pander-s4-engine-run

Pander S.4 Postjager Trimotor Mailplane

By William Pearce

In the early 1930s, Dutch pilot Dirk Asjes was disappointed with the slow development of Dutch airmail flights and Fokker aircraft. Asjes sketched out an aircraft design and asked the aircraft manufacturer Pander to build a special mailplane to compete with KLM (Koninklijke Luchtvaart Maatschappij or Royal Dutch Airlines) mail and passenger service. Officially, Pander was called the Nederlandse Fabriek van Vliegtuigen H. Pander & Zonen (H. Pander & Son Dutch Aircraft Company). Pander was a furniture company that had expanded to aircraft construction in 1924 when its owner, Harmen Pander, purchased the bankrupt VIH (Vliegtuig Industrie Holland or Holland Aircraft Industry).

pander-s4-engine-run

The Pander S.4 Postjager displays its clean lines. The trimotor aircraft was purpose-built as a mail carrier to fly from Amsterdam to Batavia.

Airmail 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). To improve service, KLM ordered the Fokker F.XX Zilvermeeuw, which had a top speed of 190 mph (305 km/h). While the F.XX was being built, Pander took up the challenge to build a faster aircraft solely to transport mail. Pander’s new design was the S.4 Postjager, and financial support came from a few Dutch shipping companies who hoped to break KLM’s monopoly on air transport to the East Indies.

The Pander S.4 Postjager was designed by Theodorus (Theo) Slot, who was originally with VIH. The aircraft was a low-wing trimotor with retractable main gear. The S.4 was made almost entirely of wood. The aircraft was powered by three 420 hp (313 kW) Wright Whirlwind R-975 engines. The aircraft’s interior was divided into three compartments: cockpit, radio room, and mail cargo hold.

pander-s4-takeoff

On paper, the S.4 appeared to be an impressive, purpose-built aircraft that could improve airmail service for the Netherlands. In practice, the aircraft never had an opportunity to fully demonstrate its capabilities without outside difficulties hindering its performance.

The S.4 used external ailerons that mounted above the wings’ trailing edge. Sometimes called “park bench” ailerons because of their appearance, they are often mistaken for Flettner tabs. A Flettner tab is a supplementary control surface that attaches to and assists the primary control surface. By contrast, a “park bench” aileron is the primary control surface, and there is no other control surface integral with the wing. External ailerons operated in the undisturbed airflow apart from the wing and were more responsive during minor control inputs or during slow flight. In addition, external ailerons allowed the use of full-span flaps to give the aircraft a low landing speed. 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). On the S.4, the flaps extended from the engine nacelles to near the wingtips.

The S.4 had a wingspan of 54 ft 6 in (16.6 m) and was 41 ft (12.5 m) long. The aircraft had a maximum speed of 224 mph (360 km/h), a cruising speed of 186 mph (300 km/h), and a landing speed of 60 mph (97 km/h). The S.4 was designed to carry 1,102 lb (500 kg) of mail. It had an empty weight of around 6,669 lb (3,025 kg) and a loaded weight of around 12,125 lb (5,200 kg). Six fuel tanks, three in each wing, carried a total of 555 gallons (2,100 L). The aircraft had a range of 1,510 miles (2,430 km) and a ceiling of 17,717 ft (5,400 m).

pander-s4-underside

This underside view of the S.4 shows its PH-OST registration. Also visible are the external ailerons attached to the wings’ upper surfaces. The aircraft’s slot flaps (not visible) extended from the engine nacelle to near the wingtip.

Cleverly registered as PH-OST, the completed S.4 mailplane made its public debut on 23 September 1933. The Fokker F.XX also made its debut at the event, which was attended by Prince Henry of the Netherlands. The S.4 flew the following month, when Gerrit Geijsendorffer and Funker van Straaten made the maiden flight on 6 October 1933. Flight testing went well, and on 9 December 1933, the S.4 departed on an 8,700-mile (14,000-km) flight from Amsterdam to Batavia (now Jakarta, Indonesia). Flown by Geijsendorffer, Asjes, and van Straaten, this flight was a special run to demonstrate the aircraft’s speed and range and also to deliver 596 lb (270 kg) of Christmas mail (made up of some 51,000 letters and postcards) to the Dutch colony. At the time, the Fokker F.XX was being prepared for the same flight.

The S.4 had made a scheduled stopover in Rome, Italy and was proceeding to Athens, Greece when the right engine lost oil pressure. The aircraft made an emergency landing in Grottaglie, Italy, and inspection revealed that the right engine needed to be replaced. With no engines available anywhere in Europe, one was shipped from the United States and set to arrive on 22 December. This setback put the Christmas mail service in jeopardy. To make sure the mail was delivered, arrangements were made for the F.XX to pick up the S.4’s mail and continue to Batavia. But, the F.XX had its own engine issues before it even took off. This left the Fokker F.XVIII, the aircraft the S.4 and F.XX were meant to replace, as the only alternative. A F.XVIII picked up the mail and continued to Batavia with enough time for Christmas delivery. The failed Christmas flight was a huge embarrassment for both the S.4 and F.XX programs.

pander-s4-ground-side

This side view of the S.4, now named Panderjager, shows the aircraft as it appeared in the MacRobertson Race. Note the “park bench” aileron extending above the wing.

The repaired S.4 set out for Batavia on 27 December and arrived on 31 December. It made the return flight, leaving Batavia on 5 January 1934 and arriving in Amsterdam on 11 January. Although the S.4 averaged 181 mph (291 km/h) on the flight from Batavia, the aircraft’s mail flight failed to impress, and the S,4 was not put into service. Pander decided to prepare the aircraft for the MacRobertson Trophy Air Race flown from London to Melbourne, Australia.

The MacRobertson Race started on 20 October 1934 and covered some 11,300 miles (18,200 km). For the race, the S.4 was flown by Geijsendorffer, Asjes, and Pieter Pronk and carried race number 6. The aircraft had been renamed Panderjager, but some referred to it as the Pechjager (“pech” meaning “bad luck” and “breakdown”). After leaving Mildenhall airfield in England, the S.4 arrived in Bagdad, Iraq in third place at the end of the first day of the race. The next day, the aircraft proceeded to Allahabad, India, still in third place. Upon touchdown in Allahabad, the left gear collapsed, resulting in bent left and front propellers and a damaged left cowling and main gear.

pander-s4-rear

This rear view of the S.4 shows the external brace on the horizontal stabilizer and the elevators’ trim tabs. The image also provides a good view of the “park bench” ailerons.

Allahabad did not have the facilities to repair the S.4. Geijsendorffer took the propellers and traveled by train to the KLM depot in Calcutta (now Kolkata), India to make the needed repairs. This delay took the S.4 out of competition, but the decision was made to finish the race. Repairs were completed, and the S.4 was ready to fly on the evening of 26 October 1934. A service vehicle towing a light was positioned across the field from the S.4 to illuminate its path. The S.4’s crew found the light distracting and asked for it to be shut off, as the aircraft could provide its own lighting.

Once the service vehicle’s light was shut off, the S.4 prepared for takeoff. Unfortunately, the crew of the service vehicle misunderstood the instructions. They thought they were to proceed to the S.4 and illuminate the aircraft from behind. As they made their way toward the S.4 in darkness, the aircraft began its takeoff run. At about 99 mph (160 km/h), the S.4’s right wing struck the service vehicle. Fuel spilled from the ruptured wing and quickly ignited as the S.4 skidded 427 ft (130 m) to a stop. Pronk was uninjured, and Geijsendorffer and Asjes escaped with minor burns, but the S.4 was completely destroyed by the fire. The two operators of the service vehicle were severely injured.

Pander planned to convert the S.4 to a scout or bomber after the race and sell it to the military. With the loss of the S.4, there was no aircraft to sell, and Pander was not able to recover its expenses. The company went out of business a short time later.

The S.4 sits at Allahabad, India with bent propellers on its front and left engines. The de Havilland DH 88 Comet “Black Magic” suffered engine trouble, and work to repair its engine was underway as it sat next to the S.4. The S.4 never left Allahabad.

The S.4 sits at Allahabad, India with bent propellers on its front and left engines. The de Havilland DH 88 Comet “Black Magic” suffered engine trouble, and work to repair its engine was underway as it sat next to the S.4. The S.4 never left Allahabad.

Sources:
Nederlandse Vliegtuigen Deel 2 by Theo Wesselink (2014)
Jane’s All the World’s Aircraft 1934 by G. G. Grey (1934)
Blue Wings Orange Skies by Ryan K. Noppen (2016)
“High-Speed Mail Machine” Flight (7 September 1933)
“The Aerial Phost” Flight (5 October 1933)
“Opening of Amsterdam Aero Club’s New Clubhouse” Flight (28 September 1933)
“The Pander Postjager Pauses” Flight (14 December 1933)
http://www.aviacrash.nl/paginas/panderjager.htm
https://de.wikipedia.org/wiki/Pander_S4
https://en.wikipedia.org/wiki/Pander_%26_Son

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