Engine out!” After climbing through 8400 feet msl, the engine’s hum became a sputter and I immediately set to work. I pulled back on the yoke until the sky filled the windscreen and danced on the rudder pedals to keep the wings level. The nose bobbed up and down as I fought for the highest angle of attack without stalling the plane. The propeller slowed until I could easily count the blades passing by and finally came to a halt. Things got quiet.
I lowered the nose to pick up 60 knots and as the altimeter needle swept through 8000 feet msl, yelled, “Time!” Directing all my attention to keep the airspeed at 60 knots, I reminded myself to breathe as I often do when I’m concentrating on getting something just right. The altimeter smoothly unwound and passed through 7200 feet msl. “Stop!” Bill clicked his stopwatch. Mixture – FULL RICH, throttle – CLOSED. I yelled, “Clear!” and hit the starter. After easing the throttle back to full, we heard the engine hum once again and chuckled as we made our way up to altitude for many more glides.
Multi-engine pilots know the drag incurred by a failed engine with a windmilling propeller can be devastating. So these planes are equipped with a mechanism by which a propeller can be feathered; that is, put in coarse (high) pitch/low rpm until the engine comes to a halt. Although many single-engine aircraft do not feature such a mechanism, is it possible to stop the rotation of the propeller? Is there any reason why you would? My mentor, the late aviation author Bill Kershner, and I routinely took our planes up to answer questions such as these. During a sequence of flights, we sought to estimate the penalty that a windmilling propeller exacts during a glide in a single-engine Cessna. Before discussing the details of the experiment, though, let’s review the aerodynamics of gliding flight.
GLIDING 101: BEST AIRSPEED
In a stabilized glide, the forces acting on the airplane (lift, weight and drag) sum to zero. If the angle of attack is too high or too low, the aircraft will descend at a faster rate. The minimum sink airspeed, VMS, is the speed resulting in the lowest descent rate and, therefore, the longest time aloft when gliding. To find VMS, we use some basic math to establish the glide polar, a function associating to each airspeed the vertical velocity of the aircraft in still air (where airspeed equals groundspeed). For each aircraft configuration (weight, flaps, propeller pitch, etc.), there is such a polar that can be generated from a sequence of test glides.
For any point (v,g(v)) on the polar, the glide ratio is -v/g(v):1. (If your glide polar is represented with airspeed in knots and vertical velocity in ft/min, then it’s approximately –100v/g(v):1.) The velocity with the largest glide ratio corresponds to the flattest of those lines—the one tangent to the glide polar at the point (VBG, g(VBG)). That makes VBG the best glide airspeed, and it guarantees the maximum forward travel per unit altitude loss without taking into account winds. The value -VBG/g(VBG) is the maximum lift-to-drag value, L/DMAX, of the airplane and is a measure of its aerodynamic efficiency. Larger values of L/DMAX mean greater efficiency.
This all moves from an academic discussion to the real world when the engine decides to take the day off. Of course, having that happen over an airport is an unreasonable expectation and it’s more likely a good landing spot is going to be some distance away. The task at hand is to get there. Rather than maximizing the time in the air, it’s more important to maximize the glide ratio; that is, the forward distance traveled per unit of altitude loss.
The glide polars shown above represents a fictitious general aviation airplane for which the minimum sink airspeed is 53.5 knots and the best glide airspeed is 60.4 knots. Its L/DMAX is 11.3, a value typical for such a plane. In contrast, gliders have maximum lift-to-drag ratios starting in the 20s and can be higher than 60. You may see the glide ratio instead represented as the forward distance (in nautical miles) that the plane will travel in still air for every 1000 feet of altitude loss. Such a format can make the calculation easier in an engine-out situation.
PRACTICING GLIDES
During the certification process, test pilots generate a glide polar for the aircraft, thus providing data for the pilot’s operating handbook. Using the procedures above, the minimum sink airspeed, which is seldom published, as well as the best glide airspeed are obtained. Advisory Circular AC 23-8C, Flight Test Guide for Certification of Part 23 Airplanes, says that fixed-pitch propellers should be windmilling during the tests. For airplanes with variable-pitch props, the best-glide speed is determined by assuming that “the means to change the propeller pitch is still operational and, therefore, the propeller should be set at the minimum drag configuration. For most installations, this will be coarse pitch or feather.”
The FAA’s Airplane Flying Handbook, FAA-H-8083-3A, recommends practicing glides with the propeller in flat pitch/high rpm. I found this especially interesting given that glide ratio and speeds are calculated with propellers in coarse pitch/low rpm or feather. Among operating handbooks from several single-engine airplanes with variable pitch propellers (Cessna 182T, Beech A36, Piper Malibu, Meridian and Saratoga) I consulted, only the Piper manuals specified a propeller condition (coarse pitch or feather) during a descent after engine failure. If you practice glides with a propeller in coarse pitch/low rpm, be sure to return it to flat pitch/high rpm before increasing manifold pressure.
What if your POH doesn’t specify the propeller condition? Although generating your own polar would be informative and fun, that’s not necessary to figure out in what condition your propeller should be during a glide. Try 1000-foot glides in both conditions, starting from an appropriate altitude, and see which one gives you results closer to your POH values. Choose a day with calm winds but don’t rely on groundspeed information. Instead, calculate the time t (minutes) it takes to descend 1000 feet at VBG. Your glide distance in still air is approximately t*VBG /60 nm. Why not make determining that part of your next flight review? Knowing your airplane better equips you in emergency situations. With the wrong propeller condition, you might not get expected performance.
STOPPING THE PROP
That a windmilling propeller causes considerable drag due to engine friction is part of multi-engine training. With the same principles at work in a single, there is every reason to believe that stopping the propeller in an engine-out situation will increase glide distance.
While stopping the propeller of a single may sound a bit unusual, I had already found it second nature. For several years before his passing, I taught spins with Bill in his 1979 Cessna 152 Aerobat, N7557L. Once the basics are covered, some students enjoy experiencing the dynamics of a developed spin. During this phase, the centrifugal force sends the fuel outboard in the tanks. After 13 rotations, the fuel in the lines is exhausted and the nose has pitched up sufficiently for the propeller to lumber to a stop. The spin recovery technique is the same as any other and, in this Aerobat, diving to 120 knots during the recovery returns enough flow over the propeller to make the engine restart every time. We knew we had a good airplane for the test.
Over the course of several flights and 54 glides, we gathered data that proved statistically significant and concluded that gliding with a stopped propeller is more efficient than with one that continues to windmill. The glide distance for N7557L was improved by 8.3 percent with a stopped prop. The sidebar on the opposite page gives the details of the test.
During the experiment, we used the 60-knot best glide speed for both propeller stopped and windmilling conditions. But since each aircraft configuration results in a potentially different glide polar, a natural question is whether we could have achieved more dramatic results by using a different airspeed in the prop-stopped condition. Maybe. I recently gathered data to create a glide polar for N7395L (my matching 1979 Cessna 152 Aerobat) but didn’t see much difference in the best glide airspeeds. If there were a difference, it would imply that the efficiency difference is even greater than we found.
PRACTICAL SIGNIFICANCE?
I’ve learned there is a benefit to stopping the propeller that has nothing to do with glide ratio. The airplane becomes very quiet and smooth with a stopped propeller and on a certain aerobatic training flight, that proved fortuitous. N7557L didn’t have an intercom and in the days before I had my airplane, two days of teaching aerobatics left me hoarse and with ringing ears. On this occasion, my student was particularly hard of hearing and I couldn’t seem to convey my advice on improving his aileron rolls. “Start the roll with more left rudder!” I screamed. He shook his head like someone who had no facility with my language. “More left rudder!” yielded only a shrug of his shoulders and an apologetic expression. Finally, I cut off the mixture, raised the angle of attack and flirted with a stall while I forced the propeller to come to a halt. With peace and quiet, I was able to communicate my suggestions and resumed the lesson.
Stopping the propeller in an engine-out glide makes sense in an academic way. What I mean by that is the next time you’re out hangar flying, it’s a fun topic to throw out while relaxing safely on the ground. But does it really make practical sense with a true emergent engine failure? During Part 23 certification of general aviation and commuter aircraft, glide performance must be determined and AC 23-8C specifies that for a fixed-pitch propeller, “stalling the airplane to stop the propeller from windmilling is not an acceptable method of determining performance because the procedure could cause the average pilot to divert attention away from the primary flight task of gliding to a safe landing.” Indeed. If you’ve never done it before, giving it a try during a real emergency is not a recipe for success.
Note there is some glide inefficiency while stopping the propeller as the left side of the glide polar attests. It can be minimized with a good technique but not completely eliminated. So it only makes sense if the engine failure occurs at a high enough altitude to offset it. Sure, stopping the propeller can be helpful in extending the glide, but only for the pilot who is experienced at the technique and has encountered engine failure at a relatively high altitude.
PUTTING IT ALL TOGETHER
Aviation provides opportunities for lifelong learning. We’re all student pilots, continually acquiring new tools for the box. Thinking about potential emergencies is a necessary part of our development. Upon engine failure, I would employ the technique advocated by my POH.
The glide portion might look something like: Trim for best glide speed with landing gear and flaps up, propeller in coarse pitch/low rpm. Find the most suitable landing area using what I know about wind information. Update airspeed using speed-to-fly information. If altitude permits, attempt to diagnose the problem to see if a restart is possible. If there is absolutely no way that I can imagine the engine could be restarted and I had sufficient altitude, I would stop the propeller. It would help with glide distance, I’m comfortable with the procedure and the plane flies much more smoothly that way. I’m not advocating it for anyone else, but it’s a tool I’ll certainly keep in the box.
Catherine Cavagnaro, ATP and CFI-I, owns and operates an aerobatic school in Sewanee, Tenn. (www.aceaerobaticschool.com) She also holds a Ph.D. and is Professor of Mathematics at the University of the South.
