Τρίτη, 19 Απριλίου 2016

The flying anti-aircraft battery: The Bell YFM-1 Airacuda…it had some design flaws!







During one of the test flights of the first FM Airacuda, on September 28, 1939 one of the turbo-superchargers exploded. source
During one of the test flights of the first FM Airacuda, on September 28, 1939 one of the turbo-superchargers exploded. source
“Flying the Bell Airacuda was a new experience for me, since it was the first pusher aircraft I’d ever flown. Its handling characteristics were foreign to anything I had ever had my hands on. Under power it was unstable in pitch, but stable with power off. While flying straight and level, if a correction in pitch was required, a forward push on the control resulted in the airplane wanting to pitch over even more. Pitch control became a matter of continually jockeying the controls, however slightly, even when the aircraft was in proper trim. The same applied if pulling back on the control. It would tend to continue pitching up, requiring an immediate corrective response. The same happened in a turn with power off, the Bell became stable in pitch. This was fortunate because during approach and landing, it was very stable, and a nice flying airplane.” – Test pilot Erik Shilling

The Airacuda was Bell Aircraft’s answer for a “bomber destroyer” aircraft. The concept of placing aircrew away from the fuselage wasn’t just a German idea. The Bell Aircuda YFM-1 had similar wing mounted crew cabins – in fact, two of them! Developed in the late 1930s as a sort of flying anti-aircraft battery for use against enemy bomber formations, the YFM-1 featured manned forward-facing gun turrets on both wings, each packing a 37mm cannon. To make room for the cabins, the plane’s engines faced aft.
It was the first military aircraft produced by Bell. Originally designated the "Bell Model 1," the Airacuda first flew on 1 September 1937. source
It was the first military aircraft produced by Bell. Originally designated the “Bell Model 1,” the Airacuda first flew on 1 September 1937. source
In an effort to break into the aviation business, Bell Aircraft created a unique fighter concept touted to be “a mobile anti-aircraft platform” as well as a “convoy fighter.”Created to intercept enemy bombers at distances beyond the range of single-seat fighter interceptor. It was an innovative design incorporating many features never before seen in a military aircraft, as well as several never seen again. Using a streamlined, “futuristic” design, the Bell Airacuda appeared to be “unlike any other fighters up to that time.”
According to Major Alexander De Seversky’s 1942 book, Victory Through Air Power, the Bell Airacuda “represents a great engineering achievement. But its designation as ′convoy fighter′ is erroneous, since that requires different disposition of armament. With its maximum firepower directed forward, it really offers a preview of an effective long-range interceptor fighter.”
It was designed during a time when heavy escort fighters such as the Bf-110 were in development. source
It was designed during a time when heavy escort fighters such as the Bf-110 were in development. source
A forward-firing 37 mm (1.46 in) M4 cannon with an accompanying gunner was mounted in a forward compartment of each of the two engine nacelles. Although capable of aiming the cannons, the gunners’ primary purpose was simply to load them with the 110 rounds of ammunition stored in each nacelle.
The crew of five included the pilot and gunners; a copilot/navigator who doubled as a fire-control officer, using a Sperry Instruments “Thermionic” fire control system (originally developed for anti-aircraft cannon) combined with a gyro-stabilised and an optical sight to aim the weapons; and a radio operator/gunner armed with a pair of machine gunsstationed at mid-fuselage waist blisters for defense against attack from the rear.
An unusual feature of the Airacuda was the main door for entry. The door was opened and pulled down and hinges folded in on three steps for the crew to climb into the aircraft.
Bell's FM Airacuda was slower and less maneuverable than the bombers it was supposed to protect, leaving it useless as an escort fighter. source
Bell’s FM Airacuda was slower and less maneuverable than the bombers it was supposed to protect, leaving it useless as an escort fighter. source

Design flaws

The Airacuda was plagued with problems from the start. The lofty performance estimates were unobtainable as, despite its sleek looks, the Airacuda was heavy and was slower than most bombers. In the event of interception by enemy fighters, the Airacuda was not maneuverable enough to dogfight, while the meager 600 lb (270 kg) bombload was of little use in the intended fighter-bomber role. Even the 37 mm cannons were of less value than predicted. The cannons had a tendency to fill the gun nacelles with smoke whenever fired and, additionally, fears persisted as to how the gunners would escape in an emergency, with the propellers directly behind them. An emergency bailout would have required both propellers to be feathered, though additional provision was made with the use of explosive bolts on the propellers to jettison them in the event of a bailout.
The Allison V-1710-41 engines, though relatively trouble-free in other types, had no additional cooling systems. Like many pusher designs, they were prone to overheating. On the ground, the aircraft had to be towed to and from the runway and could only be started when the Airacuda was able to take off immediately. Even in the air it was not uncommon to experience overheating problems. Although designed for turbo-supercharging, the first flights were made with V-1710-9 carbureted engines that only delivered 1,000 hp each. Despite the 5 ft-long shaft extensions, there were no problems with this feature. When the turbos were fitted to the later YFM-1, they were plagued by cranky turbo regulators that backfired continuously. An explosion during a September 1939 test flight made it apparent that the teething engine troubles would not be solved easily.
The Bell FM Airacuda was a twin-engine long range escort fighter which could also be used as an interceptor and light bomber. source
The Bell FM Airacuda was a twin-engine long range escort fighter which could also be used as an interceptor and light bomber. source
Additionally, Marshall Wainwright notes that other sources indicate the first eight aircraft were to originally have been powered by Allison V-1710-13 engines fitted with GE Type B-6 turbo superchargers. These aircraft were eventually delivered with improved V-1710-23 engines. Wainwright further states that two of the YFM-1 airframes were changed on the production line to accept the V-1710-41 without turbo supercharging, becoming YFM-1Bs. This is noted in a contract change dated 19 October 1939 which shows that aircraft 34-489 and 38-490 had their turbos, all associated ducting, and controls removed and V-1710-41 “Altitude Rated” engines installed instead. The (D2A) was essentially a -23 with higher supercharger gear ratios, which allowed the motor to develop around 1,090 horsepower up to 13,200 ft ASL. They used the same ratings and components as the Altitude Rated V-1710-33 Allison fitted to the original Curtiss XP-40. Allison was paid $1,690 to modify each engine.

The Airacuda was marked by bold design advances and considerable flaws that eventually grounded the aircraft. source
Initial flight testing by Lt. Ben Kelsey proved the Airacuda virtually impossible to control with only one engine, as the aircraft would go into an immediate spin. Problems with stability in pitch were also encountered, and had to be corrected by reducing power. Test pilot Erik Shilling described his experiences in a later book, Destiny: A Flying Tiger’s Rendezvous With Fate as:
Flying the Bell Airacuda was a new experience for me, since it was the first pusher aircraft I’d ever flown. Its handling characteristics were foreign to anything I had ever had my hands on. Under power it was unstable in pitch, but stable with power off. While flying straight and level, if a correction in pitch was required, a forward push on the control resulted in the airplane wanting to pitch over even more. Pitch control became a matter of continually jockeying the controls, however slightly, even when the aircraft was in proper trim. The same applied if pulling back on the control. It would tend to continue pitching up, requiring an immediate corrective response. The same happened in a turn with power off, the Bell became stable in pitch. This was fortunate because during approach and landing, it was very stable, and a nice flying airplane.”

Later the Airacudas were dispersed to various airfields so that pilots could fly the strange aircraft and add it to their log books. source
Later the Airacudas were dispersed to various airfields so that pilots could fly the strange aircraft and add it to their log books. source
The Airacuda was also saddled with a complex and temperamental electrical system and was the only aircraft ever built to rely on an independent auxiliary power unit to power both engine fuel pumps, as well as all aircraft electrical systems. Systems usually powered by an aircraft’s engines were instead powered by the single generator. The generator, with its own supercharger, was located in the belly of the aircraft. In the event of a failure (and they occurred frequently), the crew was instructed to begin immediate emergency restart procedures as the aircraft basically shut down. When the APU failed, the pilot had “NO fuel pressure, NO vacuum, NO hydraulic pressure, NO gear, NO flaps and NO ENGINES”.
Despite the aircraft’s many faults, only two were lost in accidents. The seventh aircraft (38-492) was on its final test flight from the Buffalo factory prior to delivery to the Air Corps when pilot John Strickler, a Bell pilot and engineer, and co-pilot Brian Sparks, who was Bell’s chief test pilot at the time, encountered problems recovering from a deliberate spin attempt which was part of the test flight profile.
Despite every effort to emerge from the spin, the aircraft would not respond, and it appeared that the rudder had locked. Co-pilot Sparks shut down the engines and waited for the propellers to come to a stop before bailing out. Because of the tandem seating, it was necessary for Sparks to exit the aircraft first, and in doing so he struck the empennage, breaking his legs — and in the process, freeing the rudder. Strickler decided to stay with the aircraft and attempt an emergency landing. By this time, the aircraft had lost sufficient altitude that there was not time to restart the engines. Strickler put the Airacuda down hard in a farmer’s field, and walked away unhurt. The Airacuda was so badly damaged it had to be scrapped.
Bit hard to improve it considering it was slower than the bombers. source
Bit hard to improve it considering it was slower than the bombers. source
All three Airacudas with tricycle landing gear encountered problems and were damaged at one time or another. The most serious accident occurred to YFM-1A (Model 8) 38-497, on a flight from Chanute Field, Illinois, and Keesler Field, Mississippi, when a broken oil line started an inflight fire. The cause of the broken line appeared to be serious airframe vibration encountered during the flight. With no way of extinguishing the fire, both the pilot and crew chief agreed to bail out. The pilot was killed when his parachute failed to deploy (he may have struck the tail while bailing out). This was the only fatality to occur during the flying of Airacudas. The accident investigation report stated “inherent defects in design caused constant maintenance difficulties and the flying of this type has been very limited.”
The engines were bad, the overheating issue and the pusher props were difficult to master. The dual 37mm cannons were meant as a bomber destroyer, but it was hard to get up to altitude. There were many other planes to chose from by the time this plane arrived...the jet age was almost here! Bell, the creator of this had many novel concepts for its time, and this plane was one of them. source
The engines were bad, the overheating issue and the pusher props were difficult to master. The dual 37mm cannons were meant as a bomber destroyer, but it was hard to get up to altitude. There were many other planes to chose from by the time this plane arrived…the jet age was almost here! Bell, the creator of this had many novel concepts for its time, and this plane was one of them. source

Operational history

Despite these problems, one fully operational Airacuda squadron was eventually assembled, and operated from 1938 until 1940. Funds were appropriated, but never released, for the purchase of two groups of Airacudas. Continuing problems gave the aircraft a reputation as “hangar queens”.Towards the end of the type’s operational life, the aircraft were flown primarily for photo opportunities and always accompanied by a chase plane for safety. Eventually the decision was made to disperse the aircraft to various airfields to give pilots an opportunity to add the unusual aircraft to their log books. Airacudas were sent at various times to Langley Field, Virginia; Maxwell Field, Alabama;Hamilton Field, California; and Wright Field, in Dayton, Ohio. YFM-1 38-488 was displayed at the 1940 World’s Fair in New York, finished in the markings of the 27th Pursuit Squadron. During this time, the aircraft saw limited flight time, as few pilots were interested in flying the unusual aircraft.
Several plans were made to modify the Airacudas to give them operational status, including modifying the airframe and adding more powerful engines, but all proposals were eventually rejected. In early 1942, despite fears of enemy bomber attacks against which the Airacuda was intended, the aircraft were stricken from inventory

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