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From the Ray Prouty Archives: Low Speed Power Failures

By Ray Prouty | October 20, 2017

All good helicopter pilots — especially those who fly single-engine helicopters — are aware of the serious consequences of an engine failure while flying low and slow. Manufacturers put their best recommendation of the low-and-slow region to avoid in operator’s manuals as height-velocity diagrams — or the Deadman’s Curve. A sample curve (for the Hughes 500D) is shown in Figure 35-1.

There are some situations, however, when the pilot must ignore these recommendations and enter the warning area if he is to do the kind of job that only helicopters can do. In those rare instances when a power failure occurs inside the Deadman’s Curve, a crash is to be expected. How much damage is done depends on how well the pilot has managed his meager energy reserves.

Saving rpm

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The objective of energy management in this situation is to keep the rotor speed up as long as possible so it can be used to develop thrust to cushion the crash.

For the purpose of illustration, rotor rpm can be compared to the water level in the leaky bucket of Figure 35-2. The size of the leak corresponds to the power required.

Normally, the loss of water leaking out is exactly balanced by the water being pumped in by the engine. But when this source fails, there are two reserves that can be called on. The first is the potential energy corresponding to the height above the ground and the second is the kinetic energy corresponding to forward speed.

RWP Low speed PF 1

Neither reserve comes automatically to the rescue. Each requires quick pilot reaction to bring it online before the rotor speed drops below a minimum value where the rotor would stall if asked to produce thrust equal to the eight of the helicopter.

Power failure at high hover

IN hover, of course, the forward-flight kinetic energy reserve tank is empty, so the pilot must initially do whatever he can with his potential energy. For guidance, let us look at the techniques that assure successful landings from the point marked “High Hover Height” on Figure 35-3. From this point, the management of energy has two objectives. The first is achieved by rapidly dropping collective pitch. This does two things. It reduces the power extracted from the rotor, thus saving precious rotor speed, and it gets the helicopter going down through the air toward an autorotative condition.

The second objective is to build up some forward speed. Even though the transfer of energy from the potential reserve to the kinetic reserve is not 100% efficient, it has an overall benefit in reducing the power required. Even in autorotation, the rotor requires about as much power as in level flight. However, instead of coming from the engine, the power is supplied by using up the potential energy or altitude. For most helicopters, a speed of about 60 kts is the speed for minimum power and thus also the speed for minimum rate of descent.

Upon reduction of collective pitch, the rotor will flap down and forward. While one might think this a powerful means of pitching the helicopter nose-down and accelerating it forward, the rotor thrust will initially be low and both the ability to pitch the helicopter forward and to accelerate it forward will be degraded.

The low control power will be especially noticeable on helicopters with teetering rotors. These rotors will happily flap in response to cyclic-pitch commands but little pitching [movement] will be applied to the fuselage until rotor thrust builds up as the helicopter approaches autorotation. Helicopters with offset flapping hinges will not suffer as much loss of control power but their ability to accelerate forward initially will be no better.

The final flare

Even before getting into autorotation or to the speed for minimum sink rate, the pilot should be able to make a cyclic flare to take advantage of his newly acquired forward-flight kinetic energy to develop a rotor thrust higher than the gross weight. This thrust, tilted back in the cyclic flare, will kill off both forward speed and the vertical speed to the point where the landing gear can cope with both.

During the first part of the flare, a slight rotor overspeed might be obtained, which will help develop the rotor thrust required to cushion the touchdown. The “High However Height” is the minimum altitude at which the pilot can accomplish all of this without bending anything.

A bit of speed helps

RWP Low speed PF 2For a power failure with some forward speed along the upper boundary, the pilot has a head start toward achieving the desired flight conditions and so can go through the process faster. Thus, he requires less altitude — as reflected in the shape of the Deadman’s Curve. If his speed is beyond the nose of the curve, he should be able to make a satisfactory touchdown from ay altitude more than a few feet above the ground.

At low altitude, the pilot’s reactions are also influenced by how much speed he has. A successful landing form the “Low Hover Height,” which is usually with the landing ear only 10 or 15 feet off the ground, can be made by maintaining a level altitude and letting the helicopter settle, using the collective only to soften the final impact. With forward speed, the use of a cyclic flare to keep the rotor speed up is possible. This explains why the boundary goes up with speed.

High-speed segment

The high-speed segment of the height-velocity diagram simply calls attention to the fact that when flying low and fast, it is no time for anything to go wrong Combat pilots flying nap-of-the-earth might have to fly in the region. But most others can easily avoid it.

The rotor has an automatic reaction characteristic that is both good and bad in this situation. As it loses rpm following the engine failure, it will flap nose-up. This pitches the helicopter up and gets it started into a climb — but it might also slam the tail into the dirt if the ground clearance were too small to start with.

The crash situation

So far we have discussed making successful landings from outside the Deadman’s Curve. But the question we started with was, “What should the pilot do if the power failure occurs inside the manufacturer’s height-velocity diagram?”

His primary objective should be to hit in a level altitude to take the maximum advantage of the energy-absorbing characteristics of the landing gear and fuselage belly. To accomplish this, he should try to do — as nearly as possible — what he would do if he were just outside the boundary at that speed. That is, from the upper portion of the diagram, drop collective, try to gain some forward speed, and then make a cyclic flare, followed by a leveling maneuver just before touching down.

If the rotor thrust is still low after the collective reduction and the pushover to pick up speed, the ability to do the final cyclic flare will be degraded because of the low control power And, the impact might be more on the nose than on the landing gear.

For this reason, the descent from low heights and low speeds within the Deadman’s Curve should avoid extreme nose-down attitudes. From this low corner of the curve, let it settle in; only using collective at the last moment to get as much thrust as the rotor can develop before stalling.

Obviously, these recommendations are full of “ifs” and “buts” depending on the exact situation. And in the final analysis, I can only say, “Good luck.”

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