Nakajima Ki-44 CGI Project

[John D]

It's page 138 in the Ki-44 manual.
Basic shape does not necessarily mean the NN-21. The DC-2 gave strong influence to Nakajima's wing design for Ki-27, 43 and 44. This was written somewhere in a book of series Mechanic of World Aircraft published by Kojinsya.

View attachment 495397

It's page 138 in the Ki-44 manual.
Basic shape does not necessarily mean the NN-21. The DC-2 gave strong influence to Nakajima's wing design for Ki-27, 43 and 44. This was written somewhere in a book of series Mechanic of World Aircraft published by Kojinsya.

View attachment 495397

Thanks very much for clarifying things, Shinpachi. As a primary source, I guess the Ki-44 manual has to take precedence over secondary sources. In fact, I've found it difficult to get completely consistent information on the Ki-44's airfoil from the available secondary sources. Here's a list of what the ones I've seen say:

The Incomplete guide to airfoil usage
The Incomplete Guide to Airfoil Usage
Nakajima Ki-27 NN-2 mod (16%) NN-2 mod (8%)
Nakajima Ki-43 Hayabusa NN-12 mod (18%) NN-12 mod (8%)
Nakajima Ki-44 Shoki NN-2 mod (14.8%) NN-2 mod (9%)

WWII Fighters, Design with Precision Book 1 by Matsuba Minoru, page 14
Nakajima Ki-44-II Otsu "Shoki" - NN2 14.8% ~ 9.9%

Nakajima Ki-44 Shoki by Martin Ferkl, page 6
"The designers adopted an airfoil similar to the one employed on the Ki-43. The NN-22 was used in the wing root and NN-21 on the wingtip. The wing thickness in the root was 14.5% compared to 16% of the Ki-43 wing".

Famous airplanes of the world No. 16, Army Type 2 Fighter "Shoki", page 11. (Text in Japanese)
I can't read Japanese, but going by the description of airfoils and percentages on page 11, this book seems to say the same thing as Ferkl's book, except instead of NN-22 it says "NN-2 was used at the wing root and NN-21 at the wingtip". If you have this book, perhaps you can help me here Shinpachi with a more precise translation.

The Koku-Fan, August 1970, vol.19, no.11, page 16. (Text in Japanese)
Again, I can't read Japanese, but the text mentions "NN 2" and "14.5%"

From these sources, the consensus would appear to be the airfoil was NN-2 with the wingtip possibly NN-21. Is there any chance the KI-44 manual is referring to the wingtip when it says NN-21?

I'm currently designing a 1/5 scale flying model of the Shoki on Turbocad and NN-2 suits me because page 114 of Maru Mechanic No.12 gives all the co-ordinates for NN-2 in its discussion of the Ki-27 fighter. If the airfoil for the Ki-44 was actually NN-21, then where might I find the co-ordinates for that airfoil?

 

[Shinpachi]

Thanks for your detailed information, John.
They are interesting but please allow me to make my answer as simple as possible because I don't rely on the secondary sources either no matter how they may look plausible.

As the image attached herewith shows, the Ki-43 manual tells -
Airfoil NN12(thickness change: 18% to 8%)

You will easily understand that the number NNxx includes both root and wingtip as a set.

As a common knowledge since the ww2, the Ki-44 is famous for its thinner wing than the Ki-43/Ki-27. Also, its root thickness is told as 14% (I estimated it 14.5% three years ago) clearly by Mr Ichiro Naito (Aviation historian. Born in 1921 to serve as an aviation engineering officer in IJN).

If I may trust Mr Naito and the thickness change ratio between root and wingtip of the Ki-44 is same or similar to the Ki-43, the wingtip thickness of the NN21 can be estimated like -
14% x 8%/18% = 6.22...%

I would take 6% to draw the natural lines in my CGI now.

God knows the truth after all

 

[Shinpachi]

I'm currently designing a 1/5 scale flying model of the Shoki on Turbocad and NN-2 suits me because page 114 of Maru Mechanic No.12 gives all the co-ordinates for NN-2 in its discussion of the Ki-27 fighter. If the airfoil for the Ki-44 was actually NN-21, then where might I find the co-ordinates for that airfoil?

I would scale-down to 14/16 = 0.875 vertically.
For wingtip, 6/8 = 0.75 ?

 

[GrauGeist]

Shinpachi-san, I have missed your progress on this project in my absence, as I was there from the start.
It has come along wonderfully and I am amazed at the beauty of the final product.

I am curious however, and perhaps you may know the answer to my question: Do you think that the Northrop Gamma 5D may have had any influence on the KI-44's design?

 

[Shinpachi]

Thanks for your kind comment, Dave.
I am glad again you are back

IJN imported a 2E(Gamma 5A) as BXN1 and a 2F(Gamma 5D) as BXN2 in 1933 and 1937 respectively.
Based on them, Nakajima developed a dive bomber D3N but IJN adopted Aichi D3A.
Retractable landing gear type of 2F was not imported but gave big hint to development of the B5N.
As the army was also interested, in 1938, it ordered Nakajima to modify the D3N as an army dive bomber Ki-52 but was not adopted.

I find no direct influence of the Gamma 5D to the Ki-44.

 

[John D]

Thanks very much for your advice, Shinpachi. That's a neat way of deriving the NN-21 from the co-ordinates of the NN-2 and the known thickness to chord ratio of the NN-21.

I'm so far down the track with my Shoki plan, however, it would mean too much work revising things now. Also, given the rather short wingspan of the Shoki, from a model flying point of view it may be safer to stick with the thicker airfoil of the NN-2 to avoid the higher stalling speed of a thinner wing. The difference should not be so noticeable as to make the plane less scale-looking. Actually, since I based my plan on that in Maru Mechanic, my plane will not be exact scale anyway in light of your discoveries regarding the actual Nakajima wind-tunnel model! By the way, thanks so much for posting the story of that model. It's worth a trip to Tokyo just to see it. I hope the museum has it professionally scanned so its dimensions can be accurately recorded and made available to all.

Regarding the question above on the Gamma, I looked it up and noticed its airfoil was a NACA 2215, which is the same as that used on the DC-2. Given your comments earlier that the NN-21 was derived from the airfoil used on the DC-2, perhaps Nakajima was influenced by the airfoil on the Gamma as well?

Just a final question on Nakajima airfoils. Just as NACA airfoil numbers signify camber, point of maximum thickness and thickness to chord ratio, do the numbers following NN signify anything? Does the 2 indicate camber, for instance?

 

[Shinpachi]

Thanks for your interesting information again, John.
Your decision for adopting NN2 looks realistic and best.

I think the xx of NNxx simply indicates a serial number of the root/wingtip coupling.
IIRC, a Nakajima engineer recalled like - As the army urged us "Hurry, hurry", we had little time to develop the airfoil from the beginning.
They would be encouraged to know if the NACA of Gamma was same as DC-2 to be confident.

I have just found out an original diagram of the NN-2改(kai=improved) in the Ki-79 manual.
May be good to compare.

Let me wish you good luck, John.

 

[John D]

That's a very interesting document, Shinpachi. Thank you for posting it. Maru Mechanic must have used it to create the diagram of NN-2 on page 114 of its Book 12, which you also kindly posted above. I've spotted one mistake in it, though, which is surprising given that it's a primary document. The thickness measurement at 90% of the chord states 84.8, which cannot be correct. The Maru Mechanic version corrects this and gives the measurement as 34.8.

One the other hand, the primary document gives the measurement 84.5 at 70%, whereas Maru Mechanic gives this measurement as 86.5. Similarly, the primary document gives the measurement 161.8 at 60%, whereas Maru mechanic gives this measurement as 161.3.

Who is right? When I plotted the airfoil on Turbocad using Maru Mechanic's figures, I got a fairly smooth outline. The discrepancies at 60% and 70% seem fairly minor and may not affect the outline that much, I guess, but the discrepancy at 90% definitely goes in Maru Mechanic's favour. So, as you say, God knows!

Anyway, I look forward to any more of your fine work you may wish to post in future. Your video of the Shoki's butterfly flaps in operation is particularly useful to anyone trying to get their head around how they actually worked. 2D diagrams are one thing, but a 3D working model is something else entirely. Regards.

 

[Shinpachi]

Thanks for your kind advice, John.
You have God's eyes at least.
No army which urges designers to hurry, no mistake. I would rely on the Maru Mechanic's numbers.

The linkage between the flap and the actuator could have looked like this.
Just for references.....

 

[fubar57]

Lovely Shinpachi

 

[Gnomey]

Great work so far shinpachi!

 

[John D]

Superb work, Shinpachi. Thank you very much. Those images make the operation of the Shoki's flaps so much clearer!

 

[CommanderBounds]

I have been taking an interest in Nakajima aircraft recently and your work helps me to appreciate the Shoji's design. Although it is somewhat strangely proportioned when viewed from certain angle, it's one of my favorites. Excellent stuff!

 

[Wurger]