OK, I have too much time on my hands but someone had to figure out this pitch x rpm stuff.

Here is how this Patriot made it to 170 mph in a dive with a 10x8 and why I predict 201 mph in a dive with a 9x10 prop. The theoretical speed considering pitch and rpm must also consider the propeller airfoil and airframe drag to be accurate. Since the propeller airfoil and the airframe drag is usually ignored and counteract each other, the straight pitch x rpm is often not to far off. As we try to go faster we use sleeker airframes to over come the ever increasing drag with speed. This often keeps the simple pitch x rpm some what accurate in the speed range our models fly. Put the same prop and engine on a 60 trainer and we see the formula is not even close.

To calculate speeds I used information from http://www.apcprop.com/ and http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html . I think I can trust APC props and NASA. I would like to start with a quote from the APC site that says

“The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more.”

The NACA 4412 airfoil has a camber of 4% and if we put that in the foil simulator on the NASA site we can see that it has a lift factor equal to an AOA of about 4 degrees. We then add 1 degree because of the altered zero-lift angle as stated on the APC site. We then have an airfoil lift factor equivalent to 5 degrees pitch. A 10 x 8 prop is twisted 15 degrees at the tip to give it an 8” pitch. Therefore, because of the airfoil shape, we have a pitch equivalent to 1.33 times 8 (15 degrees for pitch and 5 degrees for airfoil) or a 10 x 10.64 prop.

The Rossi 53 is not timed for high rpm and gets a static 15,500 rpm with a 10 x 8. A typical unloading of about 1500 rpm would take it to 17,000 rpm. After that the engine timing and the pipe would act as a governor and not allow much more, even in a dive. At this point the prop would act as a huge break if the plane tried to go faster. Calculating a theoretical speed using 17,000 rpm, using our equivalent prop size of 10 x 10.64 and not losing anything to drag because of the steep dive you come up with, you guessed it, 171 mph. Now where have we heard that # before?

Do the same thing with a 9x10 prop and we come up with an equivalent pitch of 12.5. The same rpm is used because of engine and pipe timing and no loss to drag because of the dive. The speed will be 201 mph if flutter does not take it out. It is interesting to note that the actual speed of 133 mph in level flight would be an actual prop efficiency of 78%.

No arbitrary views but any other thoughts are welcome if you made it this far