i have a pandora with 3 bladed prop, i would like to use a 2 bladed prop. has any one done this , and what size worked for you and what sized spinner , and where did you buy them from. thanks rusted & busted
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Re: prop swap
For our power output and plane size in RC use, the fewer the number of blades the better when you are trying to get as much thrust as possible at the highest speed. The more blades, the more inefficient the prop is. The race guys used single bladed props that were counter weighted for balance. Full scale planes have to add blades for high horsepower output to try to keep the blade tip velocity below the speed of sound.
You can use a rough power index guide to compare props based on pitch and diameter for a given RPM.
Power index = Diameter x Diameter x Pitch
It's just a pure number but it will compare the power used by two different props at the same rpm.
example: 10" diameter prop with 6 pitch would be 10 x 10 x 6 = 600
9 x 9 x 9 = 729
The 10" will be like a lower gear and pull more in climbs and through turns. The 9 inch will be like a higher gear and pull harder at higher speed if the air frame doesn't have too much drag to get to the higher speed.
To fine tune the setup you are on your own. Turbulence is not something that can be predicted very well. You kind of have to let the air frame / motor / prop combination tell you what they like at the speeds you want to go.
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Re: prop swap
If you don't have one, buy a watt meter, before altering the power system on any electric. It will tell you whether a new prop will smoke the ESC or motor, long before you find out the hard way. And, while the comments about prop efficiency are valid, the difference in the size props we use are hardly enough to notice, IMO.Comment
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Re: prop swap
The rule of thumb for going from 3 or 4 blades down to 2 blades is to increase the diameter by 1-2" and decrease the pitch by 1-2". Eg, from a 3-blade 8x8, a 2-blade 9x7 would be a good starting point.
Consideration should be given for ground clearance when making this conversion, on models where it applies - eg 'ROG' or Rise Off Ground models vs hand/mechanical launch, & fuselage clearance on multi's. And as alluded, confirmation with a watt-meter (for electric powertrains of course) is essential, making sure you stay within the bounds of both the motor AND the ESC, and in some cases your choice of LiPo as well. You also want to ensure you don't exceed the chosen prop's RPM limits.
And finally, of course the spinner (where used) will also have to be changed to accommodate the blade count.Comment
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Re: prop swap
The formula I found for the load on small props is based on the cube of the diameter, times pitch, times the square root of the blade count minus 1, or D*D*D*P*sqrt(B-1). Running the stock 11x6x3 and evaluating for two blades with a similar pitch, I get 12x6.5x2 as the nearest equivalent, so a 12x6 should draw slightly less power at the same rpm, but the rpm may increase because of the lighter load, giving a net increase in actual speed. I've used this formula for conversions between props of the same blade count but different diameters, also.
Another variable is the blade profile and stiffness of the blades; I've found that two props with the same specs can vary as much as 20% in the maximum power drawn from the battery. Again, trust the math, but verify with a good watt meter.Comment
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