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

And the point of this copyright infrigement is?

On Aug 26, 7:30*am, Ron wrote:
And the point of this copyright infrigement is?

Well, sorry for posting it in it's entirety.
Selected paragraphs would've been better.

The point was to ask whether tractor vs pusher
is simply a matter of personal taste, or really a
matter of performance.

You don't see that many people talking about
Cozys and Velocities. The Long ez kit isn't
offered anymore.

---
Mark

On Wed, 26 Aug 2009 19:17:36 -0700 (PDT), Mark
wrote:

On Aug 26, 7:30*am, Ron wrote:
And the point of this copyright infrigement is?

Well, sorry for posting it in it's entirety.
Selected paragraphs would've been better.

The point was to ask whether tractor vs pusher
is simply a matter of personal taste, or really a
matter of performance.

You don't see that many people talking about
Cozys and Velocities. The Long ez kit isn't
offered anymore.

---
Mark

problem with pushers is one single dropped anything in the engine bay
will eventually impact the prop in flight.
I can point you to an mt prop with the perfect impression of an
aircraft washer about 6" out from the hub..

the other problem is that a canard can never extract as much
performance from the wing.

sideslips in a tractor aircraft are easy. you never hear of the
deviations from heading that occur in a canard. I'm told they can be
attention getting.

crashworthiness of an aircraft is directly related to the speed at
landing/impact. a tractor aircraft can extract more from the wing and
can be landed slower than a canard.

separately from that the pushers were all composite aircraft.
the crashworthiness of some composite aircraft has been a little less
than tin aircraft shall we say.

so without the evangelists out there doing the sell on the designs
interest tends to focus back on the aspects that are less than ideal.
plus also vans has produced some good flying aircraft that almost
anyone can build with assurance that they will end up with an aircraft
worth the build.

the lightest aircraft you can build has a prop up front.

Stealth Pilot

On Aug 27, 6:49*am, Stealth Pilot wrote:
On Wed, 26 Aug 2009 19:17:36 -0700 (PDT), Mark





wrote:
On Aug 26, 7:30*am, Ron wrote:
And the point of this copyright infrigement is?

Well, sorry for posting it in it's entirety.
Selected paragraphs would've been better.

The point was to ask whether tractor vs pusher
is simply a matter of personal taste, or really a
matter of performance.

You don't see that many people talking about
Cozys and Velocities. The Long ez kit isn't
offered anymore.

---
Mark

problem with pushers is one single dropped anything in the engine bay
will eventually impact the prop in flight.
I can point you to an mt prop with the perfect impression of an
aircraft washer about 6" out from the hub..

the other problem is that a canard can never extract as much
performance from the wing.

sideslips in a tractor aircraft are easy. you never hear of the
deviations from heading that occur in a canard. I'm told they can be
attention getting.

crashworthiness of an aircraft is directly related to the speed at
landing/impact. a tractor aircraft can extract more from the wing and
can be landed slower than a canard.

separately from that the pushers were all composite aircraft.
the crashworthiness of some composite aircraft has been a little less
than tin aircraft shall we say.

so without the evangelists out *there doing the sell on the designs
interest tends to focus back on the aspects that are less than ideal.
plus also vans has produced some good flying aircraft that almost
anyone can build with assurance that they will end up with an aircraft
worth the build.

the lightest aircraft you can build has a prop up front.

Stealth Pilot- Hide quoted text -

- Show quoted text -

Thank you very much.

---
Mark

Stealth Pilot wrote:
On Wed, 26 Aug 2009 19:17:36 -0700 (PDT), Mark
wrote:
The point was to ask whether tractor vs pusher
is simply a matter of personal taste, or really a
matter of performance.

You don't see that many people talking about
Cozys and Velocities. The Long ez kit isn't
offered anymore.

---
Mark

problem with pushers is one single dropped anything in the engine bay
will eventually impact the prop in flight.
I can point you to an mt prop with the perfect impression of an
aircraft washer about 6" out from the hub..

And I know of cases where the spinner of a tractor was hit dead-on by
birds. There is nothing in accident statistics to indicate this alleged
problem has been an issue.

the other problem is that a canard can never extract as much
performance from the wing.

First, the subject of the thread is pusher versus tractor. There are and
have been pushers that don't use canard wing layouts.

Second, got a cite for your "performance" claim? Because I've got
references that claim just the opposite, such as:

"Theoretically, the canard is considered more efficient because using the
horizontal surface to the horizontal surface to help lift the weight of
the aircraft should result in less drag for a given amount of lift."
From: "Pilot's Handbook of Aeronautical Knowledge" FAA publication, 2003.

sideslips in a tractor aircraft are easy. you never hear of the
deviations from heading that occur in a canard. I'm told they can be
attention getting.

Again, a pusher is not a canard. You're confusing wing configurations
with engine configurations.

crashworthiness of an aircraft is directly related to the speed at
landing/impact. a tractor aircraft can extract more from the wing and
can be landed slower than a canard.

Sigh. See above.

separately from that the pushers were all composite aircraft.
the crashworthiness of some composite aircraft has been a little less
than tin aircraft shall we say.

Congratulation on making statements with high-density baloney that aren't
even related to the pusher/tractor issue.

First, pushers have been around at least since the Wright brothers and
were built in wood and metal long before fiberglass composite aircraft
were built.

Second, what does the "crashworthiness" of "some" composites have to do
with anything? And by "tin" do you mean "metal" or do you really think
there are aircraft made mostly of tin?

so without the evangelists out there doing the sell on the designs
interest tends to focus back on the aspects that are less than ideal.
plus also vans has produced some good flying aircraft that almost
anyone can build with assurance that they will end up with an aircraft
worth the build.

Huh?

the lightest aircraft you can build has a prop up front.

Do you believe this for real or are you just trolling?? The lightest
aircraft you can build don't even have an engine (e.g. hang-gliders,
cloud-hopper balloons, and cluster balloons.) And the lightest powered
aircraft one can buy (ultralight powered parachutes and trikes) _are_
generally pushers!

There are pros and cons to putting engines in front, in back, on top, out
on the sides, and maybe even the bottom of the fuselage, but the final
determination is influenced by the intended mission of the aircraft - an
issue that wasn't addressed.

On Aug 27, 1:11*pm, Jim Logajan wrote:
Stealth Pilot wrote:
On Wed, 26 Aug 2009 19:17:36 -0700 (PDT), Mark
wrote:
The point was to ask whether tractor vs pusher
is simply a matter of personal taste, or really a
matter of performance.

You don't see that many people talking about
Cozys and Velocities. The Long ez kit isn't
offered anymore.

---
Mark

problem with pushers is one single dropped anything in the engine bay
will eventually impact the prop in flight.
I can point you to an mt prop with the perfect impression of an
aircraft washer about 6" out from the hub..

And I know of cases where the spinner of a tractor was hit dead-on by
birds. There is nothing in accident statistics to indicate this alleged
problem has been an issue.

the other problem is that a canard can never extract as much
performance from the wing.

First, the subject of the thread is pusher versus tractor. There are and
have been pushers that don't use canard wing layouts.

Second, got a cite for your "performance" claim? Because I've got
references that claim just the opposite, such as:

"Theoretically, the canard is considered more efficient because using the
horizontal surface to the horizontal surface to help lift the weight of
the aircraft should result in less drag for a given amount of lift."
From: "Pilot's Handbook of Aeronautical Knowledge" FAA publication, 2003.

sideslips in a tractor aircraft are easy. you never hear of the
deviations from heading that occur in a canard. I'm told they can be
attention getting.

Again, a pusher is not a canard. You're confusing wing configurations
with engine configurations.

crashworthiness of an aircraft is directly related to the speed at
landing/impact. a tractor aircraft can extract more from the wing and
can be landed slower than a canard.

Sigh. See above.

separately from that the pushers were all composite aircraft.
the crashworthiness of some composite aircraft has been a little less
than tin aircraft shall we say.

Congratulation on making statements with high-density baloney that aren't
even related to the pusher/tractor issue.

First, pushers have been around at least since the Wright brothers and
were built in wood and metal long before fiberglass composite aircraft
were built.

Second, what does the "crashworthiness" of "some" composites have to do
with anything? And by "tin" do you mean "metal" or do you really think
there are aircraft made mostly of tin?

so without the evangelists out *there doing the sell on the designs
interest tends to focus back on the aspects that are less than ideal.
plus also vans has produced some good flying aircraft that almost
anyone can build with assurance that they will end up with an aircraft
worth the build.

Huh?

the lightest aircraft you can build has a prop up front.

Do you believe this for real or are you just trolling?? The lightest
aircraft you can build don't even have an engine (e.g. hang-gliders,
cloud-hopper balloons, and cluster balloons.) And the lightest powered
aircraft one can buy (ultralight powered parachutes and trikes) _are_
generally pushers!

There are pros and cons to putting engines in front, in back, on top, out
on the sides, and maybe even the bottom of the fuselage, but the final
determination is influenced by the intended mission of the aircraft - an
issue that wasn't addressed.- Hide quoted text -

- Show quoted text -

Thanks. I believe however that Stealth Pilot understood
my considerations here based on the fact that I mentioned
the Cozy, Long ez, and Velocity. But just to clarify, I'm
asking within the category of a nice personal aircraft that
is suitable for cross country, 1 person, a little luggage.

Some fellows that fly Vans had commented at an
airshow that canards were "kinda squirrely". I see them
more as being *pitch sensitive*, until you get used to
their characteristics, although landing they do come
fast. Again, I assume familiarizing oneself with that
glideslope cures that.

As far as "tin" planes, I didn't take that any more literally
than actually trying to fly a tuna can.

---
Mark LLC

Regardless of what is fact and what is fiction, most of what has been
said about composite canard pushers vs. metal classic tractors is
comparing apples to oranges (IIRC, in Germany we usually compare
apples to pears). Advantages and disadvantes have been pointed out,
but do not relate to the original pusher-vs.-tractor question. The
Cessna Skymaster example in the original text suits it much better.

Apart from the problem of aft CG and hence the difficulty to make a
single-engine pusher in a classical wing layout, there are two
opposing factors:

1. The tractor engine works more efficiently since the prop is in an
undisturbed air stream. The slipstream may be able to increase maximum
lift on parts of the wing, but can induce a rolling moment. The
turbulence created creates more drag, especially on the fuselage.
Also, putting the engine up front makes it less likely to have an
aerodynamically optimized fuselage.

2. The pusher engine works less efficiently since the prop sits in an
airstream that has already passed fuselage and wing. OTOH, the
fuselage can be shape-optimized more easily and sees an undisturbed,
laminar airflow. Maximum lift is likely to be a little lower.

Now, which of these effects is the dominant one? Also, if you have a
twin engine airplane, the fuselage is out of the equation, so the
final result may be different!?

Oliver

On Aug 28, 3:13*am, Oliver Arend wrote:
Regardless of what is fact and what is fiction, most of what has been
said about composite canard pushers vs. metal classic tractors is
comparing apples to oranges (IIRC, in Germany we usually compare
apples to pears). Advantages and disadvantes have been pointed out,
but do not relate to the original pusher-vs.-tractor question. The
Cessna Skymaster example in the original text suits it much better.

Apart from the problem of aft CG and hence the difficulty to make a
single-engine pusher in a classical wing layout, there are two
opposing factors:

1. The tractor engine works more efficiently since the prop is in an
undisturbed air stream. The slipstream may be able to increase maximum
lift on parts of the wing, but can induce a rolling moment. The
turbulence created creates more drag, especially on the fuselage.
Also, putting the engine up front makes it less likely to have an
aerodynamically optimized fuselage.

2. The pusher engine works less efficiently since the prop sits in an
airstream that has already passed fuselage and wing. OTOH, the
fuselage can be shape-optimized more easily and sees an undisturbed,
laminar airflow. Maximum lift is likely to be a little lower.

Now, which of these effects is the dominant one? Also, if you have a
twin engine airplane, the fuselage is out of the equation, so the
final result may be different!?

Oliver

So let me sum it up thusly.... 6 of one, 1/2 dozen of the other.

BobR wrote:
On Aug 28, 3:13 am, Oliver Arend wrote:
Regardless of what is fact and what is fiction, most of what has been
said about composite canard pushers vs. metal classic tractors is
comparing apples to oranges (IIRC, in Germany we usually compare
apples to pears). Advantages and disadvantes have been pointed out,
but do not relate to the original pusher-vs.-tractor question. The
Cessna Skymaster example in the original text suits it much better.

Apart from the problem of aft CG and hence the difficulty to make a
single-engine pusher in a classical wing layout, there are two
opposing factors:

1. The tractor engine works more efficiently since the prop is in an
undisturbed air stream. The slipstream may be able to increase maximum
lift on parts of the wing, but can induce a rolling moment. The
turbulence created creates more drag, especially on the fuselage.
Also, putting the engine up front makes it less likely to have an
aerodynamically optimized fuselage.

2. The pusher engine works less efficiently since the prop sits in an
airstream that has already passed fuselage and wing. OTOH, the
fuselage can be shape-optimized more easily and sees an undisturbed,
laminar airflow. Maximum lift is likely to be a little lower.

Now, which of these effects is the dominant one? Also, if you have a
twin engine airplane, the fuselage is out of the equation, so the
final result may be different!?

Oliver

So let me sum it up thusly.... 6 of one, 1/2 dozen of the other.

On the other hand..... there are more fingers.

Dan, U.S. Air Force, retired