Nakajima Ki-84 advanced versions and derivatives

[Airborne2001]

I understand but, focusing on the aircraft and ignoring the biological squidgy bit, leaves the aircraft on the tarmac. Not a lot to compare between two parked up airframes.

No I definitely agree with that. I think they just were more specifically referring to just the stats of the aircraft, and not the full schematics of an air battle. You are right though.

 

[Foo Fighter]

I agree that there are folk who misunderstand logic.

 

[dan_inbox]

I think they just were more specifically referring to just the stats of the aircraft, and not the full schematics of an air battle.

And experts should waste their time and competence to answer such a pointless question because ...?

 

[Deleted member 2278]

I wouldn't have ordinarily commented but the disparity in training by the point a Ki-84 and Spit XIV are likely to encounter one another in combat (summer of '45) was so large, it bore pointing out. There is also a wider strategic disparity to consider. In a contrived scenario where both combatants are same height, on reciprocal courses and sight each other simultaneously, the notional Spitfire pilot might legitimately question why the rest of the squadron got the day off.

 

[Airborne2001]

I think they just were more specifically referring to just the stats of the aircraft, and not the full schematics of an air battle.

And experts should waste their time and competence to answer such a pointless question because ...?

Can you please be a bit more polite with your responses? I was just talking about specs.

 

[GTX]

Cool it guys.

 

[spicmart]

How do you think would a Ki-84 powered with Allied high-quality-fuel have fared against a late-war Spitfire XIV?

Cheers,
Spitfire XIV still wins, it's engine power above 20000 ft was about 20% greater. Hi-oct fuel is good, having a better supercharger is better (on the roughly comparable engines), and that was what Giffon 65 had over the Homare.

And if the Homare had had a supercharger (on par with the XIV's) together with high-octane fuel and good reliability, what then?

 

[tomo pauk]

And if the Homare had had a supercharger (on par with the XIV's) together with high-octane fuel and good reliability, what then?

Then the performance of the Ki-84 goes up in all altitudes, and it is evenly matched with Spitfire XIV.

 

[Foo Fighter]

....and if this were in the speculation etc division it might even be relevant.

 

[spicmart]

Is it true that the Ki-84 only had a load limit of 5 to 6 Gs? That's less than most other fighters late in the war.

 

[HoHun]

Hi Spicmart,

Is it true that the Ki-84 only had a load limit of 5 to 6 Gs? That's less than most other fighters late in the war.

What's your source?

Regards,

Henning

 

[spicmart]

Hi Spicmart,

Is it true that the Ki-84 only had a load limit of 5 to 6 Gs? That's less than most other fighters late in the war.

What's your source?

Regards,

Henning

I read it to be in one of Greg's videos.

 

[HoHun]

Hi Spicmart,

I read it to be in one of Greg's videos.

Hm, he has 4 longish videos on the Ki-84 ... do you happen to have a more accurate attribution?

Generally, I like the research part of Greg's videos, but he sometimes goes off the rails in the conclusion parts. Usually, that's only evident if you happen to know more about the topic than Greg does.

In the case of the Ki-84, he has made up his mind that the famous 427 mph figure is from actual US test flights, for which as far as I remember his videos he has presented no evidence at all.

Check out this video ... not that I agree with everything the guy says, but I agree with thr basic premise that 427 mph figure is based on a US calculation done before they even had a clear idea of what the Ki-84 looked like:

View: https://m.youtube.com/watch?v=L12PNyii7hw


Regards,

Henning (HoHun)

 

[HoHun]

Hi Spicmart,

I read it to be in one of Greg's videos.

He mentions a G limit here (at 559 seconds in, in case the forum doesn't render the link second-accurate):

View: https://youtu.be/PUwcAbuvLOU?t=559


He doesn't mention what the source of the datasheet he shows on screen is, but I suspect it's an American document prepared for pilots flying the captured Ki-84. In that case, it seems perfectly reasonable to me that the Americans came up with a reasonably low G limit to be on the safe side with a captured aircraft type about which they had limited knowledge of less-than-perfect accuracy, and with a specific airframe of which previous service life they probably knew next to nothing.

In other words, from all I can tell, that limit could have been pulled out of thin air by the US.

(I believe Greg's method of using stall speed figures to calculate turn rates is flawed, by the way. The instrument error at stall speed was big enough that most manuals don't even try to quantify it, and as Gs available increase to the square of airspeed, going from a 1 G stall to a high-speed stall also squares the size of the error.)

Regards,

Henning (HoHun)

 

[spicmart]

Hi Spicmart,

I read it to be in one of Greg's videos.

Hm, he has 4 longish videos on the Ki-84 ... do you happen to have a more accurate attribution?

Generally, I like the research part of Greg's videos, but he sometimes goes off the rails in the conclusion parts. Usually, that's only evident if you happen to know more about the topic than Greg does.

In the case of the Ki-84, he has made up his mind that the famous 427 mph figure is from actual US test flights, for which as far as I remember his videos he has presented no evidence at all.

Check out this video ... not that I agree with everything the guy says, but I agree with thr basic premise that 427 mph figure is based on a US calculation done before they even had a clear idea of what the Ki-84 looked like:

View: https://m.youtube.com/watch?v=L12PNyii7hw


Regards,

Henning (HoHun)

I see you found it already. A guy in another group mentioned the G load and somehow Greg's video.

Thanks a lot for the video about the performance calculations! Just finished it. Very enlightening although I sure didn't remember or understand everything I got an overview about the matter now.

The propeller issue was startling that they would have to equip such a high performance aircraft with a totally inadequate product.
Can you make a calculated guess how much performance has suffered because of this?

 

[HoHun]

Hi Spicmart,

The propeller issue was startling that they would have to equip such a high performance aircraft with a totally inadequate product.
Can you make a calculated guess how much performance has suffered because of this?

I didn't follow that discussions, but I think the points were too small a propeller with too small a pitch range.

I don't think that would have much impact, as the Me 109 and the Fw 190 have small propellers as well and manage to transfer 2000 HP just fine. You would see overspeeding (or reduced boost to prevent that) in the climb table otherwise, and it shows full boost. The pitch range is not all that decisive either, you might sacrifice a bit of take-off performance to mount the blades at the pitch angle that gives full top speed. Generally, it's climb where propeller size matters, not so much top speed. Aerodynamic forces increase to the square of (local) velocity, so when you're going slow, blade area is more important than when going fast.

However, there might be points either in the video or in the discussion that I missed, so that's just some off-the-top-of-my-head remark that might be invalidated by additional complications I am not aware of

Regards,

Henning (HoHun)

 

[spicmart]

Thanks for the details!

HoHun,

In the Ki-84 calc video it is mentioned that the Homare and the Griffon which was powering the Sptifire XIV were in about the same ballpark. I was wondering if an "optimized" Hayate could have been on par with the latter as weight, dimensions, armament are similar, not sure about drag of the airframes?

 

[HoHun]

Hi Spicmart,

I was wondering if an "optimized" Hayate could have been on par with the latter as weight, dimensions, armament are similar, not sure about drag of the airframes?

Adding more variables to an already confusing data set tends to make things even more confusing

I just found this graph on my hard disk, which I prepared for some reason I don't remember, back in 2016 ... Spitfire LF.VIII tropicalized as tested by the RAAF (and extrapolated to the F. by me) vs. Ki-84 performance based on data from the tests of prototype #4 (not in fully operational configuration), with the parameters adjusted to reflect a production type with +500 mm boost @ 3000 rpm and individual ejector exhausts for additional thrust:



The Ki-84 compares favourably, but mainly due to its more powerful engine, and of course the Spitfire XIV has a Griffon where the Spitfire VIII "makes do" with the less powerful Merlin. On the other hand, the Spitfire XIV might be a bit heavier than the VIII, though I haven't checked the numbers, but then again, the weight of the Ki-84 would go up if you added the guns the prototype #4 was lacking.

I suppose the "optimized Hayate" you are suggesting might indeed have less drag. While the type certainly looks very clean, it's really difficult to judge that by simply eyeballing the shape of an aircraft, and I believe the Japanese had very little in the way of wind tunnels to actually verify their design decisions. Quite possible that with a bit of wind tunnel testing, it could have gained a worthwhile amount of extra speed. I'd probably look at other radial-engined aircraft for comparison though, as that's more apples-to-apples than comparing it to inline-engined aircraft.

For perspective, Focke-Wulf stated (in the "Widerstandsdaten für Flugzeuge" comparison sheet) that the drag of the Fw 190D-9 was down by about 10% compared to that of the Fw 190A-8/A-9.

This was broken down into:

- 0.6 % drag increase for the tail,
- 3.4% reduction for fuselage and cabin
- 2% drag increase for the supercharger air intake
- 7% reduction for cooling drag
- 2% reduction for the armament installation.

Considering that leaving out the two outer wing cannon was possible for the A-8/A-9 as well, it would probably be fair to consider an 8% difference in drag between an inline-engined and a radial-engined aircraft realistic, all else being equal.

Regards,

Henning (HoHun)

 

[spicmart]

The Ki-84 compares favourably, but mainly due to its more powerful engine, and of course the Spitfire XIV has a Griffon where the Spitfire VIII "makes do" with the less powerful Merlin. On the other hand, the Spitfire XIV might be a bit heavier than the VIII, though I haven't checked the numbers, but then again, the weight of the Ki-84 would go up if you added the guns the prototype #4 was lacking.

The Spitfire XIV has a normal gross weight of between 8400 lbs/3810 kg, fitted with fuel, guns and ammunition according to spitfireperformance.com. A Frank with armament installed but without ammo would be heavier (3914 kg). Its guns weigh 120 kg.

Max. true air speed in FS supercharger gear

446 mph at 25,900 ft.

Max. rate of climb in MS supercharger gear

5,040 ft/min. at 2,100 ft.

But then again the XIV had a dual-stage supercharger. It would have needed such for the Ki-84 to feature for an apt comparison.

I suppose the "optimized Hayate" you are suggesting might indeed have less drag. While the type certainly looks very clean, it's really difficult to judge that by simply eyeballing the shape of an aircraft, and I believe the Japanese had very little in the way of wind tunnels to actually verify their design decisions. Quite possible that with a bit of wind tunnel testing, it could have gained a worthwhile amount of extra speed. I'd probably look at other radial-engined aircraft for comparison though, as that's more apples-to-apples than comparing it to inline-engined aircraft.

The Homare 45-21 (1180 mm/810 kg) had a smaller diameter than the BMW 801 (1290 mm/1012 kg) and was astoundingly light. The designers sure had to accept some drawbacks with this.
Yet the diameter of the fuselage cross section seems larger with the Japanese fighter, at least by guessing when taking a glance at profiles. Would have to print frontal view drawings and overlap them or use some app to calculate the max cross section area.

For perspective, Focke-Wulf stated (in the "Widerstandsdaten für Flugzeuge" comparison sheet) that the drag of the Fw 190D-9 was down by about 10% compared to that of the Fw 190A-8/A-9.

This was broken down into:

- 0.6 % drag increase for the tail,
- 3.4% reduction for fuselage and cabin
- 2% drag increase for the supercharger air intake
- 7% reduction for cooling drag
- 2% reduction for the armament installation.

Considering that leaving out the two outer wing cannon was possible for the A-8/A-9 as well, it would probably be fair to consider an 8% difference in drag between an inline-engined and a radial-engined aircraft realistic, all else being equal.

Interesting that the tail (fuselage drum and enlargement of the fin) which is mostly wetted area, I guess, adds such drag. And a blown hood seems to have a positive effect, too.

I know from a Lednicer article, which you surely know of, that the Spitfire had about the worst radiator arrangement dragwise.
For the D-9 the equivalent flat plate area is 8.66 % less than that of the A-8's so it's roughly in the middle.

 

[HoHun]

Hi Spicmart,

But then again the XIV had a dual-stage supercharger. It would have needed such for the Ki-84 to feature for an apt comparison.

Thanks for providing the comparative weight data! With regard to the dual-stage supercharger, the Ki-84 might actually be competitive without one as it relied on water-methanol injection. Much as a second supercharger stage (normally, plus inter-/aftercooler), water-methanol injection was a method to improve supercharger efficiency.

Here's a comparison of two data sets for the engines (big can of worms, as there is a lot of not always conflicting data around, but these are two sets that I consider reasonbly realistic):

Engine - Griffon 65 - Ha-45 Model 21
Boost - +21 lbs/sqin - +500 mm Hg
Low Gear Full Throttle Height - 1680 m - 800 m
Low Gear Full Throttle Height Power - 2230 HP - 2040 HP
High Gear Full Throttle Height - 5700 m - 5200 m
Low Gear Full Throttle Height Power - 1860 HP - 1850 HP

(All HP values are metric to go with the metric altitudes.)

So, the Griffon 65 is better at low altitudes, and it is also better at high altitudes, but the advantage is about 10 % power down low, and +500 m altitude up high. The price the Spitfire pays for this is greater drag from the big intercoolers, but the price the Ki-84 pays for the elimination of intercooler drag is a weight increase because it has to carry a lot of methanol-water to achieve the power it has.

The Homare 45-21 (1180 mm/810 kg) had a smaller diameter than the BMW 801 (1290 mm/1012 kg) and was astoundingly light. The designers sure had to accept some drawbacks with this.
Yet the diameter of the fuselage cross section seems larger with the Japanese fighter, at least by guessing when taking a glance at profiles.

It's worth noting that the drag of streamlined bodies of a certain volume is not dominated by frontal area, and a fairly low aspect ratio actually gives the best results. This is pointed out for example by Raymer's "Simplified Aircraft Design for Homebuilders", but I believe this is also mentioned by David Lednicer in the article in which he discusses the F8F.

I suppose with radial engines, frontal area actually was of some concern for the internal aerodynamics, as even with the baffling, the internal shapes weren't streamlined. However, a well-streamlined fuselage minimizing overall drag for the aircraft might well exceed the radials frontal area and still be better than a slimmer fuselage. In fact, the Fw 190 rear fuselage had a simplified shape for ease of manufacturing ... there's an anecdote, I believe in Alfred Price' book on the Fw 190, that the original concept design of the Fw 190 had a nicely streamlined rear fuselage with a lot of compound curves, and a senior designer just took a ruler and drew straight lines on the threeview.

And a blown hood seems to have a positive effect, too.

Hm, do you mean "positive" as in "increasing drag"?

Regards,

Henning (HoHun)

 

[spicmart]

Thanks for providing the comparative weight data!

Actually I've seen various gross weight figures for the Ki-84 ranging from 3600 to almost 3900 kg but never exceeding last figure.
I took the weight of the Ki-84 from your graph which is 3794 kg and added 120 kg gun weight to it. With ammunition it approaches 4 tons. It seems too heavy.
In post #15 of this thread in the "imperfection table" 3770 kg of gross weight is given.

Engine - Griffon 65 - Ha-45 Model 21
Boost - +21 lbs/sqin - +500 mm Hg
Low Gear Full Throttle Height - 1680 m - 800 m
Low Gear Full Throttle Height Power - 2230 HP - 2040 HP
High Gear Full Throttle Height - 5700 m - 5200 m
Low Gear Full Throttle Height Power - 1860 HP - 1850 HP

(All HP values are metric to go with the metric altitudes.)

So, the Griffon 65 is better at low altitudes, and it is also better at high altitudes, but the advantage is about 10 % power down low, and +500 m altitude up high. The price the Spitfire pays for this is greater drag from the big intercoolers, but the price the Ki-84 pays for the elimination of intercooler drag is a weight increase because it has to carry a lot of methanol-water to achieve the power it has.

The 2230 hp for the Griffon 65 are with 150 octane fuel, I guess. Only 2030 hp with "normal" fuel (100 octane?). So the Ha-45-21 compares favorably, generating 2040 hp with 92 octane.
Are there numbers for the amounts MW carried?

However, a well-streamlined fuselage minimizing overall drag for the aircraft might well exceed the radials frontal area and still be better than a slimmer fuselage.

I seem to have read that the drag of the J2M Raiden's fuselage was lower than the A6M's although the former's diameter was about 20 cm wider. They achieved it with a carefully shaped cowling. Not sure how valid this is.

In fact, the Fw 190 rear fuselage had a simplified shape for ease of manufacturing.

You mean its rear fuselage would have been more aerodynamic when more bulbous and curvy?

For perspective, Focke-Wulf stated (in the "Widerstandsdaten für Flugzeuge" comparison sheet) that the drag of the Fw 190D-9 was down by about 10% compared to that of the Fw 190A-8/A-9.

This was broken down into:

- 0.6 % drag increase for the tail,
- 3.4% reduction for fuselage and cabin
- 2% drag increase for the supercharger air intake
- 7% reduction for cooling drag
- 2% reduction for the armament installation.

Considering that leaving out the two outer wing cannon was possible for the A-8/A-9 as well, it would probably be fair to consider an 8% difference in drag between an inline-engined and a radial-engined aircraft realistic, all else being equal.

Thanks for the breakdown. Very interesting.

And a blown hood seems to have a positive effect, too.

Hm, do you mean "positive" as in "increasing drag"?

I mean it is less draggy. Something they found out with the Malcom hoods of Spitfires and Mustangs, too.

 

[spicmart]

The Nakajima fighters Ki-43/44/84 seem to have a very slight, hardly noticeable.negative sweep at the forward leading edge of the wing. Anyone know why that might be other than perhaps for CoG reasons?

 

[sienar]

The Nakajima fighters Ki-43/44/84 seem to have a very slight, hardly noticeable.negative sweep at the forward leading edge of the wing. Anyone know why that might be other than perhaps for CoG reasons?

Nakajima started doing this with the Ki-27 and it became a common feature on their aircraft. It was to reduce tip stall.

 

[spicmart]

The Nakajima fighters Ki-43/44/84 seem to have a very slight, hardly noticeable.negative sweep at the forward leading edge of the wing. Anyone know why that might be other than perhaps for CoG reasons?

Nakajima started doing this with the Ki-27 and it became a common feature on their aircraft. It was to reduce tip stall.

Yet hardly any other aicraft company chose this solution. The other major case that comes to my mind is the P-36/40.

 

[spicmart]

The Nakajima fighters Ki-43/44/84 seem to have a very slight, hardly noticeable.negative sweep at the forward leading edge of the wing. Anyone know why that might be other than perhaps for CoG reasons?

Nakajima started doing this with the Ki-27 and it became a common feature on their aircraft. It was to reduce tip stall.

Is the effectiveness of such a wing design known?

 

[tomo pauk]

Thanks for providing the comparative weight data! With regard to the dual-stage supercharger, the Ki-84 might actually be competitive without one as it relied on water-methanol injection. Much as a second supercharger stage (normally, plus inter-/aftercooler), water-methanol injection was a method to improve supercharger efficiency.

People were installing 2-stage superchargers for good reasons on engines - these were pumping more air on the engines than what was capable with 1-stage S/cs. Water-alcohol injection was not cutting it at high altitudes if the S/C was running out of steam.
We can compare the Jumo 213A with water-alcohol injection with 213E without that, or R-2800-34 vs. 32, or DB 605AM vs. 605L - the 2-stage version is always the better engine above ~7 km. Of course, the 2-stage engines can also employ the water-alcohol injection for even greater power advantage at high altitudes.

 

[HoHun]

Hi Tomo,

People were installing 2-stage superchargers for good reasons on engines - these were pumping more air on the engines than what was capable with 1-stage S/cs. Water-alcohol injection was not cutting it at high altitudes if the S/C was running out of steam.

Well, unless you'd like to debate the numbers for the Griffon 65 and the Ha-45 Model 21 I quoted above, the Griffon 65 does have an altitude advantage of merely 500 m in full throttle height for the same brake horsepower at that height.

However, the background of this surprisingly small difference might warrant a closer look: The Griffon 65 operates at 2.46 hPa boost, the Ha-45 at just 1.68 hPa. As ambient pressure at 5500 m (about mid-way between both engines' static full throttle height) is 0.505 hPa, that means the Griffon's supercharger needs to achieve a total pressure ratio of 4.9, while the Ha-45 needs one of just 3.3 at the same altitude.

The Griffon is a 36.7 L engine running at 2750 rpm with a compression ratio of 6:1.

The Ha-45 Model 21 is a 35.8 L engine running at 3000 rpm with a compression ratio of 8:1.

The approximate post-compression charge pressure for the Griffon would be 14.8 hPa, that for the Ha-45 would be 13.4 hPa if no heating or cooling of the charge were involved, which is an unrealistic assumption, so don't take these numbers too serious as they are only meant to illustrate that the Ha-45 gets basically similar results from its single-stage supercharger as the Griffon from its two-stage one.

There certainly were good reasons to install a two-stage supercharger on the Griffon, but the choice to equip the Ha-45 with a single-stage supercharger and water-methanol injection seems to be based on valid reasons, too. Of course, in practice, the Ha-45 might have been a bit too ambitious to really live up to the expectations, but I'm not sure it couldn't have been a fine engine if the Japanese industry and its supply chain hadn't been subjected to relentless bombing attacks.

Regards,

Henning (HoHun)

 

[Nicknick]

Hi Spicmart,

But then again the XIV had a dual-stage supercharger. It would have needed such for the Ki-84 to feature for an apt comparison.

Thanks for providing the comparative weight data! With regard to the dual-stage supercharger, the Ki-84 might actually be competitive without one as it relied on water-methanol injection. Much as a second supercharger stage (normally, plus inter-/aftercooler), water-methanol injection was a method to improve supercharger efficiency.

Here's a comparison of two data sets for the engines (big can of worms, as there is a lot of not always conflicting data around, but these are two sets that I consider reasonbly realistic):

Engine - Griffon 65 - Ha-45 Model 21
Boost - +21 lbs/sqin - +500 mm Hg
Low Gear Full Throttle Height - 1680 m - 800 m
Low Gear Full Throttle Height Power - 2230 HP - 2040 HP
High Gear Full Throttle Height - 5700 m - 5200 m
Low Gear Full Throttle Height Power - 1860 HP - 1850 HP

(All HP values are metric to go with the metric altitudes.)

So, the Griffon 65 is better at low altitudes, and it is also better at high altitudes, but the advantage is about 10 % power down low, and +500 m altitude up high. The price the Spitfire pays for this is greater drag from the big intercoolers, but the price the Ki-84 pays for the elimination of intercooler drag is a weight increase because it has to carry a lot of methanol-water to achieve the power it has.

The Homare 45-21 (1180 mm/810 kg) had a smaller diameter than the BMW 801 (1290 mm/1012 kg) and was astoundingly light. The designers sure had to accept some drawbacks with this.
Yet the diameter of the fuselage cross section seems larger with the Japanese fighter, at least by guessing when taking a glance at profiles.

It's worth noting that the drag of streamlined bodies of a certain volume is not dominated by frontal area, and a fairly low aspect ratio actually gives the best results. This is pointed out for example by Raymer's "Simplified Aircraft Design for Homebuilders", but I believe this is also mentioned by David Lednicer in the article in which he discusses the F8F.

I suppose with radial engines, frontal area actually was of some concern for the internal aerodynamics, as even with the baffling, the internal shapes weren't streamlined. However, a well-streamlined fuselage minimizing overall drag for the aircraft might well exceed the radials frontal area and still be better than a slimmer fuselage. In fact, the Fw 190 rear fuselage had a simplified shape for ease of manufacturing ... there's an anecdote, I believe in Alfred Price' book on the Fw 190, that the original concept design of the Fw 190 had a nicely streamlined rear fuselage with a lot of compound curves, and a senior designer just took a ruler and drew straight lines on the threeview.

And a blown hood seems to have a positive effect, too.

Hm, do you mean "positive" as in "increasing drag"?

Regards,

Henning (HoHun)

Here we have a nice table for the parasitic drag for different engine configurations with different frontal areas. Note, that this is for engine nacelles and not for single engine fighter. The influence by single engine fighters will be lower than for nacelles, as the minimum cross section if often not defined by engine alone. I think I’ve seen something similar in Harry Ricardos “fast running combustion engine” or somewhere else, I forgot about it…

MORE POWER from LESS ENGINE (How to develop 5000 hp from 3000 lb with 7 sq ft frontal area) on JSTOR

E. S. HALL, MORE POWER from LESS ENGINE (How to develop 5000 hp from 3000 lb with 7 sq ft frontal area), SAE Transactions, Vol. 35 (1940), pp. 504-519

www.jstor.org

 

[HoHun]

Hi Nicknick,

Here we have a nice table for the parasitic drag for different engine configurations with different frontal areas.

Hm, this seems to be behind a paywall, I'm afraid.

Regards,

Henning (HoHun)

 

[Nicknick]

the graph on the first page is for free, at least in my browser

 

[HoHun]

Hi Nicknick,

the graph on the first page is for free, at least in my browser

Ah, thanks - if that's the one you meant, it gets displayed for me too!

Regards,

Henning (HoHun)

 

[tomo pauk]

Well, unless you'd like to debate the numbers for the Griffon 65 and the Ha-45 Model 21 I quoted above, the Griffon 65 does have an altitude advantage of merely 500 m in full throttle height for the same brake horsepower at that height.

At 21000 ft, Griffon 65 was supposed to make 1780 HP per this data sheet. (Wilkinson gives a bit more for the 60 series Griffons, ~1800 at 21000 ft). In high gear and on 150 grade fuel, it was supposed to do a bit less than 2100 HP (per this chart; note that it is with 400 mph 'worth' of ram)
TAIC data gives 1675 HP at 19680 ft for the Homare 21 on 3000 rpm and 43.9 in Hg. On 49.6 in Hg, it was good for 1830 HP.
1675 HP at 19680 ft works to about 1600 HP at 21000 ft? To be on the safe side, I reckon that Griffon 65 was making about 10% greater power above 21000 ft, and in similar fashion at low level. At medium altitudes, and without 150 grade fuel for the G65, the fully-rated Homare was better.

However, the background of this surprisingly small difference might warrant a closer look: The Griffon 65 operates at 2.46 hPa boost, the Ha-45 at just 1.68 hPa. As ambient pressure at 5500 m (about mid-way between both engines' static full throttle height) is 0.505 hPa, that means the Griffon's supercharger needs to achieve a total pressure ratio of 4.9, while the Ha-45 needs one of just 3.3 at the same altitude.

The Griffon is a 36.7 L engine running at 2750 rpm with a compression ratio of 6:1.

The Ha-45 Model 21 is a 35.8 L engine running at 3000 rpm with a compression ratio of 8:1.

Griffon 65/s S/C was fit for the purpose of pushing as much of the mass of the air in the cylinders as possible at high altitudes, as one can expect when dealing with 2-stage superchargers.
Note that Americans measured the CR of the Homare 21 at 7.17:1 per this.

The approximate post-compression charge pressure for the Griffon would be 14.8 hPa, that for the Ha-45 would be 13.4 hPa if no heating or cooling of the charge were involved, which is an unrealistic assumption, so don't take these numbers too serious as they are only meant to illustrate that the Ha-45 gets basically similar results from its single-stage supercharger as the Griffon from its two-stage one.

Abilities of a supercharger are just one of factors of the altitude performance - engine that runs high revs, or is a big engine (or a combination) with a good 1-stage S/C can give a good run for it's money to a 2-stage supercharged, smaller engine (or the one that does not rev as high). Eg. DB-605AS or D vs. Merlin 60 series, or Jumo 213A vs. low-level Merlin 60 series.

Granted, the best deal is to have a big engine, with as good S/C as possible, and to make it doing good/great RPM.

There certainly were good reasons to install a two-stage supercharger on the Griffon, but the choice to equip the Ha-45 with a single-stage supercharger and water-methanol injection seems to be based on valid reasons, too. Of course, in practice, the Ha-45 might have been a bit too ambitious to really live up to the expectations, but I'm not sure it couldn't have been a fine engine if the Japanese industry and its supply chain hadn't been subjected to relentless bombing attacks.

Nakajima (and the Japanese engine companies in general) were not that into 2-stage supercharging as it was RR or P&W - so they were probably were making the bet based on what they know? Butt indeed, by the time Homare was in full production, the Japanese logistic and support was crumbling, with nickel supply especially starting to bite the engine companies by late 1944.

As for the Homare - I'd slash the CR down, perhaps to 6.5:1, or even to 6:1 - sacrifice perhaps 50 HP high up, but have an engine that will run well even on 87 oct fuel.

 

[HoHun]

Hi Tomo,

In high gear and on 150 grade fuel, it was supposed to do a bit less than 2100 HP (per this chart; note that it is with 400 mph 'worth' of ram)

That's in fact the chart that my Griffon numbers are based on - I calculated the static altitudes equivalent to those achieved with the ram pressure increase as stated on the chart. I didn't make any corrections for the temperature rise though, so without the temperature increase due to ram effect, the Griffon should yield slightly more power than I quoted, but the exact difference is difficult to establish once an intercooler with unknown parameters enters the equation.

TAIC data gives 1675 HP at 19680 ft for the Homare 21 on 3000 rpm and 43.9 in Hg. On 49.6 in Hg, it was good for 1830 HP.

I hadn't seen this one ... 49.6 in Hg are +500 mm Hg, as used in my comparison. I credited the Ha-45 with 1850 HP (metric) @ 5200 m, the new datasheet you quote states 1855 HP (metric) @ 4900 m ... according to this, the altitude advantage of the Griffon 65 would be 800 m rather than 500 m.

To be on the safe side, I reckon that Griffon 65 was making about 10% greater power above 21000 ft

As power was dropping off on both engines approximately linearly, the power advantage of the Griffon would increase with altitude, so using a fixed percentage might not give an accurate impression.

Note that Americans measured the CR of the Homare 21 at 7.17:1 per this..

Quite interesting ... do you have more pages from that that perhaps would allow the document to be dated? Because in F-IM-1119C-ND ("FRANK-1 Interim Report No. 3") from November 1946, WEP data still is 1875 HP (metric) @ 5450 m.

Of course, these 1900 ft difference between the two documents are not trivial as far as the comparison to the Griffon is concerned.

Nakajima (and the Japanese engine companies in general) were not that into 2-stage supercharging as it was RR or P&W - so they were probably were making the bet based on what they know?

Or on what they could produce, I guess. I suspect supercharger impellers are expensive precision parts, and intercoolers are producing both external and internal drag, so a water-methanol injection system has several advantages to make it attractive, especially if you're having to use low-octane fuel anyway.

Regards,

Henning (HoHun)

 

[tomo pauk]

Quite interesting ... do you have more pages from that that perhaps would allow the document to be dated? Because in F-IM-1119C-ND ("FRANK-1 Interim Report No. 3") from November 1946, WEP data still is 1875 HP (metric) @ 5450 m.

The doc can be downloaded from here.

 

[HoHun]

Hi Tomo,

The doc can be downloaded from here.

Thanks a lot! Having had a first closer look at it: Assuming a constant pressure ratio, the full throttle height at WEP should actually be 600 ft higher than indicated by the estimate in that document. As twice the amount of water-methanol mixture is injected at WEP than at MIL, that's probably a conservative assumption, and the extra charge cooling probably would result in an even higher WEP full throttle height estimate.

So, I'm not sure what the reason might have been for the intelligence guys to come up with that 16000 ft estimate ...

Regards,

Henning (HoHun)

 

[tomo pauk]

As twice the amount of water-methanol mixture is injected at WEP than at MIL, that's probably a conservative assumption, and the extra charge cooling probably would result in an even higher WEP full throttle height estimate.

Why do you think that it would?

 

[HoHun]

Hi Tomo,

As twice the amount of water-methanol mixture is injected at WEP than at MIL, that's probably a conservative assumption, and the extra charge cooling probably would result in an even higher WEP full throttle height estimate.

Why do you think that it would?

Cooling the charge increases its density. A cooler, denser charge should tend to be more efficiently compressed, preserving or even increasing the original pressure ratio, and thus achieving a higher full-throttle height than without injection.

If you see the full throttle height dropping below what you'd expect from a constant pressure ratio, that's usually because of the charge heating that's the inevitable byproduct of the compression.

Hooker pointed out the benefits of having the carburettor before the supercharger, which improved compression efficiency ... injecting anti-detonant before the supercharger has pretty much the same effect.

(Of course, the full throttle height at WEP still is lower than that at MIL, but I think the estimate in the document you provided is pessimistic. It's worth noting that the US post-war documents on the Ha-45 don't reproduce this low number.)

Regards,

Henning (HoHun)

 

[tomo pauk]

Cooling the charge increases its density. A cooler, denser charge should tend to be more efficiently compressed, preserving or even increasing the original pressure ratio, and thus achieving a higher full-throttle height than without injection.

Please note that Homare was using the water-alcohol injection also for military power. A S/C will provide less of the compressed air at 20000 ft than it will be the case at 16000 ft, thus the required ADI flow will be lower.

 

[HoHun]

Hi Tomo,

Please note that Homare was using the water-alcohol injection also for military power. A S/C will provide less of the compressed air at 20000 ft than it will be the case at 16000 ft, thus the required ADI flow will be lower.

I'm not quite sure what the last "lower" in your sentence refers to, but WEP at low supercharger gear full throttle height uses about 15% more pressure, yet provides 100% more anti-detonant.

No anti-detonant flow rate is given for WEP at high supercharger gear full throttle height, but as the flow rate at MIL is the same under all stated conditions, I'd assume it probably was similar for WEP.

Regards,

Henning (HoHun)

 

[tomo pauk]

I'm not quite sure what the last "lower" in your sentence refers to, but WEP at low supercharger gear full throttle height uses about 15% more pressure, yet provides 100% more anti-detonant.

'Lower' means 'lower flow rate' there, for how much is needed at 20000 ft vs. how much is needed at 16000 ft, for max boost at 3000 rpm.
WEP does not provide anti-detonant, it requires it.