Can Vmg exceed the V of a jet exhaust?

[ChristiaanJ]

Keith,

In VERY bief, I think you're still confusing a supersonic intake being 'dragged through the air' as such and the same intake with an engine behind it.

CJ

[ChristiaanJ]

Originally Posted by Keith.Williams

Could it be that the supersonic intake produces thrust in an indirect way...

As said before, there's nothing "indirect" about it.
The forces acting on the complete intake structure, which finally are part of the forces that propel the aircraft, are acting forward.

... rather like the convergent propelling nozzle?
The force on the propelling nozzle acts rearwards, but the overall effect of the nozzle is to increase the total thrust. But the extra thrust does not act directly on the propelling nozzle.

You're slightly confusing the issue, because we're now talking about supersonic con-di nozzles, where again the resulting forces on the nozzle structure act forward.

CJ

[Keith.Williams.]

As said before, there's nothing "indirect" about it.
The forces acting on the complete intake structure, which finally are part of the forces that propel the aircraft, are acting forward.

I am quite prepared to accept the possibility that the supersonic con-di intake produce sthrust directly, but I would like to hear an explanation of how this is achieved. If it simply a matter of the balance of static pressure ahead and behind the intake, then an explanation would help.

You're slightly confusing the issue, because we're now talking about supersonic con-di nozzles, where again the resulting forces on the nozzle structure act forward.

My only reason for including a reference to the convergent propelling nozzle is because it produces additional thrust indirectly. As we have already obeserved the force acting on the convergent nozzle is rearward acting.

I am aware of the fact that the divergent part of a con-di propelling nozzle produces thrust directly. In effect it is recovering some of the energy that would otherwise be lost when the high pressure exhaust gas expands in all directions behind the aircraft.

But even in a con-di propelling nozzle the force on the front (convergent) section is drag and only that on the rear (divergent) section is thrust. The overall resultant force is the sum of these two forces.

Similarly I accept that the force on the rear (divergent) section of a supersonic intake is thrust. But the force on the front (convergent) section is drag. Once again the resultant is the sum of the two.

What I have a problem with, is the idea that the static pressure inside the intake is somehow greater than the total pressure ahead of it.

[Mr Optimistic]

pictures here, but not very exciting

Jet engines: fundamentals of theory ... - Google Books

[barit1]

Keith.Williams.

What I have a problem with, is the idea that the static pressure inside the intake is somehow greater than the total pressure ahead of it.

I suspect you also have a problem with the curious fact that a fore-aft-sliding bubble canopy, left in an unlatched state, is likely to be forced forward by the difference in static pressures.

[Mr Optimistic]

easiest case is a ramjet

http://web.mit.edu/ga3/www/qual_exams/Exam-2002.pdf

see page 12 (MIT asking similar question)

Not sure I believe intakes ever produce thrust of themselves, however always ready to learn !

[Keith.Williams.]

Thanks for the link Mr Optimistic.

I was already familiar with most of it, and I am quite familiar with how a supesonic intake compresses the air.

The link confirms my earlier comment that the TOTAL PRESSURE reduces as air flows through a shockwave.

But nothing in the link explains how the thrust exerted by the STATIC PRESSURE acting on the rear surfaces of the intake, can be greater than the drag exerted by the TOTAL PRESSURE acting on the front face.

I don't think that anyone can sensibly deny that the increased pressure and density caused by the intake will increase the thrust produced by the engine.

[ChristiaanJ]

Originally Posted by Mr Optimistic

Not sure I believe ...

It's a fact, and belief doesn't enter into the equation.
That's why I insisted earlier some of it is totally counter-intuitive.

Originally Posted by Keith.Williams

...the thrust produced by the engine.

Not sure this was already mentioned, but in the SR-71, in a slightly off-design state (not standard Mach 3 cruise), ALL the thrust comes from intake and exhaust and NONE from the engine. The engine actually causes a small amount of drag, and moves backwards on the engine mountings.

CJ

[Mr Optimistic]

Engines

The engine increases the enthalpy of the flow: whilst the engine may take momentum out of the gas flow the subsequent expansion in the nozzle gives the thrust. The intake prepares the flow but unless the engine increases the pressure against a surface it can't produce thrust. Otherwise I look forward to a diagram showing how this miracle can happen (hence the ram jet example which makes the principles clear and also begs for a nozzle).

[Keith.Williams.]

barit1

I suspect you also have a problem with the curious fact that a fore-aft-sliding bubble canopy, left in an unlatched state, is likely to be forced forward by the difference in static pressures.

Well, in this situation the front facing (inside surface) of the canopy is shielded by the windscreen so it is not being subjected to total pressure pushing it aft.

And the slipstream will probably be sucking air out of the cockpit. So the pressure in front of the canopy will be less that that behind it.

If you think about it for a moment you will see that this siutation is totally different to that of an intake.

Now imagine that the windscreen blows off and the canopy becomes unlocked. Which way will it now slide?

You will of course need to switch off your sarcasm mode for a moment to see what I mean.


CJ

Not sure this was already mentioned, but in the SR-71, in a slightly off-design state (not standard Mach 3 cruise), ALL the thrust comes from intake and exhaust and NONE from the engine. The engine actually causes a small amount of drag, and moves backwards on the engine mountings

I believe that at that point it is using reaheat so it is effectively using a ramjet. The ramjet then becomes the engine.

Now if the pilot were to shut down the fuel flow to the reheat would it still produce thrust?

[RJM]

I'm the OP, also responsible for post #5:

Thanks all. You've answered all aspects of my question.

In the light of recent posts, I'll retract that...

[Mr Optimistic]

welcome back !

The trouble is that in trying to focus on essentials the arguments become diffuse. Hence the supersonic intake producing thrust argument. If the engine can hold the shock at an advantageous position in the inlet then yes the net force obtained by integrating the pressures over the inlet can be a positive thrust but not without the engine ! The thrust is not an intrinsic characteristic of the inlet nor of the nozzle and I think that's the source of confusion.

[Brian Abraham]

We might dwell for a moment on the designers (Bob Rich) statement that the compressor is a pump to keep the inlets alive. It goes without saying that thrust produced by the inlet requires the turbo machinery in order to function, and by extension, fuel.