Heli tail rotor drives

[David E]

I'm quite new to all this, and looking around at various helis, I see that some have belt driven tail rotors, some are shaft driven, and some have a motor on the end of the boom. What are the pros and cons of each type? (Hoping this won't start a war between different camps or anything!)

[Owen Hailey]

Hi David

 

I have had both tail motors and belt driven tail rotors, and both are prone to there difrent problems, motors burn out fairly easy, and belts need to be ajusted fairly often, but are quite reliable, but a lot of heli flyers say shaft is best as there is less to go rong, but there is more power loss with gears, and more noise comes from gears, so its down to what suits you at the end of the day.

 

the choice is yours Owen.

[Ant Sisley]

Motor driven tails are only found on small electric heli's. They are used normally because there is no room for a belt or shaft and, in the days of brushed motors, the main drive wasn't strong enough. The downside was you needed a mixer to drive both motors or alternatively two ESC's. A typical example would be the 4 in 1 module on the Hummingbird FP.

 

Belt drives tend to be cheaper to replace and have a higher survivability in the event of a crash. If you put a model in and bent the boom, there was a good chance the belt would be ok. Obviously a shaft drive would have to bend with the boom. Many helis up to 30/50 size utilise belt  drives, whereas 60 size  models and upwards would be shaft drive. This is probably down to the power output of 60/90 motors. However, there are a few 60/90 models around with belts as there are 400 size electrics with shafts. The main disadvantage with a belt is that it stetches requiring monitoring and tensioning.

 

The manufacturer will decide what drive is suitable for a particular model. For example, the Century Hawk sport 30 used to have a 2mm piano wire whereas the Falcon, Raven 3D had either a CF or aluminium tube. Thunder Tiger uses a belt on the Raptor30/50 which were generally regarded as training models and therefore expected to be crashed more often, so cost was a big issue. The Raptor 60 and 90 were then considered performance models and have shaft drives.

 

Personally I've always preferred shaft drive, but a belt works just as well.

 

Hope this helps.

 

[JohnH]

speaking from experience, the current Century Hawk Pro 30 has a shaft (or, more accurately a  tube) drive

[David E]

Interesting. So they don't really have any effect on handling or anything?

[Aero120]

There's pro's and cons for both systems. One is as good as another if it is well designed and appropiately maintained. Just as a belt can stretch under load, so a shaft can wind up. The belt is more efficient as it doesn't require the bevel gears at each end to turn the drive through 90 degrees and hence doesn't have these additional  frictional power losses. Its lighter too as there are four less gears and associated bearings and so more reliable as there are four less grubs screws to potentially vibrate loose!
However the belt has to handle the loads imposed and still be flexible enough to go round the small drive pullies to there is a finite load it can carry before it becomes too thick to bend easily. However a thicker shaft doesn't cause a proportionate increase in the frictional loses in the gears and bearings. So as stated above the larger models generally have a shaft drive to transmit enough power whereas smaller models can benefit from a lighter simpler design.

[Delta Whiskey]

Raptor 50 as a training model !!

 

I guess it is really stable if setup right, but isn't any model ............The Raptor 50 can equally do 3D as would the 30 I suspect, again if setup right as any model would have to be!

[Delta Whiskey]

I'd be interested in your reply Ant, as to why you stated the Raptor as a training model?

[Andy Dockerill]

David,

I’m no more than a novice myself and all the info above relates to belts and shafts. I think it’s worth pointing out that the electric motor tails are only able to spin one way(like belt and shaft tails) but as far as I know none have adjustable pitch on the tail rotor blades.

What this means is that unlike a belt or rotor driven tail that is able to adjust the pitch of the tail rotor blades to spin the heli nose left or nose right the electric tail rotors simply speed up more in relationship to the main rotor. This pushes the nose to the right. It cant push the nose to the left - it just slows down or stops and the torque of the main rotor turns the nose.

Does this really matter if you are just hovering around like me?? Probably not, but you will have much more control of the nose of the heli if it has variable pitch on the rear rotor.

Andy

[David E]

Interesting. And presumably that means that the maximum rate of yaw on an electric tail drive is going to be affected by throttle/CP setting than it would on other types?

[Andy Dockerill]

David,

Not really sure what you are asking.

The heli can be driven into nose right yaw because inputting right rudder will increase the tail rotor speed pushing the nose round. So the yaw rate is not directly linked to the main rotor speed in that sense.

Inputting left rudder does not push the nose round in the same sense. It reduces the tail rotor speed. The laws of physics then do their stuff and the torque from the main rotor pulls the nose to the left. So in a way I guess it is linked to the main rotor speed.

The piro rate( which I think is what the experts call this) can be adjusted on larger / more expensive helis but for these electric driven tail helis its set in the programming of the 4 in 1 or what ever the relevant heli has.

Any of the guys above with more experience who can explain this better??

Andy

[wlfk]

Interesting. And presumably that means that the maximum rate of yaw on an electric tail drive is going to be affected by throttle/CP setting than it would on other types?

 

If you put a 'more' or 'less' in that sentence, it would be easier to give a yes/no answer.

 

An electric motor spins one way by stopping, and spins the other direction by overcoming the torque of the main rotor. The ratio of the power of the electric motor to power of the main rotor is arbitrary and just depends on how the helicopter was designed. In practice, most motor-driven tails are found on low-performance helicopters and my limited experience is that their tail authority tends to be pretty marginal.

 

In contrast, a tube/belt driven tail has to have a collective pitch mechanism, and these tend to have both positive and negative pitch. So you can rotate in the 'passive' direction faster with such a tail than with a motor-driven tail. Unless of course the tail rotor can spin backwards, and as Andy noted most of them don't.

 

The issue here is that a collective pitch rotor needn't produce any torque at all - for example if you are descending or autorotating. In this situation, a motor-driven tail would only give you tail authority in one direction. Probably another reason you only tend see motor driven tails on collective pitch helis.

 

But the last and most significant difference is the amount of time it takes to change the tail torque on a fixed-pitch motor driven tail. To do this, you have to spin the tail rotor faster or slower, and this takes time. In contrast, a tube/belt driven tail keeps going at the same speed and just changes the angle of attack of the blades. This can be done much faster and makes a belt/tube tail much more responsive than one where the tail authority is regulated by a speed controller. There's no theoretical reason why you shouldn't use a collective-pitch tail on a motor-driven tail, but I haven't seen it done.

 

Finally, torque-tubes make less static than belts, which is a significant advantage in Arizona but less so in the UK.