| No. | Cause | Fix | Comment |
| 1 | Wheelbase error. Distance between main wheels too large. | Reduce the wheelbase width. | See below. |
| 2 | Wheelbase error. Wheelbase too far forward of CG point. | Move wheelbase back towards CG point. | See below. |
| 3 | Fin/Rudder area too small. | Increase Fin/Rudder area. | See below. |
| 4 | Castoring tailwheel | Fix the tailwheel, or make it steerable. | Suggested by Gordon Whitehead: No initial resistance to torques present in the yaw axis as the aeroplane begins to move. This leads no directional control and strong ground loops before the aeroplane gets moving on the takeoff run. In full size aircraft which use castoring tailwheels, these problems are overcome by fixing the tailwheel during takeoff and landing, and allowing castoring again when the pilot is taxiing. |
| 5 | Tail skid on tarmac runways. | Replace tail skid with a fixed or steerable tailwheel. | Suggested by Gordon Whitehead: Lack of friction between the skid and the tarmac allow the yaw torques to dominate as the the aircraft starts to roll and so the problem surfaces. |
| 6 | No wind. | Any of the above. | Suggested by Gordon Whitehead: With a head wind for takeoff there is some rudder/fin authority at standstill, i.e. weathercocking. This means on calm days, the problem, if it exists, will worsen and so manifest. |
| 7 |
Over zealous use of rudder, pilot error. Idea for point 7. contributed by Adrian Smith. |
i.)Take care with rudder use. ii.)Reduce rates on rudder. iii.) Add some exponential on rudder. |
While this point will not usually lead to ground loops, it can definitely lead to weaving, and at worst, loss of control. Where dual rates or flight modes are available the pilot will usually switch to low rates on rudder and test ground handling before committing to a takeoff run. When ground handling is good the takeoff should also be good. As the takeoff run proceeds and ground speed builds up the rudder will become more effective aerodynamically, and adjustments to tracking should now be gentle, often dabs of rudder being all that is necessary to keep on track before takeoff. |
Comment:
Efforts with rudder to control the turning motion during the takeoff run lead to the aircraft first turning to the left, then right, then again left etc., i.e. weaving, and in extreme cases will ground loop, i.e. spin around the vertical or yaw axis in the horizontal plane.
During the takeoff run, engine reaction torque creates extra force on left wheel and less force on right wheel. This causes angular acceleration of the aircraft's Center of Gravity about the left wheel initially, i.e. the left wheel behaves like a fulcrum or pivot, and so the aircraft swings to the left.
If the model pilot is able to correct the initial turn to the left, with right rudder, then as soon as the correction begins to occur the aircraft will tend to ground loop in the opposite direction, i.e. to the right.
Essentially the aircraft is exhibiting instability about the vertical or yaw axis, while on the ground, and the root cause is the buildup of angular momentum, which is greater than the stabilising forces the fin/rudder can counter.
If the model pilot is able to get airborne, the fault disappears as the fulcrum is no longer present. The risks of taking off with this condition are considerable however and are deliberately shown in the video. The model pilot should first fix the ground looping problem.
The torque developed to produce this angular acceleration about the yaw axis is proportional to the fourth power of the radius, where the radius concerned is the distance between the aircraft's CG and the left wheel. This fourth power effect means that even small reductions in this critical radius will make a very significant reductions to the weaving and ground looping tendency, i.e. a small reduction in wheelbase width, or small rearward movement of the under carriage, (as this again reduces the critical radius), will greatly reduce the ground looping tendency.
Comments
April 24th. 2016
Point 7 added in line with text submitted by Adrian Smith.