Explain Dissymetry of Lift in a helicopter. How does this apply to Retreating Blade Stall?

Question

Dissymetry of Lift and what compensates
Retreating Blade Stall
No Lift Regions
Blade comppressability

Answer 1

Disymmetry of lift doesn't happen on modern helicopters. The potential cause of disymmetry of lift is the advancing blade generating more lift and the retreating blade generating less lift due to the addition of the forward motion vector of the helicopter.

All helicopters use flapping hinges that allow the blades to flap up and down, reducing the angle of attack on the advancing blade and increasing the angle of attack on the retreating blade. Thus lift is equivalent around the entire rotor disc, regardless of forward velocity..... almost.

As we go faster, the retreating blade will require an ever increasing angle of attack. Eventually (After Vne is reached) the angle of attack will become so great the blade will stall. This results in the disc moving rearward rapidly (due to gyroscopic precession) and loss of control of the helicopter (due to mast bump, rotor contacting the tail boom, etc).

No lift regions are sections of the rotor disc that aren't generating any lift due to no relative airflow. Generally in cruise this is a small area aft of the hub on the retreating blade side.

Compressibilty happens when the rotor tip (usually the advancing side) encounters sonic airflow. A small amount is normal (it gives Bell helicopters that SNAP SNAP SNAP sound on a cold day) but it forms shockwaves and reduces efficiency of the rotor. An extreme case would cause the rotor tips to pitch down and a massive loss of lift due to Mach tuck.

For this reason blade RPM is kept at 400 RPM or lower.

Answer 2

As the blades rotate, they are constantly changing relative direction from the incoming airflow.
While the blades are moving forward, the incoming air strikes them directly, and they create maximum lift. As the blades are directly perpendicular to the incoming air, they create less lift for a brief moment seeing that blades dont create lift as well while air is moving diagonally instead of directly over the blades. As the blades are moving with the incoming air, they produce less lift which creates dissymetry.
The blades are made of flexible materials which alow the blades to bend up when lift is maximum and flex downward while lift is decreased, otherwise the helicopter would wobble.
Im not completely sure about no lift reasons, but I believe that they are caused by compression of the air at the leading edge of the blade. the area would extend from areas of the blade that are transonic, ending in areas that are supersonic. Im not completely sure, I fly fixed wing aircraft, Not helicopters.
Are you actually going to fly a chopper, or is this for some science class?

Answer 3

Dissymetry of Lift
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It has been suggested that this article or section be merged into Dissymmetry of lift. (Discuss)
Dissymetry of Lift is a phenomenon that affects single-rotor helicopters in lateral flight, whether the direction of flight be forwards, sideways or in reverse. To simplfy the analysis, forward flight will be considered here, but the analysis applies regardless of the direction of flight because circular geometric symmetry applies. This is to be contrasted with asymmetry of lift, which is a phenomenon analysed while the helicopter is in the hover condition (see the appropriate article for discussion of that phenomenon).

Let a single-rotor helicopter be in forward flight, and let the viewpoint from which the helicopter is analysed be a point some distance above the rotor hub (this distance is irrelevant to the analysis, save that it allows the helicopter to be viewed in toto). Let the angular speed of the rotor disc (in radians per second) be ω, and let the forward linear speed (in metres per second) of the helicopter be v. The diagram below shall illustrate this. Furthermore, let the radius of the rotor disc (the distance from the centre of the rotor hub to the tip of any rotor blade) be r.



Now, in the diagram, let the state of the rotor tip be considered at points A and B, given the conditions stated in the diagram with respect to speed and direction of rotation of the rotor disc. When a rotor tip reaches the point A in the diagram, the linear speed through the air will be a combination of the linear speed imparted to the rotor tip due to its rotation about the rotor hub, and the forward speed of the helicopter. Since the quantities being added are properly velocities rather than speeds, the addition will be a vector addition, in which direction is important. At point A in the rotation cycle of the rotor blade, the linear speed of the tip through the air will be equal to rω+v. When the rotor blade has rotated further in the cycle, so that the rotor tip is at position B, the rotor speed will be equal to rω-v.

Since the lift generated by an aerofoil is proportional to its linear speed through the air, the rotor tip at position A in the diagram will be producing a lift proportional to rω+v, while the rotor tip at position B in the diagram will be producing a lift proportional to rω-v, a linear speed of lower magnitude. Therefore, the rotor disc will be, in the case of the hypothetical helicopter illustrated, producing more lift on the right hand side than on the left hand side, all other conditions being equal.

To reduce dissymetry of lift, modern helicopter rotor blades are mounted in such a manner that the angle of attack varies with the position in the rotor cycle, the angle of attack being reduced on the side corresponding to position A in the diagram, and the angle of attack being increaed on the side corresponding to position B in the diagram. However, there exists a limit to the degree by which dissymetry of lift can be diminished by this means, and therefore, since the forward speed v is important in the phenomenon, this imposes an upper speed limit upon the helicopter. This upper speed limit is known as VNE, the never-exceed speed. This speed is the speed beyond which the aerodynamic conditions at the rotor tips would enter unstable régimes - if v was sufficiently fast, the rotor tip at position A would be travelling fast enough through the air for the airflow to change radically as the rotor tip became supersonic, while the rotor tip at position B might have insufficient net linear speed through the air to generate meaningful lift (the stall condition - known as retreating blade stall. Needless to say, entry of the rotor tip into either of these aerodynamic régimes is catastrophic from the point of view of the pilot, and the maintenance of stable forward flight.

The situation becomes more complex when helicopters with two sets of rotor blades are considered, since in theory at least, the dissymetry of lift of one rotor disc is cancelled by the increased lift of the other rotor disc - the two rotor discs of twin-rotor helicopters rotate in opposite senses, thus reversing the relevant directions of vector addition. However, as entry of the rotor tip into the supersonic aerodynamic realm is one of the unstable conditions that affects forward flight, even helicopters with two rotor discs rotating in opposite senses will be subject to a never-exceed speed. In the case of tandem-rotor helicopters such as the CH-47 Chinook, additional factors such as the aerodynamic drag of the entire design, and the available engine power, may conspire to ensure that the helicopter is incapable of achieving the VNE imposed upon it by dissymetry of lift. In the case of the Kamov Ka-50 Werewolf, which is a coaxial design, it is possible for the helicopter to enter this aerodynamic régime as it has sufficient engine power, and pilots of this machine need to take this into consideration during the operation of the helicopter.

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