Ditto with others. You can get some information from ground RPM tests and with a semi-educated guess you can estimate what will happen in the air. But some factors such as unloading and engine power curves are often difficult to "guess." In the example I mentioned in post #12, I'm 95% sure the "confusing" issue was engine power curve. I was using a piped YS45 on very high nitro. Without getting into all the ins and out, I just wasn't "on the pipe", i.e. the engine wasn't in a RPM range where the tuned pipe gave the boost I was looking for. All my on the ground testing was worthless, as I never hit enough RPM to get to the pipe. But in the air, as the prop unloaded and the RPM went up, the pipe started to kick in and then the RPM really jumped. If I had to estimate, I figure 3K more RPM in the air vs the ground. It took awhile to find a prop that would unload properly, yet had high enough pitch to get the speed I was looking for, and the only way to really test was in the air. Cheers.