JETMOTOR

James Watt: notera att hp inom det metriska systemet bara är en översättning av hästkraft till horsepower, dvs. hk och hp har samma värde. Ditt angivna värde gäller för enheten Bhp (British hp, ej samma som bhp=brake hp). Bhp används av dem som inte gillar metersystemet och är definierad som 550 fotpund per sekund, dvs 33000 fotpund per minut.

Wow! Excellent ??

As others have said, aircraft jet engines are measured in THRUST not horsepower.

One Horsepower is the ability to lift 33,000 lb one foot in one minute. You can also say that one horsepower is equal to moving I lb 33,000 feet in one minute. There is a TIME element in horsepower, so if you know the speed, you can convert thrust to hp.

Let’s take a B737–800 with 2 x 27,000 lb thrust engines. If they are at full throttle with the aircraft stationary, then they are making 27,000 lb thrust but NO horsepower, because there’s no speed involved.

Now it happens that 33,000 feet per minute (from the figures above) equals 375 miles per hour. So each lb of thrust used to move an aircraft at 375 mph equals one thrust horsepower.

With me so far?

Ok. Now the normal cruise speed if the B737–800 is a maximum of Mach 0.82, which at the altitude and temperature (the speed of sound is related to the temperature) is around 473 knots or 551 mph.

So for every lb of thrust used at 551mph, that results in 551 divided by 375 = 1.46 hp.

So how many lb of thrust do we have at those speeds. This is the $64 dollar question. At the cruising altitude of jet aircraft, the thrust produced is roughly 1/4 that of the thrust at ground level, due to the thinner air. But there are two engines, so the result of that is that the aircraft has about 13,500 lb of thrust (27000 x 2 divided by 4) pushing it along if it’s at full power. We have to assume that the crew will have the engines set at about 80% thrust (correction from B737 pilots welcome) so we now have about 10,800 lb of thrust in total.

So the total horsepower is now 1.46 x 10,800 = 15,700 horsepower.

So there you go, a B737–800 at its cruising altitude has both engines producing a total of roughly 15,700 hp.

That HAS to be a rough calculation as the speed of sound alters with temperature at different flying altitudes, and the actual thrust produced by the engines can vary with weight and altitude.