Rotorcraft Speed Limitations

Rotorcraft can't fly as fast as fixed wing aircraft. This essay explains what the limiting factors are in rotorcraft speed. I will try to add diagrams to make it more clear when I get a chance, but I don't know when that will be.

A spinning rotor causes a lot of drag. In fact, the drag is proportional to the cube of the rotor rpm. So, the slower the rotor spins- the less drag it creates. Unfortunately, for conventional rotorcraft, there is a limit to how slow the rotor can spin. For the aircraft to stay in equilibrium, both sides of the rotor must produce the same amount of lift. The two big reasons for this are structural and dynamic. If one side produced more lift than the other, it would create a large moment that would have to be carried by the rotor head, making the head much heavier. But even if the rotor head was strong enough, the moment created would cause the aircraft to roll to the side producing less lift.

So- because of that decreased airflow over the retreating blade- its angle of attack must be increased to compensate. Well, there gets to be a point where the angle of attack can't be increased enough to create enough lift- the blade would stall from being at too high of an angle of attack. But by increasing the rpm of the rotor, the velocity over the retreating blade is increased, so it can produce more lift at a lower angle of attack and maintain lift equilibrium.

Now to move on to what limits their high speed flight- which I didn't explain fully in my autogyro essay. Because of the high drag- they're not suited to it (a fixed wing plane could fly fast much more efficiently), but at a certain point, the drag actually goes up much faster than the cube of the rpm. Remember that the velocity over each blade is the vector sum of the rotor velocity and the aircraft velocity. This is what decreases the velocity over the retreating blade. It also increases the velocity over the advancing blade. Well, because of that high rotor rpm to keep enough airflow over the retreating blade, the advancing blade ends up going much faster than the rest of the aircraft. It will reach the speed of sound a lot sooner. In fact, even before an airfoil reaches the speed of sound, local pockets of supersonic flow appear because the airfoil acclerates the air. That supersonic flow really increases drag on the rotor. Not only does this translate into more drag on the aircraft- it means that more power must be used to drive the rotor.

So, while it might be possible to fly fast with a rotorcraft if you had a big enough engine for propulsion, the drag penalties just make it so that it's not worth it. One notable exception to this (and this is not an unbiased opionion, as I currently work for the company) is the CarterCopter (http://www.cartercopters.com). It adds wings to aircraft in addition to the rotor. Now, the wings can take over the lift, and the rotor only needs to produce just enough to keep it in autorotation. That means we can slow the rotor down and still maintain lift equilibrium, because neither side needs to produce a large amount of lift. We plan on slowing the rotor to about 100 rpm in flight, which is just fast enough to keep it stable. We're still limited by the upper limit of the advancing blade reaching the critical Mach number, but because our rotor spins so much slower, we can go a lot faster before that happens (theoretically 500 mph with a jet).

Anyway, I don't mean to turn this into a commercial, so back to conventional rotorcraft. I went to the website of the Fédération Aéronautique Internationale to look up the official speed records. For an autogyro, they had 207.7 km/h, or is 129.1 mph, set in 2002 by Kenneth Wallis in one of his aircraft. The record speed for a helicopter is 400.87 km/h, or 249 mph, set in 1986 by John Trevor Egginton and Derek James Clews in a modified Westland Lynx. But remember, those are only the official records - meaning that they count because some representative of the FAI was there to witness them. The fastest I've ever heard of a helicopter flying was the Bell Model 533, a highly modified Huey tested by Bell Helicopters. It had small wings to help unload the rotor, and jet engines to further unload the rotor by providing all of the thrust. It got up to 315 mph, but from what I hear, burned fuel so fast that it could only fly at high speed for about 15 minutes.

So, practically speaking, a pure helicopter (no wings or auxiliary thrust) will be limited to a top speed of around 200-250 mph, though most current helicopters fly at less than 200 mph. A compound (rotor + wings) with auxiliary thrust could go faster, theoretically up to very high subsonic speeds. A few notable attempts at high speed rotorcraft include the Bell Model 533, the Lockheed Cheyenne, the Fairey Rotodyne, the McDonnell XV-1, the Bell V-22 and other Tilt Rotors, the Boeing-Vertol 347, the Boeing CRW, and the CarterCopter. And I'm sure there have been other attempts that I didn't list.

As far as long distance- if you have too much drag, you can't fly efficiently- so you use a lot of fuel. You can only add so much fuel before the aircraft won't be able to produce enough lift to support its weight- so that's where the limitation comes in (actually, that's the limitation for all aircraft- it's just that conventional rotorcraft are less efficient that airplanes). The longest recorded flights for autogyros and helicopters are 543 miles and 2213 miles, respectively. It's probably possible to fly farther- but why when an airplane can do it better? And a conventional rotorcraft will never be able to fly as far as an efficient fixed wing aircraft. (Once again, that's something we're trying to remedy with the CarterCopter.)

I hope this essay helps you to understand the limits of high speed flight in rotorcraft. If you have any questions, e-mail me, and I'll do my best to answer them.