The high price of aviation fuel is causing aircraft owners and pilots to review their operations in search of ways to keep operating costs down. Those operating aircraft with controllable propellers have been requesting information on cruise operation in the low RPM range - 1800 or 1900 RPM for example. The number of queries received indicates a great deal of interest, and therefore it seems appropriate to share the information on this subject with all of our readers.
The Textron Lycoming Engine Operator’s Manual has performance curves applicable to each engine series. The curve for the IO-540-K series, 300 horsepower engine is printed here as a reference for this article. The curve does provide data on the maximum manifold pressure (MP), which may be used with any particular RPM at sea level and at altitude. The limiting manifold pressure line clearly restricts high manifold pressures with low RPM settings. There is a good reason for this; high manifold pressure and low RPM is similar to allowing your automobile to lug up hill in fourth gear. The pinging you hear in your automobile tells you that detonation is occurring and you should shift down to a lower gear. In an aircraft, detonation is not likely to be heard as damage occurs in the engine and it is then too late for preventive measures. For this reason, engine operation should be within the limitations established in the Pilot’s Operating Handbook (POH).
Although there are restrictions, it is quite apparent that operation is possible in the 1800 to 1900 RPM range. Lower RPM will result in less friction horsepower with a resultant fuel savings, but most of the fuel flow reduction experienced will be the result of a much lower power setting and therefore reduced performance. This raises a question about the amount of benefit in terms of cost savings that might actually be achieved by using the lower RPM settings for cruise.
One of the first considerations of low RPM cruise is that power settings this low should not be used during the engine break-in period. During the break-in period, normal climb power as specified in the Pilots Operating Handbook should be used. To seat the piston rings in a new or overhauled engine, cruise the aircraft at 65% to 75% power for the first 50 hours, or until oil consumption has stabilized. Low power for break-in may result in glazed cylinder walls and high oil consumption that can only be cured by cylinder removal and rehoning.
There are some other considerations of low power cruise operation. Low manifold pressures, below an arbitrary point of perhaps 18 inches for continuous cruise, may cause excessive oil usage, and oil buildup in the valve guides which could lead to sticking valves.
Particularly during cold weather operation low power operation may allow both the oil and cylinder head temperatures to fall below the normal range. This is detrimental to good engine health. Oil temperature in particular should be maintained between 165oF and 220oF to achieve maximum service life. At lower temperatures, the moisture which gathers as a result of combustion will not vaporize and be expelled. This can cause dilution of the oil which detracts from its lubricating properties.
The Pilot’s Operating Handbook for each aircraft provides a variety of power settings that most often show 2100 or 2200 RPM as the minimum for cruise. The table shown here is for the IO-540-K series engine which was illustrated in the curve shown earlier. Using that curve, note that cruise flight at 6000 feet using 1900 RPM would be limited to approximately 55% of power with manifold pressure set at 24 inches. As shown in the curve, 24 inches of MP is very near the limiting manifold pressure line and therefore close to the maximum available.
Quite frequently, someone will ask if the engine will last longer if it is run at a slower RPM setting. The answer must be qualified. Operation at the recommended cruise RPM settings should allow the engine to reach TBO if it has regular oil changes, is operated within normal temperature ranges and is well cared for by pilots and maintenance personnel. Longer engine life may be expected from most engines when the operator is willing to sacrifice maximum performance for conservative cruise operation in the 60% to 65% power range. For many engines these power settings are achieved at 2100 or 2200 RPM rather than the 1800 or 1900 RPM mentioned earlier in this discussion.
In summary, it is possible to run an engine at cruise using 1800 or 1900 RPM. A curve from the Engine Operator’s Manual should be consulted to insure that manifold pressure limits are not exceeded. In reality, the recommendations of the Pilot’s Operating Handbook provide the best guidance for operation of an aircraft/engine combination, and therefore the recommendations and limitations of the POH should be observed.