B Questions on how helicopters fly

[cmb]

FWIW, to solve the issues of stall on the retreating rotor blades, of course the other solution is to use contra-rotating blades.

By having two rotors, the retreating blades can be allowed to stall out (retreating tip speeds drop to zero relative velocity) because the two balance each other and the net centre of pressure from both rotors will then always remain along the midline of the airframe.

Thus, whereas a single rotor with sub-sonic rotor tips can achieve a maximum of 1/3 speed of sound or so (because there still has to be a finite rearward tip velocity on the retreating blade), a contra-rotating helicopter could theoretically be run close up to speed of sound tip velocity with the retreating blade generating no lift at zero air speed, therefore can get up to the maximum possible for a rotary wing aircraft of 1/2 speed of sound.

e.g. Sikorsky X2.

 

[Lnewqban]

Ah, so that's what I wanted to know. As you said, if you were to reverse the rotation, there would still be an imbalance, in this case, if the rotor rotated from the left to the right, at 9 o'clock it will create more lift, but at 3 o'clock it would reduce the lift, correct?

Exactly!

For fixed wing airplanes, we can always keep the same lifting force by playing with the AOA of the wing and the velocity forward respect to the mass of air.
In other words, a Cessna 150, for example, can keep 1,600 lbf of weight at level flight in a range of speeds between 125 mph and around 50 mph (which corresponds to a stall AOA with deployed flaps of about 14 degrees).
In order to achieve that, the pilot must modify the engine power and the AOA via elevator input in order to keep level flight.

Note that the above speeds are relative to the incoming air hitting the leading edge of the wing, not relative to ground.
In case of a strong tailwind, for example, an observer on the ground will see a faster moving Cessna than when no strong wind is present.
The opposite applies in case of the airplane facing a strong headwind.

 

[Hornbein]

Gosh, I just learned all kinds of neat stuff.

 

[N1206]

Normally air flows downwards through the helicopter rotor...
With an autogyro, air flows upwards through the rotor causing autorotation as the aircraft is propelled forwards. The blades glide and so support the weight of the autogyro.

This is almost certainly in error.
Newton will not be denied. If you want an object to resist a force, you must supply a force of equal magnitude vectored in the opposite direction. A boat floats because the force pulling it downward is offset by an equal weight of water lifted upward. An object hovers because a mass of air is given a downward acceleration that matches the mass of the object and its acceleration due to gravity. The equal and opposite reaction is the lift given to the objects (aerofoils) imparting that acceleration. There's no magic. The forces involved must balance.
Instead of gravity pulling the object down, air is driven down instead. Instead of the ground supplying the normal force to hold the vehicle up from falling to the center of the earth, the force of lift holds it in position.

Autogyros rotate because the drag on the advancing aerofoil is less than the drag on the retreating one. When the forward groundspeed and rotational airspeed become great enough, the autogryo flies. You are trading the energy expended to drive the vehicle forward for the lift needed to raise it through the mechanism of drag. But the air is moving <downward>

You can test this for yourself. Build a taketombo. Spin it with your hands or a string, and it will move upward. Take that same taketombo, stick it in a tube and hold it out the window of a car as you drive along. Once you move along fast enough the taketombo will begin spinning and will lift out of the tube.

When a helicopter loses power, the blades are still rotating and air is still being forced downward, but the blades slow down. Lift is reduced accordingly. The helicopter begins to accelerate downward. The air accelerated downward by the still-spinning-but-slowing blades meets the air still there from moments ago. There is viscosity. It has to go somewhere and that takes time and crucially creates a whack of drag as it works its way out from under the blades. If you are lucky, the drag created and ground effect add up to enough force to keep the downward velocity of your vehicle down to a survivable prang speed. If the blades stop spinning, almost all the lift force is lost and you prang in at something between zero and terminal velocity depending upon how high above the deck you were.

But in all cases, the air is moving <downward>

 

[DaveC426913]

: abruptly stops in the middle of Googling "taketombo" to more urgently Google "prang" :

 

[256bits]

When a helicopter loses power, the blades are still rotating and air is still being forced downward, but the blades slow down. Lift is reduced accordingly. The helicopter begins to accelerate downward. The air accelerated downward by the still-spinning-but-slowing blades meets the air still there from moments ago. There is viscosity. It has to go somewhere and that takes time and crucially creates a whack of drag as it works its way out from under the blades. If you are lucky, the drag created and ground effect add up to enough force to keep the downward velocity of your vehicle down to a survivable prang speed. If the blades stop spinning, almost all the lift force is lost and you prang in at something between zero and terminal velocity depending upon how high above the deck you were.

Is this when a helicopter makes a normal controlled landing with the engine still functioning?

If the engine looses power, autorotation of the blades allows the pilot to land safely with the pilot adjusting blade pitch angle and forward air speed. Just as good as a parachute.
Descent with engine failure can be catastrophic if it occurs with no or little forward airspeed at a height particular for type of helicopter - generally speaking around 500 feet - for the air flow to be set up correctly from downwards movement to upwards movement over the blades.

 

[A.T.]

Autogyros rotate because the drag on the advancing aerofoil is less than the drag on the retreating one.

In autorotation both blades can have sections that act as turbines to support the rotation of the rotor. This works theoretically even without forward motion (just sinking), so there is no advancing & retreating blade. With forward flight the balance between turbine section and propeller section shifts and becomes asymmetrical. See video below for a visualization: