Jet engine air intake

[172_driver]

Tried to search the subject before, but couldn't find any thread that really answered my question. It is relative simple, maybe better suited in the Engineers section, but I give it a try!!

Assuming subsonic aeroplanes, as far as I've read through litterature the engine intake is "usually designed divergent" to allow for reduced velocity at the compreesor face. Is this generally the design, on let's say a CFM56 or JT9? Being up in Victorville the other day, having the possibility to drool over some fancy aircraft compared to my Duchess, I couldn't say the intake looked divergent to me on A320, MD80, B737.

Anyone knows, from testing or calculation, what is a typical velocity the air enters the compressor face?? I seem to recall from my ATPL-theory M0.5, but are modern engines with VIGV able to cope with higher velocities than that? Is there any advantage/disadvantage with having high/low velocity to the compressor, considering ram pressure recovery perhaps?

Thanks in advance!!

[HarryMann]

Yes, they're divergent. The normal assumption is that all the dynamic head is converted to pressure at the compressor face (fan face in a turbofan) and then that first stage compresses further.
This means that the engine experiences a Momentum Drag equivalent to the nacelle intake area x density x free-stream velocity (a/c speed)
This is subtracted from the gross thrust, which is why the nett thrust of a big fan drops off with speed much faster than a turbojet.
A high speed at the first-stage face (or fan face) would seriously compromise the fan blade design, due to shocks or excessive Mach drag, as well as noise.

[barit1]

Said divergent design of course may compromise the takeoff performance a bit, and so many early jets had auxillary "blow-in" (or "suck-in") doors to provide more airflow at low IAS.

Examples are the JT3D 707's, early 747-100's, the C-5A/B, T-33 etc. They were terrific noise generators, too.

[172_driver]

Thanks for that! While increasing the momentum drag, the "ram pressure recovery effect" does increase the thrust as well at higher speeds, doesn't it? 1) Off-loads compressor and 2) increases mass flow? I suspect the advantages are higher for turbojets though.

[barit1]

Strictly speaking, it doesn't "Off-load" the compressor, but it increases the static pressure at the compressor inlet and thus increases air density and mass flow of the working fluid (ie air). Thus the whole gas turbine cycle is boosted.

[leewan]

They are divergent, taking advantage of Bernoulli's theorem which is, speed goes down, pressure goes up. As mentioned, this intake also has the effect of slowing down the airspeed for the compressor to handle.

[172_driver]

Guess I have to look closer next time... but I believe you they are divergent

Off-load was wrong word, perhaps, what I meant simply that the ram recovery pressure increases the inlet pressure to the compressor and part of the job is already made.

[barit1]

...part of the job is already made.

Well, no; The increased mass flow permits the engine to do MORE work. Pressures thoughout the engine are higher, not merely the same.

[punk666]

High by pass engines usually use a pitot type inlet. If you look at a jet engine from the side you will see that the inlet is slightly divergent, the advantage of this is the fact that it decreases the ram velocity and increases the pressure.

[HarryMann]

punk666

That is what we said above, and it applies to all types, not just high-bypass..

172driver
I suspect the advantages are higher for turbojets though.

Again, yes, what was said above*, there is an increasing divergence between pure-jet and by-pass jet net thrust as speeds approach the transonic regime. Clever inlet & first stage design has however, allowed an increasing by-pass ratio to be used at transonic and supersonic speeds with lower penalties.

* Momentum drag is very large effect, pressure recovery less

Yes, blow-in doors for low speed mass flow increase. Also spill-doors for excess mass-flow (surge conditions) can/have been incorporated, sometimes the two combined possibly

[lomapaseo]

* Momentum drag is very large effect, pressure recovery less

..... Also spill-doors for excess mass-flow (surge conditions) can/have been incorporated, sometimes the two combined possibly

I suspect that with very high bypass inlets that the drag increase of a surge condition at early climb power could result in very high drag conditions on one side for several seconds coupled with the loss of thrust. Then again the inlet spillage effect would wash over the wing surfaces hopefully with not the same detrimental effect as an inflight reverser deployment.

[HarryMann]

You at the least have most of the momentum drag without the thrust! As you say, there are probably some secondary effects from spillage, hopefully some good as well as bad

[172_driver]

Thanks again for taking your time, I am quite new to this and doesn't know anything more than in the ATPL-book. So I apologise if I don't get the big picture.

I found this picture as a reference,



Even though it's a generic picture, it seems that for pure turbojet (by pass 0) the thrust output dips initially to recover later on at higher Mach speed. My own guess, which I'd like to have confirmed, is that the increased ram pressure is dominating both the momentum drag (loss of thrust) and the reduction in acceleration through the engine (Voutlet - Vinlet).

For a turbofan, the increased front area will produce a great deal of momentum drag why it suffers quite some thrust loss with forward speed.

It's interesting that the JAA learning objectives specifically states that thrust from jet engines should be considered constant with increasing speed.