Does anyone know where to find thrust figures for engine-propeller or prop-rpm combinations? I could work out the math myself, but I would hope that someone helse has already done the work on that and posted it somewhere. Thanks for any leads.

Silent J

you can get a rough estimate or maybe even a close match by going to DMA and typing in the prop size then pick a good brushless motor or anything that will turn what your gas engine does.....then just keep adding cells until you get the rpm your glow engine turns with that prop. it will give you static and in flight. www.flydma.com
then go to p calc. oh and I hope you do know a LITTLE somin about cells and electric motors. if so this should be no prob. and fairly accurate.

Well, this isn't related to anything electric. (not part of that Electric Vtol post) But thanks for the help. I've been interested in doing a VTOL design on my own, and now possibly for my college senior design project. So these figs should really help.

Silent J

does the calculations work for ducted fans as well?

For models, I believe you’ll fine the calculations are a dead end street.
As I’m sure you are aware, the formula for thrust is:

Thrust=Tc (density ratio) (velocity squared) (diameter squared)

Where Tc is the propeller coefficient. This term is empirically determined and requires considerable test data. It not only depends on the propeller but the shape of the body it is attached to. Unfortunately since power curves for model engines are not generally available (except for single points) nor are propeller characteristics readily found. A usable figure for Tc is just not possible.

Your best bet would be to contact the engineering department of a propeller manufacture and see if they have any test data they would be willing to share. Both APC propellers and Bolly props have done considerable testing in development of their lines although the approach of each is different. However I believe you will find that most of their work is in flight test rather than static thrust.

Any data you are able to acquire will only be applicable to the propellers tested. Attempts to extrapolate to propellers of other designs, or manufacturers are at best a wild guess.

Aerodynamics is an empirical discipline. Sprinkled through all the complicated formulas that make it seem like science, there are “coefficients” derived from test data that make them work. For full scale aircraft there is a very large body of such data that enables the designer to design with confidence, but us modelers are left pretty much to a cut and try approach.

I guess the best way to determine thrust still ends up being testing. Kinda sucks though, all this stuff I'm learning in engineering--although useful later on--seems a little difficult to apply to the model industry because of lack of data. Prop efficiencies and power curves would be most useful. Maybe some polite emails to manufacturers could prove beneficial. Thanks for the help everyone.

Silent J

I can understand your dilemma. I graduated from Ga. Tech in Aeronautical Engineering more than fifty years ago and had an interesting career in the field with the USA Air Force, a major aircraft manufacturer, and an airline. Along the way I also became a pilot and flight instructor. As a long time model builder I find that the theoretical knowledge is a great help in understanding what effects what and how to make changes that improve things, but when it comes to designing from scratch, I’m stuck with TLAR (that looks about right). When you plan to build a hundred airplanes at thirty million dollars each, you can afford to do a lot of testing and research but for a $150 model, a little educated cut and try will usually produce perfectly acceptable results.