Prop Efficiency

[Reverand Blue Chip]

Right now I run a APC 6X4 on my E* and Funjet.

What is the difference in efficiency between that and say a 5.5X4.5 or 7X3...

Or is it all simple experimentation?

Tell you why I ask - looking at the new E* II with the folding prop - I am considering putting it on the new ship - I just dont know if it will be worth it.

[aeryck]

A lot depends on the rpm you're swinging the prop at. Diameter vs pitch is kind of like low and high gear on a vehicle.. lower pitch but larger diameter will move more volume but at less speed.. think high torque here. Smaller diameter but larger pitch will try and move the air faster - high gear; lower torque but higher speed. It all depends on what kind of flight envelope you want. Using theoretical numbers, if a 6x4 gives you 30oz thrust with a pitch speed of 60mph, a 5.5x5.5 may only give 25oz of thrust but a pitch speed of 80mph. Likewise, a 7x3 may give you 35-40oz thrust but only a pitch speed of 45mph.

Higher pitch also increases probability of prop stall... that is, it creates a partial vaccuum around the prop blade because it literally runs out of air to push.

[londonguy]

Unless you are just interested in theoretical maximum efficiency of a given prop you also need to take into account the plane, motor and the speed range you want to fly at because that will determine what is the best prop to get the best overall efficiency.

I also fly a 6x4 with a Mega 16/15/4 on a 1 meter wing. Back before FPV I used to fly with a 5x5 on the same motor plane and used to get higher speed and better run times than the 6x4. Now that I have loaded it with FPV, laminated it and doubled battery size, the 5x5 is completely useless because I have to run higher throttle to maintain speed and I get rubbish run times.

I then switched to a 6x4 and it's now better suited and I get the long run times and good cruise speed.

So it's not easy to say which is better

[Reverand Blue Chip]

What is the equivelant of a 6X4 or 6X5 APC pusher in folding form?

[Vitamin J]

Unless you are just interested in theoretical maximum efficiency of a given prop you also need to take into account the plane, motor and the speed range you want to fly at because that will determine what is the best prop to get the best overall efficiency.

How much more pitch speed should I aim for over actual airspeed for a slick plane like a wing? I also assume the loss is less at lower speeds than high, is that correct?

[londonguy]

How much more pitch speed should I aim for over actual airspeed for a slick plane like a wing? I also assume the loss is less at lower speeds than high, is that correct?

I would only use the pitch speed a relative guide when comparing two combinations of setup, rather than trying to convert it into something real world. And you are right, the % loss goes up the faster you go because as you double your speed you quadruple your air resistance.

The theory says that a larger diameter prop with a lower pitch will be more efficient than a smaller prop with a higher pitch but you really need to do some real world testing because no amount of number crunching will tell you what the perfect prop is.

[Reverand Blue Chip]

So is the above something of a general rule?

Larger diameter lower pitch = more efficient but slower flying?

[Vitamin J]

Now that I have more experience I'd say the easiest way to think about it is Pitch = Speed and Diameter = Thrust and you have to balance the two to keep the motor running within spec.

[wanderer]

Hello Everybody,

Sorry for what might sound some very dumb questions, it's just for me to clarify and be sure I understand correctly:
When you're talking of pitch speed, you're referring to the speed of the propeller blade going through the air for a given engine RPM, correct?

And the prop stall is it comparable to the cavitation (even the same) can occur on speed-boats for instance? Is there a way to "feel" it? Like a sudden increase of engine RPM for example (as it's the case when you're on a boat and got the propeller into cavitation) ?

Thanks for your answers
Have a great day

Alain

[londonguy]

Hello Everybody,

Sorry for what might sound some very dumb questions, it's just for me to clarify and be sure I understand correctly:
When you're talking of pitch speed, you're referring to the speed of the propeller blade going through the air for a given engine RPM, correct?

And the prop stall is it comparable to the cavitation (even the same) can occur on speed-boats for instance? Is there a way to "feel" it? Like a sudden increase of engine RPM for example (as it's the case when you're on a boat and got the propeller into cavitation) ?

Thanks for your answers
Have a great day

Alain

I dunno about feel but you can definitely hear it. If you wanna know what it sounds like going from normal to stalled state you can try going vertical, assuming you don't have enough power you should hear at some point the prop speed up and a distinctive sound as it stalls.