Dual Rates & Expo

What are rates?

For background... "Rate," in this context, refers to the amount of servo movement you get with a given movement of the control stick. The key thing is this:

The higher the rate, the most sensitive the controls are and the more rapidly the model will respond. You'll have a more 'twitchy' hover, tighter loops, faster rolls, and so on. Lower rates make the sticks less sensitive, which makes it easier to hover and to fly smoothly but more difficult to do most aerobatic maneuvers.

Rates are measured in percentages. 100% is just a middle-ish value that the radio manufacturers picked arbitrarily, as a point of reference. Most futaba radios allow you to choose a rate between 30% and 140% - I believe JR radios allow 10% to 150%. This doesn't make JR radios somehow better; Podunk radios might just as well pick a 100% rate that allows 180 degrees of servo movement, and ask you to pick values between 5% and 100%.

Rates are related to ATVs (adjustable travel volume), but rates affect both servo movement equally in either direction. ATVs have a separate adjustment for travel in either direction.

Adjusting the rate changes how far the servo will move from its center when you move the stick to one side or the other. If the rate is too high, the linkages or other parts of the model may bind; if the rate is too small, the linkages has not move enough to make any difference at all.

What are dual rates?

Dual rates indicates that the transmitter has a switch that can select between two different rates, typically referred to as "high" and "low."

Every channel on the radio has a rate, but not all have dual rates. Typically, dual rates are available on the rudder, elevator (or fore-aft cyclic), and aileron (side-to-side cyclic). Dual rates for the throttle just wouldn't make much sense - why would you want to limit your high throttle setting and raise your idle at the same time?

What do you do with dual rates?

Personally, I spend 99% of my time flying in the "low" rates, which I gather is different from most folks. Using "low" rates most of the time, I feel less at risk should I accidentally find myself flying with the switch in the wrong position.

One of the most convincing arguments against the use of dual rates is that if you think you're in your normal "high" rates, you can get into trouble if you start a knee-high roll and find that you're actually flying on "low" rates. This has led to many a crash and many a curse to the sky.

So, for me the "low" rates are normal. I find the "high" rates useful when I'm trying to learn something new. Faster response allows me to bail out of a botched maneuver more quickly. The drawback is that I can't fly at all smoothly this way, and it takes a bit more concentration to keep the heli on the path I have in mind. The advantage is that if I start a maneuver in the "wrong" rate, I can return to a safe orientation faster than I expected, so I'm not likely to crash as a result of using the wrong rates at the wrong time.

How do you set up dual rates?

The high cyclic rates are set up so that the swashplate will almost bind if I move the cyclic stick all the way forward, all the way back, or all the way to either side. The swashplate will bind if I move the stick into the corners of the gimbal (top-left, bottom-right, etc).

The low cyclic rates are set so that the swashplate will almost bind when I move the cyclic stick into the corners. There is a bit of unused travel available when I move the stick straight forward or to either side.

My radio - Futaba 8UH - allows me to put both the aileron and elevator cyclic rates on the same switch. With the radio configured his way, I'm either using high rates for both channels or low rates for both channels. This makes one less switch to mess with, and it frees up another switch for another purpose (for example, to turn my cyclic->throttle mixing on and off, or to adjust the governor settings, depending on the model).

The nice thing about the low rate is that I never need to worry about binding anything. With high rates, it's possible to bind the swashplate against the main shaft if the stick moves too far into the corners. I find that a little scary because with most machines, when the swashplate is locked against the main shaft, collective control binds up as well. Concepts and Barons are immune to this, but virtually every other machine on the market depends on a (smoothly) sliding swashplate for collective control.

Once I'm satisfied with the radio setup, I adjust the mechanical setup to get the amount of cyclic authority I want. So far, with my Concept and Futura, I've found that when I'm happy with the authority when the low (unbindable) rates are in effect, I'm also happy with the extra authority provided when the high rates are in effect.

What about the rudder rates?

Back in the days of mechanical gyros, I set the "high" rate to the maximum allowed by the radio (with maximum ATVs, too) and was barely satisfied with the pirouette rate. This led to lots of binding while the heli was on the ground of course, so I used the low rate to set the travel to where it wouldn't bind if I bumped the rudder stick.

Today, in the age of "yaw rate demand" or "heading hold" gyros, I have a little more flexibility...

My initial "heading hold" setup used a low rate that was just fast enough for a clean 540 stall turn, and a high rate that allowed extremely fast pirouettes for zing-zing-zing-zing stall turns and ripper-esque plummeting pirouettes. I found that pirouetting fast for the sake of pirouetting fast wasn't very interesting to me though, and didn't use high rates much.

I spoke briefly with Curtis Youngblood about pirouetting aerobatics a couple of years ago, and he offered the following practice suggestion: set your low dual rate to get slow pirouettes that you're comfortable with when you hold full rudder. It seemed like a good idea - if you commit to holding full rudder, you have only three controls to worry about, not four. I set the "low" rate for very slow pirouettes, and set the "high" rate for a clean 540. Then practiced holding full rudder and keeping the helicopter in one place, later circling around myself, and later flying circuits. Then I gradually started turning up the pirouette rate...

Then I had an idea of my own, for learning pirouetting tumbles. I set the low rate for pirouettes that were exactly as fast as my tumbles. This way, a forward flip with full rudder results in a flip with a single pirouette. At the halfway point, the helicopter is inverted and pointed in the same direction it started in (nose-in or tail-in, depending). The cyclic stick basically moves in a single smooth circle over the course of the tumble. With some simulator practice, this helped me take a big step forward.

What to do with high rates? Why, set the pirouettes to be exactly twice as fast as the tumbles. One full pirouette - and one circle with the cyclic stick - per half flip. More simulator practice, and another big step forward! I'm almost starting to get comfortable doing this in real life, three mistakes high.

What about expo?

Exponential reponse allows you to make the controls softer around the center (for smoother flying) without reducing the total servo throws (so you don't lose aerobatic potential). The stick sensitivity increases at the extremes, which some people find unsettling. Personally, I'm all for it.

When I switched to lighter flybar paddles, I cranked up the expo to about 55% at first, then down to 45% and later to 35%. It was a nice way to compensate for the change until I became more comfortable with the increased response. Now I find that 35% is a nice compromise that allows smooth hovering and predictable aerobatics.

Conversely, you can also use expo make the stick more sensitive around the center, and less sensitive at the extremes. I have no idea why you'd want to, though.