European executive.Page 2
Since I did not get any flying in the P.166 and it was rather early days to describe its luxury values - it was full of automatic observer and only two pilot's seats- the soundness of the structural design would appear to be the most interesting point to make at this time. I might add, however, that I saw the second aeroplane approaching completion, while the main components of a third were well advanced in production type jigs. I also examined a fuselage which had been subjected to full static testing and which showed no signs of failure or of wrinkles indicating that the material had in any place been stressed beyond its elastic limit.
Before going on to the details of the structural design itself, I would add that I have seen Piaggio production over the past seven or eight years, ranging form the P.136 in its original form, through the P.148 and the later P.136L to the P.149 - and on every occasion I have been impressed by the care taken over interchangeability of components. In point of fact, the exercise of supplying the P.136L airframes from Italy for furnishing and assembly in the USA would have been impossible without an exceptionally high standard of component interchangeability. The Piaggio Company fully realises the importance in the executive market of spares which require no individual fitting, but can just be bolted on. The P.166 has been designed from the start for just this interchangeability and the jigs on which it is being built are of production type, with very sturdy fittings for all attachment points. The general constructional principle is to hold the components by their attachments, as well as by the main spars or structural members and to ream the holes full size as the final operation before removal. Reaming fixtures are attached direct to the jigs.
The superficial difference between the P.136L and the P.166 is that the latter has flush riveting throughout, a concession to American ideas rather than to aerodynamic requirements. Furthermore, the fabric-covered control surfaces of the earlier aircraft have been replaced by metal skins with indented stiffening beads as used on the military trainers. The fuselage is a characteristic Piaggio semi-monocoque without longerons. The bulbed L-section extruded stringers are laid in notches cut in channel section ring frames, the whole forming a skeleton over which the skin is laid. The skin panels are cut and formed to size and curvature, but the drilling of skin and framework is done together after marking off when the skin has been temporally located by tooling holes. Four deep beams under the floor of the cabin give stiffness to the structure and carry the nosewheel drag loads through it. They also form a strong area against a belly landing and, of course, compensate for the reduction in stiffness due to the large window openings. The latter have deep channel-section frames to carry the stresses round them and also have internal stiffening plates with large pierced-and-flanged lightening holes.
The fittings of the main spars of the wings are bolted to a bulkhead - the one which separates the cabin from the toilet and galley zone - of very robust design. The diaphragm of the bulkhead is a sandwich which is reinforced to a thickness of 4mm, where the spar loads are accepted and the latter are further diffused by the fact that the steel spar attachment fittings are bolted through the light alloy channel sections. The opening in the bulkhead is strongly reinforced by a T-section formed from back lipped L-section channels of material about 3mm thick.
Beneath the wing are two half-bulkheads, i.e. they are rather more robust ring frames than usual with diaphragms closing the lower half of the fuselage. Between these is mounted the undercarriage assembly of steel tubes which is attached to the bulkhead diaphragms by bolted plates. The volume containing the undercarriage is boxed over so that it is completely isolated and no dirt can enter the fuselage. The rear fuselage is quite conventional and ends in two partial diaphragm frames that extend upward to take the tail plane and fin mounting brackets. Although the dorsal fin appears to be integral with the fuselage, it is in fact a light separate structure permanently riveted to it. The tail cone fairing is located by four spigot fittings and is readily removable by the use of quick-release toggle fasteners. Beneath the rear fuselage is a small steel like bumper with a skid and a shock absorbing dashpot.