The McDonnell XP-67 was a radical design for a twin engine long range fighter most notable for the extensive blending between the fuselage, wing and engine nacelle.
We start with a look at the development of a new high powered but small profile engine in response to a USAAC Request for Data issued on 11 September 1939. One of the engines to emerge from this was the Continental I-1430, one of the many technically promising but unsuccessful engine designs of the Second World War. As with so many interesting aircraft, the failure of its original engine would doom the XP_67.
We next look at Request for Data R40-C of February 1940, which called for a single-engine single-seat pursuit-interceptor aircraft, with the potential to outclass the existing P-38s, P-39s and P-40s of the USAAC. This was clearly a rather confused programme, with on the one hand an expectation that a large number of radical designs would be submitted and on the other hand a desire to get an aircraft into production relatively quickly. McDonnell’s first submissions were for single engine designs with a large engine in the fuselage driving two pusher propellers (all of the favoured projects were pusher designs). The most interesting feature of this McDonnell Model 1 was the blended design, where the fuselage blended seamlessly into the wings (as see much later on the ultra-high speed SR-71 Blackbird).
We follow the design process in some detail. One of the interesting feature of this is that the design began with a layout similar in appearance to that of a twin engined jet aircraft, with pusher engines and a swept back wing, but evolved into something somewhat more conventional, with tractor propellers in engine nacelles and a straight wing. A nice feature of the book is the author’s creation of simple plans showing the layout of the XP-67 overlaid on earlier versions of the same design, and over its rival aircraft (Vultee XP-54, Curtiss-Wright XP-55, Northrop XP-56).
By the time MacDonnell were given a contract for two prototypes, in April 1941, it is clear that the XP-67 wouldn’t arrive in time to make a major contribution to the war – the first prototype wasn’t expected until October 1943, the second April 1944.
I do think the author is a little too defensive about the flaws with this aircraft. It’s first taxiing test and first flight both had to be cut short because of fires in the engine nacelles. The author repeated comments that it wasn’t the engines themselves that caused the problem, but the peripheral equipment. However in the case of the first flight that include the supercharger, which I would say was a key part of the engine setup rather than a peripheral. The engine itself might not have caught fire, but it was the engine that produced the heat that triggered the fires in the nacelle and it appears to have been the nacelle design that was at fault, probably because it was designed to be as small as possible for the engine.
On the fourth flight the engines began to overspeed and new engines were needed. However the biggest problem was that the Continental engine wasn’t going into production, and the aircraft was very carefully designed around the dimensions of that engine. Any change would have needed bigger nacelles, and thus a total redesign of the entire carefully blended wings and fuselage. All this at a time when the aircraft was under-performing compared to the existing service fighters of 1944.
Quite a few variants of the basic design were considered, including the use of combined piston and jet power, an attack aircraft with internal bomb bay, one or two man reconnaissance aircraft, but all of these required the original aircraft to have performed rather better than it actually did.
Although the XP-67 project wasn’t a success, it did provide McDonnell with a great deal of experience of fighter development, and many aspects from it (in particular the wing and nacelle blending) were used on the FH-1 Phantom I and F2H Banshee,
This is an excellent study of this unusual aircraft, showing how significant it was for later McDonnell designs, but also how dangerous it could be to rely on an untested, experimental engine which might never reach production.
1 – Origins
2 – Prototype Design
3 – Test and Development
4 – Influence of Frontline Types
Author: Steve Richardson and Peggy Mason