Tuning pitch and throttle curves

Setting the pitch and throttle curves "correctly" for the different flight modes took me a while. If the messages in rec.models.rc.air and the heli mailing list are any indication, this is a common issue for neophytes in general. In an effort to keep people from spending any more fuel than necessary while tuning their own pitch and throttle settings, I thought I'd document what I learned here. If you have suggestions, or would like to share your experiences, please use the form at the bottom of this page.

What are the different flight modes?

This depends upon which transmitter you're using. It is my understanding that older and/or inexpensive radios had two modes, referred to as "normal" and "idle-up," at the other end of the spectrum, the newer, more expensive computer-based radios may allow for several different modes. My own Futaba 8UH has three flight modes, labelled to as normal, idle-up-one, and idle-up-two. I tend to refer to normal mode as idle-up-zero, but it might just be me.

Each flight mode has its own settings for the throttle and collective pitch curves, as well as a tail rotor compensation curve (a.k.a. revolution mixing, or anti-torque system). With my radio, these curves are set be assigning servo positions to the position of the left stick at five points (top, bottom, mid-stick, and the 1/4 and 3/4 positions).

There is also a throttle hold mode, which is to facilitate autorotation practice. You might think of this as a degenerate case of a flight mode: this mode has its own pitch curve, but the throttle is held at a pre-set value (typically just below the point at which the centrifugal clutch engages) and there is only one rudder trim value (rather than a 5-point curve as in the other flight modes). While I was learning autoroatation, I would set the throttle to a point just above where the clutch engages - this effectively gave my Concept 30 a driven tail, without affecting the main rotor. It's cheating, but it helped a little bit.

When I first created this page, I was flying with an old-fashioned mechanical gyro, which required quite a bit of tail rotor compensation to perform well. I'm now using heading-hold piezo gyros in all of my aerobatic helicopters, which require no tail rotor compensation at all. In fact, any tail rotor compensation curve will only make things more difficult than they should be.

The tail rotor compensation curves shown here are only useful if you're using a standard gyro. If you're using a heading hold gyro, you should set all of your tail rotor curve values to zero. The curves shown here are only here to serve as guidelines for people who still use standard (i.e. non-heading-hold) gyros.

The curves shown below all assume that your helicopter maintains a reasonable head speed at 100% throttle and 8 degrees of pitch. This is not unusual for a 30-class helicopter using 15% nitro and a muffler. If your engine is stronger, increase the pitch range accordingly.

What are the different flight modes for?

With my Concept, normal mode is used when starting the helicopter and hovering. Idle-up-one was for forward flight and mild aerobatics. As I got more and more into inverted flight and hovering, and 3D aerobatics, I began spending more and more time in idle-up-2. Now, I spend almost all of my time in this mode. Idle-up-1 is now a mode I only use for experimental setups - currently I've got it set up with a slightly reduced pitch range for slow-and-low 3D flight.

With my Futura, I only use normal mode when starting and spooling up after an auto. I used idle-up-one for hovering and idle-up-two for everything else at first, but soon realized that the 60-size machine hovers very comfortable in idle-up-2... so, like the Concept, I use normal mode to spin the blades up to speed, I use idle-up-2 for everything else, and idle-up-1 is basically just a scratchpad for new ideas. I haven't had any new ideas lately, so it's identical to idle-up-2.

The Concept's collective control feels somewhat more more sensitive than the Futura's - 30-class helis are like that because they are lighter and therefore quicker to respond to tiny changes. I compenate for this by using an idle-up-zero flight mode that has a reduced pitch range as compared to idle-up-2. The reduced pitch range is similar to a 'low rate' on aileron or elevator but here it makes the collective a bit softer. With the Futura, I can do most of my hovering exercises in idle-up-two without much trouble, so I don't use the milder flight modes very much at all.

Keep your pitch curves linear. Forget half-stick.

Once upon a time, someone suggested setting up your pitch curves so that you hover at half stick. Sometimes, that's a great idea. Other times, it just causes trouble. Forget you ever heard that suggestion. There are really only two variables when you're setting up a flight mode:

1. How much positive pitch do you want? This usually is just a compromise between how fast you want to climb (this means more pitch), and how fast you want the rotor to spin (this means less pitch). 8 to 10 degrees is typical.
2. How much negative pitch do you want? If you're learning to hover, you don't want very much. The more negative pitch you have, the more sensitive the collective will feel (you don't want that), and the faster you'll be able to descend (you probably don't want that either). Zero to negative two degrees is typical. If you're into fast foward flight, loops, and stall turns, you'll want more negative to allow you to slow the helicopter down and transition from fast flight upstairs to hovering downstairs; -4 or -5 is typical. If you're into inverted flight or 3D aerobatics, you'll want just as much negative pitch as positive pitch, and again, -8 to -10 degrees is typical.

Once you've decided on the positive and negative extremes, the middle points are easy to set up.		Let's assume you've picked -4 and +8.
Figure out the total pitch range.		12 degrees total (8 + 4)
Divide the total by two, and subtract that much from your top end pitch, and this tells you how much pitch to use at center stick.		12 / 2 = 6 8 - 6 = +2 at center stick -4 + 6 = +2 at center stick
Divide that number by two, and use it to determine how much pitch you want at the 1/4 stick and 3/4 stick positions.		6 / 2 = 3 -4 + 3 = -1 at 1/4 stick +8 - 3 = +5 at 3/4 stick
Just to be sure, make sure that the pitches you're about to set up will give you the same amount of pitch change between any two points on the curve		start with -4 on the bottom -4 + 3 = -1 -1 + 3 = +2 +2 + 3 = +5 +5 + 3 = +8

(I like to start by setting up idle-up-2 with -4 to +8 just because you get a nice whole number at each step, but it's not necessary.)

Mailing lists and message boards will show lots of people with awkward pitch curves like -5, 0, +5, 7.5, 10. OK, so you hover at half stick, but below half-stick you have 10 degrees of pitch range and above half stick you have only 5 degrees of pitch range. This means you'll get a very soft feel above half stick, and a rather twitchy feel below half stick. And you'll be hovering right on the edge... If the helicopter starts descending slowly, you'll need to move the stick quite a bit to stop it... but if the helicopter starts rising slowly, you'll need to move the stick half as much to get the same effect. This just makes things harder than they need to be. Do not worry about hovering at half stick. Worry about the top-end and bottom-end extremes, and just set the rest up for a linear response.

And now, some examples...

After you've had some time to experiment with different flight modes, you will decide for yourself what works best. In the meantime, you should try some different setups... Following are the setups that I used while I was learning.

idle-up-zero a.k.a. "normal" mode

Since I use this mode when I'm starting the engine, the throttle curve has to start with a 0% value at low stick. As with all flight modes the throttle is at 100% when the stick is pushed all the way forward, to facilitate rapid climbing in case I find myself approaching terra firma too quickly. To keep things simple, the throttle curve started out as 0, 25, 50, 75, 100%. The middle values were later changed slightly to maintain a constant rotor speed, and the end result was something like 0, 30, 65, 80, 100%.

It's difficult to do stall turns and figure eights in this mode, so I I started running -3 or -4 degrees pitch in this mode... Then I found it difficult to hover because of the increased sensitivity (more degrees of collective pitch change for the same stick deflection). Eventually I realized that what I needed to do was learn to switch to idle-up-one when not hovering. It seems obvious now, but it actually took me a while to realize the real purpose of the idle-up switch - I can have the 'soft hover' and the forward flight performance, I just had to learn to toggle the idle-up switch as appropriate.

throttle:       0,   30,   65,   80,  100
pitch:          0,    2,    4,    6,    8
tail rotor:   -30,  -20,   -1,   +4,   +9

If you use a heading hold gyro, all tail 
rotor mixing values should be set to zero.

idle-up-one

When I was doing a lot of forward flight and some basic aerobatics, I would switch into this mode as soon as I left the helipad, (well, not 100% of the time, I must admit...). I wanted to be able to do loops, rolls, and stall turns, the collective pitch is -4 degrees at low stick. To keep the rotor speed up at in the middle of a roll or at the top of a bit loop, the low-stick throttle is 60%. To keep it from overspeeding in upright descents, the throttle goes down to 25% at 1/4 stick, when the pitch is closest to 0 degrees, and where the rotor is actually autorotating a little bit in a descent.

The pitch "curve" is linear, so the point at which the helicopter hovers is a between 1/2 stick and 3/4 stick. Note that there are three degrees of pitch change between each of the points - this gives a consistent feel to the collective control.

throttle:      60,   25,   40,   65,  100
pitch:         -4,   -1,    2,    5,    8
tail rotor:    +4,   -3,   -1,   +5,   +8

If you use a heading hold gyro, all tail 
rotor mixing values should be set to zero.

idle-up-two

This is the mode I use for 3D flight, so I want the response to be identical whether I'm right side up or inverted. This means 0 degrees pitch in the middle and equal amounts of positive and negative pitch at either end. Smooth hovering in this mode is more difficult than the others, since the same stick movements now yield almost twice the pitch changes.

I initially had this set to +/- 10 degrees, but the motor was definitely struggling at the extremities. So, I dialed in the following +/- 8 settings and found that the helicopter was much happer this way.

throttle:     100,   60,   50,   60,  100
pitch:         -8,   -4,    0,    4,    8
tail rotor:    +8,   -2,   -7,   -2,   +8

If you use a heading hold gyro, all tail 
rotor mixing values should be set to zero.

After I upgraded to 30% nitro and a tuned pipe, my Concept was able to use all of the available pitch range comfortably, so I expanded all of the curves to use +/- 9.5 degrees. Coupled with a head speed of about 1950 RPM, this makes for a very sensitive helicopter, but that sensitivity serves to boost my confidence as I learn new maneuvers.

My idle-up-1 pitch range is now set up with +/- 8 degrees, exactly as idle-up-2 is described above. I use it sometimes to remind myself that you don't really need tons of power to do 3D well. I'm hoping to get more comfortable flying this way. The collective control is a bit less sensitive, which helps smooth out my flying a little bit. The maximum pitch is reduced, which reduces my top speed, which helps me keep the helicopter closer and lower. I am still most comfortable in idle-up-2, with the extra pitch at my disposal, but I'm working on using the extremes less.

Throttle Hold

I've used two very different approaches to my throttle hold pitch curve. The first, which I used to learn autos and continued with until I got into inverted autos, is all about keeping things simple. Low stick provides about -3 or -4 degrees of pitch, and the curve is linear up to about 9 degrees of pitch. From the start of an auto until the flare begins, I left the stick all the way at the bottom. If I wanted to adjust my rate of descent, I did it between flights, by adjusting the pitch setting at low stick.

Later, following a suggestion from Gary Wright, I moved to a symmetrical pitch curve - currently set to +/- 9.5 degrees (all that my Concept allows). This is identical to my idle-up-2 curve. The advantages are that I can enter and exit throttle hold from idle-up-2 without collective hiccups, and I can vary the rate of descent all I want whether I'm upright or inverted. The drawback is that I must now actively pay attention to the collective during an auto. It requires a little more brain-power to do this, so I don't recommend it until you can work the rest of the controls in your sleep. However, when you're ready for the extra control, you'll appreciate it. Keeping the head speed up during a rolling auto is all about collective control, and the more familiar that curve is, the better off you will be.

Some folks advise +12 degrees of pitch at high stick in your throttle hold pitch curve. I'm not one of them. By the time you've got that much pitch in your blades, they will probably be generating more drag than lift. After more than a half-second at this pitch, they will almost certainly have stalled out. I've experimented with extra pitch at the top, and have not found it useful. The theoretical advantage of additional lift is offset by the rapidly decaying rotor speed, and you do yourself no favors by making the collective control more sensitive.

I've also experimented (accidentally...) with a range as limited as -3 to +6, and surprisingly enough, I've never done smoother autos. Sure, there was no hang time at the end, but it touched down as gently as you could ask - which is not consistent with my normal flying style, I admit! The extra-fine collective control made things a little bit easier, and the limited negative pitch made for a relatively slow descent.

I think that using the same amount of pitch as your idle-up-2 pitch curve is a good compromise. For starters, remember to descend with about the same pitch you'd use to hover if the skids were on the other side of the rotor disk. If you're descending upright, put the stick in the inverted-hover position. If you're descending inverted, put the stick in the regular hover position. While you're rolling, put the stick in the center, at zero pitch, to conserve rotor energy during the roll. With practice you'll find yourself adjusting the pitch in response to the rate of descent, the sight and sound of the spinning rotor blades, and so on.