On the S.4, the flaps extended from the engine nacelles to near the wingtips.

However, external ailerons had a tendency to flutter at higher speeds, potentially causing catastrophic damage to the aircraft (but flutter was not well understood in the 1930s). In addition, external ailerons allowed the use of full-span flaps to give the aircraft a low landing speed. External ailerons operated in the undisturbed airflow apart from the wing and were more responsive during minor control inputs or during slow flight. By contrast, a вЂњpark benchвЂќ aileron is the primary control surface, and there is no other control surface integral with the wing. A Flettner tab is a supplementary control surface that attaches to and assists the primary control surface. Sometimes called вЂњpark benchвЂќ ailerons because of their appearance, they are often mistaken for Flettner tabs. The S.4 used external ailerons that mounted above the wingsвЂ™ trailing edge. In practice, the aircraft never had an opportunity to fully demonstrate its capabilities without outside difficulties hindering its performance. On paper, the S.4 appeared to be an impressive, purpose-built aircraft that could improve airmail service for the Netherlands. The aircraftвЂ™s interior was divided into three compartments: cockpit, radio room, and mail cargo hold. The aircraft was powered by three 420 hp (313 kW) Wright Whirlwind R-975 engines. The S.4 was made almost entirely of wood. The aircraft was a low-wing trimotor with retractable main gear. The Pander S.4 Postjager was designed by Theodorus (Theo) Slot, who was originally with VIH. PanderвЂ™s new design was the S.4 Postjager, and financial support came from a few Dutch shipping companies who hoped to break KLMвЂ™s monopoly on air transport to the East Indies. While the F.XX was being built, Pander took up the challenge to build a faster aircraft solely to transport mail. To improve service, KLM ordered the Fokker F.XX Zilvermeeuw, which had a top speed of 190 mph (305 km/h). The trimotor aircraft was purpose-built as a mail carrier to fly from Amsterdam to Batavia.ДЄirmail service to the Dutch East Indies involved using the relatively slow Fokker F.XVIII, which had a top speed of 149 mph (240 km/h). The Pander S.4 Postjager displays its clean lines.

Mailplane support