On this page, some background information on the development of our LDS linkages shall be given. It was a process triggered by the random shortcomings of the RDS system installed in the series production Freestylers.

Part of these were attributed to the degraded centering performance of the DS3288 servos in many cases. But it was obvious, that the manufacturing tolerances of the metal parts in combination with the large wiper bend angle desired by customers were responsible for occasional excess free movement and return error.

Anyway, a system was desired, that mainly provided more precision around the center position of the flaps (in combination with low actuation forces), high stiffness, robustness, flexibility to mount different servos. This should still come with a sufficiently small height profile to allow realizing an all-internal linkage. Inspired by Jaro Mueller’s system on the Espadita, thoughts went back to a pushrod system. Starting around Christmas 2010, the first prototypes were constructed.

Working principle of LDS-Systems

The “Herrig/TUDM”-System

LDS-Set System “Herrig/TUDM”

Reasons that traditionally prohibit an internal linkage with a conventional pushrod design are deformation of the pushrod, slop in the servo output shaft and excess stresses on the flap hinge. All these problems were tackled with the LDS system. Double shear connections avoid flexible bending of the pushrod. Also, the neutral axis of the pushrod was kept as straight as possible to reduce the risk of buckling in case of a crash.In order to keep the bending stiffness high in the area where the rodneeds to be curved at the excenter, this resulted in the special shape. To avoid peeling of the hinge, when accidentally landing with extended flaps, the force from the horn is distributed via a carbon rod over a larger length into the hinge. This way the maximum peeling stress on the peel-ply hinge is drastically reduced. The rod solution is also particularly helpful when retrofitting existing wings.

Max.deflection of flaps with system TUDM and Servo DS 3288

Complete installation of system TUDM with Servo DS 3288

The pin diameter was selected thin, to get maximum radius of the horns. Still, just a few mm’s of arm length are possible at the servo – for which a special excenter solution was developed. To avoid bearing of the holes, the horns were made sufficiently wide. The second servo excenter is coupled to the main excenter via the pins, in order to allow it carrying some load as well. A separate bearing support rib prevents the servo output from longitudinal movements, but also reinforces the area of force introduction into the wing. Glued on both sides…

The “Herrig/TUDM”-System ist available for Servos DS 3288/JR189HV, MKS 6125 mini/Glider, Futaba S 3150 / S3173SVi and Graupner DES 587.

The “Helfrich”-System

LDS System “Helfrich”

A little bit in advance, but in close cooperation and with mutual fertilisation, Uli Helfrich developed an LDS system, too. Uli started by modifying normal metal servo horns. But manufacturing with drilling templates was too time consuming. And the tolerances of the servo splines of the horns often left much to be desired…

In the course of time, solutions for the bearing support were simplified. The initially laminated servo frames were replaced by a more pragmatic plywood-GFRP plate solution. Finally, the milled servo horns, enabled by the CNC machine, were the way to go and also allowed a reduction of the part count…
Still, the second servohorn is only coupled via friction from the pressure of the screw, but it seems in connection with the aluminium pushrod this solution works in practice. So far, the two systems coexist – one with a slightly larger focus on control precision under all circumstances, the other with more focus on user friendliness and max deflections.

The “Helfrich”-System is available for Servos MKS 6125 mini/Glider, DS 3288/JR189HV and Futaba S3173svi.