Helicopter electric tail rotors

Oskar

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I hate changing the subject but what impresses me most about those RC drones is the fantastic job of stabilization that has been accomplished with several tiny slivers of vibrating piezoelectric material used as rate gyros.

I don't trust those gyros enough yet with my bum in the seat, but this guy clearly does 😲

 

Smack

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It is inspiring that there are those out there that are pushing forwards with technology rather than standing pat on the 'old ways'.
 

Oskar

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Some more progress on the electric tail and a very interesting discovery.

All seven tail motors are now installed, tail rotor authority is fantastic! The CAN bus now also runs from the instrument panel all the way back to the tail, so tail battery status can be monitored while flying.

The interesting discovery happened when the main drive tripped while hovering. In a conventional helicopter this would have resulted in strong yaw, but this time there was virtually no yaw. I couldn’t work out what had happened until I watched the video, and then it all began to make sense.


The first thing that can be seen is the tail motors slowing down. Some had just about stopped before the rudder pedals had even started moving. The next thing that happens is roll to the left.

Of course what happened first is that main rotor torque went to zero. The tail rotor controller has feed forward (see post #21), picked that up, and automatically reduced rail rotor thrust to zero, which is why the tail motors stopped. Because both main rotor torque and tail rotor thrust disappeared there was negligible yaw, so at this point the pilot still has no idea that there's a problem.

Then comes the roll to the left. The tail rotor had been pushing right (it’s not European) and the thrust disappearing causes the left roll. Having the tail rotor thrust in line with the rotor is good for efficiency, but not so good for dynamics. Because tail rotor thrust is much higher than the cg of the helicopter the left roll is amplified.

A question that arises has to do with instinctive reactions in the event of an emergency. Helicopter pilots will instinctively associate unexpected left yaw with loss of power, all other effects being secondary. If the tail rotor has feed forward control the main reaction to power failure is not unexpected yaw but unexpected left (or right for European) roll, which can be very confusing the first time it happens…
 

Smack

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Oskar, what tripped the main drive ?
 

Oskar

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Unfortunately had the main drive monitoring temporarily disabled, it's back on now so will just have to keep flying until it happens again! Without batteries I'm running close to the high voltage trip level so was initially getting the occasional overvoltage trip, but this time it would have been something else.
 

hillberg

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With the normal arrangement of a shaft driven tail rotor when the main rotor rpm drops the thrust in the t/r also drops (rpm related)-
you only use a little peddle as needed the action/reaction is less violent .

In your system it reacts extremely quick pilot reaction is half seconds - the electrics are at the speed of light,,,, Interesting unplanned test.

Will you lower the props to the c/g? reducing the "shuffle"
 

Oskar

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Will you lower the props to the c/g? reducing the "shuffle"

On the one hand you want the tail rotor thrust to be in line with the main rotor, that's what all the BIG helicopters do. On the other hand you want the tail rotor thrust to be in line with the cg. You can't do both, might just leave it as it is and practice engine outs in the hover until I get used to the dynamics involved.
 
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All_In

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May I ask a question for those like me who are ignorant?
What is the shuffle?
 

hillberg

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May I ask a question for those like me who are ignorant?
What is the shuffle?
Helicopters with the tail rotor above the c/g will input a rolling action with peddle inputs and abrupt torque changes - Starts PIO with some pilots
R-44 .
Jet rangers, (Long Rangers use a lateral bob weight to dampen it.)
loaded Hueys and sky cranes get it.
The Sikorsky S-58 has a hydraulic damper in the t/r controls limits force / rate. no issue

The later large helicopter designs cant the t/r thrust down at an angle to augment the thrust for wider c/g range. Shuffle too was eased.
 

Oskar

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May I ask a question for those like me who are ignorant?
What is the shuffle?

Nice answer hillberg, didn't know that some helicopters even have dampers for that.

The effect is similar to a gyro where prop thrust is not in line with cg. If you increase or decrease prop thrust there will be a resultant pitch change which needs to be compensated for by the pilot. In a helicopter if tail rotor thrust changes and the thrust is not in line with the cg, there will be a resultant roll which has to be compensated for by the pilot.
 

hillberg

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The main reason to mount the tail rotor high wasn't for aerodynamics but ground clearance - most helicopter in low gross weight conditions sit tail low in a hover.
With the 212 I flew the heals of the skids touch with the stinger a foot and a half off the ground (we had tall skid installed or the tail would have 8"}

S-58 always handed tail wheel 1st.
Bell OH 58 always landed toe first on the skids
 

All_In

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Thank you both so much. I love this forum.
Makes learning so easy.
 

Kevin_Richey

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Does "Shuffle" refer to the required adjustment on the other three controls in a heli that need adjusting when the pilot moves the first one of those four controls?

Is this in all phases of flight?

Or, like in this case where the main rotors lost power, you had to overcome the sudden power loss & torque being removed because of an equipment failure, & not something you initiated?

In watching your video, it appears that the collective arm moves down @ a split second after the mains lost their torque?

Please excuse my ignorance, as I've never had the controls of a heli completely in flight, only the cyclic, while the pilot maintained control of the other three, & that was while moving forward @ speed. So, for that one time, it just felt like flying a gyro's cyclic as I made turns.
 
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hillberg

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Does "Shuffle" refer to the required adjustment on the other three controls in a heli that need adjusting when the pilot moves the first one of those four controls?

Is this in all phases of flight?

Or, like in this case where the main rotors lost power, you had to overcome the sudden power loss & torque being removed because of an equipment failure, & not something you initiated?

In watching your video, it appears that the collective arm moves down @ a split second after the mains lost their torque?

Please excuse my ignorance, as I've never had the controls of a heli completely in flight, only the cyclic, while the pilot maintained control of the other three, & that was while moving forward @ speed. So, for that one time, it just felt like flying a gyro's cyclic as I made turns.

Everything is force (thrust) against force (gravity and torque)

tail rotor thrust against main rotor torque adds translating tendency (sideward movement) helicopters either tilt the mast [Enstrom]-rig the cyclic [huey] or let the pilot take care of it [Robinson]

moving cyclic too much you can wash out lift (stering the pot) - or in autorotation add or subtract rotor rpm by fore and aft inputs (short life)

moving collective changes thrust, torque, translating tendency and with some helicopter a pitching moment around the c/g and RPM droop and yaw when power / torque changes
(Pilot is lazy on the peddles)

Moving peddles change heading - can also droop main rotor RPM or add main rotor RPM (t/r also uses power) and if the power plant you'll get a slight pitching moment from precession of the crank mass (R-22)

Normal auto rotation (power loss) cruise flight

Heading is the first sign [no torque t/r thrust =uncomanded yaw] add peddle for yaw

Rotor RPM drops [no power] add aft cyclic to load the rotor system reduce the collective RPM is life (forward cyclic & unloaded rotor = ☠)

steady state autorotation


Hovering auto

heading change loss of Main Rotor Torque (also translating tendency) yaw and a lateral drift (tail rotor thrust) cyclic adjust for zero drift'

peddles for heading

freeze the collective until the helicopter settles add collective before contact with ground to soften landing


So much is going on , With time all of this becomes second nature like breathing .
 

Oskar

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So much is going on , With time all of this becomes second nature like breathing .

Exactly!

Pilots constantly monitor their surroundings to determine what is happening. An important parameter is noise, if the noise suddenly stops the brain will automatically go into engine failure mode. The pilot of an electric helicopter is at an immediate disadvantage, because there is no engine noise the pilot cannot use noise as an indicator of power loss.

Helicopter pilots are also very sensitive to uncommanded yaw, any sudden unexpected yaw is a strong indication of power loss. Large uncommanded roll or pitch is tricky, if there is not much turbulence pilot workload quickly goes through the roof. In a split second you need to work out what is going on, and then decide what to do about it.

In the video the normal indications of power loss were missing. The noise level did not change, and there was no uncommanded yaw. So when the uncommanded roll came there was a very confused pilot sitting in the seat. It was a case of “We have a problem, don’t know what it is, so let’s get this thing on the ground as quick as we can.” Even back on the ground I wasn’t quite sure what had just happened.

What caught me out was a small software change, adding feedforward to the tail controller. It makes normal flying much easier, but has an unexpected side effect on an emergency procedure. Boeing found something similar with the 737 MAX...
 
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