Many times I see people from the airplane fraternity trying to use an airplane engine in their R/C heli or doing airplane engine mods to their heli engines or trying exhaust systems on their heli engines that work well on their airplane engines. In almost every instance, they are left disappointed. Why? Because there is a significant difference to how a heli engine operates and how an airplane engine does. This requires differences in how they are designed.
The obvious questions therefore are:
1 - What are the difference between R/C model airplane engines and R/C model Helicopter engines?
2 - What requires these differences?
Of course, one of the obvious differences will be the big heat sink head that one finds on the heli engines, compared to the smaller one on the airplane version. As all the power in the airplane engine is used to turn a fixed pitch propeller. This propeller is not only pulling the airplane forward but is also moving a lot of air backwards over the engine's cylinder head to provide cooling. On the other hand, in a helicopter, most of the power is used to turn the main rotor which has a constantly changing pitch (load) profile. As little of the air moved by it can find its' way over the heli engine's cylinder head, a small, relatively efficient fan and shroud will be used to do so. Consequently a head with cooling fins that can take advantage of this will be provided. Of note is that some of the engine's heat can also be dissipated through the heat sink/transfer effects of a metal motor mount and a lower metal frame.
The second most noticeable difference is that most heli-engines do not come with mufflers and most airplane engines do. The reason most heli engine manufacturers do not provide mufflers for their engines is not a coincidence. It is not only because it would take different flange mountings, but mostly because of how differently a heli engine is required to "breath". This results in a different optimum chamber volume and different baffle locations from those for their airplane version.
How a heli engine breaths is also why they require a different and more precisely machined carburator. This is the one main component that low cost engine manufactures, with inadequate experience in heli engine, get wrong.
What makes heli engines breath differently? They are required to provide widely different amounts of power without changing the rpm at which they are operating. Consequently , the system that controls their inflow of fuel (carburator) and their system that controls the expulsion of the burned fuels (exhaust) must be different.
In a hover they will only be required to deliver enough power to suspend the weight of the heli in a stationary position at a desired rotor speed (e.g.1850 rpm.). Due to the gearing of the helicopter, this rotor speed will dictate a specific engine rpm. On the other hand in a full power climb, they will then be required to produce their maximum power at the same rpm. As you can see, the power requirement may have doubled, but the rpm stays the same.
The requirement for a constant rotor speed is the reason a heli radio has throttle curves, pitch curves and other throttle mixes. A constant rotor speed results in a constant and predictable collective and cyclic response. This is why we should also use cyclic to throttle mixing to maintain rotor speed through manoeuvres that require heavy cyclic commands. This is also why we use a tail rotor (rudder) to throttle mix to maintain a constant rotor speed through pirouetting manoeuvres. Any heli with all of these set up properly will fly a quantum leap better then one that is not. One that is not, will always have the poor engine struggling to keep up with the power demands, resulting in poor collective and cyclic response. This is why so many flyers feel they need more power. Many times it is more an issue of setting up proper curves and throttle mixes then the need for a different engine, more nitro or a different exhaust system.
How does this relate to what is happening in the engine? We know the constant rotor speed will demand a constant engine speed, simply because the gear ratio cannot be changed in flight. So if we take a helicopter that has a 9:1 gear ratio and combine that with a desired head speed of 1850, we will find that the engine speed will have to remain constant at 16,550. In the heli, we will have it set up so that from about 1/3 throttle barrel opening on up to a full throttle barrel opening, the engine will maintain 16,650 rpm.
Now let us take this engine and put it in a direct drive situation, such as it is in an airplane with a fixed pitch prop. At full throttle it may rev to 16,650 and also turn that prop at 16,650. On the other hand, if you close the carburettor to 1/3 it would only rev to about 7,000 or so. There is a substantial difference in the fuel draw capability of the engine when it is turning at 16,650 compared to at 7,000. But this is all fine because less fuel is required to produce the power required to turn that prop at 7,000 then at 16,650. Less fuel draw, less fuel, less power, less power required.
As the fuel draw of an engine is directly related to the RPM that it is turning, one can see that in the helicopter, as the power requirement and air supply provided by the opening of the carb barrel changes, the fuel draw stays the same - very high. On the other hand, in the airplane, as the power requirement and air supply changes, so does the fuel draw. It drops from that of an engine turning at 16,650, to one that is turning at only 7,000. In one. the fuel draw stays high. even as the power requirement and air supply drops, in the other, as the power requirement and air supply drops so does the fuel draw of the engine.
This constantly high fuel draw characteristic of an engine in a helicopter requires a far different and more sophisticated needle valve system. These needles have to precisely provide the correct fuel/air mixture to a system with an unchanging high fuel draw, but with a constantly changing air supply and power requirement.
On the exhaust side, we also know that exhaust back pressure changes with RPM and the amount of burned fuel being expelled. Here we now see how the amount of burned fuel per minute will constantly be changing, while the engines capacity to purge itself of burned fuel (engine rpm) stays constant. This is very different then what happens in an airplane engine. Consequently, it also requires a different exhaust system to do so properly and consistently at all throttle/power settings.
Because an airplane engine is usually operating in the 11,000 rpm range and these rpm's fluctuate substantially during a flight, the wear on it's components is substantially less. Let us assume the average rpm over a 10 minute flight would be 8,000. That means the engine bearings and piston/cylinder moved through 8000 x 10 = 80,000 cycles. On the other hand, the heli engine was up around 16,650 rpm for the full 10 minutes. That means it went through 166,500 cycles in that time! More then double that of the airplane engine. Needless to say, the wear factor and heat generation here is far greater. In fact more then 4 times greater as doubling rpm actually increases loads by 4 times or more, not just two times.
As you can see, what may look simple on the surface, can be much more complex when taken in detail. There is a lot more involved in our heli engines then meets the untrained eye. Hopefully now you have a better understanding of these little gems and can better understand why those that are user friendly, develop good power, and last a long time, may well cost substantially more then their airplane sibling or the less advanced, low cost heli engines that do not.