alienteabagger

This has been on my mind for quite some time now. I am considering slowly switching all my glow powered planes to gasoline. Glow fuel prices always seem to skyrocket dummer the summer months and then slowly come back down towards the end of the fall. Gasoline is just so much cheaper.

How does one determine what size gas powered engine to use in any given sized model?
For example: Does a plane that calls for a 1.20 4stroke engine need a 40cc gasser?
Is there some kind of mathematical formula for converting glow engine sizes to their equivalents in gas engines?

Thanks for any info.

Dr1Driver

A plane that uses a 1.20 4 stroke will work with a 23cc gas engine. Watch your weight and balance, gas engines are a lot heavier then glow.

ggraham500

My Feedback: (4)

well, I'm learning this as well.

What I have learned so far is not use a gasser below a .61 two stroke size. So I convert the .61 two stoke to an equivalent 4 stroke. (For example ... a .61 two stroke would approximate a Saito FA-100 four stroke.) Then to convert to a gas engine a basic conversion seems to be methanol four stroke fuel size engine multiplied by 16.3. That suggests that a .61 two stroke would use a 17 cc gas engine. A 120 would use a 20cc gasser. A 150 two stoke is equivalent to a 26 cc.

That is why many people are interested in learning more about the new Fox 15 cc gas engine. Another way to look at comparison though is weight differences. A gas engine probably is 15%-20% heavier than a methanol fuel engine. I'm not sure why. That's why I suppose the minimum gas engine (technologically speaking) so far seems to be a 15cc.

I've also learned that when anything is larger than a 1.50 cu four stroke equivalent, a gas engine is the best choice.

But I'm still perplexed by net power ... for example power-wise speaking, is a 26 cc gas engines as powerful as a 1.50 cu alcohol fuel engine?

I guess the other way to do a conversion is weight. A 9-12 pound model is a 26cc gas? A 12-16 a 30cc gas? etc. Still learning that.

ggraham500

My Feedback: (4)

why are gas engines heavier? Is the big difference the carb?

Lnewqban

Check this related thread out:

http://www.rcuniverse.com/forum/m_57...tm.htm#5798609

JOHNS3D

My Feedback: (5)

cause you need standoff, ignition and extra battery for the ignition.

Lnewqban

Great tips, ggraham500.

In order to make it easier for some of us to compare apples and pears, I have put together the attached comparison chart.
Please note that this is not a comparison of power, but only one of engine displacement volumes.
Having the same displacement volume, a two-stroke nitromethane engine will produce more power than a two-stroke gasoline engine....and weighting less.

Dr1Driver

You're getting a lot of good advice here.

Weight: Take a look at a glow engine. Now take a look at a weedeater engine. The whole design is different. The gas engine has a flywheel, magneto, spark plug, diaphragm pump carb, and some other things that quickly add weight.

Power: Hard to compare there...for example, my last 1/4 Fokker Dr.1 used a 23 cc gas engine. The recommended engine was a 1.20 4 stroke glow. I wouldn't want to have flown it with the glow engine. The prop that engine could have turned would have been too small to give me the reliable flying characteristics I wanted.

Model weight: Not that easy. It depends on what flying characteristics you want from your plane. A giant scale Corsair might use a Zenoah G-62, let's say, but a Cub the same size might only need a G-38.

Back to power for a minute. You're looking at two stroke glow, four stroke glow, and two stroke gas. They all have different power ranges and prop capabilities. In picking an engine, decide how you want your plane to fly, factor in the weight, wing loading and drag, and choose an engine/prop combo that will do what you want it to. On my Dr.1, I wanted to swing a large diameter/low pitch prop. This would give me plenty of torque (in the large diameter), and limit the top speed (in the low pitch). For cleanliness, reliability, and sound, I wanted to use a gas engine. I looked at the gas engines available that fit the model's needs and chose the Zenoah G-23. Anyone who ever saw it fly said it looked great in the air and flew in a very scale-like manner. I'm not pushing the Zenoah, just citing an engine and model I have extensive experience with.

Keep asking, we'll keep helping.

Tired Old Man

My Feedback: (1)

"alienteabagger"??

I think this name requires a little expansion to better define the applicability

alienteabagger

Nice chart!!!!
So if I understand everything correctly, a simple displacement conversion is NOT enough to determine gas engine size? Is flying weight (as pointed out by Dr1Driver) a better piece of information to use than glow engine displacement? Or is it best to use the displacement conversion and THEN take flying weight into consideration?

I love learning new stuff.


p.s. My World Models Extra 330L (.75 size, ±8.2lbs AUW) is getting gassed with everything I learn here.

Lnewqban

Then, this old thread is for you:

http://www.rcuniverse.com/forum/m_87...tm.htm#8771351

alienteabagger

I think I may have sprained my brain hahahahaha!

TimBle

the major difference in weight is the engine block design. Gasline nearly twice as much energy content as Methanol+nitro fuel. That energy is converted to heat to do work.
the hat needs more metal to dissipate it to the environment.

then theres teh extra components like Spark plug, ignition control and timing and battery.


Because of the bigger bang, a gasoline engine can swign a bigger prop and generate more thrust.

Its also more fuel efficient so it'll burn less fuel in a lfight so you use a smaller tank (saving weight).

ultimately you need tolook at the aplication and decide which engine will meet your needs.

Personally for scale aircraft gasoline engines cannot be beat. The question is which engine.

Swept volume comparisons tell you very little about performance in an airframe. MAx poer output comparisons also tell you very little.

For a pylon racer, perhaps a 2Stroke glow is best since it can rev a smaller prop very high.
For a aerobaticplane, the torque of 4strokes glow (Methanol), electric or gasoline is sought after but noise is a problem, hence electricis making a big impact in precision aerobatics. the throttle curve for a gasoline is also different to a glow or electric.

this and many more factors can be taken into account or likeme you can just like the sound and low cost of gasoline.

I am going gasoline on all my future airplanes that require anything larter than a .90 size 2stroke glow.