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I want to know whether it is possible to make a helicopter with blades that rotate counter-clockwise (CCW) seen from below. I have difficulty understanding the angular momentum in this situation.

As we may see in the picture below, the rotor rotates clockwise (CW) seen from the pilot seat look up. So far, the rotations I have seen. To extend this question: is it possible for a single engine airplane to have propeller rotating counter-clockwise (CCW) seen from the rear, looking in the direction of the nose (as so far, what I found is that every single engine airplane is rotating CW). As we know from physics, the angular momentum of a CW helicopter will be pointing up, and the angular momentum of a single engine with CW rotation is pointing forward. To be clearer: Is there any impact (i.e. power efficiency, etc) if a helicopter's blades rotate CCW (seen from below), or if a single engine plane's propellers rotate CCW? Of course their torques will be different, but it is not the case in my question as long as it is not affecting the performance (i.e. reduce the power efficiency, etc).

Marine One with the blade shows it is CCW rotation Angular Momentum with right hand rule

Picture source: Picture 1, Picture 2;

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    $\begingroup$ Note that angular momentum is not a force. $\endgroup$
    – Bianfable
    Aug 21 '20 at 16:48
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    $\begingroup$ I do not understand the question. $\endgroup$
    – Fattie
    Aug 21 '20 at 16:54
  • $\begingroup$ @Bianfable, I understand that it is not a force. If it is a force, then will no question. But however, it is a vector. $\endgroup$ Aug 21 '20 at 17:01
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    $\begingroup$ @AirCraftLover vectors are just mathematical means to represent a physical value. For speed (or forces), we use vectors; for rotation (or torque) we also use vectors. speed, force, rotation and torques are not a vectors, but we usually use vectors to represent them. $\endgroup$
    – Manu H
    Aug 21 '20 at 18:44
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    $\begingroup$ @AirCraftLover I fail to understand your point. My point is that mathematical tools used to work with a model are not the physical objects they represents but a representation and only one representation for one possible model. Thus you cannot say "a force is a vector" but you can say "you can represent a force by a vector in Newtonian mechanics" $\endgroup$
    – Manu H
    Aug 24 '20 at 9:55
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Well helicopters are designed with both types of rotor systems. American built helicopters have main rotors which rotate counter clockwise whereas European built helos have clockwise turning rotor systems. Near as I know there is no particular advantage to one over the other, though you do have to reverse anti torque pedal corrections for collective inputs on a CW turning rotor system vs a CCW system, transverse flow effects are reversed and LTE events are more likely from right vs left crosswinds.

Just to be clear, torque is VISUALIZED as a vector perpendicular to the plane of rotation; it is not a force. If the latter were true, you could propel craft to the moon and beyond using just a gyroscope.

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In an inertial frame of reference, with a balanced propeller, there is no fore or aft forces except for the aerodynamic forces, so it will not make any difference if it is a left handed or right handed prop.

Some very strange effects come into play though when you are not in an inertial frame of reference though, if you have a bicycle wheel lying around, hold the axle steady and spin it, you should feel no forces. Now try repeating this while moving around... The forces you feel are sometimes referred to are a result of something with angular momentum moving in a non-inertial frame of reference. In a constantly rotating frame of reference, you have the coriolis and centripetal 'fictitious' forces. When there is acceleration, it gets even more complicated (Eular force).

Given the coriolis effect from the earth's rotation on the scale of a rotating propeller is so small, it is fine to completely disregard it, however there is a tiny difference in forces depending on which direction you are flying.

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  • $\begingroup$ Is there any difference in forces? I don't think angular momentum can produce forces, just torques (and there is no fixed point pivot off). Note that linear and anguar momentum are both conserved, but each on its own. $\endgroup$
    – Jan Hudec
    Aug 21 '20 at 20:40
  • $\begingroup$ Correct. Angular momentum can only produce torques when it is changed (Newtons second law applied to rotation). The other body forces that arise from non-inertial reference frames (centripetal, coriolis & Eular) can be considered as fields with a resultant force that acts on the objects COG. $\endgroup$ Aug 21 '20 at 20:58
  • $\begingroup$ @StuartBuckingham, maybe I am not clever enough to express my aim. Just allow me to ask another question but still related: Will any different if a helicopter or a single engine propeller rotate CCW? Different here, means, CW will be better (or probably also worse) from CCW. $\endgroup$ Aug 22 '20 at 6:08
  • $\begingroup$ @AirCraftLover - As stated by others, here are plenty of airplanes and helicopters that have props and rotors rotating in either direction. There are even aircraft with multiple props and rotors rotating in opposite directions on the same aircraft. Except for clearance issues and counteracting unwanted gyroscopic effects (especially during the loss of an engine), there is no advantage of CW vs CCW. Which one is chosen is more out of convention and the simplification of the fabrication, repair, and maintenance logistical supply chain than anything else. Interchangeability has value. $\endgroup$
    – Dean F.
    Aug 24 '20 at 6:03
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You are mixing torque with force. Torque is prependicular to the plane of rotation, but torque CAN NOT lift you up, force can.

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  • $\begingroup$ A rotation direction determine the torque and angular momentum. You can not separate them. They are just like X, Y, Z axis. $\endgroup$ Aug 22 '20 at 6:04
  • $\begingroup$ @AirCraftLover - I believe what Noah is saying is that torque is relative to two moveable bodies that have a rotational force applied between them. Since the plane of the rotation is horizontal, it can not contribute to a force that counters gravity (lift). If the rotating bodies were in the vertical plane like a Ferris wheel, it would be a different story. The force that counters gravity is the lifting force provided by the airfoils of the rotors moving through the air. The torque of the engine on the driveshaft acts to spin the rotors. It will also spin/yaw the helicopter if not counteracted $\endgroup$
    – Dean F.
    Aug 24 '20 at 5:41
  • $\begingroup$ @AirCraftLover - in the case of a single engine airplane, the torque of the engine spins the propeller in the vertical axis. The airfoil of the propeller moving through the air provides Thrust (lift in the direction of the airplane’s longitudinal axis). If torque is not counteracted by the wheels contact with the ground or by aerodynamic forces, torque would just cause the propeller to spin in one direction and the fuselage to spin/roll in the opposite direction. Gyroscopic forces like precession will occur if force is applied to the sloping prop to move it out of its current plane of movement $\endgroup$
    – Dean F.
    Aug 24 '20 at 5:48
  • $\begingroup$ @AirCraftLover - Worth noting is the fact that there are plenty of multi-engine airplanes that have counter rotating props. In most cases, the engine and props are identical to each other except for they are mirror images. Otherwise, every specification of the engine and props are the same. They are just made to run in opposite directions. This type of set up usually helps to diminish any unwanted gyroscopic effects. $\endgroup$
    – Dean F.
    Aug 24 '20 at 5:55
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As we know from physics, the angular momentum of a CW helicopter will be pointing up

No, you have completely misinterpreted what the thumb is in your right hand rule diagram is and have put more stock into its physicality than there is. If we wanted to we could use the lefthand convention instead of the right hand convention in our math instead. Would you be asking if the lift is decreased from the angular momentum in that case?

The upward pointing thumb is just a mathematical definition we use for convenience since it allows us to uniquely define the direction and orientation of rotation in 3D space with just three numbers (one vector) instead of many, many more. By this, I mean that the other way to represent angular velocity is a set of linear velocity vectors all pointing tangentially to the rotation equidistant from the axis of rotation such that translation/linear motions cancels out. That's difficult to work with mathematically and doesn't provide any more unique of a definition of direction than the right or lefthand rule does. That math has then been arranged such that using that convention produces results that make sense in the real world.

What you are doing is a bit like writing down the number 5 to record that there are 5 apples, but then thinking that number 5 is the apples.

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Force is a vector, but not all vectors are forces. Vectors that are not forces include:

  • Torque
  • Linear momentum
  • Angular momentum
  • Velocity
  • Displacement
  • Electric field strength
  • Vorticity

and many many many more.

Some of them may be kinematically/dynamically coupled to forces, or may produce forces via aerodynamic means (e.g. linear momentum through fluid creates aerodynamic forces). But they are still not forces; You can't simply add them up.

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Clockwise versus counter-clockwise propellers and rotors are nearly irrelevant. They don’t matter to the performance of the aircraft. They only matter in a difference in flying technique. The Angular Momentum does not contribute to lift in a helicopter nor thrust in an airplane.

The site you reference in your question does not present any force of lift. There is no force implied acting parallel to the plane of motion of a spinning object. Neither the L (angular momentum) nor the omega (angular velocity) produce a force that will act upon an object outside of its frame of reference. In other words, they will not lift a helicopter nor push/pull an airplane. Don’t let the L fool you into thinking that it stands for Lift. It does not.

Take the weighted bicycle wheel attached to handles experiment as an example. Once you start the wheel rotating in open space, its angular momentum will want to keep its position (or at least direction) rigid in space. As long as you keep the handles pointed in a consistent direction, the spinning wheel will provide no force until it makes contact with some surface (torque). The wheel will not push nor pull the person or object holding it. Only if you apply force to change the position of the wheel’s axis (the delta in the equations), will the wheel apply a force to remain rigid in space. Or, it will transfer the force in a direction according to the direction of its spin (gyroscopic precession).

In real world application, this means that you can ride a bicycle without worrying about a force pulling you to one side or another. It is actually easier to balance staying upright the faster the wheels are rotating. You can even ride with no hands touching the bicycle. And, when you do want to turn the wheels, all you have to do is lean to one side or the other. Gyroscopic precession will turn the wheels for you.

Another example is how motorcyclists can stay upright without much motion forward. They rev the engine while engaging the clutch while relatively motionless or at extremely low groundspeeds. Revving the engine spins the flywheel attached to the crankshaft. The flywheel acts like a gyroscope. Again, this provides rigidity in space. It does not matter which direction the motor spins. All that matters is the rotational and angular velocity of the flywheel and the flywheel’s mass.

So, yes both clockwise and counter-clockwise engine-propeller/rotor combinations exist in aviation. The only advantages of one over the other is logistics, not performance.

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