Here's my take on what Busa's trying to say:
Ki84 redesigned a plan to install 2000H.P. engine in Ki44.
Ki84 weakened effectiveness of an elevator to the limit, and it was designed so that it might battle at high speed.
"Ki-84 was essentially a redesign of an original plan which the Ki-44 was to be fitted with a 2000hp engine. With consideration to combat at higher speeds, the Ki-84 elevator efficiency(=turning capability) was given less emphasis than before(as compared to aileron efficiency)."
A tail gear was not grounded under the influence of downwash of a flap by the prototype model at the time of landing. Of course, the flap, the horizontal stabilizer, and the elevator were improved. But, it did not solve.
It is because it was judged that it was not useful for Ki84 to solve it.
"The prototype had problems with the tail gear not extending properly during landings. The reason was traced back to the influence of the downwash casued by flaps in landing position. While subsequentially the design of the flaps, horizontal stabs and elevators were enhanced, the tail gear problem was never really solved. It is probably because there were other priorities and redesigning the whole rear-end to solve this problem was out of the question."
The pilot said that he could not raise a nose of an airplane even if it pulls a stick at a low speed.
In order to accelerate at combat speed, even if it pushed the stick, it was said that a nose of an airplane could be lowered only about 20 degrees. Clearly, the Controllability which is different in Ki43 or Ki44 was able to be given.
"The elevator efficiency (compared to other JP planes) was reduced to the point that a slight pull of the stick would not effect the pitch of the plane at low speeds. It is said that while the plane was accelerated to combat speed the pilot pushing slightly on the stick would achieve only 20 degrees lower pitch. Clearly, this kind of elevator response was very different from the Ki-43 or the Ki-44."
The test pilots of Nakajima and IJA thought that Ki84 should equip a control system with the elevator and rudder of high control force,And aileron of low control force. It was the controllability similar to the foreign airplane. (The result was the middle grade of foreign airplanes and IJA airplanes. )
"The test pilots of Nakajima Aviation and IJAAF retained opinions that the Ki-84 should emphasize in light controls with the ailerons, and heavy controls with the elevators, as per the contemporary example of the fighters of the other major combatants of the war. The result achieved was at a somewhat middle point between a typical Japanese fighter and "foreign" fighters."
Directional stability at the high speed was made so good that it is imbalanced at the time of turn, it raised shooting accuracy. (The vertical stabilizer with low height had a problem in Ki44. However, it is unknown whether there was any problem by Ki84.)
"Directional stability at high speeds were greatly improved, almost causing over correction, which greatly helped improving shooting accuracy and gaining gun solution. (There were problems reported with the Ki-44 due to its short vertical stabs, but there is no reference as to if such was also the case with the Ki-84)"
As compared with more heavily built with the same engine, the@small propeller has not offered the outstanding climbing performance.
"When compared with the more heavily built fighters using the same engine such as the C6N or the N1K2-J, the short propellers installed on the Ki-84 resulted in not much of an increase in climbing performance(despite the Ki-84 was more lighly built). "
IJA wanted to make the airplane similar to Fw190A5.
Though it was regrettable, since the thrust was insufficient, Fw190A8 was resembled.
But, the IJA was able to get what they wanted.
"In a summary, the IJAAF intended to make a plane resembling something like an AH Fw190A-5, but the insufficient thrust has caused it to be somewhat like the AH Fw190A-8. However, all in all, the IJAAF was generally pleased with what they've achieved."
Though regrettable, I cannot explain why Roll Rate of Ki84 is not excellent.
We think that Roll Rate of N1K2-J is not good, either.
"Unfortuately, I cannot explain why the Ki-84 modelled in AH2 does not have excellent roll rate. In our opinion, we also think the roll rate of the AH2 N1K2-J is too low."
