Beginners Guide to Gas Engines

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I have put this thread together to help anyone trying to get into gas powered planes. You may have heard about them but are not sure where to start. You may be asking yourself why would I do it? What are the advantages and disadvantages? Who do I buy from? What manufacturer should I go with? What do I need to make it work right? If you are not asking yourself these questions, don't worry, I will give you lots of info that will bring up more questions than these.

If you have experience with glow engines, you are 50% there. A 2 cycle engine is a 2 cycle engine, no matter what fuel it is running. So what is the difference between Gas and Glow?

Let's start with the fuel differences. Glow fuel is made up of Alcohol, Nitromethane and oil. Alcohol (methanol) is the major combustable component by volume in glow fuel and has ~45% less energy per liter than gasoline, but is more volitile, so it can be more explosive than gas and more fuel can be injected because it has an air to fuel ratio ~55% lower than gas. Then when you take into account nitromethane, the nitromethane component of glow fuel increases it's explosive nature and increases available oxygen in the fuel because nitromethane has an oxidizer. Approximately 8.7 times more Nitro will burn in one piston stroke than gasoline. Nitro also has less energy per Liter than gas, but it is also very volitile, which means it burns more completely. Finally, Nitro has a higher combustion velocity, which makes it a perfect fuel for high rpms. What this all means is that gas is not as good for high RPM motors and makes lower peak horsepower, but it will have equivalent midrange torque and better fuel economy, approximately 9 times better for the same cubic inch 2 cycle motor. A CRRC-Pro 26cc V2 gas engine is rated for 2.42 hp at 9000rpm. A comparable glow engine, OS 160FX, is rated at 3.7 hp at 9,000rpm. CRRC-pro 26cc v2 is one of the lightest gas engines, but average on power. 26cc can go to 3.0+ hp.

Weight is another issue to consider. There is some debate on this, but glow motors are best used on small to medium planes and gas motors are best used on medium to large planes. The reason is weight. Gas engines are heavier than glow for a number of reasons, but mostly the added ignition system. The old conversion engines with magnetos were very heavy, but these days, manufacturers are building purposed gas engines for airplane use and using lightweight materials to minimize the weight penalty. The following information will show the general sizes of gas engines available today and the best size.

15cc gas =.91 glow
20cc gas = 1.20 glow
26cc gas = 1.40 glow
30cc gas (25% scale) = 1.60 glow
45cc gas = 2.00 glow
50cc gas (33% scale)
...

The smallest gas motor on the market is the RCGF 15cc engine, and it is a scale flyer on a 90 size plane. If it can be fit, it would be a rocket on a 60 size. There is no option for any smaller planes at this time. For comparison, a 26cc engine will weigh in at 1100g, where as an OS 160FX weighs 925g.

Before I go further, I really should have made the comparison of power between gas and glow engines with better engine examples. I will stick with the OS 160FX example for glow rated at 3.7 hp at 9,000rpm and weight of 925g. For gas, it is more realistic to compare a 30cc engine, so we will use a DLE 30cc, which is 3.7HP at 8,500 RPM and weight of 1030g.

Reliability is another factor to consider. Glow engines have a few things that make them wear out faster. Higher nitro content decreases engine life. Glow 2C (2 Cycle) engines run at higher RPM's, which means they will have a higher piston speed. Gas engines are usually run under load below 8,500 rpms for the smaller displacement engines. Now, I know you may be thinking that your nitro engine has lasted you X number of years and is still going strong. Think of the last time you saw a gardner with his backpack blower. He puts about 4 hours a day, 6-7 days a week on that thing, and it runs for 5 years+ with no issues (I know, I used to work landscaping). On a gas engine, it takes 20-30 hours of runtime just to fully break it in! It usually takes about 2 hours on a glow engine. Gas will last last longer if cared for properly.

Ease of Tuning is better for a Glow engine than Gas. The biggest reason is now you have an ignition system timing to take into consideration. Obviously, a nitro engine is pretty easy to tune. So is a gas engine, but the carb on a gas engine is more complicated, and different from glow in many ways. I will discuss tuning a gas engine later on.

Finally, economy of use. This is the single biggest reason for running a gas over glow. Gas engines are by far cheaper to run than glow. First reason is fuel prices. If you run your glow engine on cheap 15% nitro 18% oil fuel, it will cost you ~ (approximately) $22 minimum (excluding sale prices). An average 1.40 glow engine will consume approximately 3.3 oz of fuel per minute at WOT (wide open throttle). A gallon of liquid is 128 fluid ounces, that means at 100% WOT, said glow fuel will cost you ~$.57 per minute. A comparable gas engine is 26cc. A 26cc gas engine on 91 octane and synthetic 2 stoke oil will cost you ~$4 per gallon. At WOT, the average 26cc engine will consume ~1.4oz of fuel a minute, which converts to $.04 per minute. You gotta love it!

One thing though, gas engines are more expensive to buy initially and the parts needed to set up the plane cost more. I estimate 15-30% more for gas engines initially.

So I have explained the positives and negatives of Gas compared to Glow, so now I will go into choosing the right gas engine.

AVAILABLE GLOW ENGINES

There are quite a few manufactures on the market, especially with the introduction of cheap chineese engines. Some of the most notable engines are from Desert Aircraft (DA) and Mentor Motori (3M), but these are the most expensive and only available in 30% + size aircraft. DA is manufactured in the US, which makes it very desireable within the US. 3M is Italian.

With that said, I will assume you want to get into gas in smaller than giant scale aircraft, so we will discuss 22cc - 30cc engines. You will be best going with a known company with a reputation for service, especially if you are buying a chineese engine. Below are retail companies I have read or experienced positive feedback about.

• Paragon RC (Canada) - http://www.prcmodel.com/ - CRRC, SV, JC, DLE
• Valley View RC (US) - http://www.valleyviewrc.com/ - CRRC, DLE, MLD
• BP Hobbies (US) - http://www.bphobbies.com - RCGF
• Troy Built Models (US) - http://www.troybuiltmodels.com/ - DL, Aerovate/RCGF
• Cermark (US) - http://www.cermark.com/ - SPE, MLD
• Esprit Model - http://www.espritmodel.com - OS, Saito, Evolution, Zenoah
• Horizon Hobby (US) - http://www.horizonhobby.com - Zenoah

*JC makes engines for RCGF and Aerovate

Please also refer to the excel spreadsheet attached to this post for a comparison of different engines from different manufacturers to help you decide. It does not include Saito or Evolution engine, which are very pricey but excellent engines.

I have experience with CRRC-Pro gas engines, but many like the DLE-30 & RCGF. JC also look really good. If you want a 4 stroke gas, go with the Saito, but be ready to drop over $400.

Attachment: 02.03.2010_gas_engine_comparison.xls (18.0KB)

GAS ENGINE AIRFRAME & ELECTRONICS PREP

A gas engine has a few considerations beyond a glow powered plane to get it ready for flight. Gas planes vibrate considerably more than glow, so much so, care must be taken to prevent things from vibrating apart. I have already witnessed this in my own plane when my throttle servo stopped working in flight. It turns out the combinations of vibration and too short of throttle servo lead caused the wire to pull tight and break off the circut board in the servo. The servo literally vibrated apart.

First, the airframe must be prep'd. Mix tightbond (wood glue) with a bit of water so it will flow a bit better (not so much it is runny) and paint the glue on all the wood joints you can get to. Then, it is a good idea to look over the airframe and picture places of high stress. One of the biggest stress locations is in the motor box. A good practice is to "pin" the motor box where the sidewalls of the box meets the firewall, especially if the manufacturer used a tongue and groove joint. Drill small holes and glue dowels into the holes with wood glue and let dry. Then shave the dowels flat and it is almost like they are not there. You may also need to place some balsa triangles in the corners of some places. Remember, it is not uncommon for gas planes not prep'd properly to come back with the motor box barely hanging on.

While your at it, use some fuel resistent paint or 30 minute epoxy mixed with isopropyl alcohol to seal the airframe. Remember, gas planes have a much lower oil content and don't mess wood up like glow fuel. In fact, if you are using synthetic oil in your gas, the oil coming from the exhaust will be more like droplet sludge while the motor is breaking in. The exhaust oil is less likely to soak in the wood, which is a very good thing. Unfortunately, ammonia will not remove it (ie windex, etc), it takes a stronger solvent like an orange degreaser or other engine degreaser.

Also, consider the vibration damage to your servos. It is highly suggested you use metal gear servos with gas planes, as the vibrations will create slop in the gears in a short amount of time. You don't have to spend alot though. A 120 size plane or smaller will do fine with 120oz/in servos, and hobbyking has a line that is excellent. I use them in my Seagull Edge 540 60 with no issues, decent speed for a 3D plane, and moderately of light.

HXT 10kg Servo (metal gear) 55g / 10kg / .16sec
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=2

also carried through hobbypartz out of City of Industry, Ca

http://www.hobbypartz.com/servo-mg996r.html

Also, with those servos, you should get some aluminum servo arms. again, this is for the vibration, but most people run aluminum for the precise feel with no arm flex. This is not required in 90 size 3D and pattern planes. I run plastics that came with the HXT's and they work well. If you want to go aluminum, MPI is the best value and work really well.

Ball joints are highly recommended all around. Standard clevises will vibrate and slop all over the place, especially after things start wearing out (doesn't take long with plastic). 2-56 size is enough, but I have used 4-40 size for 3D just in case. The response with ball joints is phenominal. Don't forget, always secure the outside of the ball with a washer in case the arm pops off the ball.

It will be really importaint to get lots of foam for all your electronics and batteries.

You need to secure all connectors on your electronics. You can use the higher priced removeable connector like this:

http://www.espritmodel.com/index.asp?PageAction=VIEWPROD&ProdID=7940

or you can use heat shrink tubing around the connector, which works equally well.

Now, for the throttle cable. Quite possibly the most important part of prep'ing a gas engine airframe is making sure your electronics are isolated from electrical noise eminating from the ignition module. It is really easy if you know what to do. You can use a standard metal push rod or flex cable, a plastic push rod is not necessary (like the Dubro Laser series). For the connection to the engine, use a 2-56 PLASTIC ball joint. This will take care of everything. You can also use a plastic throttle servo arm, but this is not required, but it does add insurance against ignition noise. There is one other thing to protect against electrical noise, but we will cover that later, as it covers the kill switch we will install.

Finally, when you install your electronics, you MUST put high density foam (high density silicone foam is really good) around all electronics and other vibration sensitive units. These things include the ignition coil, batteries, RX, servo reversers, etc.

Alright, let's put the plane together.

INSTALLATION

I assume you know how to put a plane together per the instruction manual. I will focus on gas engine specific modifications.

1. Mount the engine. This is straight forward except one thing to consider, gas engines run the same in any orientation you install it, even upside-down. Most people install it inverted because it fits under the cowl the best.

2. Fuel tank and fuel lines. Fuel tanks are actually an easy thing to mount. Because gas engines have built in pumps that suck the fuel in, the fuel tank can be mounted in any orientation desired within the airframe, including moving the tank under the CG point. You can use any fuel tank you want, but you must change the tank plug to a gas fuel type: http://www.wholesaletrains.com/Detailair.asp?ID=200412395

A fuel line needs to be fitted to the tank as a breather line for it. The easiest and best thing to do is take about 2 feet, route it above the tank, loop 2 times around your 4 fingers, then Velcro the looped breather line to the top of the fuel tank. Then take the end of the breather line, route it to the outside of the plane, usually under the front landing gear or down the fixed landing gear exiting at the wheel. You want there to be a neutral pressure created in the tank.

Fuel lines CANNOT be silicone, like what is used on Glo. It will swell and separate when exposed to gasoline. Use Neoprene (opaque), or even better, Tygon tubing (transparent):http://www.wholesaletrains.com/Detailair.asp?ID=200303827

These clips are nice too. Yes you can use zip ties, but these are removable, work well and look good:http://www.wholesaletrains.com/Detailair.asp?ID=20015469

Don't forget a fuel filter. An inline fuel filter is mandatory as gas is not filtered as well as glo fuel. After 6 flights, I saw a noticed gunk in the filter, so it is a real concern. Gas carbs have zero tolerance for gunk in the fuel. Also, a filter in the gas tank is a good idea but not mandatory: http://www.wholesaletrains.com/Detailair.asp?ID=200454613

You will also need a gas capable fuel pump for filling the tank. Dubro makes a cheap hand crank one:http://www.shopatron.com/products/productdetail/part_number=911/101.0.1.1

For a gas tank, you can use any gas tank you want, even a cheapie one gallon like I got from home depot. You can get a fuel bottle fitting and adapt it to the tank: http://www.wholesaletrains.com/Detailair.asp?ID=20017194


3. Ignition Installation. The ignition module needs to be mounted a minimum 5" from the RX to be safe. 2.4GHz systems have little to no issues with ignition noise except some possible reported issues from a loose spark plug cap (not substantiated). The ignition noise is mostly outside the frequency of 2.4GHz. Mount the ignition as close as possible to the engine, usually on the outside or inside of the motor box. Make sure to wrap the ignition with high density foam. Ignition modules are vibration sensitive and do not like being mounted directly to wood. Zip ties work really well as mounts.

Now, you need to install the power to the ignition. The ignition and RX systems MUST be isolated from each other. You CANNOT use 1 battery for your ignition and RX, unless you use an expensive power module ($200+) from companies like 42% products, Smart-Fly or Fromeco. AMENDED: WRC Wike R/C makes a module to eliminate the ignition battery. It is cheaper to run 2 batteries, one for the RX and one for the ignition. Ignition modules from almost all companies require no more than 5.8v input. Anymore than 5.8v and they will fry. A few ignitions will accept A123 (6.6v full charge) batteries directly to the igition, but you need to check with your engine manufacturer. More voltage will not give more power, it is tested true. Lipos will never work directly connected.

To setup the ignition. you will need a battery (lipo, A123, NiMH), possibly a linear voltage regulator and a high capacity switch. To fly all day, ignition batteries need to have at least 1300 mAh on 2S lipo or 2S A123. 2200 mAh is required for 4.8v NiMH batteries. Voltage sag is a real issue to consider, so you must have enough capacity. If the ignition is required to stay at 4.8v, use a linear voltage regulator (LVR / BEC). Voltage regulators are also called BEC's. Ignitions will not work with switching regulators.

Any gas powered plane has a special consideration for a kill switch. A gas engine will not always shut down by closing the throttle, as there is a air bypass on all newer carbs. To guarantee a complete shutdown of the engine in failsafe or mechanical failure of the throttle, an electronic kill switch should be used for the safety of everyone. A consideration is that the AMA has on the books that gas engines at AMA competitions are required to have an kill switch installed. The kill switches to look for are Opto-kill switches. They use optic systems to isolate igition noise and shuts down the ignition if it detects the RX goes dead (RX power loss, reset, lost connection with TX, accidentally turn off your TX while the engine is running). A couple companies make them, but the cheapest is made by 42% Products but works great. The Opto-kill switch goes on the ignition side and RX side.

An excellent LVR is from 42% products: http://www.troybuiltmodels.com/items/42IGNREG.html
Heavy Duty Switch: http://www.troybuiltmodels.com/items/42SWITCHJR.html
Opto-Kill Switch: http://www.troybuiltmodels.com/items/42OPTOGASENGINEKILL.html

IGNITION WIRING:

Using Lipo or A123 batteries, you will connect the ignition it as follows:

ENGINE >> IGNITION >> KILL-SWITCH >> LVR / BEC >> SWITCH >> BATTERY #1

Using 4.8v NiMH and A123** batteries, you will connect the ignition it as follows:

ENGINE >> IGNITION >> KILL-SWITCH >> SWITCH >> BATTERY #1

** Only if manufacturer allows

RECEIVER (RX) WIRING:

Using Lipo, you will connect the ignition it as follows:

RX >> KILL-SWITCH >> BEC >> SWITCH >> BATTERY #2

Using NiMH and A123 batteries, you will connect the RX as follows:

RX >> KILL-SWITCH >> SWITCH >> BATTERY #2

REMEMBER: The Kill Switch is designed to bidge between the Ignition side and RX side.

LINKS TO MORE INFORMATION:

Excellent newbies to gas engines thread:
http://www.flyinggiants.com/forums/showthread.php?t=40091
&
http://www.flyinggiants.com/forums/showthread.php?t=39258

OK Engine breakin procedure thread, but be careful, a lot of flamers here:
http://www.flyinggiants.com/forums/showthread.php?t=23079

Good info about venting the gas tank:
http://www.flyinggiants.com/forums/showthread.php?t=48815