High-end V-12 engines in Ki-84/Ki-61

[Howard Gibson]

The Ki-84 had little room for growth because its short legs forced a small prop, and I doubt a 5 bladed prop would have helped much in harnessing the power, as sweep efficiency is a big loss in smaller diameters.

That is the magic of narrow track gears: Even with short legs and deeply cranked wing dihedrals, the prop still clears, as it did in Spitfires. The criticism of narrow track landing gears never shows the slightest understanding of the massive benefits... In the 109 it allowed a forward gear position that gave full power for short field sideway take offs, straight into the wind, without 4 guys holding the tail down... The Spitfire could not do that but it should have. Yes the 109 had a lower tolerance for side winds than most (15 knots vs 25 knots), but it could go in any direction on a wide field...

Those short legs is why the true performance of the Ki-84, as measured by actual Japanese data, is around 395-410 mph top speed and not 427, while the climb rate was about like that of a 190A-8 at 1.58 ATA. Power to weight is irrelevant when your prop cannot deliver the power.

The Ki-84 was always flown with 92 octane fuel and automatic MW-50 when above 425 km/h, so it did not drastically underperform because of poor build quality, as is often claimed. It just could not get much better. And when ferried on delivery it had no MW-50, so it could only reach 425 km/h: This was then used by historians as an example that they were being badly built!

The Ki.84 had 2000hp, and the kind of performance that 2000hp allows. The prop was up to the job. If you switch to any V12 other than a Rolls Royce Griffon, you get less power than that. You can work to design an efficient radiator. You need to make the fuselage skinny like that of a Bf109, Spitfire, or P51.

The width of the landing gear does not help propeller clearance. The height does. I assume that the Spitfire five blade propellers were developed to work around short landing gear. What the Ki.84 needed was landing gear that did not break.

 

[wrathofatlantis]

The Ki.84 had 2000hp, and the kind of performance that 2000hp allows. The prop was up to the job. If you switch to any V12 other than a Rolls Royce Griffon, you get less power than that. You can work to design an efficient radiator. You need to make the fuselage skinny like that of a Bf109, Spitfire, or P51.

The width of the landing gear does not help propeller clearance. The height does. I assume that the Spitfire five blade propellers were developed to work around short landing gear. What the Ki.84 needed was landing gear that did not break.

Sure the landing gear of the Ki-84 was designed strong enough (though defects in manufacture caused them to often break), but so was the gear of the 190.

No the Ki-84 prop wasn't quite of a proper size, even with 4 blades, since the N1K1 had the same engine, only slightly weaker, 8 inches more prop diameter with the same 4 blade shapes (surely if it can use 8 inches extra, so can the more powerful Ki-84?), and yet the N1K1 climbed better despite being 630 pounds heavier in a combat-loaded condition. (!)

Again, estimated climb rates are not Japanese data: Original Japanese data shows a Ki-84b climb rate identical to a 190A-8 for a very late model Ki-84, which is to say very low performance for the power/weight ratio. The real top speed of only about 640 km/h (400 mph) is also abnormal for 1900 hp. Please don't tell me it could do 427 mph because the US re-built the whole engine after a strip-down and added high octane fuel... Even with that it did not do 427 mph (TAIC calculated figure), and the US flights were short and devoid of any measurements.

The N1K1 topped at 618 km/h with 630 more pounds and only 1800 hp, only 22 km/h slower than a best case Ki-84 at 640 km/h (more often quoted as 635, but I'll assume the drop tank shackles are off).

An additional indication that the prop was undersized was a serious Ki-84 tendency to over-rev in dives, so you had to be cautious down-throttling the Ki-84 in dives.

Your point about gear width being unrelated to prop clearance ignores the height effect of the dihedral. Most WWII fighters had significant dihedral, and this means a narrow track gets more prop clearance for a given gear length.

 

[Howard Gibson]

Your point about gear width being unrelated to prop clearance ignores the height effect of the dihedral. Most WWII fighters had significant dihedral, and this means a narrow track gets more prop clearance for a given gear length.

When you design an aircraft, you know the clearance required for your propeller blades, and you know the dihedral angle. You make the landing gear long enough.

 

[Howard Gibson]

Again, estimated climb rates are not Japanese data: Original Japanese data shows a Ki-84b climb rate identical to a 190A-8 for a very late model Ki-84, which is to say very low performance for the power/weight ratio. The real top speed of only about 640 km/h (400 mph) is also abnormal for 1900 hp. Please don't tell me it could do 427 mph because the US re-built the whole engine after a strip-down and added high octane fuel... Even with that it did not do 427 mph (TAIC calculated figure), and the US flights were ridiculously short and devoid of any measurements.

We had a discussion around here somewhere about Ki.84 performance, and author Rene J. Francillon Ph.D.

All of my books quote a Ki.84 top speed of 388mph. According to Profile_70, written by Francillon, early production Ki.84s with 1800hp Ha-45-11 engines did 388mph. We are interested in the methanol water injected engines that put out 1990hp. That ought to work out to around 400mph. A 35.8l engine putting out 1990hp at 2900rpm works out to a BMEP of 249psi. A Merlin 61 with 27l at 3000rpm puts out 1570hp, for a BMEP of 251psi. The Merlin is running on 130 octane gas, as opposed to the Homare's 92 octane. The M/W injection definitely is turned on.

The 427mph happened with American testing in 1944. The Americans wanted to know how fast the Ki.84 "Frank" was compared to American aircraft. If they did this in 1946, they would be comparing Japanese technology with American, and using the same fuel is a valid exercise. The curves on WWII Aircraft Performance show military power and war emergency power. I am not sure what other tweaks the Ki.84s had.

 

[wrathofatlantis]

We had a discussion around here somewhere about Ki.84 performance, and author Rene J. Francillon Ph.D.

All of my books quote a Ki.84 top speed of 388mph. According to Profile_70, written by Francillon, early production Ki.84s with 1800hp Ha-45-11 engines did 388mph. We are interested in the methanol water injected engines that put out 1990hp. That ought to work out to around 400mph. A 35.8l engine putting out 1990hp at 2900rpm works out to a BMEP of 249psi. A Merlin 61 with 27l at 3000rpm puts out 1570hp, for a BMEP of 251psi. The Merlin is running on 130 octane gas, as opposed to the Homare's 92 octane. The M/W injection definitely is turned on.

There is no working Ki-84 Ha-45 Homare without water-injection... Water-Injection is an integral part of Ha-45 operation in the Ki-84 (and probably on all of the Ha-45s, not sure).

The FW-190A-8 is 1200 pounds heavier at normal loaded weight, has similar power and has the exact same top speed and climb rate.

I would say the Ki-84 is mildly underperforming in speed, but much more noticeably so in climb rate... The climb rate figures I have are Japanese and are time to altitude, so pretty indicative. Matching the A-8 climb rate at 1.58 ATA had this Ki-84b at a very high speed of 410 mph, the highest reliable figure I have ever found on the Ki-84: A very late 4 X 20 mm cannon model... (The last 10% of them made, supposedly.)

In comparative dogfight tests the Ki-100 is described as vastly out-climbing the Ki-84. (Which is odd, but not described as abnormal in terms of what it could do.)

The 427mph happened with American testing in 1944. The Americans wanted to know how fast the Ki.84 "Frank" was compared to American aircraft. If they did this in 1946, they would be comparing Japanese technology with American, and using the same fuel is a valid exercise. The curves on WWII Aircraft Performance show military power and war emergency power. I am not sure what other tweaks the Ki.84s had.

It did not because the Americans flew it for less than an hour and took no measurements. 427 mph is TAIC calculated.

There is literally no flown hard data by the Allies for the Ki-84. 427 mph has been debated for decades now, and the conclusion is it is not real.

One of the errors the American made is they took the best-looking Ki-84 and ignored the beat-up ones. The beat-up ones were beat-up because they worked... They took home the hangar queen lemon, and the result is they got very little flying time out of it. They did get it to work right after rebuilding the engine, but I read the original flight test and it is explicitly stated that there was no time for hard performance data. I don't know if this is the right version, but the different versions are similar:

ohttps://www.wwiiaircraftperformance.org/japan/K-84_MATSC.pdf