On May 18, 7:30 am, "Martin X. Moleski, SJ" wrote:
The most effective area of the prop is the outer areas, not just the tip. And shortening a prop doesn't get rid of tip vortices; we just get new tips with vortices of their own. Shortening most props will increase the tip vortex, since the chord increases, and tips with longer chords generate larger vortices. Pointy tips generate smaller vortices. There are some full-scale props that have special tip treatments to control the vortex, and they run quieter as well as more efficiently. McCauley's Q-tip prop is an example. Shortening the prop unloads the engine and allows it to turn at a higher RPM, therefore generating higher HP. But there are tradeoffs: a shorter prop is less efficient, especially at lower airspeeds, and a faster-turning prop loses more HP to drag, which increases by the square of the increase in speed. Shorter props are usually found on faster airplanes, longer props on slower aircraft. The fast airplane will have a longer takeoff roll due to the thrust losses of the shorter prop, and the slow airplane will be limited in its cruise speed by the longer prop, whose tips get too fast and create too much drag. The tip speed of any prop in the static condition is 3.14 x diameter x RPM, so a 6" prop, being roughly 19" in circumference and turning at 15,000 RPM is doing
285,000 inches per minute, or 270 MPH. In forward flight, the tip speed increases even if the RPM stays the same, The tip speed becomes the square root of the sum of the square of the tip speed plus the square of the forward speed. If the model is doing 75 MPH, the tip speed is about 280 MPH. In full-scale airplanes, tip speeds in the static mode can be over 700 MPH (as in a Cessna 185 floatplane with the longer seaplane prop, turning at 2850 RPM) and the noise becomes fearsome as sonic shock waves are generated by the tips. The drag increases with forward speed as the tip speed rises, so this sort of airplane will have a constant-speed prop in which the pitch is increased and RPM decreased for cruising. (The term "constant-speed" refers to the propeller governor's ability to vary the prop pitch to maintain a set RPM by loading or unloading the engine as necessary to keep RPM constant.) Tip speeds this close to the speed of sound cost a lot of HP, but the longer prop is necessary to get the airplane up on the float step for takeoff within a reasonable distance. The propeller's pitch is usually measured at 70% of the distance from the hub to the tip. The angle at that point, and the circumferential distance described by that point, will give us a theoretical distance of travel when we use a little trigonometry. You can do it on paper without the trig: Draw a baseline, and another line angling upward from the left end of the baseline at the angle you find on the prop blade. Measure along the baseline the circumferential distance, and then measure straight upward to the angled line: that's your pitch. If you take the angle anywhere along the prop's blade, and work a circumferential distance for that point, you'll find about the same pitch, though some FS props have the pitch distributed a little differently to take into account such things as cowling drag or cooling issues.Dan