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2: The CW-21 described

II. THE CW-21 DESCRIBED

Most sources state that the CW-21 mounted one .50 caliber and one .30 caliber machine gun [10]. In contrast, the American Aviation article of April 1940 described the initial production version of the CW-21 as mounting four synchronized machine guns. The article stated maximum speed was "over 300 mph." This jibes with later sources that generally place the CW-21's maximum speed at 304 mph at 12,000 feet [11]. The CW-21B's speed is cited as 312 mph at 12,000 feet and 315 mph at 17,000 feet. Both the American Aviation article and a more extensive description of the CW-21B published in Aeronautics in August 1941 mention a "guaranteed maximum level speed" of 333 mph. This was attained at 18,000 feet. It seems possible Curtiss obtained faster speeds with an aircraft flown without a radio and other gear referred to as "optional equipment" in the Aeronautics article. If a speed of 333 mph was obtained, the engine may have been operated above its normal maximum rating of 2,100 rpm.

In describing how the CW-21B was equipped Aeronautics said normal equipment included provision for two Colt .50 caliber machine guns and two Colt .30 caliber machine guns synchronized to fire through the propeller disc. Optional armament was said to include two pairs of the same weapons mounted in the wings. Additional fuel tanks could be fitted. Radio and oxygen equipment was also described as optional.

CW-21 three-view

The design incorporated a "tell-tale" warning system to advise the pilot of improper operating conditions. This was illustrated in American Aviation in March 1940. It was simply a series of eight panel lights that indicated "fuel press low", "oil press low", "mixture lean", or "high blower" and so forth. At the time this was considered an advanced feature.

Curtiss engineers took pains to keep the aircraft's weight low by carefully selecting each piece of equipment to be incorporated. All major structural components other than landing gear and engine were constructed of stressed aluminum alloy sheet generally 24ST Alclad. Where aerodynamics were involved flat head rivets were used.

The wing was of internally braced cantilever construction made in three sections. The center section incorporated the landing gear and was integral with the fuselage. This allowed the aircraft to be rolled around with the outer wing panels removed. The wing incorporated a split trailing edge flap and ailerons that were both of all metal construction.

The fuselage was of semi-monocoque construction covered with aluminum alloy. The fuselage was riveted to both the wing center section and fixed parts of the empennage to form a single integral unit. The engine mount was made of chrome molybdenum tubular steel with a steel tube engine-mounting ring. This was attached to the fuselage by four "quick detachment" fittings.

The main landing gear were 27 in. smooth contour tires with wheels mounted on cantilever shock absorber struts. Toe pedals mounted on the rudder pedals could individually control hydraulically operated brakes. In contrast with the original CW-21 design the CW-21B featured a retractable, steerable tail-wheel.

The Wright Cyclone R-1820-G5 direct drive engine was fitted with a two-speed supercharger and non-icing carburetor. The propeller was the three bladed automatic constant speed type.

A stainless steel exhaust collector directed exhaust gases out of the bottom of the engine cowling. The cowling, made of alloy sheet reinforced with channel and hat section stiffeners, was constructed in three 120-degree sections and fixed to the upper section of the fuselage so that no exhaust gases entered the cockpit. A sealed fireproof and heatproof bulkhead of aluminum alloy with an asbestos core separated the power plant from the fuselage.

The pilot's seat was adjustable and fitted with a safety belt. Complete engine controls including propeller and supercharger speed control were within easy reach. The cockpit was lighted and equipped with standard instrumentation. There was an internal battery and starter switch, flap and wobble pump controls. The cockpit was enclosed with an acrylic windscreen and sliding canopy. Immediately behind the pilot was a 1/4 in. steel bulkhead made both to protect him from enemy fire and for head protection in case of a turnover.

With a wing span of 35 feet and length of 27 feet 2 inches the CW-21B was not particularly small for its era but its empty weight of 3,382 pounds and loaded weight of 4,500 pounds testified to the weight saving measures taken by Curtiss engineers. Even so the CW-21B's loaded weight was 250 pounds heavier than the CW-21.

CW-21B
The CW-21B

The R-1820-G5 was rated at 1,000 hp at 2,200 rpm for take off and its normal sea level maximum rating at 2,100 rpm was 850 hp [12]. Normal maximum rating in first supercharger gear was 850 hp at 2,100 rpm and 6,000 feet and 750 hp at 15,200 feet in high supercharger gear. Curtiss credited the fighter with a maximum speed of 333 mph at 18,000 feet and 314 mph at 5,600 feet. Cruising speed was 282 mph at 12,200 feet. Cruising range was 630 miles. Climbing time to 13,120 feet (4,000m) was 4 minutes and to 16,400 (5,000m) was 5 minutes.

Here it might be worth commenting on the claims of an initial climb rate of over 5,000 feet per minute (Curtiss advertising) or even the slightly lower figure (4,500 feet per minute) mentioned in some sources [13]. The figures above show an average climb rate to both 4,000m and 5,000m as well as between those heights of 3,280 feet per minute. If the CW-21 actually climbed at initial rate approaching or exceeding 5,000 feet per minute but averaged 3,280 feet per minute to both 4,000 and 5,000 meters, its climb performance curve would look very strange when graphed out. In order to average 3,280 feet per minute to 4,000 meters after a start of nearly 5,000 feet in one minute its climb rate between 5,000 feet and 13,200 feet would average barely 2,700 feet per minute. The rate would then increase again to 3,280 feet per minute between 4,000 and 5,000 meters. This unlikely scenario appears even stranger when considering that at 5,600 feet the supercharger would bring the engine rating (850 hp at 2,100 rpm) up to exactly the same figures for sea level. In all likelihood claims for an initial climb rate much in excess of 4,000 feet per minute are probably vastly overstated and fail to represent the aircraft's performance under normal operating conditions. On the other hand an average climb rate of 3,280 feet per minute to 16,400 feet would certainly constitute outstanding performance.

The 1940 edition of Jane's All the World's Aircraft published climb figures of 4,800 feet per minute for the CW-21 and 4,500 feet per minute for the CW-21B. Maximum speed for the CW-21B was given as 333 mph consistent with the Curtiss "guarantee." In the 1941 edition of Jane's the speed for the CW-21B was revised to 314 mph at 12,200 feet. The 1941 Jane's made no mention of climb performance!

continued in part 3