Jet Engine RC Helicopters

RC Helicopter Direct Drive Turbines

RC Helicopter jet engines spin their axels at far to great a speed to directly hook up to the rotors, so the most obvious way to slow them down is a gear box. In the direct drive design, the turbine shaft is directly connected to a gear box, which slows down it’s rotation speed and powers the main rotors. This is the most obvious and simple design, but it has a few problems that have caused another, more sophisticated one to gain popularity. For example:

1. All the thrust produced by the jet engine is wasted. Because the only work done by the engine comes from it’s main turbine shaft, all the thrust from the engine is left unused. The thrust from the engine is usually channelled outside the RC helicopter.
2. The wasted thrust can lead to unwanted motion. RC jet engines produce a lot of thrust, and forcing it outside the RC helicopter creates unwanted forces that have to be cancelled out.
3. Direct drive jet RC helicopters are very inefficient. Only 20% of the total power produced by the rotor blades actually goes into turning the rotor blades.

With all the problems present in direct drive turbines, most jet RC helicopter enthusiasts are turning to another design:  two stage turbines.

RC Helicopter Two Stage Turbines

Two stage turbines use the thrust produced by the engine, instead of simply venting it as exhaust. Recall that a jet engine has a main turbine shaft, which is used to compress air and keep the engine running. A two stage turbine leaves the first shaft free to spin, and then adds an additional turbine to the end of the engine, where the exhaust is emitted and thrust is produced. The second turbine spins another shaft, which then drives the gearbox that powers the main rotors. What’s the benefit of using two turbines instead of just one? There are actually quite a few:

1. A two stage jet engine is about 80% more efficient than the direct drive design.
2. Less thrust is wasted, so there are fewer unwanted forces caused by venting it.
3. The engine has to carry less load. Overall, it’s better if the jet engine doesn’t have to directly power anything. Harnessing the thrust produced by the engine reduces vibrations and wear.

The only problems with the two stage design are that two stage turbines tend to be heavier than their direct drive counterparts, and they’re also a little more expensive.