Fuel consumtion of a 582 in a Cheetah

[John Boucher]

Help me here please... The operators manual states that the 582 Blue top burns fuel at 20.5 litres / hour..... in cruise ! Is this correct?

I have a Cheetah and fly at 5800rpm as I was taught. The operators manual says 6000rpm.... Now what is it to be?

I have a E-type gearbox with a 3.47:1 reduction. This means the prop only spins at 1600rpm at 5800rpm. That's slow!

Bad Nav

[Aerosan]

This is interesting.................I have the same question


Chair-sit down -grab beer- wait for reply

[Morph]

Niren

Good to see you on the forum...

Help me here please... The operators manual states that the 582 Blue top burns fuel at 20.5 litres / hour..... in cruise ! Is this correct?

I have a Cheetah and fly at 5800rpm as I was taught. The operators manual says 6000rpm.... Now what is it to be?

I have a E-type gearbox with a 3.47:1 reduction. This means the prop only spins at 1600rpm at 5800rpm. That's slow!

Bad Nav

This all depends on the prop. The 582 motor will run up to 6800rpm. The Rotax manual will give you fuel consumption based on many assumptions, i.e. altitude, aircraft, prop pitch etc etc. It cannot tell you specifics about your plane. Factors that govern the fuel consupmtion are, load, drag, wing design, speed, propeller pitch, altitude, density altitude, i.e. temperatures, air pressures etc etc.

Assuming you are happy with the plane, then except for flying lighter, i.e. kick the pax out, or reducing drag by adding fairings etc the only thing you do have control over is the prop (if it's ground adjustable of course).

Here you can pitch it lower, giving you a higher static rpm (don't exceed the 6800). This is great for takeoff and climbing. Unfortunately you will then have to run at a higher RPM to maintain straight and level flight. so higher fuel consumption.

Alternatively you pitch it higher, reducing the static rpm, this will reduce you climb ability but it will also reduce the rpm at cruise, improving the fuel consumption. It won't really give you better speed as this is governed more by the wing design than the prop pitch.

The absolute ideal of course is to have an in-flight adjustable prop. This you set for low pitch at take off and during climbing and then once at cruising altitude, increase the pitch to improve the economy rate. Unfortunately these are damned expensive.

I had a 503 in my old Challenger and I had the static set to 6150, i.e. max rpm at takeoff. The climb was not the best but at 5400 I would be at best cruise speed of 75mph and burning 14l/h. At 4800rpm I would be flying at 60mph and burning 10l/h . However you are flying in a nose high attitude which was not comfortable. if I increased to 5800 then fuel consumption would be at 16l/h but then you could not trim her for straight and level and she would just want to climb. You would have to push and hold the stick forward to try to increase speed and maintain straight and level. I would add a miniumum of 2 l/h at each rpm on to these figures if I had a 582 on the plane. (NOTE: the Challenger is a sleek tandem 2-seater so the drag is significantly lower then that of a side by side like the Cheetah. Expect higher consumption because of this.) The 582 wouldn't give me more speed just better climbing ability.

Wrt the gearbox and prop speed. The most important thing to remmember is that the prop tip must not exceed the speed of sound. This will cause vibrations and the prop to fail. The length of the prop is critical here. The longer it is the faster the tip will be moving for the same rpm. Remmember the box ratio is a step down. So, a lower ratio box turns the prop faster than a higher ratio box. This means lower ratio boxes are limited to shorter fatter props vs longer thinner ones on the higher ratio box.

[John Boucher]

Hi Morph...

Thanks for the reply...

I have a P-Prop that Pieter de Necker built for me. Let me just say that this is a good prop and it delivers what I want it to. I get 6200max rpm at altitude 5800ft straight and level. Throttling back to 5800rpm, trimming out I can maintain altitude ( s & l )

Maybe I should have the carbs serviced by an authorised service centre and decoke?

BAD NAV

[Morph]

p-props are great all rounders. Do you have any idea what fuel consumption you are getting. I doubt if it is 22l/hour except aybe at WOT during a climb

[John Boucher]

Morph... maybe this should be thread on its own?

I get 20l/hour as the handbook says. The problem I have is that when we flew with DPZ in Kimberley, we used 17l/h. A friend of mine has the same configuration as I do and he also gets a better fuel burn than I do....

Something amiss!

[Morph]

What are your plugs burning like? Are they a nice deep chocolate brown or are they black and oily? Do you have a lot of oily residue on the fuse behind the exhaust?

If black plugs then you are running too rich. Check your jet needle circlip position.

582
Jet Needle 11G2 - Circlip position #3
Needle Jet 2,72
Main Jet 165 at sea level
Main Jet 158 at Highveld
Idle Jet 45

If you are getting a lot of oily residue on the fuse check that the carbs aren't overflowing during flight, this happened to me once. The float wasn't closing off the float needle properly.

Setting up your carbs
Make sure that the needle circlip is below the plastic spring holding cup and NOT ABOVE. Also check that you have a small black rubber O ring on top of the circlip, preventing vibration.

Turn big idle adjustment screw completely out until it no longer makes contact with the piston. Make sure that the piston is completely closed (down) and not hanging on the cable.

Turn the idle screw in until the piston rises by about 5 mm, make sure that both pistons are at the same height.

Turn the idle air adjustment screw completely in and then 1 full turn out.

Open the throttle (engine off) and stop when the bottom of the piston just meets the top of the port. Make sure that both pistons are at the same level, this can be adjusted at the adjustment screw through which the cable goes.

The throttle cable should have approx 1mm free play at idle.

Also check that the choke cables have the same free play when closed.

Start the engine and get it up to operating temp. Adjust both idle screws equally to get an idle of 2000rpm. You may fine tune the idle air adj. screws if necessary to try and get it running smoothly.

Adjustment of the float lever - remove the float chamber and the float lever should be parallel to the float chamber edge, adjust by being the part that activates the float needle valve.

[DieselFan]

Morph... maybe this should be thread on its own?

I get 20l/hour as the handbook says. The problem I have is that when we flew with DPZ in Kimberley, we used 17l/h. A friend of mine has the same configuration as I do and he also gets a better fuel burn than I do....

Something amiss!

Does he cruise at EXACT same rpm? ie not a different pitch? All up weights the same? I've had two trikes have 3-4 l/h diff and neither one had fairing.

[John Boucher]

Just spoke to my friend....

Cheetah, p-prop, no spats, dacron covered, same engine, flies at same rpm as I do - 16l / hour

The plugs are burning fine.... Maybe the carbs just need to be set up properly?

I think I must pop around to Morning Star the weekend of the 24th and we can meet. Would like to say hello to Reg V. with the silver Jab as well!

BAD NAV

[Morph]

Cool by me.

I see you have a CT in your avitar. We have one at Morning Star belonging to our club chairman. Awesome plane

If your carbs are set up properly then the only other thing I can think of is your plane has more drag than the others.

[John Boucher]

I'll PM you for contact details Morph.

The CTSW is a great aircraft. I flew the second one that came to SA and was delivered to a chap in Hoopstad. Had a wing leveller in etc. Elmo did the demo in Kimberley. Have been in contact with Francois and would have liked to fly the newer model - bigger rudder etc. That will still happen! It's just a bit pricey.....

If only the Rand / Euro was closer to what the Rand /$ is then it could be on!

Check out the Rans website. they have developed a new aircraft for the new sport class.... the Rans S-19. Spoken to Diesel Fan about it and want to fit a Raptor 105 diesel engine with full FADEC control to this project if it materialises!