Vector,
You are correct. P-47's engine was rated for higher altitudes because of the nature of the war it was designed to fight. High-altitude combat against the Luftwaffe. Much of the Thunderbolt's bulk and weight is attributable to the complicated ductwork of its tubo-supercharging system.
Addressing the matter of drag, insofar as wing area is concerned, wing thickness is the greatest drag-producing factor. There was very little difference in wing thickness. The Hellcat had a wing thickness factor of 16 percent. That is, it's thickness was 16 percent of the width of the wing. The Thunderbolt's wing thickness factor was 15 percent. The Hellcat's wingspan was almost exactly 2 feet wider than the Thunderbolt's, hardly enough to be a major factor in the production of drag. The above information was published in an article by Corky Meyer in Flight Journal magazine, August 2001 edition.
By the way, that article is a great read. It's the tale of Grumman's attempts to solve the Hellcat's compressibility problems. The test dives were supposed to be limited to .75 mach, or a speed of 485mph (580mph true air-speed), with the dive starting at 28,000 feet. Corky inadvertently entered the dive at a 75 degree angle instead of the 60 degree angle mandated by test parameters. The resulting dive reached .77 mach, with a true airspeed of over 600mph! With compressibility forcing the Hellcat into a dive angle near to the vertical, he passed through 9,000 feet at more than 700 feet per second. His controls were immovable, as though set in concrete. Corky saved his life by reducing throttle to idle, thus greatly increasing the drag of his aircraft. The resultant 7.5 g pullout at 2500 feet bent both horizontal stabilizers at their midspan; the right one up at a 15 degree angle, and the left one down at a 15 degree angle. A complete inspection of the aircraft didn't show anything else bent or any rivets pulled! Grumman redesigned the elevator structure to withstand the 7.5 g buffet boundary as a result of the elevator failure. Grumman wasn't called "The Iron Works" for nothing.
But I digress!
The Thunderbolt was definitely faster than the Hellcat because of its complicated and powerful turbosupercharger. Nevertheless, at it's rated altitude of 21,600 feet, the Hellcat was perfectly capable of 400 mph in level flight. Of the two sources have quoted to substantiate that fact, one came from one of Grumman's main test pilots for the Hellcat, and the other was the result of tests run by Allied Technical Air Intelligence. Both sources agree that the -5 Hellcat's top speed exceeded 400mph at it's rated altitude.
Shuckins
