Helicopter electric tail rotors

[Oskar]

This thread is about an electric tail rotor conversion on a Mosquito helicopter.

Measurements on the conventional tail rotor showed that it was very power hungry, and calculations indicated that drone motors driving fixed pitch props would use a lot less power. The mechanically driven tail rotor was thus removed and replaced with a few battery powered electric motors. The design ended up with seven motors arranged like this, the large circle is the old tail rotor and the thick dark line the main rotor.


The obvious place to put the battery is close to the motors, thus at the end of the tail boom. That’s what I did, the battery shown gives about 30 minutes of endurance.



For tail rotor control the existing foot pedals are used. At some point pedal movement needs to be converted into an electric signal, this is done using a potentiometer which is then fed into a microprocessor.



The micro decides what signal should be sent to the tail (more about that later) and that then goes all the way to the back with a single twisted pair.



The control signal is sent to each motor controller. The motors and their controllers are standard off the shelf drone stuff, nothing fancy there.

The battery management system in the first picture keeps an eye on the battery and is used to accurately measure the tail rotor power. A comms wire still needs to be installed so that the tail rotor battery can be monitored while flying.

So far I’ve had the heli in the air with only 4 motors, it works but they are working reasonably hard. The last 3 motors have since arrived and been installed, so testing with 7 motors will happen soon.

 

[Greg Vos]

I enjoy inovation but I’m not sure I would trust this set up with a life if you were planing on going up with the chopper?
why not use one large electric motor as used in large scale RC aircraft? Leave the pack at the cg and run thick supply cables to the electronic speed control that needs to be close to the actual motor, for neatness these cables can be in the boom, one motor with a single speed control offers less to go wrong IMO


I do enjoy that you guys can tinker like this without the CAA interfering

 

[Oskar]

I do enjoy that you guys can tinker like this without the CAA interfering

The CAA registration ball is already rolling, it's marginally easier in NZ than ZA.

 

[Smack]

Oskar, that experiment is fabulous; thank you for the update.
It seems that the microprocessor sends the signal to all 7 tail rotors; can the 7 thus use just one motor controller instead of 1 controller/motor?
I agree with the use of multiple motors rather than Greg's 'one big one' thoughts for redundancy; maybe the same for the controllers or maybe pare down to just 2 tail rotor controllers?
Regarding lag time, have you noticed any control input-to-control response lag time compared to the old mechanical version?
What is the role of the microprocessor; it would seem that the potentiometer's signal could go straight to the tail rotor motor controllers?
Keep up the good work !!
Brian

 

[DennisFetters]

Igor Sikorsky was right.

An electrically powered tail rotor is much less efficient than a traditional shaft-driven and gearbox combination. To equal the full duration of flight time that a petrol-powered helicopter would offer, your battery or electrical generator would be far heavier than the traditional tail rotor system. Not to mention the ugly mess you have going on with all the motors and props back there.

There is absolutely no benefit to be gained, and in fact, is a giant step back...... in my humble opinion drawn from designing, building, and flying both manned and UAV helicopters for the past 30 years.

Hope it saves you some time.

 

[Oskar]

An electrically powered tail rotor is much less efficient than a traditional shaft-driven and gearbox combination.

Dennis,

I don't know how to measure efficiency of a tail rotor, but I have measured the actual power that the tail rotor consumes.

In the hover with AUW of 190kg the old mechancial tail rotor used 2.8kW. Four electric motors used 1.1kW, seven motors should use less than 1kW.

Maybe Igor Sikorsky used mechanical shafts and gearboxes because drone technology wasn't availalble in those days.

 

[hillberg]

You are only adding complexity and weight. Not saving anything as the power requirements do not change. You now add a very unsafe design feature.
The lack of heading control when drive line drag is evident during autorotation / cross wind landings as those props are fixed pitch.

You can't thrust with the torque wise direction during power off events Maybe Igor used and designed things that worked ?

You only added a dangerous feature on an otherwise system that worked.

 

[Oskar]

why not use one large electric motor as used in large scale RC aircraft?

Greg,

That was what I originally wanted to do. There are two options when using a single motor, either variable pitch or fixed pitch.

What I first wanted to do is use the existing variable pitch tail rotor and drive it with a single motor replacing the rear gearbox. Searched for weeks but couldn’t find a suitable motor with anything close to the correct speed, power and weight requirements.

The other option is fixed pitch which gives more freedom. Again I searched for weeks, smaller and bigger motors were freely available but there’s a gap around the 2 to 10kW range with very few options. Just couldn’t find anything suitable.

As soon as you divide the numbers by 7 the world opens up as now you’re sitting in drone world. It’s still the top end for drones, which until a few years ago wasn’t freely available but now is.

Looking back I now firmly believe that multiple motors is the best way to go for two reasons, firstly redundancy and secondly dynamics. But more about that later.

 

[DennisFetters]

Dennis,

I don't know how to measure efficiency of a tail rotor, but I have measured the actual power that the tail rotor consumes.

In the hover with AUW of 190kg the old mechancial tail rotor used 2.8kW. Four electric motors used 1.1kW, seven motors should use less than 1kW.

Maybe Igor Sikorsky used mechanical shafts and gearboxes because drone technology wasn't availalble in those days.

On a correctly designed conventional single main-rotor and tail-rotor helicopter will typically use 15% of overall engine power for anti-torque for normal operations. But, you need to design the capability of using up to 25% total power for anti-torque for adverse conditions or flying at gross weight.

Maybe Sikorsky didn't have experience with drone technology, but I do.

Nothing wrong with experimenting, even only to end up reinforcing the original concepts were right.

 

[Oskar]

You are only adding complexity and weight.

Complexity yes, weight no.

The original tail rotor and rear tail rotor gearbox weigh about the same as what was added to the tail, and that includes the weight of the battery. Once the front gearbox is removed the total weight of the electric tail rotor (including battery) will be much less than the weight of the mechanical tail rotor it replaced.

 

[Oskar]

On a correctly designed conventional single main-rotor and tail-rotor helicopter will typically use 15% of overall engine power for anti-torque for normal operations.

That's exactly what I measured on the original tail.

In the video below though the main rotor was using 19kW and the tail 1.1kW which works out to close to 5% of total power.

 

[Martin W.]

Igor Sikorsky was right.

An electrically powered tail rotor is much less efficient than a traditional shaft-driven and gearbox combination. To equal the full duration of flight time that a petrol-powered helicopter would offer, your battery or electrical generator would be far heavier than the traditional tail rotor system. Not to mention the ugly mess you have going on with all the motors and props back there.

There is absolutely no benefit to be gained, and in fact, is a giant step back...... in my humble opinion drawn from designing, building, and flying both manned and UAV helicopters for the past 30 years.

Hope it saves you some time.

Bell is giving it a try .... however they may not go public with the results .... unless it is really successful then I suppose they will build some.

Bit more info here . https://www.rotaryforum.com/threads/power-needed-to-spin-rotors.1145537/page-3

 

[hillberg]

Complexity yes, weight no.

The original tail rotor and rear tail rotor gearbox weigh about the same as what was added to the tail, and that includes the weight of the battery. Once the front gearbox is removed the total weight of the electric tail rotor (including battery) will be much less than the weight of the mechanical tail rotor it replaced.

Mo problems and less capability

 

[grevis]

Good progress Oskar. For the potentiometer that picks up the pedal movements have you built in some redundancy there (like multiple pots and let a micro compare signals to detect faults). Also hall effect sensors may be a bit more reliable.
Gary

 

[grevis]

As your testing progresses it will also be interesting to see if the multiple rotor idea works better when hovering side on to wind, it might suffer less from vortex rings which could be another positive.
Gary

 

[hillberg]

Bell is giving it a try .... however they may not go public with the results .... unless it is really successful then I suppose they will build some.

Bit more info here . https://www.rotaryforum.com/threads/power-needed-to-spin-rotors.1145537/page-3

Bell only did it to lock up a patent on electric t/r systems - Like they did with the electric powered S 300 it's not about technology, Just the $$$$

 

[Smack]

Mo problems and less capability

Yeah, mechanically-driven tail rotors never fail......

 

[Oskar]

Good progress Oskar. For the potentiometer that picks up the pedal movements have you built in some redundancy there (like multiple pots and let a micro compare signals to detect faults). Also hall effect sensors may be a bit more reliable.
Gary

Redundancy is definitely high up on the priority list.

Working backwards there is already redundancy with the motors and controllers. Even if three motors or controllers fail there will still be enough tail rotor authority left to hover.

Next is the battery. At the moment I only have one battery, but it will be very easy to install two smaller batteries and run some motors on one battery and the rest on the other battery.

Next is the information transfer from front to back. The overall controller, main drive controller and battery management system already run on a CAN bus, which is what all modern cars and many aircraft use (including the new Rotax 91X iS engines). The same CAN bus could be used to send required thrust information to the tail in addition to what I'm using now.

At the moment the overall controller is a single point of failure. For redundancy a second micro fed from a second pot would be the answer. But for now there are too many things above that on the list of things to do.

 

[hillberg]

Yeah, mechanically-driven tail rotors never fail......

40+ years doing it commercially no T/R failures here. Engine failures - See some stuffed in the ground, through the reeds no issues -They work.

1st autorotation with a cross wind with fixed pitch electrics and you'll land with no yaw control (thrust) where you need it.

 

[DennisFetters]

40+ years doing it commercially no T/R failures here. Engine failures - See some stuffed in the ground, through the reeds no issues -They work.

1st autorotation with a cross wind with fixed pitch electrics and you'll land with no yaw control (thrust) where you need it.

In my 35 years of flying and building helicopters, I have to say that I have had one tail rotor failure. It was on my first Mini-500 prototype, and I was trying a new-fangled hydraulic cylinder for anti-torque pitch control that I had built.

I was hovering it down the runway and the exhaust system heated the thermally protected hydraulic line and it bubbled out, losing pressure and control. I spun in 3 circles and sat it on the runway still spinning about another turn and a half.

I had that junk yanked out by the roots, and before the day was over, I was flying the new push-pull cable that was in every Mini-500 after that. Never looked back.

You can see in the video below how much punishment the tail rotor, gearbox, and shaft can take and still fly.