Introduction

1.   The same basic laws govern the flight of both fixed and rotary wing aircraft and, equally, both types of aircraft share the same fundamental problem; namely that the aircraft is heavier than air and must, therefore, produce an aerodynamic lifting force to overcome the weight of the aircraft before it can leave the ground.  In both types of aircraft the lifting force is obtained from the aerodynamic reaction resulting from a flow of air over an aerofoil section.  The important difference lies in the relationship of the aerofoil to the fuselage.  In a fixed-wing aircraft, the aerofoil is fixed to the fuselage as a wing while in a helicopter, the aerofoil has been removed from the fuselage and attached to a centre shaft, which, by one means or another, is given a rotational velocity.

2.   Helicopters have rotating wings, which are engine-driven in normal flight.  The rotor provides both lift and horizontal thrust.

Rotor Systems

3.   Helicopters may be single or multi-rotor, each rotor having several blades, usually varying from two to six in number.  The rotor blades are attached by a rotor head to a rotor shaft which extends approximately vertically from the fuselage. They form the rotor, which turns independently through the rotor shaft.  The axis of rotation is the axis through the rotor head and about which the rotor blades are permitted to rotate.  The plane of rotation is at right angles to the axis of rotation at the head of the main rotor shaft

Definitions

1.   General.    Certain terms are used in explanation of helicopter aerodynamics which differs from those used in fixed-wing flight. It is therefore necessary to define these terms before considering the principles of helicopter flight.

2.   Shaft Axis.    The shaft axis is the axis through the main rotor shaft, about which the blades are permitted to rotate.

3.   Axis of Rotation.    The axis of rotation is the axis through the head of the main rotor shaft about which the blades actually rotate. Under ideal conditions, the axis of rotation will coincide with the shaft axis. However, this will not always be the case since the rotor disc is permitted to tilt under certain conditions of flight.

4.   Plane of Rotation.   The plane of rotation is at right-angles to the axis of rotation at the head of the main rotor shaft, and is parallel to the tip path plane.  

5.   Tip path Plane.       The tip path plane is the plane described by blade tips during rotation and is at right-angles to the axis of rotation. The area contained within this path is referred to as the rotor disc.

6.   Lift.    The lift produced from the wing of a fixed wing aircraft results from a combination of many factors and is commonly expressed in the formula CL ½ p V² S. (see basic aerodynamics thread) Lift from a helicopter rotor blade can generally be expressed in the same terms but because the blade moves independently of the fuselage, the velocity (V²) in hovering conditions (in still air) is purely the result of the blade rotation.

7.   Blade Pitch.    The angle between the blade element chord and the plane of rotation is called the blade pitch angle.

8.   If the blade had a constant value of pitch throughout its length, blade-loading problems would arise because each section of the blade would have a different rotational velocity and would therefore produce a different value of lift. As lift is proportional to V2 each time the speed is doubled, the lift would quadruple and the lift pattern or loading, on the blade would be as shown by the red line in. To avoid this considerable load variation, lift must be increased at the root end and decreased at the tip; the blade is therefore either tapered, given a washout, or a combination of both. Lift from the blade will still have its greatest value near the tip but its distribution along the blade will be more uniform .

9.   Relative Airflow.   If a rotor blade is moved horizontally through a column of air, the effect will be to displace some of the air downwards. If a number of rotor blades are travelling along the same path in rapid succession with a three-blades rotor systems rotating at say 240 rotor rpm (Rrpm), a blade will be passing a given point every twelfth of a second, then the column of still air will eventually become a column of descending air . This downward motion of the air is known as induced flow.  The direction of the airflow relative to the blade is the resultant of the blade’s horizontal travel through the air and the induced flow.

10.   Rotor Thrust and Rotor Drag.    If the force acting on the aerofoil (total reaction) is split into the components of lift and induced drag, then lift, which is at right-angles to the relative airflow (RAF), is providing a force in direct opposition to the weight, as in the case of the fixed-wing aircraft. The lifting component of the total reaction must therefore be that part of it which is acting along the axis of rotation. This component is known as rotor thrust. The other component of total reaction will be in the blade’s plane of rotation and is known as rotor drag .

11.   Total Rotor Thrust.   Provided each blade produces equal rotor thrust, the total rotor thrust will act through the hub at right angles to the plane of rotation.

12.   Control.    A helicopter is able to climb and descend vertically, move horizontally in any direction and while hovering over a spot on the ground, turn onto any selected heading.

13.   Vertical Movement.    To achieve vertical movement, the total rotor thrust must be increased by increasing the pitch angle of each and every blade, which, in turn increases the angle of attack. The pitch angle is increased collectively by use of the collective pitch lever. The reverse takes place in a vertical descent.

14.   Control of Rrpm.    An increase in total rotor thrust necessitates an increase in total reaction, which, in turn, will give an increase in rotor drag. Engine power must therefore be increased to maintain Rrpm when increasing total rotor thrust, and vice versa. The designer normally incorporates some form of cam-operated linkage between the collective lever and the engine in order to maintain Rrpm.

15.   Horizontal movement.    The thrust required to move the helicopter horizontally must be obtained from the total rotor thrust. This can be achieved by tilting the disc so that the rotor disc is tilted in the direction of the required movement. To enable the disc to tilt, the pitch angle on one side of the disc must, at the same time, be decreased by the same amount, causing the blades to descend. To keep the rotor disc in the tilted position, the pitch must vary throughout the blades 3600 cycle of travel. This change in pitch is therefore known as a cyclic pitch change and is achieved by the pilot moving a cyclic pitch stick.

16.    Torque Reaction.    Unless blanked in some way, the fuselage will yaw in the opposite direction to the main rotor as a result of torque reaction. There are a number of ways by which this reaction can be overcome, but the only method considered here would be the fitting of a tail rotor. When the moment of the tail rotor thrust equals the torque reaction couple, then the fuselage will maintain a constant direction. As the torque reaction is not constant, some means must be provided to vary the thrust from the tail rotor. This is achieved by the pilot moving yaw pedals which collectively change the pitch, and thereby the angle of on the tail rotor blades.

17.   Additional Tail Rotor Functions.    The tail rotor has the following additional functions:

(a)   To alter the direction of the fuselage while hovering.

(b)   To maintain a balanced condition of fight.

(c)   To stop the fuselage rotating in power-off (autorotative) flight. When the rotors are being turned purely by the reaction of the air and without any assistance from the engine, there will be no torque reaction; friction will cause the fuselage to rotate in the same direction as the rotor. Directional control is maintained by changing the pitch on the tail rotor to such a degree that the tail rotor produces a thrust in a direction opposite to the required when the rotor is being driven by engine power. The tail rotor blades are symmetrical in shape and must be capable of being turned to produce plus or minus values of pitch.

18.   Flapping.    Flapping is bending stresses are absorbed by designed deformation of the rotor / hub combination.  

19.   Coning.    Rotor thrust will cause the blades to rise up. In normal operation the blades are said to be coned upwards, the coning angle being measured between the span-wise length of the blade and the blades tip path plane.  The coning angle will vary with combinations of rotor thrust and rotor rpm . If rotor thrust is increased and rotor rpm remains constant, the blades cone up.  If rotor rpm is reduced, centrifugal force decreases and if rotor thrust remains constant, the blades again cone up.  The weight of the blade will also have some effect but for any given helicopter this will be constant.

Limits of Rotor RPM

26.   Because the area of the rotor disc reduces as the coning angle increases, thereby decreasing thrust the coning angle must never be allowed to become too big.  As centrifugal force gives a measure of control of the coning angle through rotor rpm, provided the rotor rpm is kept above a laid down minimum, the coning angle will always be within safe operating limits.  There will also be an upper limit to rotor rpm due to transmission considerations and blade root loading stresses.

Overtorqueing

27.   Overtorqueing is possible on turbine driven helicopters when the transmission system can not absorb the high torque that turbine engines are capable of producing. Overtorqueing can be avoided by careful monitoring of the torque gauge and careful use of the helicopter controls.  

Overpitching

28.   Overpitching is a dangerous condition reached following the application of pitch to the rotor blades without sufficient engine power to compensate for the extra rotor drag. The rotor rpm decays, coning angle increases, disc area and rotor thrust reduce. A need is felt to increase the collective further which again decays rotor rpm. This can be prevented only by reducing the pitch angle of the blade by reducing collective. This entails a loss of height which can be dangerous close to ground.

Limits of Rotor RPM

Because the area of the rotor disc reduces as the coning angle increases, thereby decreasing thrust the coning angle must never be allowed to become too big.  As centrifugal force gives a measure of control of the coning angle through rotor rpm, provided the rotor rpm is kept above a laid down minimum, (that's why engine consideration of tuning, fuel config is so important in RC Flying) the coning angle will always be within safe operating limits.  There will also be an upper limit to Nr due to transmission considerations and blade root loading stresses.  

Overtorqueing

Overtorqueing is possible when the transmission system can not absorb the high torque that engines are capable of producing. (That's why it is important to choose the right engine/motor for your heli) Overtorqueing can be avoided by careful monitoring noise created by the rotor, this comes with experience  

Overpitching

Overpitching is a dangerous condition reached following the application of pitch to the rotor blades without sufficient engine power to compensate for the extra rotor drag (During quick stop and Tareeq like flying   ). The rotor rpm decays, coning angle increases, disc area and rotor thrust reduce. A need is felt to increase the collective further which again decays rotor rpm. This can be prevented only by reducing the pitch angle of the blade by reducing collective. This entails a loss of height which can be dangerous close to ground.

rcpilotacro really loved your write up very technical and precise information for all level of pilots .....its ur thread so please do add effects and drawbacks of ground effect and also pitch up/down effect  at 0 degree  ELV And AIl when the rotocraft is in motion ........ really loved ur post     

There were a few takers , yes will add more , just for you Shannon , topic of next discussion is IGE/OGE

Cool

While i write IGE/OGE here are some vids (from another post)

and

Anymore updates on this

Lift -Off and Climb to a Free Air Hover

1.   To lift a helicopter off the ground, a force must be produced greater than the weight, which acts vertically downwards through the aircraft's centre of gravity (CG).  On the ground with minimum pitch set, the total rotor thrust is small, and on some aircraft can even be negative, and the aircraft remains on the ground.  As the collective lever is raised blade pitch and the angle of attack are increased and the total rotor thrust becomes equal to AUW and the helicopter is resting only lightly on the ground.  A further increase in angle of attack causes total rotor thrust to exceed the AUW (See image1) and the helicopter accelerates vertically.

2.   As the Rate of Climb (ROC) increases there is a relative airflow down through the rotor.  This adds to and increases the induced airflow (IF).  The angle of attack and total rotor thrust (TRT) are automatically reduced by the increased IF and the acceleration decreases until a steady ROC is achieved with TRT = AUW, (See Image2).

3.   In the climb the Total Reaction Vector is tilted away from the axis of rotation because the direction of the RAF has changed.  Rotor drag is increased and more power is required to maintain rotor rpm (See Image3).

4.   To stop the climb, collective pitch and angle of attack are reduced and the TRT is now less than AUW. The helicopter's ROC decreases, IF reduces, angle of attack re-increases and TRT increases until a steady hover is achieved with TRT equal to AUW.  The helicopter is now said to be in a Free Air Hover.

Vertical Descent

5.   At low rates of descent the sequence is the reverse of the vertical climb, that is, due to downward movement, IF will be opposed and angle of attack will increase, (see Image4).  

6.   When climbing or descending there will be some parasite drag from the fuselage but the amount is small since a ROC or ROD is barely enough to create drag enough to reduce ROC/ROD.

Nice post

nyc post  ...For people who wanna know how the heli feels in ground effect ...the air hockey disk is a perfect example or even a hovercraft ...its like the heli is just resting on a cushion of air 

Loved the write-up

Any Plans to make one for the newer VTOL TiltRotors ?