Originally by Dean Pappas for Sport Aviator
As it turns out, there aren’t all that many things to check when test-flying a new airplane, but if one of the tests described here shows a problem, you are working harder than you need to when flying your aircraft. If your model is intended for the all-important one of training mission, that’s a bad thing. If you are a more advanced flier, you are simply missing out on flying and looking better than you already do.
The mission of this series is to describe the tests and corrective actions, in a systematized way, to help you make your airplane fly better. None of it is any great effort, and you don’t have to attack it all at once. As a starting point, it is important to know that a good ground stance has the wing sitting at a small but positive angle of attack: somewhere between 0Þ and 3Þ positive. Your model’s pitch behavior can be investigated separately from something such as the unfortunate tendency the airplane has to turn left immediately after takeoff.
The proper ratio between rudder throw and nose-wheel throw is usually had when the rudder pushrod is connected to the outside of the rudder-servo wheel/arm and the nose-gear pushrod is connected to the innermost hole on the servo wheel (above) and the outermost hole of the nose-gear arm (below).
A nose-down attitude forces the model to accelerate for longer on the runway, until the airspeed is greater and the elevator control becomes powerful enough to lift the nose. This is bad because it leads to a sudden leap into the air as the full up-elevator finally takes hold. What almost always follows is a too-steep climb and a loss of airspeed and control.
The last tricycle-gear problem is the fore and aft location of the main gear. If the main gear is placed too far aft, the airplane has a great deal of weight on the nose wheel. This also makes the high-speed steering more sensitive and requires a lot of up-elevator input to break ground. Try pushing down on the stabilizer to lift the nose wheel, to get a feel for how much force the up-elevator control has to make.
Again, this can lead to an overly steep departure after an excessively long takeoff roll. It also causes the airplane to “slap” onto the ground during landing; that can add to the wear and tear on the nose gear.
The ideal location for the main gear makes the nose wheel very light when the fuel tank is empty. Either bend or shim the main gear so that the wheels move forward. The model should almost sit on its tail when the tank is empty.
There is one problem that afflicts tail-draggers and tricycle-geared models: overly springy landing gear. Sometimes the kit comes with wire landing gear that is too springy for the airplane's weight. That can make bounce-free landings difficult; anything less than a grease job is turned into a roller-coaster ride.
The solutions to this problem range from wire and rubber-band reinforcements to replacing the gear with a beefier aluminum unit.
So Why Are You Dragging Your Tail Around?
Tail-draggers have different versions of the same problems as tricycle-gear airplanes, with one interesting difference: the fore-and-aft location of the main gear.
If the main gear is mounted too far aft, the airplane tends to nose-over easily. That's embarrassing at the very least.
What is not as often appreciated is that if the mains are mounted too far forward, you get that high-speed wheelbarrow problem that was previously discussed. The airplane will be difficult to keep straight at high speed just before liftoff.
The "ideal" location is just far enough forward to not nose-over easily. It's that simple. This means that if your home field is paved, you will be setting up your landing gear farther aft than someone who calls a grass field home. Since most aluminum landing gear are made with a small amount of rake, or angle, you can move the wheels a half inch or so just by turning the gear around. You can also change the wheel position with tapered shims between the landing gear and fuselage.
Toe-in and camber adjustments are interesting, but they are secondary to the position of the main gear itself. Keep the wheels lined up straight, and you'll do fine.
Tail-wheel linkages are simpler than nose wheels. Too much control throw is rarely a problem, but sloppy or overly springy connections to the rudder can make precise corrections on takeoff almost impossible. You want just enough give to act as a servo saver.
The other trick that helps save servos is to put fewer casters in the tail-wheel assembly. Although it looks "real" to have the wire tail-wheel strut bent way back, it also strengthens the mechanical advantage of any sideward landing impact to rip the guts out of the rudder servo, beat up the bottom rudder hinge, and mangle the rudder pushrod or cables.
A short, nearly vertical strut with a small spring coil works well. For airplanes weighing 5-10 pounds, a 3/32-inch-diameter-music-wire 1/2A nose-wheel strut works beautifully.
If you still find it difficult to keep the airplane straight on takeoff because of over-control, the tail-wheel throw needs to be reduced. This is actually easy to do. For those of you using the "two springs"-type steering linkage, all you need to do is hook up to the inner end of the rudder horns and the outer end of the tail-wheel horns.
If you used the "tiller arm"-type linkage, where a single piece of wire runs along the bottom of the rudder and is attached with some kind of clip, it's a bit tougher to do this unless you are still assembling the airplane; then it is easy.
All you need to do is move the tail-wheel pivot forward and find a location for the clip on the bottom of the rudder where the steering throw is reduced. This is simple and offers positive steering control.
Takeoff, Climbout, and the Center of Gravity (CG)
Let's cover what happens on takeoff when the airplane is nose-heavy. A severely nose-heavy model will require lots of up-elevator to lift the nose wheel and break ground. The problem could also be landing-gear position, the ground stance, or the center of gravity (CG).
The last two are easy to eliminate, but you need the information you gathered in the air to tell whether to move the landing gear or not. If the CG is in the right spot, holding a constant climb angle is easier. If the airplane is nose-heavy, you will find yourself needing a quick elevator adjustment a split second after liftoff.
Let's look at the other, more urgent side of the problem. On takeoff, tail-heaviness often shows itself as climbouts that quickly become too steep, even when they did not start out that way. If you find yourself chasing the elevator in a pilot-induced oscillation (PIO), you've probably got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll too, and that is usually how they get turned back into their component parts. Try moving the CG forward temporarily, and see if it's easier to fly a smooth departure climb.