Should you "stop the prop" for a forced landing?

One flying philosophy I believe in is the importance of knowing the characteristics of your aircraft inside and out so that should the unthinkable occur, you the pilot are well-prepared to handle the situation with the highest likelihood of a successful outcome.

The Mooney M20E Owner's Manual includes a chart depicting glide range, and suggested indicated airspeed for best glide at maximum gross weight. The chart depicts a glide ratio of 10.3:1 with the propeller windmilling, and 12.7:1 with the propeller stopped.

Ok, well hold on a second... First, how do you stop the prop? Second, is it worth it do gain that additional glide performance in a power loss situation? Third - lets say you lose power, but still have engine rotation, oil pressure, and the ability to control the prop: What is the glide ratio then?

These are questions I'd had in mind for some time. Yet, its very seldom anyone in primary or transition flight training discusses these issues. Several years ago, when I began to instruct I decided to conduct an experiment to answer some of these questions.

Before I go further, I want to emphasize that yes, I took a small risk in these tests: I intentionally shut down the engine of my airplane a few times. I'm not advocating that you go out and do this unless you know what you're doing. For each and every test I was a few miles from a long runway on a nice day. I practice power-off landings from downwind frequently, and KNOW I could land it, on a runway, if I had to. I care about my cylinders and took quite a bit of time at the top and bottom of each descent to cool/warm the engine in a controlled, healthy manner. On the other hand, if my engine ever quits over some tall mountains... I think I am better equipped!

First, lets discuss the experiment setup and conditions:

• Aircraft: 1966 Mooney M20E
• Lycoming IO-360 engine
• "Standard" Hartzell (non-schimitar) propeller
• Engine controls are rigged such that it idles about 750 rpm on the ground, prop full forward
• Atmosphere close to standard temperature
• Aircraft weight 80% of gross weight, with glide speed adjusted accordingly* to 95mph indicated airspeed

*The Mooney Owner's manual publishes a best glide speed of 107mph indicated airspeed, but says elsewhere that L/D max is 105mph indicated. Like all airspeeds that attempt to capture a desired angle of attack, best glide needs to be adjusted for any variance in aircraft weight versus the weight the speed was published for (max gross). I adjusted the 105 mph speed for actual weight, so the target speed was 95mph indicated. For further reading on the topic, Aerodynamics for Naval Aviators is a good reference.

Experiment method:

1. Established the aircraft in a power off descent slightly above 5000' MSL
2. Trimmed for target airspeed, allowed the aircraft to stabilize
3. Started a stopwatch descending through 5000' MSL
4. Measured time after 500 vertical feet of descent

Here were the test cases, and results:

So, what does it take to stop propeller rotation?
The method used was to establish a glide at idle cut-off, propeller windmilling, propeller control at "Max" RPM. I started a stopwatch once target glide speed established and stabilized. I simultaneously increased pitch to slow the prop. It took several seconds, at minimum control airspeed with slight stall buffet (a full stall was not necessary) to stop rotation. I then reduced pitch to regain calculated glide speed. At this point the elapsed time and altitude loss from starting the maneuver were noted: 25 seconds and 300 vertical feet.

Conclusions:

1. If I lose the engine and making a safe landing sight field is questionable, it is probably worth stopping the prop, provided that I did not have oil pressure to use the governor to do so. You can cover about 25% more ground with propeller rotation stopped vs. windmilling with the propeller control set to "Max" RPM. So its not a slam-dunk that will double how far you can go, but if the best option for a SAFE landing sight looks "iffy", I would, without question, stop the propeller
2. If oil pressure and propeller governing are available, the difference in performance between windmilling at "Min" RPM and the propeller stopped are quite small, about 10%; stopping the propeller may not be worth the additional distraction from dealing with the emergency
3. The Mooney owner's manual glide ratios appear very close to the data I captured
4. The effect of an idling engine on perceived glide performance is substantial. It does lend credit to the idea that when practicing this in a normal environment with the engine still running at idle power, that you do derive some benefit that will not be there in an actual emergency