Design Aspects of the Japanese Hamp

by Major Doy L Hancock
Chief, Evaluation Branch, Technical Data Laboratory, Engineering Division, AAF Material Command, Wright Field

Captured and flight tested by the AAF, the Jap Hamp is found to be structurally strong but extremely vulnerable

In a comparison between the Mitsubishi Hamp and Zeke Type O, it is noted that the former is equipped with higher horsepower, provided with a two-speed supercharger, and a downdraft carburetor; the wing span is about 4' shorter and has square wing tips. It is also much smoother in performance than the Zeke. The two planes are identical with the exception of these differences. Landing gear, canopies, etc, are completely interchangeable.

Much confusion created both by the lack of knowledge concerning the Japanese Air Force and the involved Japanese method of designating their aircraft has persisted, and it is seldom that one can feel with certainty that the Japanese aircraft being referred to in a document or in conversation is the one under consideration. Perhaps the following will clarify the misinterpretation encountered.

When the war with Japan broke out, it was discovered that the Japanese used a fighter whose partial designation was "Type O." The press began immediately to use the "catchy" term Zero. Unfortunately, there are at least seven Japanese airplanes designated as Zeros, including fighters, bombers, and seaplanes. The term Zero indicates the year the aircraft was commissioned; consequently, all Japanese aircraft commissioned in 1940 are Zeros. After much discussion, it was decided by the United States Army Air Forces that the only satisfactory solution was to use as much of the translated Japanese designation as practicable and append a suitable code name. Thus, the proper designation for the aircraft being discussed here is Type O F Hamp, the Type O F (1940 fighter type) being the portion of the translated Japanese designation, and Hamp being the assigned code name.

The Hamp is of all-metal construction except for the fabric-covered control surfaces. The general workmanship is good and cleanness of aerodynamic design is enhanced by generous fairings, flush riveting, and a smooth paint finish. Construction is light but not flimsy, and, contrary to popular belief, is strong, Japanese spar and skin strengths comparing favorably with our own. Probably the factor that has contributed most to the misconception of Japanese airplane flimsiness, is their extreme vulnerability. Japanese aircraft are not fitted with leak- proof tanks, bullet-proof glass or armor plate and are not capable of taking even a small amount of the punishment that our own aircraft can sustain in combat. Therefore, hits in the unprotected fuel tanks will often result in fire and explosion, and the resultant disintegration of the aircraft may be misinterpreted as a structural weakness.

This aircraft is powered by a Nakajima Sakae 21, a development of the Sakae 12 used in the Zeke. Production of this engine began in 1942. The principal modifications made to the Sakae 12 to develop the 21 was to substitute a down-draft carburetor for an up-draft type and a two-speed supercharger for one of single speed design. Following are some of the important characteristics of the Sakae 21:

Cooling mediumAir
Cylinders14 radial
Displacement1700 cu in
Compression ratio7:1 (est)
Weight1175 lb (est)
Propeller gear ratio12:7
Blower ratios6.38:1
Take off power950 hp at 2600 rpm
Rated power1020 hp at 6400 ft
885 hp at 15,700 ft

The three-bladed propeller is all-metal, constant-speed, and modeled after a Hamilton Standard design.

The aluminum alloy fuselage is divided into two sections. The vertical division occurs at the bulkhead, located just forward of the end of the cockpit canopy. The fuselage section is secured to the front section at a butt joint by means of 80 bolts. The forward semi-monocoque section is not structurally complete until riveted to the wing which forms the floor of the cockpit. The rear fuselage section of semi-monocoque construction has no hand holes or openings in the skin. Access to the rear fuselage is possible only by swinging the pilot's seat forward.

The full-cantilever wing is of two-spar construction. Wing and fuselage are built as an integral unit, the extruded spars being continuous from wing tip to wing tip. Although this is a carrier-borne fighter, no portion of the wing folds. All-metal split type flaps are hydraulically actuated.

The full-cantilever tail surfaces are metal-covered except for the fabric-covered control surfaces. A metal trim tab, adjustable on the ground only, is used for rudder trimming. The elevators are the only control surfaces equipped with trim tabs adjustable from the cockpit.

The wide tread landing gear (11'6") is hydraulically actuated. The main gear retracts into the wings towards the center of the fuselage and is completely enclosed by fairing. The tail wheel is not completely retractable, a small portion being visible after retraction.

Three internal fuel tanks are provided, one in each wing and one in the fuselage just forward of the cockpit. The tanks are of all-metal construction and are not provided with leak-proof protection. A permanent fitting located in the center portion of the front spar, is provided to take the steel supporting pipe of jettisonable fuel tank. This arrangement is clean aerodynamically since there are no projections to disturb the airflow when the tank is jettisoned. Almost all Japanese jettisonable fuel tanks are of all-metal construction. Wooden tanks have been used in smaller quantities and even compressed paper tanks have been reported.

Also contrary to popular belief, the Hamp is equipped with a full complement of instruments. Japanese instrumentation suffers from a lack of quality rather than a lack of quantity. The Japanese in the Hamp used many obsolescent instruments as judged by our standards.

Since this aircraft is designed for carrier-borne operation, it is equipped with arresting gear. Two airtight compartments have been formed in each wing by sealing off two boxes whose sides are formed by:

  1. the front and rear spar and two ribs; and
  2. the front spar, the wing leading edge and two ribs.

A large canvas bag situated in, and conforming to the shape of, the rear fuselage section serves as flotation gear for the fuselage. Flotation is effected by shutting the valve in the cockpit (normally open) which serves the function of trapping atmospheric air in the entire flotation system, thus making it airtight.

A small wing tank ventilating door is located on the under side of each wing. Cooling of the wing tanks is necessary to help prevent fuel vapor lock at high altitudes.

The theater of operation will determine the procedure in identifying a foreign plane since it is necessary to know the types of planes in that area. The Hamp has the round blunt engine nacelle and the square wing tips that serve as splendid identifying characteristics.

There is no armor plate or bullet proof glass of any description on this type of aircraft. As a result of this deficiency, the pilot is extremely vulnerable and can be considered a good target.

No attempt has been made to leak-proof the three aluminum fuel tanks; therefore, in an attack against the Hamp, concentrated gunfire should be brought to bear on the wing roots. Unprotected fuel tanks present an excellent target and hits will invariably result in complete destruction of the airplane by fire or violent explosion.

The Hamp is armed with two 7.7-mm (equivalent to our .303-cal) machine guns, synchronized to fire through the propeller arc, and two 20-mm cannons mounted in the wings. The machine guns are identical to the British Vickers Mark V. The Japanese gun will fire British .303-cal ammunition and most components of both guns are interchangeable. The 20-mm Oerlikon automatic cannon is not considered as formidable a weapon as the American .50-cal gun. In actual combat, it was found that the cannon was inaccurate and had a shorter range than the American .50s. The 7.7-mm machine guns are equipped with 600 rounds of ammunition each, and the 20-mm cannons are provided with 100 rounds apiece.

Provisions are made to carry one 132-lb bomb under each wing. Bomb equipped Hamps are used principally for the purpose of air-to-air bombing attacks.

The Hamp is employed both as a carrier-based and a land-based fighter. In combating the Hamp or any other enemy fighter, it is extremely important to be well acquainted with both its weak and strong tactical qualities. The Hamp is very maneuverable and retains good control characteristics right up to the stall. Climb characteristics are good but not exceptional, its high rate of "zoom" having created the erroneous notion of extraordinary climb often attributed to this airplane. As compared to these desirable tactical qualities, the Hamp is impaired by the following undesirable ones: Its high speed is considerably below that of most of our first-line fighters; it has a low diving speed; at high speeds, the controls tend to "freeze" and maneuverability is materially reduced; and the engine cuts out when subjected to negative gravity accelerations, such as are encountered in the beginning of a dive.

From the above, it should be apparent that the Hamp is a dangerous adversary in a dog fight and a "clay pigeon" when fought intelligently. Actual combat experience has shown this to be true and has proven that fighting the Hamp intelligently entails the following:

  1. Use hit and run tactics. Once having altitude superiority, make a pass, dive away and climb quickly to regain altitude superiority.
  2. Always combat the Hamp at high speed since its maneuverability advantage disappears at high speeds that cause control "freezing."
  3. Never dog fight the Hamp because of its superior maneuverability and stall control characteristics make it capable of remaining outside the attacker's gunsight in a dog fight. Furthermore, by capitalizing on these two advantageous characteristics through skillful handling, the Hamp can easily get on the tail of any allied fighter.
  4. Always break away from a Hamp by diving. This puts the Hamp at a disadvantage in two ways; first, it cannot enter a dive as quickly as allied fighters, since the engine will cut out when subjected to negative gravity acceleration; and second, once in the dive, it cannot stay with the allied fighters due to its lower terminal velocity. The Hamp, on the other hand, breaks combat by a climbing turn which is usually made to the left.

Against the Hamp, as against any enemy fighter, the bomber's best defense is mutual support with its attendant concentrated fire power. The straggler will be pounced upon immediately and weak defense points will be exploited. The air-to-air bombings that have been reported recently are designed, not so much to damage the bombers, but rather to break up the formation and create stragglers that should fall an easy prey to the attacking fighters. Thus far, these tactics have not proven successful, principally because they have failed to break up the formation.

Empty weight3913 lb
Normal gross weight5750 lb
Maximum gross weight6331 lb
Span36' 2"
Length29' 9"
Height9' 2"
Wing area (sq ft)238
Aspect ratio5.5
Wing loading (lb/sq ft)24.2
Propeller diameter1O'
Maximum internal fuel capacity134 gal
Capacity of jettisonable tank94 gal
Oil capacity13 gal
This article was originally published in the October, 1944, issue of Industrial Aviation magazine, vol 1, no 5, pp 60, 62, 93.
The original article includes a three-view silhouette and 3 photos:
Hamp in US markings in flight, seen from 3 o'clock,
Hamp cockpit instrument panel,
Hamp in US markings on the ground, seen from front left.
Photos are not credited but are probably from USAAF Wright Field.