Math Wiz Quiz

[AndyT]

Here is one for you math wizards.

How many foot pounds of torque does a 6 bladed prop generate?
52 inch diameter
Total Prop weight = 28.4 pounds American
2500 rpm

And if you had two 3-bladed props with the following;
52 inch diameter
Total weight each prop = 14.2 pounds American
2500rpm

[Mike.Powell]

Andy,

The torque a prop presents to an engine depends on the aerodynamics the blades experience. If the rotational speed is constant, the prop mass doesn't enter into it. Looking at torque from the prop end can get pretty complex.

Another approach is to look at the torque delivered by the engine. If the RPM is constant, the torque presented by the prop to the engine must exactly match the torque the engine delivers. If there is a difference, the RPM will change to put them in balance.

Looked at this way, the question becomes something like: How much torque is required to deliver 620 shaft horsepower (for example) into a load rotating at a constant 2500 RPM?

College physics was long time ago, and I don't recall how to solve this. However, my gut feeling is that it's pretty straightforward. I suspect that if you thumbed through a physics study review guide on your next trip to a bookstore, you'd likely find a similar worked example.

[AndyT]

Mike,

Thanks for the response. I was thinking it would be something along those lines. I know there is an industry standard formula to figure this kind of problem but I don't remember it (Suprise!) and I was not able to find it anywhere online either.