Can someone here explain engine thrust pounds to me?
Musicman59
an explanation in layman's terms if at all possible.
Ok for example, a fighter engine has 20,000 pounds of thrust. Just
what does that mean? What relation is that to the weight of the
aircraft? If the aircraft is heavier than the thrust of all engines
used, how does it move, not to mention getting to 1000 mph, etc.
Thx - Craig
Ron
Well thrust is a force, one of the the four on an aircraft. Thrust
propels it, drag works the opposite way, gravity tries to pull it
down, lift tries to keep it up.
As long as thrust is greater than drag and friction, it will continue
to accelerate. Of course greater the thrust compared to its mass,
the faster it will accelerate - F = MA
Thrust does not have to be more than the weight to get it moving, just
has to be more than the force holding it back.
What ultimately determines the top speed of an aircraft, is where
thrust = drag, and there are many factors in that.
Now if you want to take the plane vertically and look at it roughly
like a rocket, 20,000 lbs of thrust will accelerate it vertically, as
long as the mass is less than that. If the mass is the same, it will
not accelerate but will hold its velocity. And if the mass is greater
than the thrust, it will decelerate.
Jeff Crowell
> Well thrust is a force, one of the the four on an aircraft. Thrust
> propels it, drag works the opposite way, gravity tries to pull it
> down, lift tries to keep it up.
>
> As long as thrust is greater than drag and friction, it will continue
> to accelerate. Of course greater the thrust compared to its mass,
> the faster it will accelerate - F = MA
> Thrust does not have to be more than the weight to get it moving, just
> has to be more than the force holding it back.
>
> What ultimately determines the top speed of an aircraft, is where
> thrust = drag, and there are many factors in that.
>
> Now if you want to take the plane vertically and look at it roughly
> like a rocket, 20,000 lbs of thrust will accelerate it vertically, as
> long as the mass is less than that. If the mass is the same, it will
> not accelerate but will hold its velocity. And if the mass is greater
> than the thrust, it will decelerate.
And, keep in mind that the word 'acceleration' can also mean the
forces used to change the direction the aircraft is moving (Newton
and all that). That's what Ron meant about many factors being
involved in drag effects. I'd nitpick to say that drag and
friction are one and the same.
Keeping to layman's terms:
If an aircraft is in level, non-turning flight (often referred to
as "unaccelerated flight"), it will reach a speed at which total
drag equals total thrust; these forces are in opposition. That
change in speed is usually referred to as acceleration, whether
the plane speeds up (positive acceleration) or slows down (negative
acceleration)--so why do we call it "unaccelerated" flight?
The "unaccelerated" term acknowledges that an aircraft which is
maneuvering experiences a G load other than 1.0. A level turn
(neither climbing nor descending) requires an increased G load
on the airframe. This G loading increases the amount of work
the wing needs to do to keep the aircraft at the same altitude--
it increases drag. The amount of added drag is proportional to
the amount of added G. So if I am at steady-state speed in level,
straight-ahead flight and start a level turn, I will slow down
until drag once again equals thrust.
If I am flying a 20,000 pound plane with a 30,000 pound thrust
engine in unaccelerated flight (not climbing/diving, not turning),
I'll reach a steady state speed (how fast depends on the design
of the plane, my altitude (affects air temperature/density), etc.
If I then enter a 2G level turn, the airplane now "weighs" 20,000
pounds (2Gs times 20,000 pounds) and I'll slow down. I'll slow
down (decelerate) because I accelerated (added G). ;-)
If I enter a climbing spiral at 2Gs, I'll slow down even faster,
due to what's called "God's G."
This can work in your favor--if I need to speed up to get out of
a dangerous situation, to "bug out" of a fight, I can go to max
power and fly straight and level; the plane will speed up quickly.
But if I "unload" (push forward on the stick to reduce G below
1.0) the wing doesn't have to work as hard. Drag decreases and
I'll accelerate even more quickly than I did at G = 1.0. It
also helps that I am now descending--but the decrease in altitude
is a separate effect from the decrease in drag caused by the
unload. An unload to 0.5G means I will gain speed more quickly
than if I unloaded to only 0.75G. There is an upper bound to the
benefits of unloading--for example, at a sustained zero G the
airplane would speed up like a raped ape, but my fuel system
probably wouldn't work and the engine would quit (likewise the
oil pressure would drop to zero). Not good. It's also
considered to be Bad Form to crash into the ground whilst bugging
out. ;-)
That's probably a whole lot more than you wanted, but there it
be.
Jeff
--
Murphy's Laws of the gunfight:
If you are bleeding, say something witty.
Jeff Crowell
> If I am flying a 20,000 pound plane with a 30,000 pound thrust
> engine in unaccelerated flight (not climbing/diving, not turning),
> I'll reach a steady state speed (how fast depends on the design
> of the plane, my altitude (affects air temperature/density), etc.
> If I then enter a 2G level turn, the airplane now "weighs" 20,000
> pounds (2Gs times 20,000 pounds) and I'll slow down.
Curses! Damned proofreaders....
That should read:
If I then enter a 2G level turn, the airplane now "weighs"
40,000 pounds (2Gs times 20,000 pounds)
^^
Jeff
--
A people that values its privileges above its principles will
soon lose both.
Ed Rasimus
And, just to keep it pure. The "friction" referred to in the OP should
be termed "parasite drag" or the drag of the shape of the aircraft
being resisted as it passes through the air molecules.
The drag caused by increased G is "induced drag" or the drag which is
a byproduct of the production of lift. To produce additional lift and
make the wing continue to support that accelerated "heavy" aircraft
you must increase angle of attack or increase speed through the air.
Thrust limits G-available if trying to increase speed to counter G
forces. Drag limits G-available if trying to increase angle-of-attack
to produce more lift.
Better to just consider four forces--thrust/drag in the longitudinal
and lift/weight in the vertical. Soon you must enter turns and that
makes the whole mess unworkable. A generous application of magic and
incantations is then required to sustain flight.
Ed Rasimus
Fighter Pilot (USAF-Ret)
www.thundertales.blogspot.com
LIBERATOR
The way I understand it is if your plane weighs 2 tons, and you have
30,000lbs of thrust, the weight of 30,000lbs to the center of the
earth is the weight pushing the craft.
So, if someone says 30,000lbs thrust, you have what 30,000lbs falling
to earth would be like, pushing your craft.
Jim Wilkins
Wings are efficient enough that they create maybe 10 - 15 pounds of
lift for each pound of drag, and the engine has to overcome only the
drag. This is why unpowered sailplanes are possible. Their wings are
so good that the drag is less than 5% of the lift.
jsw
Eunometic
Static thrust (ie where the aircraft is stationary) is given by
F = dm/dt x Ve where
F = force in Newtons (about 102 grams)
dm/dt is the mass flow rate in kg/sec of the air (plus fuel)
Ve = exhaust gas velocity in meters second.
If you want that data in feet, pounds, seconds be prepared for lots of
scaling factors.
If the engine is moving then the thrust is reduced by the inlet
velocity.
F = dm/dt x (Ve - Vi)
If you fly at say 36000ft the air density is half that at sea level
and the jet thrust will redcue accordingly.
Engine thrust is normally measured in
Newtons (becuase its easy and usefull)
kP (kiloPond) which is the force exerted by 1kg of mass on the earths
surface (a kg is a unit of mass not force)
Roger Conroy
"Ed Rasimus" <rasimus...@verizon.net> wrote in message
news:i1e1c6pftq7fgp9ic...@4ax.com...
Don't forget the lemon fondant or else a blood sacrifice may be necessary.
Jeff Crowell
>> Better to just consider four forces--thrust/drag in the longitudinal
>> and lift/weight in the vertical. Soon you must enter turns and that
>> makes the whole mess unworkable. A generous application of magic and
>> incantations is then required to sustain flight.
Roger Conroy wrote:
> Don't forget the lemon fondant or else a blood sacrifice may be necessary.
Probably just as well we don't get into the whole lift demons thing.
Jeff
--
It's no use accusing Islamic terrorists of being medieval. In
1350, Islam underlay the most glittering, tolerant, hopeful and
advanced societies on Earth. It was not, repeat not, Cantor Fitzgerald
who reduced them to the whimpering tyranny, corruption and poverty
that distinguish them on this planet today.
Alan Lothian
Roger Conroy
"Jeff Crowell" <jeff.crow...@hp.com> wrote in message
news:i9s43j$ae7$1...@usenet01.boi.hp.com...
> Ed Rasimus wrote in message
>>> Better to just consider four forces--thrust/drag in the longitudinal
>>> and lift/weight in the vertical. Soon you must enter turns and that
>>> makes the whole mess unworkable. A generous application of magic and
>>> incantations is then required to sustain flight.
>
> Roger Conroy wrote:
>> Don't forget the lemon fondant or else a blood sacrifice may be
>> necessary.
>
>
> Probably just as well we don't get into the whole lift demons thing.
>
>
> Jeff
> --
Insofar as there may be a few readers who are baffled by the above brief
exchange, the path to enlightenment is thattaway >
http://www.messybeast.com/dragonqueen/liftdemon.htm
Here endeth the lesson.
Eunometic
Threse kinds of thrust to weight ratios i.e. 30,000lbs of thrust to
20,000lbs weight are extraordinary and were reached by fighter
aircraft untill aircraft like the F-15 and F-16 apeared. Fighter
aircraft such as the Mirage III and Phantom flew combat with thrust to
weight ratios of around 0.6:1.
Aircraft like the B-47 flew with 12 times more weight than thrust.
A typical wing section might have a lift to drag ratio of 50:1.
So, as Ed Rasmus points out, most drag comes from parastic drag
(mainly on the fueselage) rather than induced drag (from producing
lift on the wing).
There is an exception: aircraft conducting high-G manouvering of
course have a high components of induced drag that can quickly bleed
of speed.
L/D ratios deteriorate at high angles of attack. L/D ratios for a
wing sections can be improved by using high aspect ratios (or
elliptical wings). Slats only extend Coefficients of Lift to higher
angles of attack before stall, they do not increase Coefficients of
lift at a particualr angle of attack in fact L/D ratios in fact
deteriorate at high angles of attack.
Tankfixer
jeff.crow...@hp.com says...
> Ed Rasimus wrote in message
> >> Better to just consider four forces--thrust/drag in the longitudinal
> >> and lift/weight in the vertical. Soon you must enter turns and that
> >> makes the whole mess unworkable. A generous application of magic and
> >> incantations is then required to sustain flight.
>
> Roger Conroy wrote:
> > Don't forget the lemon fondant or else a blood sacrifice may be necessary.
>
>
> Probably just as well we don't get into the whole lift demons thing.
>
Drag Demons...
Lift Pixies..
Jim Wilkins
> ...
>
> Drag Demons...
> Lift Pixies..
Satellites are held aloft by funding, and will fall down on you if
it's cut off. .
Jeff Crowell
> Drag Demons...
> Lift Pixies..
Depends on whose research you go by.....
Jeff
--
Choose your death carefully--you'll be stuck with it for a long time.