Sunrise Aviation

This column is the fourth in a series dealing with the theory, mechanics, and operation of constant speed propeller mechanisms.
Most manufacturers-- in order to ease conditions for the engine--recommend reduced settings of MP and RPM after the first thousand feet of climb. Unless you use both hands, this presents an important choice: which first--MP or RPM? Instead of memorizing a procedure, it is best to understand the principle involved.

A reduction in power (MP) at a constant RPM poses no problems. Things are simply getting easier for the engine. However, reversing the process--decreasing RPM first--significantly increases internal engine pressures and temperature.

To understand exactly why high power and low RPM are a bad combination, it is necessary to understand that as you increase prop loading and decrease RPM, the engine has less and less chance to convert its combustion energy into motion (in autos, this is called “lugging”). The “trapped” energy converts directly into heat, resulting in a rapid increase of internal temperatures, usually seen first on the cylinder head temperature gauge (CHT). The resulting high temperature is the enemy of performance and engine life.

All this theory gives rise to a simple procedure: reduce the source of potential heat--the power--before loading up the prop, or, avoid high power and low RPM.

PROCEDURES
This rule points the way for proper procedure during power changes: using the first “enroute climb” reduction as an example, with intended targets of 25" MP and 2500 RPM, first bring back the throttle, and then follow with the RPM. This choice--reducing throttle first--will momentarily give you 25" in combination with 2700 RPM: harmless. Reversing the sequence, on the other hand, would leave the throttle at 30" while the RPM was reduced to 2500--undesirable. Thus, for power reductions, always reduce MP first, then follow with RPM.

For an increase in power, for example from a cruise setting of 22" and 2200 RPM back to climb power, raise the RPM first. This produces a momentary condition of 22" and 2500 RPM--perfectly acceptable. The reverse is undesirable: increasing MP before RPM would momentarily result in 25" and 2200: high power, low RPM. For power increases, increase RPM first, then follow with MP.

Although the power settings discussed above are not universal, they are fairly representative for most non-supercharged engines. A quick look at them presents a striking numerical coincidence: it appears that as long as the pilot chooses a manifold pressure number that is the same as or lower than the RPM, ie. 21" and 2200 RPM, all will be well. Make the MP number bigger than the RPM, and heating and engine stress problems may start to develop.

This approach works fairly well, but it's important to note that it is simply a convenient rule of thumb, not Words From On High. Remember, one of those numbers is the measure of inches of mercury in an evacuated glass tube, the other is hundreds of crankshaft revolutions a minute --a true case of apples and oranges.

This point becomes dramatically obvious when dealing with supercharged engines, where power combinations as high as 41" and 2700 RPM can be acceptable on takeoff. As a result of the great variety of potential situations, care must be taken in every case to consult manufacturer's recommendations before choosing power combinations.
Next month, climb and cruise.