View Full Version : Power Equation

23rd Apr 2010, 17:29
This is a small paragraph from my book.

The power required for the flight depends on the thrust required and flight velocity.By definition,the propulsive horsepower required is related to thrust required and flight velocity by the following equation

Pr= TrV/ 325 where Pr is power required in hp,Tr is thrust required(total drag) in lbs and V is TAS in Knots

By inspection of this relationship,it is apparent that each pound of drag incurred at 325 knots requires one horsepower of propulsion power.

I am not able to make sense of the above statement.How would they have come to the magic figure of 325 in the denominator.Does it apply to all Jets in general.

23rd Apr 2010, 17:56

that is basic physics and applies to about everything in the universe about 30 seconds after the big bang which moves slow compard to the speed of light.

Power = Force * Velocity.

Unfortunately as European I am more used to Si where 1 Watt = 1 Newton * 1 m/s

With 1hp = 745W and 1lb = 4,448N and 1kn = 0,51444 m/s

745/(4,448 * 0,51444) = 325,5

Here you have your fudge factor for this weired US system.

23rd Apr 2010, 17:58
A bit of a throwback but one horsepower = 550ft-lb/sec. I knot is also 1.69ft/sec. So divide 550 by 1.69 and the answer is 325 or near enough. In old money. So the story goes James Watt was often asked how many horses would the steam engine replace. So he took a stab at what a reasonable cart horse could be expected to do and halved it to take into account unskilled labour and management of said steam engine. Hence the 550 ft[lb/sec.

John Farley
24th Apr 2010, 17:37

Cor! That is exactly wot I was taught back in 1950. Since we are on opposite sides of the earth I reckon it must be right then!

24th Apr 2010, 20:55
Oh, it's Saturday night so why not:

Start with the basic definition of power:

Power = Work/Time

But in any organisation, as we all know, Knowledge is Power, and Time is Money.

So, substituting, we have:

Knowledge = Work/Money

and, transposing:

Money = Work/Knowledge

Thus, for any given amount of Work, the less Knowledge a person applies to it the greater the Money accruing to that individual. :E

I respectfully submit the above as an explanation of engineers' salaries world-wide (I am one), and possibly those of a number of other professions too. :ok:

24th Apr 2010, 22:27
A classic, D120, and it worked for me:)

enicalyth - have you ever met an UNreasonable cart horse?