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Lift vs Rotor Thrust


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Total rotor thrust is the sum result of the forces, and is considered to act like a big arrow pointing upwards, at 90 degrees to your tip path plane / rotor disk. Pulling pitch makes the arrow bigger, and moving the cyclic to tilt the disk makes the arrow tilt in the relevant direction.

 

Lift is the component of TRT which acts on the blade element, at 90 degrees to the Relative Air Flow.

 

One is big picture, one is small picture.

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In Wagtendonk's book (and the Rotorcraft Handbook as well), this was confusing to me too--maybe because for fixed wings thrust is the force that is generated, and lift is a consequence of thrust production. When Wagtendonk threw in "total rotor thrust", I couldn't figure out where thrust came from when lift was the only thing he talked about creating. As I understand it (and realizing I might still not have it right), helis generate lift; other forces that permit vertical and horizontal movement are a consequence of lift production:

  • Lift is the force generated by individual blades and acts at right angles to the relative wind; induced drag is a consequence of lift and acts parallel to relative wind.
  • You want to know how lift and other forces combine to affect flight, so you need to combine the forces for each blade to get a total force, and then dissect that total force so you can see how it overcomes forces opposing flight. Weight is the easiest to understand--if the helicopter weighs x pounds, you need x pounds of force to overcome weight. Lift isn't necessarily in-line with weight, so you have to covert it to a force that is.
  • The first step is to combine lift and induced drag and get the net force (Lift + Drag, or the "total reaction").
  • Now you want to know how (Lift+Drag) is related to weight, so you translate (Lift + Drag) into horizontal and vertical components relative to the rotation of the rotor disk (the rotor mast is the point where the force of the blades will act on the heli).
  • The horizontal component is torque, and has to be overcome by the engine to keep the blades spinning.
  • The vertical component is the Total Rotor Thrust, and acts vertically out of the rotor mast.
  • When hovering, TRT opposes weight and the engine force opposes torque.
  • Tilt the disk with the cyclic and you redirect TRT, but when you do so TRT is no longer completely opposite weight--it has a vertical component that opposes weight and a horizontal component ("thrust") that gets you where you want to go.
  • Thinking about it this way, lift doesn't oppose weight, but through a few manipulations provides a vertical force that does.

Hope this helps, but I'll be just as happy if it is correct. (Non-noobs feel free to set me straight--I'm still learning this stuff too.)

--c

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I rather think this confusion is why the U.S. Army doesn't buy into the "Total Rotor Thrust" term. I would get tired of explaining the term vs. aerodynamic thrust (opposite drag) and vs. aerodynamic lift.

 

Total Rotor Thrust=the combined force produced by all the blades together.

 

Lift=aerodynamic force that acts opposite of weight, or, in the case of rotor blades, the aerodynamic effect that manifests perpendicular to the rotational relative wind (as modified by the induced flow; i.e. "resultant relative wind").

 

See how that clears...err, muddies the waters?

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TRT is simply saying that wherever you point the rotor disk, that is the direction the forces of the rotor will act. TRT is the resultant of the lift and thrust vectors.

 

What I've previously said in my post above is to demonstrate that there are two competing definitions for lift, one that is used to understand basic aerodynamics (list as an aerodynamic force) and one used to understand how the rotor blade produces its lifting effect and the direction it acts upon the blade. Stating the obvious, if you confuse these two, you will be confused.

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Slick, you have sort of got it.

 

Lift is part of the small, detailed picture like Raymond posted. You talk about Lift when examining the forces on a blade element (cross section) and there are other little arrows marked Rotational Flow, Induced Flow, Relative Air Flow, Total Reaction, Weight, Drag, vertical component of TR which balances weight, horizontal component of TR which balances Drag, and so on. Lift is the part of Total Reaction which is at 90 degrees to the RAF.

 

Zoom back out, and look at the 3-d picture of a couple of blades whizzing around the mast. If you could see them, all the coloured arrows from the small picture would be there too, some pointing up and back, some pointing down and back, some forward, a real mess. But if they all added together, you would see one big green arrow pointing upwards from the centre of the disk at 90 degrees, and some blurry red arrows dragging backwards through the disk.

 

The green boy is your man, Total Rotor Thrust. Point him where you want to go, make him as big as you can.

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