|The Myth of High RPM|
|Written by Larry Schlasinger|
|Friday, 16 December 2011 11:17|
The first time I pushed the throttle forward on my newly acquired Cessna 185 in 1986, I noticed something strange. Instead of the tach going to the expected 2,850 rpm, it went all the way to 3,000. Wow! Because I had no experience with 185s or their engines, I decided to seek the advice of a friend who had been flying them for years. He asked where the plane had come from, and when I answered, “Alaska,” he started to laugh and said, “Figures.” I inquired as to what was so funny, and he stated, “Those guys up there will do anything to increase the performance, whether it’s approved or not. The increase in engine speed will increase the horsepower and reduce your takeoff; just don’t let an inspector see that, as it should only go to 2,850.”
His explanation seemed to make good sense, and I started to make takeoffs with the engine speed at 2,850 rpm, unless I thought I needed the extra performance. Any time that I was heavy or it was hot, the prop control went into the panel, the prop screamed, and away I went. It did seem to have better power, and really a lot more noise. I flew this way for many years, and I never thought much about the engine speed; I just made it high for more power and less when I didn’t need it.
Then, in 1998, I was introduced to Gerd Muehlbauer, the owner of MT-Propeller. I was negotiating to be his United States sales rep when the subject of high engine speed and, specifically, the Cessna 185 came up. I wanted to STC an MT-Propeller three-blade propeller for the 185, and I wanted a long prop that could be turned at least 2,850 rpm. Gerd started to laugh (sound familiar?) and told me a story about horsepower vs. engine speed vs. prop length vs. thrust. He gave me a good lecture about what makes a plane fly and how power is converted to thrust, and it is thrust that really counts.
He stated that my idea of more engine speed making more power was correct, but when the prop is turned beyond about .9 Mach, it starts to become inefficient, because of the Mach buffet and turbulence on the prop tips. When this happens, the propeller is converting power to noise instead of thrust, and real performance decreases. I found this hard to believe (why should I believe one of the best propeller engineers in the world?), as I had been flying with a long prop and high engine speed for so long. Gerd saw the doubt in my face, so he decided to prove to me his point. He set up a standard thrust testing scale and attached it to the stinger on my 185. We then proceeded to measure the thrust applied at full power and engine speed starting at 2,200 rpm. To my chagrin, he was right. Using the standard-equipment 86-inch aluminum prop, we saw an increase in thrust up to about 2,600 rpm, and then it started to decline after that. We could only get 2,800 rpm static, but the thrust was considerably less than the 2,600-rpm reading. I can just imagine what 3,000 rpm would have been; much, much less.
After this testing session, I invited some of my other seaplane friends to come to a propeller testing party. We assembled several propellers suitable for the 185: a standard metal 86-inch two-blade, a new 86-inch metal three-blade, an 83-inch MT-Propeller three-blade composite, and also an 83-inch MT-Propeller two-blade. We ran all the props on the same plane, on the same day, in the same conditions (keep in mind that the actual thrust numbers produced are somewhat irrelevant. The particular scale, the attachment point on the aircraft, and ambient conditions make these numbers vary greatly. An aircraft producing 1,100 pounds of thrust on one scale may actually have less thrust than another aircraft that only produces 1,000 on a different scale). Everyone there, except me, was really surprised by the results. All of the propellers produced more thrust at lower engine speeds, and the 83-inch MT-Propeller three-blade produced the most. The MT-Propeller two-blade and metal three-blade were about the same, and the two-blade metal produced the least. The MT-Propeller three-blade was also almost 25 pounds lighter than the metal three-blade prop.
So, what does all this mean? It means that I’d flown under a misconception for many years and made my neighbors mad at the same time. It also means that when someone with as much expertise and experience as Gerd Muehlbauer tells me something, I’ll be more inclined to believe him.
Aluminum 86-inch three-blade MT-Propeller 83-inch three-blade
Engine Speed (rpm) Thrust (pounds) Engine Speed (rpm) Thrust (pounds)
2,200 696 2,200 735
2,300 738 2,300 772
2,400 810 2,400 845
2,500 903 2,500 943
2,600 1,050 2,600 1,092
2,700 1,001 2,700 1,126
2,800 920 2,800 1,075
Note: The MT-Propeller at 83 inches was optimized for operation at 2,700 rpm with standard temperature; operating in lower-than-normal temps will reduce the high-engine-speed performance even more than indicated here.
|Last Updated ( Friday, 16 December 2011 11:25 )|