Proper mechanical setup is a very important factor in how well an RC helicopter will perform. With all the low-cost and very capable computer transmitters available it’s easy to lose sight of the basics of mechanical setup and rely on programming to overcome issues that should have been addressed mechanically.
Many heli pilots I know spend a lot of time setting up collective pitch and spend very little time setting up their throttle. They think that as long as the throttle goes from zero to full throttle it is properly set up. That couldn’t be further from the truth! It is critical that when we set 50 percent on our throttle curve that we actually have 50-percent barrel opening on the carburetor, otherwise the numbers on our throttle curves will make no sense, and it will become next to impossible to get the correct relationship between throttle and pitch throughout the entire stick range. The result will be head speeds that are impossible to tame and vary by hundreds of rpm throughout the collective range. And if the head speed is constantly changing, the gyro will have a very hard time trying to control the tail of your heli.
Fortunately, there is a very straightforward method of getting the mechanical setup correct on the throttle and this month we are going to look at how to mechanically set up throttle on a nitro heli. This setup will work with any heli and radio, but for this example I will be using an Align T-Rex 600N and a Spektrum DX7 radio.
The easiest way to begin this setup is with a fresh model on your transmitter or by performing a data reset. That way, we are sure there are no unwanted trim, subtrim, curves, or mixes that will prevent a perfect mechanical setup. If you really want to use an existing model, please make sure the throttle travel adjust is at 100/100, throttle subtrim is at zero, the normal throttle curve is linear (0, 25, 50, 75, 100), and there are no throttle mixes active. In addition, move the throttle trim tab all the way to the bottom; do not leave the trim in the default center trim position! The center position will wind up being very close to our idle position when we are through and the bottom position will be a fully closed throttle. This setting is very important to make sure that later we can get a stable idle at around the center of the trim adjustment.
LINKAGE ROD LENGTH
Set the linkage length so the centers of the ball links line up exactly over the centers of the throttle and servo arms.
The first thing we must do is set the length of the linkage rod. To do this, either measure the distance from the center of the servo output shaft to the center of the carburetor output shaft and adjust the ball link until the centers of the links match this measurement, or adjust the linkage rod until the center of the rod end holes are exactly over the centers of the servo and carburetor output shafts by holding the linkage over the output shafts (see photo 1). This adjustment must not change during the rest of the setup. Next, put a linkage ball about 13mm from center on the carburetor arm (this is the outside hole on an O.S. Hyper 50 engine). This distance may vary a bit if you have another engine, and that’s OK. The idea is to have the ball far enough from the center to allow the travel adjust to be at somewhere around 100/100 later in the setup process. It’s important to not have too little resolution by being less than 80/80 travel adjust and not to have too much of over 110/110 so you don’t over drive the servo if you decide to use throttle mixes.
SERVO ARM SETUP
Set the servo arm so it is at 90 degrees to a line from the centers of the servo and throttle arms.
Now it’s time to set the position of the servo arm on the servo. This is a very important step to make sure we don’t get strange differential in the throttle that would be impossible to mix out with only a 5-point curve. Fortunately, Radio Control Helicopter magazine has provided a tool for this. Find one of the subscription card inserts and use it by aligning the edge covering half the output shaft holes of both the carburetor and the servo and use the top of the card to show the position of the servo arm by aligning the servo arm holes. Photo 2 shows how this easily allows us to set the perfect 90-degree relationship of the servo arm to the carburetor, which is critical! If you need to use a small bit of subtrim to get this perfect it’s OK. Try to keep the subtrim to 15 or less and pick another orientation of the arm or use a different arm if you exceed 15. Now that the servo arm is positioned, install a linkage ball 13mm from center (install the linkage ball on the inside of the servo arm for the T-Rex 600N. This may differ if you have another helicopter. Follow the manufacturer’s instructions if you don’t have a T-Rex 600N).
Adjust the arm so that the left carb body mark lines up with the throttle barrel mark at 50-percent throttle.
Shoot for between 80 and 110 on the travel adjusts to maintain good servo resolution.
Now comes the tricky part of the setup. Connect the linkage to the servo ball but leave the carburetor end disconnected and tighten the carburetor arm just snug enough to hold position, but loose enough to turn with just a little force. We’ll tighten it fully after we are done. Go to the normal throttle curve screen on the DX7 and move the throttle stick to exactly half. The reading should display in 50 and out 50 on the bottom of your throttle curve screen. Now adjust the throttle barrel on the carburetor to the left of the center by three marks (see picture 3); this is the mechanical 50-percent barrel opening of O.S. Hyper 50 engine. For some reason, many pilots seem to think the center mark is the 50-percent position but it’s not, so please don’t make this mistake or it will induce differential into your setup.
With the carburetor barrel marks lined up on the left mark, adjust the arm so that you can rest the ball link directly on top of the ball and tighten the carburetor arm. Change to the travel adjust screen of the DX7 and move the throttle stick to full open (the ball link should not be connected yet) and adjust the travel adjust until the ball link fits on the top of the carburetor arm when you manually open the carburetor to its fully open position. Just for reference, my travel adjust is at 83 on my setup. Yours may vary a bit, and if you want to achieve a perfect 100 just move the servo ball in a little on the servo arm. In my experience, anywhere from 80 to 110 will work very well.
Make sure the throttle is at 50 percent when adjusting the servo and throttle arms.
Adjust the throttle trim all the way down on the DX7 radio before you begin the setup.
If you were accurate with your 50-percent throttle adjustment earlier, you can move the throttle to the fully closed position manually and perform the same adjustment procedure as the
fully open throttle for travel adjust. You will wind up with exactly the same travel adjust number for fully closed. If the high and low throttle travel adjust numbers are off by more than three or four, take another look at your half stick adjustment because it is slightly off and needs to be readjusted. When you get the half stick right, the fully open and closed positions will fall into place perfectly. Now you can put the link on the carburetor ball and make sure the arm is tight (it’s a good idea to Loctite this screw). You can also move your throttle trim tab back to the center position. You may have to advance the trim tab a few clicks for a good idle, but this will be very close. Your throttle setup is not only complete now, it’s right.
After doing this procedure several times it will become very easy and very quick, and the difference it makes is huge. It is really important to know that when your throttle curve shows a certain number, like 35 or 50 or whatever, you know you have exactly that amount of carburetor opening. It also means that if you run a governor it will have an easier time keeping a constant rpm because it doesn’t have to compensate for unintended differential in your throttle setup. The first time you do this procedure it may take time, but after the first time you fly after taking the time to set up your throttle correctly you’ll always take the time for it in the future. And show your friends at the field, too; even the plank flyers.
TEXT BY JOHN GILMORE; PHOTOS BY JOHN REID