Drag of Aileron and Flap Linkages of a Sailplane Model

(Results are presented with permission of Dr. Würz)

This artists impression shows all three pushrod setups
on the lower side of the wing at the same time.


This paper is a based on work by Werner Würz, published in the papers of the ISF-Seminar¹ in December 1989 in Baden, Switzerland. Dr. Würz built and performed drag measurements on linkages, as they are used on the wings of F3B models. These models are usually equipped with flaps and ailerons, which are driven by 2 micro-servos in each wing, resulting in a total of four control horns and pushrods. Although these parts have only a small frontal area, they cause additional drag, which had not yet been examined on model aircraft.

Test Setup

The test setup consisted of the pushrod assemblies, mounted on a wing segment, which had been manufactured in the moulds of the F3B model «Wizard». This model is smaller than a current F3B model, but the wing section MH 42 can be considered as a representative section for these models. The tests were conducted in the "Modellwindkanal" of the Institute for Aero- and Gas Dynamics (formerly under the head of Prof. F.X. Wortmann), which is an excellent low Reynolds number wind tunnel. The combination of the wing chord of 0.2 m and the wind tunnel operating range leads to Reynolds numbers of 100'000 and 200'000 for these tests. Thus no high speed tests (speed task) were possible, but the results will be very similar at higher Reynolds numbers.

Side view of the three linkages on the lower surface of the wing. The wing chord was 200 mm and the size of the flap was 23% of the wing chord. Standard 2 mm diameter push rods of different lenghts were used.

The Tests

The linkages were tested on the upper wing surface as well as on the lower surface. The drag measurements were performed by traversing a wake rake in spanwise direction behind the trailing edge of the model. The resulting spanwise drag distribution was the integrated and after subtracting the basic airfoil drag, the additional drag of the setup was calculated. The lift coefficient for each Reynolds number was chosen to represent a wing loading of 40 kg/m² (Table 1).

Reynolds number Lift Coefficient Cl
100'000 0.75
200'000 0.30

Table 1: Reynolds number and corresponding lift coefficient.

The plots below show four typical drag distributions, as they have been plotted from the wake surveys. First, it can be seen, that the drag is never constant along the span, even outside of the influence of the pushrod or fairing. Similar results have been found later by M. Selig [31].

At the borders of the area of influence, a clearly visible reduction of the local drag occurs for all cases where the linkages are mounted on the upper surface. This is the result of vortices, which develop at the sides of the front linkage horn or in the corners of the fairing, respectively. These vortices destroy the laminar separation bubble on the upper surface, leading to a reduction in local drag. When the linkage is mounted on the lower surface, no such effect is visible.

This schematic image shows how the laminar separation bubble is destroyed on both sides of the fairing. The drag in this region is reduced, whereas it increases behind the fairing due to the turbulent wake of the fairing itself.

Spanwise distribution of the drag coefficient for two pushrod configurations attached to either the upper or the lower side of the wing.


In order to make a comparison possible, the drag of each setup was normalized with an area of 0.01 m². The resulting drag coefficients are shown in the figure below.

Drag coefficients of the different pushrod types (for a reference area of 0.01 m²).

It can be seen, that the covered pushrods have the lowest drag, and that in most cases it is better to place the linkage on the lower surface of the wing. Only in one case, for the short, open linkage at a Reynolds number of 200'000, the location on the upper wing surface showed lower drag. This was due to the fact, that the base airfoil has a laminar separation bubble at this low Reynolds number, which is partially destroyed by the servo lever, thus reducing the local drag coefficient.
There is a remarkable benefit from covering the pushrod, even if a simple fairing with an open rear end is used.

If we add the drag of four pushrods to a typical F3B plane, we find, that their contribution to the total drag is approximately the same as that of the horizontal tail plane. It might be worth, to consider internal pushrods and levers again, despite their mechanical difficulties and the risk of flutter, if the stiffness is not sufficient.

The drag components of an F3B model show the contribution of the individual parts.

¹ During the 1980s, the ISF-Seminars ("Internationales RC-Segelflug Forum") have been an excellent platform for the exchange of theoretical and experimental results for application in F3B models. Building techniques as well as aerodynamic aspects have been presented there as well as specific topics for the competition pilots, like winches and tactics.

Last modification of this page: 21.05.18

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