Why can't a Helicopter fly faster than it does ?
Wrote for the Helicopter History Site by Glenn Beare
In the following paragraphs, the reasons for this will be discussed in detail. For ease of explanation, all descriptions will be based on a simple two bladed rotor system. which rotates counter-clockwise when viewed from above. This makes the advancing blade on the right side of the aircraft swinging toward the front of the helicopter.
The explanations will deliberately be kept fairly basic. For the more advanced out there, please don't send e-mail saying that there is more to it than has been stated. However, do comment if you consider that any of the explanations are fundamentally wrong.
There are a number of factors that govern the maximum speed of a helicopter.
Drag In aerodynamics, drag is the force opposing thrust. Drag is present in helicopters in two main types:
a. Parasite drag Parasite drag is the drag forces created by the components that protrude into the airflow around the helicopter. Because this drag is opposing thrust it is reducing the amount of thrust available to make the helicopter fly faster. Parasite drag includes the landing gear, antennas, cowlings, doors, etc. The shape of the fuselage will also produce parasite drag. On later helicopters where the manufacturer has attempted to raise the speed of the helicopter, the landing gear is retractable to reduce the amount of parasite drag produced. Generally, for a given structure, the amount of parasite drag is proportional to the speed that the structure is passing through the air and therefore parasite drag is a limiting factor to airspeed.
b. Profile drag Profile drag is the drag produced by the action of the rotor blades being forced into the oncoming airflow. If a rotor blade was cut in half from the front of the blade (leading edge) to the rear of the blade (trailing edge), the resulting shape when looking at the cross-section is considered to be the blade "profile". For a rotor blade to produce lift, it must have an amount of thickness from the upper skin to the lower skin, which is called the "camber" of the blade. In general terms the greater the camber, the greater the profile drag. This is because the oncoming airflow has to separate further to pass over the surfaces of the rotor blade. The blade profile for a
given helicopter has been designed as a compromise between producing sufficient lift for the helicopter to fulfill all of its roles, and minimising profile drag. To alter the amount of lift produced by the rotor system, the angle of attack must be altered. As the angle of attack is increased then the profile drag also increases. This is generally referred to as "induced drag", as the drag is induced by increasing the angle of attack.
Have you ever stuck your hand out of the window while travelling in a car? If so, did you notice that if you kept your hand flat with your thumb leading then you could keep you hand in that position fairly easily with some effort. What happens if you turn your hand so that your palm is facing into the wind? It is not as easy now to keep you hand still and it requires far greater effort to keep it there. This can be related to profile drag and induced drag.
Retreating Blade Stall To understand retreating blade stall it is first necessary to understand a condition known as "Dissymetry of Lift" . Consider a helicopter hovering in still air and at zero ground speed. The pilot is maintaining a constant blade pitch angle with the collective pitch control lever and the aircraft is at a constant height from the ground. The airflow velocity over the advancing blade and the retreating blade is equal.
If the tip of the advancing blade is travelling at 300mph then the tip of the retreating blade must also be travelling at 300mph. The velocity of the airflow over the blade is progressively reduced as we look closer toward the root end of the blade (toward the rotor hub) as the distance that the observed point has to travel around the circle is reduced.
In this condition the amount of lift being generated by each blade is the same because the amount of lift produced is a function of velocity and angle of attack. However, if the helicopter started to move forward then the airflow velocity over the advancing blade would be increased by the amount of the forward speed as the blade is moving in the opposite direction to the flight. If the helicopter was then travelling forward at 100mph, then the airflow at the advancing blade tip would be:
Velocity induced by the blades turning: