Flight, October 1933

THE FOKKER F.XX

   BRIEF references have been made from time to time to the two new types of aeroplane which N. V. Nederlandsche Vliegtuigenfabriek of Amsterdam have had under construction, and illustrations of the first machine of the smaller of the two types have been published (see our issue of August 24, 1933). This week we are able to publish a more detailed description and a number of photographs of the F.XX, a three-engined high-wing cantilever monoplane of typical Fokker construction (welded steel tube fuselage and wooden wing), in which high performance has been obtained without sacrificing comfort. The only direction in which greater comfort might possibly have been secured would be by doing away with the engine in the nose of the fuselage. The Fokker designers are, however, of the opinion that a twin-engined machine would not be suitable for the particular work for which the F.XX is intended, and the extra complications of a four-engined installation were not considered justified in a machine of this size; hence the three-engined arrangement was chosen. A larger machine is being built, in which the four engined disposition of the power plants will be used. The F.XX has a gross weight of approximately 9 tons.
   The high performance (maximum speed 186 m.p.h., cruising speed 154 m.p.h.) has been attained partly by fitting powerful engines, partly by using a streamline fuselage of oval section, and partly by using retractable undercarriages. The great depth of the cantilever wing has been utilised to give headroom in the cabin - there is a minimum height of 6 ft. - so that the fuselage proper is of relatively small depth. The seating accommodation provides for 12 passengers, the chairs being placed in the orthodox way, in a row along each side of the cabin, with an aisle between them. With this cabin arrangement the machine carries petrol for a range of approximately 400 miles in still air. As the cruising speed is 154 m.p.h., the duration is less than 3 hr. The tankage is actually sufficient for approximately 1,000 miles, but in that case the pay load must, of course, be correspondingly reduced.
   The general layout of the Fokker F.XX is well shown in the scale drawings and photographs. In the main it follows the well-known Fokker formula, and it is not until one sees the machine in flight with the undercarriages retracted into the wing engine nacelles that the new type obtrudes itself, although the rounded sides and bottom of the fuselage, as well as the N.A.C.A. engine cowlings, indicate careful cutting down of head resistance. If one examines the figures given in the table on the previous page, it is found that the power loading is 10 lb./h.p., and the wing loading 18.8 lb./sq. ft. With these loadings the maximum speed is very good at 186 m.p.h., giving a value of the Everling "High-speed Figure" of 23.53, which is exceptionally high for a three-engined aeroplane, and reflects the advantage of retracting the undercarriages. Wing camber flaps are fitted to the trailing edge between the fuselage and ailerons, and result in a reduction of about 9 m.p.h. in the landing speed, giving a speed range of very nearly 3 to 1. The ratio of gross weight to tare weight is 1.654, so that the machine carries as disposable load 65 per cent, of its tare weight. This is a fairly common ratio, but indicates that the cantilever wing need not lead to an unduly heavy structure if a fairly high wing loading is employed.
   The engines are Wright "Cyclones" of 640 b.h.p. each, at 1,900 r.p.m., and when the machine is fitted up for 12 passengers, and carries a crew of three, and fuel and oil for 400 miles, the actual pay load is 4,235 lb., or 2.2 lb./h.p., based on maximum power. The machine cruises on 1,280 b.h.p., however, and the pay load per cruising h.p. is therefore 3.3 lb. In view of the high cruising speed this cannot be considered an excessive power expenditure.
   As already mentioned, the Fokker F.XX is structurally similar to other Fokker machines, with a welded steel tube fuselage and an all-wood wing. The fuselage is covered with fabric, except immediately behind the engines, where the covering is electron sheet. The wing covering is plywood.
   The Wright "Cyclone" engines are installed in the orthodox fashion, but the petrol tanks, which in previous Fokker machines have been placed above the cabin, are housed in the wing on each side of the fuselage, as the central part of the wing forms part of the cabin. The tanks are of welded aluminium, and each of the four has a capacity of 143 gallons, giving a total petrol capacity, when the tanks are completely filled, of 572 gallons, which is sufficient for a flight of about 1,000 miles. Although the petrol tanks are situated above the engines, direct gravity feed is not employed, the fuel being conveyed to the carburetters by engine-driven pumps. There is a hand pump for emergency, and also for use when starting up.
   The pilot's cockpit is totally enclosed, but the side windows, which are large enough to serve as emergency exits, are of the sliding type. The seats for the two pilots are placed side by side, and are adjustable for angle of tilt as well as for position. The first pilot sits on the left, and has to the left of him the wheel for setting the camber gear flaps. Between the seats is a knob by means of which all electric leads are rendered "dead" in order to minimise the risk of fire in a crash.
   In the same compartment as the pilots', but between their seats and the front wall of the cabin, is the engineer's and wireless operator's post, which contains all the wireless equipment and the engine instruments, switches, etc., as well as the handwheel for raising and lowering the undercarriages.
   A sliding door gives access to the cabin which, as already mentioned, has seating accommodation for 12 passengers, four in the bay between the wing spars, and eight aft of the rear spar. In the wing on each side of the fuselage is a luggage compartment, while aft of the lavatory at the back of the cabin is a large luggage hold.
   The retractable undercarriages consist of forks formed by Messier telescopic legs, and of a steel tube framework for bracing the forks. The wheels can be locked in a number of intermediate positions between "fully up" and "fully down" Aluminium fairings in front of the wheels were tried, their purpose being to close the opening in the engine nacelle when the wheel was up, but they were found to disturb the flow over the tail, and are not now used. Dunlop wheel brakes are fitted. The tailwheel, which is fitted with a low-pressure tyre, is also retractable.