A beginner's doubt on jet engines

Ramapriya wrote:
I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the
jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward
motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit
of
a tiresome affair by the time it's over...

As others have said, the tug can certainly over power it. However, I
have read accounts of pilots from the 727 days saying that at idle, the
engines did put out a lot of thrust and that on landings it added a lot
of float. Will most jets roll on the idle engines if the brake is not
set?

-Robert

Robert M. Gary wrote:
Will most jets roll on the idle engines if the brake is not set?

I spoke with an F-15 pilot at Nellis AFB and he said the F-15 will reach
about 80 knots at idle.

Hilton

PS: Don't know if they trolled me, but both the pilots today didn't
seem to know much of the CFM56's internals. Possibly they didn't want
to bother answering, but I'll have to look elsewhere to figure the
compressor's and ignition chamber's construction...

too many questions.. to the wrong person or persons.. may label you a
suspected terrorist..
and why would they give information like that.. to someone they don't know..
and information that is readily available in a public library?
some one may come knocking on your door to check you out...

BT

For what it's worth, a 757 at ground idle produces around 3000 pounds thrust
per engine - you can back them using the reversers, but you can't go full
power as the re-ingest the exhaust gasses.

And when you're backing you must always use forward thrust to stop, and not
the brakes - anyone guess why?


"Ramapriya" wrote in message
oups.com...
I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
a tiresome affair by the time it's over...

Sorry if this is infuriatingly elementary, but I need to ask somewhere


Ramapriya

"Cockpit Colin" wrote in message
...
For what it's worth, a 757 at ground idle produces around 3000 pounds
thrust
per engine - you can back them using the reversers, but you can't go full
power as the re-ingest the exhaust gasses.

And when you're backing you must always use forward thrust to stop, and
not
the brakes - anyone guess why?

The disk brakes would roll our of the calipers?
--
Jim in NC

"Morgans" wrote

The disk brakes would roll our of the calipers?
--
Jim in NC

Make that "out" of the calipers.

"Cockpit Colin" wrote

And when you're backing you must always use forward thrust to stop,
and not the brakes - anyone guess why?

Sure...brakes will set it on its tail. I've backed-up
B-727s frequently.

Bob Moore

10% is usually about half way to light off for a turbine engine. Starter
usually cuts out around 50% and stabilize at 60-70% core speed.
Michelle

Morgans wrote:

"Ramapriya" wrote in message
roups.com...


I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
a tiresome affair by the time it's over...

Sorry if this is infuriatingly elementary, but I need to ask somewhere


Ramapriya




At idle, turbo fan engines, or turbo jet engines do not make a tremendous
amount of thrust. The push-back tugs are very powerful, with very low
gearing. They simply push harder than the engines are pushing.

Sometimes, the engines are not started until after push-back, or while the
push-back is taking place.

Some of the heavy metal pilots can tell you better than me, but it is my
understanding that it takes well over 50% RPM to get 50% thrust, so it
goes to follow that 10% RPM is way less than 10% thrust.

Where have you been? Kinda' lonely around here, without your constant
questions! g

Tail strike! Seen it happen. Brakes grab more in reverse leading to to
quick a stop and rotation over the mains.
Michelle

Cockpit Colin wrote:

For what it's worth, a 757 at ground idle produces around 3000 pounds thrust
per engine - you can back them using the reversers, but you can't go full
power as the re-ingest the exhaust gasses.

And when you're backing you must always use forward thrust to stop, and not
the brakes - anyone guess why?


"Ramapriya" wrote in message
roups.com...


I know that a car's engine needs to be mated to the transmission for
propulsion. This I know isn't the case with an aircraft with a jet
engine; if the engine is turned on without the brakes applied, the jet
of air leaving the engine will hurl the craft forward.

My doubt is, why does this forward motion effect not occur during
push-back, when the engines are normally turned on? Is it because at
low revs the engine would lack the punch to initiate the forward motion
of a heavy aircraft? If not, I'd imagine the push-back becomes a bit of
a tiresome affair by the time it's over...

Sorry if this is infuriatingly elementary, but I need to ask somewhere


Ramapriya

Ramapriya wrote:

PS: Don't know if they trolled me, but both the pilots today didn't
seem to know much of the CFM56's internals.

Perhaps they don't know that much about the internals. Ernest Gann said he never
understood why American Airlines felt it necessary that he be able to answer
detailed questions about the workings of the radial engines he flew behind. He
couldn't imagine climbing out on the wing to affect repairs in flight, though he
said "there would be times when I fervently wished it were possible."

George Patterson
The desire for safety stands against every great and noble enterprise.