Flight, January 1922
THE WIBAULT NIGHT BOMBER
An Interesting French All-Metal Aeroplane
AMONG the examples of metal construction found at the recent Paris Aero Show, there are few which could be said to show real merit. Either they were too costly in manufacture,
unsatisfactory in design, or of very inferior workmanship. Exceptions there were, of course; and first and foremost among these was the Wibault all-metal night bomber, "B.N.2." Unfortunately, this machine was not exhibited complete, but from a specimen wing shown on the Duralumin stand, it was possible to form a very good idea of at least the wing construction. In view of the good workmanship, it was not surprising to find that the wing - and indeed the whole machine, which was at the time of the Show undergoing its acceptance tests - was built by Pierre Levasseur, whose workmanship is always a joy to behold.
A set of excellent photographs on the Pierre Levasseur stand showed the general lines of the Wibault "2 B.N.2." Unfortunately, we have not been able to obtain good photos, of the machine, but the accompanying general arrangement drawings should indicate the general lines.
As the letters "B.N." indicate, the machine is intended for night-bombing, although it is claimed that it could be modified for commercial work, if desired. As a result, it is claimed, of the metal construction, it has been possible to effect a great saving in the structure weight of the machine, which forms a relatively small percentage of the total weight. The engine fitted is a 600 h.p. Renault, which naturally weighs a good deal. Nevertheless, the weight of the machine empty, but with cooling water, is only 4,620 lbs., while the total loaded weight is 9,450 lbs. The useful load, which in this case does not include tanks and fuel, is 3,100 lbs. Sufficient fuel is carried for a four hours' flight at full throttle.
Apart from this feature of small structure weight, the "Wib.2, B.N.2" is chiefly remarkable for the fact that its wings are placed very far aft on the fuselage, and that, in spite of its comparatively large size, only one pair of struts is employed on each side. Another remarkable feature is that the upper plane is of shorter span than the lower.
To deal with the various features in the order enumerated, the placing of the wings has probably been decided upon as a result of the necessity of centralising as much as possible a heavy load of varying weight. With such a long cargo space, there is a possibility of so arranging the load - in this case bombs - that by dropping the central ones first and then, working towards front and rear simultaneously, the others in sets from front and rear, the trim is not seriously upset. Large space is required, and in the Wibault this has been attained by spreading the bomb chamber in a fore-and-aft rather than in a lateral direction. Also, for night-flying, it is possibly an advantage to have the crew placed far aft in the fuselage, where the view downward is but little obstructed. For use as a commercial aeroplane, which the designer may have had in mind, the central placing of a large cabin or cargo space is also advantageous. It is claimed that as a commercial machine, the "Wib.2," would carry a pilot and 13 passengers. Allowing 180 lbs. for each occupant, and about 30 lbs. for the luggage of each, this should be possible, while still carrying wireless outfit and other paraphernalia. In that case, the power expenditure per passenger would be about 46 h.p., which is very reasonable, especially as the speed at 6,500 ft. is stated to be 125 m.p.h.
As regards space in the cabin, we understand that when stripped of its bomb cradles, etc., the cabin space available is 14 ft. 9 ins. long, by 5 ft. 2 ins. wide, by 5 ft. 11 ins. high, or approximately 450 cubic ft.
The fuselage is also, we understand, built of metal throughout, with the exception of the covering, but as it was not exhibited we have not had an opportunity of examining its constructional details.
The employment of only a single pair of struts on each side has been made possible by using wing sections of great depth. For the section used, it is claimed that this, which is the result of experiments made by M. Wibault personally, is more efficient than ordinary thin sections, although of the deep, high-lift type. The small amount of external bracing results in a low wing resistance, and the inward slope of the interplane struts in conjunction with the short top plane has the effect of giving equal loading in upper and lower spars, which is not the case where upper and lower bays are equal.
As regards the wing construction, this is indicated in the accompanying sketch. The somewhat unusual view is the result of the fact that the wing from which the sketch was made was standing on its leading edge. The wing spars are built up of Duralumin sheet, flanged over as shown. It will be noticed that the flanges of the lightening holes are produced by simply bending at right angles along straight lines, and that where a radius occurs the flange is made up of a separate piece. This form of construction has the advantage that the flanges can be made without incurring the expense of costly dies for stamping out the flanges. The spar flanges themselves are flat strips of Duralumin riveted to the flanged-over edges of the webs. The number of rivets required is not excessive, and altogether this spar construction is much more of a commercial proposition than the majority of those exhibited. For greater compressive strength, it would probably have been an advantage to have the spar flanges rolled to form a corrugation along their ridge, but even with the flat flanges the spars have withstood a sand load equal, we are informed, to a factor of safety of 7-5, without showing any permanent deflection.
The ribs are entirely tubular, the flanges consisting of tubes, bent to the desired curvature, and the webs formed by a series of N-trusses, attached to the flanges by clips and rivets in the manner shown.
The main characteristics of the "Wib.2, B.N.2” are as follows: Length o.a., 41 ft. 10 ins; span, 55 ft. 6 ins.; height, 16 ft. 5 ins.; wing area, 1,035 sq. ft.; engine, 600 h.p. Renault; weight empty, but with cooling water, 4 620 lbs., useful load, 3,100 lbs.; fuel for 4 hours at 6,500 ft.; total loaded weight, 9,450 lbs.; speed at 6,500 ft., 125 m.p.h.; wing loading, 9.13 lbs./sq. ft.; power loading, 15.8 lbs./h.p.