Power Equation
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Joined: Sep 2008
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From: RIGHT SIDE OF GOD
Power Equation
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.
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.
Joined: Aug 2005
Aviation Qualifications: PPL
Posts: 612
Likes: 387
From: EDLB
Hello,
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.
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.
Joined: Jul 2004
Posts: 513
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From: Sydney NSW
horsepower
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.
Joined: Oct 2002
Posts: 372
Likes: 7
From: Surrey
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.
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.
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.
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.
