February 1, 2011
By Barry Schiff
Every pilot who flies a single-engine airplane with a constant-speed propeller is instructed by the emergency checklist to position the propeller in high pitch (low rpm) following engine failure. This is because a windmilling propeller creates less drag in high pitch than in low. Glide performance is improved. In some high-performance singles, the decrease in drag is so dramatic that you literally feel the airplane accelerate as you pull back the propeller-pitch control (and vice versa). Don’t do this, of course, unless the engine has failed or is idling.
A windmilling propeller is loosely analogous to a transmission used to slow a speeding automobile. Windmilling the propeller in low pitch is like coasting a car in low gear (maximum drag); windmilling in high pitch is like coasting in high gear (minimum drag).
A propeller, however, creates the least drag when it is not moving at all. This leads to something else a pilot can do to further improve his glide ratio, and that is to stop the propeller completely. This procedure also can be used by those who fly singles with fixed-pitch propellers.
I first heard about this many years ago when Bill Thompson, the chief flight-test engineer for Cessna, told me that stopping the propeller of a Cessna 172 improved its glide ratio by 20 percent. I decided to perform a flight test of my own using a Cessna 182. The 182 has a glide ratio of 9.3:1, which is typical. Most singles have glide ratios between 8.0:1 and 10:1.
While high over a large airport (just in case), I closed the throttle, pulled the mixture control to idle cutoff, and pulled the propeller-pitch control all the way aft (to reduce windmilling rpm). I then raised the nose until the airplane approached a stall. This is when engine compression overcame windmilling forces, and the propeller shuddered to a stop. I then lowered the nose and accelerated to the best glide speed of 70 knots. The tail buffeted slightly. With the propeller in the horizontal position, it interfered with airflow across the tail. This was easily resolved by tapping the starter and flicking the prop to the vertical. This also is the minimum-drag position of a stopped propeller and results in the maximum possible glide ratio. Three-blade propellers should be positioned with one blade in the vertical position, and four-blade propellers should be positioned to form an X, not a cross.
Once the burbling across the tail stopped, it was glass smooth and very quiet and peaceful—much like flying a sailplane.
Using the time required to descend through 2,000 feet of altitude and the average true airspeed in this descent, the glide ratio was 11.12:1, a 20-percent increase in glide performance. Thompson was right (not that I ever doubted him).
Stopping the propeller can be a lifesaving procedure, but consider this advice:
There are times, of course, when increased glide performance and distance are not important, such as when the desired landing site following engine failure is nearby. This is when a pilot might need additional time more than he needs additional glide range—time to attempt a restart, to brief his passengers, and to simply gather his wits and plan a safe approach. By reducing airspeed to approximately halfway between stall and best-glide speed, sink rate is decreased (as is glide ratio). This delays the inevitable landing. Be careful, though, to return to best glide speed when 1,000 feet agl (or above). This provides a greater margin above stall and more closely approximates a normal landing approach.
Do not attempt to stop the propeller when flying IMC with vacuum-powered, gyroscopic instruments. Stopping the prop also stops the vacuum pump (and the alternator).
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