View Full Version : Thrust Conversions Ancient And Modern

29th Oct 2007, 21:57
Hope this is the correct place to ask...

Essentially I`m creating a little software utility to make comparisons between the performance of historic and modern aircraft. The user inputs various parameters eg weight (sorry mass) ... power, wing area etc and various results are displayed for comparison to other types.

The results need to meaningful rather than super accurate.

I`m struggling a bit to work out the best way to compare engine performance (power) Specifically a quick and dirty way to convert HP, ESHP, lb/kg st.t. etc to a common denominator.

I`m thinking just convert to Newtons and leave it at that? Would this be a valid assumption.

30th Oct 2007, 01:25
Thrust (in newtons or lbf) is a meaningful yardstick, but for a jet (or turbofan) it's measured in a static test stand, and that means little at cruise.

Further, thrust for a prop-driven plane isn't easy to find in print, but a rough rule is that static thrust (in lbf) might be 2.5 or 3 times the shaft HP.

I'm open to any refinements others might suggest. :8

30th Oct 2007, 10:34
Thanks Barit1 .... the 2.5 to 3 factor is a starting point for a rough and ready comparison.

Since my original post I`ve been doing a bit more reading and find this topic has been discussed quite extensively on the Web in a variety of locations. The general view being that you can`t compare apples with pears.

For the time being I`m going to confine myself to comparing like with like but give a bit more thought as to whether I can come up with some qualified notional (rather than scientfic) comparison for illustrative purposes. So any more thoughts along those lines on the subject would be welcome.

Nathan Parker
30th Oct 2007, 18:13
The general view being that you can`t compare apples with pears.

Yes, thrust and power are different things. For a jet, thrust is relatively constant with airspeed, but power increases.

For props, assuming a constant speed prop, power is relatively constant with airspeed, but thrust decreases.

30th Oct 2007, 21:10
Thanks Nathan I should have been more careful with the wording of my original post in respect of power/thrust.

Looks like Barits' suggestion about applying a factor to generate a notional static thrust number might be the way to go when comparing different propulsion methods, especially as I`ll only be using basic data .. eg 150 h.p ... 700 s.h.p ...20,000 lb.st.t

30th Oct 2007, 21:52
Not an easy task, windriver.
For example.
Lets look at two engine types on one substantially the same airplane....the B707 long body intercontinental model.
The earlier type had P&W JT4A straight-pipe engines.
The later airplane had JT3A fan engines.
Both about the same thrust at sea level.
Yet, because the fan engine moved a much larger volume of air on takeoff, runway performance was quite good, for the time.
OTOH, the straight-pipe JT4A, not being a fan engine, suffered severly with runway performance...rolling 11000+ feet on a 12,000 foot runway was not uncommon.
And yet, up high, the straight-pipe engine was very efficient...FL370 and above
The fan powered airplane suffered greatly at these higher levels...it simpy ran out of gas (performance-wise) at these higher altitudes.

30th Oct 2007, 23:22
But the JT4A-powered machine had to carry a couple tons more fuel for the same mission, didn't it? With the higher TOGW, performance HAD to be hurting...:eek:

31st Oct 2007, 03:14
Well actually, no, baret1.
The 707-320 airplane (JT4A engines) had a MTOW of 317,000 pounds.
The 707-320B airplane (JT3D fan engines) has a MTOW of 333,600 pounds.
Approximately the same fuel capacity (23,000 USG) for both, as each had very large center fuel tanks.
Now, IF the JT4A powered airplane was 'stuck' at a low cruising altitude, range suffered greatly, whereas the JT3D powered airplane was not quite so critical.
Now, having said all this, the JT4A powered airplane did not have full span LED's (except for a few ordered by SouthAfrican) so takeoff performance (runway length required) was....very poor.
Up close and personal with the far end of the runway was a regular occurance.

Many different variables, as you can see with these old airplanes.

1st Nov 2007, 02:32
saith 411A: Many different variables, as you can see with these old airplanes

Right, and without the LED's on the JT4A machines, I don't think you can say what was the only (or even the primary) cause of the looonnng TO roll.

Dick Whittingham
1st Nov 2007, 17:11

How early do you want to go? The very earliest aircraft could only just stay aloft - even rough air would bring them down. So we could say that their thrust was not much different from their drag. With a figure for weight and a guess at L/D ratio you would have some sort of answer.

I think I recall from Rolls Royce legend that the 1000lb static thrust Whittle engine was accepted for development on the basis that 1000lb was about the thrust a Merlin was giving at operational height


1st Nov 2007, 17:31
So we could say that their thrust was not much different from their drag.Dick,
I think Windriver is looking at some kind of overall "comparator", which would at least assume steady level flight. In that case thrust and drag are equal.