Jet engines - why thrust decreases with speed

[powdermonkey]

Hi all,
Can anyone explain this:
" The thrust will decrease with an increase in speed"

1. any increase in forward airspeed will compress the air at the intake, which will lead to a larger MASS flow per unit volume. So mass flow increases with speed increase.
2. Due to intake momentum lag, air looses momentum, which decreases the overall thrust of the engine. But if air is slowed down, does the static pressure not increase and does static pressure increase not mean denser air ( this is where I might get this wrong)

Anyway, then my notes just state that increase in speed means reduction in thrust which seems to negate no. 1 above.

Can anyone shed some light on this for me, I would be very grateful.
Cheers
Powdermonkey

[powdermonkey]

STR, Thank you for that, they omitted the thrust equation, and with it it is very obvious, I'm sure I had it in AGK,but forgot it now.
My problem is my notes state " any increase in forward airspeed will compress the air at the intake increasing the mass per unit volume"
But then according to the formula, any increase in forward speed/intake velocity reduces thrust.
So is it a case of one having the greater overall effect of thrust reduction?

By that token, should not intake lag help with thrust according to the formula, if exhaust velocity is constant then if intake velocity is reduced, thrust should increase relatively.

Very confused:confused:

[HotDog]

The net thrust or resultant force acting on the aircraft in flight is the difference between the gross thrust and the momentum drag. The momentum or intake drag is the drag due to the momentum of the air passing into the engine relative to aircraft velocity, expressed as WV divided by g.

W: Mass of air passing through engine (lb./sec.)
V: Aircraft speed (ft/sec.)
g: Gravitational constant 32.2

Does this make it any clearer?

[powdermonkey]

OK
So total thrust is thrust at the exhaust - intake drag , the faster you go, the greater the drag force therefore total thrust is reduced?

So intake momentum drag then has a stronger negative effect on total thrust than the compression of the air at the engine intake due to increase in forward airspeed?
Please tell me that is correct!!!

[powdermonkey]

Sorry guys, I'm more confused than ever, think will sleep on it but I am VERY grateful and appreciative of your hard work in explaining it to me!

[HotDog]

OK

Please tell me that is correct!!!

That's the way I see it. Cheers, HD

[Algy]

Everything said above is essentially correct I believe, but I think altitude matters too.

The statement that thrust decreases with speed is only true up to a point. And that point is inside the normal operating envelope.

As stated, momentum effect/drag has a negative effect on thrust with increasing speed. But ram effect (ie the rise in massflow due to the rise in pressure at the inlet) counteracts that with increasing speed.

At a certain speed the lines cross and if you accelerate past that you eventually reach a subsonic Mach where the net effect is that thrust increases with speed. Because (relatively) high Mach is more easily achievable at (relatively) high altitude, engines perform better at typical long-distance cruise levels.

"Handling the Big Jets" is not big on aerodynamics, but has a graph that seems to suggest the crossover is slightly below FL300 in practice.

In the book, Dai Davies simply says: "...at low altitudes engine thrust decreases with speed because of the increasing engine internal drag with increasing aircraft speed. At high altitudes there is a restoration of this loss with increased speed due to the ram effect at higher Mach numbers".

[HotDog]

Yes, you are right up to a point. However, you will reach a curve in the graph called coffin corner at which point you are likely to fall out of the sky as we did one day, pushing the envelope at 410 in a Convair 880 with CJ 805 engines.:{

[Algy]

Hmmm, not really I think - coffin corner is on a different graph altogether. Speed vs alt right?

And in that regard it doesn't matter what engines you've got (unless they're too feeble to even get you up there I suppose.)

[powdermonkey]

Thanks everyone, me thinks I've got it, enough to pass exams anyway!!!:ok:

[Old Smokey]

Two factors are at work simultaneously, namely -

(1) Thrust decreases with forward speed, due to the thrust equation :Thrust=
W (Vo - V1)
(Where W = mass of air, Vo = Exit Velocity, and V1 = intake velocity, as
stated earlier), and

(2) Ram recovery takes place proportional to the Mach Number Squared. This
increases the intake pressure, thus unloading the work required of the
compressor (which is considerable) leaving more of the internal work
produced available for producing thrust, and, significantly, increasing Mass
Flow.

If you refer to the diagram below, static thrust (i.e. at zero speed) is
represented by the horizontal dashed line.

http://static.flickr.com/53/109651157_e654ea38ae_o.jpg

Speed effect (due to the thrust equation) causes actual thrust to reduce
with increasing TAS (the descending line), quite similar in concept to
thrust decline with increasing speed for the propeller aircraft. Decline is
quite uniform.

Ram Recovery (the upward curving line) exponentially causes thrust recovery
as Mach Number increases, being zero at zero speed, mediocre at medium
speeds, and significant at high speeds.

The Actual or Net Thrust is the sum of the two values, the sum of these
being shown as the upper curved line dipping below the Static Thrust line,
eventually crossing and exceeding it at high Mach Numbers. (The concept is
very similar to the addition of the Induced Drag and Form Drag values to
form the familiar "U shaped" Drag Curves).


The MINIMUM Net thrust is encountered when the Speed Effect and the Ram
Recovery effect line cross, and thrust begins to recover to static values
thereafter. For earlier generation jet aircraft (B707 etc.) minimum thrust
occurred at about M0.5, with full recovery to static values by about M0.75.
Later generation High Bypass engines do not enjoy quite the same benefits
from Ram Recovery as earlier aircraft, due to more of the Fan and less of
the Core receiving the increased dynamic pressure, but nevertheless data for
the B747 shows (at least) full recovery to static thrust at typical Climb
and Cruise Mach Numbers.

(Note that the diagram is a generic one for instructional purposes, and not
applicable to any particular engine, but the relationships are correct).

If it were not for Ram Recovery, jet aircraft would be hard pressed to make
it to much more than 300 knots.

Regards,

Old Smokey

[powdermonkey]

Old Smokey and everyone else,
Thank you very much for your considerable efforts......ever think of getting together to write some manuals?
Anyway, thanks a lot, I really appreciate it and more importantly I GET IT!!!:ok:

[Algy]

Ah well, so you say, but did we mention the test? :ok:

[powdermonkey]

Algy, go on, do your worst..........is it open book exam?:8

[novicef]

Old Smokey are you ex- Queens Airforce??

[Ignition Override]

Higher speed means less induced drag. And you know that up high, thinner air with lower drag (but less thrust available).

No fancy theory needed.

Is this for engineering design or a pilot job? This from an unwashed, unlettered colonial.

[Old Smokey]

Ignition Override makes a good reminder, the diagram which I posted is for level flight, basically applicable at any level (in the generic sense).

novicef, afraid not, civilian to the bone.

Regards,

Old Smokey