Supersonic Speeds Inside Engine...
 

Brian...yes the tip speed of the compressor and fan will almost always be supersonic in a modern turbine engine...typically from M1.2 to M1.7...even a turbocharger compressor will go supersonic at its higher pressure ratios...



The reason is that high wheel speeds are necessary to achieve high rates of work...remember kinetic energy is a function of speed squared...and compressor and turbine work are a function of rotational speed...so the higher our wheel speed the greater the work output per given mass flow...


The sonic shock waves do contribute to losses of course...although the shock energy actually helps to compress the air...the axial gas speeds through the compressor and fan will generally be ~M 0.4 to M 0.5...


The axial gas speed through the first turbine nozzle guide vane ahead of the turbine wheel will usually be choked...ie Mach 1...although this will typically increase to just slightly over sonic speed just aft of the NGV...about M1.1 or 1.2 at most...as the annulus increases in size...giving a bit of converging-diverging nozzle effect...


The turbine wheel tip speeds are generally subsonic...about M 0.8...or M 0.9...however since the speed of sound increases with temperature...and at hot section temps it will be more than double that of freestream speed of sound...the turbine actual speed in m/s (or ft/s) will be similar to the cold section...


It is really quite something to stop and consider the amazing power that modern gas turbines produce...a single turbine wheel of ~0.75 m diameter will make close to 40,000 hp...with a mass flow of about 100 kg/s through the engine core...about the size of a 50,000 lb thrust fan engine you would see on a widebody airliner...


Regards,


Gordon.


PS:One more thought about the intake thrust...the very first post said it best...with the intake making over 60 percent of thrust...the airplane is basically “sucking” its way through the air at M2...that's it in a nutshell...

Not closer than the point at which the streamtube that contains the engine flow is first affected by the forward influence of the intake. Otherwise as far ahead of the intake as you wish to define it.
For subsonic intakes the closest point will vary with the ratio of engine mass flow to intake capture mass flow for that flight Mach number and intake area.If in doubt go further forward.
For supersonic intakes it will be the point of the cone for axisymmetric intakes or the leading edge of the ramps if 2D

"Inlet Thrust"
 

Hello All,
I have been reading and studying for many, many years the topic of "Supersonic Inlet Thrust. I do not hold a degree in Aeronautical Engineering or physics, but feel that I have my head around the physics of propulsion, at least to a degree.

I am not posting to irritate others or create a flame war, I only seek the truth, and there is MUCH confusion regarding the issue of how supersonic inlets work, even among the engineering staff at NASA. I know, because I have spoken with them and exchanged much mail and documents regarding this interesting issue.

First of all, in trying to get of grasp of this, one must have an understanding of supersonic and subsonic fluid flow. Supersonic fluid flow is not magic, but it behaves very differently than fluids at subsonic flow, and the transition from one to the other must be kept in mind too. To state that "Most of the thrust of the SR-71 or Concord" comes from the intake is a misunderstanding of jet propulsion. Yes, there is positive pressure recovery in the inlet, and yes it is greater than free stream static pressure. Jet engine propulsion creates thrust thru momentum change, accelerating gas out of the exhaust. The "positive pressure" in the inlet system or that acting on the rear faces of compressor blades is what is know as "PRESSURE THRUST". It is used to explain rocket and jet propulsion to high school students. It is a VERY INEFFICIENT way of producing thrust, and is avoided by engineeers.

If you look at rocket nozzle designs, you will see a different flow regions the relationship of "pressure thrust" to change in momentum. Engineers would like to convert all pressure thrust to a change in momentum, however some pressure thrust will always be created. It is NOT the lions share of thrust, even in the case of the SR-71 or the Concord. The supersonic inlet on the SR-71 is very efficient and it is thru that (at high mach speeds) it performs the lions share of compression, freeing up compressor stages of the J58 engine. At the 4th or 5th stage (can't remember which one), 6 bypass ducts pass the compressed air directly into the afterburner inlet, so a large portion of the airflow is acting as a ramjet engine. One could state that (at high mach) the inlet is responsible for the compression and ramjet action, BUT it is NOT producing the greatest portion of total thrust anymore than the Pistons in you car create the greatest portion of the horsepower generated.

This misconception has been perpetuated and is a misunderstanding of how the entire propulsion system works together. When some state that "The engine falls back in it's mounts while the intake system transmits forward thrust to the airframe", you should be willing and able to offer proof. DO the math, you will not find "positive pressure" in the inlet system "pushing" on the rear end of inlet components, or doors to amount to "75% or whatever number" of the total thrust. If you are careful to study ALL of the inlet system, you will find, again at high mach much of the "overpressure" air in the inlet is dumped overboard thru ducts and vents, as the inlet can take in more mass flow than the J58 can effectively deal with with some bypassed around the core and mixed in the complicated exhaust system again with it's blow-in doors and de-laval type noxzzle.

I am not posting to offend anyone, I like most everyone here am interested in the exact workings supersonic inlets, and propulsion. If, I have overlooked anything or failed to take into account forces not considered, please call me on it and educate me. The supersonic inlets and exhaust systems on the SR-71 are extremely interesting and work together, but it is not "Sucking itself thru the sky. I find it incredibly that all of this was conceived, worked out and tested in the 1950's with slipsticks.................
Tomtech

Thrust Distribution in a Jet Engine...
 

Tom...please refer to my previous posts in this thread...I have even drawn little sketches to illustrate the physics...I do have a degree in aerospace engineering and I work in the industry...

It is no secret how an intake produces thrust...this subject is covered in all textbooks...here is a book from Rolls Royce that has an entire chapter devoted to this...you can view the entire book online here...

Rolls Royce - The Jet Engine

Go to Chapter 20...Thrust Distribution...Figure 20-1 on page 208 shows the thrust distribution in the engine...the compressor...diffuser...and burner each make a forward thrust force...or gas distribution as the book calls it...

The turbine section and the jet nozzle make a rearward force or drag...when you subtract all the drag forces from all the thrust forces you end up with the engine's overall thrust...

This is the principle I have tried to explain in detail...it has to do with the fact that the sum of momentum and the pressure times area product is greater at the aft end of the compressor and diffuser and burner...than it is at the forward end...creating a force in the forward direction...

An engine inlet is a diffuser...that is why it makes a forward force...please read the chapter and try to do the math in there and you will understand how it works...

Regards,

Gordon.

Gordon,
Many thanks for replying to my post, I will, when I have time look at the materials you mention. No disagreement on the inlet being a diffuser. I think the mail area of confusion lies in the statements that point to the inlet static pressure being much above the outside static pressure, no argument there. The aircraft is not standing still, the inlet or the entire engine is not "thrusting against static pressuer outside the aircraaft". It is, rather thrusting against the "ram pressure", whic is MANY times geater than static pressure. It is the energy in ram pressure that the inlet can recover and convert to pressure rise, this, is one measure of inlet performance. As the (mach 3.1 or so) air is slowed thru a series of shocks, (in the case of the SR-71) it is terminated in the inlet section. The diffusion process builds pressure but at the expense of DRAG. Inlets produce drag, not thrust. You are not thrusting against outide pressure (which at 50,000 feet ain't much), but against the RAM pressure. If you want to say the the air compressed by the Sr-71 that is bypassed and fed to the afterburner inlet produces the majority of thrust, so be it, I am sure that is correct. I have "done the math", seems like everyone posting is not familiar with PRESSURE THRUST, which is covered in elementary text on any type of jet or rocket propulsion. To go even further and state that the engine is just processing airflow, while the inlet is doing the lion's share of the work is a false statement. It was stated by Kelly Johnson himslef, but later he was corrected by engineering and he, among friends admitted he was initailly wrong. Yes, I have done the math, and adding up and summing drag and positive pressures inside the engine is not necessary or even correct, unless you want ot calculate Pressure Thrust. In the end it is momentum exchange in the exhaust gases that creates thrust, and yeah, some state the "exhaust system " produes X amount of thrust. All propulsion components work together to provide thrust. I will study the book you mentioned, and try to see what you are saying. Again, thank you for the response, I know you for sure have done the math, and I will continue to educate myslef in this area. I will get back with you when I have done some more studying. I still find this an extremely intersting subject!

Gordon,
I have taken time to look at the book you referenced. I must say, I hope, as a practicing engineer you are not using that material, it was written for the "Sunday Reader". In turbojet engineering school, you would have worked out velocity triangles for axial flow compressors? Well, the bulk of energy put into the air is swirl energy, that is slowed (diffused) by the stator blades, resulting in compression correct? The stator blades absorb forces in the rearward direction releiving what the compressor "rotor" blades put into the foward direction (Pressure thrust again). Yeah, there is an overall foward pull on the compressor shaft. If so much "foward thrust" was generated by the compressor blades, then the Harrier would be moving foward while it is in hover? The book even shows gas flow for the Harrier. I have seen thrust distributions for turbojet engines before in far greater detail than the Rolls Royce book, If I get a chance, I will dig thru some and foward them to all for study. I am not trying to poke fun, I am glad you took the time to respond, as I my learn something. I will be back, thank's for taking time to read and think.

Concorde Inlet Thrust
 

Tomtech,

As one of the original Concorde inlet design team and as someone who has spent a fair bit of time recently trying to unpick the intricacies of the SR-71 design, I have to disagree with you gently.

The 'split' of Concorde's cruise propulsive forces generally referred to first appeared in Ken Owen's "New Shape in the Sky" many years ago. The data was given to him by members of the design team, and I think it is stretching things a bit to suggest that they 'misunderstood' jet propulsion principles.

I won't try to answer for the SR-71, but on Concorde (this is after all a Concorde thread :)) the intake, engine and final nozzle all had their own attachments to the airframe so it is reasonable and possible to enumerate their relative contributions. They all produced thrust. How you divide the total up between them is a matter of book-keeping conventions and as any accountant will tell you those are convenience variables!

Specifically, the 'split' will depend on how one divides the momentum drag out between the components. I can't remember (if I ever knew) the book-keeping that went with the Ken Owen's picture, but any reasonable division results in a considerable part of the total thrust acting on the intake mounts and another sizeable chunk on the final nozzle mounting.

What I think you may be missing is that although, of course, the thrust is obtained by imparting momentum to the mass flow through the engine, and although the design objective is to make sure that the static pressure of the flow exiting the final nozzle is as near ambient static as possible, the forces which must be applied to that mass to accelerate it to its final velocity are reacted as pressures on solid surfaces on the aircraft (or engine of course). In fact all forces on the aircraft (other than gravitational) ultimately come from pressures acting on solid surfaces.

So "pressure thrust", far from being an undesirable, is in fact an essential.

I'm going to be away for a few days, but if you feel you want more detail feel free to PM me.

I think some ealier posts had the view that the engine could be deconstructed into individual parts which could be characterised separately with the total engine being the sum of the parts.

Gentlemen,
Well, I have gone back over all the fine posts made on this forum and re-introduced myself to aircraft propulsion and must offer my apology for my prior post (#143). I STAND CORRECTED and thank all of you for having patience with me. My misunderstanding was thinking of the intake as lone device "creating" thrust, not, rather as contributing to thrust by virtue of the great pressure recovery at high speeds. Also, I will take this time to correct myself, I completely miss applied the term "pressure thrust". As Gordon, and others have pointed out, ALL reaction forces in the engine and intake must be eventually felt as pressure forces distributed throughout the engine, intake and exhaust. I was using the "pressure thrust" term of positive pressure at the exhaust opening, which was not fully expanded to velocity, thereby contributing to thrust, but not as efficiently if it were converted to gas velocity. It was my miss application of the term that led me to a "serious miss understanding" of propulsion. I feel I have moved my understanding forward by quite a leap, and have you all to thank. I will read more in an effort to rid myself of ignorance and thank you all again for putting up with my lack of understanding and taking time to post and offer diagrams, it helped me much.
Tom