It seems that no one wants to stick their neck out, so here is a way to approximate it - which is probably all you need:
The prop load is proportional to the square of the rpm. So pick what you think will be the hp and rpm that hp is developed. Since you haven't run the engine yet, just guess. Then calculate the predicted torque at that rpm: T = hp * 5252/rpm, where torque is in ft-lb. Then you can calculate the torque to be set at other rpms by the equation T = (rpm / (rpm for max power)) ^2 * Torque at max power. Example: Say you predict 200 hp at 6000 rpm. That would require 175 ft-lb of torque. At 3,000 rpm the toque would be 1/4 of that, or 44 ft-lb. Other speeds and torques: 4,000 - 78, 5,000 - 122.
My
guess is that what you might want to do is start off at the lighter loads and then gradually work up to the max power condition. Use the predicted loads and then as you get close to the max power point you will find out how far off your prediction was and can correct and do it again. I'm sure you will want to run WOT at a range of rpms so you can pick the best target speed.
Most people that are limited to running engines with only a prop would give anything to have a "real" dyno where speed and torque can be set independently. Why not run a whole variety of speeds and loads to get a better idea of how it performs? Hope it goes well - good luck!
Gary Casey
Note: Forwarded message is
attached.
ey Troops;
Finally in the dyno room building the mount plate to begin testing. Tracy says that the engine should be tested with the prop on, and this is kind of hard in a dyno room. We are mounting the engine without the PSRU, so that we don't tear it up in testing, and instead hooking directly to the shaft which is loaded by means of a water brake. We can put any load on it that we want, problem is, how to calculate that prop load in foot-pounds, at different settings. Talked to Craig Cato, and he is leaving for Europe, so doesn't have time to run the calcs, and I am just a dumb high country nail-banger. Any thoughts?
Greg Ward
Lancair 20B N178RG in progress