# Homework Help: Power efficiency problem

1. Jan 14, 2006

### endeavor

A 3.25 * 103-kg aircraft takes 12.5 min to achieve its cruising altitude of 10.0 km and cruising speed of 850 km/h. If the plane's engines deliver, on the average, 1500 hp of power during this time, what is the efficiency of the aircraft engines?

I initially believed the power out would be: (Kinetic energy + Potential energy)/Time

And efficiency would be 1500hp/power out. But after my calculations, I got 14.6% efficiency, when it should be 48.7% efficiency.

What did I do wrong?

2. Jan 14, 2006

### Pengwuino

I calculated 48.8% efficiency.

Did you....

1) Change minutes to seconds?
2) Change km to m?
3) Change km/h to m/s?
4) Convert J to hp? ( 735.498 75 watt = 1hp/s)

You have to do each of those in this problem.

3. Jan 14, 2006

### Gamma

Check your units. Did you convert the power in hp to watts?

1 hp = 746 watts.

4. Jan 14, 2006

### Gamma

Looks like we typed at the same time.

5. Jan 14, 2006

### endeavor

I don't know what I did wrong.
I'm pretty sure I converted everything.
It must have been a calculation error, because I just got the right answer.