simonini reduction

Hi, I'm not getting great fuel consumption and on speed bar i'm on nearly full throttle. Will changing the reduction to 3.1.1 give more thrust lower down? Can the reduction be changed to 3.1.1 by the small pully like most of the other reductions?

Many thanks

Danny

Danny

How big is your prop ? I have a simmo 200 turning a 130 prop through a 3.1 to 1 and it is powerfull and efficient compared with friends units which run 2.6 to 1 or 2.8/1. The engine revs a bit more, especially with the 2 blade prop.

Not sure about whether you can do it by just changing to a smaller crank pulley.

Dave

hi dave it is a 13o prop

thanks danny

Seems to be a bit of confusion here ? The standard Simonini mini2 engine comes with a 129mm Crown Gear (the big ring) and ratios are changed by swapping the small pinion pulley.

None of the ratios quoted are the factory ones offered though : http://www.simonini-flying.com/mini2plus_eng.htm

A bigger reduction ratio means the prop will be spinning slower for a given engine speed (rpm) - ie. for a 1:3 reduction (like on the Evo engine) the prop will rotate at 1/3 of the engine speed.

Lack of power and poor fuel consumption are more likely to be a result of poor engine tuning (fuel/air mixture or ignition timing), a worn engine or incorrect propeller - blade length, width and pitch angle all have an effect on the engine loading.

A standard Simonini mini2 hits peak hp / torque around 7000 rpm with performance dropping off again after 7500 rpm. The first step is to check whether it reaches those figures and if the power increases smoothly from idle to 7500rpm (max revs only for a few seconds).

Any prop should match the engines reduction ratio and power range, rather than the other way round ....

Sorry Aquatix. My fault for making the assumption that MrDanny's engine was set up properly.

But I'll keep my 3.1 to one drive ratio and my 130 prop with it's steeper pitch turning more slowly because it's been shown to work well. The only explanation I can find for it's superior thrust and economy is what I heard elsewhere, and that was that if you try to turn a prop too fast it produces alot more drag for the little extra thrust. Inspite of this, I understand that Simonini will supply about 4 different ratios of redrive, according to what the customer feels they require for the prop they are using.

Hope this helps to clear up the confusion

Dave

Hi Dave - I assume you have the 138mm crown & 44mm pinion reduction gear, so probably a 200 super or other slightly improved version of the stock mini2 engine ? What is the exact model of prop you are using, and is it 2 or 3 blade ?

It is true that overspinning a prop will create problems but generally not an issue with a well designed prop until the tip speed exceeds 300m/s. Generally a bigger prop with more pitch angle will perform better at lower rpm, but most of our 2-stroke engines have tuned exhausts to perform best at a certain rev range - which the prop & reduction ratio should exploit best use out of. Whilst it is sometimes possible to improve performance or economy by changing propeller and/or reduction, most people don't have the facility to test such changes so may be best sticking to manufacturer recommended set-up.

One other potential problem with bigger props is lack of airflow cooling over the engine, so a CHT gauge comes in handy to monitor this. I've noticed quite high cylinder head temperatures even flying in this snowy weather with my big 3 blade Helix prop, so I'm going to test a smaller 2 blade to see how that performs.

PS - there is also a longstanding problem with the Simonini pulleys on the 3.1:1 ratio that it may be worth checking. Apparently the machined grooves do not match the poly-v belt correctly, leading to premature belt failure. The attached pics show my belt after just 10 hours (shown inside out for clarity).

Crikey. There was a problem there. Hope it is resolved now. Yeah, my mates Sportix shredded a belt and I had to retrieve him. My last units belt was a bit worn at 30 hours run time so I popped a new one on before the sale of it. Nearly 150 hours on 'Ol Flattie now and still on the first belt, and still looking good.

I found the info on prop size and it's possible influence on engine temperature very interesting. Never thought of that before but I do see the logic in it. I did think about getting a CHT gauge, but thats as far as I got. I still go by smell and the colour change of the exhaust (chrome plate) I tend to set the low screw very lean and the high setting just to the rich side. that's where I got down to 3.3 ltrs/hour

I'll measure the pulleys if your interested. Something else I've meant to do is take up a rev counter to find out what revs are needed at different trim settings / wings.

Do you get any engine vibration with the evo ?

The belt problem was after just 10-15 hours on the stock Simonini pulleys - 138 & 44mm (1:3.1 ratio) - but clearly not a problem on yours if thats what you are running ? Kobra were quick to respond with some new belts and a set of remachined pulleys are en route so all resolved now. They said the same thing had happened with a few motors.

Did yours come with that ratio fitted ? Most mini2 engines are around 1:2.4, so thats quite a difference. 1:3.1 is standard on the Evo but then it has more power to cope without needing to rev high.

Vibration seems very good for a 2-stroke & certainly better than my old F200 Walkerjet which shook the baffle out of the silencer !! The Evo is massively constructed though (and heavy with it) so doesn't have much scope to move about, but if I lean back and touch my helmet against the frame (when checking the wing) it still feels like a visit to the dentist.... Fuel economy is good - consistently below 3.5 litres per hour - although I've not measured it exactly yet.

Like you I run the low screw lean (otherwise it sounds lumpy with excessive 4 cycling) and the plug shows no sign of overheating (in fact its quite a dark colour) but CHT still hits 200 degrees C on a moderate sustained climb, well under maximum rpm. It may be the platinum tipped plug I'm using so will have to do some back to back testing (an excuse for more flying).

Matching different sized (pitch & reduction ratios) props to a particular engine is a bit of a dark art - and hard enough with powerboats where you can at least hear it cavitating - so worth a bit of discussion before anyone ruins an engine by going large & fitting small pulleys. Even if you had facilities (and an assortment of props) to static thrust test them all it would only be part of the story as maximum rpm is rarely needed and the load it presents to the engine changes once moving through the air. A different prop would likely require different carburettor settings, making back to back comparisons impractical.

(an excuse for more flying).quote]

As if you need it

An interesting topic

Cheers,

Alan

PS - there is also a longstanding problem with the Simonini pulleys on the 3.1:1 ratio that it may be worth checking. Apparently the machined grooves do not match the poly-v belt correctly, leading to premature belt failure. The attached pics show my belt after just 10 hours (shown inside out for clarity).

[attachment=1]Kobra Evo 006.jpg[/attachment]

[attachment=0]Kobra Evo 005.jpg[/attachment]

Kobra have already sent me some new belts (upgraded quality) and have remachined their own pulleys to permanently cure the problem, with a set on their way to me now. It may be worth keeping a close eye on your belt if you are running this reduction ratio .....

Them there Belts look very familiar.

Had the same problem with my cors-air. But not directly caused by the Grooves in the pulleys.

More by the way the two pulleys were fractionally out of alignment after I had changed the reduction ratio

It appears I had pushed the small pulley further onto the crank than factory intended. After two quickly destroyed belts. I removed the thin shim between the eccentric cam and the crank case (there for such adjustment I assume) and all was back in line.

Moral of the story. Check the alignment before fitting £60 worth of belts

Added bonus. I`ve found a supplier for Cors-air / Parajet belts at only £16 each

Colin B

Good point - incorrect tension or alignment is the usual culprit for damaged belts, but that was the first thing I checked. Kobra said it had occurred on a few of the Evo motors (only 3.1:1 ratio, something about Simonini using an old stock of pulleys ?) hence they were remachining them themselves. I will compare them when the new pulleys arrive.

They also said they are looking to build their own complete engines in the near future so that should prove interesting. As a company they seem quite committed to constant improvement and customer service seems as good as the best.