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Interested Passenger
7th Nov 2014, 16:02
I wouldn't want to darken the tech log with such a silly question, but two of my friends are heading south, one set of from Bedfordshire in his BMW M3 and is currently at Dover waiting for the ferry.

The other set off many many hours later, overtook him after 30 minutes and is about to touch down in Nimes


I know jet engine performance is thrust, and there isn't any easy way to get from that to bhp, but just very roughly, rule of thumb, good enough for a wind up on facebook etc how much would 27300 lbf equate to?

Checkboard
7th Nov 2014, 16:20
The problem is that power is work done over time. Work done is Force times distance.

So - if the force (thrust) the engine is providing is with the brakes on, at the end of the runway, then the distance the aircraft is travelling is zero and the power is zero, so the "horse power" is zero.

If the force the engine is providing is at 250 knots (a low level cruising speed), then the power is 15,618,071.1050 watts. It is equivalent to:
20,944.1747 mechanical horsepowers.
21,234.6599 metric horsepowers.
20,935.7522 electrical horsepowers.
1,592.1373 boiler horsepowers.

So - ballpark for your friend - 20,000 horsepower (if the aircraft is climbing at full power).

Interested Passenger
7th Nov 2014, 16:26
Cheers Checkboard, I appreciate the two measuring systems aren't compatible, but that approximation is good enough for my needs :)

603DX
7th Nov 2014, 16:29
Assume that the aircraft travels at 500 mph, and that the total engine thrust is 27300 lbf to sustain that as a steady speed in level flight, with thrust exactly equal to total induced plus parasitic drag resistance.

One Imperial horsepower is 550 ft-lbs/second, so an approximate total equivalent horsepower being developed would be about

500x5280x27300/550x3600 = 36,400 bhp (quite a lot!)

(I await incoming! ;) )

Interested Passenger
7th Nov 2014, 17:00
car manufacturers will give the absolute max power under best conditions, no matter how unrealistic, so the jet can do the same :)

the Bugatti Veyron famously has 1000BHP to do 250mph.

quick googling says air density at sea level is 1.2kg/m3 and at 30000 it's .356kg/m3 so roughly 1/4

as drag is dependant upon the square of velocity, that same 1000bhp should push a veyron along at 500mph at 30,000ft

(forget rolling resistance, forget car engines won't work at that altitude) an object roughly Veyron shaped with 1000bhp at 30,000ft would do 500mph

now a 737 has quite a lot more surface area than a Veyron, but surely not 72 times as much.

please point out my obvious errors

:ok:

mini
7th Nov 2014, 23:09
how much would 27300 lbf equate to?

Your stated figure relates to turning effort ie Torque. You can have a stupendous amount of torque and have nil power - think of a guy leaning on a big wrench straining against a seized nut - huge torque, but no power unless it moves...

Power (kW, bhp etc, ) relates to the rate of doing work.

Its always a conundrum with my students.

603DX
8th Nov 2014, 15:17
Your stated figure relates to turning effort ie Torque.

Afraid not, mini, the OP clearly stated the 27300 lbf to be a thrust (i.e. a force), not a torque. :uhoh:

funfly
8th Nov 2014, 15:23
As a comparison...
The Spitfire engine started off at around 1000 hp although it was improved as time went on.

megan
8th Nov 2014, 23:42
A very simple formula to make the calculation

Horsepower = thrust (pounds) * velocity (feet per second) / 550

Ignores propeller efficiency though

603DX
10th Nov 2014, 10:37
Ignores propeller efficiency though

I love the little inconsequential comments that many of us make within our posts - a beauty here by megan, in a thread which we all know is about jet turbine engines!

:D:D:D

Interested Passenger
10th Nov 2014, 11:38
so Megan's formula

Horsepower = thrust (pounds) * velocity (feet per second) / 550

so at 325kt, which is 550fps thrust = hp

and at just under 500kt it's 1.5x thrust, 40k hp, under each wing.

or quite a bit more than an M3 :O

megan
10th Nov 2014, 12:05
I love the little inconsequential comments that many of us make within our posts - a beauty here by megan, in a thread which we all know is about jet turbine engines603DX, when we make reference to horsepower, at least in my mind, we are referring to a piston engine equivalent. The thrust provided by a propeller is somewhat less than the output provided by the engine due to inefficiencies. Generally propellers are in the order of 90% efficient. Stanley Hooker of Rolls Royce had a moment of epiphany when he realised that the output (thrust) of Whittles jet was in the order of the Spitfires drag, and so the promising future potential of the jet engine. He had made the connection between the Spitfires horsepower and drag, and the thrust of the Whittle jet.

603DX
10th Nov 2014, 12:22
603DX, when we make reference to horsepower, at least in my mind, we are referring to a piston engine equivalent.

Quite so, if that's how you want to look at it. But when calculating power as simply thrust times speed, as I and others here have done, propellers on piston engines are not relevant.

Blacksheep
10th Nov 2014, 12:27
I was watching a programme yesterday about building the US "X-Craft". It's a development ship powered by diesel [low speed] and gas turbine [hi-speed] engines. The gas turbines appeared to be CF6s and the Chief Engineer said they each developed 33,000 shaft horsepower.

So there's a definite number for a gas turbine with both air and ground use.

603DX
10th Nov 2014, 12:56
The gas turbines appeared to be CF6s and the Chief Engineer said they each developed 33,000 shaft horsepower.


So did that figure include actual shaft horsepower plus the equivalent horsepower of the residual thrust of the high speed gas efflux, in the same way that turboprop aircraft engines generate motive power from both?

AdamFrisch
10th Nov 2014, 14:55
There's a very easy conversion that's close enough.

Thrust in lbs x0.7 = hp

Flash2001
10th Nov 2014, 15:18
Dunno who told you that one but the dimensions are wrong!

After an excellent landing etc...

G-CPTN
10th Nov 2014, 15:24
Has anybody remembered to double-up the result, as most airliners (travelling from the UK to Nimes) usually have two engines producing around 27300 lbf each.

Boeing: 737-800 Technical Characteristics (http://www.boeing.com/boeing/commercial/737family/pf/pf_800tech.page)

How many engines does a BMW M3 have?

I would also suggest that calculations based on the speed (of the aircraft) attained at cruising speed is not an accurate guide of the power required (as I doubt that this would be at maximum thrust) - you would have to consider MTOW performance, Shirley?.

wrmiles
10th Nov 2014, 19:05
The problem with the thrust times velocity approach is that you have to know the thrust at the particular velocity and altitude, it varies with both altitude and airspeed for a turbofan.


For several smallish turbofans which also have a turboprop version, the turbofan thrust is about 30-40% more than the turboprop horsepower.

Interested Passenger
10th Nov 2014, 19:12
in post 14, i did say under each wing, and even with my minuscule aviation knowledge, I knew a 73 is a twin:ok:

megan
11th Nov 2014, 08:45
603DX, I beg forgiveness for making a post giving a formula that covered all motorised fixed wing aerial vehicles. Be as pedantic as you wish, but threads often supply more information than the bare minimum asked for. I think the OP has far more intellect to know that jets, as the term is generally used, don't have props - except those engines that operate on the identical principal and drive props ie turbo props. He may in fact now realise he can calculate the prop thrust and drag of a piston powered airframe.

603DX
11th Nov 2014, 10:34
megan, I seem to have annoyed you in some way, from the tone of your responses. I assure you that this was not my intention, and plead that after a working lifetime as a chartered engineer, I find it hard to avoid being strictly factual in my approach to technical questions. Indeed, some might call this being pedantic. So I kept rigidly to the OP's specific request for a "ball park, rule-of-thumb" indication of the equivalent horsepower of aircraft gas turbine thrust, as here quoted:

I know jet engine performance is thrust, and there isn't any easy way to get from that to bhp, but just very roughly, rule of thumb, good enough for a wind up on facebook etc how much would 27300 lbf equate to?

I made no assumptions regarding number of engines or type of aircraft, just the bare bones of what was asked for. Then I gave an example of the crude calculation for a hypothetical aircraft at a given speed, in level flight at an unspecified altitude, to keep things simple. Some other posters did the same sort of thing, to maintain the same generalised approach.

So I am sorry if I have given unintentional offence by being over-simplistic, I'm quite a nice fellow, really! ;)