Greg's Airplanes: Nakajima Ki-84 Hayate "Frank" History

[qaz]

TAIC 687kph at 1850hp at 20000 feet doesnt seem anything out of ordinary to me.
F8F-1 does 698kph at 1850hp at 20000 feet.
Is Ki-84 really that much draggy than F8F-1?

Also this document is from Aug 1945 and states:

So they had a flyable ki84 before the end of the war and probably when the TAIC was done.
Clark Air Base was recaptured in January, TAIC dates as of March.

Hi,
I'm aware that TAIC had flying Ki-84s during the war. However, this calculation was done by March 1945 as written in the TAIC Manual. It's quite doubtful that any comprehensive max performance test was performed on a captured Ki-84 by this time, and the TAIC manual makes no note that it is derived from real data.

In terms of the drag, I can't really give an answer because I don't have this data. But as an aside, I would say that it is quite likely the F8F had superior propeller design than the Ki-84, which is often cited as a regret in Japanese books.

If you have speed and WEP power charts for the F8F, I'd like to see them if it's no problem.

Furthermore, the TAIC figures for the power @ alt of the Ha-45-21 seem to be based on early targets. The Army/Navy data often gives a 2nd speed full pressure mil-power alt of 20,000ft/6100m and mil power of 1620 metric HP (1600 HP) at this alt. Whether this power was reduced by a lower CR or not is hard to say.
WEP figures are rarely published in Japanese sources, but the WEP full pressure alt will be below 20k ft in that case.

In general I am averse to using a calculation, especially because we can't verify exactly what factors went into it, and it's far from the existing Japanese data at full rated power. I'm hoping there are other Japanese tests yet to be uncovered.

Thanks for the informed rely. I've read your analysis before and very much enjoyed your thorough discussion of the topic.

There are a few things that I'd point out:

So getting back to TAIC and Middleton (MAMA) testing standards, the information that we're missing is the Homare 21 bench testing under controlled conditions. Do you have access to these documents? There are two AFAIK, one relating to the controlled bench test and one relating to the aircraft evaluation. AFAIK, these are the documents that Greg is primarily referencing and they are absent from your website's analysis. They've been linked to multiple times on this forum.

I linked the reports used in the video a few posts up. The Middletown report doesn't have a cover page, so perhaps I cited it oddly, but I have been using it.
The Middletown report doesn't include any flight performance other than the basic handling characteristics and the brief mention of climb performance that I posted above.
The 1946 engine overhaul text is included in the report, however the power specs given are simply a copy of the TAIC figures from early 1945 or before:

The actual overhauled engine was bench tested just up to 2900 rpm +470mmHg, but there's no power results given.

There are a few points that need to be made:

First, WW2-era pitot tube speedometers aren't always accurate due to differences in wind, heat, humidity, altitude, airflow over the tube, and other variables that can't be controlled for. So TAIC is likely testing engines in isolation, outside of aircraft, in order to avoid two issues. The primary issue is standardization and the secondary issue may be cost. However, Greg doesn't speculate as to why TAIC's engineers used controlled external bench testing conditions to generate the synthetic speed estimates of the Frank, but it's cheaper and it's more standardizable to run the engine up on the ground and then synthetically generate its top speed.

In fact, TAIC basically states in the beginning of their intelligence reports that they do not calculate speed in the air and that all calculations, unless otherwise stated, are done synthetically. But that said, their report on the Homare 21 is actually one of the most thorough and comprehensive reports available on a WW2 engine, according to Greg.

TAIC had no choice but to make estimated calculations for the max performance of Japanese aircraft. It simply wasn't possible to actually test most newer types, and captured planes were often unflyable, or not outputting their intended performance, safe to do so, or so on. The Ki-84s captured in the Philippines were likely subjected to restricted operating conditions even if they were in good conditions.
An actual flight speed test will always be more accurate than a calculation, but it wasn't possible or practical.
In the speed video IIRC he states that he thinks the T-2 Report (TAIC) numbers were an actual performance test. It's an easy assumption to make because it's labeled 'factual data', but it seems certainly to be calculated estimates.

Second, Greg also gets into the 624 KPH numbers and explains why they're not as authoritative as MAMA's data. Basically, those numbers aren't as well understood as the Middleton data. Again, the Homare Ha-45-21 that was tested at Middleton was removed from the Ki-84, completely disassembled and rebuilt to the factory standard (likely using machined components if such components were bad on the original engine). The engine was then tested at Middleton WITHOUT an aircraft attached to the engine using 96 octane (so the issue with knock disappeared). Because it was run with 96 octane, the dash 21 was run at its maximum horsepower using full RPMs and manifold pressure. In other words, this was not indicative of an aircraft flown in the Philippines or Japan, where the majority of air combat occurred. There were only three synthetic fuel factories that could produce 100 octane fuel under Japanese control IIRC. One in Manchuria, Korea, and Taiwan. So if there were Frank fighters capable of similar performance, they would have likely been outside the major zones of conflict (except Taiwan). It's possible that some better conditioned Franks were flying in the 1944 Japanese offensive in China, but IDK.

I don't think the climb data similarity is proof that TAIC's information is incorrect. If it is incorrect, or a result of a patchwork methodology, it would take an aerospace engineer to figure out where the mistake occurred. We lack the calculations used by TAIC and only have their final analysis. Is it possible? It is, but we don't have authoritative data or primary sources to confirm that point. At present, we only have (thanks to your work) a working hypothesis that suggests there might be a mistake in their calculations. However, I believe that the MAMA report might shed additional light on this incongruity.

The 624km/h number is pretty well understood in Japanese sources. It was a mil-power performance test done with one of the initial prototypes with Ha-45 Special (Ha-45-12) which is weaker than an unrestricted Ha-45-21. I'd say it's authoritative, but only for that context.
If you meant the 634km/h 'fully rated' data, I agree that it's not well contextualized, but it's still more tangible than a TAIC estimate in my perspective.

Middletown data does not exist in the report used. There is no flight speed performance given in that report. The other T-2 report just has TAIC's calculated estimates, as explained.
And the output of the engine is not provided in the Middletown report, just TAIC's power numbers from early 1945.

The fact that we lack the data that actually went into TAIC's calculations (apart from HP numbers, which seem potentially inaccurate) and can't verify their accuracy is why I don't think they should be used. In the first place, they have a huge disparity of up to 50km/h higher than even other existing numbers stated to be from fully-rated Japanese tests. Even if there are some unknown factors working against here, like reduced CR, I think that 50km/h is a huge gap to recover.
As TAIC also states, when data is not certain every benefit is given to Japanese performance. This is natural, it's better to overestimate than accidentally underestimate an enemy.

I do believe that the 'best possible' Ki-84 condition could potentially mark speeds a couple tens of km/h higher than even the fully rated JP tests, but this TAIC speed that is calculated with unknown factors and hugely disparate from the data we currently have doesn't really seem a good reference.

Third, the 96 octane fuel's impact on test data should have (according to Greg) lowered knock as it wasn't increasing manifold pressure or increasing RPMs. TAIC still ran the engine at its designed power settings. The performance-at-altitude curve that you observed was based on the supercharger settings and should, AFAIK, be pretty much the same. Greg even got into this and mentioned that the critical altitude was suggestive of (IIRC) good knowledge of the Homare's supercharger design. You'll have to watch the video to understand his point about this because I can't remember the details.

In summary, I think that Greg's data only indicates the theoretical maximum performance of the Frank and (as he claims) is not the actual combat performance of most Franks, which was likely heavily watered down by operational limitations. I do not think you are wrong but Greg's point is that he is interested in the theoretical maximum performance of the aircraft as that is the standard that we use to judge the engineering of WW2 aviation designs.

By the way, I believe that TAIC's last report was September of 1945 and that the TAIC report was based on interviews with captured aircrew/pilots and that the Middleton report was generated in 1946 based on benchtesting. So the MAMA report is the final say in all post-war analysis on the Frank.

It's incredibly unlikely that the Japanese Army exam department was conducting tests with fuel that would knock at rated power.

I am restating a bit here, but as I previously wrote the engine power results from the overhaul test are not actually given in the report, if they were obtained. Rather the TAIC numbers based on intel or estimates from early 1945 are provided.

So overall, my point is that there is no newer data, engine power or flight speed, given in any report presented than what TAIC calculated based on at least partial intel data in early 1945. *With the exception of the vague climb speed estimated in the Middletown report, which seems more in line with Japanese values.

 

[Metrallaroja]

In terms of the drag, I can't really give an answer because I don't have this data. But as an aside, I would say that it is quite likely the F8F had superior propeller design than the Ki-84, which is often cited as a regret in Japanese books.

If you have speed and WEP power charts for the F8F, I'd like to see them if it's no problem.

Ki-84 propeller diameter 3.1m
F8F-1 propeller diameter 3.8m

Furthermore, the TAIC figures for the power @ alt of the Ha-45-21 seem to be based on early targets. The Army/Navy data often gives a 2nd speed full pressure mil-power alt of 20,000ft/6100m and mil power of 1620 metric HP (1600 HP) at this alt. Whether this power was reduced by a lower CR or not is hard to say.

Are you sure that they give 6100m crit alt for max speed?
Because in the other archive you sent crit alt was at 6650m

 

[Metrallaroja]

I dont think Ki84 propeller has any problem tbh.
It seems good enough. Yes F8F propeller might be better. But I doubt the difference in top speed is that much to be what, 50kph slower than F8F-1 you say?
With the same power at the same alt. While having less wing area, slimmer fuselage and bit more of span and lenght.
Oil radiator doesnt seem very efficient but I dont know much about it.

 

[AerialTorpedoDude69]

qaz I'm going to respond without the quotes as I feel we are both on the same page but differ in terms of what we believe are valid sources. I tend to weight the Middleton (MAMA) reports more than the official Japanese sources because of differences in test methodology between Japan and the US. But you are right that I don't see more than a few performance metrics and no top speed indicated.

You may have noticed more than a few discrepancies in the TAIC report, there still remains the fact that (IIRC) there is another report on the Homare after it was rebuilt which we are not linking to. Do you know which report I'm talking about (hopefully I'm not having a false memory).


It was that report which strongly suggested to me that the Homare in peak condition could hit its rated horsepower with 92 octane. I want to say it was the report from this thread:

Japanese Aircraft Performance

Found this thought a few might be interested in it. Enjoy!!!

ww2aircraft.net

But I can't say for certain. Re-reading it, it seems not very compelling, although it does say that the HP was valid at 1,990 for the dash 21. I want to double check with I Ivan1GFP to see whether he knows if there is a report on the on-the-ground benchtest of the Homare 21 that's out there. Someone else who probably knows about the Homare's MAMA and TAIC testing is Howard Gibson and others. As far as I can tell, the MAMA report does go into the Homare's benchtest on the ground although they don't run it up to 3,000 RPMs. I think 2,400 is as high as they went, which is its military rating not its WEP.


Regarding fuel type, you can imagine that an engine designed for 100-octane, which was using 91/92, must have had knock problems even if being tested in a facility with elite test pilots and ground crew. There's just no way to get around it reliably without a redesign of the engine.

I dont think Ki84 propeller has any problem tbh.
It seems good enough. Yes F8F propeller might be better. But I doubt the difference in top speed is that much to be what, 50kph slower than F8F-1 you say?
With the same power at the same alt. While having less wing area, slimmer fuselage and bit more of span and lenght.
Oil radiator doesnt seem very efficient but I dont know much about it.

As horsepower increases, so too should the size of the prop. There are a number of calculations that exist, such as prop pitch, blade dimensions, blade numbers, etc.. that go into calculating the sort of prop that should be used. Horsepower is in that calculation as is altitude, but I don't know it or understand it and couldn't find any online resources on the subject. I did find this Quora post but it was for turbofans (which I assume is the same as props).


The calculation is pretty simple: It's 13(EPD)^2/3 where E = efficiency, P = power, and D = outlet diameter (which the poster basically says should equal the prop diameter x 0.90.)



Another poster said to use the following:

Thrust (lbs.) = 10.42 x [dp x Eta x SHP}^0.6667 x (RHO/RHOs)^0.3333

where dp = prop dia. (ft.), Eta ~= 0.53 (= 0.8 x Cf^1.5/Cp for a Cp of 0.2, Cf of 0.26)

and SHP = shaft HP of the motor driving the prop, RHO = air density at altitude and RHOs = air density at sea level = 0.00237 slugs/ft^3.

But in other words, so matter how you look at this situation, prop size makes a large difference in thrust. A shorter prop is going to have a major impact on maximum speed.


I would point out that MAMA and TAIC both were using engineers to calculate top speed and so it's likely they were using a factory spec prop. However, in their report, they claimed that the Japanese used a Curtiss Electric type of prop. Is it possible they used the wrong prop when calculating maximum speed? That's possible. Many of the components had to be repaired or replaced. In the official bench test report, they mentioned that they used a wooden prop. Specifically the 41K3929, which has a diameter of 92 inches. So clearly it's not the original prop and was used as a stand in.


Regarding the Frank's oil cooler, it's not amazing in its design. Greg thinks the Frank is beautifully streamlined with the exception of the oil cooler, which looks like it was just stapled on. However, the oil cooler is a Meredith Effect type cooler that uses ram air to generate thrust out the back of the cooler. The problem is that it's not ventrally mounted and so isn't optimally positioned.

 

[Metrallaroja]

qaz I'm going to respond without the quotes as I feel we are both on the same page but differ in terms of what we believe are valid sources. I tend to weight the Middleton (MAMA) reports more than the official Japanese sources because of differences in test methodology between Japan and the US. But you are right that I don't see more than a few performance metrics and no top speed indicated.

You may have noticed more than a few discrepancies in the TAIC report, there still remains the fact that (IIRC) there is another report on the Homare after it was rebuilt which we are not linking to. Do you know which report I'm talking about (hopefully I'm not having a false memory).


It was that report which strongly suggested to me that the Homare in peak condition could hit its rated horsepower with 92 octane. I want to say it was the report from this thread:

Japanese Aircraft Performance

Found this thought a few might be interested in it. Enjoy!!!

ww2aircraft.net

But I can't say for certain. Re-reading it, it seems not very compelling, although it does say that the HP was valid at 1,990 for the dash 21. I want to double check with I Ivan1GFP to see whether he knows if there is a report on the on-the-ground benchtest of the Homare 21 that's out there. Someone else who probably knows about the Homare's MAMA and TAIC testing is Howard Gibson and others. As far as I can tell, the MAMA report does go into the Homare's benchtest on the ground although they don't run it up to 3,000 RPMs. I think 2,400 is as high as they went, which is its military rating not its WEP.


Regarding fuel type, you can imagine that an engine designed for 100-octane, which was using 91/92, must have had knock problems even if being tested in a facility with elite test pilots and ground crew. There's just no way to get around it reliably without a redesign of the engine.

About fuel octane we dont have to forget that WEP is using water injection, which really helps to make small differences in fuel totally irrelevant.
For example P47N engine was rated at 72'' on 100/130 fuel using water injection.
Same thing with other R2800.
This is a comparation of the same powers with 100/130 and 115/145.
100/130 just demands a higher water flow

As horsepower increases, so too should the size of the prop. There are a number of calculations that exist, such as prop pitch, blade dimensions, blade numbers, etc.. that go into calculating the sort of prop that should be used. Horsepower is in that calculation as is altitude, but I don't know it or understand it and couldn't find any online resources on the subject. I did find this Quora post but it was for turbofans (which I assume is the same as props).


The calculation is pretty simple: It's 13(EPD)^2/3 where E = efficiency, P = power, and D = outlet diameter (which the poster basically says should equal the prop diameter x 0.90.)



Another poster said to use the following:


But in other words, so matter how you look at this situation, prop size makes a large difference in thrust. A shorter prop is going to have a major impact on maximum speed.

I would point out that MAMA and TAIC both were using engineers to calculate top speed and so it's likely they were using a factory spec prop. However, in their report, they claimed that the Japanese used a Curtiss Electric type of prop. Is it possible they used the wrong prop when calculating maximum speed? That's possible. Many of the components had to be repaired or replaced. In the official bench test report, they mentioned that they used a wooden prop. Specifically the 41K3929, which has a diameter of 92 inches. So clearly it's not the original prop and was used as a stand in.

Yes prop efficiency calculations are also out of my knowledge. afaik prop blade and prop airfoil section also have an important role.
But what I know is that propeller tip speed being near or over supersonic speeds affects a lot the prop performance at high speeds. In fact many reno unlimited have propeller tips cut to reduce prop tip speed.
Ki-84 engine running at 3000rpm with a propeller ratio of 0.5 makes so that the diameter of the Ki84 propeller is crucial. Meanwhile F8F runs at 2800rpm with a 0.45 reduciton ratio.
In Ki-84 this is probably well thought and the propeller diameter is the exact needed for the max speeds of the aircraft.
After all we are talking about a 4 blade late war propeller. The efficiency between the propellers is probably minimal and would not explain big differences in speed.
This ofc is my opinion, maybe someone with more knowledge can do some calcs and proof me wrong.

Regarding the Frank's oil cooler, it's not amazing in its design. Greg thinks the Frank is beautifully streamlined with the exception of the oil cooler, which looks like it was just stapled on. However, the oil cooler is a Meredith Effect type cooler that uses ram air to generate thrust out the back of the cooler. The problem is that it's not ventrally mounted and so isn't optimally positioned.

I do also think it is well streamlined. My fav streamlined radial is La-9 tho, which has the oil radiatior Ki-84 should use.
But I guess having the oil cooler near the engine has many other advantages like weight saving and maintenance ease.

 

[qaz]

Are you sure that they give 6100m crit alt for max speed?
Because in the other archive you sent crit alt was at 6650m

I dont think Ki84 propeller has any problem tbh.
It seems good enough. Yes F8F propeller might be better. But I doubt the difference in top speed is that much to be what, 50kph slower than F8F-1 you say?
With the same power at the same alt. While having less wing area, slimmer fuselage and bit more of span and lenght.
Oil radiator doesnt seem very efficient but I dont know much about it.

The full-pressure alt of the Ha-45 changed over time as the engine was developed. It seems that the final number of actual fully-rated engines may have been 6,100m full pressure for mil power, rather than 6,500-6600m range, but it's on sparse data I would not consider conclusive.
Basically, depending on which source you use and what date the numbers are from, the full pressure alt is different by hundreds of meters.
For example, in that same Ki-84 634km/h report it also seems to be supposed at the beginning of the report that the full pressure alt is 6,000m, but the actual test is 6,650m. The fact that the report isn't dated or anything adds more confusion into it.
*I acknowledge the ram effect of course, but there is disparity even in the same aircraft type.
The 4th Ki-84 prototype which had a fully rated -21 seems to have had a full pressure alt of 6,100m and gave 631 km/h.
Furthermore, when the engine was 'restricted' in service the full pressure alt probably returned to a few hundred meters higher due to the lower manifold boost cap, if I am not mistaken.

I'm going to respond without the quotes as I feel we are both on the same page but differ in terms of what we believe are valid sources. I tend to weight the Middleton (MAMA) reports more than the official Japanese sources because of differences in test methodology between Japan and the US. But you are right that I don't see more than a few performance metrics and no top speed indicated.

You may have noticed more than a few discrepancies in the TAIC report, there still remains the fact that (IIRC) there is another report on the Homare after it was rebuilt which we are not linking to. Do you know which report I'm talking about (hopefully I'm not having a false memory).


It was that report which strongly suggested to me that the Homare in peak condition could hit its rated horsepower with 92 octane. I want to say it was the report from this thread:

But I can't say for certain. Re-reading it, it seems not very compelling, although it does say that the HP was valid at 1,990 for the dash 21. I want to double check with I Ivan1GFP to see whether he knows if there is a report on the on-the-ground benchtest of the Homare 21 that's out there. Someone else who probably knows about the Homare's MAMA and TAIC testing is Howard Gibson and others. As far as I can tell, the MAMA report does go into the Homare's benchtest on the ground although they don't run it up to 3,000 RPMs. I think 2,400 is as high as they went, which is its military rating not its WEP.


Regarding fuel type, you can imagine that an engine designed for 100-octane, which was using 91/92, must have had knock problems even if being tested in a facility with elite test pilots and ground crew. There's just no way to get around it reliably without a redesign of the engine.

I don't think there's another known report presented by anyone yet, but if there is I would love to see it. As for now, I think all the existing numbers are TAIC's estimates or calcs from the war.
The Ha-45 was originally designed for 100 octane fuel, but to meet the specified performance it was redesigned to introduce water-methanol injection with 92 octane fuel. WM was injected at WEP, mil power and even below. In service, I can certainly believe it still had knock issues easily (especially because even 91/92 octane wasn't guaranteed), but in the state of performance testing prototypes, it's natural that better fuel would be used, not to mention that the engine will be maintained much more thoroughly.
In the end, to be clear, the operating fuel of the Ha-45 *was* intended to be 91/92 oct with WM injection.

Yes prop efficiency calculations are also out of my knowledge. afaik prop blade and prop airfoil section also have an important role.
But what I know is that propeller tip speed being near or over supersonic speeds affects a lot the prop performance at high speeds. In fact many reno unlimited have propeller tips cut to reduce prop tip speed.
Ki-84 engine running at 3000rpm with a propeller ratio of 0.5 makes so that the diameter of the Ki84 propeller is crucial. Meanwhile F8F runs at 2800rpm with a 0.45 reduciton ratio.
In Ki-84 this is probably well thought and the propeller diameter is the exact needed for the max speeds of the aircraft.
After all we are talking about a 4 blade late war propeller. The efficiency between the propellers is probably minimal and would not explain big differences in speed.
This ofc is my opinion, maybe someone with more knowledge can do some calcs and proof me wrong.

s horsepower increases, so too should the size of the prop. There are a number of calculations that exist, such as prop pitch, blade dimensions, blade numbers, etc.. that go into calculating the sort of prop that should be used. Horsepower is in that calculation as is altitude, but I don't know it or understand it and couldn't find any online resources on the subject. I did find this Quora post but it was for turbofans (which I assume is the same as props).


The calculation is pretty simple: It's 13(EPD)^2/3 where E = efficiency, P = power, and D = outlet diameter (which the poster basically says should equal the prop diameter x 0.90.)



Another poster said to use the following:



But in other words, so matter how you look at this situation, prop size makes a large difference in thrust. A shorter prop is going to have a major impact on maximum speed.


I would point out that MAMA and TAIC both were using engineers to calculate top speed and so it's likely they were using a factory spec prop. However, in their report, they claimed that the Japanese used a Curtiss Electric type of prop. Is it possible they used the wrong prop when calculating maximum speed? That's possible. Many of the components had to be repaired or replaced. In the official bench test report, they mentioned that they used a wooden prop. Specifically the 41K3929, which has a diameter of 92 inches. So clearly it's not the original prop and was used as a stand in.


Regarding the Frank's oil cooler, it's not amazing in its design. Greg thinks the Frank is beautifully streamlined with the exception of the oil cooler, which looks like it was just stapled on. However, the oil cooler is a Meredith Effect type cooler that uses ram air to generate thrust out the back of the cooler. The problem is that it's not ventrally mounted and so isn't optimally positioned.

It's also a bit beyond my depth. Japanese books often say that the propeller was inefficient, they chose a small one with four blades in order to minimize weight/size and meet their other design goals. However these are just secondary sources.
For reference though, even other Japanese planes with 2,000hp class engines like Shiden and Saiun had prop blades longer (3.3-3.6m). And of course, American planes in that power range had blades significantly longer.
I can imagine this to be a factor why the speed was not significantly increased by the bump in horsepower from -12 to -21, but this is of course just a thought.
I've also read in US reports after the war on the Japanese propeller industry that they did not have the means to design propellers of the same efficiency as the United States, but this is of course possibly biased.

Mainly, if we all agree that the TAIC number is a calc, I am wary of using it just because it is not really verifiable in accuracy and doesn't coincide with the (admittedly sparse) real data we have ATM. That's just my opinion of it. Even if the data was decent, there is potential for a calculation to be off.
TAIC estimated the Raiden's speed to be as much as 670+km/h, but no plane ever demonstrated this.
I think it's likely that Ki-84 could see performance improvements, perhaps even significantly if the fully rated test had other factors against it like the reduced CR, old-type exhaust thrust, but I feel like the gap of 630s to 680s is too big, unless there were unknown huge issues at play.

 

[DarrenW]

This has turned out to be one excellent thread. Great exchange of ideas and different viewpoints which adds nicely to what's already been discussed on this forum about the Frank. I wonder if Greg even bothered to check us out, as it could have helped him gain a better perspective and understanding of the airplane during his research.

One can only guess..

 

[AerialTorpedoDude69]

M Metrallaroja your observation makes a lot of sense regarding the Ki-84 prop. It doesn't look that much smaller (and isn't ) than a Bearcat's prop. however, in the calculation, because it's a critical component in generating thrust, even relatively trivial differences in size can result in significant differences in top speed.


Out of all the arguments here, prop size is the only one that we can prove using a relatively simple mathematics equation. And it is absolutely possible that with some stand-in values, such as a prop efficiency of around 70%, that a few inches of prop length can make a difference in thrust (which we'd have to calculate into speed using drag). However, large prop sizes only absorb the power output of an engine. If you attached a huge propeller to a 100 HP engine, you wouldn't have any gain in thrust (and you'd probably lose thrust as a consequence of prop weight).


The question is whether the Frank's propeller adequately absorbed the power output of a 2,000 HP engine. I don't know whether it did but you can imagine that it wasn't large enough as aircraft with similar engines tended to have slightly larger props.


So something for the Frank might look like Thrust (lbs.) = 10.42 x [3.1 meters x .70 x 1,990]^0.6667x0.00237 slugs/ft^3
My calculator can't do square roots or powers but without those the basic formula equals 4318.3.


And a Bearcat might have something like =Thrust (lbs.) = 10.42 x [3.8 meters x .70 x 2,250]^0.6667 x0.00237 slugs/ft^3
Without the SR/powers it looks like 5985. That's a huge difference in thrust. However, because we're not doing the math on this, we can only guesstimate. Some of the people here are math wizards (like Howard Gibson ) so if they can step in and explain how this formula means large differences in thrust, I'd much appreciate it.


qaz The La-9 is one of the best designed aircraft of the post-war period IMO. I couldn't find information on its prop legnth btw. But I'm going to guess it uses a larger, three-bladed prop and huge landing gear, such as you might find on a corsair.

 

[DarrenW]

M Metrallaroja your observation makes a lot of sense regarding the Ki-84 prop. It doesn't look that much smaller (and isn't ) than a Bearcat's prop. however, in the calculation, because it's a critical component in generating thrust, even relatively trivial differences in size can result in significant differences in top speed.


Out of all the arguments here, prop size is the only one that we can prove using a relatively simple mathematics equation. And it is absolutely possible that with some stand-in values, such as a prop efficiency of around 70%, that a few inches of prop length can make a difference in thrust (which we'd have to calculate into speed using drag). However, large prop sizes only absorb the power output of an engine. If you attached a huge propeller to a 100 HP engine, you wouldn't have any gain in thrust (and you'd probably lose thrust as a consequence of prop weight).


The question is whether the Frank's propeller adequately absorbed the power output of a 2,000 HP engine. I don't know whether it did but you can imagine that it wasn't large enough as aircraft with similar engines tended to have slightly larger props.


So something for the Frank might look like Thrust (lbs.) = 10.42 x [3.1 meters x .70 x 1,990]^0.6667x0.00237 slugs/ft^3
My calculator can't do square roots or powers but without those the basic formula equals 4318.3.


And a Bearcat might have something like =Thrust (lbs.) = 10.42 x [3.8 meters x .70 x 2,250]^0.6667 x0.00237 slugs/ft^3
Without the SR/powers it looks like 5985. That's a huge difference in thrust. However, because we're not doing the math on this, we can only guesstimate. Some of the people here are math wizards (like Howard Gibson ) so if they can step in and explain how this formula means large differences in thrust, I'd much appreciate it.


qaz The La-9 is one of the best designed aircraft of the post-war period IMO. I couldn't find information on its prop legnth btw. But I'm going to guess it uses a larger, three-bladed prop and huge landing gear, such as you might find on a corsair.

This discussion on the importance of propeller design in regards to speed is very interesting. Even propellers of very similar size and design by the same manufacturer can give different results.

Case in point, the F4U-1 started with a propeller with a particular blade design and size but was eventually retrofitted with the very similar F6F propeller because it improved maximum speed by 5 mph. So it is easy for me to see how performance of an aircraft could be greatly improved if a far superior propeller was actually utilized.

http://www.wwiiaircraftperformance.org/f4u/f4u-1-02334.pdf

 

[Metrallaroja]

qaz The La-9 is one of the best designed aircraft of the post-war period IMO. I couldn't find information on its prop legnth btw. But I'm going to guess it uses a larger, three-bladed prop and huge landing gear, such as you might find on a corsair

Three-blade propeller VISH-105V-4 with a diameter of 3.1 m, same diameter as Ki-84.
Nothing special, in fact looks pretty bad for the standards of the era. Ki-84 prop at least has 4 blades.

This discussion on the importance of propeller design in regards to speed is very interesting. Even propellers of very similar size and design by the same manufacturer can give different results.

Case in point, the F4U-1 started with a propeller with a particular blade design and size but was eventually retrofitted with the very similar F6F propeller because it improved maximum speed by 5 mph. So it is easy for me to see how performance of an aircraft could be greatly improved if a far superior propeller was actually utilized.

View attachment 703699
View attachment 703700

http://www.wwiiaircraftperformance.org/f4u/f4u-1-02334.pdf

Smaller diameter and reduced gear ratio means a reduced propeller tip speed. Maybe the 3 blade design was running into high mach speeds and that is why we see the improvement on max speed.
Of course it is also a different blade design and 4 bladed instead of 3 bladed.

 

[tomo pauk]

This has turned out to be one excellent thread. Great exchange of ideas and different viewpoints which adds nicely to what's already been discussed on this forum about the Frank. I wonder if Greg even bothered to check us out, as it could have helped him gain a better perspective and understanding of the airplane during his research.

One can only guess..

Hate to be that guy, but I would not hold my breath on that.

 

[Metrallaroja]

This discussion on the importance of propeller design in regards to speed is very interesting. Even propellers of very similar size and design by the same manufacturer can give different results.

Case in point, the F4U-1 started with a propeller with a particular blade design and size but was eventually retrofitted with the very similar F6F propeller because it improved maximum speed by 5 mph. So it is easy for me to see how performance of an aircraft could be greatly improved if a far superior propeller was actually utilized.

View attachment 703699
View attachment 703700

http://www.wwiiaircraftperformance.org/f4u/f4u-1-02334.pdf

We can do some rough calcs for this propellers: Propeller Performance: An introduction, by EPI Inc.

Mach 1 at 20000 feet = 1036.85 feet/sec
13 feet = 156 inches with 0.4 ratio
13.25 feet = 159 inches with 0.5 ratio
F4U-1 engine rpm = 2700
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2700*0.5*159/229.2 = 936.5183 feet/sec
Vr2=2700*0.4*156/229.2 = 735,07 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 936.5183^2) = 1129.11 feet/sec
Vht2 = square root (630.74^2+735.07^2) = 968.58 feet/sec

Tip Mach of 3 blade 13.25 feet 0.5 ratio propeller at 430mph at 20000 feet = 1129.11/1036.85 = 1.08 Mach Supersonic
Tip Mach of 4 blade 13 feet 0.4 ratio propeller at 430mph at 20000 feet = 968.58/1036.85 = 0.934 Mach Subsonic

And probably that is why a smaller diameter and a reduced propeller ratio was choosen.

We can do a similar calc for Ki-84 and F8F-1 propeller:

Mach 1 at 20000 feet = 1036.85 feet/sec
F8F-1 propeller 12 feet 7 inches = 151 inches with 0.45 ratio
Ki-84 propeller 10.16 feet = 121.92 inches with 0.5 ratio
F8F-1 engine rpm = 2800rpm
Ki-84 engine rpm = 3000rpm
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2800*0.45*151/229.2 = 830.1 feet/sec
Vr2=3000*0.5*121.92/229.2 = 797.90 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 830.1^2) = 1042.54 feet/sec
Vht2 = square root (630.74^2+797.9^2) = 1017.09 feet/sec

Tip Mach of F8F-1 propeller at 430mph at 20000 feet = 1042.54/1036.85 = 1.0055 Mach Barely Supersonic
Tip Mach of Ki-84 propeller at 430mph at 20000 feet = 1017.09/1036.85 = 0.981 Mach Barely Subsonic

Very small difference in propeller tip speed between the two airplanes, both propellers are designed for the same speeds. Goes to show how meticulously studied the propellers are designed to achieve max performance at low and high speeds.

 

[Metrallaroja]

We can do some rough calcs for this propellers: Propeller Performance: An introduction, by EPI Inc.

Mach 1 at 20000 feet = 1036.85 feet/sec
13 feet = 156 inches with 0.4 ratio
13.25 feet = 159 inches with 0.5 ratio
F4U-1 engine rpm = 2700
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2700*0.5*159/229.2 = 936.5183 feet/sec
Vr2=2700*0.4*156/229.2 = 735,07 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 936.5183^2) = 1129.11 feet/sec
Vht2 = square root (630.74^2+735.07^2) = 968.58 feet/sec

Tip Mach of 3 blade 13.25 feet 0.5 ratio propeller at 430mph at 20000 feet = 1129.11/1036.85 = 1.08 Mach Supersonic
Tip Mach of 4 blade 13 feet 0.4 ratio propeller at 430mph at 20000 feet = 968.58/1036.85 = 0.934 Mach Subsonic

And probably that is why a smaller diameter and a reduced propeller ratio was choosen.

We can do a similar calc for Ki-84 and F8F-1 propeller:

Mach 1 at 20000 feet = 1036.85 feet/sec
F8F-1 propeller 12 feet 7 inches = 151 inches with 0.45 ratio
Ki-84 propeller 10.16 feet = 121.92 inches with 0.5 ratio
F8F-1 engine rpm = 2800rpm
Ki-84 engine rpm = 3000rpm
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2800*0.45*151/229.2 = 830.1 feet/sec
Vr2=3000*0.5*121.92/229.2 = 797.90 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 830.1^2) = 1042.54 feet/sec
Vht2 = square root (630.74^2+797.9^2) = 1017.09 feet/sec

Tip Mach of F8F-1 propeller at 430mph at 20000 feet = 1042.54/1036.85 = 1.0055 Mach Barely Supersonic
Tip Mach of Ki-84 propeller at 430mph at 20000 feet = 1017.09/1036.85 = 0.981 Mach Barely Subsonic

Very small difference in propeller tip speed between the two airplanes, both propellers are designed for the same speeds. Goes to show how meticulously studied the propellers are designed to achieve max performance at low and high speeds.

I can also add P-51H propeller:

P-51H propeller 11 feet 1 inches = 133 inches
Vr3=3000*0.479*133/229.2 = 833.86 feet/sec (surprsingly close to F8F-1)
Vht3 = square root (630.74^2 + 833.86^2) = 1045.54 feet/sec

Tip Mach of F8F-1 propeller of 3.84m diameter at 430mph at 20000 feet = 1042.54/1036.85 = 1.0055 Mach Barely Supersonic
Tip Mach of Ki-84 propeller of 3.1m of diameter at 430mph at 20000 feet = 1017.09/1036.85 = 0.981 Mach Barely Subsonic
Tip Mach of P-51H propeller of 3.37m of diameter at 430mph at 20000 feet = 1045.54/1036.85 = 1.00838 Mach Barely Supersonic

Stiletto as a bonus

 

[Peter Gunn]

Wasn't this the same Greg that chuntered on about the Thunderbolt being far superior to the Mustang and how politics got in the way of the Thunderbolt being the premier long range escort in the ETO? Totally flying in the way of pesky "facts"?

Or am I mistaking him for some other YouTube "authority"?

 

[tomo pauk]

Wasn't this the same Greg that chuntered on about the Thunderbolt being far superior to the Mustang and how politics got in the way of the Thunderbolt being the premier long range escort in the ETO? Totally flying in the way of pesky "facts"?

The same Greg.

 

[DarrenW]

We can do some rough calcs for this propellers: Propeller Performance: An introduction, by EPI Inc.

Mach 1 at 20000 feet = 1036.85 feet/sec
13 feet = 156 inches with 0.4 ratio
13.25 feet = 159 inches with 0.5 ratio
F4U-1 engine rpm = 2700
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2700*0.5*159/229.2 = 936.5183 feet/sec
Vr2=2700*0.4*156/229.2 = 735,07 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 936.5183^2) = 1129.11 feet/sec
Vht2 = square root (630.74^2+735.07^2) = 968.58 feet/sec

Tip Mach of 3 blade 13.25 feet 0.5 ratio propeller at 430mph at 20000 feet = 1129.11/1036.85 = 1.08 Mach Supersonic
Tip Mach of 4 blade 13 feet 0.4 ratio propeller at 430mph at 20000 feet = 968.58/1036.85 = 0.934 Mach Subsonic

And probably that is why a smaller diameter and a reduced propeller ratio was choosen.

We can do a similar calc for Ki-84 and F8F-1 propeller:

Mach 1 at 20000 feet = 1036.85 feet/sec
F8F-1 propeller 12 feet 7 inches = 151 inches with 0.45 ratio
Ki-84 propeller 10.16 feet = 121.92 inches with 0.5 ratio
F8F-1 engine rpm = 2800rpm
Ki-84 engine rpm = 3000rpm
max plane speed lets say about 692kph/430mph = 373.66 ktas

Vr1= 2800*0.45*151/229.2 = 830.1 feet/sec
Vr2=3000*0.5*121.92/229.2 = 797.90 feet/sec
Vt at 430mph = 373.66*1.688 = 630,74
Vht1= square root (630.74^2 + 830.1^2) = 1042.54 feet/sec
Vht2 = square root (630.74^2+797.9^2) = 1017.09 feet/sec

Tip Mach of F8F-1 propeller at 430mph at 20000 feet = 1042.54/1036.85 = 1.0055 Mach Barely Supersonic
Tip Mach of Ki-84 propeller at 430mph at 20000 feet = 1017.09/1036.85 = 0.981 Mach Barely Subsonic

Very small difference in propeller tip speed between the two airplanes, both propellers are designed for the same speeds. Goes to show how meticulously studied the propellers are designed to achieve max performance at low and high speeds.

Propeller tip speed may have been a factor to some degree but there's also reason to believe that improved blade design was the biggest reason for the increased efficiency. An informal comparison on this forum of F4U-1As in similar drag configurations showed up to an 8 mph improvement in maximum speed at higher altitudes while utilizing the 13ft 1 in three-blade F6F propeller in place of the standard 13 ft 4 in variety, with no real difference noted at lower altitudes.

Perhaps the fatter blade design of the F6F propeller helped improve thrust in the thinning air as altitude increased?

 

[AerialTorpedoDude69]

Wasn't this the same Greg that chuntered on about the Thunderbolt being far superior to the Mustang and how politics got in the way of the Thunderbolt being the premier long range escort in the ETO? Totally flying in the way of pesky "facts"?

Or am I mistaking him for some other YouTube "authority"?

Greg's highly technical analyses indicate his bias toward highly technical designs. The Thunderbolt was heavily engineered and was among the most expensive aircraft of the war, like the P-38. But his obsession with engineering is the entire reason why a lot of people watch his Youtube channel. It may bias him toward certain kinds of aircraft but he still can explain things about planes that I didn't know or fully understand. And I appreciate his unique analysis.

But the man-hour production time of the P-51 pretty much blows the doors off all other aircraft of the war. Have you seen his analysis of the P-51 vs the Bf-109K? It's pretty outstanding IMO.

M Metrallaroja thanks for that information on the prop size. So if Soviet engineers were able to fully absorb the HP of a 1,850 engine with a three blade 3.1 meter prop, it's highly likely that the Ki-84 could absorb 1,990 with a four blade.

If I understand it correctly, it's the surface area of the blade that matters the most. However, four-blade props are less efficient due to turbulence than three blade props. (The two-bladed props are the most efficient IIRC.)

But as others have said, prop selection is a science with lots of factors involved. Going off all the data in this thread, it seems likely that while the Pe-32 prop wasn't the best, it was probably able to fully absorb the 1,990 HP output, even with its 3.1m length. I wish we could get Greg to look at your data because he's probably experienced in doing the math on prop thrust output.

 

[Howard Gibson]

But I can't say for certain. Re-reading it, it seems not very compelling, although it does say that the HP was valid at 1,990 for the dash 21. I want to double check with I Ivan1GFP to see whether he knows if there is a report on the on-the-ground benchtest of the Homare 21 that's out there. Someone else who probably knows about the Homare's MAMA and TAIC testing is Howard Gibson and others. As far as I can tell, the MAMA report does go into the Homare's benchtest on the ground although they don't run it up to 3,000 RPMs. I think 2,400 is as high as they went, which is its military rating not its WEP.

I have not yet posted on this thread, but please keep calling me a math genius! I enjoyed Greg's YouTube video.

A really useful way to make sense of engines is to work out brake mean effective pressure.

BMEP = BHP / Displacement / RPMs times all sorts of unit conversions. BHP is brake horsepower. This tells you what is happening inside the engine cylinder.

My tables are in N/m^2. The Nakajima Homare engine was 35.8 litres displacement, it ran at 2900rpm, and it put out 1990BHP. This works out to a BMEP of 1710kN/m^2. This is consistent with engines running on 100/130 octane fuel. The Japanese were using 92 octane. The Japanese had the methanol water injection flipped on. There is no other explanation.

I was surprised to see that the 427mph Ki-84s were tested by the Americans in 1944. My original theory was that the Americans were testing in 1946 to see the state of Japanese technology, and 100/130 octane fuel was a reasonable thing to use. They may not have had anything else. 1944 makes this meaningless.

Consider the American's ability to read Japanese manuals, and the willingness of Japanese pilots to cooperate with their American captors. Again, the American fuel was 100/130 octane. Gas up your captured Frank and go flying. Open the throttle and watch for engine knocking. This is your maximum boost, and this is how you hit your maximum speed.

I have Profile 213 on the Kawanishi N1K Kyofu/Rex and Shiden/George by Rene Francillon. This aircraft was tested by the Americans at speeds in excess of 400mph (http://www.wwiiaircraftperformance.org/japan/George-107A.pdf), and Francillon claims it was slower than the F6Fs and F4Us. This is consistent with the Japanese using what passed late in the war for 92 octane, and the Americans testing with 100/130 octane. Francillon quotes a top speed of 363mph at 19355ft.

 

[AerialTorpedoDude69]

I have not yet posted on this thread, but please keep calling me a math genius! I enjoyed Greg's YouTube video.

A really useful way to make sense of engines is to work out brake mean effective pressure.

BMEP = BHP / Displacement / RPMs times all sorts of unit conversions. BHP is brake horsepower. This tells you what is happening inside the engine cylinder.

My tables are in N/m^2. The Nakajima Homare engine was 35.8 litres displacement, it ran at 2900rpm, and it put out 1990BHP. This works out to a BMEP of 1710kN/m^2. This is consistent with engines running on 100/130 octane fuel. The Japanese were using 92 octane. The Japanese had the methanol water injection flipped on. There is no other explanation.

I was surprised to see that the 427mph Ki-84s were tested by the Americans in 1944. My original theory was that the Americans were testing in 1946 to see the state of Japanese technology, and 100/130 octane fuel was a reasonable thing to use. They may not have had anything else. 1944 makes this meaningless.

Consider the American's ability to read Japanese manuals, and the willingness of Japanese pilots to cooperate with their American captors. Again, the American fuel was 100/130 octane. Gas up your captured Frank and go flying. Open the throttle and watch for engine knocking. This is your maximum boost, and this is how you hit your maximum speed.

I have Profile 213 on the Kawanishi N1K Kyofu/Rex and Shiden/George by Rene Francillon. This aircraft was tested by the Americans at speeds in excess of 400mph (http://www.wwiiaircraftperformance.org/japan/George-107A.pdf), and Francillon claims it was slower than the F6Fs and F4Us. This is consistent with the Japanese using what passed late in the war for 92 octane, and the Americans testing with 100/130 octane. Francillon quotes a top speed of 363mph at 19355ft.

Thanks for chiming in Howard!

Greg mentioned that TAIC/MAMA were using 96-octane fuel in their US tests rather than 92-octane. So it appears that all references to 92-octane in the US documentation were derived from translated Japanese documents and it wasn't the actual fuel that they were using. I'm not sure why this discrepancy exists in the records. My guess is that they used 92-octane and then added TEL to it to get it up to 96 octane. But going off your analysis, as well as qaz it might be possible that tests added tetra-ethyl lead to 96 octane. TEL additives may have resulted in better performance as tetra-ethyl lead increases octane ratings (I'm not sure how much).

By the way, Rene Francillon derived his data from a single Japanese source that was published in the 1950s in English (which I think S special ed linked to a while back). This source almost certainly used military rating and not WEP values for maximum speeds, as was standard for Japanese service trials. In other words, the Japanese records are slightly slower than what a US test would find.

Additionally, combined with what we know from Qaz's data on the detuned, 7:1 compression ratio Homare-21s (which Bueschel's book references but doesn't cover much in detail) many Homares were simply not performing on par with the model included in the Middleton tests. So the MAMA and TAIC data (which included Military tests as well) isn't comparable to the Japanese test data. As far as I'm aware, most sourced Japanese documentation lists the top military-rated speed of a production Ki-84, with a detuned engine, at 380 MPH. As tested by Nakajima, it's about 390 MPH.

So the main issue is that using WEP, most aircraft gain between 10-20% more power, over military ratings. A Corsair gets about 17% more power using WEP. Is it irrational to believe a Frank might get something similar? Combined with low drag, that might translate into a substantially faster top speed.

Overall, if our discussion is on top condition aircraft using WEP I'd say that the 687 KPH values are probably close to correct. However, we also know that the Homare engine was a high RPM design and that the Japanese engineers struggled with implementing a lubrication system and finding a suitable lubricant. My guess is that their WEP was usable for shorter periods of time due to lubrication issues colliding with high RPMs.

 

[qaz]

Hi everyone, I'm trying to gather varying Japanese performance data on the Ha-45 so we can get a better idea of its output in different conditions.
I'm not sure I can quite keep up with the discussion but I already outlined my position on it well enough probably.
In the mean time I just want to point out a few things:

-However Middletown actually tested the airplane or engine isn't really involved, because that took place after the creation of the actual final TAIC calc in early 1945. The TAIC calc was done supposing 92 octane fuel and normal Japanese operating conditions (on a fully-rated, full CR Ha-45).

-The TAIC calculation is able to be contrasted to Japanese test data because they also give mil-power numbers (up to 686km/h @ 7km). TAIC also estimated nearly no difference between WEP and mil power max speed because of the alt difference (although TAICs mil-power Vmax alt of 7000 is higher than any test number). It's not even clear that the Japanese test numbers 631-634km/h with full mil-power rating (+350mm) actually used a reduced-CR engine either, only speculation.

-I still think it's very difficult to assume the TAIC numbers are even close to credible. We have essentially no idea what went into them (or how satisfactory this data was), it's almost certain much of it was intel data, not even real engine test data from the US. And again the disparity in speed is massive, over 50km/h even if we speculate that the Japanese tests had other problems.

I want to introduce this post on warbirdperformance's blog, it has one document I don't possess:

四式戦闘機一型(キ-84I)「疾風」の性能/Ki-84I Frank : WW2航空機の性能:WarbirdPerformanceBlog

データ①：『「キ八四」操縦マニュアル』昭和19年1月3日Data 1: "Ki-84 Pilot Manual" 3/Jan/1944全幅(span)　　　　　11.238m全長(length)　　　　　9.870m全高(height)　　　　　3.385m (水平/horizontal)翼面積(wing area) 21.000m^2空虚重量(empty weight

warbirdperformance.livedoor.blog

'キ84機体説明書 Ki-84 Airframe Manual'

At the end of the war the US requested materials from Nakajima concerning its aircraft, some were reproduced due to being destroyed or other reasons.
In this material Nakajima considers the top speed of Ki-84 with full mil-power +350mm boost and 3,000 RPM to be 631km/h.
Most likely taken from the 4th prototype test, which is written in books as 631km/h @ 6,120m, so either the books or this report got an alt number swapped (6120 vs 6210).
Regardless, this demonstrates that Nakajima was comfortable considering the top speed (mil power) of full boost/rpm Ha-45-21 Ki-84 at 631km/h.

Whether this test had the reduced 7.17CR or not is not known, but 50km/h will not appear that easily.

-
Also, I've been looking for Nakajima's Ki-84 performance calculations. I wouldn't consider them authoritative in comparison to real test data either, but would still be done on better data than TAIC and would nonetheless be interesting to compare to TAIC's estimate. However at the moment I could not locate it.