Development, construction and flight testing of the N-20.2 Arbalète

The brief debut of the N-20.1 glider caused some consternation. A large part of the flight enveloppe, especially in the high speed range remained untested. The proposal to buiId another aircraft was brought forward and was immediately accepted. This time it was to be powered with Turbomeca Piméné jet engines. Four if them.

The basic construction of the aircraft was to be identical to the N-20.1 glider. With some alterations:

– No slats
– No trim flaps
– No canard wings
– No solid rocket motor
– No towing attachments
– Observer seat replaced by fuel tank
– Main landingear wheels replaced by Dunlop wheels and disk brakes
– Modified canopy geometry
– Aileron compensation through rudder tab
– Flight control wires replaced by control rods
– dihydral of outer wing sections deleted

Projekt Arbalete N-20.2 Modell im Windkanal
The engines were positioned at the trailing edge of the wings. Two on top, two below the wings. Concerns about possible aerodynamic interferences between the engine nacelles and the fuselage proved unfounded during subsequent wind tunnel tests. What came as a surprise was a slight flow delamination outboard of the engine nacelles at high angles of attack. It was considered negligable however.
The wing structure remained unchanged with the exception of the ribs. The former version was cut from a solid block. Now they were built from plywood sheets with reinforcement stringers.
Projekt Arbalete N-20.2 Flügelbau
Projekt Arbalete N-20.2 Triebwerkeinbau

The installation of the jet engines in the wood structure was a challenge for itself. Heat dissipation under the very tightly designed cowling needed some extra engineering.

The fuel tank was a welded aluman construction. It contained 235 Liter fuel and was installed in the fuselage mid section between the wing spars. 

The cockpit was widened by 20mm and received a revised canopy.

The four engine throttles were positioned on the left hand side and the elevon trim wheel on the right hand side. 
The rudder controls are linked through control rods to a chain drive, which diverts the control cables to the left fuselage side.

Projekt Arbalete N-20.2 Cockpitbau
Projekt Arbalete N-20.2 front view
The building process of the entire aircraft took 11 month between October 1950 and September 1951.
Prior to its first flight the take off calculations were performed. Unfortunately there was a tree in the take off path which appeared to be a problem. Consequently the tree was felled.
On 16. November 1951 the Arbalète took for its first flight. Take off was at 10:24, followed by an uneventfull landing after 16 min flight time with 80 liter remaining fuel.

Test pilot Mathez stated that the aircraft has nicely balanced elevon controls but only very little rudder pressure. Further more the synchronisation of the four engines is demanding.

During the follwing 91 test flights a total of 33hrs and 27min was accumulated. The aircarft was taken through full aerobatics and reached its maximum speed of 387kts.

All pilots certifed the flight performance and handling characteristics as outstanding.

Unfortunately the original flight protocolls are missing.

Die Erprobung erstreckte sich im Geschwindigkeitsbereich 137 km/h (Abkippen) ≤ Va ≤ 650km/h (entspricht Veff = 720 km/h). Hervorzuheben ist, dass nach einer Totalflugzeit von ca. 10 Stunden die Reglage des Flugzeuges soweit durchgeführt war, dass von Piloten keine Beanstandungen mehr erfolgten. Ebenfalls bis zu diesem Zeitpunkt war die Erprobung so weit gediehen, dass ein vollständiges Akrobatikprogramm geflogen werden konnte. Die Beurteilung durch die Piloten war über Erwarten günstig. Hervorgehoben wurde die grosse Wendigkeit, rasche und genaue Wirksamkeit der Steuerung bei kleinen und gut abgeglichenen Steuerkräften. Knstflugfiguren konnten mühelos und präzis geflogen werden. Das Stabilitätsverhalten wurde ebenfalls als gut und angenehm taxiert.
Start und Landungen waren mit keinen besonderen Schwierigkeiten verbunden. Die erflogenen Leistungen waren durchwegs besser, als aufgrund von Windkanalmessungen und diversen aerodynamischen Untersuchungen voraus bestimmt waren.

The Arbalète was last flown on 20.Mai 1054. Which is well past the date of the cancellation of the entire project in 1953.

The original aircraft was on prominent display for many years at the Verkehrshaus Luzern. It was transfered to Dübendorf in 2019 and is on display at the Fliegermuseum today.

Detailed report of the performed flight test program.
Detailed description of the N-20.1 Arbalète.