Technicalities

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Our Flying Mail column is an equal-opportunity zone. Truth and falsehood mingle freely there.

In the April issue a reader, Hal Stiles of North Miami, Florida, wrote to assert that "Pusher props are superior." He pointed out that the Cessna Skymaster, the push-pull twin with one tractor and one pusher engine, performs better on the rear engine alone than it does on the front engine alone. Indeed, the evidence appears quite persuasive: "The Cessna 337 Skymaster can climb at 200 fpm on the front engine only," Stiles wrote, "but it can climb at 300 fpm on the rear engine only. A nonturbo 1967 model has a single-engine ceiling of 7,500 feet with the rear engine out, and 9,500 feet with the front engine shut down … The VariEZ [sic], the Velocity, the Cozy and Pushy Galore all demonstrate the superiority of pusher props."

Stiles was commenting on a February letter from Evan Mortenson, an aeronautical engineer who had written to respond to Mac's carefully weaseled statement (note the word "theoretical") that the pusher arrangement of the propellers of the Piaggio Avanti "gives the propellers a theoretical efficiency edge over a standard tractor arrangement." Mortenson, who has designed and certified a pusher twin himself, wrote that "a pusher propeller will always take a hit in the efficiency department because the blades must pass through the wake of the wing or body that is ahead of it."

Another reader, retired US Navy Commander John Pfeiffer of Monterey, California, also wrote in February about the Avanti's propellers. He lives and works near the airport, and reports that you can hear an Avanti coming from a long way off. They are, he says, short of military jets, the loudest things around. Ed., the shadowy fellow who scribbles the italicized replies to letters, agreed that a particularly harsh sound is characteristic of pusher propellers.

The pusher/tractor debate is an old one, but it continues to inflame passions. One reason for its persistence, I suppose, is that propeller efficiency, considered scientifically, is a very complex topic involving many different variables and subtleties, and it is hard for a nonspecialist to understand. From the specialist's point of view, most of the interesting aspects have to do with blade geometry -- number and shape of blades, speed, spanwise distribution of area, twist and downwash, airfoil shapes, compressibility effects and so on. These are characteristics of the propeller itself, removed from the influences of the airframe to which it will be attached. Compared with a propeller spinning in aristocratic isolation inside a wind tunnel or a computer's central processor, the practical device attached to an engine and mounted on an airplane is a slum dweller.

An isolated propeller can convert as much as 92 or 93 percent of the power supplied to it into usable thrust. But interactions with an airframe erode some of that performance, and a proper degree of cynicism prohibits ever mentioning efficiencies greater than 87 percent in professional company. The principal disadvantage of a tractor propeller is that some portion of the airframe is bound to be within its wake or "slipstream." Since the propeller accelerates the air in the slipstream (just as a wing supports the airplane by accelerating air downward, a propeller drives it forward by accelerating air backward) those portions are in effect going faster than the rest of the airplane, and experience increased "scrubbing" drag. Drag is a function of the square of speed, and so this penalty sounds as if it ought to be significant. It should be especially significant for single-engine planes, a large portion -- half or more -- of whose total surface area is immersed in the slipstream.

Scrubbing drag in the slipstream is not negligible, but it is not as large a factor as you would suppose, because the slipstream is not going that much faster than the surrounding air. This seems intuitively implausible to anyone who has stood behind an airplane that is running up; but that is because slipstream acceleration is greatest when an airplane is standing still. It decreases as airspeed increases. At 200 ktas, a two-bladed propeller absorbing 250 horsepower accelerates the air within the slipstream by only about 13 knots.

Now, with a pusher propeller there is no scrubbing drag increment. But there is a different problem. Propeller blades are designed by carefully adjusting the shape and angle of the blades to interact most efficiently with the air they encounter. The air encountered by a pusher propeller, however, has been somewhat jumbled up by the passage of the airplane. In the wake of the fuselage, for instance, air that has passed close to the airplane's skin has slowed down. If the fuselage were perfectly round and symmetrical, it would be possible to adjust the blade twist to account for this variation in inflow velocity. But fuselages are never pure bodies of revolution, and wings produce their own turbulence and downwash; and so different portions of the flow field meeting the propeller are moving with different speeds and at different angles. A blade twist distribution that is ideal at one point is wrong for another. The result is that the blade of a pusher propeller cannot be optimized for all points in its rotation. It has to be a compromise, accepting a small loss here to prevent an even larger loss elsewhere.

To give you an idea of the magnitudes of the costs involved, take a hypothetical single-engine plane with a 200 hp engine that can do 170 knots at 7,000 feet. Let us suppose that the acceleration within the slipstream is 7 percent, that half of the airplane's wetted area is within the slipstream, and that half of the airplane's total drag is due to skin friction (as opposed to induced drag, pressure drag, leakage drag, cooling drag, etc). The resulting drag increase is something like 3.5 percent of the total. This slows the airplane by slightly less than two knots, and is equivalent to a loss of about 2.5 percent points of propeller efficiency (say, from 86 percent to 83.5 percent).

Now, how to compare this with the efficiency of a pusher propeller? For this I'm obliged to turn to books, since it's not possible to do a rule-of-thumb calculation from basic physical principles, as it was for the tractor. In Roskam and Lan, Airplane Aerodynamics and Performance, there is a chart showing the effect of what you might call the occlusion ratio -- the ratio of fuselage diameter to propeller diameter -- on the efficiencies of tractor and pusher propellers. The efficiency of the pusher prop is lower than that of the tractor, but negligibly so, until the fuselage diameter reaches half the prop diameter, at which point the two curves begin to diverge. When the fuselage diameter is 60 percent of the prop diameter the pusher has dropped behind by about 3 percent; by the time the fuselage diameter is 70 percent of the prop diameter, the decrement is 8 percent. An equation is provided (it is also found in Dan Raymer's Airplane Design: A Conceptual Approach) that yields roughly the same result.

If this is the case, then how to account for the performance of the Skymaster cited by reader Stiles, which does so much better on its pusher propeller than on its tractor one? Indeed, to judge from that example alone one would wonder why there are any tractor-propeller airplanes in the world at all.

Actually, what the case of the Skymaster demonstrates is why it is impossible to infer general rules from particular airplanes. The poor performance of a Skymaster when its aft engine is shut down is due not to the inferiority of a tractor propeller but to flow separation on the extremely blunt aft end of the fuselage. When the rear propeller is powered, it draws in air around the aft end of the body, greatly reducing its drag, somewhat as turbulating dimples improve the performance of golf balls by shrinking the diameter of their wake. The Skymaster is a case in which the installed efficiency of the pusher propeller is indeed much greater than that of the tractor. But the effect cannot be generalized to other airplanes, because very few airplanes have fuselages shaped like the Skymaster's.

And what about the other pusher designs that Stiles mentions -- VariEze, Velocity, and so on? Well, they are certainly efficient airplanes, but one could list many tractor airplanes that match them pound for pound, dollar for dollar or horsepower for horsepower. In any case, you can't compare two different airplanes in terms of a single parameter. How could you filter out the effects of all the other differences?

One way, I suppose, would be the statistical approach -- by evaluating a lot of different airplanes against some common standard. Pushers might be at a disadvantage because there exist fewer pusher types than tractor types, but on the other hand if pushers were truly superior they would still stand out, just as rotaries, though few in number, stand out in the road races in which they compete. The nearest thing I have to such an evaluation is a listing of the results of 10 years of CAFE 250 and 400 races. Those races, which ran from 1979 through 1988, attempted to measure overall "efficiency" by a formula incorporating payload, speed and fuel burn. The list comprises 423 competitors (many of whom flew in more than one race) representing a wide spectrum of certified and experimental types. The top 20 scores all belong to homebuilts; eight of them are pushers. The top two were Mike Melvill in the tractor Rutan Catbird, purpose-built for the competition, and Dick Rutan in his pusher LongEZ, which edged the phenomenally slick pusher VariEze of Gary Hertzler only because Dick figured out a way to cram four people into his two-seat airplane. The next-highest score after Hertzler's VariEze was posted by Nick Jones' White Lightning, a tractor. And so it goes. These data are admittedly old; but nothing has happened in the intervening years to alter the general impression that a pusher propeller does not confer any particular advantage.

As you might guess from the comparative rarity of pusher types, there are arguments against the arrangement from considerations other than propeller efficiency. Since airplanes land tail-down, the back end of an airplane is not the ideal location for its propeller, and tail props are vulnerable to foreign-object damage. It's easier to cool engines on the ground if the fan is blowing toward them. Mid-engines with drive shafts present a whole medley of weight-augmenting difficulties involving vibration, cooling and access. Balance is served by putting the engine and propeller at the opposite end of the airplane from the stabilizing surfaces, and most airplanes have their stabilizing surfaces in the back; it's no coincidence that most modern pusher airplanes are canards. Conversely, tractors tend to have superior landing and takeoff performance because of blowing of the wings and tail surfaces. On the other hand, tractor props are detrimental to longitudinal stability while pushers enhance it.

And then there is always the matter of noise. Propellers are inherently noisy, but pushers add to their basic noise various dissonances generated as the blades slice through disturbed air. Those sounds travel faster than the airplane, and so they are audible to the occupants as well as to Commander Pfeiffer and his neighbors on the ground.

As rational discussions of the pusher vs. tractor controversy always conclude, in terms of efficiency alone it really is "a wash." Yet, though it may sound stodgy to say so, if the great majority of airplanes -- not counting jets -- throughout the past century have used tractor propellers, they've probably done so for some reason. It's not that one type or the other possesses an absolute superiority. It's just that the tractor arrangement entails fewer small annoyances.