I was bored and sketched these together. They are all ideas I've discussed with people during race preparation for Reno this past year. Finally took some time to sketch them out. No real plans or movement on any of these ideas. They were just fun to draw and helped with waiting for next year.

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Twin Tandem
Unlimited racer concept

Derived from the DO-335 and the constraint of not using 1930-40s era engines. Since this would be designed with composites it would come in underweight and it would have less horsepower than any other racer in the field. These weight and power shortfalls would be made up by adding another engine and bolstering the support structure. Once you decide on using a twin you have more drag unless you look to the Macchi 72, DO-335, or Cessna Skymaster.

DO-335 seemed the cleanest in terms of aerodynamics. Started by sizing the powerplants with the Falconer V12 from the Thunder Mustangs as a placeholder. The goal would be to hit an empty weight of 4,501lb to hit the minimum weight limit with 1,200 horespower. This would give a power loading ratio of 0.25 and a wing loading of 35 lb/sqft.

Goal was an average race speed of 400 mph to match the Pond Racer. While racing, the back seat would hold tanks for spray bar/ADI fluids. Otherwise the plane could travel the airshow circuit painted up as a fake DO-335 and sell airplane rides.

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"TF's Revenge"
An unlimited racer concept.
A converted TF-51 with a turbo charged V12 (preferable an Aardema V12 with all the bugs worked out) with the turbos mounted where the front cockpit used to be. A new firewall gets put in front of the rear cockpit. Chin scoop feeds the turbos or gets replaced by an air intake similar to the BF-109/P-38. Extra space in the front cockpit goes toward spray bars/ADI.

Other wi$hli$t item$ are new prop design and an optimized wing/airfoil

The turbo charged TF concept would also apply to an airplane like the Yak-11, or any plane where their is a rear cockpit.

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If the unlimited/warbird class allowed turboprops (I know, I know BLASPHEMY BURN HIM AT THE STAKE!! yadda yadda yadda) the main contenders would be the assortment of modern "warbirds" (trainers and light attack aircraft like the T-6, Super Tucano, PC-21, etc)
The PC-21 has a max operating speed of 425 mph. While this would be enough to place in the silver, if the turbo props were to ever unseat the reciprocating engines significant modifications would need to be made.
Easy one is pull the back seat. Between the ejection seat, avionics, and controls would save a lot of weight. This makes room for a new turtle deck which would also help with profile drag. The empty rear cockpit could hold any expendable fluids.
As a trainer, the fuselage is very long and is probably overly stable for racing. This can be remedied by shortening the fuselage or reducing the tail area. The tail area requires less work. Dorsal and ventral fins removed. Add fillets at the wing roots and at the tail cone (like Race#5 "American Spirit") and improve the wing tips.
By reducing weight and improving the aerodynamics the PC-21's wing and power loading could be improved making it more competitive as a racer.
Additional work could be done to move the cockpit to the back while keeping the height down, remove the second ejection seat, unnecessary avionics, etc to improve the competitiveness. It'd be nice to convert it to a tail dragger and lose the weight of the nose wheel, but by then I think this would be a scratch built.