Tail Rotor Adjustment

In mid-June 1996, the helicopter mailing list just about exploded with discussion (to put it politely) over how best to dial in one's tail rotor. One reason this is such a hot topic is that there are so many variables involved. Another is that tail rotor adjustments seeks to achieve two seemingly incompatible goals: on one hand, we want the tail to 'lock in' and remain as steady as possible when hovering. On the other hand, we want the tail to be powerful enough to pirouette like a banshee in fast forward flight.

We want to be able to do big high speed sideways loops with the tail sticking out... stable enough to keep the heli from yawing in mid-loop, yet strong enough to keep the the tail from 'weathervaning' into place behind the canopy, thus straightening out the formerly sideways loop.

Variable

Effect upon stability

Effect upon authority

Gyro gain
positive
As the gyro gain is increased, the tail rotor becomes more resistant to wind buffeting, changes in engine speed or main blade pitch, and so on. This is the reason we use gyros in the first place.
If the gyro gain is set too high, the tail will wag back and forth, oscillating and overcompensating for itself.

negative
With a "standard" gyro, as the gain is increased, the gyro 'robs' the pilot's authority as a result of its attempt to keep the heli from pirouetting too fast.
With a "heading hold" gyro, there is no drawback to increased gain, unless you turn it up far enough to induce oscillation (wagging).
Servo arm length
negative
As the servo arm length is increased, the effective gain of the gyro is also increased, leading to the same problems noted under the gyro gain section.
With a standard gyro, the control will become touchier from the pilot's point of view for the same reason - small movements of the servo cause larger changes in tail rotor pitch.
With a heading hold gyro, the control sensitivity is determined by the gyro itself so the difference here is negligible.
Lengthening the servo arm will typically require a lower gain setting in the gyro, but the two will balance out and the overall stability will changed very little, if at all. Because the servo will be able to cause a greater tail rotor pitch change in a given time period, the servo is effectively made faster this way, which can effect a net increase in stability even though the electronic gain is reduced.

positive
As the servo arm length is increased, the servo is able to cause larger changes in the tail rotor pitch. With a standard gyro, this allows for faster 540 stalls, pirouettes, and the like. With a heading hold gyro, the over overall effect is typically negligible.

Tail rotor diameter and/or blade chord
negative
As the area of the tail rotor is increased, the tail rotor's strength is increased. This causes gyro inputs to be magnified as if the gyro gain had increased. This also causes pilot inputs to be magnified in the same way. Both factors contribute to a tail boom that doesn't want to sit still.
Increasing the tail rotor diameter, like increasing the servo arm length, will require a corresponding reduction in gyro gain. However, as with a longer servo arm and a reduced gain, the overall effect is usually a small increase in stability.

positive
As the tail rotor area is increased, the tail rotor is better able to force the helicopter to yaw, which allows for faster pirouettes and so on.

Vertical tail fin area
negligible
In theory, the vertical fin should help keep the helicopter pointed straight ahead in forward flight. However, in my experience (flying with solid fins, skeletal fins, and even no fin at all), the fin's effect is negligible. They just aren't large enough to matter.

negligible

The only practical purpose of the vertical fin is to keep the tail rotor blades from contacting the ground in less-than-perfect landings.

Servo travel (using ATV) With a heading hold gyro, the transmitter ATV values determine the maximum pirouette rate of the helicopter. In conjuction with the exponential setting, they also determine the sensitivity of the rudder control around center.
With a standard gyro, an increased ATV percentage will have a negative effect on preceived stability, as since smaller control movements lead to more servo movement.
With a heading hold gyro, the transmitter ATV values determine the maximum pirouette rate of the helicopter. In conjuction with the exponential setting, they also determine the sensitivity of the rudder control around center.
With a standard gyro, more servo travel means more authority to yaw harder and faster. Because the gyro will, in flight, reduce the servo throw (due to the rotation of the helicopter itself), it is common to set the travel volume of the rudder servo to go 20% to 40% past the limits of the rudder linkages.
This will allow you to stall the servo when testing on the bench, but unless you really overdo it, the servos won't hit their limits in flight unless you turn the gain down low enough to get blindingly fast pirouettes.

Servo speed
positive
A faster servo will allow the gyro system to react more quickly to destabilizing forces such as torque, wind, and the like. Faster servo response means faster tail rotor correction, which means faster damping of undesired yaw.

positive, but insignificant
A faster servo will allow you to enter pirouettes more rapidly (i.e. go from standstill to pirouette in less time), but... The 0.1 second you save entering the pirouette isn't likely to make a noticable difference. Moveover, the maximum rate of yaw (how fast the heli spins pirouettes) will not be affected at all.

Standard Gyro Setup Examples

Note that the following setup ideas predate the advent of heading hold gyros. They are retained here in order to assist in the setup of standard gyros only.

If you are setting up a gyro for heading hold operation, follow the instructions provided by the gyro manufacturer.

Gary Wright, a competitive helicopter flier of some note, shared some information on his standard-gyro tail rotor setup scheme with the heli-list. Gary suggested using smaller tail rotor blades with standard gyros because:

you can raise the gyro gain without causing wagging

you can use a longer servo arm, again without causing wagging

you can overdrive the tail rotor servo further

it takes less power overall to cause the helicopter to yaw

Chief among his claimed advantages of the smaller tail rotor disk was a lessened "translational lift." The air flowing over the tail rotor disk at high forward speeds causes an increase in tail rotor strength, which can cause the heli to wag in forward flight. A smaller tail rotor will have an ameliorating effect.

He also believes in higher gyro gain (presumably made possible with the smaller tail rotor) and a smaller tail fin area. The higher gyro gain reduces the tail rotor strength, but I assume that the the longer servo arm and even-more-overdriven rudder servo make up for this.

It seems to work for him; he came in second at a national event recently. Now, how did the first place guy set up his tail rotor?

Another heli-list member put it another way. Doug Adams suggested using the smallest tail rotor that will deliver "enough" authority [for pirouettes, 540s, and the like]. When using smaller tail rotors, it is key, he said, to get the maximum possible tail rotor pitch when in flight. This combination, he says, will allow the gyro to work better, will reduce transitional lift (the natural tendency of the tail rotor to yaw the helicopter when in fast forward flight), and will reduce weathervaning.

And then there's the scheme I'm using as of this writing (3 Oct 1996)...

From nathan@metareality.comThu Oct  3 13:21:44 1996
Date: Thu, 3 Oct 1996 09:47:05 -0700 (PDT)
From: Nate Waddoups 
To: R/C Heli Mail List 
Subject: the one true gyro gain, was Re: Rudder to gyro

On Thu, 3 Oct 1996, Lance Robb wrote:
> At 08:45 AM 30/09/1996 -0700, Nate Waddoups wrote:

> >However, as of this weekend, I've pretty much stopped 
> >using the low rate setting at all (I leave the 
> >aforementioned p-mix turned off).  I found that I can 
> >do 540s even with the gyro gain set as high as possible 
> >without wagging the tail in forward flight.  The yaw 
> >rate starts slow, but if I keep the rudder pushed all 
> >the way over, the yaw accelerates to a 540-able rate.
> >It looks kind of neat, too. :)  With the rudder 
> >centered, the heli locks straight again nicely.
> 
>         I am surprised that you have enough gain to hover 
> as stable as the gyro will let you and still have enough 
>rudderauthority to give you a good rate for a 540 ST.
> 
>         Very interesting......is there a secret you 
>aren't telling us about???

I'm not sure which part is the secret, but my setup is pretty straightforward. Well, I guess the dual rate part might be a little odd... maybe that's it.

Concept 30 SRX with unmodified tail rotor
Futaba 153BB gyro,
S3001 servo,
ATV is set so that with dual rate at 100% I can't bind the linkage
I generally fly with the dual rate setting at 140%
gyro gain is just under 50%, probably 45% or so
The tail wags at 50% gain...
...probably due to the length of the servo arm - it used to be at about 75% gain, without wagging, when I was using a small servo wheel on the rudder servo. It was wagging all over the place after I put the arm on, until I turned the gain down. I don't think I ever did 540s with that setup without using low gain (~40% at the time).

When talking about overdriving the rudder, people usually suggest something in the +20% area. +40% might be my 'secret.' It still doesn't pirouette anywhere near as fast as it can, so I'm not too worried about overdriving the linkage.

When doing range checks, I set the rudder back to the 100% rate so I can wiggle the rudder stick with impunity.

This might be part of the secret too:

> >The yaw rate starts slow, but if I keep the rudder 
> >pushed all the way over, the yaw accelerates to a 
> >540-able rate.

Leave your heli in high gain, and try a 540 at something like a 45 degree angle (this way, if you don't make it all the way around, you won't lose too much alititude (hopefully)). That's how I discovered the secret. It starts yawing at a rate so slow it doesn't look like it will ever make 540 degrees. By the time it's gone 180, the rate is fast enough that a 540 looks possible. By the time it's gone 540, the rate looks fast enough that a 900 might not be out of the question, but I haven't tried that yet.

Maybe this only works with old-fashioned mechanical gyros and slow rudder servos. I guess it also helps to have lots of forward speed, to maximize the 'stall time.'

It's nice to be able to do 540s without adjusting the gyro. Last time I went flying, I set the gyro gain to channel 8 (this is the mixture channel on the 8UHx). Someone mentioned this idea on the list a while back, and I think it's pretty neat.

What was my 'high' gain is now my 'low' gain, since I never used the low gain setting anymore. High gain now means 'so high you can't do anything but hover.'

I've programmed it to be at low gain in idle-up, and extra-high gain in normal mode, for hovering. In normal mode, it switches to lower gain at full collective so I don't wag when I'm launching into forward flight.

It's a little bit more resistant to crosswinds now when I'm hovering, but that's all. Low gain is high enough for a stable hover anyhow, so high gain does not make a very pronounced difference...

Other heli fliers are encouraged to send their tail rotor schemes to the heli-list, to me directly, or to simply describe them below.

click here to send me a message
To link directly to this page:
http://www.natew.com/rcheli/frames.cgi/html.TailRotor.Std

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Variable	Effect upon stability	Effect upon authority
Gyro gain	positive As the gyro gain is increased, the tail rotor becomes more resistant to wind buffeting, changes in engine speed or main blade pitch, and so on. This is the reason we use gyros in the first place. If the gyro gain is set too high, the tail will wag back and forth, oscillating and overcompensating for itself.	negative With a "standard" gyro, as the gain is increased, the gyro 'robs' the pilot's authority as a result of its attempt to keep the heli from pirouetting too fast. With a "heading hold" gyro, there is no drawback to increased gain, unless you turn it up far enough to induce oscillation (wagging).
Servo arm length	negative As the servo arm length is increased, the effective gain of the gyro is also increased, leading to the same problems noted under the gyro gain section. With a standard gyro, the control will become touchier from the pilot's point of view for the same reason - small movements of the servo cause larger changes in tail rotor pitch. With a heading hold gyro, the control sensitivity is determined by the gyro itself so the difference here is negligible. Lengthening the servo arm will typically require a lower gain setting in the gyro, but the two will balance out and the overall stability will changed very little, if at all. Because the servo will be able to cause a greater tail rotor pitch change in a given time period, the servo is effectively made faster this way, which can effect a net increase in stability even though the electronic gain is reduced.	positive As the servo arm length is increased, the servo is able to cause larger changes in the tail rotor pitch. With a standard gyro, this allows for faster 540 stalls, pirouettes, and the like. With a heading hold gyro, the over overall effect is typically negligible.
Tail rotor diameter and/or blade chord	negative As the area of the tail rotor is increased, the tail rotor's strength is increased. This causes gyro inputs to be magnified as if the gyro gain had increased. This also causes pilot inputs to be magnified in the same way. Both factors contribute to a tail boom that doesn't want to sit still. Increasing the tail rotor diameter, like increasing the servo arm length, will require a corresponding reduction in gyro gain. However, as with a longer servo arm and a reduced gain, the overall effect is usually a small increase in stability.	positive As the tail rotor area is increased, the tail rotor is better able to force the helicopter to yaw, which allows for faster pirouettes and so on.
Vertical tail fin area	negligible In theory, the vertical fin should help keep the helicopter pointed straight ahead in forward flight. However, in my experience (flying with solid fins, skeletal fins, and even no fin at all), the fin's effect is negligible. They just aren't large enough to matter.	negligible The only practical purpose of the vertical fin is to keep the tail rotor blades from contacting the ground in less-than-perfect landings.
Servo travel (using ATV)	With a heading hold gyro, the transmitter ATV values determine the maximum pirouette rate of the helicopter. In conjuction with the exponential setting, they also determine the sensitivity of the rudder control around center. With a standard gyro, an increased ATV percentage will have a negative effect on preceived stability, as since smaller control movements lead to more servo movement.	With a heading hold gyro, the transmitter ATV values determine the maximum pirouette rate of the helicopter. In conjuction with the exponential setting, they also determine the sensitivity of the rudder control around center. With a standard gyro, more servo travel means more authority to yaw harder and faster. Because the gyro will, in flight, reduce the servo throw (due to the rotation of the helicopter itself), it is common to set the travel volume of the rudder servo to go 20% to 40% past the limits of the rudder linkages. This will allow you to stall the servo when testing on the bench, but unless you really overdo it, the servos won't hit their limits in flight unless you turn the gain down low enough to get blindingly fast pirouettes.
Servo speed	positive A faster servo will allow the gyro system to react more quickly to destabilizing forces such as torque, wind, and the like. Faster servo response means faster tail rotor correction, which means faster damping of undesired yaw.	positive, but insignificant A faster servo will allow you to enter pirouettes more rapidly (i.e. go from standstill to pirouette in less time), but... The 0.1 second you save entering the pirouette isn't likely to make a noticable difference. Moveover, the maximum rate of yaw (how fast the heli spins pirouettes) will not be affected at all.