Seems like 6 of one, 1/2 dozen of another?

Pusher Pusher


By Peter Garrison
June 2009

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.

---
Mark