A feature of aircraft design which improves performance and simplifies landings
THE DOUGLAS DC-4E AIR LINER is the first large aircraft to have a three-wheel undercarriage as a standard fitting. The rear wheels fold inwards into recesses in the lower surface of the wings; the front wheel retracts backwards into the nose of the fuselage through an opening which is afterwards closed by doors. The landing and take-off runs are shorter than those for many smaller commercial aircraft.
ALTHOUGH the tricycle undercarriage is fitted as standard to one or two types of aircraft that are produced in small numbers, it is still in the experimental stage. Its possibilities are far-reaching. Because it tends to change the fundamentals of the designing and flying of aircraft, the tricycle undercarriage is the subject of many varying opinions, some favourable, some unfavourable. But its advantages are such that there can be no doubt that it will eventually be adopted for many types of aircraft.
The tricycle undercarriage as such is not new. Three-wheel undercarriages were used on some of the earliest aircraft of aviation’s pioneers. What is new is the way in which the use of three landing wheels is applied to modern aeroplanes.
There are two schools of thought about the arrangement of the three wheels. Some designers have preferred the single wheel in front of the two wheels, and others have adopted an arrangement with the single wheel behind the pair of wheels. The single wheel in front has, however, appeared on most designs which have been tested and have proved to be practicable on aircraft.
The shape of the aircraft has some bearing on the question which arrangement of the wheels is better. With modern high-performance aircraft retractable undercarriages are necessary, and the third wheel has also to be retracted.
On first consideration it might appear that a normal undercarriage and a tail wheel constituted a tricycle undercarriage with the single wheel behind. But there are several differences. An orthodox aeroplane has only two landing wheels. The tail wheel is merely a wheel on which the tail of the aeroplane rests when the landing is made and while the aircraft is on the ground. Moreover, it performs no part in the take-off. In a tricycle undercarriage of either type, all three wheels are landing wheels, and all three wheels are used for the take-off run.
When an ordinary aeroplane is resting on all three wheels it has a nose-high attitude and the fuselage makes a steep angle with the ground. With a tricycle undercarriage the aircraft rests on all three wheels in a position approximately the same as that of the aircraft when in flight. The fuselage is about parallel with the ground.
The position of the pair of wheels of a tricycle undercarriage is different from the position in which they would be on an aircraft with a normal undercarriage. They are either farther forward or farther back, according to whether a back or front single wheel is used. In an ordinary undercarriage the centre of gravity of the aeroplane is approximately over the landing wheels, so that the aircraft has to be balanced on the two wheels during the take-off. In a tricycle undercarriage the centre of gravity comes at some point between the pair of wheels and the single wheel. Thus the weight of the aircraft is distributed between all three wheels and no balancing effect during the take-off is required.
Varying arrangements for braking and steering have been adopted in the tricycle undercarriage with the single wheel in front. Braking on the pair of wheels and braking on all three wheels have been tried. Braking on all three wheels is most likely to be finally adopted.
Steering of all three wheels and steering of the front wheel only have also been tried. All three wheels fixed, and a castoring front wheel with fixed other wheels have also been tried. A fixed pair of wheels and a steerable front wheel is the most generally adopted.
From the point of view of the light aeroplane the advantages of the tricycle undercarriage lie in easier flying. Landing, the most difficult part of flying, is remarkably simplified. This means a considerable reduction of the number of hours tuition required before a pilot can fly solo. Not only is the landing itself simplified, but also the danger due to a bad landing is almost completely eliminated. It is virtually impossible for an aeroplane with a tricycle undercarriage to turn over on to its nose, no matter how bad a landing is made. But the tricycle undercarriage must be stronger than a normal undercarriage if full advantage is to be taken of its safe-landing characteristics.
When taking off an aeroplane with a normal undercarriage, the pilot has to reduce the angle of incidence of the wings by raising the tail of the machine. When this is done, the aircraft is able to pivot about its two wheels in a fore-and-aft and a left-to-right direction; as speed is gathered it can pivot in a lateral direction also. The aircraft must be kept straight in all directions by the pilot. If the pilot pulls the control stick back before the aeroplane has gathered sufficient speed, he may lift it off the ground with the wings at too large an angle of incidence so that the aircraft will stall and drop back on to the ground.
With a three-wheeled undercarriage all these difficulties are removed. The aircraft normally rests on its wheels at a suitable angle for taking off. The three wheels are in contact with the ground the whole time until the aircraft has gained sufficient speed to take off. It is thus stable in both fore-and-aft and left-to-right directions. When speed is sufficient the aircraft automatically lifts itself into the air.
The pilot cannot cause the aircraft to rise before it has sufficient speed. To do this he would have to increase the angle of incidence of the wings, and to increase this angle the tail of the aircraft has to be lowered. With the weight of the aircraft distributed over three wheels the tail cannot be lowered until the wheels are free of the ground and the aircraft has attained sufficient speed for a take-off.
It is this distribution of the weight of the aircraft over three wheels which enables the tricycle undercarriage to simplify landing. The normal way in which to land an aeroplane with a tricycle undercarriage is to bring the aircraft down straight to the ground at a flat angle of glide. The undercarriage will do the rest.
Good View while Taxying
The front wheel of’ the undercarriage will touch a little before the back wheels, but the weight distribution will cause the back wheels to drop on to the ground quickly; then the aircraft will run along the ground without any bounce occurring. Wheel brakes may then be applied as hard as desired, the weight distribution preventing the aircraft from nosing over. The flatter the final glide before the ground is touched the gentler the landing.
If a landing of this type were attempted with a normal undercarriage, the front part of the aircraft would be bounced into the air. As there would be no rear wheels to prevent the tail from falling, the wings would increase their angle of incidence and the aircraft would rise several feet into the air before stalling. If the aircraft is flown on to the ground with its fuselage level it will have to be balanced on its wheels until sufficient speed is lost to allow the tail to drop of its own accord. Until this occurs wheel brakes cannot be used even gently.
The only limit to the speed at which a tricycle-undercarriaged aircraft may be glided on to the ground is the strength of the undercarriage. If the tricycle-undercarriaged machine is brought in to land like a normal aircraft with the tail low (see the chapter “Performing Practice Circuits”), as soon as the back wheels touch, the nose of the aircraft will drop, so that the third wheel is in contact with the ground as well. Thus, no matter how the aircraft is brought into the aerodrome, a satisfactory landing can be made. The view of the pilot while taxying is good because of the aircraft’s level position.
A NON-RETRACTABLE TRICYCLE UNDERCARRIAGE on an American twin-engined aircraft. The aircraft was designed by the brothers Otto and Wally Timm and tested in flight at Glendale, California. One of the main objects of the design was to produce a commercial aircraft suitable for operation from rough country. That is why the three-wheeled undercarriage was adopted.
The advantages of the tricycle undercarriage as applied to commercial aircraft may well prove of considerably more importance than their assistance in the simplification of flying tuition. In commercial aircraft pilots attain such a high degree of skill that there is little point in simplifying the landing of commercial aircraft. But if the degree of skill required can be maintained constant while the wing loading of the aircraft is increased, then something will have been attained. This is made possible by the tricycle undercarriage.
High-performance aircraft with high wing loadings have a flat angle of glide, and for safety the speed must be well above stalling speed. A considerable distance is thus taken up during the levelling out process for a landing and, after landing, brakes must be used with care. All these considerations make landing in any but large aerodromes difficult, and further increases of wing loading would increase the difficulty.
With a tricycle undercarriage the position is different. The aircraft may be glided straight on to the ground at a speed well above the stalling speed and as soon as it has touched down the wheel brakes may be applied hard. Increases in stalling speed due to higher wing loadings thus become of small importance and it becomes comparatively easy to get into even small aerodromes.
The possibility of gliding straight to the ground and yet making a good landing can also be of considerable value when blind landings are to be made. In fog it would no longer be necessary to obtain a glimpse of the ground at the last moment, so that the levelling out movement of the control could be made. With the tricycle undercarriage completely blind landings become possible.
There is another way in which the tricycle undercarriage may assist the commercial aeroplane pilot. With modern heavily-loaded aircraft long runways are necessary and the surface must be strong enough to support the weight of the aircraft. These requirements have produced the concrete runways which are becoming more and more popular. They provide the desired long and strong runways without the expense of suitably preparing a large area of land.
Cross-Wind Landings
There is one drawback to the concrete runways. A sufficient number of them cannot be provided to suit exactly all wind directions. Because of this, cross-wind landings have often to be made. With a tricycle undercarriage it has been found that the jar on the wheels when landing is considerably less pronounced, the tendency being for the aircraft to swing into the direction in which its centre of gravity is travelling.
With an undercarriage in which all three wheels are steerable even greater advantages for out-of-wind landings may be obtained. In one undercarriage it is suggested that the three wheels be permanently locked by the pilot, out of line with the aircraft. The angle at which the wheels are locked is to depend on the amount of drift of the aeroplane. Thus the aircraft would make a smooth landing and run straight along the runway although pointing slightly to one side or the other of the runway. Indicative of the way in which designers have appreciated the advantages of the tricycle undercarriage is the fact that this type of landing gear is fitted to America’s latest commercial aircraft, the Douglas DC-4E. This aircraft is the largest commercial aeroplane in America and has been produced to the order of five important air lines in the United States. The gross weight is 63,000 lb.; the aircraft carries forty-two passengers, a crew of five. and about three tons of mail. The maximum speed is over 240 miles an hour.
The three wheels of the undercarriage are each higher than a man and all three retract. The two rear wheels fold inwards into recesses beneath the wings; the front wheel folds backwards into the nose of the fuselage. The opening through which this wheel passes is afterwards closed by doors. The nose wheel is steerable.
During extensive flight tests, the tricycle landing gear proved entirely satisfactory. With an all-up weight of 61,200 lb. the DC-4 took off in the creditable distance of just over 1,000 feet. The engineers expect this to be bettered. Pilots of various air lines who flew the DC-4Erepeatedly remarked that the take-off and landing run were shorter than on many smaller aircraft at present in use.
Special tests of the tricycle landing gear have shown that the DC-4E has exceptional stability when on the ground. The landing gear has proved entirely satisfactory with the centre of gravity of the aircraft moved to different fore-and-aft positions, and with different loads.
AN EXPERIMENTAL MONOSPAR AIRCRAFT used in Great Britain to test the possibilities of the tricycle undercarriage. This photograph illustrates how the three-wheeled undercarriage maintains the aircraft in its flying attitude when on the ground. There is thus no necessity to get the tail down when making a landing, nor to raise it when taking off.
