When preparing Brent Regan's IVP for the Aircraft Spruce race from Colorado to OSH in 1996, we did a lot of flight testing, drag reduction work, and research including discussions with Lancair and the structural analyst for the LIV, Martin Hollman.
In the early days fuel tanks were enlarged to 92 gallons by adding additional tank volume near the wing tips. Prior to the tank enlargement, Vne was 274 knots indicated up to 25,000 based on his flutter analysis, but the effect of the tank volume increase was unknown.
A group of builders chipped in to do the revised analysis which showed that the added fuel was detrimental, but burning fuel off during climb was adequate to maintain required margins as long as the fuel came out of both tanks.
The Mach limit was not explored, but dive testing to Mach = 0.6 had been completed.
In preparation for the race, I computed a series of tables that would provide Mach numbers from OAT, corrected for compressibility and frictional heating, these being the days pre-Chelton. We established hard limits of 274 knots IAS and Mach = 0.58 for the descent.
Brent's plane uses a modified Lycoming 540 with 8.5:1 pistons (if memory is correct) and specially selected turbos that allowed us to race at 90+% power, 30 GPH, 320 knots TAS at 27,000 feet, Mach 0.52 when racing in temperatures well above standard. As I recall, Brent said his engine produced about 375 HP at 100%, corrected to standard conditions.
Mixture management was crucial in our attempt to break the San Francisco to Denver record (set by the original Lancair IV) as we had barely enough fuel for the planned trip after the long climb to 27,000 with way above standard summer air temperatures before passing over San Francisco. We constantly diddled mixture, temperature and speed and the engine situation was never stable. Flying through cirrus clouds we got a big static discharge that whacked everything and even reset the digital clock. But when it became clear we would be two minutes late, we canceled, reduced power, and descended in a civil fashion to Jeffco Airport. We did not have the required tailwinds to break the record which was set with a better boost from the winds aloft.
The NExt day was the race to OSH. It was a shorter leg, so we could pour on the fuel, keep things cooler, the OAT was also a bit lower, so we could nudge up the power a bit. The descent was very flat at first to stay below Mach 0.58, steepening as the air got thicker and warmer which allowed still higher power setting and higher IAS yielding better cooling. The speed was adjusted by controlling pitch to yield Mach 0.58 at 100% power until the IAS reached 274 knots, and that was held the rest of the way down.
A Lancair IV going down hill at full power and 274 knots (in and out of IMC) sounds very different than it does in cruise. It made the hair on the back of my neck stand on end. We won the race that year at just a hair under 300 knots from brake release to destination.
Fred Moreno