I've mentioned a couple of times lately that an airplane with a control-lable-pitch propeller will glide farther with the propeller in coarse pitch than in fine pitch.
These terms coarse and fine aren't exactly intuitive, but they are analogous to screw threads of coarse and fine pitch. A screw with fine threads needs more turns to advance a certain distance; likewise, an airplane engine turns faster, at a given forward speed, when the prop is in fine pitch. Fine pitch is also called flat pitch. "Flat" may give a clearer idea of the position of the blade, but unfortunately it does not have a suitable antonym. Tilted pitch? Bumpy pitch? You hear "steep" pitch sometimes, but that's confusing because a more steep pitch gives a less steep climb or descent.
Anyway, the gist of the idea is that a windmilling propeller is driving the engine, it takes a certain amount of power to do this, and the faster the engine turns the more power it takes. This power has to be supplied by the airplane's descent; the more power is required to spin the engine, the greater the rate of descent has to be.
Curious about the magnitude of the effect, I ran a quick test. In the vicinity of 7,000 feet, at 100 kias (which is 5 knots above my best glide speed, but a convenient number to hold on the ASI), I throttled back to idle. My rate of descent was 1,200 fpm with the prop in fine pitch and 1,000 fpm with it in coarse pitch. This is quite a significant difference -- a 20 percent increase in gliding distance just for moving the prop control back.
This simple experiment stimulated my curiosity further. What would be the rate of descent at zero thrust -- in other words, if the prop weren't there at all? There's no way to identify zero thrust without special equipment: either a thrust meter or, more simply and ingeniously, a microswitch touching the back of the propeller flange and detecting the crankshaft's small fore-and-aft movement as the prop passes from drag to thrust. Maybe one of these days I'll install one of those switches -- just for the fun of it -- but in the meanwhile I resorted to my computer simulation of my airplane, which generally seems to be pretty accurate. It said that the rate of descent at zero thrust should be around 750 fpm.
Is this reasonable?
A tentative answer comes from a 1972 Lycoming paper entitled Performance Characteristics of the Continental IO-360-D Model Engine. Lycoming tested Continental's six-cylinder competitor -- which I have -- to its own 360-cubic-inch, 200 hp engine in order to "note the significance of the unlikenesses to Lycoming engines." The main conclusions were that the rival engine breathed well at high rpm, had very even mixture distribution among the cylinders, cooled marginally at takeoff power, produced 5 fewer horsepower than claimed and achieved a specific fuel consumption "in the range of .40" (which is very good). Lycoming also noted that the Continental engine had various features that simplified manufacturing and reduced its cost. This will come as news to anybody who has ever bought one.
Anyway, this handy piece of corporate data gathering (I will not say espionage, since they bought the engine fair and square) includes a plot of engine friction against rpm. I'm not sure whether it includes pumping losses -- the work done sucking in air and blowing it out -- but at any rate the range is from around 12 horsepower at 2,000 rpm to 28 at 2,800. The variation is pretty linear -- about 2 horsepower per 100 rpm.
Horsepower is readily convertible into rate of climb or descent. One horsepower equals 33,000 pounds going one foot per minute. The pounds, feet and minutes can all vary, so long as, when you multiply the pounds times the feet and divide by the minutes, the result is 33,000. A 3,000-pound airplane, for instance, climbing 11 feet per minute in still air is absorbing 1 horsepower (3,000 x 11 = 33,000). By the same token, a 3,000-pound airplane descending 1,100 feet per minute yields 100 horsepower. It works both ways; the airplane requires horsepower to climb, and returns the investment when descending. Descent is an engine: It is the only engine of a glider.
Taking Lycoming's 12-friction-horsepower figure for 2000 rpm, and jacking it up to 15 or 16 for prop losses (because the prop does not convert aerodynamic force into work with perfect efficiency; quite a bit is lost to random turbulence), it turns out that a 2,100-pound airplane, which mine was at the time I was doing this test, has to lose around 236 fpm just to keep the engine spinning. This matches the difference between the computer's 750 fpm rate of descent and the observed 1,000 fpm pretty well.
(Did that go by too fast? It's simple: 15 horsepower = 15 x 33,000 or 495,000 pound-feet per minute. Divide this by the weight of the airplane, 2,100 pounds, and you get 236 feet per minute.)
So far, so good. But now you have to wonder about a stopped prop. Suppose the engine seizes up and doesn't windmill. Is the glide better or worse than with a windmilling prop?
For a lot of different reasons, it's hard to guess the drag of a stopped prop. The flow field around the nose of the airplane is distorted, the blades are not exactly flat plates and they get more streamlined toward the hub. The drag may depend on the position in which the prop stops. The prop's wake may affect the drag of the rest of the airplane. But let's stipulate for the sake of this discussion that my propeller, stopped and in fine pitch (because a constant-speed prop automatically goes to fine pitch when it stops turning), has an equivalent flat plate area of 1.7 square feet.
