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Airfoils for helicopters
Safety & Ops
Vaughan Askue
Airfoils for Helicopters
As we discussed in the last article, fixed-wing aircraft designers typically start with an airfoil that is optimized for the flight condition at which the aircraft spends most of its time or the one that is the most demanding aerodynamically. Then they use leading and trailing edge high-lift devices to reshape the airfoil for low-speed operations, such as takeoff and landing. Unfortunately, the helicopter designer has no such luxury. Although some experimentation has occurred with airfoilchanging devices, none have been able to cope with the extreme centrifugal loads and very rapid aerodynamic changes experienced by the working portions of a main rotor blade. The helicopter designer also has to deal with an additional problem called “pitching moment”—the nose-up or nosedown twisting force that an airfoil exerts as it experiences different aerodynamic conditions. In the fixed-wing world, these moments occur slowly and are small enough that the large tail with a long moment arm can easily cope with them. In a rotor blade, the twisting occurs very rapidly, altering the blade’s characteristics and creating large loads in the control system. This factor actually drove the selection of the airfoils for early rotor blades. Early helicopters were small and did not have hydraulic servos to absorb the loads created by the blades. Whatever happened at the rotor came directly down through the control system to the pilot. I remember watching a film of an R-4 pilot, circa 1944. The pilot’s right hand was moving rhythmically in 4-inch circles as if he were stirring something. He could do nothing about it; these movements were the minor imbalances from the rotor moving the stick. To minimize the control feedback, the early designers had to select airfoils that had minimum pitching moment. They used what is called a “symmetrical airfoil,” shown in Figure 1. As you can see, the midline is straight, so the top and bottom shapes are mirror images of each other. If the hinge line is placed properly (about a quarter of the way from the leading edge to the trailing edge), the theoretical pitching moment is zero. To control retreating blade stall, the designers used relatively thick airfoils with a large leading edge radius. This type of airfoil worked fine until about 1970. At this point, helicopters were getting fast enough that the designers were faced with a difficult set of compromises. On the retreating side of the rotor system, blade stall was the big issue. 8
Figure 1 Symmetrical airfoil
Figure 2 Airfoil with S midline
Controlling blade stall encouraged the use of thick airfoils with large camber. The advancing blade near the tip was approaching transonic speeds, which required a thin airfoil with a small leading edge radius and little or no camber. At the same time, pitching moment had to be kept small to keep control system weights under control. The aerodynamicists dug into their bag of tricks. First of all, the airfoil had to be thin to cope with the transonic airspeeds near the advancing blade tip. To improve the lowspeed lift to cope with retreating blade stall, the designers had to increase camber. They did this by curving the leading edge of the airfoil downward, effectively creating a leading edge high-lift device called a cusp. Unfortunately, increasing the camber also increases the pitching moment, which is not good. The designers responded by curving the trailing edge upward. This is called a reflex, and it tends to counteract the pitching moment produced by the leading edge cusp. The result is a thin airfoil Air Medical Journal 23:2
with an S-shaped midline, shown in Figure 2. These shapes actually are much more subtle than shown in the illustration. Helicopter airfoils have to be efficient in such a wildly varying set of different conditions that their design is an art in itself. Although they generally follow the pattern described above, tiny changes can make a big difference, and each manufacturer has expended substantial amounts of design effort to develop families of airfoils optimized for their specific operating conditions. Rotor designers have another set of tools they can use to improve main rotor performance. This involves the blade shape as seen from above (normally referred to as the planform of the blade). We’ll discuss that next time.
Note: This column is written in an attempt to increase the understanding of medical personnel who work around helicopters and their pilots. I cannot do this in isolation. I need your comments both to help me understand how well I am communicating and to find subjects that are interesting and helpful to you. I can be reached at [email protected] or by phone at (203) 386-6451.
Vaughan Askue is the S-76 technical support manager at Sikorsky Aircraft Corp. in Stratford, Connecticut. 1067-991X/$30.00 Copyright 2004 by Air Medical Journal Associates doi:10.1016/j.amj.2003.12.012
March-April 2004
9
Vaughan Askue
Airfoils for Helicopters
As we discussed in the last article, fixed-wing aircraft designers typically start with an airfoil that is optimized for the flight condition at which the aircraft spends most of its time or the one that is the most demanding aerodynamically. Then they use leading and trailing edge high-lift devices to reshape the airfoil for low-speed operations, such as takeoff and landing. Unfortunately, the helicopter designer has no such luxury. Although some experimentation has occurred with airfoilchanging devices, none have been able to cope with the extreme centrifugal loads and very rapid aerodynamic changes experienced by the working portions of a main rotor blade. The helicopter designer also has to deal with an additional problem called “pitching moment”—the nose-up or nosedown twisting force that an airfoil exerts as it experiences different aerodynamic conditions. In the fixed-wing world, these moments occur slowly and are small enough that the large tail with a long moment arm can easily cope with them. In a rotor blade, the twisting occurs very rapidly, altering the blade’s characteristics and creating large loads in the control system. This factor actually drove the selection of the airfoils for early rotor blades. Early helicopters were small and did not have hydraulic servos to absorb the loads created by the blades. Whatever happened at the rotor came directly down through the control system to the pilot. I remember watching a film of an R-4 pilot, circa 1944. The pilot’s right hand was moving rhythmically in 4-inch circles as if he were stirring something. He could do nothing about it; these movements were the minor imbalances from the rotor moving the stick. To minimize the control feedback, the early designers had to select airfoils that had minimum pitching moment. They used what is called a “symmetrical airfoil,” shown in Figure 1. As you can see, the midline is straight, so the top and bottom shapes are mirror images of each other. If the hinge line is placed properly (about a quarter of the way from the leading edge to the trailing edge), the theoretical pitching moment is zero. To control retreating blade stall, the designers used relatively thick airfoils with a large leading edge radius. This type of airfoil worked fine until about 1970. At this point, helicopters were getting fast enough that the designers were faced with a difficult set of compromises. On the retreating side of the rotor system, blade stall was the big issue. 8
Figure 1 Symmetrical airfoil
Figure 2 Airfoil with S midline
Controlling blade stall encouraged the use of thick airfoils with large camber. The advancing blade near the tip was approaching transonic speeds, which required a thin airfoil with a small leading edge radius and little or no camber. At the same time, pitching moment had to be kept small to keep control system weights under control. The aerodynamicists dug into their bag of tricks. First of all, the airfoil had to be thin to cope with the transonic airspeeds near the advancing blade tip. To improve the lowspeed lift to cope with retreating blade stall, the designers had to increase camber. They did this by curving the leading edge of the airfoil downward, effectively creating a leading edge high-lift device called a cusp. Unfortunately, increasing the camber also increases the pitching moment, which is not good. The designers responded by curving the trailing edge upward. This is called a reflex, and it tends to counteract the pitching moment produced by the leading edge cusp. The result is a thin airfoil Air Medical Journal 23:2
with an S-shaped midline, shown in Figure 2. These shapes actually are much more subtle than shown in the illustration. Helicopter airfoils have to be efficient in such a wildly varying set of different conditions that their design is an art in itself. Although they generally follow the pattern described above, tiny changes can make a big difference, and each manufacturer has expended substantial amounts of design effort to develop families of airfoils optimized for their specific operating conditions. Rotor designers have another set of tools they can use to improve main rotor performance. This involves the blade shape as seen from above (normally referred to as the planform of the blade). We’ll discuss that next time.
Note: This column is written in an attempt to increase the understanding of medical personnel who work around helicopters and their pilots. I cannot do this in isolation. I need your comments both to help me understand how well I am communicating and to find subjects that are interesting and helpful to you. I can be reached at [email protected] or by phone at (203) 386-6451.
Vaughan Askue is the S-76 technical support manager at Sikorsky Aircraft Corp. in Stratford, Connecticut. 1067-991X/$30.00 Copyright 2004 by Air Medical Journal Associates doi:10.1016/j.amj.2003.12.012
March-April 2004
9