composites vs. aluminum

I used to have a friend that built metal aircraft. He had an interesting
way of explaining his preference.

"When you build a composite aircraft", he said, "first you make the plug and
then the mold and then the aircraft." "It's like building the same damned
airplane three times."

"When I'm through pounding a rivet", he said, "I'm through with the damn
thing". "I don't have to wait for it to dry or cure". "I don't need any
particular temperature to work either." "If I'm comfortable, the airplane
is too."

I fly a composite aircraft.

Bill Daniels

wrote in message . ..
... turbulent. In addition, it was found that the area within the
propwash was not laminar. The Mustang had a mighty big prop that
washed about a quarter to a third of each wing.

So achieving laminar flow isn't easy. Getting attached laminar flow
is one of the big reason (as I understand it) why Burt Rutan designed
the rear engined EZ series of airplanes....

Yes. pushers can get good prop thrust efficiency easier since the
"thrusted" air behind is not impeded by any structure. Air molecules
move randomly at about 1000KTS and will easily fill in the void in
front of a pusher prop to be used for thrust. In a tractor the
thrusted air is deflected by the fuselage resulting in more thrust
loses. Thats why tractors generally have bigger props to "reach"
around the cowling and over wings.

---------------------------------------------------
Paul Lee, SQ2000 canard: http://www.abri.com/sq2000

On 2 May 2004 17:02:05 -0700, (Paul Lee) wrote:

Yes. pushers can get good prop thrust efficiency easier since the
"thrusted" air behind is not impeded by any structure. Air molecules
move randomly at about 1000KTS and will easily fill in the void in
front of a pusher prop to be used for thrust. In a tractor the
thrusted air is deflected by the fuselage resulting in more thrust
loses. Thats why tractors generally have bigger props to "reach"
around the cowling and over wings.

Not sure that prop efficiencies are that much greater for the pusher
than for the tractor engined airplanes Paul. While it's true that the
pusher prop doesn't throw it's thrust against the fuselage, the
fuselage is nevertheless affecting things. You always know when a
pusher flies by because of the characteristic whapping rasping sound
the prop makes. It makes this sound because the airflow to the prop
is masked by the shape of the fuselage and wings at various places.
Around the bottom of the fuselage the prop sees relatively clean air,
but when it passes the wing, it hits a mass of downwash from the wing.
Then clean air, then turbulent air again. Plus, the mass of the
fuselage itself masks off some of the air the prop sees. The
turbulence is so severe that it's my understanding metal props are not
recommended for EZ's.

In addition, the diameter of the prop on tractor airplanes isn't
generally larger because it has to be to generate thrust around the
fuselage, it's larger because it can be. Props on pushers generally
have to be smaller in order to not grind it off on the ground in case
of inadvertant high AOA. Over rotating with an EZ risks a prop
strike.

Most tractor engined airplanes don't have that issue. Rotating for
takeoff moves the prop away from the ground, not closer to it.

My understanding is that Burt Rutan has said that front engine or
pusher, the efficiencies and design advantages pretty much cancel each
other out if you design around them, in other words neither design
offers a clear advantage over the other.

Corky Scott

wrote in message . ..
On 2 May 2004 17:02:05 -0700, (Paul Lee) wrote:
................. You always know when a
pusher flies by because of the characteristic whapping rasping sound
the prop makes. It makes this sound because the airflow to the prop
is masked by the shape of the fuselage and wings at various places.
Around the bottom of the fuselage the prop sees relatively clean air,
but when it passes the wing, it hits a mass of downwash from the wing.
Then clean air, then turbulent air again..........
In addition, the diameter of the prop on tractor airplanes isn't
generally larger because it has to be to generate thrust around the
fuselage, it's larger because it can be. Props on pushers generally
have to be smaller in order to not grind it off on the ground in case
of inadvertant high AOA. Over rotating with an EZ risks a prop
strike.
......

You raised some good points. The actual sound difference doesn't
bother me as a pilot. In fact the "behind" sound in a pusher is more
bearable. But the fact is that even turbulent air, while not ideal,
can be pushed back with the prop - those molecules moving randomly at
1000kts can easily fill in the void - unless you are moving near
1000kts. In a sense you have "turbulent" (dead) air in front when
taking off. Best possible laminar flow, from what I gather, is more
crucial for wing lift than for prop thrust. So I still think there is
some advantage to pusher props - although, as you pointed out, it may
not be very significant.

Just a further brainstorm curiosity. Not sure if I can express this
clearly. The greatest net force you have is on takeoff. The prop grabs
the molecules and throws them backward. The change in molecular
momentum results in the thrust (F = dP/dt). When the air is moving
fast backward (high air speed) there is much smaller change in
momentum - granted there is more molecules pushed. Would the turbulent
air (slow air) in front of a pusher prop help the thrust somewhat? A rocket
engine, with molecules relatively at initial 0 speed, has more thrust
than a jet.

On 3 May 2004 16:11:01 -0700, (Paul Lee) wrote:

Just a further brainstorm curiosity. Not sure if I can express this
clearly. The greatest net force you have is on takeoff. The prop grabs
the molecules and throws them backward. The change in molecular
momentum results in the thrust (F = dP/dt). When the air is moving
fast backward (high air speed) there is much smaller change in
momentum - granted there is more molecules pushed. Would the turbulent
air (slow air) in front of a pusher prop help the thrust somewhat? A rocket
engine, with molecules relatively at initial 0 speed, has more thrust
than a jet.

My understanding, which is limited I admit, is that pusher props are
less efficient than the exact same engine and prop mounted on the
front of an airplane. It's one of those characteristic give and takes
that happen throughout the relm of aviation design. True, the pusher
gets to push air directly to the rear. The downside is that the air
it gets to accelerate is dirty (turbulent) air and that degrades
performance. The tractor engine sees clean air for it's prop, but it
smacks some of the air it accelerates against the nose of the
fuselage, the windscreen, wings and appendages. There's good with the
bad though, the column of air blasted to the rear gives the elevators
and rudder effective air movement to become operational even at very
low forward airspeeds. Ever see a Pitts taxiing in with it's tail up
in the air? With a pusher, the air being blown rearwards doesn't pass
over any of the control surfaces, they must wait for forward motion to
become fast enough for them to have enough airflow before responding
to input. This is one reason why the EZ's have a relatively long
takeoff roll.

You are absolutely correct that a front mounted prop pushes a certain
amount of air against the nose of the fuselage, and that's thrust
wasted. But remember, props don't produce a whole lot of thrust down
near the hub anyway so percentage wise, there isn't much thrust there
to waste. In fact in some props, there isn't really an airfoil near
the hub. This would be true with the pusher props too, since the
fuselage also masks the area near the hub in that instance too.

What is bad is the prop passing through the two downwash area's and in
some cases the column of exhaust from the engine. Remember me saying
that in the P-51 the area of the wing within the turbulence from the
prop was non laminar due to the turbulence? Well think about what's
happening to the airflow around the prop when it whaps through the
downwash from each of the wings. And it's the entire prop that passes
through this downwash.

I agree that during takeoff, this affect would be minimised, at least
until lift off, then the airplane would be producing maximum lift,
which means the prop would be passing through the strongest downwash
off the wings that it ever encounters.

So if you managed somehow to fit your engine in front of your SQ, I'm
guessing that performance would be virtually identical. Well mayby
not, it might takeoff quicker because you'll be able to lift the nose
to get a higher angle of attack sooner. It's the tiny size of the
fuselage the minimal wetted area and the smoothness of the surfaces
and finish, plus the laminar design of the airfoils that give Rutan's
designs their performance, not necessarily the rear placement of the
engine.

The airplane with the lowest coeficient of drag ever recorded was a
tractor design, albeit a small one. :-)

Corky Scott

wrote in message . ..
On 3 May 2004 16:11:01 -0700, (Paul Lee) wrote:
.............
The airplane with the lowest coeficient of drag ever recorded was a
tractor design, albeit a small one. :-)

Corky Scott

Scott,

Lowest CD very much depends on the design. There are "many" tractor
designs and much fewer pusher ones to compare to. But really overal
CD of a body has little to do with pusher/tractor prop effectivenes
issue here. I think a good way to settle this is to put a model
tractor body in a wind tunel and then reverse the same thing and
test it as a pusher. Anybody's got a spare wind tunel and lots of time?

On 4 May 2004 20:09:32 -0700, (Paul Lee) wrote:

Lowest CD very much depends on the design. There are "many" tractor
designs and much fewer pusher ones to compare to. But really overal
CD of a body has little to do with pusher/tractor prop effectivenes
issue here. I think a good way to settle this is to put a model
tractor body in a wind tunel and then reverse the same thing and
test it as a pusher. Anybody's got a spare wind tunel and lots of time?

I didn't make myself clear enough, sorry. What I meant to say was
that pusher or tractor, the plusses and minuses pretty much cancel
themselves out. One can optimize a pusher to make it a really
efficient airplane, or optimize a tractor to do the same. I doubt you
will find enough differences in the performance of such machines to
measure.

Corky Scott

(Paul Lee) wrote:
I think a good way to settle this is to put a model
tractor body in a wind tunel and then reverse the same thing and
test it as a pusher. Anybody's got a spare wind tunel and lots of time?

Or, if one is looking to "prove" the opposite result, put a model
pusher body in a wind tunnel and then reverse the same thing and
test it as a tractor! g
--
Alex
Make the obvious change in the return address to reply by email.

On Wed, 5 May 2004, alexy wrote:

(Paul Lee) wrote:
I think a good way to settle this is to put a model
tractor body in a wind tunel and then reverse the same thing and
test it as a pusher. Anybody's got a spare wind tunel and lots of time?

Or, if one is looking to "prove" the opposite result, put a model
pusher body in a wind tunnel and then reverse the same thing and
test it as a tractor! g
--

Actually, this question has been answered.

The Cessna SkyMaster has both a pusher and a tractor engine.
The single engine climb rate and speed are both higher for the
rear engine alone, than for the front engine alone.

The pusher wins!

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage http://bfn.org/~ca266

"George A. Graham" wrote
Actually, this question has been answered.

The Cessna SkyMaster has both a pusher and a tractor engine.
The single engine climb rate and speed are both higher for the
rear engine alone, than for the front engine alone.

The pusher wins!

George Graham

Actually, no. One would have to test without the prop on the non-operating
engine and with the cooling inlets streamlined. With the dead engine prop
still on, it is largely a test on which position has less drag than the
other.
--
Jim in NC


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