Ground School: Principles of Helicopter Flight

Principles of Helicopter Flight
 

Welcome to the world of R/C Helicopters. The helicopter is probably the most challenging form of radio control model, being mechanically complex in nature, and requiring 100% concentration while operating.

Flying a model chopper is not unlike balancing a metal bearing or a marble on a piece of glass. If the mechanics of the chopper have been adjusted and aligned very well, it is similar to having a flat piece of glass. If the chopper is not set up just right, it is like having a convex piece of glass where the bearing wants to keep rolling off to one side.

The first thing the budding helicopter pilot must realize is that the model works on the very same principles as the full-size and controlling the chopper is just as difficult, if not more-so due to size and orientation. It is not simply a matter of pushing one button for up, and another for forward flight, etc. Flying a helicopter, just like flying a model aircraft, is a skill that must be learned and that can only happen with practice . . . now that I have you thoroughly discouraged! Although building and flying a model chopper can be complex, it is also extremely satisfying. Being able to accurately control a vehicle which you can hover, fly forward, backward, sideways, and do all kinds of interesting maneuvers and aerobatics, as well as land at your feet, is very exciting.

Unlike learning to fly a model airplane where flying with an instructor is a must, you basically learn to fly helicopters by yourself. Before you start flying, however, some time with an experienced helicopter pilot will be invaluable. He can help you set up your helicopter (it is extremely important to have the mechanics set up accurately for safe and easy flying) as well as give you some tips on flying; what to expect from your model and how to operate the controls.

 

How does a Chopper Work?

princ There are basically two different types of helicopters, those that have collective pitch and those that do not. Collective pitch is where the pitch of the main rotor blades may be simultaneously increased or decreased to change the amount of lift. This gives a quicker response to changes in vertical thrust as controlled by the pilot. On helicopters without collective pitch, the amount of lift is controlled entirely by the speed of the rotor blades or in other words the speed of the engine (throttle control). The reaction time is longer and thus the control is less responsive.

There is a trade-off, however, and that is the cost and complexity of the rotor head. There are a great deal more moving parts in a collective pitch rotor head and thus, they are more expensive. Most current choppers are of the collective pitch variety.

On a standard, collective pitch helicopter there are four controls and these are operated by five channels of your radio system. These controls are the collective pitch, the fore and aft cyclic pitch, the side to side cyclic pitch, and the tail rotor pitch. The collective pitch must also be coupled with the throttle of the engine so that when more load is put on the main rotor blades by increasing the pitch, more throttle is applied to help overcome the additional drag.

Helicopter flight is governed by the pitch, or angle, of its rotor blades as the sweep through the air. When climbing or descending, the pitch of each blade is changed simultaneously and to the same degree. To climb, the angle or pitch of the blades is increased. To descend, the pitch of the blade is decreased. Because all blades are acting simultaneously, or collectively, this is known as collective pitch. For forward, backward and sideways flight, an additional change of pitch is provided. By the means the pitch of each blade is increased at the same selected point in its circular pathway. This is known as cyclic pitch.

When a helicopter is started up and the rotors begin to turn they are maintained in flat pitch, with no angle, or bite on the air. As the engine warms up and the rotors turn faster, the collective pitch is increased and the helicopter lifts vertically. To make the aircraft fly forward, the collective pitch is retained, keeping the aircraft in the air, while the cyclic pitch is adjusted to enable each blade to have more bite as it passes over the tail. To stop the helicopter and hover, the cyclic pitch is returned to neutral, causing the rotor blades to have the same pitch thought-out their cycle, allowing the collective pitch to retain the helicopter hovering in the air.

The left stick of your radio transmitter controls the collective and throttle in the vertical direction and the tail rotor pitch in the side to side direction. Your right stick controls both cyclic operations; up and down for fore and aft control and side to side for the cyclic side to side control. There are also mixing functions which mix the throttle and collective functions, and the throttle/collective and tail rotor functions.

 

Operation

The engine of a helicopter drives both the main rotor shaft and the tail rotor via a series of gears and a clutch. As the motor comes to speed, the clutch engages and begins to turn both rotor systems. Generally, at this point, there is no pitch on the main rotor blades and thus no lift. The throttle is increased until the main rotor blades are brought up to speed. To lift the helicopter collective pitch is applied.

princ_rudder Because, for every action there is an equal and opposite reaction, when the engine is forcing the rotor blades to turn in one direction, the body of the helicopter will want to rotate in the opposite direction. The function of the tail rotor is to correct this tendency. The tail rotor blades provide enough thrust to the side to keep the helicopter pointing in one direction. By increasing or decreasing the pitch of the tail rotor blades the direction the helicopter is pointing can be changed.

princ The cyclic control permits the main rotor blades to be varied independently making the helicopter move in a horizontal direction. If one of the rotor blades increases pitch as it approaches the rear while the opposite blade decreases in pitch while approaching the front during its rotation, more lift will be produced in the rear, tilting the helicopter forward, and thus moving the helicopter in a forward direction. The same principle applies for side to side and rearward, allowing the helicopter to fly in any direction.

princ_leftright The control of the cyclic and collective pitch is transferred from the radio servos to the rotor blades via the swash plate. Part of the swash plate is stationary while the other part is allowed to rotate with the rotor head. Control linkage is connected from the servos to the stationary part of the swash plate as well as from the rotating part of the swash plate to the rotor head.

When flying a chopper, small control inputs are continually required by the pilot to correct for deviations in the flight path. That is why 100% concentration is required in chopper operation. The more accurately the chopper is set-up, the fewer the number of corrections that are required by the pilot.

What Happens if the Engine Stops?

Auto-rotation is a way for helicopters to land successfully after a loss of power from the engine to the rotor drive systems.

This is accomplished with the aid of a special device known as an auto-rotation clutch which allows the rotor blades to free-wheel. As soon as power has been cut, the throttle/collective control is brought back all the way.

This will usually bring the main rotor blades to have slightly negative pitch. As the helicopter starts to descend, the air moving through the blades will keep them spinning. The spinning blades will act like a parachute in reducing the helicopters decent. When the helicopter nears the ground, the pilot increases the collective pitch making the pitch of the blades again positive.

The momentum of the blades is converted to lift, slowing the descent of the helicopter down further, enabling it to land softly.