A technical question about horsepower and thrust

[Guest rocketdriver]

I’m trying to do some estimates of the performance for my KR2 that has yet to fly in Au and for which I have very little information from the previous owner ......

 

Regarding thrust .......

 

In steady state level flight, thrust = drag (near enough)

 

Using imperial measure for the moment, 550 ft.lbs/sec = 1 horsepower. (it can also be expressed in metric terms of course .... 1 Kw = 1 Newton metre/second .. but back to imperial as, even today, we use hp to measure engine power, mostly)

 

Also 1 knot True airspeed, 1 KTAS = 6072 ft/hour = 6072/3600 ft/min = 1.687ft/sec.

 

Analysing the dimensions of the various units, which, if either, of these is true please, where KTAS is the actual measured true airspeed at a particular power setting.

 

1. Thrust = (Prop hub horsepower * 550)*propeller efficiency/(KTAS*1.687)

 

1. Thrust = (Prop hub horsepower * 550)/(KTAS*1.687)

 

 

[facthunter]

You're going to give me a head ache RD. There are too many unknowns. Things don't stay constant. Ie the thrust and how do you measure it?

 

Jet engines don't measure horsepower, they measure thrust. ( Well they quote it) and a certain thrust does give you a power equivalent at a certain velocity. OK enough of the thinking aloud.

 

Using Shaft horsepower and the efficiency of the prop and balancing it with the drag would work in principle. You should be able to get a total drag/speed graph for the plane from somewhere. Every plane has two limiting speeds where the power is not enough to maintain height. If you know how other similar planes perform all you need to do is apply the available horsepower you have from your engine/prop combination, calculations. The only way I could do it would be graphically.. Nev

 

 

[Guest Baphomet]

The other method you can employ is to tie it to something solid, with a spring balance in series. Run it up to full power, and measure the static thrust directly in lbs or Kg's. Sounds dangerous, but isn't if approached sensibly. Used to do this often, when playing with Gyro powerplants, gives a reliable real-world result and is ideal for testing different engine/prop combinations.

 

Baph

 

 

[Guest rocketdriver]

Hi Nev .... re headache, imagine how I feel ! :big_grin:.

 

I'm figuring that if I know the engine horsepower from the dyno graph, and if I know the airspeed actually achieved at a specific engine revs, I can work out the thrust if I use the right formulae. Then, I can calculate the L/D, knowing the weight .......

 

What is puzzling me is this. I know the pitch of the prop (72inches), so I know what speed should be achieved at any specified rpm if the prop is perfectly efficient. The actual speed will be lower, and the ratio of the two speeds will tell me the prop efficiency. So do I need to include efficiency again in the equation for thrust? ... my feeling is no, its already accounted for in the KTAS achieved by the aircraft under test, but I'm not sure ...

 

hm ...headache or Migraine ... I think I'd better take a bex and have a nice lie down:laugh:

 

 

[fly_tornado]

The variable is the airframe, a pusher will get a different result to a tractor. light plane versus heavy plane, biplane versus monoplane.

 

 

[Guest Maj Millard]

Keep it simple as always. Do as we have always done in the past with unknown aircraft/thrust issues. Tie the aircraft to a post with a stout rope. Insert a butchers' scale which reads up to about 300 lbs or more into the tiedown rope. Run the ring out of the engine at full power, and have someone take a reading on the scale.

 

Don't know what would be applicable to your particular aircraft as I don't have a weight for it, however in the past with the average machine (UL), we found that 120 lbs of thrust was a little risky but 200 plus would probabily get you off the ground ok.

 

This is a very basic measurement of the actual lbs of thrust your prop is producing, in a static condition.

 

As the aircraft gains speed, the propellor becomes more efficient, and the RPMs will increase, often in the region of 200-300 rpms, thus producing more thrust.

 

When you think the prop may be producing sufficient thrust, go do a couple of low hops down a long runway and if you lift off faily quickly you'll probabily fly ok.

 

Be aware however that performance in ground effect may be quite different to performance out of ground effect, in respect to prop thrust/performance and wing lift. Always have plenty of runway in front of you when running these test, in case you find performance is not what was expected..............................................................Maj...

 

 

 

[facthunter]

Prop efficiency is NOT the difference between the geometric pitch and the "effective' pitch. It almost is but there is more to consider. You could just have a poorly constructed prop dimensionally that was not an efficient producer of "lift" along the blade aerofoil. Propeller manufacturers give different pitch figures from each other, so you can go wrong making comparisons. Also, an example regarding efficiency, it is accepted that wooden props don't produce the efficiency values that metal props do , as they don't have the surface finish and thinner tip sections.. They run into compressibility at lower tip speeds as well.

 

I'm going to have to admit that when I talk about selecting a prop for a plane I pick a similar (on cruise speed) example and get a prop recommended for say an 0-200 that might cruise at 2300 rpm and deliver say 75 HP in it, and then compare my engines prop speed for cruise. You are using an EA 81 with redrive aren't you?. Work out what you would like your cruise RPM to be. Hopefully it will give you the required HP at that RPM and if your redrive ratio is OK and your U/C is long enough, go from there. Whatever you engine is, there must be a range of props that are used with it (as a starting point) on other aircraft if not on yours. Nev