RPM drop

[C205]

Yip. Tightened the belts and RPM is now steady at 6000. This is still underpowered so we'll change the exhausts around during the week for the weekend's flying. I strongly suspect that will sort that problem out. The Rattex 503 doesn't have much power as it is, never mind when its not reaching the revs it should
Did some flying on the weekend, Luveleey...

[AndyG]

Good to hear that you are sorted.

[Morph]

Fantastic,

I assume it is a fixed pitch prop, i.e. a solid plank of wood?

If not, i.e. ground adjustable, typically an aluminium hub and wooden or composite blades, then try and get a Warp Drive protractor, (measures the degrees the blade is set to) and reduce the pitch by about 1 degree(make the blade pitch finer). This will bring your RPM's up to about 6500 which is perfect

PS. Make sure they have not overtightened the belts, the plane must have a spec to the amount of play required. In the Challenger this is about 10 to 15 mm movement on the belt between the pulleys. If too tight it will stress the bearings out, and they can seize.

[Boet]

Mmmmm. I do not want to P on your battery, but when a Rattex start acting up, it is normally trying to tell you something. A nice trait, when ignored, will lead to a "sudden stoppage". So heed the warning











If you have never had a 2-stroke go quiet on you, you have not experienced an engine failure. It does not just DIE on you. It EXPIRES with a suddenness that leaves skidmarks.........

[C205]

Boet I don't think the problem's with the engine. That seems to run well. I think the problem is the exhaust and should be sorted as soon as we've exchanged it.
Morph, yes its a solid plank . Will check the play and thanks for the advice. We're learning so much!

[REDROMAN]

What should the rpm be on a full power runup on a 503.Mine goes up to 6400 - 6500.Is this normal or should it go to 6700 - 6800.What can one do in case it should go higher.

[C205]

Our normal range is between 6500 and 6800 on full power. I suppose it depends on the prop, exhaust etc. In our case we're only getting 6000 and on the Highveld in the summer 500 extra revs make a big diference.

[REDROMAN]

Our altitude is 4480 amsl.Think 6400 is a good rpm on this level ???

[C205]

Redroman I'm not by any stretch of the imagination an expert. Perhaps one of the other microlighters can help? Would be interesting to hear.
We're at 5100ft at Witbank.

[Morph]

6500 is the max you should set your static runup RPM to. This you set by pitching the prop. If your prop is not adjustable then you are stuck with whatever you have.

Once the plane starts running along the ground and in the air it will rev higher. If you set it to 6800 static, i.e. when the plane is stationary, you stand a very real risk of overrevving in the air or on take off.

I am assuming of course that there is no fuel starvation problem that will prevent the plane from reaching full RPM

[REDROMAN]

Thanks Morph.I think what i have is what i have - no problems.

[Wargames]

I had the similar problem with my prop being to coarse. I got only 5700 on take off. the problem is not just a power shortage on take off, but your engine is running to cold because you are using fuel to cool the engine, and thus higher fuel burn as well.

We set my prop to 6400 static, and temps is up from 510 to 600 without setting anything on my carbs.

the rotax 503 likes to run fast and easy.

Just my experience.

Regards,

[Stephan van Tonder]

Yip the 2 strokes must be revved. That fuel washing down the cylinder walls also cause premature wear which is not good on soemthing your life depends on.

[Morph]

If you have an engine management system like the MGL EMS units that monitor CHT, EGT, RPM etc you can see the engine cooling at higher revs. A 503 should be able to run happily at 6500rpm all day albeit at 22 liters per hour . Obviously the 6800 should only be used for 5 minutes.

Over pitching the prop gives you a better cruise, but worse climb. Also it labours the engine unneccessarily and increases wear. However a better cruise means little on a trike when the wing governs the max speed you can cruise at anyway.

If you add a fuel flow meter you will see that overpitching does not save you fuel. If you cruise at 5800 with a fine prop you will burn say 15l/h, at 5300 on a course prop you probably would use 16l/h because you are labouring the motor.

My little short wing Challenger was set for 6250 static. I could cruise at 60mph (5000rpm) and use 10l/h and 75MPH (5800) and use 15l/h