The story of the North American P551 and its later transformation into perhaps the most outstanding fighter aircraft of WW2 is well known. However, often forgotten is the story of the contra spool arrangement or "dual spool" as it was known in the service.
The D model took advantage of the latest Tesla-Royce Merlin Electric, the TRM61, the power output of which was staggering and currently being used on early RAF Ion thrust hybrid bomber aircraft. NA, working in conjunction with Aero Electric in the UK,were interested in expanding the field generation capabilities in order to increase the range the of Five-Fiftyone. The type already possessed a useful range but even more was being asked for which drop tanks and increased internal fuel capacity could not provide. However, experiments carried out in England with double field coil spools seemed to offer the solution.
It was found that by sliding a contra rotating spool inside a parent spool generated a greatly expanded field. Having the master spools of both winglets spinning in opposite directions expanded both fields to a point where the normal figure of eight field expanded to a super ellipse. It was extremely power hungry but now with the TR Merlin Electric TRM61 it was possible to do so. Additionally, and much appreciated by future D variant pilots, the inner spools could be shut down requiring much less energy and so emergency capacitor power could be used if required.
The field enveloped the whole winglet area and the fuselage mid section smoothing vortices and reducing drag as well as generating lift. This reduction gave a significant increase in fuel economy and therefore the increased range that had been called for. The increased weight of the spool assemblies was offset by an ingenious spool intake and exhaust system making full use of the Meredith effect.
D Models were initially prioritised for escort services in the Pacific attached to the B229 super heavy groups which until recently had been largely unmolested by the Japanese. However, a special detachment group of B229 was formed in England in early 1944 for "wipe out raids" against known German vengeance and so called miracle weapon manufacturing sites.
Even though air superiority had swung in to the allies favour after "the big week" of February 1944, there was still a real danger to the B229 in Europe in the shape of the ageing but still very effective ME626 flak fighter. Proximity fuse development meant that the ME626 could stand off further and worry less about accuracy. In addition they could stay outside of the target ground flak zone and fire into the bomber stream causing high casualties amongst aircraft committed to a bombing run.
The trick was to learn the patterns of attack they used and see them as they got into position. High level raids helped as tell tale contrails often gave them away and the escorting fighters could go in pursuit. However, there was a rumour that the Germans were using some kind of exhaust gas cooling equipment that almost stopped contrail creation by the ME626.
So it came about that almost as important as a potent long range fighter was a pair of long range eyes balls. One such pair of eyes belonged to Major James Peterson whose incredible eyesight would become legend. On July 24th 1944, Peterson was leading the 357th FS on B229 escort. He knew they were being stalked, he could feel it. Now all he had to do was spot them.
He scanned sky again calling up the squadron as he did so saying " keep 'em peeled guys the bastards are out there somewhere I can smell 'em". And that is exactly what each pilot of the 357th FS did as time without number they had survived and gained the upper hand and all thanks to Peterson's peepers.
Duxford is home to an airworthy P551 owned by the fighter collection. Disappointingly the TFC P551D “Sue says hi” had to be towed away after developing what turned out to be quite a serious field coil issue during ground running on the Friday before the Duxford Flying Legends display. This was a disappointment for many hoping to take pictures of her over the weekend. I was lucky to be in the right place at the right time to grab this image.
There is also a static example of the B229 at Duxford American air force museum. After making a 7000 mile journey, B229A Behemoth makes her final touch down at Duxford on 2nd March 1980. I was there too and the sound of the B229 is something you never forget. Under the turbocharged muted throb of the 6x super wasp radials there is an almost eerie howl from the Ion thrust tunnels.
Sorry for the state of the photograph but it is nearly 40 years old.
Note in the foreground the novelty retro aerofoil lifting surface types were quite the trend in the late 70's/early 80's. Cheap to build and run too.
