Sunrise Aviation

As promised last month, this article will concentrate on making a variable pitch prop continue to work efficiently through climb and cruise, picking things up after the first power reduction to “cruise climb.”
For the remainder of the climbout, periodic observation of manifold pressure is required: as outside air pressure reduces with increased altitude, it’s necessary to periodically open the throttle to maintain constant MP--once every thousand feet is reasonable. Assuming a desired value of 25", this periodic throttle adjustment will continue to work until you pass 5000'--after that, you will be limited to whatever air pressure is made available by nature.

CRUISE
Once cruising altitude is reached, it is common to recommend a reduction in power from the “cruise climb” settings, both for efficiency and for extended engine life. What settings should you use? You may be aware that from a theoretical stand-point, cruising airspeeds require a big prop bite for maximum aerodynamic efficiency; still, it’s pretty difficult to figure out exactly what to do on your own. Out comes the owner’s manual, where you will find a number of interesting facts. Lets take a C172-RG at 4000' pressure altitude as an example:

According to the book, 24" and 2400 RPM will give you 133 kts and a fuel burn of 9.9 gph. Reducing power to 23" and 2300 RPM will produce 127 kts at 9 gph, a 5% reduction in speed with a better than 10% reduction in fuel burn. Leaving the MP at 23" and further reducing the RPM to 2200 looks even better: 125 kts at 8.7 gph, less than 2% more speed reduction with an additional 3% increase in fuel economy.

Although the lower airspeeds produced by each of the two reduced power settings do result in lower airframe drag, the majority of the increase in fuel efficiency comes from using bigger and bigger prop blade angles (evidenced by the lower RPM)--angles that take full advantage of prop airfoil design, working the blades close to an ideal lift-to-drag ratio. The primary benefit? Lower costs at the gas pump.

As a secondary advantage to lower RPM selections, you might consider the very substantial reduction in cabin noise--well worth the trade-off of a few minutes of extended flight time in terms of pilot and passenger fatigue.

THE WHOLE PICTURE
The variable pitch prop, and specifically the constant speed arrangement, is designed to overcome the disadvantages of fixed pitch props at both ends of the performance range. During takeoff and climbout, where emphasis is on maximum horsepower, the governor permits the pilot to choose low prop blade angles that permit the engine to accelerate to and maintain the high RPM necessary for maximum horsepower production. Although blade angles are too low during this stage for ideal aerodynamic efficiency, that aerodynamic loss is easily overcome by the very significant increase in total horsepower produced.

In cruise, at the other end of the performance range, emphasis naturally shifts away from maximum power. Now, constant speed props prove superior by permitting the pilot to compensate for the changes in prop angle of attack that accompany aircraft acceleration. By making extremely high blade angles available, the mechanism guarantees improved prop aerodynamics.

Next month, I’ll consider descents, landings, go-arounds, and emergencies.