22 Jul THE FLAWED AND MISCALCULATED FISHER XP-75 EAGLE
It’s a fair statement that the P-75 project was a sad story. There were numerous failed or poorly-performing fighter aircraft during the Second World War. These include the Messerschmitt Me 210, the Lavochkin Gorbunov Goudkov LaGG3 and the Bell P-39 Airacobra.
However, the General Motors (Fisher) XP-75 Eagle is a strange exception, as it truly had the potential to be one of the best fighters of its time. Unfortunately, it was conceived from a hungry demand for large numbers of vastly improved aircraft and thus the design was rushed. The results were unsatisfactory, to say the least, even as it began to make progress towards the end of its development period.
After all, the P-75 was built under the constraints of a tight deadline and budget. Its designer, Donovan Berlin, had notable successes in his career and was responsible for several excellent P series models. Unfortunately, he was forced to maintain a low production cost and time. For this reason, he resorted to constructing the P-75 from a variety of parts adopted from pre-existing aeroplanes and automobiles.
“The XP-75 Eagle is a strange exception; it indeed had the potential to be a great fighter”
In hindsight, this was probably a bad idea, but it seemed to be the only viable choice. Each individual part was optimized for its specific carrier and a combination of (mostly) random parts was bound to fail in some ways. So why wouldn’t the Fisher Division of General Motors develop the aircraft from scratch and perfect it – instead of picking different parts and hoping they’d supposedly work in unison?
General Motors was inexperienced and under the expectation to fulfil a long list of requirements. USAAF issued a Request For Proposals in 1942, demanding an aircraft that could reach a maximum speed of 440mph and maintain an operational ceiling of 38,000 feet. Its purpose was to act as an interceptor, countering both bombers and fighters of the German Luftwaffe. For most aviation companies, this was a project of “high priority”.
However, the Fisher Body of General Motors had inadequate experience in building the functioning systems of aircraft. Despite the success yielded from their early attempts, they were yet to perfect the process of building vehicles purely from scratch, perfectly designed for their purposes. Perhaps most importantly, they were adapted to a production-line style of work in the automobile-manufacturing industry, making only routine modifications upon original models.
A design and contract put forward to USAAF was accepted and two prototype models were planned. The powerhouse of the P-75 was the largest and most powerful liquid-cooled engine available at the time – the twenty-four-cylinder Allison V-3420. Situated behind the cockpit and connected to the fighter’s nose by an extension shaft, the Allison drove a pair of contrarotating propellers, making it seem as if the P-75 had six props. The engine output of 2,600 horse-power was controlled by a reduction gearbox.
Initially, the P-75 used inverted-gull wings adapted from the P-51 Mustang, but these were quickly replaced with straight wings from the Curtiss P-40 Warhawk instead. The tail was stolen from the Douglas A-24. Other sections such as the underbody were based upon previous aircraft examples.
“GM used a production-line style of work, making only routine modifications upon original models.”
The XP-75 Eagle had a climb rate of about 4,200 feet per minute, and “Eagle” was a patriotic title given to the XP-75 sometime early during the project.
The first XP-75 made its test flight on the 17th of November 1943 and the pilot found numerous issues. A few examples include a terribly miscalculated centre of gravity – making for awful spin problems – slow and problematic liquid-cooling of the engine after it had overheated, and sluggish controls. On a whole, the prop system did not help the aircraft’s speed and was often found to hinder it, cancelling out the airflow of its oppositely-rotating propellers. Moreover, the sharp-edged cockpit canopy was not given a good treatment and was generally unsatisfactory.
By the time the results were evaluated by USAAF, the aircraft needs had changed. It was 1943 and they wanted an aircraft that was better suited to long-range penetration, one that could travel deep behind enemy lines and escort bomber aircraft to their target sites. On July 6th, USAAF issued a request for six prototypes of a new fighter model, the P-75A.
The P-75A was equipped with a bubble canopy, which was the improved and modern cockpit of its time. Furthermore, its engine was upgraded to an Allison W-3420-23, with the hopes of eliminating many of the engine problems of the earlier design. Additionally, the engineers saw a few of the design issues disappear with the modifications to the tail assembly.
“USAAF needed an aircraft to escort deep behind enemy lines”
However, the P-75A was still a fighter aircraft and needed excellent armaments in order to carry out combat effectively. The wings were armed with six .50 machine guns, with four of the same armaments in the plane’s nose. The serious problem with installing guns in the nose was that all four would need to be perfectly synchronised to fire through a set of contrarotating props, which – as one could image – was a particularly awkward and complex problem to solve. It could also carry two 500-pounder bombs if necessary, but this was likely to only exacerbate the centre of gravity issues.
The flying specs of the P-75A were a speed of 434 mph at an altitude of 20,000 feet and 389 mph at sea level. USAAF agreed to the revised design and placed an order for 2,500 aircraft should the test flights be successful. By the Autumn of 1944 it was finalised and prepared for test flights, but it was already significantly later, and a lot had changed throughout the war.
The P-75A still had its problems. It remained extremely difficult and annoying to handle and the engine wasn’t capable of outputting as efficiently as it should have done. Speed was significantly below that which the manufacturers had guaranteed. In fact – even though the design work on the P-75A was reputable – similar planes that were arriving from the factories and development facilities were achieving better results with many less problems.
For example, the P-38 Lightning, P-47 and P-51 Mustang were able to sustain high speed and long range, making them ideally suited to the escort role. The USAAF was already purchasing multiple of these aircraft and putting them into action. The P-75A offered little advantage over these newer designs and was still very sluggish at the controls in comparison.
“The P-75A was extremely difficult to handle and the engine didn’t output as it should have”
Following a P-75A flight by Col. Mark E. Bradley, he made a comment hinting towards the severe disadvantages of this fighter aircraft and advised USAAF to cancel production since it had notable inferiorities to enemy German fighters. The window for research and development of the P-75 had practically ended, also. Most importantly, the state of war had shifted in the favour of the Allies and many of the original requirements had been dropped.
To a certain degree, the XP-75 was very rushed. Even its model name was the result of two previous numbers (P-73 and P-74) being dropped in order to title an aircraft with a “clean” model number, supposedly one that would be remembered in history.
The P-75 never reached its full potential but was badly flawed right from the start. Should the designers have spent more time ensuring that the system was built specifically and perfectly to USAAF’s demands, perhaps it would have seen active service.
However, it was too late and only six of the aircraft were produced. Only one remains today, in the National Museum of the United States Airforce, Dayton, Ohio, after going through a full restoration.
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