What a plane fishy old bird
Posted by apgaylard on May 30, 2008
“As best demonstrated by Nature in the case of the aerofoil maple-seed, today’s propeller is a pressure-screw and therefore a braking screw, whose purpose is to allow the heavy maple-seed to fall parachute-like slowly towards the ground and to be carried away sideways by the wind in the process. No bird has such a whirling thing on its head, nor a fish on its tail. Only man made use of this natural brake-screw for forward propulsion. As the propeller rotates, so does the resistance rise by the square of the rotational velocity. This is also a sign that this supposed propulsive device is unnaturally constructed and therefore out of place.”
This statement exhibits a strange inflexible insistence that concepts can only be applied to one end: once a brake always a brake! This is self-evident nonsense: the fastest propeller-driven aircraft ever to fly, the Tupolev Tu-114 had a top-speed of 541 mph (Mach 0.73). Not bad for something relying a “braking screw”.
How do propellers work? Their blades rotate at an angle to the intended direction of motion. Essentially, they are wings moving at an angle of incidence (pitch) to the local flow. As they move through the air they push it out of the way it; causing a ‘pinching’ of the flow streamlines around the “suction” surface (rear) resulting in a reduced pressure (and increased local airspeed) compared to that on the “pressure” surface (front). This pressure differential gives rise to lift and drag forces.
These forces act parallel (drag) and normal (lift) to the blade chord. As the blade is inclined, or pitched, the lift and drag forces have components both normal and perpendicular to the intended direction of travel.
Taking the components of these forces in the direction of travel gives rise to a net thrust. Summing this over the length of the propeller blades gives rise to the net available thrust which moves the aircraft forwards.
Where has Schauberger gone astray with this idea that the airscrew is a brake and as such out of place as a propulsion device? He’s led himself astray by focussing on the changes in only one of the key physical forces at play here.
While it is true that the drag of a blade increases with the square of the air velocity flowing over it, so too does the lift. As a propeller is – oddly enough – designed to propel, the aerofoil sections used to form the blades are shaped to give the maximum thrust for minimum loss. Therefore the propulsive force dominates over the resistive.
As a result of careful profiling, and the automatic control of blade pitch (so-called “constant-speed” propellers have automatic systems to continuously vary the blade pitch to maintain engine torque such that the rate of engine revolution is constant over the range of flight speeds) most modern propellers have efficiencies in the range of 83% to 90%. Not too shabby for a braking device!
It may be that, “Only man made use of this natural brake-screw for forward propulsion.” But the results can hardly be argued with: whilst the peregrine falcon has been clocked at over 180 miles per hour, and the mighty sailfish can manage an impressive 68 miles per hour in water, neither of these natural record holders travel at anywhere near the speeds obtainable by propeller-driven aircraft.
Perhaps more importantly, citing the increase in resistance of a propeller rising with the square of rotational velocity as, “a sign that this supposed propulsive device is unnaturally constructed and therefore out of place” is plain bonkers: the aerodynamic drag of anything in nature varies with the square of velocity, once a critical speed for that object in a particular fluid (actually, Reynolds Number) has been exceeded.
This is as true of the peregrine and sailfish as it is of a propeller blade. If Schauberger had any logic he’d be complaining about their propulsive systems. The only things, “unnaturally constructed and … out of place” are Schauberger’s ideas. It’s baffling that this crank is still taken seriously in some quarters.
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