info from the web:-

•  

I mainly fly gassers although I have had tonnes of glow engines before. Initially, gas engines were very heavy, hence substituting them with a glow engine was not suitable unless the planes were designed for 50cc and above.

Anyway, today, zenoah has quite a good line up and the zenoah 20cc petrol replaces any .91-1.30 size glow engine. Its weight is lighter than an OS120 surpass. The engine is cheap to run, in-expensive(almost the same cost as an OS91 2 stroke), gives you about 7800RPM on a 15-5 and a 16-8.

Zenoah 20cc ~ OS/TT/ASP etc .91/1.20
Zenoah 26cc/RCS 26cc/RCGF 26cc etc ~ OS 1.60FX
Zen 45cc/3W50cc/ZDZ 50cc/or any other 50 cc ~ OSBGX1(35cc)/Moki 2.10 etc. Basically any plane that has a wingspan of 80" can accept a 50cc engine.

There is no glow engine bigger than the OS BGX1(35cc) and the super tigre 42cc glow engine. However, using such big glow engines increases the cost of fuel tremendously as their consumption is extremely high. For example if you use a 420cc tank, the OS BGX1 will give you about 8-10 minutes max, but a 50cc petrol engine will give you 18-25 minutes depending on if the engine has been leaned out or not.

•  

I beg to differ with the comparison made, from which it appears to show that a 20 cc gas engine is equivalent to a 1.20 size (roughly 20 cc ) size glow. However a 20 cc may be substituted for 90 size ( 14.74 cc) glow

The glow would be able to deliver more power for the same displacement on same prop etc, this is where BTU, air volume required to burn a unit of fuel, calorific value, energey density etc of different fuels come in. Yes however I do agree that fuel consumption would be much higer in a glow than a gas.

Again replacing same size glow with same size gas may work if you are looking for sedate scale (not even scale warbird capability) type of flying but then this equation takes on different dimesion if you want any decent aerobatics capability.

•  

There are various threads on this topic at RC Gropups and RC Universe. 


http://www.rcuniverse.com/forum/m_8673009/anchors_8673009/mpage_1/key_/anchor/tm...

•  

DLE 20 out pulling the saito 125 glow FS
prop and weight are factors

•  

converted all of my 140-180 glow 2 strokes to the SAP 30cc (180) gassie 2 stroke on pipe just about 1 year ago

•  

a 1.20 four-stroke would be about 20cc gas. A 1.4 four-stroke would be about 26cc. A 1.6 four-stroke maybe a 30cc.

•  

But consider the aircraft weight, too. You can get away with a smaller fuel tank which might equal out the weight increase due to the ignition and battery. Sometime you get lucky ... DLE-20 ends up being about 2 ounces lighter than a Saito 1.50 and produces about the same hp. [again prop comparison...]

You need to go about 10% higher with gasoline as a gasoline engine of the same displacement does not produce as much power as a nitro/alcohol eng

Dave
The guy's have given you good info, I'd like to add that it never hurts to search the specific aircraft on the rc forum's to see what other's have put in it and their results. It also can help you with fitting issues and other potential problems.
There's lots of smaller gas power plants out there now including several in the 15-17-20cc range that are all meant to fit in 60-120 size airframes. I have a dle20 at home here right now that I can't wait to put into a plane (hanger 9 Hellcat or Thunder tiger bearcat). I'd be happy to show it to you if you think it's something you'd consider for your project.
P.S. I went with this engine because it is one of the most compact one's you can get, and it even has beam mounts so you can retrofit it where you had a glow engine.
Hope this helps.

Dave
Just let me know and you can borrow it and lay it over your plans, or check it against your firewall. It'll still be in the box until I need it, so you can have it whenever you need it.