3D Aerobatic Setup

Model Airplane News, Sep 2004 by Somenzini, Quique

Instead of explaining how to do a 3D maneuver, I decided that this time around, I'd describe the setup of my airplanes. The mechanical setup is the foundation from which a great flying model starts. The 3D flight mode is probably the simplest mode to set up. Why? Because in order to perform the full range of 3D maneuvers, the airplane must have the control surfaces move to their maximum travel. That's when different flight modes or dual rates become so important. When the control surfaces move to extreme throws, it's nearly impossible to fly smoothly and precisely at high speeds. So when you use dual rates or-even better-a dedicated flight mode, you can easily adjust the airplane for different flight realms, e.g., precision or 3D aerobatics.

MY TECHNIQUE

To have the very best setup for 3D, you must think about it from the beginning. When I design and build my airplanes, I work the setup into the construction process. Most of you will never design a dedicated 3D airplane, but most likely, you will assemble the latest 3D "whiz-bang" and install the radio system, control horns, etc.

The "soul" of 3D is making sure that the servos travel to their maximum to provide a minimum of 45 degrees of control-surface (aileron, elevator, rudder) movement-especially the elevator.

First, you need to verify that the control surfaces can deflect 45 degrees; if they can't, you must correct that. If the rudder or ailerons deflect only 35 degrees, that should be OK (it really depends on the airplane's design, though), but most airplanes require a minimum of 45 degrees of elevator travel. On my airplanes, I like the control surfaces to have around 55 degrees of travel. Next, make sure that the clevis is attached to the control horn directly over the hinge line. This important step ensures that the control surfaces travel equally on both sides of neutral. This is also very helpful for when you use a pull-pull cable system on the rudder because it helps the cables remain tight as they travel from one extreme to the other. When you program the radio for maximum servo travel, the servo arm should travel 60 degrees to each side. You'll understand later why this is important.

Next, I check how long the control horns need to be when the control surfaces are deflected at 45 degrees. I also make sure that the pushrod(s) don't touch or rub anywhere along the length of the wing (see "Control-surface geometry" illustration). Check the distance from the hinge line to the hookup point on the control horn. This measurement is an excellent reference to help you choose the correct servo-arm length. For example, if the measurement at the control horn is 1 ½ inches and you use a servo arm that's 1 ½ inches long, then the control surface will be deflected 60 degrees to each side. This 1:1 ratio method is a very simple way of determining servo-arm length. If you have too much deflection, move the clevis (in) closer to the servo output shaft. When you move the clevis inward, you'll gain better resolution and more torque. You might need to do a few tests until you have the throw you want.

The control horn's location on the control surface in relation to the servo is very important. A popular setup is to have the pushrod at a 90-degree angle on the servo arm when the servo is centered. This is wrong! When the servo arm travels from one extreme to the other, the pushrod moves in an arc, and with the 90-degree angle, the servo will be pulling the surface control more than it will be pushing it. If this were the aileron servo, you would have to adjust the servo travel to have the same degree of aileron travel up and down, and this most likely would result in less servo travel. The same would happen with elevator. To avoid this problem, the control horn must be placed in toward the servo centerline (see "Control-horn placement" illustration). As you can see, when in this position, as the servo arm reaches its maximum travel, the pushrod is parallel to the servo's centerline. This setup allows the control surface to travel the same amount when the servo travels to either extreme.

Another way to compensate for the servo are is to offset the servo arm at its neutral position. This way is OK, but it might take you more time through trial and error to find the best angle. If you can't relocate the control horn, your best option would be to offset the servo arm.

Here's a detail that many modelers overlook: when you use more than one servo per aileron or elevator, the height of the control horns will vary as the aileron or elevator tapers (thins) towards the tip. You must adjust each pushrod's height by measuring from the hinge line-not from the control surface. As the control surface tapers, the pushrod will appear to be higher, so don't be fooled.

HOOKING UP THE CONTROL SURFACES

When you've accomplished all of the above, use a piece of clear UltraCote to seal all the control surface's gaps from the bottom. This makes the model look better, but more important, sealing the gaps helps the control surfaces be more effective as well as have the same feel (left and right, up and down); this is very important for ailerons.