John L. McCloud III
United States of America
John L. McCloud II, a Senior Scientist at the NASA Ames Research Center and one of the helicopter industry's most respected research engineers died in 1982. He gave significant contributions in the areas of rotorcraft performance, rotor aerodynamics, dynamics, and control.
McCloud was born in 1922 in Detroit and after attending the University of Michigan for two years, he enlisted in the United States Navy and served as a pilot and flight instructor during World War II. After the war, he graduated from Stanford University with a B.A. and M.S. degree in Aeronautical Engineering. He began his research career at the Ames Laboratory of the National Advisory Committee for Aeronautics in 1952.
Among his many research programs, McCloud investigated high advance ratio stall characteristics of helicopters in the 40-by 80-Foot Wind Tunnel. He established the performance gains possible using cambered airfoils to deny stall. His research showed a 15 percent increase lift capability at a given airspeed and increased maximum speed of 20-25 percent at a given lift. He compared experimental results with calculated stall results and identified shortcomings and needed improvements in analytical theories.
Over several years, McCloud established rotor performance characteristics for five different rotor systems emphasizing very high-speed flight. Some of the data were taken at advance ratios greater than 1.0 and with an advancing tip Mach number of 1.0. The database generated has been and will be used for many years by rotor system designers and analysts for correlation with predictions.
McCloud was one of the leading experts on rotor airfoil blowing for boundary layer control, power for rotor rotation and rotor control. His early work showed that leading edge blowing could be used to delay retreating blade stall at high advance ratios (greater than 0.34). His later investigations established the full-scale feasibility of using large amounts of blowing to also provide the rotor driving torque with a series of tests with the Giravion Dorand Jet Flap Rotor. Lift coefficients twice those achievable with conventional rotors at advance ratios of up to 0.50 were obtained. He showed that for representative lift coefficients, a helicopter with jet flap rotor operation up to 300 knots was feasible.
In the past ten years, McCloud experimentally and analytically investigated the promise of multicyclic rotor control to improve the ride quality and enhance rotor system reliability. His first full-scale wind tunnel tests investigating multicyclic control used the modified Jet Flap Rotor System. Substantial reductions in vibratory blade flapping stresses and oscillatory vertical hub forces were obtained using an open-loop control strategy. He later assessed the multicyclic control potential of a rotor employing a servo-flap to effect blade torsional oscillations. Again, pylon vibratory loads were reduced with concurrent 50 percent reductions in blade bending moments and blade root control actuator loads.
His analytical studies of multicyclic control included open-loop optimal control to minimize vibration calculated using linear quadratic regulatory theory. He investigated the nature of the transfer matrix between multicyclic controls and the resultant vibration levels, particularly for changing flight conditions. For such a case, closed-loop control including a control lag was shown to significantly improve system identification and control.
In addition to these research areas, McCloud participated in and contributed to numerous full-scale experimental and developmental rotor programs. Aircraft programs included the XV-1, the XV-3, the UH-1, AH-56A, and OH-6 helicopters and the XV-15 tilt-rotor aircraft. Research on full-scale rotor systems included a number of UH-1 rotors, the Advancing Blade Concept, the Controllable Twist Rotor, the Circulation Control Rotor, and the Bearingless Main Rotor. In his last wind tunnel test, the X-Wing system, he brought his years of experience in rotor blowing technology and multicyclic control.
McCloud was an active member of the AHS and was one of the founding members of the Society's San Francisco Bay Area Chapter and served as its President in 1975. He also served on various Technical Committees, including the Dynamics Committee from 1977 to 1981.
His accomplishments will always be remembered.
AHS Update: Vertiflite January/February 1983