Did the Japanese Kawanishi N1K2-J "George" fighter have double-slotted flaps?

[AerialTorpedoDude69]

EDIT: The George did not have double-slotted Fowler flaps. It had a unique flap system which combined single-slotted Fowler flaps with split flaps on the aft flap. The flaps were substantially different from other Fowler flaps in that it was designed for fighters rather than for bombers. The George's flap system would have made it one of the best dogfighters for its weight class. Thanks to MiTasol for answering this question!

Hi everyone, I found something interesting while looking over the George's unique flap system and hoped to get your help.

Does the N1K2-J George's combat flaps look like they're double-slotted Fowler flaps? Or am I seeing things? For more information on the George's flap system, check out this amazing post at the old War Thunder forum.

This is highly significant because it could mean that the George was the first aircraft to use double-slotted Fowlers. Regular Fowler flaps go back to the 1920s. They increase wing area which decreases stall speed, while limiting drag. The first aircraft to have electrically activated Fowler flaps is the Lockheed Model 10 Electra, which is what Amelia Earhart and Ted Noonan flew on their ill-fated flight in 1937. But I can't find what the very first aircraft was to use double-slotted.

Unfortunately, I can't verify that the George's flaps are double-slotted. There's a single image I think taken during a restoration which could suggest a double-slotted design. There are several N1K2-J models that clearly sport double-slots, but those could have been made in error.

The reason double-slotted flaps are so interesting is because of their complexity and effectiveness. The reason they are still used today is because they allow for large-bodied aircraft to substantially increase lift, with the only tradeoff being an increase in complexity and a slight increase in weight. For aviation buffs, it's fascinating to know where this design originated from. If it does originate with the N1K2-J, then the designer deserves recognition for this achievement.

 

[Shortround6]

A-26

Better photo of the lower left

To be double slotted you need two slots for the air to go through.
Even if they use a two piece flap with a hinge, it is not double slotted unless there is enough airflow through the hinge/gap to affect the air flow on the upper surface of flap assembley.

Lockheed 10 and 12 used split flaps. The 14 used Fowler flaps.

 

[AerialTorpedoDude69]

A-26
View attachment 769712
Better photo of the lower left
View attachment 769713

To be double slotted you need two slots for the air to go through.
Even if they use a two piece flap with a hinge, it is not double slotted unless there is enough airflow through the hinge/gap to affect the air flow on the upper surface of flap assembley.

Lockheed 10 and 12 used split flaps. The 14 used Fowler flaps.

Outstanding, thanks for your insight!

According to this paper, the Lockheed Model 10 Electra and Martin Model 146 had the first Fowler flaps. But the paper doesn't have any citations for this information. Furthermore, if the source is correct, it doesn't say whether Fowler flaps were added to later production models as is common with novel aircraft features.

The N1K1 float fighter is an example of this. While it first flew before the A-26, it first flew with a different set of flaps. This video, however, seems to indicate that it did have double-slotted Fowler flaps, although the aircraft in the video probably isn't a prototype.

Going off the diagram you provided, it looks like the George does have a double-slotted flap system. I highlight this in the image attached to my original post.

 

[MiTasol]

Japan Center for Asian Historical Records have (or at least had) a set of N1K2 blueprints on their site that I downloaded in 2013. I see nothing there to suggest that the flaps were double slotted but do note that the outboard end of the flap is a shape that could mistakenly suggest that. A close look at the photo on the Warthunder page shows the same as the blueprint.

If you cannot find the files on JACAR PM me and I will send you my PDF of part two of the drawings - unfortunately part one is corrupted.

EDIT - part one opened on my spare computer (sometimes happens with old PDFs for some reason) so I saved it again as the same name and it works fine now.

 

[MiTasol]

Just tried a search of JACAR and could not find part 1 but did find this which seems to me to solve the question. The document is for the N1K1, not the N1K2, but this looks like what the photo the OP posted shows

空技報－第０３５８号・研究実験成績報告・空戦フラップニ関スル研究（其ノ一）（十五試水上戦闘機）（飛行機性能ノ研究第四十四回報告）

 

[AerialTorpedoDude69]

MiTasol brilliant work, thank you so much for your insight!

It's pretty clear now, thanks to what you've provided, that the George's flaps combine a Fowler with a split. I don't know what this arrangement is called but the function is clear:

The extra wing area and camber from the Fowler and camber/lift from the split flap probably gave the George extraordinary low-speed handling for its size and weight. It appears that this system was unique to the N1K1/2 because the Ki-43 doesn't appear to have this feature, although Wild_Bill_Kelso would know for certain. This image strongly suggests that the Ki-43's Butterfly flaps were a variation on the Fowler and had the potential for high camber/lift.

In comparison, the N1K1/2's flap system was more complicated but also capable of generating higher lift at the expense of higher drag of the split flap were popped.

 

[Wild_Bill_Kelso]

MiTasol brilliant work, thank you so much for your insight!

It's pretty clear now, thanks to what you've provided, that the George's flaps combine a Fowler with a split. I don't know what this arrangement is called but the function is clear:

The extra wing area and camber from the Fowler and camber/lift from the split flap probably gave the George extraordinary low-speed handling for its size and weight. It appears that this system was unique to the N1K1/2 because the Ki-43 doesn't appear to have this feature, although Wild_Bill_Kelso would know for certain. This image strongly suggests that the Ki-43's Butterfly flaps were a variation on the Fowler and had the potential for high camber/lift.

In comparison, the N1K1/2's flap system was more complicated but also capable of generating higher lift at the expense of higher drag of the split flap were popped.

I actually don't know for certain, but my understanding is that the Ki-43 flaps were pretty simple. They do seem to extend outward, but not exactly like fowler flaps, it's a different design, as you can see in these models. I've never been able to determine if these were truly 'automatic' flaps (as is sometimes claimed) in that they deployed at a certain bank angle or something, or if they were manually triggered by a lever in the cockpit.

And this photo of a wrecked aircraft

Another shot with extended flaps from 'Flying Heritage Collection'

 

[MiTasol]

The Ki-43 flaps are actually very complex. I have a manual with some diagrams of them but am not at home at present so do not have access to them. The centre drive of each flap is a rack and pinion set up with lots of cables and pulleys and the hydraulic actuator also drives a rack.

I will try and post them tomorrow.

 

[ThomasP]

I may be mistaken, but I have always though of the Ki-43's 'butterfly' flaps as being similar to the Fairey-Youngman flap, or maybe a retractable Junkers flap? In the drawing below, the Fairey-Youngman flap system allows either the Fowler configuration (top), or the Junkers configuration (middle), or some combination of the two (bottom).

To me it looks like the 'butterfly' flaps on the Ki-43 are deployed in a similar manner to the middle image for the Fairey-Youngman in the image below.

 

[MiTasol]

Interesting but very different

 

[MiTasol]

This is the flap system on the Ki-43. You will need to study the drawings carefully unless you are familiar with Technical Drawings

I posted the link to the manual a fair while back under a title of something like Japanese aircraft documents.

Actuator rack and main drive pulleys in the fuselage

Wing actuator rack

Outboard track

 

[captain Frey]

Hello,
I don't understand the use of the small flap (41°). Does it descend systematically or can the pilot choose to use it or not. In the photos we see N1K2J flaps extended without this open rear flap.
Thank you for your reply.

 

[AerialTorpedoDude69]

I've never been able to determine if these were truly 'automatic' flaps (as is sometimes claimed) in that they deployed at a certain bank angle or something, or if they were manually triggered by a lever in the cockpit.

Wild_Bill_Kelso thanks again for chiming in, those were great photos.

AFAIK the Ki-43 flaps were almost certainly not automatically deployed. Nakajima fighters of the time had moved the flap deployment trigger to the control column, which normally contains gun control.

The inventor of the automatic flaps was a Kawanishi factory manager and engineer. In an NHK video on the George N1K2-J, Saburo Kiyomizu, the original designer was interviewed about the design. (Here's the time-indexed link.)

Part 1: Shiden Kai Documentary
Part 2: Shiden Kai Documentary

If I understand the schematics that MiTasol kindly provided, the Oscar's butterfly flaps are hydraulically activated Fowler flaps designed to increase camber and lift, when fully deployed and minimizing drag while increasing wing area in the intermediate deployment position. So they'd lower stall speed, improve turn, and otherwise make an incredible dogfighter even more incredible. It makes sense that it had more victories attributed to it than the Zero, with a much lower production run.

captain Frey I could be wrong but I'm almost certain those are split flaps on the aft flap of the Fowler. It increases drag and lift so it's probably used for landing along with the Fowler flaps. It might have been used while making low speed turns, although the drag could be problematic.

 

[Wild_Bill_Kelso]

Very interesting that the flap control ended up on the control column. I would think that would make it much easier to deploy and close them rapidly during combat, such as when executing tight turns. Some other aircraft had the same feature.

Are 'butterfly flaps' just another word for 'fowler flaps' or are they considered a subtype? I notice that the flaps on the Ki-43 seem much smaller and less draggy, without the raised slots on the wing etc. that you see in some other aircraft with fowler flaps. They also have that unique wing or flower-petal type shape.

 

[Shortround6]

Just a guess but "butterfly flaps" maybe a subset of Fowler flap.

It would appear (could be way wrong) that the Lockheed 14/Hudson flaps started coming down fairly soon in the travel while some of the other users may have wanted more reward travel before adding a lot of camber (downward angle). There could have been a number of different arrangements as to what the flap did where in it's travel. Due to mechanical linkages most of the time the pilot didn't have any control over what happened. If he selected 8% (or what ever) the flap/s moved a certain amount and the linkage used whatever downward angle the designers/builders had decided on and more travel (15-20%) got a different result until full travel was reached. Perhaps some planes used dual actuators so there could be more than one "curve"? But there better be interlocks so one actuator is fighting the other or bending things where they are not supposed to bend.

As an aside, note the slots/rectangles in forward part of the outer wing in the top picture. These were fixed (or letter box) slots that acted a lot like slats except they didn't move. They maintained airflow over the alerions at high angle of attack to maintain control. Spitfires used washout (wing twist) so that the wing root stalled before the wing tips preserving alerion control in that last few degrees of area where the wing started to stall.
3 different methods of achieving the same goal/result.

 

[AerialTorpedoDude69]

S Shortround6 as Fowlers were originally designed to shorten landing rolls on heavy aircraft, they probably increased camber and lift but also drag.

However, in a fighter, they'd want to minimize drag to improve sustained turn rate. With drag in mind, it looks like the Butterfly flaps have additional aerodynamic streamlining.

 

[MiTasol]

The big difference between the the Fowler flaps and the Butterfly flaps is in the tracks and rollers.

Fowler flap tracks are curved and two roller sets (one extending out the front of the flap and one a little further back on the flap spar) on each end of each flap segment start to force the flaps to droop and increase drag as soon as they leave the full up position.
These diagrams are for the Lockheed transport and bomber aircraft of the time but the N1K1 flap uses the same geometry.
Rigging flaps on a Hudson is a pain as every one of the cables has to have the same tension as well as be the correct length for the flaps to move smoothly. The P-38 flaps are a similar setup except the panels are not connected to each other so that makes rigging far easier,

The Ki-43 Butterfly flap tracks are straight and each flap has only one roller per track (A and D below) so that the flap moves purely fore and aft (so virtually zero increase in drag) until they reach the end of the track and then, because the actuator (B outboard) and cable (C inboard) is connected above the flap, all further actuator movement causes the flap to droop and increase drag.

IF I am reading the geometry right, in a hard right roll the left flap can float down under aerodynamic forces and the right flap can float up. This would help increase the roll rate.

 

[AerialTorpedoDude69]

MiTasol your knowledge and understanding of flight dynamics is amazing, thank you for taking the time to explain the flaps' probable function.

There's a video of a Ki-43 doing snap-roll, or a roll that's been assisted by a wingtip stall:

Nakajima Ki-43 Type 1 Hayabusa Peregrine Falcon (Oscar) GIF by zinzinzibidi | Gfycat

Watch and share Peregrine GIFs and Hayabusa GIFs by zinzinzibidi on Gfycat

web.archive.org

Unfortunately, it does not demonstrate the flaps in action as far as I can see.

 

[Wild_Bill_Kelso]

That's a nimble looking critter, I can see why Allied pilots would be nervous around it...