Steve Cote
Some time ago Bob Steward told us that the most efficient way to run our 360 equipped Sundowners was, basically, wide open as long as we were up high. I have taken this to heart and she seems to work just fine.
Well, some folks saw the pix of my 1400 mile trip recently and, when they saw the shot of my dash at 8500′ and 2650 RPM they went a little over the top.
“Are you crazy? You’ll bust the engine that way? You can “over-rev” if you even slightly nudge the wheel down!”
A calibrated tach being a pre-requisite, of course, just what does “redline” or “overspeed” mean at altitude for this engine, fixed prop, and cruise attitude? (Of course, here I am, away from the books and I cannot specify my prop pitch. It’s somewhere in the middle to cruise range.) At what point does that red paint on the gauge mean “danger”?
Bob Steward, A&P IA
Since I started the controversy, let me explain. The engine produces full rated power at sea level, 59 degrees F and 2700 RPM. This combination is 100% of the continuous HP that Lycoming and the FAA agree the engine can produce.
As you climb the engine loses manifold pressure as the air thins out. This means that at less than 29.92 in hg the engine no longer puts out full rated power. You can set the RPM by closing the throttle part way and artificially thinning the air going into the engine, or you can climb to a high altitude and let the atmosphere provide the thinner air automatically with the throttle wide open.
If you look at the tables in the Lycoming Engine Operator’s Handbook ($19 from Lycoming, and everyone ought to have one), you’ll see that at full rated RPM (2700) and about 8000′ Density Altitude the engine produces 75% power.
This means that the way your instructor taught you in primary training to cruise at a set RPM (2500?) is gradually lessening the power the engine has as you climb to higher altitudes. Above 8000′ you have to have the throttle all the way in to have whatever % HP is left below 75%. This means that the engine will be running 2650-2700 just to provide cruise power. There is nothing wrong with turning the engine 2700 all day long if the manifold pressure is such that the engine produces 75% or less power. Down low you have to do this manually by throttling back. Up high the air is already thin, and you can run wide open throttle just TRYING to get 75%.
As to the “over speed” issue, Lycoming defines several levels of overspeed, and a “momentary overspeed” (defined as less than 10% and for less than 30 seconds) is no problem. That is 270 RPM over the 2700…. 2970 is the limit for a “momentary overspeed” and is of no consequence in the life of the engine. So accidentally letting the RPM cross 2700 is not any cause for concern.
If you want the performance that the book says your plane will deliver, then you have to fly it like the factory test pilots did. That means you have to know that “2500 RPM” is not cruise power, its just a power setting at a particular ALTITUDE.
A lot of people fly down low and bump around in the rough air, when they could fly higher, get above the turbulence and have cool air (3 degrees F / 1000′ is the lapse rate, so 8000′ air is 24 degrees cooler than sea level, think about that next time its 90 degrees on the ground…) and you’ll get the ~2% / 1000′ boost in TAS. Want to see a Sundowner scoot? Got to ~7500′-8500′ and get that 15% boost in TAS. Sure you are going to be running 2700 RPM, and having cool air come in the cabin vents, and you’ll also be seeing 10.2 gph fuel flow at ~120 knots.