Two stage turbines and direct drive turbines – the two ways model jet engines convert thrust into horse power and torque.
After learning all about how model jet engines work , we now know they spin very fast and produce lots of thrust.
This is fine for a RC jet powered airplane that relies on thrust to push it through the air, but a RC helicopter needs to spin a rotor - we need rotational horse power called torque.
The first method of converting turbine thrust power into rotational power for a RC turbine helicopter is to extend the turbine/compressor shaft out the rear or front of the engine and attach it to a gear reduction box or series of reduction gears & belts (as shown below).
The gear reduction then reduces the turbine shaft speed of
around 100,000 - 150,000 RPM by at least a factor of ten for a more usable speed most main gear pinions spin at (in the range of 15,000 RPM).
This is the least expensive way to convert turbine power to rotational horse power, but there are some problems with this method.
First, all the thrust that the turbine produces is wasted since it is simply just blown out the exhaust nozzle/s. This wasted thrust also can push your helicopter around so you have to use up even more power to compensate for the push of the thrust.
This leads to the biggest problem of direct drive turbines, they are very inefficient. Most of the power (about 80%) is just blown off. Shorter flight times and/or larger fuel tanks are the end result.
Direct drive turbines are on the way out as far as I am
concerned. The top model turbine manufacturers have all started getting away
from direct drive and offer 2 stage turbines now.
A two stage turbine is exactly what is used on real helicopters and real turboprop airplanes. Instead of wasting all that thrust, it is used to turn another set of turbine blades mounted right behind the turbine blades in the jet engine.
In essence, the jet engine's sole purpose when combined with a second stage turbine is to act as a high volume air pump or more correctly called a "gas generator". At any rate, these two turbines (the engine's and the second stage's) are not connected to each other with a shaft – they are completely independent of one another.
Think of it like two fans facing each other. If one fan is turned on, it causes the other fan to spin. For you car guys and gals out there, this is the same principle that is used in an automatic transmission torque converter.
The secondary turbine converts most of that thrust back into rotational power and is hooked up to a reduction gear box to reduce the speed down to a usable level – about 15,000 rpm for RC helicopters. The secondary turbine doesn’t spin as fast as the primary turbine in the jet engine. This reduces the amount of heat and wear in the reduction gear box.
The above picture shows a two stage MW44 Wren model jet helicopter engine. Notice the collar that is bolted onto the back of the jet engine. This is where the secondary turbine is located. Bolted onto the back of this collar is the jet exhaust diverter. Behind the diverter, you can see the gear reduction box. This is a 90 degree gear box that allows for the more conventional "vertical" engine shaft placement that is used on most RC helicopters to drive the main gear.
The picture below shows the individual components of the second stage.
Inside the gear box below...
This method of capturing the thrust to spin another turbine increases efficiently tremendously. A model helicopter two stage turbine engine is about 80% more efficient than a direct drive turbine engine.
By using up most of this thrust/energy to spin another turbine, there is much less thrust force exiting the exhaust nozzle/s and the effects of jet exhaust pushing the heli around is greatly reduced. In fact, the air flow out the exhaust nozzles is not much greater than what you would feel out of a hair dryer - albeit much hotter ;-)
The other big benefit is you are not loading down the engine as you would with the direct drive method. There is no mechanical connection between the engine and the second stage/gear box. This makes for a much smoother running model turbine engine, better engine management, better speed control, longer bearing life, reduced parts count, and reduced vibrations.
Of course there is a bit more weight with a two stage turbine and
it is more costly. These are insignificant concerns next to the many
advantages 2 stage turbines provide IMO and why I personally chose to fly with a two stager over a direct drive.