How does KW relate to lbs thrust??
 

I was recently accosted by an enthusiast who wanted to know how many times more powerful my CFM56-3B1 engines (each 20,000lbs static thrust) were, compared to a car engine rated, say, at 190 bhp. After persuading him that thrust can't be compared to horse power directly as one is a measure of force and the other is a measure of power, we were left without a convincing answer. Is there, however, a possible comparison?

YOu are right about the difficulties in associating horsepower with thrust. Piston engines are basically power producing engines, that is the power they can produce varies closely with the fule flow. Jet engines are thrust producing, the thrust varies with fuel flow.

As the Power is directly proportional to the thrust multiplied by the velocity, a jet on the runway, on brakes at full thrust is not producing any power at all! For a better comparison, look at the results achieved:

A standard production V8 will do the quarter (mile) in about 16 seconds or so. That's 1320 feet, or about the distance required to get a 737 with two 20,000lb engines airbourne.

The production car weighs about 2000kg and finishes the quarter at around 150 kph.

The 737 will weigh around 55,000kg, and finishes the quarter at around 130 kn (240 kph) in about 12 seconds. (NB These figures are educated guesses.)

So tell your mate to imagine sitting at the lights next to a road train (ie a semi trailer with a couple of extra trailers on the back) in his road car, and being creamed in a burn off the lights ... :)

Tell him that during take-off, the 737 would consume the contents of his 60 litre petrol tank (converting it to power) in around 30 seconds! :)

I took a similar line and told him that a typical car weighing 1,000Kg would accelerate at the same rate as a 50,000Kg 737 so therefore the latter was that many times more powerful. Another comparison was the fuel burned in unit time when cruising assuming the calorific value of kerosene and petrol are similar. The 737 is using 2,400Kg / hr in cruise whereas the average car uses just 8 Kg / hour.

I seem to recall that the RB211 based generating sets and marine gas turbines could put out about 30,000 BHP compared to 50,000 lb of rated static thrust on the aircraft engine.

I am busy completing an ATPL distance learning course, and they recon, divide lb of thrust by 2.6 to get BHP. Than means a 777 has about 80000 BHP, pretty impressive.

Some figures are waaaay off there, guys!
A 737 will take a LOT longer than 1320' to take off, that's only 400 yards!
Try about 1000+ yards as a minimum. ;)

A couple of other figures to confuse the unwary -
- A WW2 Merlin at full power will suck about 0.5 lbs of air a second to make about 2000hp. A 747 engine will suck about 200 lbs of air a second in the cruise ...
- Most times the rule-of-thumb is 2.5 lbs of thrust for 1hp.
- I put a thing called a 'G-Tec' which is an accelerometer for testing the speed of a car on the windscreen of the 747 I fly and I got 72,000hp from it.

As for the CFM 56 a very accurate rule of thumb is fuel flow (in kilograms per hour) multiplied by 5.3 is current horsepower output. But this only applies to the CFM 56 engine, taking into account the engine efficiency and all that.

I've been given this after lenghty efforts to find out conversions.

------------------
This Airbus is o.k., but why did it come with a RENAULT key ??

According to the P&W black bible the measurement of horsepower on a turbojet or, a turbo fan is equivalent to one pound of thrust equals one horsepower at 375 MPH. I would provide the formula but I can’t translate it to this forum due to a lack of computer skills.

------------------
The Cat