David:
I was talking to a guy about engine rpm in flight. As previously posted, I just completed a 476 mile hop at 9500 feel msl in under 3 hours. I was not quite at full throttle. The question regards what type of maximum engine rpm we can develop in a fixed (cruise) prop Musketeer. He thinks that the most we can get is about 2500 RPM at altitude. So told him I would ask the group. Can you red-line at 8000 ft? If we can red-line the engines, then re-pitching the prop would be one way to go faster.
Bob Steward, A&P-IA:
The question is one of “CAN you red line” or “MAY you redline” at 8000 feet.
If you read the POH/AFM in the Performance section you’ll see that at 8000 DA the engine can produce ~77% power at full throttle and 2700 RPM. So YES you MAY run full throttle at altitude and whatever RPM the engine will run (at or below 2700) is acceptable. Now whether or not you CAN get the engine to 2700 in level flight at 8000′ DA is another matter.
The prop pitch, power output, sleekness of the airframe, weight of the plane, cabin load and fuel, etc. all combine to determine IF you can get the engine RPM up to 2700. Prop pitch varies RPM at about the rate of 1″ of pitch = 50 RPM change in engine speed.
Given that there are an appalling lack of speed mods for Musketeers to help make them more sleek, washing and waxing the plane is about all you can do. Followed by not tankering fuel, to as to stay as light as is reasonable. After the Holidays, the Pilot and Passengers might also consider shedding a few pounds. After all the lift required to keep that weight in the air has a drag component, and less weight = less drag. And the less profile and induced drag the plane has,the faster it will fly, therefore the load on the prop is reduced if the plane has less drag and is sliding through the air easier. A clean air filter and properly set ignition timing are about all you can do for engine performance.
So we are stuck with having the prop re-pitched to achieve the magic 2700 RPM at 8000 DA. And don’t forget to drag out that whiz wheel that you haven’t used since the Private written. You really DO want to figure the Density Altitude in order to have meaningful numbers.
One other note of caution. 20+ year old Stewart-Warner mechanical tachs are notorious for being 100-150 RPM low at the redline. So until you can check yours with a photo tach, don’t spend big money to change anything, as you may already be at 2750 when it reads 2600!
Consult your power charts and understand that as the air gets thinner, the engine has to run faster to output the SAME HP. An RPM setting for 3500′ that gives 75% power is seriously below 75%, if flown at the SAME RPM at 8000′.
Editor’s Note:
Bob has clearly explained that you CAN run 2,700 (redline) RPM at any altitude, as well as how you can do it (reduce prop pitch). I’ll try to briefly address the unasked question: SHOULD you run 2,700 RPM at 8,000 DA? Only you can answer that question.
If you want to obtain maximum cruise horsepower, the answer can only be “Yes”. The forward velocity “unloads” the prop, but at the same time the falling air density (with heat and altitude) is reducing engine power. Torque and RPM determine horsepower, so if you are after max cruise, you need to have the prop pitched to achieve peak permissible RPM at your chosen primary cruise altitude. Reducing pitch for this purpose will have side effects that the pilot will have to manage. You will have to make certain that the engine does not overspeed near the end of a low-altitude takeoff run, and during descents. You are also not supposed to re-pitch the prop to the point that static full-throttle RPM exceeds the maximum value listed in the TCDS. Each prop also has a re-pitch limit, usually both in degree of change, and frequency of re-pitching.
If you want to obtain a balance of cruise efficiency and ease of engine management, the answer is probably “No”. A full-throttle cruise RPM of 2,500 is probably pretty reasonable. The prop is somewhat “loading” the engine, a bit like a constant-speed prop in cruise pitch, so the engine is running at full throttle but at a reduced fuel burn (and reduced power output). It will most likely be operating within the TCDS range for static RPM, and RPM will be somewhat less sensitive to changes in pitch. The engine will be less likely to exceed redline at the end of a sea-level takeoff roll.