[Mike Maas]

Mike Maas asked the Naked Scientists:
   
Helicopter forward motion - How is it done ?  It means more lift on the forward-going blade and less on the backward-going, different at each forward speed.

What do you think?

[SteveFish]

The simple answer is that all the helicopter needs to do is direct some of the air it is propelling downward, to stay aloft, rearward to propel it forward. It would have to increase rotor speed to maintain level flight. All it would take in order to do this is tilt the rotor or tilt the whole aircraft, but I think modern helicopters have some pretty fancy controls that can change the pitch of individual blades dynamically.

[Geezer]

Or, to make them move faster, you can stick a pusher prop on the back and use two contrarotating fixed pitch props for lift only - at least, that's the idea with the Sikorsky X2. Time will tell if it's a good idea, or not such a good idea.

[mikemaas]

Thanks for the two replies, Geezer and Steve Fish.

But thinking of a two-blade rotor for simplicity, if the helicopter moves forward, the forward-going blade of the lift-rotor will create more lift than the backward-going one, so the helicopter will roll over towards the side of the retreating blade. Now one thinks of some sort of adjustable cam which would increase the angle of attack of the retreating blade of the rotor or decrease the angle of attack of the advancing rotor or both. That would be a tricky piece of engineering becasue the setting of the cam or cams would have to be different for each translational speed. And there`d be a limiting translational speed some time before the total lift on the retreating blade dropped to zero.

I guess Sikorsky solved it. I wonder how.

The pattern of lift along the blades would be interesting to see too. There`d be backward airflow along some extent of the retreating blade at any forward speed.

I think we need a helicopter engineer.

Mike Maas

[Geezer]

Mike,

On the X2, the blades rotate in opposite directions, so the unequal lift of one rotor is cancelled out by the unequal lift of the other rotor.

[mikemaas]

Thanks again Geezer !

Well, that`s OK for the X2, but what about a single-rotor helicopter ? It`ll surely roll. I`ve looked at Wikipedia on this subject, but I feel there`s more. It says two things: (i) that the blades are pivoted at their root so that they can flap; (ii) that a clever device called a swash plate does indeed allow the pilot to change the angle of attack in opposite directions on opposite sides. I can see that (ii) works, but I don`t see how flapping would make any difference. Maybe I misunderstood and the flapping is to cure something else.

Think of the outward forces at the rotor-hub! I wonder if blades have ever just broken off.

[Geezer]

Yes, you are quite right. With a single rotor, the angle of attack of the blades has to be altered to equalize the lift relative to the airspeed. Ultimately, that tends to limit the airspeed of single rotor helicopters, although there is a big battle going on at the moment between Sikorsky (X2) and a European group. The European craft uses a single rotor, but I think it gets some of its lift from stubby wings at high airspeeds.