March 1998Features

Rotor RPM

Putting The Right Spin On The Situation

Airplane pilots learn early in their training how a rigid, forward-moving wing creates a pressure differential that lifts the weight of the airplane. It's not quite so simple to explain how flexible rotor blades are able to support a helicopter's weight.

A helicopter may weigh about the same as an airplane. But instead of sturdy wings with relatively stiff internal ribs and spars to support this weight, a helicopter has skinny, flexible rotor blades with little or nothing in the way of internal structure.

That rotor blade is a helicopter's wing. By whirling its wing (rotor) in a circle, a helicopter can create lift over the rotor without any corresponding forward movement of the fuselage. Being circular, the rotor disc doesn't mind what direction it flies. It can produce lift while flying sideways, backward, or forward. The spinning motion gives the rotor the necessary rigidity to support the weight of the helicopter.

Purists argue that centrifugal force is not a true force, that it's merely the reaction to the centripetal force that causes a body to follow a curved path. For the purposes of this article, we'll treat centrifugal force as a force in its own right because this makes the concept (at least for me), easier to grasp. Centrifugal force acts in the plane of rotation. It attempts to pull a rotating object, say a rotor blade, at right angles away from the rotor mast. The degree of centrifugal force generated is proportional to the mass of the rotor blade, the radius at which it is applied, and the speed of rotation.

Swinging Rigidity

Within a rotor system, centrifugal force is the dominant force. All other forces tend to modify the effects of the centrifugal force the rotor generates. For instance, a two-bladed Robinson R22 training helicopter may weigh about 1,400 pounds and thus needs to generate a continuous 700 pounds of lift per rotor blade to stay aloft. The centrifugal force generated at the root of each rotor blade at normal rotor rpm is in the region of 17,000 pounds, or about 8.5 tons. Larger helicopters can develop as much as 40 tons of centrifugal force per rotor blade.

One of the factors limiting rotor rpm is the amount of centrifugal force the rotor head and rotor blade attachments can withstand. If a rotor head is designed to handle six tons of centrifugal force per blade at normal rotor rpm, increasing the rotor rpm above the allowable range could generate seven to eight tons per blade. This would begin to damage the pitch change bearings. In an extreme situation, rotor blade separation could follow.

Rotor blade separation from overspeeding the rotor sounds (and is) fairly dramatic, but it's very rare. Usually, the damage to the pitch change bearings from the excess centrifugal loads causes a rotor out-of-balance condition, and tracking and balancing the rotor cannot correct the associated control feedback. Then further mechanical investigation usually reveals the damage before a major catastrophe occurs.

Of much greater concern is low rotor rpm.

On the ground, without the rotors turning, the blades droop because of their own weight and flexibility. Rotor blade droop is particularly obvious on, say, a Robinson R44. When the rotor blades turn at full rpm with the helicopter on the ground, the outward pull of centrifugal force holds the rotor blades rigid at right angles to the hub.

If you raise the collective pitch lever so the helicopter rises to a hover, the tips of the rotor blades rise to an angle a few degrees above horizontal. They do so because lift acting vertically on the blades opposes at right angles, the horizontal pull of centrifugal force. As these two forces come into balance, the result is a blade that is said to be "coned."

The coning angle depends on the balance between lift and centrifugal force, and it varies as either of these two forces changes in relation to the other. If rotor rpm increases, the coning angle decreases. Reducing rotor rpm causes the coning angle to increase. Increasing the helicopter's weight (or, in a turn, its apparent weight), has the same effect.

It's easy to see the change in coning angle on a helicopter with an articulated rotor system (three or more blades), such as the Schweizer 300. Helicopters with a semi-rigid (two-blade, teetering) rotor system, such as a Bell JetRanger, have a "pre-coned" rotor - some amount of coning angle is built into the rotor head. On the ground, a helicopter with a semi-rigid rotor spinning at full rpm has a relatively large coning angle, but this is not a problem. In the air, any increase beyond the built-in coning angle is the result of blade flexibility.

A Robinson R22 helicopter with its teetering rotor is unusual because it has two coning hinges fitted outboard of the rotor hub. In this respect, it is a composite of an articulated and semi-rigid rotor system.

Low-Rotor RPM

Letting rotor rpm decrease below the allowable range is one of the most dangerous situations a helicopter pilot can get into. Low-rotor rpm can occur at almost any time, and it's usually the result of improperly coordinating the collective and throttle. Often it happens at the end of an approach, where the pilot must raise the collective to arrest the helicopter's descent and add power to enter a hover. Pilots must anticipate this need for power. If they wait for the rotor rpm to decrease, it's too late because the helicopter is now on the back side of its power curve.

As the blade tips cone upwards because of the reduction in rotor rpm, the apparent area of the rotor disc, as seen from above, decreases. With less area, the rotor disc produces less lift, and the helicopter descends. If the pilot reacts to the loss of lift by raising the collective, the extra drag on the rotor blades slows them down even more.

Apart from the fact that the main rotor disc is getting smaller, the tail rotor is also losing effectiveness. The tail rotor is coupled to the main rotor at a ratio of about 5:1. Losing one revolution per minute on the main rotor means the tail rotor loses five rpm. This reduction in tail rotor rpm can soon lead to your losing directional control (if the bad things happening to the main rotor, such as loss of lift, don't get you first).

As the coning angle on the main rotor increases, the blades in an articulated system will hit the top mechanical limit of their travel and begin to bend. The bending forces can be sufficient to permanently damage and deform the rotor blades. Fatigue forces at the rotor hub increase dramatically as rotor rpm falls below the allowable range.

Of all helicopters, light, piston-engine types get into low-rotor rpm during normal flight most often. Most turbine helicopters have fuel governing systems that normally do a good job of maintaining engine and rotor rpm, which reduces the chance of rotor rpm slipping below the normal level. If the pilot of a light, piston-engine helicopter lets low-rotor rpm develop, merely opening the throttle may not produce enough engine power to overcome the rapidly rising drag on the rotor blades. If the helicopter is close to the ground, lowering the collective may be the last thing on a pilot's mind, but simultaneously lowering the collective and applying full throttle is the only sure way to recover the lost rotor rpm.

If the helicopter is hovering relatively close to a surface not suitable for landing, a pilot can sometimes recover lost rotor rpm by "milking" the collective. The pilot maintains full throttle and repeatedly lowers the collective using small movements. This reduces the angle of attack of the rotor blades while preventing the helicopter from hitting the surface. Milking the collective can be a scary business, but you have no alternative, and this can often be enough to persuade the rotor rpm to return to the green arc.

Many accidents have occurred because the pilot didn't follow the correct low-rotor-rpm procedures, or did not use them in time. The major problem is that the relative wind vector at the rotor blades begins to change as the rotor rpm slows, increasing the angle of attack regardless of any upward collective movement. Any descent caused by the loss of lift increases the angle of attack further. Stalls are not normally associated with helicopter flight, but this is one case that can put a rotor system into a full stall.

To recover from a stall in an airplane, you lower the nose - decrease the angle of attack of the wing - to restore smooth airflow across the airfoil. In a helicopter, fully lowering the collective and opening the throttle may be sufficient to regain rotor rpm, assuming the helicopter can lose several hundred feet safely. It is equally possible that no corrective action is possible, and the rotor will simply go deeper into stall.

Helicopters with low-inertia rotor systems are extremely unforgiving of low-rotor rpm. Actually, all rotor systems are unforgiving of low rpm, but a low-inertia system loses rpm faster, which requires the pilot to react quicker to prevent low-rotor rpm from reaching the critical stage.

At this point, many helicopter pilots ask "why not just add some blade tip weights to, say, a Robinson R22 helicopter, to make it into a high-inertia system?"

"It's not as easy as that," says Frank Robinson, the R22's designer. "Rotor blade tip speeds are similar for all helicopters regardless of the size of the machine, because they are all limited by the speed of sound in air. The stored energy of a tip weight is only dependent on tip speed, therefore, a one-pound tip weight added to the 50-foot diameter rotor of a large helicopter would store the same energy as a one-pound tip weight added to the 25-foot diameter R22 rotor. However, the centrifugal force produced by a tip weight is inversely proportional to the rotor diameter, so the one-pound tip weight would produce twice as much centrifugal force in the R22 rotor as it would in the larger helicopter rotor, even though it would not store any more energy. The increased centrifugal loads carried by the blades, the pitch change bearings, and by the rotor hub would all be twice as great. Clearly it is very difficult to engineer a small helicopter with a high-inertia rotor."

Turbine helicopters have a low-rotor rpm warning horn that sounds if the rotor passes a predetermined low-rotor limit. Apart from the engine failing (which would be a fairly obvious event to the pilot), this would only occur if the fuel governor failed. The horn is necessary because the whine of a turbine engine doesn't vary much with engine and rotor speed. This makes it difficult for the pilot to discern any audible difference if the rotor rpm has dropped.

On a piston-engine helicopter, it's relatively easy for a pilot to hear the changes in engine note with changes in engine speed. For this reason, the FAA doesn't require piston helicopters to have a low-rotor rpm warning horn, although some piston helicopters do have them. Most pilots of piston helicopters develop a good ear very quickly and can judge rotor rpm without constant reference to the gauge.

However you judge it and whatever type of helicopter you fly, remember - the rotor rpm limits are there for a very good reason. Abuse them at your own risk.