How they Fly?

Blades and Dissymetry of Lift

Wrote for the Helicopter History Site by Taylor Cox, Butch Lottman and Glenn Beare

blades produced lift

All rotor systems are subject to Dissymetry of lift in forward flight. At a hover, the lift is equal across the entire rotor disk. As the helicopter gains airspeed, the advancing blade develops greater lift because of the increased airspeed (for example, if your blades at a hover move at 300 knots and you fly forward at 100 knots, your advancing blade is now moving at a relative speed of 400 knots and your retreating blade is moving at 200). This has to be compensated for in some way, or the helicopter would corkscrew through the air doing faster and faster snap rolls as airspeed increased. ( See also "Why can't a Helicopter fly faster than it does ?" article )

Dissymetry of lift is compensated for by BLADE FLAPPING . Because of the increased airspeed (and corresponding lift increase) on the advancing blade, it flaps upward. Decreasing speed and lift on the retreating blade causes it to flap downward. This INDUCED FLOW through the rotors system changes the angle of attack on the blades and causes the upward-flapping advancing blade to produce less lift, and the downward-flapping retreating blade to produce a corresponding lift increase. Kinda spooky, huh? Anyway, it all balances out and the lift is equal across the disk.

In a two-bladed system, like the Bell Huey, it is easy to visualize this because of the Bell/Textron insistance on the semi-rigid, underslung, see/saw type rotor system. If you push down on one blade, the other one goes up like a teeter-totter.

Most three or four bladed systems, like the MD500, are "fully articulated" in that each blade can flap independently, without affecting the other blades. The advancing blade still flaps upward and the retreating blade still flaps downwards, they just do so without worrying about what the other blades are doing. A rigid rotor system, like the MBB Bo105, have the blades fixed rigidly to the hub and compensate for dissymetry of lift by BLADE FLEXING . The blades still flap up and down like all the other helicopters, they just do so without hinges (really spooky). The advantage of the rigid hub is that you don't have to worry about mast bumping and the helicopter is (theoretically) fully aerobatic.

The OH-58D Kiowa (Bell 406/407) has a "four-bladed-soft-in-plane" rotor system, but you don't want me to go there.

There are several trade-offs one has to make when designing or buying a helicopter. I would much rather have a rigid or fully-articulated system, because they are more maneuverable and more forgiving of abrupt (Panic!) control inputs. Most mechanics and owners/operators like the Bell rotorhead because it is easier and cheaper to maintain. Things like max gross weight vs. empty weight (cargo capacity) are much more important to determining the "quality" of lift than the number of blades.

In any event, the four aerodynamic forces of lift, weight, thrust and drag all come into play, but you have to be careful when defining your terms.

The weight of a helicopter is divided evenly between the rotor blades on the main rotor system. If the helicopter weighs 5000 lbs and it has two blades, then each blade must be able to support 2500 lbs and so on. The more blades a helicopter has then the lower the weight that is carried on each blade compared to the same helicopter with less blades.

In addition to the static weight of the helicopter, each blade must be able to accept enormous aerodynamic loads as well. For example if a helicopter pulls up in a 2g manouvre (2 x the force of gravity), then the effective weight of the helicopter doubles due to gravitational pull.

During a helicopter take off, the pilot causes the pitch angle on the rotor blades to increase slowly until it reaches a point where the lift being developed by the main rotor system (not each blade) is greater than the weight of the helicopter. At this point the helicopter will rise from the ground and will continue to rise until the lift force is decreased to a point where it is equal to the weight. The helicopter will then hover at a fixed height.

When the pilot wants to descend he will reduce the pitch angle so that the helicopter weight is greater than the lift force, the helicopter will then come down due to gravity.

What is the average flying hours before the rotorblades are due for repair ? Most metal blades have a fixed life or TBO ( Time Before Overhaul ) , which is typically in the range of 3000 to 6000 hours. The average time between repair varies greatly depending on the type of operation.

Where and who are the rotorblade repair centers in the world ? All the airframe manufacturers have their own blade repair/overhaul facilities. In addition to this, there are several manufacturer appointed service centres which are capable of doing blade repairs.

Some claimed that to buy new rotorblade is cheaper than repairing it. To what extend this statement is true ? Rotor blades are expensive items and therefore any minor damage will be repaired. However, certain damage, especially damage to the blade spar may be impossible to repair. As blades are flight critical parts, no compromises can be accepted when carrying out repairs.

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