A canna’ change the laws of physics

Scotty, The Naked Time, stardate 1704.3, Episode 7

What a plane fishy old bird

Posted by apgaylard on May 30, 2008

Viktor Schauberger - CrankViktor Schauberger had some peculiar ideas.  One of them was that aeronautical and marine engineers had got it all wrong with the propeller, or airscrew. 

“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.”

 (Viktor Schauberger, Implosion Magazine, No. 112, p. 52)

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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2 Responses to “What a plane fishy old bird”

  1. auslaendisch said

    You’ve analysed this from a different perspective than I would. I also wouldn’t necessarily argue that you sum the components of lift and drag based on the chord axes in the forwards direction to get propulsive thrust, because for a typical aeroplane you would normally define lift and drag based on the aeroplane axes, even if the wing was at a positive setting angle.

    Instead I would say that you integrate the pressure around the blade to get a force, which probably points slightly backwards of normal to the lift direction, and then you take components of this total force to get whatever you like – I would take a component in the thrust direction and call it thrust, and a component opposing the motion of the propellor, and call it drag. This drag is then the load on the engine due to the rotation of the prop, which defines the energy required to generate a certain propulsive force.

    I would also use an energy based argument to counter Schauberger. His maple-seed aerofoil is like a helicopter in the autorotation phase of an emergency landing – the maple seed converts downwards kinetic energy into rotation due to it’s shape, which has a braking effect. The force acting here is a propulsive force, upwards – the drag force in this case. The force acting in the direction of rotation is now probably more or less zero, unless the maple leaf is observed to spin faster as it falls? In the helicopter case I think if you push the blade angle very negative, the blades accelerate, implying a force increasing the rotation (but less vertical drag, so you drop faster) or you can reduce the angle or even bring it positive, whereby you get a bigger vertical force, slowing the helicopter but also slowing the blades. Of course if you supply energy into the system you can reverse the whole physics and generate an upwards force, but now you have to pay the drag penalty that opposes the rotation with engine power.

    What do you think? It’s not really different to what you wrote but perhaps the helicopter argument is equally persuasive and a bit closer an analogy.

  2. apgaylard said

    auslaendisch:
    Thanks for taking an interest. Excellent analysis – a bit more detailed than my relatively basic understanding of propeller aerodynamics (Based on John D Anderson’s Introduction to Flight) though, as you say, not that different.

    I think that your autorotation analogy is spot on (see maple-seed link in the post). Whichever way you look at it Schauberger has it wrong.

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