Nitro RC helicopters, also known as "Glow Powered" RC helicopters are still popular and serious contenders with electric RC helis. Why?
Because they are tough and can take a beating, they give good power to weight ratios, they are the lowest cost option for a fuel powered bird, and they have proven themselves time and time again. The pros and cons of nitro RC helis are explained on the Best RC Helicopter Power page.
The simple fact is without the model nitro engine (also called a glow engine), it is doubtful that RC airplanes and helicopters would have taken off – literally.
These little engines produce so much power for their size and weight, it's little surprise that nitro RC helicopters are still the number one choice for so many people.
If you are familiar with large electric RC helicopters, the basic layout and components in a Nitro heli are really not much different. The main component differences mostly revolve around the engine :-)
These include the engine itself, starting shaft, centrifugal clutch, a cooling fan and shroud, the muffler/tuned exhaust, the fuel tank, fuel tubing, an extra servo to control the carburetor, and perhaps a governor to govern engine speed (the carburetor/throttle servo is controlled by the governor). There might also be an on board glow driver if an external glow driver is not used.
Let’s first clear something up when it comes to nitro RC helicopters, more specifically nitro engines in general.
Many people confuse or call these gas RC helicopters – this is not correct.
Gas RC helicopters use 2 cycle gasoline engines, similar to the ones used in small gas powered chain saws or gas leaf blowers.
Nitro engines or glow engines are completely different. They use special fuel consisting of (methanol, nitro methane, and oil) and as the name "glow" suggests, they don’t use a spark plug - a glow plug is used. Lastly, nitro/glow engines run at much higher RPM's – usually in the 13,000 – 16,000 RPM range.
In order to build and fly a nitro RC helicopter, you have to be very familiar with how nitro engines work. The following items are some basic terms & items you'll need to understand when operating a glow engine.
Most people will only use 2 stroke nitro engines in their helicopter because of the impressive power to weight ratio. There are still a few 4 stroke nitro engines out there so you should at least know a bit about them.
The 4 stroke heli nitro engine has been around for about 35 years now. They run at a lower RPM and thus produce less noise. It looked like these would become popular for R/C helicopter use back in the day; but the added weight, extra cost, and reduced power has kept them pretty much under the radar, and now a days you hardly ever see one. OS discontinued their 4 stroke heli engine lineup a number of years back and there is no reason to think they will ever return.
There is nothing wrong with the 4-stroke nitro engine, the 2-stroke simply produces more power and doesn’t cost as much making it the preferred and more robust choice with less moving parts inside.
Unique Nitro Engine Requirements
Now that you know a bit about nitro engines, let’s examine how they apply to nitro helicopters. But before getting into that boring stuff, here's a video of a nitro powered T-Rex 700N DFC in action...
The RC helicopter relies on its engine more than any other form of RC model. RC planes can glide in for a landing, RC cars and trucks simply coast to a stop and RC boats will continue to float if their engines quit. Helicopters on the other hand depend on engine power to stay in the air.
Yes there are auto rotations, but when you are learning, an engine flame out will pretty much ruin your day and lighten you wallet.
This dependence on reliable power is key to understanding the unique ways nitro engine set-ups in RC helicopters differ from any other RC model.
What about the nitro fuel tank ? Simple right... This is a huge topic on its own, getting it wrong ensures helicopter flying and engine problems. Speaking of fuel, what about all that messy oil residue.
When Starting A Nitro RC Helicopter. The blades won't spool up until the engine speeds up and the centrifugal clutch engages.
"How does the Clutch Work?"
I get this question often enough I thought I better address it.
Here is typical centrifugal RC helicopter clutch with the starter shaft attached to it. The clutch is attached to the rotating face of the cooling fan which is attached to the nitro or gas engine output shaft. In other words, whenever the engine is spinning, the clutch is also spinning. As the engine speeds up, centrifugal force cause the ears of the clutch (circled in red) to spread outward.
The centrifugal clutch is rotating inside the clutch bell seen here. When the ears of the clutch spread out as engine speed increases, they start rubbing on the clutch liner circling the inside of the clutch bell, and the clutch bell slowly starts rotating along with the clutch. Once up to enough speed, the ears on the clutch are spread out with such a large amount of centrifugal force, the clutch and clutch bell are more or less locked together and rotating as one unit. The clutch bell has the pinion gear on the top of it and that is what in turn spins the main gear on the rotor shaft.
It is therefore very important to always start a nitro RC helicoper with the throttle stick in the idle position so the engine doesn't start at a fast speed. If it starts at speed, the clutch will engauge almost the instant the engine fires up, causing you some exciting moments as you go fumbling for the radio with one hand while holding the rotor head with great force to keep it from spooling up in your other hand. While all this is happening, the clutch ears are burning and polishing the liner inside the clutch bell... Nothing quite like the smell of burnt nitro RC Helicopter clutch on top of all the other drama going on - yummy ;-)
The photo below shows how the clutch assembly, engine, gears, and starter hub are positioned. Normally you don't see much of this from the clutch bell downward, but I have removed the cooling fan shrouding for this photo.