Reworking Eric Johnson's Airacobra

[Ivan]

Hello All,

I first encountered Eric Johnson's P-39D Airacobra a bit over 15 years ago.
Perhaps it was actually someone else's modification of his original design. It has been so long I can't remember.

This was way before I started building Aeroplanes for Combat Flight Simulator and most models of the time were adaptations of Flight Simulator 98 (or earlier) designs.
I thought the general shape was pretty good even though it was obviously a very low polygon 3D model.

It did have some obvious faults.
Some were obviously a consequence of the low resource count and a possible lack of tools at the time.
The worst fault was that it had "Plain Flaps" while the actual Airacobra had "Split Flaps".

There were a few shape issues the worst of which was the angle of the aeroplane while sitting on the ground.
The Airacobra sits at a very noticeable nose high attitude.
At the time, I was just getting into editing AIR files, so correcting the angle on the ground was not difficult.
The end result was that "my own version" of the Airacobra at that time would always sit with the Nose Wheel hovering a few inches off the ground.

.......

A couple years later not long after I started attempting my own projects with Aircraft Factory 99, I found the AFX for Mr. Johnson's Airacobra. I also found many other AFX's but really had no idea what to do with them.
With Mr. Johnson's Airacobra, I decided to try a few "simple" edits.
The Plain Flaps bothered me the most, so it was the first correction.
The next edit was to replace the 2D Wing Guns with 3D pieces.
After that, I quit. Following the Parts naming convention in EJ's AFX was quite difficult.

That is where things stood for many years. I had an updated Eric Johnson Airacobra on a couple computers but since it was not my own original work (nor was the work that great an improvement), I never uploaded it.

.......

[Ivan]

....About 6 months ago, I started getting involved in several technical discussions about the Airacobra on another forum. Over the years, I had collected a fair amount of information on various versions of the P-39 / P-400 / Airacobra and even had done a few calculations to see if I could figure out where the Center of Gravity could migrate to under operational conditions.

A few months ago, while working on edits for my P-47D-25, I decided to go poking around at other AFX's on my Development Computer. EJ's P-39D turned out to be the unlucky victim.
http://www.sim-outhouse.com/sohforum...=1#post1139933

A while later, I noticed that the dimensions for EJ's Airacobra were just a little bit off.
Everything was about 2% too small for some reason.
It was easy to prove that things matched up better if scaled up by 1.0191 x but the problem was that the textures would then be mis-mapped, so that would have to wait for the moment.

I decided to see what I would get if I did a quick edit of EJ's P-39D and fixed the biggest problems that I saw in the AF99 model.
The biggest remaining issues were the Propeller and Nose Landing Gear.
Neither one was a difficult correction and the new stance gave the aeroplane a whole new look.

A couple weeks later, Aleatorylamp decided that he also wanted to have a try at editing Eric Johnson's Airacobra, so I sent him the original AFX that I had started from. It seems like his edits to the Airacobra turned out reasonably well. Eventually I will have a look at his model to see what he actually did. It would be interesting to see how two Designers approach the same basic task with different backgrounds and techniques. I had wanted to keep the development as independent as possible so that each of us would come up with our own solutions. Things should be more interesting that way.

A while later I decided that I wanted to actually fly the Airacobra in CFS.
It seemed like a reasonable thing to attempt while messing around with editing EJ's Airacobra for a visual model.

That meant that I would try to develop a flight model based on as much published information as I could get.
There is a lot of information available in books and on the Internet, but much of it is fragmentary and somewhat contradictory and there are quite a lot of questionable anecdotal information.
Some pilots believed it was a great fighter and some believed it was a terrible fighter.
The trick here was to try to determine what was the consistent information in the reports without the value judgments.
When working on a flight model, one starts to see a lot of the inconsistencies in information that are not obvious when just reading reports and statistics. If anyone is interested in a more detailed discussion of this topic, post a request in the Airacobra thread. I believe I have arrived at results that are "reasonable" though not necessarily correct. Much of it depends on how one interprets the qualitative portios of the flight reports.

Attached are a few screenshots of Mr. Johnson's Airacobra as a starting point.
I believe the shapes are generally pretty good but there are quite a few issues with the details....

- Ivan.

[Ivan]

The original reason I began modifying Eric Johnson's Airacobra was to have a project to work on that was of no consequence.
After working on a project for a while, it is a pleasant change of pace to look at something different and perhaps make a few improvements with no particular end result or an upload in mind. I am usually working on at least two or three projects concurrently to have the choice of something different to switch to if I should get stuck somewhere, but my own projects tend to have some kind of goal and refining one's own work can be difficult.
Working on another Designer's project is a better change of pace because the design practices are certain to be different and because of the different thinking, there will be some room for "improvement".

With the P-39DEJ, its origin as a FS5 model was certain to leave room for improvement.
It was a very low polygon model with a total AF99 Parts count of only 450 and although the shapes were good, the 0.10 feet resolution meant that the Parts would be fairly crude.
As a contrast, my current AF99 projects typically go to about 1100 Parts and the resolution of a FS 98 / CFS model is 0.01 feet. (Actually it is 1/512 Meter if one is working in SCASM code.)

My own preference is not to do extensive work on another designer's project for re-release because no matter how much work is done, it is still someone else's project for copyright reasons. I prefer to release my own work which is why the only aeroplane I have ever uploaded that I did not originally design was Richard Osborne's Me 109E.

Attached is a screenshot showing the result after an increase in overall size by 1.91% in overall dimensions, the replacement of the original Propeller and a 6 inch stretch of the Nose Gear Strut.

Even at this stage, it was fairly obvious from looking at the existing AFX that EVERYTHING would need to be modified to get the correct shapes and dimensions:
If the Overall Length and Wing Span are off by 2%, it is highly unlikely that every individual piece of the model is undersize by a CONSISTENT 2%. It is much more likely that the inaccuracy was the result of "measurements by eyeball" and that there is no consistency. This was confirmed by observing that the original Propeller was considerably oversized for diameter instead of 2% undersized.

With this in mind, I decided to retain the original shapes as much as possible and only make Detail changes and modifications to reduce Bleeds and to test ideas. This model would serve as a experimental subject to develop techniques and Parts construction ideas for a new Airacobra project.

- Ivan.

[Ivan]

One of the things that bothered me about this particular aeroplane was that its markings were simply incorrect.

Early in the war, US aircraft had national insignia (stars) on both sides of the fuselage and on the top and bottom of both wings.
The star had a red circle in the middle. That circle was removed fairly early for obvious reasons.

A little later, the national insignia was painted on both sides of the fuselage but only on the top of the LEFT wing and the bottom of the RIGHT wing.
The idea was that it was natura for a gunner to aim between the dots (insignia) when shooting at an aeroplane and if th dots were only on one wing, it might mislead a gunner to aiming in the wrong place.

Note that the original paint job had the stars on the wrong side of each wing.
That was more tedious to correct than many of the actual corrections to the 3D model.

- Ivan.

[Ivan]

The worst limitation of the original design tools used for this model in my opinion was the lack of precision in specifying vertex locations. 0.10 feet (1.2 inches) is a very large step. At times, I find that even 0.01 feet or roughly 1/8 inch is too large of a step to get Parts aligned the way I want.

I must commend Mr. Johnson on what he was able to accomplish with the tools he had. If I were working with the same tools, I would probably have built one rather ugly aeroplane and then given up in frustration. He managed to build quite a few nice looking designs.
The comments below will be fairly critical but in the context of working to the capabilities of Aircraft Factory 99 which is not the tool he had to use. My intent is not to disparage the quality of his work but more along the lines of explaining why this design has limitations when translated to AF99. I am also not saying he did not make mistakes, but then again, we all make mistakes.

The first screenshot shows the Tail Structure.
As can be seen by the second image which is a comparison to a drawing by Paul Matt, the entire Tail is a bit too high in relation to the rest of the aeroplane.

The cross section at the Rudder Hinge Line on the Airacobra is very similar in appearance to an elongated Tear Drop. This is not difficult to represent in AF99 though in my own design I chose to simplify it slightly to save resources.
The cross section of the Rudder in this model is a thin Diamond.
It can be seen that the cross section of the Tail Cone is a very narrow Rectangle which doesn't really match the bottom of the Rudder but the difference on a textured model is difficult to see.
It can also be seen that the junction between the Fin and Tail Cone is a simple overlap of a triangular cross section.
The Tail Span is 13.0 feet while the actual measurement should be 13.50 feet
I believe the pieces are also a bit too thin.
If all these corrections are made, there is actually nothing at all remaining of the original pieces.

.......

[Ivan]

The strangest feature of this model is the inclusion of Plain Flaps instead of the correct Split Flaps.
This resulted in an arrangement of Wing panels designed around a disappearing Trailing Edge Flap section.
For Split flaps, this would not have been necessary.

As can be seen in the screenshot, each Wing is made up of only three top and bottom sections though there is also a flat Leading Edge Part included. The Wing Tip should be tapered upwards at the tip as was typical of American designs at the time and this is not represented. This feature might be very difficult to represent within 0.10 feet increments and this may be the reason it was not done.
The Wing Root Chord is actually quite close (about as close as it can be with 0.10 feet increments) but when I modified this model with a 1.91% stretch, this became a bit too large.

Although it is not apparent in the screenshot, there are concave sections in the Wing which should not be there.
At the Trailing Edge of the Wing Root, many polygons come together in close proximity but they really don't line up properly.
To correct all the issues would require a prety thorough rebuild but using the existing Parts as templates does not make sense because they are only accurate to at most 0.10 feet while AF99 can represent differences of 0.01 feet.

.......