how better is bigger

[Vance]

I understand that bigger is better, but in direct drive aplications adding a little rpm makes for more horsepower. I understand that bigger is less better as the speed rises. Autogyros don't go very fast, but even at 70 miles per hour the avantage of a larger prop seems to be less than it is static. Pontential horsepower goes up in a straight line so I am trying to understand the compromise in order to select the best size prop. 10% more rpm means 10% more horsepower, a 10% bigger prop means 10% less rpm which means 10%less horsepower. I am trying to understand the crossover point.

Tip speed for the best efficency seems to be open to opinion. I am reading anything form .82 mach to .92. I am not clear how the number of blades affects the best tip speed.

Two blades is the most efficent, but so many people seem to enbrace three or more. I feel lost on something that seems so clear to others.

I would be gratefull for help. My funding interuption is nearly ended and I would like to start on a simple pusher. My wife want's me to fly with a Harley and I would like to try a direct drive pusher. As you can imagine, the prop size affects the whole structure, so this is an early days decision. My particular engine is particularly happy between 3,200 and 3,800 rpm.

I have seen Hog Air and it is not how I would like to do it.

Thank you, Vance

[brett s]

Bigger prop is always better for low speed stuff if you can run it at the appropriate rpm - in your case wanting direct drive at those rpm values, tip speed is going limit your diameter though. I'm guessing you'll end up in the 4.5' range if you're looking at 3800 rpm max...

One extreme example of how much better large diameter is at low speeds is a helicopter - that's why the R22 can hover weighing almost 1400 lbs using about 130 hp. It's also why the V-22 needs far more horsepower than a helicopter of the same gross weight to hover - much higher disk loading means more power needed.

The number of blades won't affect the desired tip speed - that's more of an airfoil thing. Drag increases sharply above the critical mach number, you want to keep it below that value obviously. The prop manufactor should know where the optimum value is, another thing to take into consideration is noise - higher tip speeds are usually louder.

Where the number of blades comes in is power loading - if your diameter is fixed, you're adjusting either blade pitch or area to effectively use your available power. Too much blade pitch means poor low speed thrust because a lot of the blade will be stalled, so you'd want more blade area. The opposite of this can happen though with too many blades...great static thrust, but nothing useful with much airspeed.

[Ga6riel]

Actually rpm is a function of diameter, there are high speed rotors but they are the exception. Since the inside 40% of a prop/disc doesnt contribute much work, its easy to see how larger diameter rotors are more effective.

Also what is at work are the economies of scale, and the differences in handling the numbers between say the horsepower related arguement which is a cube law and the dimensional component which is at the square. The functions of a scaled up machine are identical, but the viscocity of the air remains the same. This is why those 60% scale fighters are so tricky and nimble to fly (providing everything is to the math of the scale, power/ailerons/wing area etc).