View Full Version : Uninstalled Vs Installed Thrust Rating

10th Nov 2005, 07:27

I read in other forum that the CF-6-50C2 has an uninstalled thrust rating of 52,500 lbs thrust at sea level. Does it mean a thrust with removed EDP & IDG (if this engine is fitted with IDG otherwise CSD & generator) and no bleed air? (engine on test cell or aircraft)

Will this thrust be reduced if the EDP & the generator (IDG) are installed on engine (engine mounted on aircraft) but their switches are positioned to the OFF position? (with the IDG connected to the gearbox and no bleed)

Thank you.
Best regards

10th Nov 2005, 11:24

My understanding of uninstalled rating is that it is derived from tests without ancillary equipment and corrected for ideal 100% efficiency intake and sea-level ISA. In other words a situation absolutely unrealisable once the engine is installed and the various bits get connected. It therefore represents an upper bound to performance. Richard Shevell used to say 3% losses were a back-of-cigarette packet figure, then he'd smile and ask what cigarettes you smoked. Gentle and funny man.

I doubt if you can replicate on an aircraft anything like the conditions under which the engine was a) tested and b) then had its figures corrected to an idealised set of rules for the record.

At times I wonder what use unattributed and unqualified figures are but manufacturers have to say something to get the ball rolling. No-one believes that their figures are anything but flattering to themselves.

But the engineers will have the specification tendered against, the agreed method of measurement and company specific implementations. They are best placed to tell you what the real installation losses are, quoting across the fleet and what effect the various mods and chips have.

Best Rgds

the "E"

11th Nov 2005, 03:19
Not all installation "losses" actually degrade thrust. For example, at takeoff N1 with no IDG or CSD or hydraulic load, the thrust is XYZ which is the sum of fan and core nozzle thrust.

If you then add on the gearbox loads, turbine temp (EGT or ITT) must rise at the same N1. The result is the core thrust INCREASES for the same fan thrust, and the total thrust is thus greater than XYZ.


11th Nov 2005, 12:52
Thanks for that but what is the axis of performance? If TGT etc have gone up that is because extra fuel has been added to compensate for something. Now you have got more thrust at the same N1 and given that installed operation is the intended mode rather than uninstalled has specific fuel consumption gone up, down or stayed the same? Has real fuel consumption gone up down or stayed the same?

11th Nov 2005, 14:30
Installation losses always degrade SFC because energy is being stolen from the machine that would otherwise create thrust. :8

11th Nov 2005, 15:20
Don't pull faces at me barit1!

A loss is a loss of course of course. But you began it with your intriguing discourse on "loss".

Let me put it this way. When engine manufacturers quote uninstalled they know that broadly speaking uninstalled performance is not their goal, ergo parameters unisntalled-wise are more likely to be off-axis than on-axis.

So now you state enigmatically that not all losses are thrust "losses" and mustering feminine irony I suggest coyly that I could've told you that. And ask about tsfc. But you immediately go back to losses again.

Obviously you put a load on a machine by adding ancillaries and to maintain N1 you have to add more fuel. I am asking if that in doing so the engine now becomes more on axis and tsfc actually improves?

Wicked woman that I am my doctoral thesis got me my job with P&W. But right now, I'm doing the interview, my rules, you're the candidate.

I'll buy lunch, you do the wine. But answer me. For once I sense fun in an otherwise dry argument.

11th Nov 2005, 15:53
Well head-girl -

I for one would gladly abandon "uninstalled" performance quotes, instead quoting some nominal or typical installation condition. It would make more sense to the user, and would reflect real-world conditions better.

The first problem is getting all the motormakers to agree on exactly what "typical" losses should be included.

The second problem is - that's not how the engine is acceptance tested. Production test people cringe at the prospect of hooking up additional gadgets in the test cell - it's time wasted, and something else to worry about.

What's your wine preference? We have some fair Merlots here in Ohio.

oh and BTW -

I have the UTMOST admiration for any E. Hartford engine built before, say, 1950!


Obviously you put a load on a machine by adding ancillaries and to maintain N1 you have to add more fuel. I am asking if that in doing so the engine now becomes more on axis and tsfc actually improves?
In my arrogance I neglected to answer this. The answer is, of course, "it all depends". If the derivatives are strong enough of course TSFC COULD improve. But the derivitaves may have nonlinear (i.e. feminine) proclivities, so any predictions of improved TSFC -

Well, I wouldn't bet my pension check on it!