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SFC per pound between piston and jet turbine.


ben87r
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So I'm trying to do a comparison between piston, small gas turbines and large jet turbine engines, I've come up with a 0.57 pounds per hour per pound for the Trent 700 series engines, now I want to see how the others stack up.

 

Basically I'm wanting to understand as to why we don't seem to be able to make small turbines efficient, and if we were to just how far it had to come. I want to try to keep the formula PPH/P if I can.

 

What I was hoping all you bright Sparks might be able to tell me is, what would be a rough standard figure between SHP/thrust and also HP/thrust for a standard 6cyl piston non turbo. Now I know that there are a huge amount of variables so I was thinking cruise power only. I can reverse engineer the C210 and C208 figures to get PPH/SHP figures of 0.66 and 0.6 using company SOP power settings and fuel flows but I'm stuck from there

 

Any ideas?

 

 

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The energy of the efflux is 1/2mv*v ( half m vee squared) and this energy comes from burning fuel.

 

Thrust comes from momentum change mv.

 

Example: 1 kg of efflux (air plus burnt fuel) gas at 100 m/sec has 5000J energy supplied by fuel. And the momentum is 100 Newton-sec, which provides the thrust.

 

compare with 2 kg gas at 50m/sec: The energy is 2500J ( half the energy) but the momentum is the same at 100 Newton-sec.

 

So accelerating a small amount of air to a high speed is always less efficient than accelerating a large amount to a lower speed. This is why low-power things like man-powered planes need big diameter props.

 

I've got a jet engine which produces 30kg thrust for 54 litres per hour and a Jabiru engine which produces 60 kg static thrust ( I think, I've never measured it ) for about 15 litres per hour. Big difference in favor of the prop huh. I'm not going to put the jet on the Jabiru anytime soon.

 

 

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Jabiru engine which produces 60 kg static thrust ( I think, I've never measured it ) for about 15 litres per hour

Thanks Bruce,

 

That will give me a start at least. That 60, is that meant to be full power or cruise? If cruise do you know the cruise power%.

 

 

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Bear in mind that (in approximate terms) the pure jet retains its thrust throughout the entire speed range of an aircraft whereas the prop will generally reduce thrust as speed increases. Hence the advantage of a jet at higher speeds.

 

 

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A turboprop (Geared turbine), is easy. You use shaft RPM and torque which can usually be read on a torquemeter sourced within the reduction gear.

 

You also have a fuel flowmeter so it's dead easy.

 

A jet engine ( 2 types) direct and bypass (Turbofan) which can be considered a thrust multiplier has a static thrust figure. No power is developed till the engine moves.. in some direction..... Work= Force x Distance, but for a rough figure 1 lb of thrust equals 1 HP at about 100 knots if you want a power comparison.

 

50,000 Lbs thrust equals about 5o,000 Horsepower at 100 knots at sea level.

 

The power drops off with height as it does with a propeller. So does the fuel flow rate as the mass of air you process in a given time reduces with reducing air density.

 

It isn't easy to work out what actual horsepower a jet engine is delivering at any point in time. You go on engine pressure measurements at various places in the engine. to set power.. Fuel flow gives a rough indication of power delivered. Nev

 

 

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Yes the turboprop sure shows how moving more air a slower amount works.

 

Getting a measure of the thrust and power from an engine mounted on a plane can be done as follows:

 

Choose some speed, then throttle right back ( or turn off the engine) and note the sink speed to maintain that airspeed.

 

For example, my Jabiru at 95 kts has about 10 knots of sink with no engine assist.

 

This means a glide angle of 95/10=9.5 to 1. It weighs 450kg, so the drag force is 450/9.5=47.3kg.

 

Now open up the engine and fly level at 95 knots. The prop will now be producing 47.3kg thrust. At a speed of 95kts (48.9m/s) the useful output power from the prop is 47.3*g*48.9 Watts

 

=22.7kW or 30 horsepower.

 

The 10 knot sink is just an educated guess at this stage, I'll check it next flight. For example, 13.5 kts of sink would give 64 kg thrust etc.

 

The 60 kg of thrust I mentioned before is what you get by tying the plane to something and measuring the pull. As Nev says, the efficiency is zero at this time with no forward speed.

 

 

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Bruce you might have to refine that method. With the engine idling or stopped there is some drag at the speed you are flying at from the prop. You would have to run the engine at a speed where it was neither helping or hindering to get a zero thrust glide ratio or ROD in the same units as your TAS. This method is used when doing assymetric . Rather than actually feathering the prop a certain thrust is used to equal the feathered condition. IF it all goes pear shaped you can re use the engine immediately rather than trust the unfeather process. The principle would be the same with your experiment. NO? You might also be able to use Potential energy figures (Mg x H) Nev.

 

 

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yes you are quite right Nev, to be accurate you do need to have the prop effectively "not there". In theory, this occurs at the "pitch speed" which is where the prop pitch times the rpm matches the airspeed.

 

For a 42" pitch Jabiru prop at 95 kts, (1.06m at 48.9 m/s) this rpm calculates to

 

48.9/1.06 rev/sec or 2768 rpm. This is a higher speed than I would have thought.

 

Next flight I'm going to experiment with this stuff. I wonder how much difference it all makes... it might be a fair bit at 95 knots.

 

 

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Everyone seems to use a different value for slip for each aircraft and where you measure the pitch and what effect the aerofoil shape has on the prop blade's effective pitch. It's probably in the order of a 15% reduction in revs. ( My guess.) You could rig up two forward facing pitot one in the propwash and the other in the freestream about 4 feet away laterally, and level a manometer tube connected to them both. It's not easy. Nev

 

 

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Thanks for the replays guys! Bit harder then what I was hoping but I've been able to get some rough figures.

 

Too much time on your hands and not enough beer? 022_wink.gif.2137519eeebfc3acb3315da062b6b1c1.gif

Well it hasn't been too much time so maybe I just need to drink more! Shouldn't take much convincing!

 

 

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