Need Ki84 HAYATE's data!

Shinpachi said:

Hi, Ivan1GFP!
Thanks for your good and kind explanation about the center of gravity.
I fully understand you are a very expert of aerodynamics.
However, please let me go on my own way for the time being as I find no remarkable reason enough to change my opinion yet.

Hi, Steven Que!
Attached image shows my answer at the moment though this is simply a reference for you.
37.7% is very reasonable because Ki-84 was famous for its super high maneuverability with the high powered engine at the initial stage of development. To prevent the disintegration in mid-air, the control stick was designed shorter than Ki-43 because even Ki-43 with the less powered engine often caused the disintegration.

As a result, Ki-84 was not so popular as Ki-43 with the pilots.

Airfoil.
I believe here is an answer for you...
http://www.ww2aircraft.net/forum/flight-test-data/need-ki84-hayates-data-18058.html#post493515

Click to expand...

Thanks
Now I have enough data to analyse
use program with airfoil to get lift coefficient
and use your data of center of gravity

then use some equations to got some coefficient
But I feel it is not easy to fly because it's cockpit is too simple
then can not let pilot know the flying motion

I still have to finish that

I want to see your flight simulation if possible because I don't think Ki-84 was so agile.
Good luck, Steven Que.

Hello Shinpachi,

I believe your method is quite good and will probably use something similar when I try to build a flight model for a Ki-84. The problem is using flight characteristics to estimate raw data and then using that raw data to analyze flight characteristics. It does seem to be the only way to address this issue at the moment though.

Personally, I believe the Ki-84 should be VERY agile, especially with partial flaps.

- Ivan.

Hi, Ivan1GFP.
Thank you very much for your cool and well-thought comments again.
Yes, I agree that my estimation about the flight characteristics is too rough and early to decide.
I would like to wait for Steven Que's result as this is his study.
At the same time, I myself will study it betterl again for our future talk.

By the way, I'm not finding out the J2M3 airfoil that you are searching for yet.
I will post if I may find.

Thank you.

PS: Please tell my best wishes and respects to your well-educated wife.
Generally, a Japanese lady who can speak English is clever, well-educated, modest but independent and lovely.

Such condition has nothing to do with men as they are simply requested to learn it for job

Thanks Shinpachi,

My wife is actually Chinese. She can read the Kanji only. She actually is quite well educated with a MD and PhD degree. She is a most independent and strong willed person which sometimes makes the inevitable arguments rather heated, but is quite a beautiful and elegant lady.

Regarding the J2M, the exact airfoil isn't necessary. I was looking for the section thicknesses at the wing root and tip. I will approximate if I can't find those and I really doubt if anyone will notice. For Combat Flight Simulator with the low polygon limits, a typical wing section only has about 6 segments to approximate the top or the bottom!

The Ki-84 is also on my build list for CFS aircraft which I will get to eventually.

- Ivan.

Oh, I see, Ivan1GFP.
My impression about your wife seems not so much different.
If I may use an extreme expression, Chinese ladies don't get old. They are slender and look
young forever.

OK. As I have exact airfoil of A6M, I will check if I can apply it to J2M as it is
or any modification may be needed.

Cheers.

Chinese ladies? Oh, Steven Que is also a Chinese.
I believe he will agree with my opinion

Hi Shinpachi,

I was figuring I would use 14% thickness at the root and 8% at the tip unless I found some reliable numbers. No big deal. These are just some "average" numbers.

BTW, My wife isn't, but I've seen plenty of rather round Chinese ladies.

- Ivan.

Jesus!

Hi Shinpachi,

Happy Birthday! I hope it was celebrated well.

The A6M and J2M airfoils won't be close. The J2M used a laminar flow airfoil. For my purpose, all I really need is the thickness of root and tip sections. For my first few models, I didn't even have those and you really can't tell from the results.

BTW, here are some more CFS models I built:
http://www.ww2aircraft.net/forum/aircraft-pictures/any-f4f-wildcat-fans-out-there-17344-3.html

The only 3D model differences are the textures and a part of the canopy framing. The rest of the differences are in the flight model and the armament.

- Ivan.

Thanks, Ivan1GFP, for the kind words for my birthday

The more I research J2M not only on the internet but the library and some bookshop, the less information that I want to know I find.

An expert here says the root airfoil is of a laminar flow airfoil as you say but the wing tip is of NACA 4 digit. I must laugh because there are a number of the 4 digits.

I will study more.
Thanks for your good advice, always.

Hello All,

This is quite an ancient thread, but I just had a thought while looking over some drawings for the J2M3 Raiden 21 in preparation for a 3D model:

I needed to estimate the center of gravity of the aircraft in a similar manner to the discussion here about the Ki-84 Hayate. My initial guess was at the second fuselage station from one of the excellent drawings by Shigeru Nohara. This is very close to what appears to be the Mainspar of the wing and is at about 1/4 to 1/3 chord from the leading edge.

I also have the Mushroom book on the Raiden and in this book there are some drawings of the aircraft carrying a drop tank on the centerline. Now HERE is what I believe to be a reasonable observation: The drop tank can be put just about anywhere under the aircraft. The designer would generally try to minimise trim changes with the addition of the drop tank and the consumption of the fuel within. Thus the designer would locate the drop tank at the aircraft's Center of Gravity or as close to it as possible on the longitudinal axis.

In other words, if you have a photo or drawing of a plane that can carry external drop tanks or ordnance, there is a pretty good chance that the location of the tank or bomb will be very close to the aircraft's CoG.

Perhaps this is obvious, but what do you all think of this idea???
- Ivan.

Howdy, Ivan1GFP.
You know, I used to have a book about the very subject of Wing Area, CG, MAC (Mean Aerodynamic Chord), etc. Unfortunately, I loaned that book out to a "friend" some years ago, and never got it back. The small things that I do recall is that the CG is a calculation that is based on the percentage of the MAC ( I forget if it is 25% or 30%). Obviously, the calculations for the MAC (as well as the Wing Area) is a factor of wing planform (ie. Rectangular, Tapered, etc). But for the life of me, I cannot recall any of the formulas. I also recall that the CG is determined on an unladen, "stock" aircraft. But even with ordinance, the CG shift cannot vary too much or the result is a nose/ tail heavy aircraft that any pilot can tell you is a dangerous condition in civilian aircraft- and any combat pilot will tell you is catastrophic in a combat aircraft. Hope my scant info is of any use to you. Based on the date stamp of the last entry, it is probably too little; too late.

Okay, I found a copy of the book that I mentioned above. I will not give specific info on the title, etc. in order to minimize any potential problems. Suffice to say that it was written for flying model aircraft; however, I have found it highly accurate when applied to full scale. Some conversions are required, but these are simple (ie. Square Inches converted to Square Feet [or Meters], etc).
A wings CG is not as easy as following a main spar, or drawing a line from root to tip at the 25% mark. It must be calculated, and this calculation is dependant on the wing's planform. The illustration below will give you some idea of how this is done. I will not include the meaning of the symbols, as this would require a lot of space and time. I include it for informational purposes only.



Since some discussion was also mentioned about stability, I am also including a diagram of a wing with that shows differing CG locations and what they mean to an aircraft's stability. It is interesting to note that the farther ahead of the wing's NP the CG is, the more stable. The aircraft will correct itself with pitch instability, but at the expense of maneuverability. Conversely the closer the CG is to the NP, the more maneuverable- but at the expense of stability. Clearly, one never wants the CG to go behind the NP!

Good Day GMan,

I didnt see this message until today. I don't really monitor this thread much. The J2M Raiden project of mine really hasn't gotten very far because of lack of time. This will be a new plane for Combat Flight Simulator if I ever finish it. You can find screenshots of some of my previous work here:

http://www.ww2aircraft.net/forum/cfs-i-ii-iii/screenshots-2737-8.html#post762958

Your first series of drawings is pretty much what Shinpachi was suggesting. The last drawing with an airfoil is pretty much what conventional wisdom would suggest. My issue with the method that Shinpachi and you suggest as far as finding the aircraft's center of gravity is that it is basically circular logic if you use the wing shape to predict the center of lift and use that CoL to predict the CoG and then use that CoG to predict aircraft behaviour.

As an example to illustrate the point, take the case of the Early and Late FW 190A. With the A-5, an extension was grafed between the engine and wing (can be seen in the front edge of the wing fillet) to correct a center of gravity that was too far aft. The external ordnance rack was also moved forward for the same reason.
With this aircraft, the wing size and shape did not change. Only a couple heavy items such as the engine were moved forward. If we calculate the aircraft CoG by calculating the CoL from the wing shape, we get the same number for both planes. The truth is obviously different. The CoG was too far aft on the early plane with all of the resulting problems which is why the modification was made. If we worked on the premise that it was correct to begin with, then our prediction of handling would be meaningless.

- Ivan.

Hi Ivan1GFP,

I've poured some thought into your response. Perhaps Shinpach-San and I are pretty much in synch on some issues about CG locations. But we diverge in the proper way to determine the 25% MAC.
I have looked into your issue with the way to determine CG locations- and to some extent, I agree with you. I am not sure why the author referred to it as CG. Personally, I would have thought that "Wing Aerodynamic Center" (WAC) might have been a more appropriate term. Perhaps it is because it is "custom" to locate the CG at the WAC. But as we all know, calculations and reality are two different things. Still, it is a fact that the more that you deviate from that 25% MAC- specifically, the WAC- the more unstable the aircraft becomes. Your example of the FW-190 variants are outstanding examples of this fact, and of the point that I was trying to make with that airfoil illustration.
Let's take an early and late model FW-190, and commit an act of blaspheme. We are going to use the method that I posted above and calculate the CG (WAC). Next, we are going to take a steel rod, drive it through each aircraft- wingtip to wingtip- being careful to follow the calculated CG (WAC). We will now suspend the aircraft by these wingtip rods. The point? That a stable aircraft will remain horizontal to the ground when it is suspended on its fulcrum. The early version of the FW-190 would probably tilt on its fulcrum towards its tail (tail heavy and unstable, as you pointed out). The A-5 variant would probably balance a lot nicer. And why might that be? Because that extension, etc. probably shifted the CG closer to the calculated CG (WAC) that I mentioned above. Same wing, different results.
But how do we determine an aircraft's CG beyond mere calculations and in the wonderful world of reality? Wind tunnel and flight testing. The latter involving taking the bird up with varying weights, and at varying locations. Now I don't know about you, but I do not know of too many Ki-84's or J2M's in existence- let alone airworthy ones to flight test. That pretty much reduces us calculations. But you are right that there are many other factors that we need to take into consideration if we want to come as close to accuracy at CG location as we can achieve through calculations alone. Tail Moment Arm, Neutral Point (the point on an Aircraft where Main and tail airfoil's total lift and drag forces effectively act), tailplane efficiency, areas of wing and tailplane, airfoils, fuselage distribution plan in view, downwash variations, etc. Or you can go with the simple generality, above. It is extremely accurate on full scale aircraft but to every rule, there are (probably unstable in flight) exceptions.

Hi Gman,

Sorry for taking so long to get back to you.

I believe your idea isn't a bad one, but the problem is still the same. We are all looking at the outline of the aircraft without knowing the weights and balances and trying to estimate what the location of the CoG would be. Now if we put it at the some percentage of the MAC, the problem is that we are ASSUMING that the designer put it where it should be to make the aircraft "Balanced".

My earlier argument was that if WE estimate the the CoG by putting it at 25% MAC or wherever the Center of Lift would be, we are STATING that the aircraft is well balanced. From this aircraft which we DEFINE as well balanced, we estimate its handling to determine if it handles like it is well balanced..... Circular Logic.

There usta be a single flyable Ki-84 but it is no longer flyable. A Japanese fellow bought the plane after restoration and gave it to the Fuji (the former Nakajima) who did not take care of the aircraft and literally sawed pieces off it to make it easier to handle.


The final determination as you mention with wind tunnel testing is something that we can't do which is why we are left with estimating.

The problem with estimating by looking at the aircraft's wing, etc is that we don't often know all the specifics such as angle of incidence of the tailplane, wings, etc. We don't know the interaction and airflow off the wing which might counteract tailplane incidence. The method also would not hold if the tailplane provides significant lift such as on Langley's Aerodrome or on a canard or delta wing.

I still believe my idea works pretty well because there will still be the attempt by the designer to locate disposable external loads at or near the CoG of the plane. The question then becomes what load condition is used to determine the CoG for the location of external racks. Also there is still the problem that the rack is located inocrrectly such as on the FW 190A. The ETC 501 rack was moved forward a bit to put it closer to CoG I believe with the A-8 series. (I believe it was only about 10 cm though.) Then again, this might have been done to counteract the aft fuselage fuel / MW50 tank but the question is whether it balanced an empty, full or partial tank and what the contents were.......

Fun, huh?
- Ivan.

Hello again, Ivan1GFP

*sigh*

Looks like this is getting more complicated than I had expected. You see, I never intended on offering a full blown tutorial due to space limitations and the matter of whether this is the proper forum for this. But let's take a look at some of your points and see if I can't satisfactorily explain some of them as close to the data that I presented as I know how (be aware that in some cases, those explanations may not be at all satisfactory).

We are all looking at the outline of the aircraft without knowing the weights and balances and trying to estimate what the location of the CoG would be

Click to expand...

We need neither weights nor balances; the world of aerodynamics revolves around the CG, and whatever weight you have- from external bomb racks to passenger luggage- needs to be distributed at the CG. You sling a 500 pounder five feet before the CG, you need to put similar weight five feet behind it (though you can use less weight the farther behind the CG you go due to tail moment [think of lever, here]). This is pretty much what you mentioned when you made the following statement:

I still believe my idea works pretty well because there will still be the attempt by the designer to locate disposable external loads at or near the CoG of the plane.

Click to expand...

The info that I stated in my first entry tells you where the CG optimally needs to be, though one can usually safely vary it by around 10% (ie. the CG should be at 25%-35% of the MAC though it can be as far back as the NP). Putting the CG behind The Neutral Point (NP) is dangerous. The farther behind the NP you put the CG, the more impossible the aircraft will be to fly. This is why the F-117, for example, MUST be fly-by-wire. The pilot's input goes through a whole slew of computers which interprets the pilot's commands, and makes it happen. SOP for a power failure? Punch out; do not pass go, do not collect $200. The F-117 is so aerodynamically unstable because of it's CG to NP relationship that no pilot alive could fly it by him/ her self- not without those computers. So you see, WWII aircraft specifically must have the CG very close to what I posted above. Now we get into the good stuff:

The method also would not hold if the tailplane provides significant lift such as on Langley's Aerodrome or on a canard or delta wing.

Click to expand...

You are correct. The info that I provided was for so called conventional aircraft only, like most WWII combat aircraft were. A conventional aircraft and a canard is an apples to oranges kind of deal. But for instructional purposes only, I will offer a diagram to help one determine a canard's CG, and (why not?) tandem wings. Enjoy, and I hope that I have come close to an acceptable explanation for you.

Hi Gman,

I believe we are discussing different things here. I am only making the assertion that visible external loads are a possible indicator of the longitudinal center of gravity of an aircraft. This can be done without taking into account wing planform or location of internal equipment.

For many aircraft, the photograph of an aircraft with external bombs or fuel tanks may be all we have to work with.

- Ivan.

Hi Ivan1GFP

I suppose that if you are comfortable with your method, then by all means go with it; however, you might want to consider the flaw in that logic. Though it may work with aircraft and drop tanks in some instances (probably through lucky coincidence), you will probably be better off eyeballing the wing at 25%. Probably way more accurate, at least if all that you are looking for is a ballpark figure. Tail moment and CG sort of go hand in hand when it comes to loads. You ever teeter-totter with your kid? Ever notice how close to the pivot you have to sit- and how close to the edge of the board that your kid has to sit just to remotely balance yourselves out? Same principle with load distribution on an aircraft. Except the board would be likened to the aircraft, and the pivot point would be the CG. You could sling a 500 pounder five feet ahead of the CG, but if you go twice the distance behind the CG, you would only need half the weight for balance (ie. a 250 pounder ten feet behind the CG). The flaw with your theory in this example is that if I drew an imaginery line from the nose of the front bomb to the tail of the rear bomb and halfed that line, it would not be even close to the CG. Worse still, what if I chose to balance that 500 pounder with 250 pounds of internal fuel ten feet behind the CG? Either way, we would be a good five feet or so off. Might be a huge mistake by going by what you see, and disregarding what you do not see. You might want to bear that in mind, and just be careful. Good luck.