Gas Engines for RC Planes
Gas (petrol) engines power large RC planes, typically from about 20 cc to well over 100 cc. They run on petrol mixed with two-stroke oil, fire with an electronic ignition, and are cheap to run for long flights. This guide explains how they work, how to size one, installation and safety essentials, and the maintenance that keeps them reliable.
By Kyle HilsonUpdated: October 20269 min read
How a gas engine works
Most RC gas engines are air-cooled two-strokes, with a spark plug fired by a separate electronic ignition module (CDI) that has its own small battery. A carburettor, often a Walbro-type pumper, mixes petrol and two-stroke oil with air; a high-speed and low-speed needle adjust the mixture. Twin-cylinder and four-stroke petrol engines also exist for scale models where sound and smoothness matter. Well-known engine makers include DLE, DA (Desert Aircraft), 3W, Zenoah and NGH.
Gas, glow or electric
| Gas (petrol) | Glow (nitro) | Electric | |
|---|---|---|---|
| Typical size | 20–150 cc and more | Small to medium planes | All sizes |
| Fuel cost | Low | High | Battery packs; costly in large sizes |
| Run time | Long; refuel in minutes | Medium | Limited by battery |
| Mess and noise | Some exhaust, loud | Oily exhaust, loud | Clean, quiet |
| Extra parts | Ignition battery, kill switch | Glow starter | Large LiPos, chargers |
For large planes, gas is usually the cheapest way to fly many flights per day. For the electric side, see sizing an electric motor.
Sizing a gas engine
Plane designers usually state an engine range, for example “50–60 cc”. Stay within it. As a rough orientation, many 2.0–2.2 m aerobatic planes are designed for 50–60 cc engines and 2.5–2.7 m planes for around 100–120 cc. Heavier scale models and those for 3D flying sit at the top of the range. Check the engine’s weight too: it sets the balance, and the wrong engine can make a plane impossible to balance without lead.
Installation
- Firewall and standoffs: mount the engine on a solid firewall with standoffs, using the right thrust angles from the plans.
- Ignition away from the receiver: keep the ignition module and its leads as far from the receiver and its antennas as possible; ignition noise can interfere with the radio.
- Separate batteries: one for the ignition, one (or two) for the receiver and servos.
- Fuel system: a petrol-proof tank with Tygon or Viton tubing and a clunk, plus a fuel dot for filling.
- Cooling: air must enter in front of the cylinder and leave behind it; outlets should be larger than inlets.
- Kill switch: an ignition kill switch reachable from outside, and often a radio-controlled ignition cut on a transmitter switch.
Fuel, break-in and tuning
- Fuel mix: use the petrol-to-oil ratio the engine maker specifies, with a good two-stroke oil, mixed fresh.
- Break-in: follow the manual; typically several tanks run on the ground or in gentle flights, slightly rich, varying the throttle.
- Tuning: set the high-speed needle for peak rpm, then back it off slightly rich. Then set idle and transition so the engine picks up cleanly from idle to full throttle.
- Propeller: start with the prop size the manual recommends; check rpm with a tachometer.
Safety
Safety: a large gas engine’s propeller can cause severe injuries. Restrain the plane or have a helper hold it from behind, never start or adjust the engine from in front of the prop, keep spectators behind the prop line, and make sure the kill switch works before every start. Clubs often have specific rules for large gas planes.
Large planes may also fall into stricter registration or weight categories in some countries; see the rules section of the RC airplanes guide.
FAQ
What fuel does an RC gas engine use?
Regular petrol mixed with two-stroke oil at the ratio the engine maker specifies. Not glow fuel.
What size gas engine do I need?
Use the range the plane’s designer gives. Roughly, 2.0–2.2 m aerobatic planes take 50–60 cc, and 2.5–2.7 m planes around 100–120 cc.
Why does my gas engine die at idle?
Usually the low-speed needle is too lean or too rich, the idle is set too low, or there is an air leak in the fuel line. Adjust in small steps and check the fuel system.
Can I convert a glow engine to petrol?
Conversion kits exist for some engines, but they need an ignition system and changes to the carburettor. For most people a purpose-built gas engine is simpler and more reliable.
More on this topic: other power types, building and learning to fly are covered in the complete RC airplanes guide.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
