Is a tail rotor really needed?
 

This has been floating in my slowly diminishing grey matter for a few months now, so if I share it with you folk out there perhaps I may get an answer that will help me.

We all know the tail rotor is there for the countering of applied power and to control the tendency for the Heli to act like a gyroscope, but could this be replaced by a counter spinning flywheel mounted on the main rotor shaft, that could/would be controlled by a clutch/brake like system. In my mind this would act as a counter measure to the energy of the spinning main rotor and G/box , as though it was a contra spinning rotor but without the complexity's of a second rotor and control rods and systems.
I realise it may sound very agricultural what I am asking, but I know that this does work on a ground operated machine, where a huge spinning drum(the main rotor on a heli) is controlled by a counter spinning flywheel which controlls the energy required to do its work, and acts as a shock load absorber.
Can you people out there say if this has ever been tried, or even whether you feel it would work.
My regards, and have a nice weekend
PRB

vfrpilotpb,

The tail rotor does not absorb or balance energy at all. It is there to exactly counter the torque that the main rotor imparts on the fuselage as it is spun. A gyro has nothing to do with it.

Basically, the main rotor does not want to spin unless pushed. The proof is how quickly it slows down if the engine is decoupled. This continuous push to keep it spinning requires that an opposite push is needed to balance the system. If the helo was a boat, and you pushed its blades around by hand, you would have to dig your shoes into the deck and lean into the blades as you push them, and your feet would push the boat around in the opposite rototion.

The tail rotor is used to allow the "boat" to grab the water and push back to keep the boat pointing straight.

You ground machinery comparison isn't correct because that machine is tied to the ground, so this side push is free, and you can't see it. Watch the engine of your car rotate in its mounts as you goose the throttle. If the car were not sitting on the ground but floating in the air, the car would roll in the opposite direction!

The gyro in that ground machine is probably used to smooth out the rotation by adding mass, a common trick.

not required
mine has not got one lol :) :)

It's possible to fly without the tail rotor-but it's very hard on the pilot. I've done it twice.

Additional to above posts-consider the fact the proposed flywheel would be heavy as heck.
Effective only as it accelerated/decelerated.
Precession's effects on flight.
Weren't there buses built with flywheels as energy sources? Seen any lately?

To:Vfrpilotpb

Let’s assume that your concept is feasible and that the spinning wheel is below the transmission. The wheel would have to be of a large mass and spin at a high rate of speed in order to counter the torque developed by the spinning main rotor. Now, let’s look a bit closer at the spinning mass. My god, it’s a gyroscope rotor and it has all of the characteristics of a gyroscope. Precession and rigidity in space. I believe it is like a Momentum Wheel that is used to stabilize satellites. Any time the pilot moved his cyclic the blades would respond due to (pick one) gyroscopic or aerodynamic precession and the spinning mass would also be effected and apply a torque load to the aircraft that may be in conflict with the pilots’ cyclic input.

To: Nick Lappos

[ 01 December 2001: Message edited by: Heliport ]

Translation:

Lu may have finally actually understood and agreed upon something Nick said.

Stay on topic. Quit the wind up's.

[ 30 November 2001: Message edited by: RW-1 ]

C'mon Lu,
Even you can see Nick is trying to explain something in simple terms, don't wind yourself up like a gyroscope, ee gads, you may precess :)

To: Sling Load

It was not a dig. I did it with a smile on my face but you couldn't see it.

Maybe I should have included a smiling gremlin.

Vfrpilotpb

It has been said that the tail rotor consumes between 6 and 12 percent of the engines power, and none of this power is producing lift or forward thrust. This loss of power still exists in fast forward flight because the vertical tail wants to swing out to the side and that creates additional drag.

This was a primary reason for developing helicopters with multiple main rotors.


Just a totally unbiased opinion, mind you. :D

[ 30 November 2001: Message edited by: Dave Jackson ]

To: Dave Jackson

“It has been said that the tail rotor consumes between 6 and 12 percent of the engines power, and none of this power is producing lift or forward thrust. This loss of power still exists in fast forward flight because the vertical tail wants to swing out to the side and thereby it represents additional drag’.

I believe that the Blackhawk and the C H-53 tail rotors produce lift because they are canted. This provides a slight lift vector and it could be more that slight. Most helicopters have aerodynamic surfaces that will maintain the fuselage in a given direction with the loss of the tail rotor. Not being a “PILOT”
It is my understanding that in the stated conditions there is minimal if no pitch in the tail rotor. Now, if you slow down under the stated conditions you will have a problem and most likely have to auto rotate or make a very fast run-on landing.

Regarding your comment about tandem rotor helicopters I don’t know if I can agree with that. To design a second rotor with its’ own drive line and a second transmission doesn’t quit equate. On a single rotor helicopter you can survive a loss of a tail rotor especially under the stated conditions above. If you lose a rear rotor and that has happened when you open your eyes you are looking into the face of Saint Peter.

Tandem rotor helicopters despite their complexity offer superior lifting power and a wide CG range.

As a matter of interest my first helicopter ride was in an HRP-1 the first military tandem rotor helicopter.

The tail rotor is there for two reasons. One is to counter the torque on the main rotor, the second is for yaw control.

A counter rotating mass could do the job of countering the torque, but at a cost: weight. If the mass turned at the same speed as the rotor and the mass was a ring about 4 feet radius with a 20 foot radius rotor (pulled numbers out of air), then the counter rotating mass needs to weigh about 17 times more than the rotors. If the mass turned twice as fast, it would weigh 1/2 that. A transmission is also needed, the faster you spin the mass the beefier the transmission will have to be.

That would counter most of the torque, but it wouldn't automatically adjust to every condition of flight. That's okay because you still need yaw control.

Conventional yaw control wouldn't rob as much power as it does now, but the weight of all the components won't change.

Yaw control in the counter rotating mass is possible if you have movable masses inside that could be moved radially using the yaw control pedals. I doubt if this would be easy to control...there would probably be a noticeable lag between input and response.

Of course, the engines must spin that counter rotating mass, so efficiency may actually drop.

___________


In short, it is possible. My new cordless screwdriver uses this technology. I doubt if any overall increase in efficiency will be realized.

Unfortunately, Lu is wrong yet again. A spinning wheel cannot counter the torque of the main rotor, because once the wheel is up to speed, it takes no torque to spin it, so it can't counter the torque of the main rotor. The main rotor is continuously making lift, so it has the drag that creates torque (torque is really rotating drag).

The fact that this silly spinning wheel concept can be considered by an individual is a mark of "technical unsophistication".

For MD600driver,
I hate to bear the bad news, but inside the tailcone of your nice little NOTAR helicopter is a perfectly fine tail rotor, with a gearbox, pitch change mechanism and driveshaft. It is based on a Eurocopter fenestron, which it strongly resembles. Also buried inside your tailcone is the nozzel mechanism, with its control links and the rotating barrel. Also, you have the required rudder stability system, and the rudder controls. In all, you have more parts, heavier weight, more failure modes, more power wasted, and a much poorer flight envelope. Other than that, it is a fine idea. ;)

Good point, Nick.

I was wrong too. :o The mass will counter changes in torque, but will do nothing for continuous application of power.


I can make this work. Add some paddles so there is drag acting on the mass. You could even vary the pitch of the paddles for yaw control. Hey, why not set up the paddles to create a bit of lift. Wait a minute....that's Kamov. :D


Seriously now, counter rotating fly wheels that balance continuous loads do exist. A mass-spring mechanism or the like that draw energy from the engines to supply the required centripetal acceleration to prevent the mass from flying off will accomodate a continuous application of power.

There are the same failure modes, weight problems, and control difficulties that I mentioned in my previous (sniff snifff...wrong) post, plus more complexity with the mass-spring or similar system.

heedm,
The spinning mass does not counter the steady torque of the rotor, it cannot, except for the tiny fraction of its power that is lost to friction. The wheel will only balance the torque while the wheel is being spun up, which is where it makes its pay in the screwdriver. It counters the twist jump (strating torque) of the tool when the tool is turned on. It cannot counter the running torque of the tool against the work (such as when a sanding disk is being pressed against a wall).

Your calculations would work to counter the torque of the rotor during rotor run-up, but not during normal powered flight.

EDIT I noticed your post after I wrote this, heedm. You are right on. One of the major problems with helo development was the fixation on torque balance via counterrotation in the early days. The simplicity of the single rotor helicopter (and NOTAR is one, too) usually outweighs the slightly greater efficiency of the double rotor (tandem or coaxial) designs that compete.

The tail rotor uses about 3 to 6% of the main rotor power in a heavy hover, and almost nothing in cruise.

[ 30 November 2001: Message edited by: Nick Lappos ]

VFR, how ya diddling?

I wouldn't have a handle without TR's!!!
So don't get rid of them :D

Darn, Nick beat me back to the NOTAR system :(

I also hear it really isn't as responsive as a tail rotor either, true?

heedm, the incorporation of 'jump takeoff' in gyrocopters is an interesting example of rotational inertia in a rotor system.
______________

Nick, are you putting a little western spin on your percentages? :)
The deputy chief designer of Kamov Company claims, with a little eastern spin :eek:, that the coaxial configuration can save 10-12% over the single rotor with a tail rotor.

Would you agree that the torque of the main rotor must be offset in both hover and forward flight? In forward flight the 'angular drag' of the vertical stabilizer 'takes over' for the tail rotor?

[ 30 November 2001: Message edited by: Dave Jackson ]

Dave,
The vertical stabaliser can take over, to a certain degree, in forward flight but what about all the poor aloutte 3 jockeys and the other helis with no vertical stabaliser. Apparently though the AIII in forward flight can lose its tail rotor and fly using the aerodynamics of the aircrafts body. Me thinks Ill not try that one.

Lu I stand to be corrected here but I believe the only reason for canting the tail rotor, I think all on Military helis, is so the the heli is level when in the hover for easier bussing and de-bussing from both sides. :)

Well.... when I'm in a hover I thank Mr. Sikorsky for the great invention of the tail rotor. If it wasn't for him, things could go seriously wrong. :D

A question for any of you guys flying fenestron T/Rs (or anyone who knows). Are they set up like conventional t/rs, thrust changed with blade pitch, or do they adjust the speed? Also I assume that they consume more power in cruise because they can't benefit from etl. true?

To: Nick Lappos

“Unfortunately, Lu is wrong yet again. A spinning wheel cannot counter the torque of the main rotor, because once the wheel is up to speed, it takes no torque to spin it, so it can't counter the torque of the main rotor. The main rotor is continuously making lift, so it has the drag that creates torque (torque is really rotating drag)”.


Please read the first words of my comment on the spinning wheel.

“Let’s ASSUME that your concept is FEASIBLE”

I started from that premise and expanded on it to show the problems that would result from the concept.

Don’t be so picky.

Lu said:
I started from that premise and expanded on it to show the problems that would result from the concept.

Earlier Lu aid:
The wheel would have to be of a large mass and spin at a high rate of speed in order to counter the torque developed by the spinning main rotor.

Nick sez:
Lu, you even critiqued the idea as needing a large mass! Then you duck what you wrote by pretending your confusing understanding was hypothetical. You are hopeless! You cannot remember what you said, how can we expect you to remember what we said. :mad:

I'd like to try a different explanation that should make this clearer (I hope).

It helps to look at torque as an accelerating force. Torque is very similar to linear acceleration, except that it acts in an arc (i.e. in a rotating direction).

Regarding the flywheel as a means of countering torque, the Hubble Space Telescope serves as an excellent example. It uses flywheels on all 3 axis to point the telescope in the proper direction. To change the attitude along an axis, that particular flywheel is accelerated (or decelerated, which is "acceleration" in the opposite direction) by torque from an electic motor. The opposing force from the motor is applied directly to the body of the telescope which starts a motion (acceleration from no motion) through that axis. To stop the motion along that axis, opposite torque is applied by the motor to accelerate the flywheel in the opposite direction.

For this application, the use of flywheels to control torque works great, since it's operating in the vacuum of space, and there is no constant application of torque, otherwise all of the pictures would be streaks of light. But for a helicopter this is an entirely different story.

First of all gravity is an accelerating force like torque, except that it's a linear force with an acceleration of 32fps/s. If a man falls from an airplane, he will accelerate at this rate until the wind resistance against him, causes his speed to stablize in balance with the accelerating force of gravity. So gravity is still trying to accelerate him against the wind resistance to increase his speed.

In a helicopter, the main rotor in trying to balance the linear accelerating force of gravity with a constant application of lift. The lift is being generated by the rotation of the main rotor, which has to have torque (an accelerating force acting in an arc) applied to it by the engine(s). So the main rotor is being accelerated by torque to both balance the lift of the helo against gravity, and to overcome the aerodynamic drag of the rotor blades. The engine(s)s in turn are applying torque to the body of the helicopter.

The problem with using a flywheel to control torque in this application, is that the torque is being constantly applied (instead of being momentarily applied as is the Space Telescope). Since torque is an acceleration force, a flywheel would have to be constantly accelerated in the opposite direction to counter it. This could work only very briefly until the flywheel was spun so fast that it burst.

So in summary for the helo application, gravity is a linear acceleration force, countered by the lift produced by the torque (rotational acceleration) of the main rotor, which applies torque to the helo body, which is countered by the horizontal "lift" of the tail rotor. A flywheel just won't work here since all of these acceleration forces are being constantly applied.

Hope this helps...

(edited for typos)

[ 01 December 2001: Message edited by: Flight Safety ]

Lu

I've edited one of your earlier posts.

Would you PLEASE stop making childish comments or 'digs' at other people.

Your 'digs' obviously amuse you. They equally obviously irritate other people, and risk spoiling good discussions.

Heliport
Moderator

Tandem rotor helicopters despite their complexity offer superior lifting power and a wide CG range.

I think you'll find that due to interference losses there is no performance gain for tandem systems - only a weight penalty and some operating advantages (rear ramp etc). The biggest lifters in the world are "conventional" designs.

Dave Jackson,
Thanks for the steer to the Kamov web site. That is a fine discussion presented there. I ran S-76 numbers to check, and the S-76B uses about 8% of its total power for the tail rotor in an OGE hover at Max Gross.

The rest of the Kamov discussion beats around proving that co-axials are the "best" configuration, but there are lots of shades of gray in the discussion.

One thing I have learned in this industry is the difference between theory and fact. As Mr. Sikorsky said, "There will be many times in your career, young gentlemen, when the theory and the facts do not agree. I heartily implore you to respect the facts!"

The actual payload of a helo relative to the size of the engines is almost a constant for all the various configurations, due to all the practical factors that complicate the basic overly simple discussion. As in all human nature, folks often fail to tell you all the facts to help serve their argument, too.

The Kamov is a case in point. While a single rotor helo must waste power in the tail rotor, the co-axial must have shorter blades (self mid-air collision is a real design problem. Two Ka-50 have been destroyed due to blade to blade impacts). The shorter blades and the height of the stacked diska allow a better flapping angle while avoiding blade contact. Unfortunately, the smaller Kamov rotor disks make their hover power efficiency less than that of a typical single rotor helo, so what they gain in one side (no tail rotor losses) they lose in another (higher disk loading wastes power.)

Nature is perverse enough to even out all the competition. :D

Nick

Thanks for your comments on the coaxial configuration.

The coaxial and the intermeshing (synchropter) helicopters represent an interesting dichotomy of ideas. They have relatively similar configurations, and yet Kamov elected a small disk area and high disk loading, whereas Kaman chose a large disk area and low disk loading.

Today's stiffer rotors should minimize many of their differences.

Good observation, Dave. Charlie Kaman's tilted syncropter disks are much less susceptable to blade contact, so he can make the disks very large, while Kamov designs must trim the blades accordingly. It never dawned on me before. This forum is keen! :cool:

The stiffer rotors might not be applicable, because the higher mast of a coaxial needs even more beef to stand the high forces of a rigid stiff blade, The KA-50 illustrates this. The lower disk has 7% hinge offset (its much closer to the transmission, so the high moments from that offset need a shorter shaft to carry them to the airframe). The upper rotor has a measly 2.5% offset, so its long mast can stay light and skinny.

[ 01 December 2001: Message edited by: Nick Lappos ]

Nick:

The comment on the KA-50's differing hinge-offsets is interesting. I understand that the Sikorsky ABC had fairly stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance. Another characteristic of the ABC was that its rotor assemblage constituted 50% of the parasitic drag.

To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.

[ 02 December 2001: Message edited by: Dave Jackson ]

Vfrpilotpb
i dont think the spining wheight would work because there is nothing for it to "twist against" once it has got up to speed( to use it for anitorque from the rotors it would have to be accellerating continuously). the rotors are spinning in the air so you need somthing in the air to counter act it unless the wheight is soooo big you wouldnt lift it of the ground anyway.
if the rotor was perpendiculare to the mast it would turn the torque from the engine into a rolling moment which the pilot could stop with cyclic but this would make everything else so hard to control. :confused:

Dave said:
I understand that ABC had reasonably stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance.

Nick sez:
Yea, the rotors were stiff, about 25% hinge offset! The Gamma reset in flight was used, more for keeping the moment down (the rotors would oppose each other to some extent) and the controls square, but bett6er tip clearance was also had. The Boeing UTTAS YUH-61A had adjustable gamma, as does the Comanche, due to the high offset. This is simple proof that the rotor rigging angle, Gamma, is set by many factors, not gyroscopic precession. (Get ready for the DRIVEL attack, Dave!)

Dave said:
Another comment about the ABC was that its current rotor assemblage constituted 50% of the parasitic drag.

Nick sez:
That may be right. The head drag is a big fraction of a helo's drag, and two heads are not better than one. An old paper by Mikheyev of Kamov estimated that the co-axials lost 10% speed and range due to extra drag.

Dave said:
To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.

Nick sez:
The intermeshing configuration could allow less stiffness, because tip clearance is not a problem at all, I think. Drag would be higher than a single rotor (more stuff in the breeze), but lower than a co-axial, because there is no long mast and upper swashplate arrangement.

I know that you favor the intermeshing configuration, Dave. There are advantages, but that transmission and head arrangement is still a handfull. Fly By Wire would help, however. Keep pushing for your dreams! :cool:

RW-1, the NOTAR tail has a slower responce then a conventional tail. It has the same authority, but it takes longer to achive it. You have to chase the tail at times, correct what you just corrected because it is slow.

Good morning Rotoheads, I have been away for a couple of days, but I am indeed pleased at the amount of replys from our worldwide collecion of heli-brains, I have quickly read through the post's and must say thank you , you have all added information that I just could not see, but I now see the failing of my pre senile rambling's, for any counter rotating weight to stand any chance it would have to stay in tandem with every move of the main rotor, that I now understand, nor had I considered the effective weight that a flywheel would need to be able to counter the spinning mass of the M/rotor at all various pitch's. So I guess that we ordinary mortals are going to be stuck with " Boring old Single Rotors"( typed with a beaming smile on his face),
However in the same vein of Rotor/counter spinning rotors, the achilles heel , so to speak, is the linkage( or so I have read ) consider this, the Avro Shackleton and various other FW type's with Counter spinning props all used a hydraulic system to alter the pitch change, this was called "The Translation Unit" and worked by pushing and pulling extremely well engineered solid steel bars, which were fixed to the rotor heads, would this system work on helicopters,or do we always have to have a physical link, for this translation unit system would do away with a lot of tiny (by comparison) control rods and connections. A swash plate would still be there, but that would work both rotors anyway.

Thank you all for your input
My Regards :D :D

Edited to remove early morning problems of co-ordination PB

[ 03 December 2001: Message edited by: Vfrpilotpb ]

Ahh, thanks H.M.

So it looks like one trades response to not having a TR out where it can get whacked against something.

They say nothing is free .... :D

Baranfin, looks like you got lost in the perennial Lu/Nick conflict (personally I think Danny should give them their own forum).

Anyway, fenestrons operate the same as conventional T/Rs, ie change pitch to alter power. If you look at fenestron aircraft, they've got large tailfins which produce sideways force in the cruise, so the pitch on the fenestron blades can be reduced same as T/Rs.

Thanks for the response MightyGem, Another question about the fenestrons. I remember reading somewhere that ducted fans produce more thrust for their size than un-shrouded. Is this true? why? I would think it would be less efficient because the tip vortices getting all confused and bunched up at the edges.

Can't help you there, old chap.
:confused:

How about this for an idea?
A tail rotor that rotates 90 degrees which during cruise points backwards and produces forward thrust and therefore unloads the head which would allow a higher forward speed. A offset tail fin which is controllable (rudder) for yaw control at cruise speed and obviously the angle of the tail rotor thrust direction would be proportional to speed.
I know there is nothing new about a pusher prop on a helicopter (Cheyane) but the mechanics and control of this mechanism wouldn't be to difficult.


Jiff :rolleyes:

To: Jiff

Your idea about the folding tail rotor was given some consideration in the design of the Cheyenne but it was decided to go with a tail rotor and the rear propeller. If I remember there was a Beta controller that distributed power to the prop and also controlled its’ pitch for increased or decreased thrust. The propeller was used during ground taxi and direction was controlled with differential braking just like a fixed wing aircraft. The Cheyenne unlike a conventional helicopter could not taxi using disc tilt. Cyclic was not available to the pilot until the weight on wheels switch indicated the helicopter was airborne and then, the cyclic control was enabled and power had been previously shifted to the tail rotor. Once forward speed was built up the tail rotor was no longer needed and the beta controller shifted power to the propeller and the thrust was adjusted. Once forward flight was established and the Beta control had shifted most of the power to the propeller providing forward thrust the helicopter was now an autogyro. Control of the helicopter was through cyclic input but the blades were operating at minimum pitch.

Of course that was 32 years ago when I worked on the program and I may have confused the facts.