Proping Question

"C J Campbell" wrote in message
news:2007051622171050878-christophercampbell@hotmailcom...
On 2007-05-15 05:59:05 -0700, "Doug Palmer"
said:

...
An engine can be made to run backwards. Model airplane engines do it all
the time, usually as a result of mixture that is too rich. Granted,

Those are two strokes and don't rely on valve timing to run. A buddy had a
Bultaco motorcycle that liked to do that - it was pretty funny when he
dumped the clutch not realizing that it was running backwards.

real airplane engines are different and have more safety systems, but I
could not say that it is impossible, especially given the enormous variety
in types of engines, magnetos, starters, and fuel systems you see on
airplanes.

If you turn a conventional four stroke engine backwards, what would have
been the exhaust stroke is now an intake stroke, and what was the intake
stroke is now an exhaust stroke - the air will flow backwards through the
engine from the exhaust to the intake so fuel will not find it's way in to
sustatin combustion. On the other hand, an engine can "kick back" for a
revolution or so - and that's enough to do the damage...

To make a conventional four stroke run backwards, you have to re-arrange the
location of the lobes on the cam.

Trivia: Kettering developed his electric starter after a friend was killed
when an automobile engine kicked back while he was starting it with a hand
crank...

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

On 16 May 2007 07:50:51 -0700, wrote:

Turning a prop backwards during hand-propping reduces the
mixture ratio in the cylinders if the engine has been overprimed. Air
is drawn through the exhaust and will absorb the fuel, carrying it
back through the carb and out. The carb will often drip fuel during
such an event.

That might explain an event that startled me at the time. While being
propped in hot weather--unsuccessfully--I was strapped in the rear
seat of the Cub, when the mechanic came by and said, "Your engine is
on fire." He leaned close to the cowling (the engine is visible
through the iconic J-3 cowling "ears"), gave a puff, blew the fire
out, and went on his way without further comment.

Blue skies! -- Dan Ford

On Wed, 16 May 2007 22:00:14 -0700, C J Campbell
wrote:

People will say it is technically impossible, but I think it is wishful
thinking. An engine may not run backwards very well,

During World War II (really!) I worked on a farm in Concord, Mass.
There was a great steel-wheeled tractor that was started with a hand
crank. One time the tractor backfired while the lad was spinning the
crank, and the engine started running backwards. He jumped aboard and
had a great time wheeling it around the yard, one speed forward and
three in reverse. Blue skies! -- Dan Ford

On Wed, 16 May 2007 21:39:54 -0700, Don Tuite
wrote:

Now you holler "Hot!, Brakes!" (assuming somebody's in the cockpit --
otherwise you switch the mags to hot and check the chocks and tiedown.

With me, it's BRAKES! CRACKED! (referring to the throttle), HOT!

Reach up, pads of your fingers just over the top of the blade. Left
leg on ground, right leg swings forward and smartly back at the same
time that you snap the blade down. The leg swing propels your body
backward, away from the propeller arc.

Huh. With me it's the left leg that swings. Then the right leg turns
me farther back and to the left, out of the propeller arc.

Blue skies! -- Dan Ford

"Cubdriver" usenet AT danford DOT net wrote in message
...
On Wed, 16 May 2007 22:00:14 -0700, C J Campbell
wrote:

People will say it is technically impossible, but I think it is wishful
thinking. An engine may not run backwards very well,

During World War II (really!) I worked on a farm in Concord, Mass.
There was a great steel-wheeled tractor that was started with a hand
crank. One time the tractor backfired while the lad was spinning the
crank, and the engine started running backwards. He jumped aboard and
had a great time wheeling it around the yard, one speed forward and
three in reverse. Blue skies! -- Dan Ford

That was a Diesel engine, correct?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

On 16 May 2007 17:22:39 -0700, Dave wrote:

I flew cubs for awhile 30+ years ago. If there was no one around to
prop yours, the drill was to stand behind the prop on the right side
and flip it with your right hand, The left was poised to adjust the
throttle when it caught. I recall that they were amazingly easy to
start (usually on the first or second try).

They still are, if the engine is warm, and that's the only occasion
when I have to prop the plane myself.

I hold onto the window frame with my left hand and, like you, pull
down with the right hand. I bounce it lightly a couple times in
advance, and one knows exactly when to pull down hard after it springs
back up.

At a minimum, I have a pair of chocks on a length of parachute cord. I
can climb into the back seat, give the cord a tug, then reel in the
chocks.

Blue skies! -- Dan Ford

On May 17, 2:52 pm, "JGalban via AviationKB.com" u32749@uwe wrote:
wrote:

I haven't yet seen a Continental or Lycoming lightplane engine
without an impulse coupling on at least one mag.

They're out there. In addition to the impuse system, there was a starting
system called "shower of sparks" that used a gadget called a vibrator to
provide the low rpm spark. Unlike the impulse system, the vibrator required
electrical power from the battery to make a spark.

I've usually seen these on older models.

Yup, you're right. Bendix made that system to provide a hot
spark for starting. The mag switch had extra terminals to provide an
AC signal from a buzzbox to the mag, but that mag had a second set of
points that gave the later spark for starting. Unison has a similar
setup now, but solid-state instead of an electromechanical buzzer to
create the signal for the primary winding.

Dan

They're out there. In addition to the impuse system, there was a
starting
system called "shower of sparks" that used a gadget called a vibrator to
provide the low rpm spark. Unlike the impulse system, the vibrator
required
electrical power from the battery to make a spark.

I've usually seen these on older models.

Yup, you're right. Bendix made that system to provide a hot
spark for starting. The mag switch had extra terminals to provide an
AC signal from a buzzbox to the mag, but that mag had a second set of
points that gave the later spark for starting. Unison has a similar
setup now, but solid-state instead of an electromechanical buzzer to
create the signal for the primary winding.

Both of those spark vibrators just make a constant sparking, just waiting
for the right combination of fuel, air and compression, don't they?

If that is the case, it would seem like that would be like a very advanced
spark, and could easily make a backfire to spin the engine backwards. Is
this what you have observed?

If that is not how they work, then...
Never Mind ! ! ! g
--
Jim in NC

Morgans wrote:



Both of those spark vibrators just make a constant sparking, just waiting
for the right combination of fuel, air and compression, don't they?

If that is the case, it would seem like that would be like a very advanced
spark, and could easily make a backfire to spin the engine backwards. Is
this what you have observed?

If that is not how they work, then...
Never Mind ! ! ! g

The system produces a much higher powered, rapid succession of sparks,
but they are only delivered to each plug via a second set of points in
the mag, that are set at a delayed timing period.

Happy Flying!
Scott Skylane

"Scott Skylane" wrote

The system produces a much higher powered, rapid succession of sparks, but
they are only delivered to each plug via a second set of points in the
mag, that are set at a delayed timing period.

OK, thanks for that info!
--
Jim in NC