Prop Unloading - Fact or Fiction?

As I've been progressing through the hobby I've heard of this effect of props unloading where a propellor spinning through moving air will draw less amps than one that is spinning at the same speed in still air (for example where you have the plane tethered by the tail and are testing the amp draw on the lounge room floor).

To be honest I was a bit dubious about prop unloading, and of course when you first encounter it with people telling you its okay to exceed the maximum ratings on equipment because "it will draw less in the air" you tend to be naturally suspicious. After all, is the prop drawing less in the air, or is it just a case that the person is working within the safety limit engineered into the product.

So, I've been doing some research and trying to apply my rudimentary physics knowledge to the problem. As I look around the web I see stuff posted like "as most flyers are aware props unload a little in the air". From my uni days I know what a statement like that means - "I'm pretty sure this is right, but I have no idea where to find a scientific explanation". Well, that's what I wanted to find.

Initially the best my efforts have uncovered is some ball park figures like props with higher diameter versus pitch (ie 3D type props) tend to unload more than props with higher pitch relative to diameter - so an 11x4.7 would unload well, whereas a 5x5 would not unload so much, depending on application.

Those that use the motocalc program have it automatically calculate how much props unload, and those that use data loggers in their aircraft definitely confirm that the prop unloading phenommenon is observable from the data. So, its definitely real - but why?

I'm still left wondering why this happens, and whether a good rule of thumb can be devised for model aviators that is not as coarse as the one currently recommended (amp draw reduces by 20% in the air).

Now - the why. Why does it happen? I've had a few different reasons put to me, and it seems that many of them play a part. The first explanation relates to effeciency. To get a handle on this I had to look into propellors in a little more detail. A propellor actually uses two different scientific principles to work. The first is what you expect - Newton's third law - we push something out the back (air) and as a result the plane moves forward in accordance with conservation of momentum. The second is obvious once someone pointed it out to me (don't you hate that) - Bernoulli's Principle. The propellor is in fact a mini wing and as it spins it creates low pressure in front of the blades, and high pressure behind the blades literally pulling the plane into the low pressure region in the same way that the low pressure above your wings holds your plane in the sky.

Now the effeciency of the propellor, just like the effeciency of the lift from your wing, relates significantly to the angle of attack the blade has with the air. The angle of attack with the air has two components - the fixed pitch of your model aircraft's propellor and the speed at which airstream is striking your propellor. This means the angle of attack of the blade changes as you vary the aircraft's speed.

Now, that explanation covers part of the phenomenon - if the effeciency of the prop varies depending on speed then clearly there will be an optimum speed where you get the most thrust. But more thrust in and of itself does not explain why your motor draws less amps.

The second explanation I've been given relates to the difference in effort between turning the prop through a static medium versus turning the prop through a moving airstream. I must admit this is the one I have the most difficulty seeing as having any relevance. The airstream in front of the propellor is partially moving anyhow, even in a static test (air has to rush in to replace the air that was just ejected out the back). The propellor has to impart force on the air as it passes, otherwise it will not generate thrust according to Newton's Third Law and as best I can see there is no particular reason to believe that imparting force (not lift - force) becomes easier as the airstream moves. After all, even in a static test there are no trully static forces (everything is in motion to some extent or another).

Here's the analogy I considered. Consider a car driving at a constant speed along a road. The road, in motion relative to the car, can be considered our airstream, and our wheels are our propellor. There are a number of forces the car's engine needs to overcome but the two biggies are friction through the drive train/axles, and wind resistance. If we take our foot off the throttle the car doesn't immediatelly stop - instead it gradually begins to lose velocity according to the forces acting against it. However, excepting static friction there is nothing about the car's velocity that makes us need to add less power to maintain speed. The fact that the car would continue to move along the road with no throttle for some distance is merely a reflection of the kinetic energy already invested in the car's body, wheels, drivetrain etc. Yes - it is easier to maintain a velocity than to accelerate, but that is scarcely news to anyone. However, when you are flying at wide open throttle you are trying to accelerate continously. Accepting the angle of attack I don't see any reason to consider that it is magically easier to impart force on the faster velocity airstream.

So, this brings us to the final consideration/explanation. If a prop with a high diameter relative to pitch is observed to unload more, does that give us a hint. Maybe. If you took a look through these notes you are familiar with the idea that a plane can be either pitch speed or thrust limited. Pitch speed limited is like a car in first gear. There is enough power to go faster, but the wheels just can't spin any faster. Thrust limited is where the force of wind resistance and drag equals the thrust produced by the prop and the plane stops accelerating - this is the car flat out on the salt flat - short of redline, but no longer accelerating.

In the pitch speed limited scenario let's say the plane can produce 500 grams of thrust. However, it only needs 250 grams of thrust to overcome drag at it's maximum pitch speed of 40km/hr. The plane can't go any faster in this scenario and it seems entirely plausible that the prop unloads substantially. Simply the motor does not need to add as much energy to the prop to maintain the velocity (just like in the car in first gear you back off the throttle and maintain velocity close to engine redline with much less fuel).

So, where does that leave my understanding - still lacking to be honest. I can grok why:

1. The prop on a pitch speed limited aircraft unloads (which aligns with why large diameter low pitch props unload more).
2. Why the effeciency of propellors changes due to the angle of attack of the blades against the airstream.

What I still can't figure out is why the propellor in a thrust limited plane unloads. The propellor becomes more effecient, but surely that would just give more thrust per watt leading to a higher top speed, but not leading to a reduction in the number of watts consumed at that top speed. And yes, you could maintain a higher speed for less watts, but that is not the same thing as drawing less watts in the air at WOT as on the ground at WOT.

So, for the moment - no rule of thumb - I guess I am hoping for someone that actually understands this stuff to come visit the site and enlighten us all.