MULTI-ENGINE airplanes may be divided into those which can and those which cannot fly with the full normal load after one or more of the engines have stopped. Until within a very short time the former class was practically non-existent. The latter class includes military airplanes in which it is desired to obtain a fuselage having a nose without an engine and those in which sufficient power cannot be obtained from a single powerplant.
Some two-engine planes that can fly with a single engine on a test flight fail to do so after having been in use for some time. Ability of a two-engine plane to fly with one engine usually necessitates a sacrifice in the pay-load to such an extent that its operation is uneconomical. Flying with one engine of a multi-engine plane idle is difficult because of the reduction of the propeller speeds of the remaining engine or engines and the turning forces involved.
These disadvantages can be overcome only by applying greater forces through the controlling surfaces, which, in turn, entails the additional disadvantage of increased head-resistance. The solution of the propeller problem lies in the use of adjustable-pitch propellers, which, however, add further undesirable complications.
Although a multi-engine plane provides insurance against forced landings, this insurance is gained only at the cost of increased fuel-consumption and greater head-resistance. In simplicity, efficiency, economy of running and initial cost, the advantages are said to be all on the side of the single-engine plane. If multi-engine planes are considered desirable because of their insurance of reliability, the minimum number of engines used should be three; and in few cases is such insurance worthwhile except in large and expensive craft.