From the Ray Prouty Archives: Ask Ray Prouty: Compound Helicopters, Compressibility

By Ray Prouty | May 1, 2005
Send Feedback


Ray, I have been a fan of yours since the 1980s, when I worked in the Gulf of Mexico. My question concerns compound helicopters. With their speed and maneuverability (the Lockheed AH-56 could quick-stop without pitching up!), how come there's not one in production today? Are there aerodynamic problems with this drive train configuration (regarding, for instance, engine failure and autorotation) that are difficult to overcome?

Jack Ogle
FAA Aviation Safety Inspector
Scottsdale, Ariz.


Much to the disappointment of those of us who worked on the AH-56 Cheyenne, the short answer to your question is there is as yet no market that justifies designing for that extra 50-60 kt. gained with a compound configuration.

The configuration is technically satisfactory and has no hidden showstoppers. The one thing that has to be considered is that, at high speeds, the power-required curve is very steep due to the parasite power increasing as the cube of the speed. The Sikorsky Black Hawk with two T700 engines can go 160 kt. By adding a wing, a propeller, and two more T700 engines, it could go to 210 kt. Thus, the extra 50 kt. comes with a hefty price tag, which might be acceptable for some applications. (But, oh, what good hovering performance this aircraft would have!)

Bell is betting the company on the hope that the market for high speed will be big enough to make its tilt-rotors pay off.

Raymond Prouty

R.W. Prouty


I am an instructor teaching the Black Hawk. Our aerodynamics manual does a poor job of describing rotary-wing compressibility. Would you expound on that subject so that I and other instructors can better our knowledge?

R. Doug Cline
Fort Rucker, Ala.

Everybody knows that air is compressible, but this has special significance to the aeronautical engineer. This is because an airplane or helicopter blade traveling at high speed through the air compresses the air ahead of it.

As the aircraft flies through the air, it sends out compression waves to warn air molecules that something is coming their way and they start moving to clear a path. The compression waves advance at the speed of sound. The effect works well up to about 300 kt. At higher speeds, the molecules don't have time to react and we start getting compression effects.

For high-speed airplanes, of course, this is a major consideration. Compressibility increases the drag on the entire aircraft. For helicopters, the effects are localized to the blades, especially those on the advancing side, where the tips may be operating close to the speed of sound to optimize performance. The factor used in both fixed- and rotary-wing aerodynamics is the Mach number, named after an Austrian physicist working in the early 1900s. It is the ratio of the speed through the air to the speed of sound. The speed of sound is a function of air temperature. It is about 690 kt. at 100F and 590 kt. at -40F.

It is not only the speed of the blade tip that determines the compressibility effect, but the local speed over the surface. As the air goes over the nose of the airfoil, it speeds up and its Mach number increases. Further back, the air has to slow down. Air doesn't mind speeding up, but it doesn't like to slow down. If the local surface speed reaches a supersonic Mach number of about 1.4, the slowing-down process is almost instantaneously made through a shock wave. On airplanes, this produces high drag and also noise--a sonic boom.

On helicopters, it produces the special noise that tells you that a Bell UH-1 is approaching. The drag effect on a helicopter does cause the power required to increase, but the primary problem is the sudden nose-down pitching moment that is generated as the shock waves on the top and bottom surfaces change the distribution of chordwise airloads on the blade. Helicopter aerodynamicists call this the Mach tuck.

Helicopters with thick blades, such as the MD500 series (whose blade thickness is 15 percent of the chord) suffer more than those with thinner blades because the air has to travel faster as it goes over the nose of a thick blade. The nose-down moment not only twists the blades, but must be reacted by the control system. The reason the Apache has swept blade tips is because with its original straight tips, the control forces at high speed were higher than those for which the actuators had been designed. The sweep fools the tip into thinking it is flying at a lower Mach number than it really is. Jet transports, of course, take advantage of the same effect.

The Mach tuck is only a minor consideration for airplane people, but because our blades are long and flexible, it becomes one of the limits for how fast a helicopter can fly. Blades tend to go out of track when shock waves are developed. Not every blade has the exact torsional stiffness as its mates.

Thus when Mach tuck comes, one will twist more than the others and generate a different spanwise-loading, leaving a different wake. That will have an effect on the others, which will soon be flying on paths of their own. The pilot will experience a rough ride. The forward speed at which this happens is a function of the speed of sound and so will be more noticeable during winter flying in Alaska than over the Gulf of Mexico.

Receive the latest rotorcraft news right to your inbox