On May 18, 7:38 am, C J Campbell
wrote:
On 2007-05-17 14:26:01 -0700, "Capt. Geoffrey Thorpe" The Sea Hawk at
wow way d0t com said:





"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...

Sounds like it was running backwards to me. Maybe it would not keep
running, but the prop only has to hit you once.
--
Waddling Eagle
World Famous Flight Instructor- Hide quoted text -

- Show quoted text -

The engine can fire when pulled through forward, but if the
prop has too little momentum to carry the piston past TDC or the
firing is advanced enough, it will kick backward. One blade,
travelling at the speed it does, will split your skull. I work on
these things all day, and when I forget that the prop is there and I
stand up after doing something on the engine and whack my head on a
stationary prop blade, it hurts big time. When it's swinging after a
kickback it's moving faster than an axe and weighs a lot more. Whether
dull or not, that trailing edge will do plenty of damage. You learn
respect for these things when you bash your head occasionally.
I used to own a '78 Dodge pickup truck, the worst vehicle I
ever encountered. It would "diesel' on after I turned the ignition
off, ignition being caused by hot carbon points in the cylinder head.
Some cylinders would fire, some wouldn't, and the result was an
exhaust system full of fuel vapours. An auto's exhaust system is a lot
longer than a lightplane's, and it can store plenty of vapour. Sooner
or later the engine would kick over backward, the exhaust would get
sucked into a cylinder, and it would run backward for a half-second or
so and finally die when the fuel vapours ran out. The hot carbon was
still doing the igniting, like a glow plug.
If an airplane does that it won't run so long, and it only
does it if there's hot carbon in the head which means that it's been
running. And that's why I consider a prop most dangerous immediately
after shutdown.

Dan