Sinus 503

The Pipistrel Sinus 503 is the two-stroke version of the Sinus motor glider. Airframe, span and aerodynamics are identical to the Sinus 912 – the same 14.97 m span, 12.26 m² wing, aspect ratio 18.3, composite construction and T-tail – but power comes from the Rotax 503, a two-stroke inline twin of 497 cc with two carburettors and dual electronic ignition. Power ratings vary by source: Rotax rates the 503 at a nominal 37 kW (50 hp), the Sinus flight manual states 45 (49) hp at 6,600 rpm for this installation, and Pipistrel quotes 53 hp at 6,800 rpm for the Taurus installation. Wikipedia sums the variant up as the "Rotax 503 powered variant with a variable-pitch folding propeller".

Drive is via a Rotax type B or C gearbox at 1:2.58 or 1:2.62 (1:3 optional). Buyers had a choice of propeller – the fixed BAM 2 or the adjustable, feathering Vario – and that choice determines soaring performance: with the Vario the Sinus 503 glides 1:30 at 95 km/h, with the BAM 2 only 1:23, and 1:18 remains at 150 km/h. Minimum sink at 90 km/h is 0.96 m/s with the Vario or 1.24 m/s with the BAM 2, and the airbrakes allow 5.5 m/s. Flap settings are -5°, 0°, +9° and +18°, with a CG range of 20–39 % MAC. The base version has tailwheel gear; the NW nosewheel version is covered by a supplemental sheet in the flight manual.

Under power the 503 is the more modest option: the flight manual quotes 150 km/h in cruise at 75 % power with the BAM 2 propeller, 186 km/h maximum level speed, VNE 225 km/h, VA 141 km/h, and stalls at 63 km/h in the second flap setting or 66 km/h with flaps neutral. Climb reaches 3.2 m/s at Vy = 110 km/h (against 6.5 m/s for the 912), ground roll is 123 m with the Vario propeller, take-off distance over 15 m is 215 m, and the service ceiling is 6,100 m. Fuel burn is 10.2 l/h, so 2 × 30 l gives 5.3 hours including a 10 % reserve and roughly 930 km. Roll time 45° to 45° is 4.2 s and load limits are +4 g / -2 g. On noise, the flight manual records that all Sinus versions were measured below 65 dB.

The Sinus 503 scores on value for money and – with the Vario propeller – on a glide ratio that comes close to the 912's. Its weaknesses sit in the powerplant: the Rotax 503 is no longer built, two-strokes need oil mixing or an injection pump (50:1 mixing ratio per the manual), shorter overhaul intervals and regular attention to ignition and carburettors. Climb performance is well below the 912's, which matters in mountainous terrain and on hot days. The flight manual lists 265 kg empty weight for the base model, a maximum crew weight of 180 kg and take-off weights of 450 / 472.5 kg; equipment and a recovery system push empty weight up, so only the weighing report is binding. A German DULV data sheet is available on the manufacturer's website only for the Rotax 912 UL series, not for the 503 series, so the individual aircraft's paperwork must be checked before purchase. No 600 kg upgrade exists; 472.5 kg remains the maximum.

The Sinus 503 suits budget-minded pilots with a soaring background who want a quiet, frugal aircraft for thermalling and unhurried cross-country flying and are comfortable maintaining a two-stroke. Anyone who wants dependable climb and a four-stroke should choose the Sinus 912.