What can you tell me about static RPM, engine management with a constant speed prop, prop pitch for climb versus cruise, and mixture leaning?
With credits to Bob Steward for most of this original text (minor editing by Mike Rellihan). This response consists of excerpts from a number or prior postings on the MM List:
STATIC RPM, CLIMB RPM, AND LEANING
What the Type Certificate Data Sheet calls “static rpm” is the highest RPM your engine will produce at sea level on a standard day. Your POH/AFM ought to show it; 2,300 RPM is usually mid-range. It’s listed as 2250-2350 in the TCDS. By the way, if you are not familiar with the TCDS information for your airplane, you should download it from the BAC site, look up the section that includes your model and serial number range, and read it from top to bottom.
Does your plane seems to take forever to accelerate, but once you climb out and lean the engine, it’s like you’ve turned on the turbocharger? Under most circumstances, especially with a fixed-pitch prop, the mixture is usually too rich while you’re on the ground.
The engine is supposed to run full rich for cooling at WOT. Lacking a C/S prop, you can’t get the full 200 HP out of the engine during static run-up and initial acceleration; you probably get closer to 140 HP at 2,300 RPM. If you have an EGT instrument (preferably all-cylinder), you can see how much leaning it takes to get the EGT to just start to rise (NOT peaked!) at full throttle. As soon as the needle starts to move, you are into the range where the “excess fuel” is
gone, and further leaning will remove the cooling needed for the engine to safely run hard (assuming you are operating below a density altitude of 7,500 feet or so).
Consider perhaps a home base at 900′ elevation, and when you start the takeoff run you are only seeing 2,300 or so RPM. Should you lean a bit on the ground, to get more power? If so, what RPM should you shoot for before brake release? When the airplane is sitting still, the prop isn’t “unloaded” from the forward component of the velocity vector, so the engine can’t accelerate the prop to produce anywhere near full power (not even 80% power). I typically lean the carbureted engine used in the Sundowner just enough to start showing up on the EGT. This improves takeoff power without (in my opinion) hurting the engine (as it is operating well below its peak power capability).
Also at 900′ field elevation it is quite possible to encounter a Density Altitude for which leaning before takeoff is appropriate. I always consider 3500′ to be the highest DA at which I use full rich in an un-instrumented plane. Lycoming says we can lean all we like, below 80% power. As an example using the O360 carbureted engine, 180 HP X 0.8 = 144 HP. 2,300 on this engine is not 144 HP according to the Lycoming power charts. Of course, you have
to rethink this once you break ground and pass 100 knots, and the RPM has risen to maybe 2,550. Its probably not a good idea to completely peak the mixture on the ground and, then have to fiddle with it again 10 seconds later as you are trying to clear the tree line. On the other hand, starting out with it a bit closer to “right” might help you clear it.
Everyone should have a copy of the Lycoming Engine Operators Manual for their engine model. They are about $19 direct from Lycoming. Its a wealth of information that answers questions that Beech may have glossed over.
RE-PITCHING THE PROP FOR BETTER CLIMB OR FASTER CRUISE (“mutually exclusive keywords”)
Fixed pitch props are like a car with only 2nd gear in a normally 3-speed manual transmission. They are a compromise between a best takeoff and a best cruise pitch. The “Static RPM” in the POH is full-throttle, stationary on the ground, on a standard day with standard temperature, and with minimal wind. It is simply an indication of the power output of the engine.
All planes with fixed pitch props climb at less than 100% power. If they could turn up 2,700 RPM in the climb, then as soon as you leveled off you’d have to throttle WAY back, to keep from over-revving the engine. The result would be too low a cruise power. Of course, if you are operating out of short strips, or doing banner tows, you might want to sacrifice cruise speed for better climb and towing capability.
Most 20+ year old planes with mechanical tachs have an error of 50-150 RPM on the low side. That means that when the tach reads 2,500 RPM, true RPM might be 2,550- 2,650. I’ve seen them with more than a 200 RPM error. Before making any great assumptions about the condition of an engine or prop, its best to have the tach checked with a photo tach. They are usually available from industrial sources, and many model airplane enthusiasts (and hobby shops) have suitable test tachs. The Chadwick prop balance guys can tell you to within 5 RPM what the engine is actually turning; a prop balance will tell you how accurate your tach is at max static RPM.
54″ is very definitely a CLIMB prop for a carbureted Musketeer. This is how to determine how much climb will be lost if you go to a 58″ Pitch on the 160 HP engine (higher pitch number for faster cruise). The rule of thumb is 50 RPM per inch of pitch change. Go to 8000′ Density Altitude, and run wide open throttle with the mixture leaned to peak RPM (not peak EGT); write down what your Tach says. If the CORRECTED tach reading is not within 25 RPM of the redline, then you may want to consider having the prop re-pitched (if you wish to achieve a higher cruise speed and slightly better climb). If you can live with throttling back in cruise to avoid exceeding redline (and thereby reducing cruise speed), you can reduce pitch even further (to improve the climb even more). If it’s at least 100 RPM over redline in cruise, then you can safely assume that you can re-pitch to a higher number and gain more in cruise, and trade off the climb. I’d have to say that for a 160 HP engine, 54 is probably too little pitch, and that 56-58 is more like it. Another factor is to look at the static RPM you can currently pull. Run the engine on the ground at WOT and lean to peak RPM. How does this compare with the TCDS required static RPM (usually 2250-2350)? If you easily exceed the minimum, then some re-pitching to a higher number is probably acceptable.
At 8,000′ DA and wide open throttle, if the engine will turn 2700 RPM, then its putting out right at 75% power because of the reduction in manifold pressure that occurs with altitude. Coincidentally. 7,500-8,000 feet is also the most efficient altitude range in which to fly a non-turbosupercharged plane. The TAS goes up about 2% per thousand feet, while the HP of the engine falls with higher altitude. Since you can’t “open the throttle” any more, if it is already wide open, then the approximately 8,000′ DA manifold pressure is as high as you can go and still achieve 2700 RPM (75% power). You will have less and less power as you climb higher, and the TAS improvement can’t keep pace with the loss of power.
Mike Rellihan Addendum May 17, 2006:
Considerable information has been published about leaning, on the BAC website, available to all members with a search on ‘mixture settings (leaning)’. GAMI has done more research on this than practically anyone else, including some of the manufacturers themselves. You can visit the GAMI website, and you can read the Pelican’s Perch series on AvWeb.
I am personally convinced that running in the range of 30-40 degrees Rich Of Peak (ROP) is the absolute hardest point at which to operate the engine. The pressure peaks the fastest, and at a time when rod angle is not optimum for leverage. The result is continuous operation under circumstances that are just shy of being called detonation (in terms of pressure spikes and impact on parts). My personal four conclusions are that:
(1) On an unsupercharged engine, you cannot get the EGT ‘too high’. You CAN get the CHT too high. CHT is the driving force in engine management, for our engines. Peak CHT will occur in the 30-40 degrees ROP range, for the reason stated above (pressure spikes, fast burn times). You can manage CHT with added fuel, added internal air (leaning), added external air (airspeed and angle of attack), and power output level. All have a role to play, under different circumstances. And you can spend a great deal of money by always blindly following the OWT’s, using only excess fuel for cooling.
(2) If you need to compromise, peak EGT is a good way to go, as long as you can ensure that all cylinders are peaked, leaving none operating ROP in the peak pressure range. It will give closer to maximum power without the pressure spikes, without the roughness often associated with Lean Of Peak (LOP) operation on carbureted engines, and without the higher fuel consumption of operating significantly ROP.
(3) LOP operation will definitely give the lowest CHTs, the lowest EGTs, and the lowest fuel consumption (for many reasons). It is difficult to achieve true LOP operation on the carbureted engines, though some seem to come quite close when flying high and at full throttle. Sometimes a tad of carb heat also helps. To run LOP, you need to be able to verify that every cylinder has ‘gone over to the lean side of peak’. That means all-cylinder EGT-CHT capability. The much more efficient operation comes at the expense of power output. There is a significant loss of power during LOP operation, and with our engines we cannot compensate by adding back some manifold pressure from a turbo. During LOP operation, you are providing internal cooling with excess air, regardless of external airspeed and cooling. For any given power level and conditions, in my experience LOP always results in the lowest EGT and CHT. During LOP operations you can also use fuel flow to directly calculate power output, regardless of other engine settings. Every bit of fuel is being efficiently burned, and the pounds per hour are directly calculable as HP.
(4) Running at least 100-125 degrees ROP will give the highest safe power levels without the spikes, and will give CHTs in the normal range at those power settings (assuming normal airspeeds, etc.). It will also result in the highest fuel consumption, as you are doing internal cooling using excess fuel flow instead of excess air.
Many people have witnessed all this for themselves, during the GAMI engine management classes. Their instrumentation enables the attendees to see exactly what is happening inside the engine, as different parameters are changed.
As an aside, virtually everyone has heard the old stories about how running too lean can cause overheating and detonation. I am convinced that the problems in question were not caused by being lean at all. They occurred when engines were normally operated full rich, or close to it. If someone experimented by leaning down a bit, they moved the operation into the peak spike range that lies just rich of peak EGT. They were ‘leaner than full rich’, but were never actually lean at all.
Once the ratio passes the optimum (stoichiometric?) air-fuel ratio, burn rate slows down; just like it does when you go rich of peak. A ‘real’ (LOP) lean mixture burns slower and cooler, not hotter and faster. As one example, automotive ignitions have had variable spark advance for two generations, so that either higher vacuum or electronics and sensors could advance the spark with a lean mix. The earlier spark helps compensate for the slow burn rate. Guess what one of the characteristics of the GAMI PRISM system will be, once they have it finalized.