The ultra-light 'plane

The Possibilities of the Construction of a “Motor-Cycle of the Air” Discussed from the Designer-Manufacturer’s Point of View.
By C. W. TINSON, F.R.Ae.S.

During the last few years the light aeroplane movement has made rapid strides in Great Britain, and is spreading both to the Dominions and abroad.

There are some thirteen light aeroplane clubs working in England, and one in Scotland, whilst others are in process of formation, conclusively demonstrating the popularity of what has been called “communal” flying. In addition, we have several flying schools which are run on slightly different lines, but which are turning out potential owners of aeroplanes, when such come within their means.

Now, the light aeroplane movement has been made possible by the advent of a two-seater aeroplane of reasonable price and low running cost, of which the De Havilland ” Moth ” and the Avro ” Avian ” are the better known examples, and the quantity of these now being constructed in series has already permitted a considerable reduction of price to be made. The price of a two seater machine of this class is still such, however, that the number of private individuals who become purchasers of them is comparatively small.

If it were possible to market an aeroplane at about half the price of the present-day two seater light aeroplane, there is no doubt that the number of private owners would rise by leaps and bounds, for the price would then begin to compare favourably with that of a good quality sports car. The object of this article is to study the problem and record a few suggestions as to possible lines along which a cheap aeroplane might be developed.

Before an aeroplane of any kind can be designed it is necessary of course that a suitable power unit is available for propelling it, and in the case of an aeroplane to sell at a fair profit at the price suggested, namely from £350 or thereabouts, the engine must be obtainable for a really moderate sum, say from £65 to £905 complete.

It has been demonstrated that 32 h.p. is sufficient to fly a two-seater aeroplane, the light aeroplanes at the Lympne Competitions of 1924 being fitted with motors of this power. The performance of these machines however, was below that required to make the aircraft, as two seaters, an attractive proposition, but when flown as single seaters, their performance was fairly reasonable. When used as single seaters these aeroplanes were carrying more wing area than was necessary of course, and consequently the sizes all round were larger than necessary for a single seater primarily designed as such. The aeroplane, however, will be discussed later.

The Engine.

The number of suitable aircraft engines of about 30 h.p. which can be purchased for less than £127 is very small, presumably because the demand has been very light, and design and development charges have therefore been borne by a small quantity.

Supposing that a demand makes itself felt, in the near future, it might be reasonably anticipated that the price of the engine would fall at any rate to the higher figure mentioned previously.

Another possibility suggests itself. Supposing that it were found that an existing motor for another purpose could be boosted up to give the power required, then the design charges are greatly reduced, whilst the development cost would be confined to alterations necessary for aircraft service—suitable shaft for a propeller coupling, dual ignition, etc., to such stiffening up of parts which failed on test, and to reduction of weight of parts subjected to less intense stress.

There have been in the past certain four cylinder air cooled motors for motorcycles which immediately suggest themselves as possible of development along these lines, and which would form ideal power units for the required purpose. Assuming the feasibility of this suggestion, the lower figure quoted for the price might be within sight.

The Aircraft Structure.

In order to produce a really cheap aeroplane, it might be necessary to depart slightly from conventional practice and by so doing to sacrifice economy of weight to a certain extent in favour of economy in money. In other words, a scheme of construction which reduces the manhours, for production is tolerable, in spite of the result being relatively heavy, so long as it is light enough to give a predetermined performance with the available horse power.

A single-seater fitted with an engine weighing not more than 100 lbs., and carrying in addition to pilot 3 gallons of fuel and 0.5 gallons of oil can be designed for a landing speed of 48 m.p.h. (KLmax. .675) for a total weight of 440 lbs. in flying order, and should be capable of at least 94 m.p.h. full out, or say 75 m.p.h. cruising, and of 600 feet per minute climbing rate at sea level. This assumes 30 h.p. as a maximum, and the performance approximates to that of a Cirrus II. “Moth.” The speed assumed is somewhat lower than might be expected at first sight, a reduction of 10 m.p.h. having been made to allow for the proportionally large size of the cockpit opening.

The quantity of fuel carried is small, but should be sufficient to give a flight of 2 hours, or about 150 miles.

Before turning to the construction of the aeroplane, it may be as well to form a mental picture of it (see illustration), in order that the imagination may be brought to bear on the relative dimensions compared to an aeroplane of more normal size.

With a wing section such that a maximum lift coefficient of .675 can be realized, we require 56 sq. feet of wing surface having a span of 18ft 8in. For balance it will be necessary that the occupant be seated between the spars, so the wing must taper in plan giving a mean chord of about three feet.

A high lift section such as is indicated implies considerable camber, and in fact will permit the use of spars about 6in, deep at the root. The wing characteristics will require a total tail unit area of 16.1 sq. feet of which 12.1 sq. feet will be tail and elevators, and 4.0 sq. feet rudder and fin surfaces, and the distance from centre of gravity to sternpost need not exceed 9 feet for these proportions.

This will give a total length of 14ft. 4¼in. whilst the height, to give a suitable propeller clearance and wing incidence on the ground, need not exceed 5ft. 6in.

Like the wing, the tail unit may be profiled with considerable camber.

Bearing in mind that it would be practically impossible to build so small an aeroplane with all members of theoretical strength (because if so they would then be too frail), and that reduction in cost will follow if the number of items forming the structure is kept to a minimum, three ply will be utilised as the principal material of construction. Generally, the dimensions are such that 3/32in. 3-ply could be used for the skin of the wings and fuselage, and 1/16in. 3-ply for the tail, with an absolute minimum of stiffening for stabilization.

Taking each half of the wing for instance. This is nominally a 3-ply shell constrained to the desired wing section by being glued and screwed to three or four formers profiled to suit. Owing to the camber of the wing, the 3-ply shell is of considerable section modulus and in fact is sufficiently strong to take the weight of the machine, provided the skin were stabilized against buckling.

It is necessary, of course, that it should be strong enough to carry ” n ” times the weight, where ” n ” is the load factor, and as it also requires stabilizatfon, two planks of spruce, about 6in, at the root and 1in. in thickness, are inserted along the span, forming spars, to which the skin is glued and screwed, the combination forming a beam strong enough and rigid enough to do its duty with the load factor superimposed. Any additional local stiffening to prevent the three ply buckling would be put in as found necessary, for the structure would have to be tested of course, because it is not amenable to pure calculation, and the points requiring further stabilization would appear in the course of test.

Simple to Make.

Now if this method of construction is feasible, the wing is composed of a minimum of parts, and no special machinery is required for its production. It remains to be seen whether such a form of construction is permissible on a weight basis.

The construction is not unduly heavy, for as estimated the weight of the wing with its ailerons comes out at 77 lbs., or 1.375 lbs./sq. ft., but assuming that the estimated weight is too low, due to an increased number of internal stiffeners being ultimately necessary, it could hardly exceed 1½ lbs./sq. ft. The construction of the tail unit would follow similar lines except that 1/16in. 3-ply would be suitable owing to the smallness of the surfaces.

The same method of construction is applicable for the fuselage, which would be formed of truncated cones of 3/32in. 3-ply, joined longitudinally by cover strips rivetted on, and to each other by laminated hoops.

The total area of 3-ply for the fuselage would be 70 sq. feet, weighing barely 18 lbs., and costing approximately £5 for material.

Thinner 3-ply could be used at the rear end, but the extra stiffening necessary would increase the production cost.

The form of construction advocated would only be possible for an aeroplane of this size : in larger sizes the number of subsidiary members for stabilization would increase considerably, and normal methods of construction would show up, as now, to advantage.

Component Costs.

Having decided that it is impossible to cut the cost of material required by any further reduction in the size of the aeroplane, or to cut the cost of labour by any further simplification of construction, it remains to tabulate a list of prices for the various components respectively representing the maximum figure allowable, and to form one’s own opinion whether it is at all possible to build down to these figures. A suggested estimation of prices for the components, allowing £80 for the purchase price of the motor complete, ready to instal, is given below :

ESTIMATED PRICES.

ITEM…… DESCRIPTION…………………………………………………….£ s. d.

1 Engine at £2.6 per h.p……………………………………………………80 0 0

2 Propeller at £2 per foot of dia. ………………………………………….9 0 0

3 Wing : Three ply ……………………………………………………………..7 0 0

…………….Other materials …………………………………………………….4 0 0

………….. Labour … …………………………………………………………….11 0 0

4 Tail Unit : Three ply … ………………………………………………………2 10 0

………………..Other materials …………………………………………………1 10 0

…………………Labour …………………………………………………………….4 0 0

5 Fuselage : Three ply ………………………………………………………….5 0 0

………………..Other materials…………………………………………………..2 10 0