All moving tailplane....

[glidaguy]

To the glider boffins...what are pros and cons of fixed and all moving tailplanes on gliders....it seems to be common on the bigger themal RES type.....is it a weight saving or less drag......and if so why dont we see all moving rudders......thanks guys......Crewe.

[Ollie]

One of the main advantages of an all moving tail plane is that it allows decalage rigging changes without shimming one of the flying surfaces. One typical arrangement is for the two sides of the horizontal tail to plug together with joiner wires with the roots butting against the sides of the fin. In this arrangement there is some gap between the stab root and the fin side which allows air to flow through and produces some additional drag. Another arrangement is to mount a one piece stab on a v-mount that allows the stab to pivot. The V-mount drag is usually less than the gap drag of the first case.

A well designed and constructed horizontal tail with an articulated elevator is as effective as the all moving stab but it is not as convenient to rig decalage. The drag of the articulated elevator can be as low or lower than the drag of the all moving stab for elevator deflection angles of less than about 3 to 5 degrees but for larger deflections, where the flow is separated, the articulated elevator drag is greater.

All moving vertical tails are rare but C R Richardson kitted a hand launch glider design with an all moving vertical tail that worked very well. I designed and built a 10 foot span thermal soarer with an all moving vertical tail that was very effective. One of the minor disadvantages of the all moving vertical tail is the need for a long control horn because the maximum deflection angles are so much less than with an articulated rudder. The long control horn can't be easily concealed in a slim fuselage. A possible disadvantage of an all moving vertical tail is that it has to be mounted entirely above the fuselage to provide ground clearance and prevent damage in a ground loop. With all the vertical tail above the fuselage, there are torsional loads on the tail boom that must be accomodated. This is the reason you will probably never see an all moving vertical tail on a successful discus launched glider.

If the pivot axis of an all moving control surface goes through the aerodynamic center of the the control surface, the load on the servo to deflect the surface is theoretically zero. It is so low in practise that the smallest micro servos can do the job. It is necessary to keep the mass of the control linkage to a minimum or, in the case of a hard landing, the inertia of the linkage can strip the gears of a micro servo. Nowadays micro servos are so light that they can be mounted in the tail and with aerodynamically balanced control surfaces are entirely practical on large thermal soaring models. The weight of tail mounted micro servos and wiring can easily be less than the weight of some popular pushrods running down a long tail boom.

All moving tail surfaces use smaller deflection angles than articulated control surfaces. This requires that any slop or elasticity in the linkages be proportionally less for the all moving surfaces.