Prop Question

[Bearcat_Pilot]

Hi,
Can I substitue different props if the pitch multiplied by the diameter are equal or very similar? For example is a 9x2 prop the same as a 6x3 in thrust and RPM because 9*2=18 and 6*3=18?

Thanks for your help.

[Dave Robelen]

Hi Bearcat_pilot,
Good try, but the two different measurements are calculated with entirely different multipliers to match the power required. When you plug the pich value into the horsepower equation, it is used as a straight multiplier. On the other hand, the diameter is raised to the 4th power in the same equation. Sooo- smal changes in diameter make a lot more difference than small changes in pitch. A common practice in matching props is to start with a pitch that is likely to match the speed range of your model, and then adjust the diameter until the engine is on it's peak. For the best results it will take several different props of different brands and similar pitch/diameter combinations to find the best match for a specific model/engine combo.
Regards, Dave

[MuddyFox]

Does anyone know what that equation would be? Is there a guide to optimizing prop sizes vs engine RPM, torque, etc?

[Dave Robelen]

Hi Muddt Fox,
A useful equation is as follows-
2.83E-12Xrpm^2XDia^4XCp=horsepower

The term 2.83E-12 accounts for unit conversions and some atmospheric standards.
Diameter is measured in inches
Cp represents the pitch of the prop. This is usually 1.0 for props 11" dia and smaller.
For props of much diferent dimensions the thrust must be measured. A fish scale tied to the tail maybe?

There are very few equations for matching a prop to a specific aiframe.
If static thrust is the main goal, move toward larger diameter props with less pitch. Where maximum airspeed is a priority, move toward more pitch and less diameter. Flight testing is still the most useful method for optimizing the prop/aiframe/engine setup. A quick search on www.google.com for "horsepower equations, propellers" will turn up more detailed material.
Regards, Dave

[Bearcat_Pilot]

Thanks for your help so far but I still have a question. I want a fairly long prop for a small .049 size engine. I want it to be between 8 and 10 inches but I don't believe I could use a regular 10x6 prop on such a small engine. Is it possible to use a long prop with a very small pitch on a small engine without damaging it? If so, do the usual prop makers make such props? Thanks in advance for you help.

[Dave Robelen]

Hi Bearcat-Pilot,
Can I assume that you understand that mounting oversize blades on one of these engines will pull the maximum power down drastically? The horsepower curve for a typical .049 favors high rpm with small loads. At present, the lowest pitch I am aware of is 4" in commercial blades. You could run an 8-4 without harming the engine if you keep the needle rich, but that is about the maximum.
Good luck, Dave