Later on in 1947 ramjets were fitted on to the wing tips of a "Mustang" again in an attempt to improve performance, tests were undertaken from 1947 to1948 with the same result as the Soviet work on aircraft with added ramjets, an increase in top speed but with the penalty of high fuel consumption and excessive drag when the ramjets were not in use..
In the late 1960's The Cavalier company (who now owned the rights to the "Mustang" design) began to explore the possibility of fitting a turboprop engine to there "Mustang II" a remanufactured P-51D with a new tail, engine, wing and second seat for an observer (which was bought by the USAF).
In 1968 the company fitted a Roll-Royce "Dart"510 turboprop engine to a F-51D (N6167U), this was so successful that it was planed to fit the more powerful Lycomming T55-L-9 turboprop engine. However by this time Cavalier sole the rights to the "Mustang" to the Piper company.
On the 4/11/1970 the prototype "Turbo-Mustang III" was delivered to the Piper factory, were Piper undertook the redesign of the "Turbo Mustang" started by Cavalier, with the fitting of the Lycomming engine and other modifications (resulting in only 10% of parts in common with the F 51D) to produced a new aircraft the PA 48 "Enforcer".
The USAF tested the PA 48 "Enforcer" in 1971 with aircraft fulfilling all expectations, but the USAF showed no interest in placing a production contract. In September 1981 the Air Force, under pressure from the US Congress ordered two new aircraft (which took to the air on 9/4/83 and 8/7/83) for a new test program which took place in 1983/84, again no ordered were forthcoming and the aircraft were placed in storage in late 1986.