Drag

[Challengedstudent]

Is propeller efficiency defined as how much thrust is produced per watt or is it how efficient the motor could convert the electric energy into thrust… Also, does aspect ratio have anything to do with propellers? I've done research and some used aspect ratios to come to the conclusion that a greater chord length would result in less efficieny but that doesn't seem right since aspect ratio considers the lift- drag ratio and not thrust - drag.

[Challengedstudent]

o, and does anybody know the ratio of a model propeller to actual propeller

[rmarz]

Challengedstudent - You seem to be having some difficulty with some terminology. If you are driving a propeller with an electric motor, you could relate power consumed by the motor to thrust developed. That could be considered a kind of efficiency. Other factors enter though. Is the motor operating at an efficient speed and voltage? You mention aspect ratio. Not sure what that is referring to. Is it diameter and pitch, or something else? It would seem that a propeller or fan design that converted rotary motion to thrust might be very different than a design to covert airstream to rotary motion (in terms of pitch and chord length). What are you trying to achieve or optimize or test? With more information we may be of more help.

Rick

[Challengedstudent]

I'm testing the science fair project - the effect of increasing the chord length of a propeller on its efficiency.. but instead of using a fan to turn the propeller and measure the electricity the motor produces, i built a thrust meter to measure the thrust produced at a given watt input. However, though i believe this project is talking about how much thrust a propeller could produce at a given watt input, i wasn't quite positive if that was the right definition of efficiency. And regarding the aspect ratio, i did research and saw that but i don't believe it ties with propellers

[rmarz]

Challengedstudent - At first I thought you might be conflicting the issue of an electric motor driven propeller creating thrust and a propeller driven by wind generating power using an electric motor as generator. Obviously two very different scenarios. If you have created a way to measure thrust developed be the propeller and electric motor, that seems to be a valid platform to vary chord length, all other elements being equal, and make your efficiency measurements and observations. There are other considerations that I mentioned before, motor speed and power consumption that are non-linear and could alter your readings. Pitch is constant, so that is controlled. Good luck, you seem to be on your way.

Rick Marz

[Challengedstudent]

Yah, in my thrust meter I have a transmitter that controls the power intake.. But other than the fact that thrust is created due to both pressure differentials and Newton's third law, the fact that drag goes against thrust, and a little info on propellers, are there any other scientific principles to this project?

[Challengedstudent]

Also, how is the propellers pitch altered and why does a propellers rpm go down with an increase in pitch

[theborg]

The pitch is altered by rotating the blades of the propeller at the point where they connect to the drive shaft. As pitch is increased the surface area of the blade presented to the air stream increases, causing an increase in drag on the blades. since I assume there is not an increase in engine power, the same force is applied to rotating the blades But must overcome greater drag resulting in a decrease in rotating speed (rpm). To maintain rpm, you would need to increase input power as drag increases with pitch angle.