Helicopter Over-Pitching

The term "Over pitching" in a helicopter is often used by people but a lot of times there seems to be a misconception as to what exactly it means. For starters let's look at coning or coning angle. When collective pitch is applied the blades will assume a position which is the resultant of two forces. In the true vertical is Total Rotor Thrust and perpendicular to Total rotor thrust in the horizontal plane is centrifugal force due to the rotation of the blades. The angle between the Plane of Rotation and the actual blade is the coning angle.

Another definition which may help in understanding over pitching is rotor disc or disc area. The imaginary line joining the tips of rotor blades which has coned up is called the tip path plane. The area contained below the tip path plane and the actual blades is called the rotor disc area. To understand it a bit better one can imagine an upside down umbrella which represents the disc area and the actual stem in the middle the Total Rotor Thrust. Simply put in order for the helicopter to fly the rotor disc area should be a certain size to support the weight of the helicopter in flight.

In order to keep the size of the rotor disc fairly constant during flight a certain Rotor Rpm has to be maintained and that value is determined by the manufacturer and displayed to pilots by the Rotor Rpm gauge in the cockpit (the rotor rpm should be maintained in The green arc). Any time the Rotor Rpm is allowed to deceay below the prescribed limit by applying collective pitch the blades will cone up more resulting in a smaller disc area and effectively less lift. In other words too Much Total Rotor Thrust and very little centrifugal force due to the low Rotor Rpm.

There are numerous ways that can lead to over pitching therefore let's look at only a few examples. When flying a helicopter without a govenor will lead to over pitching when there is no or late anticipation by the pilot to open the throttle when applying collective pitch especially when approaching the hover phase of flight. Another cause is when the collective pitch is applied to quickly from a low setting to a high pitch setting even when a governor is part of the engine system. This is more prompt when flying at high gross weights or high density altitude and even more so when the rate of descend of the helicopter is also high. Thorough planning in these conditions is obviously very important. Over pitching can happen in more advanced turbine helicopters as well so it is not limited to piston driven helicopters only though most new generation turbine powered helicopters uses FADEC fuel systems which anticipates power or collective applications more efficiently.

When experiencing over pitching in most cases the only and best recovery technique is to lower collective pitch and if throttle is under the pilot's control it should be opened and even fully opened to recover Rotor Rpm. Depending on the phase of flight the thought of lowering the collective pitch might not seem the right thing to do specifically close to the ground but it is the only way to recover low Rotor Rpm as pulling the collective pitch will result in the Rotor Rpm decaying even More.

Source by Harry Cameron

Leave a Reply