Throttle Setup by Jason Priddle This article is written around JR Radio convention. The numbers noted are for illustrative purposes, and the same principles apply to all radios Ever feel like all your motor s power is in the top or bottom part of the throttle? Having a hard time getting your helicopter throttle curves set up? There s a good chance it all boils down to simple setup errors that can be fixed in a few minutes. To correct our issues, we should first understand a few things about our radios and servos. Section 1: Radio Background First, lets talk about subtrim. In the last article, we learned that servos receive pulses from RXs in between 900μs to 2100μs. Center point is normally around 1500μs with 100% ATVs falling around default values of 1100μs and 1900μs. The most important thing to remember about subtrim is that it not only shifts the center, it also shifts the end points off of defaults (end points are floating ). The next thing to note, which may seem obvious, is that is that ATVs affect only the end points, not the center point. Further, the ATVs do not define any particular servo position, but rather the distance between the center point and the end point (that is, the default 1100μs and 1900μs 100% endpoints are not defined points, but rather the result of an offset from the subtrim- defined center point again they are floating points). An ATV of 100% means that there are 400μs between the center point and the end point. Likewise, 50% means there are 200μs between the center and end points, and so on. The final salient and most important point is the relationship between the throttle stick and the points set in the transmitter programming. When the throttle stick is at the center point, the servo goes to the position defined by the subtrim. When at idle or full throttle, it is at a position defined by the subtrim plus or minus the difference defined by the ATV. So why is this important? You cannot just hook up a throttle servo and set the ATVs to cut- off and full throttle and expect nice linear throttle response. In an extreme example, if you set the low ATV to 0% and the high endpoint to 100%, absolutely nothing will happen during the first half of throttle stick movement, your servo will not even move. Absolutely all power will be realized in the last half of stick movement. This is obviously a bad thing.
Section 2: Servo Notes Servos aren t produced perfectly. No two servos will have the output splines aligned the same when they are centered up with zero subtrim in the radio there are simply too many tiny gears, potentiometers, and motor poles to line up to make this possible. Because servos aren t perfect, we can t build our mechanical setup around the servo. We must build the mechanical setup around perfect geometry, then manipulate the radio (subtrims and ATVs) for the perfect setup. Servo arm splines are most often unevenly spaced, by design. A multi- point servo arm (like a star, or wheel) can often be rotated every 90 degrees. It will form slightly different angles with the splines and linkage in each position to help achieve a perpendicular setup before trim is used. This tactic minimizes the subtrim you need to use. This is especially helpful when using very large ATVs. In this case the potential exists to run out of servo movement if using large subtrims (because though the endpoints are floating, they can t go past certain defined limits). However, for throttles where ATVs generally fall in the 100% range, servo arm orientation perfection isn t especially important. Section 3: Putting It All Together - The Ideal Setup 1. Connect the pushrod to the throttle. Ensure that the pushrod is 90 degrees to the throttle arm at mid- stick and moves the same amount in both directions. On some setups, especially helicopters, this may require re- adjustment of the throttle arm angle on the carburetor. 2
2. Ensure servo reversing is set in the correct direction. Center the radio subtrim, and set ATVs to 50%. Put the throttle stick at its mid- point. 3. Throttle Trim a. For a gas/glow setup, set the throttle trim to full low, or; b. For an electric setup where min throttle is motor kill, put the throttle trim to the middle, or; c. For an electric setup where full low throttle results in a certain amount of low RPM operation (better motor acceleration), set the throttle trim mid- way between full low and the middle. WARNING Please be sure to remove the prop or pinion when preforming setup on electric models. 4. Power up the receiver/throttle servo. 5. Test various servo arm orientations to find one as close to perpendicular to the pushrod as possible. When the ideal location is found, secure the servo arm to the servo. 6. Remove power from the receiver/servo. 7. Adjust the servo arm length and the pushrod length so that the throttle servo has about 45- degrees of throw, in both directions, from the perpendicular point. You can carefully rotate the servo arm by hand when making these adjustments. After the ideal lengths are determined, secure the pushrod to the servo arm. 8. Power the radio/receiver. 9. With the throttle stick and trim full low, increase the low ATV value to just before binding (or just until the ESC arms in the case of electric). 10. Advance the throttle to full, and increase the high ATV value to just before binding. 11. Observe the low and high ATVs are they the same or different? If they are different (not within 2% or so), note which one is higher. 3
12. Return the throttle stick to the mid position, and adjust the subtrim in the direction of the high ATV. If the full throttle ATV was higher, add subtrim that moves the throttle toward a higher power setting. 13. Reset the ATVs to 50% and repeat steps 8 through 12 until the ATVs in both directions are equal. Well, that was a thirteen step iterative process, but it will be worth it in the end. One thing to note gasoline engines generally produce more power in the lower part of throttle travel. You can either use throttle channel expo/curve if your radio permits, or you can INTENTIONALLY set a lower value for the low ATV than the high ATV (by using subtrim toward low throttle). This will reduce stick sensitivity below mid- throttle and increase it above mid throttle. For electric setups, sometimes it s good to do exactly the opposite, or to adjust the expo in the ESC if able. Not as good as a throttle curve or radio expo, but the tactics work in a pinch!!!! Section 4: Throttle Trim Re- Attack Our discussion warrants one final subject throttle trim. In our setup procedure we specified throttle trim positions for various IC/electric setups. We set the ATV just prior to servo binding (or ESC arming): The cut position. Here s the deal on why. When you move the throttle trim away from the center position, it skews the lower part of the throttle travel. In essence, it adds a curved mix to the low throttle region of travel. Low throttle trim acts like inverse expo, high throttle trim acts like expo. We obviously don t want to fly with this curve in place. We also know that the two ATVs set the volume of travel above half- stick and the volume of travel below half- stick. In other words, they set the sensitivity of the throttle stick in each region of travel. In the pictures below, throttle stick movement is left/right and channel output is up/down. You can see the effect that trim has on output. When the artificial throttle curve created by full- down throttle trim (gas/glow setup) is added to the nice even ATVs we set up, the throttle sensitivity is increased below half- stick, right? Low Throttle Trim Mid Throttle Trim High Throttle Trim 4
Case A (Gas/glow) It just so happens that for 99% of gas/glow applications, when you advance the throttle trim to set an idle RPM, it ends up fairly close to center (after setting up the ATVs while the throttle trim is full down). This eliminates the curve created by the trim and the result is even throttle movement throughout the stick range, like the middle picture above. Case B (Electric) With an electric setup, since people most often want the motor to stop at low stick (i.e. we aren t advancing the trim to set an idle RPM), we set the trim to the center during ATV setup. This results in linear throttle response (discounting any expo built into the ESC). Case C (Electric) For the last case, where you want some RPM at low stick, I ve found that I generally use about one- quarter or less of the throttle trim to set the idle RPM. By setting the throttle trim to one- quarter during ATV setup, and advancing it to one- half to get the desired idle RPM for flight, the result is linear throttle response (again discounting expo built into the ESC). This is ESC dependent, and you might have to play with it to get the right cut- off trim position. Note that you ll need to kill the motor with throttle trim or a throttle- cut switch. The key for all three cases is that throttle trim ends up around center when you push the stick up for takeoff. Section 5: Wrap- up Hope you found this article useful. Next month (putting blind- fold on and throwing dart)... If anyone has any ideas for articles they d like researched (about RC setup or aerodynamics, not pirate ship lore) I d be happy to take suggestions! Acronyms ATV Adjustable Travel Volume ESC Electronic Speed Control IC Internal Combustion JR Japanese Remote Control Co., Ltd. μs microsecond RC Remote Control RPM Revolutions Per Minute RX Receiver 5