The big problem I see is that in autorotation the main rotor still drives the tail rotor in perfect synchronisation. That won't work for an electric driven TR.

I agree that we're getting there, in terms of fly by wire, and criticality of electric components. However, when an electric motor replaces the very commonly accepted tail rotor driveshaft and gearboxes, the criticality will be perceived at a higher level. Let alone the unusual method of control (if it's even practical), the reliability of the motor system will have to be demonstrated to a new level. Such a design initiative will be labeled as novel and unusual, and held to an unusually vigorous standard. The present motors and generators (FBW notwithstanding) are certified as secondary systems whose failure can be managed by procedure. I perceive that investors would ask why they should invest an immense amount of money to certify a novel system which really only trades known and understood problems for unknown problems, without really solving any problems.

The project I was hired to advance toward certification for a motor powered 172 considered a purpose built 150HP electric motor. Though I saw designs and detailed drawings, the project never got to the point of producing a motor for installation (they took a lot of measurements though!). It was to be about 10% heavier than the Lycoming O-320 it would replace. The battery pack was a bit more of a challenge, though not insurmountable. Certification had a path forward with the authority, I had a number of discussions as to the proposed certification basis, and general agreement. That was doable - but it was a single engine airplane, where the failure mode was no worse (and really not much different) that the original design. I am confident this will happen for airplanes, it just requires a meeting of battery capacity, and airplane utility. It costs too much to keep a training airplane offline for hours to recharge it, and changing out very heavy batteries discharged for charged is problematic. During the planning of the 172 project, I did tell my client that they should install the motor as the primary power source in an R22, and not carry batteries, just a long power cord to the corner of the apron. Of course, you couldn't fly the R22 anywhere that way, but we spend a lot of time simply practicing hovering, so it could simply be a hovering trainer, which never gets higher than ten feet, nor leaves the apron. People liked the concept, but we did not get that far. Someone will.....

In the mean time, I'm very comfortable with shaft driven tail rotors/fans, we have more pressing product improvements to work on.

P D
KISS Just on or off constant rpm with a servo controlled FBW variable pitch rotor should do it.

The word "just" is doing a heck of a lot of work in that sentence.

I don't think you understand the complexities inherent in a "FBW variable pitch rotor", of which only one - but a big one - is safety critical software and electronic hardware.

Ok perhaps "just" a pushrod then...

Or a constant (to the main transmission) RPM, pilot controlled (pushrod, no FBW) variable pitch rotor - even more simple!

Yeah, that word "just"... It always makes my ears perk up when it's not accompanied with a comprehensive plan for certification. That's because I'm one of the people who may be asked to sign a certificate approving it later, and that does not happen lightly! It's great to innovate and aspire to new technology. But when doing that pushes the thinking of aircraft certification, it's a long and expensive process of demonstration of design compliance - or worse, petitioning for a change in the design standards to enable certification of an aircraft with novel features. The Bell XV-15 was the poster child for having to evolve design standards which were outside the box, and that is still a driveshaft type design!

It may not be the daftest idea. A quick google throws up this:

Helicopter Electric Tail Rotor

Motors for model aircraft will often manage about 6-7 horsepower/Kg so if an R22 has a 120hP engine you might want a 30 horsepower motor - so 6kg for the motor and 6kg for the generator and a kilo or two for the control electronics. You would considerably simplify the gearbox and be able to do without an alternator. You might want to play with adding a small battery - large enough to transition from forward flight to the hover, then land, for added reliability and perhaps to give a little extra oomph for take off. On the back of an envelope it's feasible.

If you wanted to use fixed pitch tailrotors to reduce mechanical complexity you'd have to make them small and light in order to be responsive enough so you may wish to use several small rotors rather than one large one.

However even in electric radio controlled helicopters it's normal to link the main rotor and tail rotor mechanically and separate motors are only rarely used. The exception would be for very small models with constant pitch tail rotors where the complexity of making tiny variable pitch rotors would be prohibitive.

PD
I understand exactly. However in the current situation there is no option to disconnect drive in event of stuck / full un commanded pedal. An electric option could do this.

Funny this should come up. During my training, I asked my instructor how often a helicopter suffered a stuck pedal. He said he'd never heard of it in 21,000 hours of his flying. I asked then why so much focus on yaw control failures. He said he'd asked the same question during instructor training, and why no training for stuck cyclic or collective (also apparently extremely rare. There was no good answer to the question. The stuck pedal training seems a solutions looking for a problem. It's fun training though! 'Builds skills!

So now we introduce a tail rotor emergency turn off switch (guarded, I hope). What if a spinning pilot cannot reach it? What if it suffers unintended operation? 'Seems to introduce more failure modes than it solves!

Firstly, why wouldn't it work for an ETR?
Secondly, why do you want the tail rotor driven in "perfect synchronisation" with the main rotor?

I think that the obvious question to ask is:
If an electrically driven tail rotor could be made as reliable as a shaft driven one and it was around about the same price, same weight and had benefits such as lower maintenance costs, why isn't such a system in use already?

That's exactly what is being asked by the authorities. As well as a strive for efficiency, the push to an ETR is environmental. It can make an impact on the noise footprint, critical in approving modern air operations.

There are already electric motors and actuators on the critical parts list, you can't choose to ignore all the FBW advancements and the lessons learned. We know how to design, build and certify using redundant feeds and multiple wingdings isolated electrically and thermally. It is not a big step in to the unknown ... not saying it is cheap, but if the regulations allow for a competitive advantage to be gained then the business case will follow. I am most definitely not advocating using RPM alone or stopping the TR in flight. But changing RPM allows optimisation to particular phases of flight.

Thanks for sharing the weight estimate. I came up with a similar %delta for the ETR. The motor about 25kg, the feeds another 10kg and the generator 40kg. So 75kg replacing about 65kg. So a weight gain, but I am convinced that the MGB could be further simplified and you have greater choice over where the generator is placed.

Not quite.
When you are dealing with servos and hydraulic fluids some odd things can happen.
For example, on the UH-60 Black Hawk there is a cable/pulley/spring tensioning lash up in the Tail Rotor Servo Assembly that, if it fails, leads the TR Blades to neutral pitch. (I read up on a rare case of this occurring a few years ago. The commentary from the Blackhawk experts did note that it was very rare, and an internal bit failed ...). I am trying to recall if there was a tail servo hardover in a Seahawk back in the distant past that led to a "stuck pedals" approach ... but memory is fading there.

There were some mechanical conditions I vaguely recall that would lead to flat pitch or stuck pitch in the Huey ages ago when I went through flight training (TH-1E and TH-1L models, I don't have the manuals for them anymore).
It may be that your instructor was being "model specific" in his observations.

The simple answer is the advances in technology driven by the hybrid and electric vehicle markets over the past decade now make production of such a system feasible. The operational environment is changing which may make it attractive to the market place. Hence the new interest and why a system has been ground tested.

I'm not sure how you'd train to cope with anything that causes a stuck main rotor actuator.

Control rod failure is one that immediately comes to mind.

Stuck collective is manageable , depending on position, cyclic less so.

Lonewolf, I suspect that there is some misinformation out there, as the tail rotor control quadrant basic design purpose in life not as you described. Rather, that quadrant and associated large spring is designed to address the loss of a cable side to ballistic damage od maintenance error. As long as one cable remains intact, the pilot flies the remaining cable against the spring and almost 100% authority is retained. What you referred to was that the basic tail rotor assembly is two, crosswise mounted paddles with four blades, and the built-in blade angles are set up to generally match the tail rotor requirements between 40 and 120 or 130 ( can't remember now ) KIAS with reasonable sideslip, as in where one might be if all the tail hydraulics were lost. That situation was flown ( both stages of the TR servo depressurized ) and we were almost able ( in this failure mode one would still have the yaw boost servo which is up forward ) to get back to a hover but could not stop the right yaw drift below 10-15 kts.

More like bad memory.
Yes, your point on the neutral position is part of what I was trying to get at.
I was trying to describe the TR servo assembly. Failure there, not just a cable failure, would render any inputs moot I think. The tail quadrant caution light would illuminate if one of the two failed. (there are 1st stage and 2d stage) Not, as noted, a common problem.
As you describe it, yes, 40 and 120 ring a bell. (I seem to recall that as the number in the NATOPS).
The recent failure I tried to describe was a failure inside the servo assembly itself, actually, the quadrant, on the ground, so they never got into a hover. Physical failure. There's a point around which it all pivots, on which the cable guards are mounted. I need to go back and see if I can find notes on that. Will post if I can find them. I think it was that sleeve/post that failed.
As I remember that system, John, without hydraulics you can't move the paddles. Your point on the servos triggers some old memory.
If you lose hydraulics in that channel, I don't recall that you can move the paddles without boost.
But now I need to go and look at the old NATOPS systems diagrams. I am pretty sure that in that flight control, the Blackhawk and Seahawk are alike enough. No CILA, though, in the Black Hawk.
As to the training, you guys were test pilots.
What they didn't want people doing, operationally, was coming into a hover with inability to control the nose, and not having the tail authority to control the aircraft down close to the ground. The run on / stuck pedals approach was considered lower risk, as well as the benefit of letting the cross wind assist in holding the nose position as one got close to the ground so you could get slower before touchdown.

I guess my point, badly made, was that you need to know the specific failures that will leave you with loss of controlability even if you don't have loss of drive/thrust. Each model will have its own quirks. The blythe "we only do it because it's fun training" I don't think is a responsible position to take.

A design function that the Hawks have is a backup ( #3 ) hydraulic pump that is electrically driven, and is the same as the two primary pumps in flow/pressure capability, and this pump backs up the first stage pump pressure to the TR servo 1st stage, hence there is triple redundancy, source-wise for hydraulic pressure back at the TR servo.

As to your remark about doing it because its fun training is well taken here, as it was possible ( or at least used to be ) to depressurize both tail rotor servo stages by pulling the backup pump c/b, deselecting the #1 Primary servo and selecting the tail rotor LDI switch. . At least I think that was the sequence. It did succeed in depressurizing both stages as we intended. Anyhow, after that flight there was some reconsideration by the senior mechanical controls engineers, who were concerned that a switch back to a pressurized system with a bunch of pedal force being applied, would create a cable whiplash that might result in the cable coming off the intermediate cable attachment devices. Hence your point is very good advice.

First: With engine out what's generating sufficient energy to drive the TR?

Second: To keep the TR in the correct RPM range.

How long does an autorotation take? A couple of minutes perhaps? So not a very large capacity required then.

Or drive the generator from the transmission...