A beginner's doubt on jet engines

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

There is not enough thrust at idle to be a problem.

Mike
MU-2

"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

"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


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
--
Jim in NC

"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



Sometimes they don't even need a tug to get away from the gate...

I've been on American Airlines MD80s that have pushed back under their own
power via their reversers.

Jay Beckman
PP-ASEL
Chandler, AZ

In article . net,
"Mike Rapoport" wrote:

There is not enough thrust at idle to be a problem.

Mike
MU-2

"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



I've seen numerous times on an icy ramp when it was.

--
Ron Parsons

In article ,
"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

Thrust and RPM do not have a linear relationship. Idle can be as high as
60%.

--
Ron Parsons

On 22 Dec 2004 22:26:08 -0800, "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...

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

As others have mentioned, there is not so much thrust generated by the
jet engines at idle that the tugs cannot overcome it to push it back.
Take a look at the tugs sometime, they are massively constructed with
extra weight added on so that they have sufficient mass to push and
pull the big jets.

In addition, next time you are take a flight in a jet airliner, notice
how much power the pilot has to make the engines produce in order to
get the airliner rolling.

Corky Scott

Jay Beckman wrote:

I've been on American Airlines MD80s that have pushed back under
their own
power via their reversers.


Oops! Do they have rear-view mirrors? Prima facie, reversing blind
doesn't seem overly safe...

Ramapriya

Hi there Jim, been in the wilderness of northeastern Sudan, laying a
gas pipeline, hence the bit of silence
Returned a couple of hours ago (sneaked a visit to the A320 cockpit
until TOD), and will be returning back there in about 3 days for
another month or so, but my laptop works in the deserts too, so the Qs
might just keep comin
Cheers,
Ramapriya

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...

In article .com,
Ramapriya wrote:

Jay Beckman wrote:

I've been on American Airlines MD80s that have pushed back under
their own power via their reversers.

Oops! Do they have rear-view mirrors? Prima facie, reversing blind
doesn't seem overly safe...

They have wing-walkers to direct the reversing, just as they direct
them into the gate. They aren't doing it blind at all. I've done it
in a turboprop; it's a very strange sensation. AA seems to do it more
than any other airline; not sure why.

--
Garner R. Miller
ATP/CFII/MEI
Clifton Park, NY =USA=