Wow! It seems like it was just a few short years ago that I started flying RC. OK, it was actually 27 years ago, but who’s counting, right? I remember my first RC glow engine well: a Tower Hobbies .40 ABC engine that I mounted in a Tower Trainer .40. I was 14 or 15 years old, and thought I was on top of the RC world with my new plane! As the years flew by (No pun intended), my glow engines got larger and larger, until I bought a Zenoah G-26. Oh man! This was the coolest engine I had ever laid my hands on! I had a half-built Dynaflite Decathlon, and the G-26 was to be installed. Unfortunately, I had to sell the project and give up most of my RC modeling and flying for a couple of years while my kids were really little…
Fast forward a few years – I had just scored a new project airplane – a Lanier Giant Stinger airframe completely built. It was in rough shape, but I had to have it. At a local auction, I took the plunge and bought a used US Engines 41cc gas engine. Of course, I really didn’t know much about it at the time, but I learned a lot very quickly! I ended up spending more money to get that engine running that I paid for it at the auction.
Before I scare you off, let me say that the gas engines available now are MUCH better than they used to be. A lot of those old engines had a large flywheel and magneto ignition, making them difficult to start by hand. They were often engines converted from weed trimmers and chainsaws, and weren’t easy to mount or lightweight! So, what’s different today? Well, let’s dig in, and I’ll try to give you some pointers along the way!
Selection and Sizing
The first thing we need to talk about is engine type. Like glow engines, gas engines can be two stroke or four stroke in design. Four stroke ‘gassers’ tend to get expensive quickly in regards to size, but they are available. Unfortunately, because of their cost, I’ve been limited to two stroke engines. They have decreased in cost and weight, and increased in reliability and ease of starting. Another benefit is that gas is a LOT cheaper than glow fuel!
In the two stroke engine lineup, there are many brand names from which to choose. DA, DLE, RCGF, ZDZ, O.S., Evolution, GP, NGH, and AGM are some of the brands available. Just like transmitters and pickup trucks, everyone has their favorite. Most of them have prices that are pretty similar per size, with a few exceptions. When it comes to size, there’s been some advancement – it used to be that a 25cc gas engine was small. This is NOT the case any longer! There are now gas engines as small as the Evolution 8cc! Just so you don’t have to do the math, that’s nearly the same size as a .46 glow engine! Of course, on the other end of the spectrum, there are engines as large as the DA 200, DLE 222, and the Monstrous ZDZ 500!!! There’s most likely an engine to suit your needs!
Here’s a few of my RCGF engines. Top Row, L to R: 10cc, 15cc, 20cc, and 21cc Twin Cylinder. Bottom Row, L to R: 35cc Rear Exhaust, 40cc Twin Cylinder, 60cc Twin Cylinder, and 120cc Twin Cylinder
As you can see, these engines come with accessories – ignition module and spark plugs, mufflers, gaskets, and beam style engine mounts or radial stand-offs.
The ignition modules are separated by the number of cylinders they support, in this case a single or a twin. These modules are designed to be powered by 4.8 to 8.4 Volts – I have found that they run best on 6.0 Volts or higher.
Whether a single or twin cylinder, these engines usually include a muffler. They do some good to control the sound level coming from the engine, but to lower the level more, you’ll need to go to an aftermarket muffler. The best of these are the tuned pipe style mufflers. In some cases, the tuned pipes can also offer a slight boost in performance in addition to lowering the dB level!
The carburetor has a very important job. It mixes air and fuel to the proper ratio to burn inside the engine. There are different manufacturers of carburetors, but the most popular is Walbro. Walbro has many, many carburetors to fit nearly any small engine!
These carburetors usually have two removable covers – under one is the diaphragm fuel pump, and the other is the inlet/screen/bowl. The two work together, along with the high and low speed needles to provide fuel for the incoming air. When the fuel/air mixture is correct, the engine runs at peak performance! Learning to adjust the high and low speed needles is somewhat of an art – most manuals included with the engine will give you a ‘starting point’, but then it’s up to the modeler to fine tune for best performance.
The black plastic ‘block’ between the carburetor and the engine crankcase is an insulator. It’s job is two-fold. First, it keeps the hotter temperature of the crankcase from the cooler temp of the carburetor. Carburetors operate best at cooler temps. The block can also hold the reed valves – reed valves allow the fuel/air mixture into the crankcase, but not back out through the carburetor. The block on the left has a port in it – this port transfers the pressure inside the crankcase to the pump on the carburetor. These pulses between high and low pressure are what makes the fuel pump operate.
Most engines include either stand-offs or beam-style (black parts in far right photo) engine mounts. These provide clearance between the firewall and the engine. Now, these aren’t a one-size fits all, and you may need to modify the mounts for your particular engine installation.
Fuel Lines and Tank Preparation
Fuel lines – what do you need to know? Well, you can’t use the same fuel line that you’d use for a glow engine. Gasoline will make the silicone fuel line swell and get very soft. It is NOT suitable to use! For gas, you will need to use Tygon fuel line or a neoprene line. Tygon (yellow colored) is specifically meant for the gas/oil mix and can be purchase from many places. My favorite place to get Tygon fuel line is from DuBro. DuBro has several sizes and lengths available for purchase, and they carry a complete line of hardware and accessories for aircraft of all sizes! Fuel line retainers are recommended with gas engines due to the vibrations – you can see the fuel lines above have retainers on them in the form of wire clamps or zip ties. Also, you can see there are three different tank plugs shown. Their manufacturers have stated that they are all OK to use with gas. Brass or plastic tubes are both OK for use to pass through the tank plug, and you can also solder pre-made brass barbed rings on the brass tubing to help secure fuel line.
Once your tank has been assembled, it’s a good idea to label which line is which – you’ll be questioning yourself later if you don’t do it while it’s fresh in your mind!
When installing the Tygon fuel lines, I like to loop the vent line backward over the tank and then forward again before it exits the fuselage. This loop provides ample vent line to avoid spilling/losing fuel when the plane is inverted with a mostly full tank.
Also – Mounting the tank is important, but insulating the tank from the airframe is not quite as vital as with a glow engine. Gas doesn’t foam and bubble like glow fuel, so there is less worry about air bubbles in the fuel line.
Because most gas engine carburetors have a choke, you’ll need access to that choke. Depending on the airplane, you may have room to install a servo and pushrod inside the fuselage. This is a great option, but adds weight and requires another channel on your radio system.
A second option for rear mounted carburetors is to install a forward-facing pushrod. This pushrod exits the front of the cowl, and is pushed or pulled by hand. The key factor here is to add a ‘grip’ at the end of the rod. I do this by bending the end of the pushrod at a 90° angle. The support on the engine is an S547 – Flying Wire Steel Attachment from Sullivan.
Keep the line between the fuel tank and the carburetor as short as possible, without kinking the fuel line. This will be a benefit when you’re starting the engine for the first time. Shorter lines means a shorter distance for the fuel to travel as you’re hand starting the engine.
One final installation thought – ALWAYS USE A THREAD LOCKING COMPOUND! Gas engines produce a lot of vibrations that can loosen nuts and bolts, causing a potentially dangerous situation! To avoid this, I like to use ZAP brand Z-42 Blue Thread Locker, available at franktiano.com.
OK, OK, I know that these are all my RCGF engines, but these are the photos I had! At any rate, its a great depiction of properly installed engines, and the planes look good as well!
Running an Engine
When it comes time to start that engine for the first time, there are two distinct types of people…
A) Get the engine started, and break it in while flying.
B) Run a couple of tanks through the engine on the ground before getting airborne.
Option A gets you flying quickly, but always runs the risk of something going wrong in the air. If the engine runs smoothly right out of the box, that’s great! But, what if it doesn’t? I really don’t want to find a problem that way…
Personally, I prefer option B – running a couple of tanks through the engine on the ground with the needles set a little rich is assurance that the engine won’t die in the air. The other benefit is that running slightly rich on the ground should also give you a margin of safety for the engine leaning out in the air. Besides, it’s just a few ounces of gas anyway – not like burning that liquid gold known as glow fuel!
Wait a minute… I forgot about option C!
Yes, an engine break-in/test stand is always a good option as well. It’s also the SAFEST option! This engine stand, available from Stegallhobbies.com. The Heavy Duty Tripod Test Stand isn’t cheap, but it’s a great stand! The engine on the stand is the RCGF 60cc – Rear Exhaust. The stand handled the engine without any issues at all!
Tuning an engine can be a daunting task the first few times, but it’s not necessarily difficult. The most important thing to remember is to tune safely. You’ll need tools: a straight screw driver and a tachometer should cover most of it.
Start by reading the manual included with your engine. It should give you a start setting for the high and low speed needles. Using the screwdriver, close (turn to the right) the two needles until the just stop moving – DO NOT TIGHTEN THEM! Tightening the needles can cause damage to them. with the needles gently closed, open (turn to the left) them to the specified setting in the manual. I like to count the turns off half a turn at a time, as in “1/2 turn, 1 turn, 1-1/2 turns, 2 turns….” If you lose count, simply close the needle again and start over.
With the needles at the recommended start setting, start the engine. If it runs well, congratulations – you’re half way there! Let the engine warm up to operating temperature, and then throttle up the engine to full throttle. If the engine responds quickly without sputtering, you’re doing well. If the engine coughs and sputters and smokes a lot, lean the high speed needle about 1/16 to 1/8 of a turn. That’s the safety part – you should shut the engine off before making adjustments. If the engine hesitates then catches up, richen the high speed needle 1/16 to 1/8 of a turn. keep doing this until the engine runs smoothly. Now for the most important lesson of tuning –
ALWAYS SHUT OFF THE ENGINE WHEN ADJUSTING THE HIGH SPEED NEEDLE!!!!
Now, check the low speed needle. with the engine set to a decent idle speed (1000 – 1500 RPM is good), CAREFULLY lean (turn to the right) the low speed needle 1/16 to 1/8 of a turn. If the engine’s RPM increases, turn it another 1/16 to 1/8 turn. Keep doing this until the engine sputters and tries to stop running. When it sputters, open (turn to the left) the needle about 1/4 turn. This should set the low speed needle to the proper setting.
One more thing to consider. While gas engines are tuned less frequently than their glow engine counterparts, they will need to be re-tuned if there are large swings of temperature.
Well that’s about all I can think of right now – It’s been fun watching the transition of these RC engines over the years. The ease of operation, decreasing cost, and wide size range has made gas engines much more common at our local flying fields. I hope I have been able to help out some of the ‘new guys’ with some information on gassers – get out and grab one, and I’m sure you’ll enjoy these engines as much as I do! If you have any questions or comments, please leave them in the comments section. I’ll be happy to reply!
As always, From my shop to yours: Happy landings! -GB