...Does anybody knows how to position the main wheel of a taildragger configuration regarding to Center of Gravity or MGC?

From : Don Stackhouse

There are a number of factors that influence this. However, there is a general rule of thumb.

First, plot the C/G of the aircraft on a side view, shown with the fuselage level. It's important to show the C/G in its true position both horizontally and vertically. Draw a line downward and forwards from the C/G. The line should be angled 5 to 15 degrees forward of vertical. The center of the wheels should lie on this line, and far enough below to meet ground clearance requirements.

A greater angle, closer to 15 degrees, give more nose-over resistance, but makes the plane more prone to ground loops and bouncing at touchdown. The aerodynamic damping in pitch and yaw also influences this.

Conversely, a more aft position of the line, closer to the 5 degree angle, gives a plane that is more resistant to bouncing and ground loops, but more likely to nose-over.

If the aircraft you're working on is a flying wing, with their general tendency to have marginal damping in pitch and yaw, you should probably go for something closer to the 5 degrees setting, and also consider a light skid under the nose in case of a nose-over.

If your airplane has plenty of tail on a long tail moment arm, you can use a more forward position. Tail moment arm is probably the biggest single factor in pitch and yaw damping, and in general taildragger airplanes with very long tails tend to have very good ground behavior, as well as good dynamic stability in flight. One of the best ground-handling R/C models I've flown, taildragger or tricycle-geared, had the mains way forward of the C/G, considerably more than that 15 degree angle, which should have made it a bouncing and ground-looping monster. However, it had a huge tail on a very long moment arm, and any incipient bounce or ground loop got immediately snuffed out by the plane's natural dynamic stability before it even had a chance to get started.

As far as tailwheel position is concerned, it should put the wing at or slightly above the stall angle of attack when the plane is in the three-point position. Having the attitude lower than this can make the plane vulnerable to gusts during rollout and taxiing, and can also make the plane prone to "belly flops" if the pilot tries to do a full-stall landing, which would result in the tailwheel touching while the mains are still quite far off the ground. Conversely, setting the attitude too high can make judging the touchdown in a 3-point landing more tricky. It can also make the plane more sensitive to ground-loops and bouncing, because of the further aft position of the C/G when in three-point attitude. In some cases it may make sense to use a fairly high angle of incidence between the wing and the fuselage (higher than in-flight considerations might dictate), to get the wing stalled in three-point attitude while still keeping the fuselage at a reasonable angle, and the landing gear at a reasonable length.

Altogether it's a judgement call, based on what sorts of pilots you expect will be flying your design, and the sorts of places and circumstances where they will be trying to operate it on the ground.

Don Stackhouse
DJ Aerotech