Why can't a Helicopter fly faster than it does ?
Wrote for the Helicopter History Site by Glenn Beare
The Westland Lynx reached 217.5 Kts (402 km/h) in 1986 using specially designed high-speed rotor blades
The Eurocopter X3, an experimental compound helicopter, reached 430 km/h (267 mph) in 2011
The AgustaWestland AW609 is a tilt-rotor, and can fly at 500 hm/h
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.
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.
|Velocity induced by the blades turning:||300mph|
|Plus the velocity from forward flight:||100mph|
|Total effective velocity at the tip:||400mph|
Airflow Reversal Airflow Reversal will normally occur before retreating blade stall. You will recall that the airflow velocity is progressively reduced along a blade from being highest at the tip, to lowest at the root end.
Air Compressibility Air is a gas and therefore conforms to the properties of a gas, namely the ability to be compressed. When studying aerodynamics however, air must also be considered to have some of the properties of a fluid. A fluid has far less compressibility than a gas.
Cyclic Control Stick design Helicopter designers are forever trying to fit more equipment into the cockpit of a helicopter to satisfy market demands. At the same time, they are trying to minimise the weight of the aircraft so that it can carry and lift more. When designing the pilot and copilots workstations the designers attempt to place the controls in a position where the crew can easily and comfortably operate all controls without excessive reaching or stretching. This places limitations on the amount of movement available at the cyclic control stick.
Available Engine Power The engine system in a helicopter is required to provide power for a range of demands, not only the rotor system. In the rotor system, thrust is required to overcome drag. As speed is increased, so does drag. If more power is available to overcome drag, then potentially the helicopter can fly faster.
Summary It can be seen that from these factors that it is very difficult for helicopter designers to increase the maximum speed of a helicopter as many factors are beyond their control. Much research and development has occurred in areas such as reducing drag, better rotor blade designs and increasing available engine power.
|User Contributed Notes|
pop emil ( carbonera tv italy )
another reason the hellicopters can\'t fly faster: At a certain speed the airflow does not pass anymore vertically through the main prop, therefore the rotary wing acts only as a wing, and as a result any command given with the cyclic will have no effect. the remedy seems to be to slow down a bit the machine. I would love that someone more prepeared than I am to comment on this issue in a friendly approach, with some clear examples. Have a nice flight gentleman.
James ( Adelaide, Australia )
Very useful for a complete non-expert such as myself! I do, however, query your illustration in the \air compressibility\ section. You explain that when a rotor blade goes faster, the air tends to compress before it splits, and thus more force is needed. The illustration of the hand in water says that the force required to separate water is greater if you hit the water with an open palm, not the side of your hand. But this is because of greater surface area, _not_ greater speed. Hence, you are drawing an analogy between two separate issues, which I don\t think is useful. I don\t know if there is an alternative picture you can use - this issue doesn\t sound like one we run into in everyday life?
anonymous ( Belleview, FL )
The air compressibility is a bit wrong with its terminology. Air *is* a fluid, however it is compressible unlike most *liquids*. Also, another major but basic limiting factor is the speed of rotation at the tip of the blades. Its exponentially related to the center-speed and length of the blades. Its pretty easy (hypothetically, anyway) to design blade tips which would end up going faster than the speed of sound, at which point the chopper would probably rip itself apart from the sonic booms and other weirdness that happens to air when you break the sound barrier. If nothing else, there is a whole host of legal issues with sonic booms.
Emmit Fitzhume ( Hardy, AR )
Under airflow reversal it is stated that 100 mph is 60 kts. The correct factor is 100 mph = 87 kts.... easy to remember is that 100 kts = 115 mph.
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