Setup Throttle & Pitch Cuves

Throttle curves and pitch curves are some of the new terms you'll run into shortly after beginning the helicopter hobby. Short of a handful of RTF choppers, every model heli out there uses both throttle and pitch curves in its operation, and it's important to understand and be comfortable with the way they work. This topic was touched on in a previous issue, but we decided that it deserved another look. Don't worry, pitch and throttle curves will lose their mystery after a little practical use and experimentation.

What are These Curves?
Computer helicopter radios have many great features that give pilots a large number of options in their setup. Two of these features are the throttle and pitch curves. In most modern radios, these curves are presented as a graph, usually containing five to seven changeable points, with low to high on the X axis and 0 to 100% on the Y axis. The X axis represents the throttle/collective stick position (low being all the way down and high being all the way up). The Y axis represents the output to the servos or ESC. There are usually separate sets of both throttle and pitch curves for all flight modes. This means that for every flight mode you have enabled on your radio, you will need to set the curves for them.

The bottom line is this: The throttle and pitch curve menus allow the user to decide how much or how little throttle and pitch the heli will have at a given stick position. As a side note, after initial setup, establishing throttle and pitch curves is identical in both MPM and CCPM models.

Initial Mechanical and Radio Setup
Before sizing up the mechanical linkages and servo horns, it's important that the radio curves be in a neutral setting. To do this, set all curves as a straight line so that the low stick is at zero, the middle point is set to 50%, and the high stick is set to 100% (you can inhibit the other points and they will set themselves to line up with the others). By default, most radios will already have the curves set this way for a new model.
Next, go through the standard setup, getting the servo throws and links adjusted on the model so that the collective is zero degrees in the middle, and max pitch (most use 10 degrees or so) is the same for both full positive and full negative on the throttle stick. Do all this mechanically, leaving the servo end points at 100% wherever possible. Also, set the throttle servo so it goes from full closed to full open. Do not worry about these settings; even if you don't plan on using the full pitch or throttle range, they can be dialed out later using the curves and end points. Setting a heli up this way allows you to use the radio to change settings as you progress--without needing to change anything mechanically later on.
Your heli should now be at a point where full stick gives max pitch, and full throttle and low stick closes the throttle and gives full negative pitch. Next, we will look at the pitch curves, which are set the same way in both nitro and electric helis.


Pitch Curves
There are a few schools of thought on pitch curves as they pertain to the various flight modes. Most would agree that the Idle Up modes (often referred to as 3D modes) should leave the pitch curve at the linear 0-50-100% settings, going from full negative to full positive pitch, just as we set it up during the initial build above. Where the differences come in is with the normal and the throttle hold (autorotation) modes. For beginners, it may be best to set the normal mode collective so that low stick has very little negative pitch. A pitch range of 10 on top and -2 or -3 on the bottom works very well. This will help with gentle descents and landings, and will lessen the fall rate if the pilot panics and slams down the stick.
There are two standard methods employed to lessen the lower pitch range: using the pitch curves, or using the servo end points. It's best to use the pitch curves for this task. When you make drastic changes to the servo end points, the servo movements become non-linear and their resolution is changed. There is another huge benefit to using curves to make pitch changes instead of using end points or mechanical adjustments: Each flight mode can be changed independently of the other. If you use mechanical or end point changes, every flight mode will be affected by those changes. In other words, try to keep the servo throws as close to 100% on both sides as possible. However, there are times when the use of servo end points may be necessary for modifying pitch range, but the curve menus are there to make these changes, and can do so quite easily.

Setting Normal Mode Pitch Range:
1. Place a pitch gauge on one of the main blades and enter the pitch curve menu for the normal flight mode on the radio.


2. Bring the lowest curve point up from zero to about 35%, and check the low stick pitch with the gauge. If the pitch is too high (above -3 degrees or so), lower the percentage. If it's still below your target point, bring it up some more to get the low stick pitch to the -3 we are looking for.



3. You can leave the other points inhibited and the mid-point at 50% and the high at 100%. This will make it so that when you flip to Idle Up modes, the collective will stay in the same place as long as you are above mid-stick.


It's also simple to use the pitch curves to change the point in the stick travel where the heli will hover. Some pilots prefer to have their normal mode set up so that hovering takes place at mid-stick and with limited total pitch movement. To do this, you can change your middle curve point by raising its value until you get to about 6 degrees on the pitch gauge with the throttle stick in the center of its throw. Using the same method, you can lower the maximum pitch (full stick) to 8 or 9 degrees so that the line is as close to straight as possible. What this does is lessen the angle of the curve line, which lessens the amount of servo movement you get for a given stick movement. This is really nice for beginning pilots because the sticks are not as sensitive when set up this way. With a low of -3 degrees and a high of 8 degrees, hovering will be really docile and precise. Even if you slam the stick up or down in this mode, the heli will not change altitude too fast and you have more time to recover.
The pitch curve for throttle hold is set the same way as normal, except that you will want -4 or -5 degrees on the bottom and full pitch on the top. You need the negative degrees to get the rotor spinning nice and fast during an autorotation, and you will need all the positive pitch you can get to slow the descent and land at the bottom. Many people use the same pitch curves for both normal mode and throttle hold. In fact, some radios use the normal mode pitch curve for throttle hold as well.
Last, the other school of thought on the normal mode pitch curve is to leave it the same as the Idle Up modes; zero (FULL NEGATIVE PITCH) at the bottom, 50 in the middle (0 DEGREES PITCH), and 100 at the top (FULL POSITIVE PITCH), with all other points inhibited. This gives the heli more collective movement with the same stick input, so it is more sensitive to stick changes. But it does help your future flying greatly if you can learn to hover and land nice and stable using the same full collective range you will be using in aerobatic flight. I would recommend setting it up with the lowered pitch range first and trying that out for a while. If you then want a new challenge, try the full range. Again, the beauty of using pitch curves is that you decide how to set things up and it is all done with the radio. No changes need to be made to the heli itself after initial setup, and it only takes a pitch gauge and a few minutes.

Throttle Curves
Once you have mastered setting up pitch curves, throttle curves are a piece of cake. You will use the throttle curve to tell the ESC or throttle servo how much power to give the motor at a given stick level. Throttle hold mode usually does not have a throttle curve associated with it. Instead, you can set a flat line amount indicating where you want the throttle to go when it's in that mode (usually to motor off in electric and to idle in a nitro heli).
The other flight modes are set up differently depending on a number of factors, including whether the machine is electric or nitro, and if you are using a governor/limiter. The key ingredient is this: You generally want to maintain a constant rotor head speed no matter where the stick is positioned. Throttle curves are used to achieve this goal. The four common heli setups for throttle curves are electric non-governed, electric governed, nitro non-governed/governed, and nitro with a throttle limiter. You will need to fly the heli and have a tachometer or other way of measuring available head speed to get the throttle curves set correctly.


Electric: Non-Governed
In normal mode on an electric heli, start with zero as the lowest point - a spot where the ESC is turned off. As your pitch increases, you will need to add more and more throttle to maintain the desired head speed. Most normal mode throttle curves slope up from zero with a curve that flattens out as you get closer to full stick. The throttle curve will be pretty steep from zero stick to about one third stick. The 75% open throttle point will generally fall on the same part of the graph as your 0 degree pitch range does in the pitch curve menu (where this spot lies on your throttle curve depends on rather you are hovering at mid stick or at 1/2 stick). If you are not using full pitch in the normal mode, you will want the throttle curve to taper off below 100% as well to avoid over speeding the head.
For your Idle Up modes, use a simple V curve with 100% at the low and high ends and about 80% in the middle. This will give you full throttle at both ends of the stick for 3D flight and will lower the throttle slightly when at 0 degrees to stop any over speeding. You may find the need to adjust the middle curve point a few degrees to get a consistent RPM - again, use a tachometer to determine where the curves need to be set to. Please see the photos with each section to help clarify the meaning, like they say: "a picture is worth a thousand words."


Electric: Governed
In normal mode leave the lowest point at zero for start-up, but set the other points to an equal level, creating a flat line. Start with a 70 setting all the way across on the curve and see what kind of head speed that gives you. Raise or lower the points, leaving the first point at zero, until the desired normal mode head speed is obtained. For Idle Up modes on a governed electric helicopter the throttle curves are usually a flat line all the way across. No V curve is usually needed with the better-governed ESCs.


Nitro:
Non-Governed/Governed
Nitro helis are more complex with their throttle arrangements. In normal mode, you will start with zero as the lower point. As your pitch increases, you will need to add more and more throttle to maintain head speed. Most normal mode throttle curves slope up from zero with a gentle curve. The 50% throttle point will generally fall on the same part of the graph as your 5/6-degree pitch range in the pitch curve menu. If you are not using full pitch in the normal mode, you will want the throttle curve to taper off below 100% as well, to avoid overspeeding the head.
The Idle Up modes are set using a V curve, just like on electric helis. The exception is that instead of 80% being the middle point, most nitro helis start off with 50% as the middle point. A curve of 100-50-100 is a great starting place on a nitro heli's 3D flight modes.



Nitro: Throttle Limiter
A nitro with a throttle limiter could be set up the same as the example above. The normal mode is, in fact, set in the same way. But many will set the Idle Up modes to be 100% all the way across, letting the throttle limiter keep the heli from overspeeding while getting all the power they can out of the motor. If the throttle limiter fails, they can flip back to normal mode and land safely without overspeeding. If you notice some over speeding with this standard "V" curve, it may help to pull the intermediate points (those between the middle and ends) down in value. This will form more of a "U" shape instead of a "V". The only way to know how much or how little adjustment is needed is to get out there and test it in flight.


Conclusion
The topic of throttle and pitch curves can be a pretty complex subject. But their use becomes second nature with some practice and real-world use of the menus. To keep it simple, remember that the main goal of both of these curves is to keep the head speed constant at any given pitch range, and to deliver the amount of lift desired.
Throttle and pitch curves give the pilot complete control over how much pitch and how much head speed the rotor should deliver. They are powerful tools that allow dramatic changes to flight characteristics without making a single mechanical adjustment to the helicopter. With the information found in this article, you should have a decent starting point from which you can fine-tune these curves to get your heli flying exactly the way you want.