Theoretically it would seem that helicopters should not be able to fly without instantly crashing. Let’s think about this for a moment.
Helicopters fly in a steady hover in still air; the spinning rotor blades will all produce an equal amount of lift.
However, with a headwind, or once the helicopter starts to move forward at all, this is not the case.
The blades get unequal amount of lift, and if nothing were done to compensate, the helicopter would turn over.
Let’s look at the process in more detail…
Why do Rotating Rotor Blades Get Unequal Lift?
This is all to do with the fact that we have one rotor blade ‘advancing’ or moving towards the air flow, and the other ‘retreating’, or moving away from it. The advancing blade always gets more lift than the retreating blade. This is easier to understand if we put in some numbers.
If we assume a hover with a 10-knot headwind (a knot is just a little over one mile per hour), the advancing blade now has an airspeed of 20 knots more than the retreating blade, which is moving in the opposite direction.
To put it another way, if the rotor RPM (revolutions per minute) is X knots, the advancing blade has an airspeed of X + 10 knots, and that of the retreating blade is X – 10 knots. This means that the advancing blade has much more lift, and if nothing were done, the helicopter would roll sideways and crash towards the retreating blade side. In fact, the very early helicopters did just this: As soon as they started to move forward, they rolled towards the retreating blade side and turned over.
Autogyro: Solving the Problem of Unequal Lift
The problem was eventually solved by looking at the autogyro, which was invented back in the 1920s by a Spaniard, Juan de la Cierva. Faced with the dilemma of trying to prevent his early machines rolling over and thrashing themselves to bits, Cierva discovered that if he made his blades flexible rather than rigid, so that they could flap up and down, everything worked just fine. So, having realized this, helicopter designers now did the same thing – they designed their rotors with a flapping hinge, and the flapping cleared up the dissymmetry-of-lift issue.
Flexible Blades: Theoretical Explanation
How does this work? It is not clear whether Juan de la Cierva worked this out or not, but it is now known that there is a very good reason. With flexible blades, the advancing blade gets more lift, and it therefore flaps upwards. But as it climbs, it receives more of what is known as the ‘induced flow’.
This is a part of the airflow that is directed vertically downwards through the rotors as they turn. This induced flow reduces the lift the blade is actually producing, so it starts to fall, or flap down.
As it flaps down, it receives less induced flow, and therefore more lift. Meanwhile, the exact opposite is happening with the retreating blade. This happens continually, with the blades flapping so that a state of equilibrium is reached, with lift over the whole rotor disc being equal. It is called, unsurprisingly, ‘flapping to equality’. Once it was understood, engineers could design helicopters to fly safely.
But of course, there is yet more to helicopter aerodynamics than this. For instance, if you don’t do something to prevent it, the turning rotor blades will cause the helicopter fuselage to turn in the opposite direction. My next article will look at how you solve this particular problem.
Decoding Science. One article at a time.