Using a twin-cylinder engine in a standard gasser - RC Groups
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May 22, 2012, 08:57 AM
Registered User

Using a twin-cylinder engine in a standard gasser

Hi everyone

I'm a newbie to gassers, but they seem like the logical next step for me (coming from a 450 electric background). I'd really like to try to reduce the vibrations that appear to plague the single-cylinder gas engines.

I notice that you can get 50cc twin-cylinder engines that don't weigh much more than the 26cc Zenoah single-cylinder ones:

Has anyone investigated the possibility of mounting one of those in a conventional gasser frame? I'm looking at an RJX Xtreme 90 since that's available locally for a reasonably low price. This would leave the cylinders sticking out on the sides, which would help with cooling (they'd get the rotor downwash). If necessary it'd be easy to add a fast DC fan above each one.

The only major problem I can see is the engine speed. The RJX has a 6.64:1 gear ratio, so to get to 1800RPM head-speed (seems to be commonly recommended) I'll have to run the engine at 12000RPM. The 53cc engine linked above is only meant to go up to 9000RPM, and I can't find out what the 50cc one is rated at. I notice that the CRRCpro engines that RJX normally uses (GF26i) are meant to handle 8000RPM, but presumably they do work fine at much higher speeds since RJX continues to use them.

Now, as I said above, I'm a newbie to gassers. Is all of this just a ridiculous idea? Will the RPM limitations kill it? Or is this worth looking into? Any advice would be much appreciated.

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May 28, 2012, 12:13 PM
the great Gassif´er
Hmmmm, more likely the ridiculous overkill on power would spoil the party....
RPM would probably not be the problem, since at least in theory, a 50 cc twin should be able to run approximately the same RPMs as a 25 cc single of comparable construction.

Brgds, Bert
May 29, 2012, 10:16 AM
Registered User
Thanks for the reply. If engine speed isn't an issue then that's definitely good.

Is it really possible for excess power to be a bad thing? It might take a bit more fuel than usual, but as long as I go easy on the throttle I wouldn't have thought that I'd damage anything. Some of the big electrics have insane power ratings (Scorpion HK-5035-500 is over 7hp continuous) and they seem okay, even though the
May 29, 2012, 10:32 PM
the great Gassif´er
First: EVERYTHING excessive, is potentially bad: Ask any monstertruck builder, and he'll tell U, it can be done but isn't easy, and can be pretty, pretty dangerous...

Don't get fooled by power ratings: an E-motor basically does not pump in the power, but delivers what the system asks for: a load that absorbs 1 kW will absorb 1 kW, no matter whic E-motor you strap to it. That is because an E-motor due to its kV value, is not likely to over-rev

Unfortunately, an IC engine will try to pump in its rated power when you give it full bore, and things tend to go wrong quickly after that....

That is a slightly simpistic representation, because all is depending on how RPM's are controlled, but you can compare to spinning up an E-heli without softstart: you have to be careful not to strip the gears, because no gear can be designed to withstand the torque of an E-motor spinning up with full throttle, and still be practically light enough to be useable in a helicopter.

On a Gasser, the weight of the engine does something to the airframe too, and, well, if the rotor will not absorb the rated power of your engine at design-RPM, but you give it full throttle anyway, chances are, something will over-rev....

Brgds, Bert
May 30, 2012, 09:11 AM
Registered User
Bert, thanks again for the reply. That all makes a lot of sense.

Apart from the obvious approach of limiting the throttle curve on the TX (or limiting the throw of the throttle servo), is there any other easy way to restrict the power on a gas engine in a 'foolproof' way?

May 30, 2012, 11:55 AM
the great Gassif´er
none that makes fitting a ridiculous oversized engine a sensible thing to do

You could fit a smaller carb, but what is the point of fitting a big engine if you de-rate it to the specs of a smaller engine?

You could use a rev limiter, acting on the ignition, but that is 1) waste of fuel and 2) introducing a lot of unnecessary vibrations

You could use a governor, but again, what is the point of fitting that big engine if you are not going to utilize its potential?

Just go with the engine the helicopter is designed for.... Not all designers are morons....

Brgds, Bert
May 31, 2012, 08:52 AM
Registered User
Thanks (yet again) for the reply.

The reason for going for the bigger engine is simply to reduce vibration (since it's a twin and the two pistons will largely cancel each other out). If there was such a thing as a cheap 26cc twin-cylinder gas engine then I'd go for that; but the smallest twin I've found is the RCGF 40cc.

The reading I've done suggests that vibration is a major issue in gassers, primarily because it's not really possible to properly balance a single-cylinder engine. Putting a twin in seemed like the most obvious way to deal with the problem, even if it does add some weight unnecessarily.

May 31, 2012, 12:02 PM
the great Gassif´er
Nah.... it's not really a problem.... just something to take into account
Anyway, the main vibrations (at least the vibrations that cause damage) of single cylinders are not coming from the mechanical unbalance, but from the second order: the vibrations caused by the violent ignition and combustion process. And that's the same for a simultaneous firing opposite twin....

Any trainer can handle it, as long as you yourself take the vibrations into account when securing aal bolts and nuts, and installing the RC stuff.

As for scale, well it is no use to put too much labour in small details or an immaculate paintjob, since small details like scale antenna's, window wipers etc tend to vibrate off, and in the vincinity of doorsills and moveable service panels etc. the paint tends to rub off. Plastic doorhinges break sometimes....

I am flying petrol powered scalers from 2006 and have had once a crack in a frame plate. Other than that, still have to encounter my first real malfunction....
Not even the nuts and bolts come loose... those 2stroke singles are not exactly Harleys either...

On the subject of balanced engines and general vibration reduction: Vario supplies a worked over version of the Zenoah 230, which has better internal balancing (lightened wristpin and conrod), is tuned to operate optimal at around 10.000 RPM, and has a carburettor that is much more suitable for (heavy scale) helicopter use. Unfortunately it is a bit expensive (but I've checked, having the same modifications done in your local workshop is a lot more expensive)

Oil/fuel mixture has small influence (I use 1:40, in stead of 1:25 as per Zenoah instruction) and the carb settings have a significant influence on vibration level too.

Brgds, Bert
Last edited by Brutus1967; Jun 01, 2012 at 06:29 AM.
Jun 01, 2012, 08:35 AM
Registered User
Bert, thanks very much for the reply. You've convinced me to just go for the usual single-cylinder ones.

It appears that there are a few companies that take the Zenoah engines and modify them for better balance (eg. B. H. Hanson), so I might aim for one of those. A bit expensive, but not ridiculous.
Jun 01, 2012, 12:50 PM
the great Gassif´er
If you intend to run it in a heavy scale helicopter, make sure U have a suitable carb on it.... the standard "chainsaw" carb is not suitable for lage scale helicopters (but seem to work well in light and agile trainers and aerobatic machines)

Brgds, Bert
Jun 02, 2012, 04:37 AM
Registered User
The RJX helicopter is a light weight gasser. This makes the vibrations much more noticable. The Hanson Zenoah I have is much smoother than my stock CRRC. Not exactly a fair comparison.
Jun 02, 2012, 09:12 AM
Registered User
I was aiming to get the RJX but potentially add a fairly substantial payload (not really sure yet, but most likely starting with a camera platform and maybe adding an autopilot for return-to-base functionality). Won't be doing any acrobatics, unless something goes extremely wrong.

I figured that starting with a relatively light heli like the RJX would be best, in order to keep the total flying weight reasonably low. Presumably the extra weight would help to damp out vibrations a lot.

What sort of carbs are ideal for heavier helicopters?
Jun 02, 2012, 01:14 PM
the great Gassif´er
I do not know which carb Vario is using, but I do know, they exchange the original carb for one with a smaller venturi, that seems to keep a better mixture over the whole throttle range, if the engine is kept at constant RPM.

I do know, that several people have reported, the standard carb works good in a light trainer (most of the time relative low engine load) but not on heavy scalers (relative high and constant engine load). As I have understood, with the standard carb fitted by Zenoah, and with the needles adjusted correctly for hoover and full throttle, the engine tends to lean out during descend or landing approach, leading to sudden engine cut-out without any warning.

Please note, this is not my own experience, it is what I have been warned for by others. Personally, I am not taking the risk....

Brgds, Bert
Jun 03, 2012, 09:34 AM
Registered User
Sounds like a good place to avoid risks; carbs are pretty cheap.

The Walboro WT-257 appears to be recommended for consistent power (and as an upgrade from the stock WT-643), so that might be the way to go.

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