Thrust and Altitude - Help Needed

[Dick Whittingham]

The task is to fly at a given TAS in ISA. At low level EAS equals TAS and requires a given N1. At height EAS is lower by the density factor and so is drag. Thrust is also lower, but by how much? Does it work out that the same N1 gives the same TAS at all heights or would you have to adjust N1 up or down?

Dick

[Intruder]

A given fuel flow will yield approximately the same thrust and the same IAS through a wide altitude range. N1 will vary, though, as will TAS.

[DC-ATE]

The task is to fly at a given TAS in ISA.

Curious: Why is this the TASK?

[Dick Whittingham]

Because the set problem started by asking what would happen to sfc if you flew a constant TAS at higher pressure altitudes. I didn't want to load you with a complete analysis, and I would be happy to know just that one fact - how would N1 change?

Dick

[Old Smokey]

WRONG - At low level EAS equals TAS

CORRECT - At ISA Sea Level EAS equals TAS (and CAS).

WRONG - At height EAS is lower by the density factor

CORRECT - At height EAS is lower because of reduced Compressibility which is directly related to Mach Number

WRONG - Thrust is also lower (at higher altitudes)

Depends upon the altitude. As you are at a constant TAS, CAS/EAS/Mach are reducing, so far so good, until you climb higher and the EAS/Mach is below Vmd/Mmd, then the required Thrust will increase.

Does it work out that the same N1 gives the same TAS at all heights or would you have to adjust N1 up or down?

Depends upon the bypass ratio of your engine, i.e. the ratio of thrust produced by N1, and the thrust produced by the core (N2/N3). Too many variables here.

Regards,

Old Smokey

[DC-ATE]

I would think regardless of by-pass ratios, etc. N1 would have to be adjusted upwards to maintain a constant TAS as you increase altitude. I dunno.....always flew a constant Mach and didn't worry about it.

Edit - add: Before Mach we just set BMEP for the altitude.;)

[Dick Whittingham]

Hello Old Smokey,

What did I do wrong to be flamed in red and bold?

I plead guilty to being imprecise, but if we are being nit-picky then consider your response. EAS is the same as TAS at ISA sea level DENSITY and is "equal" to CAS only at speeds where comprssibilty is not a factor.

And I don't see your point about EAS being lower because of reduced compressibility. I was referring to the fact that if you hold a given TAS then the EAS and CAS required to achieve this at high altitude, low density, are less than that required at low level. Maybe I should have used CAS all the time

I never meant the question to wander off into the stratosphere, and in any case when I said thrust would reduce I meant thrust at a given RPM, not drag.

If I read your reply correctly you are saying that there is no general answer to my question, but that it depends on engine design.

Seriously, I have the greatest respect for you, and thank you for the answer

Dick

[Old Smokey]

Hello Dick,

Oh my God! I didn't mean to 'flame' you, in fact I'm confident that throughout all of my posts I've never sought to flame anybody! My sincere apologies!

I use emboldened print to highlight important points. Blame my lawyer for the red ink, she sends back my drafts with alterations required in RED, and she's my sister!

Now to work............ (and being nit picky / pedantic) ........

"EAS is the same as TAS at ISA sea level DENSITY and is "equal" to CAS only at speeds where comprssibilty is not a factor".

Compressibility is ALWAYS a factor, even at 1 knot. It is also a significant factor at ISA / Sea Level, but instruments are calibrated at ISA / Sea Level to remove it from display.

"reduced compressibility" - As this discussion is about constant TAS with changeing altitude, EAS, CAS, and Mach Number will be lower, hence REDUCED compressibility. (Note use of bold to highlight a point).

"I never meant the question to wander off into the stratosphere, and in any case when I said thrust would reduce I meant thrust at a given RPM, not drag".

Stratosphere has naught to do with Vmd (although it's in the region where Mmd may come into play). You can fly above or below Vmd at Sea Level.

If we consider the engine in question to be a modern generation very high bypass fan, most of the thrust comes from the fan. Thrust production from a fan is closely related to propeller characteristics. Thus, at the same RPM with increasing altitude and decreasing density, thrust will reduce. It will reduce linearly as does the density (rho). The drag for the aircraft will also be reduced by linearly reducing rho. So far it seems that constant fan speed will maintain the same TAS with flight above Vmd, but here comes the spanner in the works... Coefficient of Drag at reducing EAS (above Vmd) serves to further reduce the drag, so, hey presto, LESS engine fan speed will be required.

Lots of generalisations and assumptions, but with a TAS at EAS above Vmd, and a jet engine that produces most of it's thrust from the fan, LOWER N1 will be required to maintain the same TAS with increasing altitude!:ok:

It's now my turn to stand by to be flamed:eek::eek::eek::eek:

Regards,

Old Smokey

[Dick Whittingham]

Thanks, OLd Smokey

I had bad feelings about this from the start for the question originates in the ATPL QB. Stripped of ambiguites it asked how SFC would vary if you adjusted power to maintain 300KTAS at different heights. I thought I could figure out all but the thrust/RPM factor so that was what I asked for.

I think the examiner had in mind a basic set of circumstances that avoided Mcrit and Vmd, sticking to the middle troposphere. Otherwise I dont see how an answer could have been derived without considering specific aircraft and engines.

Anyway, I have enough to chew over now, and thanks again

Dick