The Fallacy of the Combined Aeroplane and Dirigible Balloon
T HE reason for proposing a combination of balloon and aeroplane is not hard to find. Probably the in ventor reasons something like this : "The aeroplane is inexpensive, compact and speedy ; but it is unsafe, and will not carry much weight. The dirigible is big, clumsy, expensive and slow ; but it is. safe, and it will carry a comparatively large weight. Let us combine the two principles· in such a way as to get the advantages of each without the disadvantages. Let us attach to an aero plane a
... alloon bag which is large enough to carr y part of the weight and let the machine down slowly in case of accident, but will not be so big and offer such resist ance to speed as a regular dirigible which has to lift all of the. weight unassisted. Then the aeroplane will also be big enough to carry the machine safely to the ground in case the balloon explodes." This is what is generally meant by a "Combined Aero plane and Dirigible" ; that is, a combination, each unit of which can carry a considerable part or all of the weight. Aeroplane surfaces on dirigible s for steering purposes, or as a substitute for ballast, ar e in an entirely different class and are practically necessary in some form. Then there is the entirely feasible, though perhaps not prac ticable, method of balancing an aeroplane by means of a balloon ; these forms will be considered outside of the present sphere of discussion. The arguments given above for the combination machine might seem very reasonable. They are. The only trouble is that it doesn't work out that way. The argument is all right, but the premises are wrong. Most of the fallacies in such a scheme arise from disregarding one or more of the following laws : 1. There is no object in combining two · different methods for attaining a given result unless some added advantage peculiar to such a combination is gained. Thus you wouldn't ordinarily hitch a gas engine and a steam engine in tandem to drive a dynamo, but would select the most efficient unit for that particular work, and use it exclusively. On the other hand, the combination of a reciprocating engine with a turbine is often better than either one alone because, although not so efficient at high pressures, the turbine can get energy out of the exhaust steam which would otherwise go to waste. The volume and hence the lift of a balloon varies as the cube of its outside dimensions. Much trouble is caused by a misconception of the size necessary to lift any considerable weight. There are balloons which will carr y a large weight but they are all very much bigger . than any aeroplane ever attempted. A dirigible 260 feet long by 50 feet in diameter will lift a total of 26,000 pounds when inflated with hydrogen. A balloon one tenth these dimensions would be 26 feet long by 5 feet in diameter, comparable in size to the 'Ordinary aeropl a ne ; yet it would barely lift itself, a total of 26 pounds. 3. For small sizes, the resistance and weight of a balloon vary as the square of its outside dimensions. 4. It takes exactly as big a balloon to keep a given weight from falling as it does to lift it. As a close corol lary to this, we have that it takes nearly as much gas to let a weight down easily as it does to lift it. 5. The lift of an aeroplane varies as the square of the dimensions while its weight and resistance (for structural reasons) must vary by a little more than the square. Furthermore, the efficiency of a surface is very much impaired by extending or multiplying it from front to rear, which is just the opposite to the case of the dirigible. (In spite of this, the aeroplane can probably be developed considerably above its present size.) The efficiency of an aeroplane decreases slightly with increase in size. Not so with the dirigible. It .is fundamentally and always will be a large machine. The results to be gained by even a moderate increase in size are almost inconceivable at first ; but it is founded on strict elementary laws which cannot be ignored any more than the existence of gravitation. . As an actual example, let us figure out roughly how much we can improve an ordinary Wright aeroplane by the addition of a cigar-shaped balloon which will lift half the weight. I think we may take the performance' of the aeroplane alone about as follows : Horse-power, 30; speed, 40 miles per hour ; resistance (thrust) 180 pounds ; efficiency of power unit, 64 per cent ; total weight with fuel and 2 passengers, 1 ,000 pounds. Take away half the supporting surface, which might reduce the weight to 800 pounds, and the resistance to 120 pounds, for the same speed. A balloon 60 feet long by 18 feet diameter would have a total lift of 780 pounds when fully inflated with hydrogen. Its weight with sus pension cords, framing and accessories would be in the neighborhood of 380 pounds, thus leaving 400 pounds for lift, or half the weight of the reduced size aeroplane. Its resistance, with extra bracing, etc., would be close to 300 pounds at the 40 mile speed. The speed of the combination would be cu t down in consequence to about 26 miles per hour, and the lift of the aeroplane to 1 70 pounds. We lose, in this way,. 230 pounds of lin that we had counted on from the aeroplane. It means that SCIENTIFIC AMERICAN instead of cutting . down its size to meet the new con di tions, we 8hould have actually increa8ed it by 3 or 4 times its original area. After making these corrections, we get final results something as follows : Speed, 20 miles per hour ; total weight, 1 ,600 pounds ; lift of gas-bag, 800 pounds ; lift)f aeroplane, 800 pounds. We have reduced our speed to one half that of the original aeroplane. We have a machine which is clumsier, more expensive and slightly less speedy than a regular dirigible of equal power and passenger carrying capacity. Certainly such a sacrifice should be accompanied by some very great advantage, but where is it? Take the matter of safety. Suppose · the machine were to lose headway and fall vertically. The area resisting the descent consists of the superposed aeroplane surfaces and the horizont�l surface of the balloon, which are about 1,500 and 900 respectively. The most favorable possible calculation would give a speed of descent of 20 feet per second, hardly a safe velocity at which to strike the earth. Instead of taking the horse-power the same in the above example, we could have assumed equal speeds for the two machines and increased the power to suit ; but the results in the latter case are equally significant and still less effective in producing safety. The scheme of making the balloon bag itself in the form of an aeroplane seems still less promising. If the balloon were distorted enough to give it any efficiency as 1m aeroplane surface its weight and resistance would be prohibitive. I shall touch on this a little more fully in the paper on double balloons. It might be possible to provide a regUlar dirigible with enough aeroplane surface to carry it safely to earth in case of an explosion ; but there is no need for it. Such an accident very seldom occurs, and it is easily prevented by taking proper precautions. Such a large surface would . be very heavy, and offer much resistance even when folded up. Then consider, for example, the difficulty of constructing and managing a set of aeroplanes big enough to carry a large dirigible, .which would be suddenly called u1J<!n in an emergency to support 15 to 20 tons. Such a.n accident is much better taken care of by prevention in the first place. As a last resort, I would suggest individ ual parachutes for the passengers.