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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 1:59 PM   
David Gladwin



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Status: offline 		Why do you guys think that so much time and effort etc was applied to Concorde's intakes ? The simple answer is that supersonic flow, within an engine, equals extreme inefficiency, or possibly complete flow breakdown, so much so that the local Mach number at the compressor face of Concorde's Olympus engines was reduced to a mere . 5 Mach at a true, airframe, Mach number of 2. Perhaps that is also why the intakes of the Super Hornet were completely redesigned.

Those Concorde pilots who have experienced a ramp malfunction and consequent engine surge as a result of supersonic flow into the engines tell me it was VERY alarming experience !

Keep your engines happy, keep them subsonic in relation to the LOCAL speed of sound !

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 3:22 PM   
Miniflyer


 

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Status: offline 		Treadstone, where did you get that info on the Trent 900? I just did some research, and only found one source giving out that "fact". There's a data sheet from rolls royce...however it shows many informations that are incorrect, and sometimes are contradictory to infos on the same sheet.....
I have also talked to an aeronautical engineer specializing in fluid flow....info is that blade speeds as well as airflow must be kept below mach1 (local) in order to keep from shockwave seperation (loss of efficiency, huge loads on parts, noise emission, stalled fan/compressor). I'd really be suprised if RR decided to drop out of laws of physics....but i can't get firsthand info.

As to the afterburning: it has been done, but our engines are not wonderfully efficcient, we have fuel problems anyway. Afterburners are a lot more unefficient, and thrust gain is only about 30% with 250% fuel consumption.......
the ones who have done it have had enormous heat problems, and hat to spool down the turbine about 10% in order to cope with the high back pressure....But they look spectacular to say the least

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 3:50 PM   
specialFX



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Status: offline 		This thread shows perfectly why this forum is that fantastic. People from all over the globe with different occupations, minds, opinions etc. bring in their knowledge to form an unequaled source of information. And it´s fun to read too!

And, André, it IS a fantastic hobby with many different facettes to cope with. Each one that successfully goes around all these corners and glitches has gathered many positive experiences for hs whole life and personality!

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 4:33 PM   
TREADSTONE.



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ORIGINAL: Miniflyer

..Treadstone, where did you get that info on the Trent 900?


Well the fan is 116"...thats pretty well known...and a I've seen N1 speeds over 101%. N3 speeds can be up to 12500 RPM.... we went and actually measured the diameter of a fully bladed HP turbine disc to do the tip speed maths

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 5:27 PM   
Gazzer



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Status: offline 		Miniflyer,

Better question for Treadstone could be where he works.............!!!

I guess in terms of afterburners, the early turbines were inefficient (very!), and suffered temp issues....... just what my come eh??

Wonder if injecting Nitrus Oxide as you do in cars is an option....... Sonic BABOOM I would think!!!

Gazzer

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 5:50 PM   
Rupurt



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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 5:58 PM   
Rupurt



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Status: offline 		Actually nitrous is unlikely to do much as there is already more oxygen available than required to burn the fuel.

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 6:46 PM   
Gazzer



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I thought Nitrus had a larger expanding reaction when exploding, or burning, and therefore draws more oxygen?

I qualify that by remarking on how badly I failed chemistry at school!!!

In truth not something I would entertain, too dangerous on impact........

Perhaps an intercooler would be better, more dense colder air, higher oxygen co efficient and better burning, but would the damn thing get off the ground with all that additional weight!!!?

Wren's Super Sport has a different diffuser, so I guess performance is best worked out in that manner to start with, interesting to see if that engine will be more or less fuel efficient?

Gazzer

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 6:57 PM   
Rupurt



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Status: offline 		Best thing for nitrous is to inhale it. I think most turbine manufactures use larger compressors and turbines to achieve more thrust, which is logical, if maybe a little boring. How did we kick your guys asses in the last odi cricket series it was excellent!

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 7:02 PM   
Gazzer



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C'mon, were just lousy in the heat!!

Talking of which it has started snowing again!!!

Time for a nice cup of tea.......

How about methanol instead of kerosene......?!!!!!

Gazzer

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 7:36 PM   
Rupurt



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Status: offline 		Just kidding, I was even suffering in that heat lying on the couch(sofa, whatever). I don't think methanol has a high enough energy density to be useful as turbine fuel, in other words you would need to carry a lot more fuel to have the same duration flight. Also methanol does not have the same lubrication properties as crude oil based fuels. You should get Kurt Schreckling's book "Gas Turbine Engines for Model Aircraft" its very interesting ISBN0951058916 I saw it a while back on amazon UK for 14 Pounds. It also has plans to build a FD3/64.

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 7:38 PM   
Gazzer



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Status: offline 		I've got it but it is full of funny figures and stuff, so not exactly been read....... I did look at the pictures though!!!

GAzzer

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RE: Can compressor or turbine blades exceed the speed... - 2/25/2005 7:46 PM   
Rupurt



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Status: offline 		LOL. For homework I would like you to read page 26 Chap. 2.3.1 Fuels . page 30 under cooling it also explains why there is still oxygen in the exhaust gas. Interesting stuff.

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 7:54 PM   
TREADSTONE.



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quote:

ORIGINAL: Gazzer

I have to say my interest now focus's on beyond the combustion tube and into the exhaust. The idea of an afterburner, I understand to be dumping fuel at a critical poin in the exhaust eflux, and whey hey, more expanding gas, greater velocity and Zoooooooooooooooooooommmmmmmmmmmmmmmmmmm



Gazzer Your lust for thrust will be your downfall...

In a jet without an afterburner the combustor is positioned, between the compressor and the turbine, simple but it's operating point is characterised by low mach numbers and high cycle pressures...still simple enough..
If more thrust is required there are two ways you can go about this..

Scale up the whole engine, however this has the penalty of lugging a bloody great engine around with you all the time..

Or..

Put an afterburner on the back of it. This is a low pressure combustion system which puts additional heat into the flow and provides a thrust boost Proportional to the square root of the temperature change...i.e you have to chuck bucketful's more fuel in
Afterburners run at relatively low pressure conditions and hence are thermodynamically inefficient so the resultant specific fuel consumption is very poor.
Also from a technology point of view its a challange to burn under these relatively high mach no/Low pressure conditions.

...Your lust for thrust is actually a duel with fuel..


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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 8:01 PM   
Gazzer



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I can see what you mean, its just that noise and the O rings out the back of the engine!!!!

Bigger engine, more efficiency best ways to go, clearly.

My other thoughts and this wanders further off topic, are why we insist on fuz bound tanks, when the wing space is still just that. Has our modelling technologies and cnc cutting and all the use of composites, not now given us opportunities to fill the wings with fuel as per full size??

It is a delibaretly naive question, but if you could do that, providing the weight was fine, then you possibly would have enough fuel to run turbine and afterburner. The irony of that statement is the need for more thrust to take off due to the heavier take off weight....... caused by the fuel carried to power the afterburner!!!!!!

But is all being explained to me quite well so at last I am beginning to understand the theory behind the motor in better detail!!

Gazzer

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RE: Can compressor or turbine blades exceed the speed o... - 2/25/2005 8:08 PM   
unknown



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http://www.stealth316.com/2-3s-compflowmaps.htm#cp

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RE: Can compressor or turbine blades exceed the speed ... - 2/25/2005 8:10 PM   
YellowAircraft



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ORIGINAL: Gordon Mc

quote:

ORIGINAL: YellowAircraft

Wow. I sure feel dumb. Let's talk about Shakespeare's plays and sonnets.......


To beat mach 1, or not to beat mach 1? That is the question. Whether 'tis nobler on the jets forum to suffer the ignomity and confusion of outrageous discussions, or to take arms against a sea of theories, and by opposing end them? To have your engine die; to sleep; no more (must be a RAM); and, by a sleep to say we end the confusion and the thousand natural shocks that the compressor is heir to, 'tis a consummation devoutly to be wished. To die, to sleep; To sleep: perchance to dream of flying; aye, there's the rub. For in that sleep of death what dreams may come when we have shuffled off this mortal Rossi R-8 glo-plug coil

or something like that. I learned the original about 25 years ago, and still can't get the damned thing out of my head. Kinda like the downwind-check-list that's so ingrained in my head that it will probably be the last thing ever to go through my noggin....






Now THAT was good!

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RE: Can compressor or turbine blades exceed the speed... - 2/25/2005 8:15 PM   
Rupurt



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Status: offline 		Ah! how about rocket assited take off to lift the extra fuel for afterburner. When you have filled the wings and still want more get Drop tanks.

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RE: Can compressor or turbine blades exceed the speed... - 2/25/2005 8:33 PM   
bcovish



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Status: offline 		I wish someone would write a book called "Turbines for Dummies". I'd buy one after reading all this stuff.

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RE: Can compressor or turbine blades exceed the speed... - 2/25/2005 8:45 PM   
Bobman


 

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There ya go! How a turbine works in four short words!

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RE: Can compressor or turbine blades exceed the speed... - 2/25/2005 8:48 PM   
Rupurt



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RE: Can compressor or turbine blades exceed the speed o... - 1/4/2013 10:46 PM   
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Status: offline 		It would still seem that nobody has answered the original question completely, so here goes.

No, the blades of a turbine can not exceed the speed of sound if you want the turbine to function smoothly or remain intact for long. 

In the modern Rolls Royce turbofan engine, blade speed reductions are effected by dividing the turbine into separate low, intermediate, and high pressure turbines running on independent concentric drive shafts. The large fan is thereby driven more slowly than the smaller inner compressor/turbine blades.

This is called a three-spool engine (and was preceeded by the two-spool engine).

Alternatively, the fan speed my be reduced mechanically through gearing per Honeywell, and Pratt & Whitney. This is known as a geared fan.

Engines have been designed for supersonic airflows, but these are not turbines. They are called SCRAM-jets (Supersonic Combustion Ramjets) and are still largely experimental and confined to esoteric military application, though at one time they were proposed for supersonic intercontinental commercial transport..

This explains things to my satisfaction, I hope it helps you also.


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RE: Can compressor or turbine blades exceed the speed o... - 1/4/2013 10:56 PM   
Dr Honda



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Status: offline 		First... this thread is 8 years old. Why bring it up.

Second... big engines, and little engines don't operate the same.

Third... Yes... it was answered. The tip speed of the compressor... of our little engines will go supersonic. As far as "They won't stay together for long"... well... you are right. Rebuild on most of the little engines is 25 hrs run time.


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RE: Can compressor or turbine blades exceed the speed o... - 1/4/2013 11:29 PM   
sjmoore


 

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Status: offline 		Just trying to help.

And, yes, I was going to go in to the difference between big and small engines, and, yes, I suspected many small turbines would have localized supersonic component regions, but I didn't think it pertinent.to the general concept.

Part of the discussion was how an RR t-fan engine with a 116" fan could turn at 125,000 rpm. The answer is, it doesn't..

The post may be 8 years old, but I hit it TODAY, asking the same question. It would help anyone asking 8 years from now to have a proper answer.

In short, you have added nothing to the answer, so why even bother?

I take that back - nowhere previously did I notice anyone saying their engines had blade tips routinely exceeding the SOS which was why they required rebuilds after 25 hrs.

So I think we both added something today.

Have a gleamy day.

< Message edited by sjmoore -- 1/4/2013 11:53 PM >

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RE: Can compressor or turbine blades exceed the speed o... - 1/5/2013 9:09 AM   
HarryC


 

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quote:

ORIGINAL: Dr Honda
Rebuild on most of the little engines is 25 hrs run time.


I'm not sure what you mean by that. Do you mean really really little engines such as a Kolibri? "Little" jet engines such as my Wrens certainly don't need a rebuild at 25hrs, in fact not one of them has been rebuilt and the longest serving has reached 65 hours since new and is still on the original bearings.


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