Re: Relative Engine Power

Date:         15 Apr 97 03:22:43 
From:         fintubi@navier.stanford.edu (Bill Urban)
Organization: Stanford University
References:   1
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In article <airliners.1997.910@ohare.Chicago.COM>, "john r."
<john@guava.demon.co.uk> wrote:

> Can anyone give me some approximate guidelines to compare jet engine
> thrust and rotating shaft power.
> We were recently looking over The Queen Mary at Long Beach and in the
> engine room it said that the four turbines produced about 260,000 horse
> power between them. About as much as a modern Jumbo I said but was
> greeted with disbelief.
>
> Was I that far out ?

I don't think you were far out at all.

The trick in making this comparison is deciding which energy flows in the
jet qualify as 'useful', and converting them all into power terms, which,
of course, is not usually done with thrust - at rest, thrust can be high
but power (from a propulsion standpoint) is zero.

So I'll circumvent all of this and just look at the energy flow rate (i.e.
power) INTO the engine, and show that derating this figure by any
reasonable efficiency will still gives a damn big number.

This calculation will set new standards for crudeness:

JT9D - fuel flow at SLTO is 19,600 lb/hr

(19600 lb/hr)*(20000 btu/lb)*(1hr/3600s)*(778 ftlb/btu)*(1hp/550ftlb/s)

= 154,000 HP - rate of flow of chemical energy into engine

Even for 20% efficiency, this gives 30,000 HP per engine or 120,000 HP per
plane.

Since modern engines produce about 50% more thrust (with considerably
higher efficiency) than the JT9D I used the fuel flow figure for, the
actual number is probably pushing the Queen Mary's 260,000 HP.

Anyone with better figures is welcome to speak up!

Bill