Electric to Gas Engine Equivalents

[Cherokee996]

Hello All, new to the forums, second post here.

I just finished my first foam build, a Simple Storch a couple of days ago. Still waiting on the electronics (the motor I've been eyeing is currently out of stock) but weighted glide tests are proving that this airplane certainly has potential. I've read quite a bit in the past few weeks about RC aircraft and have come up with a few questions. I'm hoping someone can shed some light on it.

I came across some info on vintage Cox Engines and it appears that there is a company in Canada that still sells brand new Cox engines. What confuses me is the comparison between gas and electric power plants and the specific notation of how these engines are labeled.

Take for example, the Cox .049. Doing a little research, I found that you can supposedly multiply the displacement by 2000 to achieve the equivalent wattage. Does that mean that a Cox 0.049 engine will produce only 98 watts?

My next plane is most probably going to be the FT Spitfire. I plan on reinforcing the foam and gas/oil proofing it as well. The recommended motor is about 350 watts, so would the approximate gas counterpart be a 0.18 cubic inch engine?

Any help is greatly appreciated!

 

[Crazy Goats]

Welcome! I would love to see some pictures and articles of this as I am interested in using those motors myself.

 

[Cherokee996]

After a few hours of searching, this is what I have come up with so far:

It seems that most American engines (COX, K&B, Fox OS, etc) do come labeled with their displacement in cubic inches. This means 1 cubic inch is roughly equivalent to 16.38 cubic centimeters. I also found some helpful info on how to do this conversion.

First of all, the conversion rate I found says that in order to get equivalent wattage, multiply the displacement by 2000. This gives an engine like the Cox 0.049 RC 98 watts.

This website provides a framework for what watts should be used in for what type of flight is being attemptedhttp://http://www.rc-airplane-world.com/watts-per-pound.html:

"Less than 50W/lb - very lightweight / low wing loading slow flyer.
50 to 80 W/lb - light powered gliders, basic park flyers and trainers, classic biplanes and vintage ('Old Timer') type planes.
80 to 120 W/lb - general sport flying and basic/intermediate aerobatics. Many scale (eg warbirds) subjects suit this power band.
120 to 180W/lb - more serious aerobatics, pattern flying, 3D and scale EDF jets.
180 to 200+W/lb - faster jets and anything that requires cloud-punching power!"


With this information, I can deduce that a plane like the Spitfire which has a recommended wattage of 350 would need an engine in the 0.18in range to achieve a relatively similar configuration. The closest brand new engine I found was the OS 25 FX engine http://www.osengines.com/engines-airplane/osmg0525/index.html. It produces about 500W, so it can run a 5LB plane fairly well. Subtract the weight of the engine itself and it leaves about 4.3lbs for the airframe and RC components.

Not to mention the plane body will need to be oil proof in order to withstand the exhaust from this engine.

My next goal is to probably pick up a couple of old used OS and K&B engines off eBay to do some tinkering. I've built enough motorcycle engines so its time to play around with the micro RC ones.

 

[flyin201]

I am very interested to see how a nitro engine will work on a foam board plane. To me I think a .25 engine sounds pretty big for a foam spitfire. A long time ago I built a Great Planes 'Slow Poke' http://www.greatplanes.com/discontinued/gpma0491.html which is a balsa slow flier. It's listed as a 2.5-3.5 lb model and it recommends a .10-.25 size motor. At the time I lived at 4,000ft elevation so I put an OS .25 motor on it and it had more than enough power. In fact it flew more like a Mustang than a slow flyer. I eventually installed an OS .15 motor and it still had enough power but flew much better in my opinion for an easy slow flying cruiser. I'm not really sure what the electric equivalent of those motors would be, but I think your multiply by 2000 method seems pretty close.

 

[Cherokee996]

I am too. I think what really sparked my interest in airplanes as a kid was seeing four F4U Corsairs do a flyby at our local airport during an airshow. I love the sound piston warbirds make when they fly real fast. Anyways, I'll probably need to build the airframe first before I actually go ahead and buy the engine. The next one down offered by OS is a 0.15. Based on your experience, that'll probably be more than enough power for a foam spitfire. I did some quick math after looking at the product page and a 0.25 engine would produce 500W of power, divided by the max weight of the airplane at 3.5 lbs yields approximately 142W/lb. That works out to be a pretty high performance airplane! The 0.15 engine gave you 83W/lb, which is more in the range of a park flyer. I'm thinking of maybe building a FT Tiny Trainer as well and possibly mounting a Cox 0.049 on it. I could go up to about 1.25 lbs and still remain in the flight envelope of a moderate park flyer.

Also, do you think it would be a feasible idea to print out the plans of the foam planes and then construct them out of balsa wood with reinforcement? I've been thinking about doing that for the Spitfire. It would probably be heavier, but I imagine a wood plane would have a much stronger airframe than its foam counterpart...or would it?

 

[Hai-Lee]

You could try removing the FB paper and coating the foam in 1mm Balsa and then cover using the usual balsa methodology.

It adds strength and with the covering film a measure of fuel proofing but adds little extra weight.

 

[flyin201]

I think you would want an airframe specifically designed for balsa from the get go. Foam and balsa are two different materials using very different build methods. Balsa construction needs a frame structure that is specifically designed to handle load and stress while using a minimum amount of material to keep it light. Foam on the other hand is much lighter and strong enough that you don't need a frame, you can simply build the external structure and the marterial is strong enough to work as long as it is not too big.

I think a better option would be to reinforce the foam structure with balsa and/or ply formers inside. For example I have used wooden dowels as spars inside of a foam wing. You could run a large balsa stick running from the motor mount to the tail and something similar for the wing spar.

If you want a completely balsa build I would look into a kit rather than converting a foam design, otherwise it is going to be a heavy and complex build IMO.

 

[Cherokee996]

Which one would be stronger?

Would a reinforced foam plane be stronger than a balsa one? Are there any good places like Flite Test where I could pick up a reasonable balsa kit?

Thanks

 

[flyin201]

It's hard to say; foam is certainly more durable but balsa when constructed properly is extremely strong structurally, just not as resistant to bumps and scrapes. There are plenty of good RC balsa kits out there, I would start with something easier to construct than a Spitfire. A spitfire is going to have very complex curves and building techniques that would be frustrating for the first time builder. Sig Mfg http://www.sigmfg.com/ has some great kits. The Sig Four star would be a great second plane after your Storch and it would be a good first time balsa build. It would also be one step closer to the flight characteristics of your Spitfire. They make them in four sizes, and the 20 size can be either electric powered or use a .15 - .25 glow motor. My brother has two of them, the .40 and .60 size and they both fly great.

 

[Cherokee996]

Interesting. I actually think I'm going to play it safe and build the tiny trainer first. I'll go ahead and paint over the body with some high quality clear spray that should theoretically protect the body from any of the nitro spray. I'll probably then go ahead and build two power pods, one with a small electric motor and one with the Cox 0.049 RC engine, that should give me a good 98 watts of power. If I can keep the weight down, it should be a decent amount of power.