your tail rotor a much more efficient mechanism of intentional yaw:

    (it's the "stick" above and perpendicular to the rotor blades in this photo:

    Last edited: Oct 27, 2011
  18. Oct 27, 2011 #17

    That was NEVER realized.

    Its weight-to-power ratio was abysmal, and its rotor design was the product of wishful thinking and lack of understanding of aerodynamic principles.
    Last edited by a moderator: Apr 26, 2017
  19. Oct 27, 2011 #18


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    That's just sooo WRONG!

    Please delete the post. Someone might actually believe you.

    This is a rotor stabilising bar and is not contra-rotating.

    You should be sure you know your subject before making such statements.
  20. Oct 27, 2011 #19


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    I meant 'realised' as in 'had the idea', contrary to your statement;

    Are you saying Leonardo ddin't have the idea to liberate the fuselage from the wings, then?
  21. Oct 27, 2011 #20


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    Wrong. This is not how lift is generated. Cambered (curved) wings and symmetric wings can both produce lift. A cambered airfoil will just provide the same lift with less drag penalty. The reason the air moves over the top of the wing faster has nothing to do with having to travel a greater distance. This is a common misconception, but still incorrect.

    Additionally, there is no density difference that, in general, explains lift. Below Mach 0.3, the flow is incompressible anyway and the density can be treated as a constant, so with that in mind, if the density really explained lift, the Wright brothers would never have gotten off the ground.

    So where does lift come from? The Kutta condition. If you place an object with a sharp trailing edge at angle of attack in a steady, inviscid flow, you will have a forward stagnation point and a rear stagnation point which would on an airfoil be located on the top side an arbitrary distance from the sharp trailing edge. This configuration produces no net lift (or drag for that matter, see: D'Alembert's Paradox). Clearly this isn't what happens in real life. However, the inviscid theory does predict that the flow successfully curves around the sharp trailing edge and finds its own natural stagnation point.

    Now, place it in a viscous flow (any real-world fluid) and you see a totally different situation. When the object initially starts moving, the flow can successfully find that natural rear stagnation point, but this is not sustainable. The governing equations predict that to do this, the velocity would have to be infinite at the trailing edge, which is obviously not possible. In essence, the trailing edge represents a singularity in the flow (and in fact is modeled as one on Kutta-Joukowski airfoils). What this means is that a vortex forms and is shed (see: starting vortex). This leads to a net circulation over the wing, as vorticity must be conserved. This circulation manifests itself as fast-moving flow over the wing and slow-moving flow under the wing, which translates to a lower pressure above the wing and a net lift force.

    That is how a subsonic wing really works.

    A supersonic wing is both more an less complicated, but suffice it to say that camber no longer matters and the lift relies on the pressure differential generated by shocks and Prandtl-Meyer expansions.

    And you should, because the way BadBrain explains it is incorrect.

    The fact is that your "boxed wing" would actually produce lift. If you built it out of a material that was light enough and you got the air moving fast enough, you could most certainly lift that box off the ground. This is the same principle as blowing over the top of a sheet of paper that is loosely hanging and watching the paper rise up. You lower the static pressure above the object when you force the air to move faster, meaning that there is a higher pressure below the surface leading to a net force. This net force is your lift. This is, of course, assuming that this box was not sitting flat on the ground where there is effectively no air under it. Then there can be no force from below.

    Again, it absolutely does matter how fast the air is moving over a given surface. Your last line there should read "If you have lift, your wing will generate a net motion downward."

    Both of the arguments you have here have the cause and effect wrong. The vacuum effect and the downward motion of the air leaving the wing are actually effects of what really leads to the velocity differential over a wing, which is the Kutta condition.


    This is a logical fallacy. Just because you don't need an airfoil to generate lift doesn't mean that the pressure differential doesn't arise on other surfaces that aren't airfoil sections. A flat plate at an angle can generate lift and yes it will deflect the air downwards, but if you measured the flow above and below it you would find a difference in velocity and pressure that can't be explained simply by saying the air was deflected. It comes from the circulation generated by having a sharp trailing edge. This sharp trailing edge is why the air is deflected downward and leads to the net circulation around the lifting body and therefore the pressure differential. You could put an ellipse at an angle in the flow and as long as it wasn't too elongated, you would not produce lift because the flow would just curl around the trailing edge and find its own preferred rear stagnation point. By adding a sharp trailing edge to something, you are enforcing your own stagnation point.
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