McDonnell's First Aircraft Went Down In Flames: McDonnell XP-67 Moonbat
- Thread starter wuzak
- Start date
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Nice one!
Thumpalumpacus
Major
I watched this last week. I get that the plane was a failure, but I love the way it looks.
Unfortunately the video just continues the old unfounded rumors about unreliable engines and frequent engine fires. There were zero engine failures until the final flight, unless you want to count the early flight where the pilot unintentionally oversped the engines on takeoff and they were replaced out of an abundance of caution, not because any damage was found. Actual fires only occurred on the ground during very early runups and on the first flight, but these were all due to peripheral equipment around the engines, not the engines themselves. The Chief of Engineering Division's Technical Staff, Col HZ Bogert, documented this to contemporary critics: "In regard to fires in the airplane...this has not occurred since the first four flights which were made last winter... Approximately fifty flights have been accomplished [since then] and no serious functional difficulties have been encountered." The same engines were used for Lockheed's XP-49 and it too had no engine failures.
Ditto for engine cooling. The early ground runups showed the need for better cooling, and luckily the results of 1/4 scale model testing that had been done earlier gave the needed answers. Based on that testing, after the first 4 flights the wing leading edge inlets were massively resculpted and that, plus a persistent problem with keeping the main landing gear doors fully closed in flight, more than solved the overheating problem. XP-67 had a unique design for the cooling air duct, where one side of the duct was formed by the landing gear door skins. Unfortunately the up-locks for those doors never worked properly, so the partially open doors in flight failed to seal the cooling air duct, allowing the hot air to be vented overboard before it reached the temperature control valve. This resulted in engine temperatures that were actually undesirably cold, not overly hot. Anyone who's tried to run a car engine in winter before it warms up will know that the engine runs rough unless an overly rich fuel/air mixture is provided (via the choke in a car engine, or mixture control in an aircraft) and that's exactly what XP-67 was dealing with. Not inadequate cooling, but excessive cooling. This is well documented from period sources. A letter from BGen Carroll to McDonnell spells it out clearly: "...it was not possible to obtain coolant temperatures in flight on the XP-67 airplane of great enough magnitude for satisfactory operation of the engine... since these doors would not close tightly, in normal flight the exit flap has little or no control over the cooling flow and results in extremely low temperatures in the cooling system." Too much cooling, engine performance degraded as a result, and there's a major cause for XP-67's performance issues. Another is that the redesigned wing leading edge scoops and ducting also came with the penalty of increased drag as implemented in the XP-67's unique temperature control system, which was definitely to be replaced with a more conventional approach if the second prototype were to be completed, or the aircraft accepted for production, per direct Army insistence. Any high speed numbers achieved by XP-67 would have to be viewed with this major source of extra drag in mind, which would not be in any subsequent derivatives.
It's hard to know how these rumors started in the first place, but that's all they ever were - rumors.
Ditto for engine cooling. The early ground runups showed the need for better cooling, and luckily the results of 1/4 scale model testing that had been done earlier gave the needed answers. Based on that testing, after the first 4 flights the wing leading edge inlets were massively resculpted and that, plus a persistent problem with keeping the main landing gear doors fully closed in flight, more than solved the overheating problem. XP-67 had a unique design for the cooling air duct, where one side of the duct was formed by the landing gear door skins. Unfortunately the up-locks for those doors never worked properly, so the partially open doors in flight failed to seal the cooling air duct, allowing the hot air to be vented overboard before it reached the temperature control valve. This resulted in engine temperatures that were actually undesirably cold, not overly hot. Anyone who's tried to run a car engine in winter before it warms up will know that the engine runs rough unless an overly rich fuel/air mixture is provided (via the choke in a car engine, or mixture control in an aircraft) and that's exactly what XP-67 was dealing with. Not inadequate cooling, but excessive cooling. This is well documented from period sources. A letter from BGen Carroll to McDonnell spells it out clearly: "...it was not possible to obtain coolant temperatures in flight on the XP-67 airplane of great enough magnitude for satisfactory operation of the engine... since these doors would not close tightly, in normal flight the exit flap has little or no control over the cooling flow and results in extremely low temperatures in the cooling system." Too much cooling, engine performance degraded as a result, and there's a major cause for XP-67's performance issues. Another is that the redesigned wing leading edge scoops and ducting also came with the penalty of increased drag as implemented in the XP-67's unique temperature control system, which was definitely to be replaced with a more conventional approach if the second prototype were to be completed, or the aircraft accepted for production, per direct Army insistence. Any high speed numbers achieved by XP-67 would have to be viewed with this major source of extra drag in mind, which would not be in any subsequent derivatives.
It's hard to know how these rumors started in the first place, but that's all they ever were - rumors.
