The thrust produced by the intake is not only a supersonic phenomenon.
A well-designed subsonic inlet will also - due to static pressure rise in the divergent annulus - create some thrust. The effect even occurs in a radial engine with the NACA cowl. I once ferried a prewar a/c with the cowl removed - it was 15 kt slower without the cowl fitted. |
Concordeski
Superb stuff guys.
Can't signoff without questioning how Concordeski designers solved the problem! |
I think the problem is the word 'thrust'
On the wetted surface theory elucidated above, a simple unknotted toy balloon will be thrust about the place and this is calculable, in theory, by looking at the unbalanced pressure acting on the inside wall. So the front of a rubber toy balloon produces thrust. Are you sure a good intake design is not more about reducing losses ?
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There are both drag and thrust benefits of the NACA cowl.
Externally, the cleaner surface (compared to cylinder heads etc.) is certainly a drag reduction. Internally, the divergent (diffuser) flow field increases static pressure on the inside of the forward cowl, which is a thrust felt by the cowl mounts. If these mounts should break, the cowl is free to shift forward, and will in fact strike the trailing edge of the prop blades. Theoretically, some additional thrust should be available from the internal air, heated (and expanded) by cylinders etc., then accelerated out the exit annulus. I'm not aware of this ever actually achieved in an aircooled engine, although the liquid-cooled P-51/Merlin combination seems to have done it. |
Landroger
Its a bypass? http://images.ibsrv.net/ibsrv/res/sr...s/embarass.gif The "power" is generated by the action of shock waves slowing the air mass and passes around the 593 to mix with the core engine exhaust within the buckets? So, it is the opposite of a High Bypass fan, in that a core of 'relatively low' velocity air from the 593, is surrounded by a tube of very high velocity/energy air from the intake? I refer to the 593, buckets and intake as components, because the whole assembly is 'The Engine'. Is that how you see it? Please say I've got it, because its been doing my head in since the statistic (75% thrust from intake) was first mentioned. :D Dude :O |
barit1
Your information on the NACA cowls, particularly the P51 are both fascinatating and enlightening, thank you for adding another dimension to this thread. It's so easy to forget what an amazing design the Mustang was, I remember reading a fascinating article a few years ago on the very area that you describe, The air inlet design was totally unique, and was lrgely responsible for the high performane of this amazing aircraft. Dude :O |
b377
Can't signoff without questioning how Concordeski designers solved the problem! Keep posting away here b377, you've started a really interesting thread. Dude :O |
Mr Optimistic
On the wetted surface theory elucidated above, a simple unknotted toy balloon will be thrust about the place and this is calculable, in theory, by looking at the unbalanced pressure acting on the inside wall. So the front of a rubber toy balloon produces thrust. Are you sure a good intake design is not more about reducing losses ? As far as having a problem with the whole thrust thing, the intake mounting assembly was designed to absorb and transmit the thrust forces from the intake assembly to the airframe, I'm afraid this is fact my friend. Dude :O |
And to think these were all done without the modern CFD codes
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Originally Posted by M2dude
Now this basic design is not uncommon, F14, F15, Tornado, MIG 25 etc., but the spilled subsonic air in these designs is ejected overboard, giving very little in the way of secondary benefits, and in fact the secondary airflow at all is technically a small waste of energy.
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Phew! Thanks Dude!
Many thanks dude, there is light indeed at the end of the tunnel - and not simply because it is on fire! :D The whole concept of 'Supercruise' is quite stunning and the thought occurs; was it played for, or a happy outcome of the way the design froze? :rolleyes:
They never did. The original TU 144 engine was an apalling lump, and the intake was crude, both aerodynamically and in terms of it's control system. One of the major problems with the TU144 was it;s inabity to supercruise without the use of afterburning, due entirely to inadequate control on inlet airflow as well as a far too simple NK144 engine. Having said that, I also read that there were external clues - to those that really know aeroplanes - that Concordski was actually a donkey. The canards, for example? :eek: Thanks again for the extra info about the intake. :ok: Roger. |
To come back a moment to the difficulty of visualising how intake and exhaust provided "nearly all" the thrust, and the engine "next to nothing"....
Think a moment of various marine and industrial gas turbines (Olympus and others). What does the engine do in those installations? It is a 'gas generator', and all the power in the exhaust is extracted by a separate turbine. As a matter of fact, the thrust of those engines is practically zero, since having thrust in a stationary installation would just be a waste. On Concorde at Mach 2, the situation is really not all that different... By sucking a huge amount of air through a very sophisticated inlet, it sets up pressures in that inlet, that provide about 75% of the thrust. By blowing that same amount of air, with added heat energy from the fuel, out of the other end, through another sophisticated convergent-divergent nozzle, we produce yet more thrust. The engine itself produces very little thrust, but by 'sucking' and 'blowing' on the right components front and back it creates the right conditions for those components to produce the thrust we're looking for. I hope this makes some sense? CJ |
MrOptimistic
And to think these were all done without the modern CFD codes The control unit's processors were based on TTL logic, as this gave superior speed and better voltage transient tolerance than the CMOS chips that were then available. Trouble with THAT was that TTL runs really hot, and cooling the eight control units was a bit of a nightmare originally. But in spite of all these and many more electronic mountains to climb, this revolutionary system was developed and test flown for the first time within TWO YEARS of the 'go digital' decision. To me that still seems one hell of an achievement. Dude :O |
DozyWannabe
Which is presumably one of the factors in why those designs can't supercruise (not that supercruise was considered a must-have in a fighter of that era), and one of the reasons that once the old girl was supersonic, none of them could catch her. The 'old girl' as you call her really was amazing though, wasn't she? :) Dude :O |
M2Dude
No-one would dare take that kind of risk now under fixed price rules. Had forgotten about TTL. Of course you would need 10^-9 now, don't suppose safety was quite the game it was now.
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Landroger
The whole concept of 'Supercruise' is quite stunning and the thought occurs; was it played for, or a happy outcome of the way the design froze? :rolleyes: Your comments about the Tu144 are astonishing and quite unexpected. I had heard, over the years, that Concordski was a triumph of the Soviet state and a very close competitor to Concorde. You are saying though, that it could not maintain Mach 2 without reheat? Roger I'm so glad that my blurbish explanations are making a little sense, the subject drives me nuts too, and I started doing it thirty six years ago. I quite like ChristiaanJ's analogy on explaing how an inlet can provide thrust, but the precise shockwave geometry that the Concorde intake required in order to do what it did best, was little more than mind numbing in terms of complexity and control; It is really difficult to imagine if it could ever be bettered aerodynamically, even now. Dude :O |
CJ
Fair try but it still seems a bit perpetual motion machine to me.
In the frame of reference where the engine is stationary, work is done on the entering gas to slow and compress it. The pressure rise over free stream sure enough gives a forward force on the intake structure (pressure higher inside than out). If the thing was a sealed unit the same would still be true (except the forward force would be greater though nothing like the net force acting backwards (aka drag)). The engine does work on the gas and expels it at higher speed and maintains a delta-P between the front and rear faces such that the pressure at the front is maintained below full recovery. So momentum taken from the air at the front and more given back at the, erm, back and it is the lower pressure at the front face compared to full recovery which underlies the force accounting does it not ? We had this discussion last year, same principle as rocket motor, the hole in the back is the big trick and yes, if you look at the forces it is the higher pressure on the casing at the front which transmits the NET force, but who ever says that a rocket motor casing provides thrust ? |
Mr Optimistic
This is one of those arguments that you could make go around and around for ever. The facts of the matter are this: The intake DID provide a sizeable amount of thrust at Mach 2, but if it makes it easier to realise think of it in terms of the engine provides ultimately all of the thrust, and a large amount is projected through the intake assembly. Without the intake, the engine would not have been able to manifest this thrust in any way. However without an extremely capable and sophisticated Rolls Royce Olympus 593 being able to be operated at maximum supersonic efficiency, this thrust would still not have been realised by the engine. We have an extremely complex powerplant arrangement here that took years to develop and gave phenomenal performance, these are facts. Dude :O |
Below is quite a nice simple but clear diagram of the powerplant as a whole, showing a VERY simplified diagram of the shock system within the intake as well as the complex path for the secondary airflow, from over the intake ramps, through the secondary air dors into the engine bay and then finally into the secondary nozzle annulus.
Dude :O http://i991.photobucket.com/albums/a...Powerplant.jpg |
I agree
The issue is one of terminology but not semantics. The idea of thrust from the intake leaves just one small step to the intake sourcing thrust and then to why do we need the hot and heavy thing just behind. This was a question here last year. Physics v engineering I guess. Should we make this an annual date ?:8
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