# Why is the rotor system of an aircraft and the rotor system of a helicopter different? [closed]

Both aircraft with propeller(variable pitch) and helicopter rotor system can change pitch.
Of course, for helicopter, there is not only collective pitch control but also cyclic control.
The aircraft rotor system seems to be heavy because it uses hydraulic.
With the exception of the cyclic control of the helicopter, it seems lighter and simpler to use the helicopter's rotor system on an aircraft.
I would like to know what happens when we replace the aircraft's rotor system with helicopter's rotor system.

## closed as unclear what you're asking by fooot♦, Sean, Manu H, ymb1, FreeManApr 9 at 13:09

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• Welcome to Av.SE. You ask one question in the title, but a rather different question at the end of your post. The format here works best with one clearly defined & focused question. You're more likely to have a well-received question if you narrow it down to one question that is specific -- "why is the world the way it is" or "how are cats different than dogs" don't tend to work well. – Ralph J Apr 9 at 1:43

The propeller moves through air mostly along its axis of rotation. This means that flow parameters stay constant along the circumference.

The helicopter rotor moves through air mostly orthogonally to its axis of rotation. Consequently, flow parameters will vary along the circumference. Especially the local airspeed changes cyclically, and with it lift and drag. On one side rotor speed and air speed add up, and on the other side one is subtracted from the other. The rotor has added hinges or flexibility in order to allow cyclic movement of its blades (actively in pitch and passively both up and down, and fore and aft) so the variations in lift and drag don't roll the helicopter to one side.

Helicopter blades use symmetrical or reflex airfoils, so pitch moments are minimized. Propellers can use airfoils with positive camber, and their inherent instability needs hydraulics in order to control their pitch. Those cambered airfoils are more efficient, however, so their better efficiency more than compensates for the added mass of hydraulic pitch actuation.

The aircraft hydraulic pitch control system is much simpler and has fewer moving parts than a helicopter's main rotor pitch control system.

A helicopter's main rotor runs at between 500 to 800RPM, but (for example) in the world of general aviation using piston engines an airplane's propeller runs at between 2400 and 2600RPM. If built to withstand the dynamic loads at 2500RPM, a helicopter main rotor would be far more massive.

Airplanes have no need of the cyclic pitch control system that helicopters require in order to be controllable.

All these things mean that mounting a collective-and-cyclic blade pitch mechanism on the crankshaft of an airplane engine would be a meaningless exercise.

• Not to mention that a lot of airplanes don't even have a pitch control system. – jamesqf Apr 9 at 4:09
• agree.......... – niels nielsen Apr 9 at 6:54
• "an airplane's propeller runs at between 2400 and 2600RPM" This is for sure not true in general. There are many aircraft out there with many different RPM ranges (Dash-8 Q400 for example runs between 850 and 1020 RPM). – Bianfable Apr 9 at 7:30
• I am talking here about general aviation using reciprocating engines. Sorry if I did not make that clear, will edit. – niels nielsen Apr 9 at 16:17