Analysing and modifying the AFX file with QBasic.

[aleatorylamp]

Hello Ivan,
OK, will do, no problem. I´ve just deleted my last post there. It appears I was
just in time. The last thing I want to do is interefere with your project.

You are totally correct in saying that the information isn´t terribly hard to find.
Actually, I had already seen all you posted on your Airacobra thread. That was
the reason behind my comment there:
"You have certainly put together a good summary of the different models."
I just thought my question would be more on topic on that thread, but don´t
worry, I´m just as happy to stick to this one.

So, as I fortunately don´t seem to be stepping on your feet, I´ll go for the
P-39D-D2, with its 1325 Hp engine, although whether I upload it or not will
depend on the success of my planned modifications.

Regarding progress with the model itself, once the negative decimal rounding
factor was solved, it looks like shifting the Centre of Rotation with AF99, or
by Modifying it with an external Moving Program, has the same effect all round,
i.e. texture mapping, landing gear locations in the .air file, and animations, as
was to be expected.

At the moment I´m working on the transparent canopy and checking the correct
height for the rear-fuselage and tail-empenage, to correct that shape.

Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
I have just found an interesting performance test document, on the different
P-39 models, hand-written in blue pencil, which you may have come across too.
http://www.wwiiaircraftperformance.org/P-39/P-39_Aircraft_Performance_Characteristics.jpg

It is the last one on a list of links at the very end of this page:
http://www.wwiiaircraftperformance.org/P-39/P-39.html

It has extensive details - manifold pressures, RPM and speeds for different powers
at different altitudes. The first column corresponds to the P-39D-D2 with its 1325 Hp
engine, so there´s more than enough information I need for the .air file!

Cheers,
Aleatorylamp

[aleatorylamp]

Hello folks,
In order to produce a more or less usable .air file for the moment,
for the P-39D-D2 Airacobra and its 1325 Hp Allison V1710-63 engine,
there is some interesting information in the 2 performance documents
I have found, i.e. the Allison Operations and Maintenance Manual .pdf
and the site with test results and links I posted before. The following
would appear to be appear quite useful:

5-minute powers, 3000 RPM:
--------------------------
Take-off: 1325 Hp, 51.0 "Hg MP, done at 104 mph.
WEP S.L.: 1550 Hp, 60.0 "Hg MP, giving 306 mph.
Standard Emergency: 1450 Hp, 55.0 "Hg MP.

15-minute powers, 3000 RPM:
---------------------------
Military Power at S.L.: 1050 hp, 44.2 "Hg MP.)
Military Power 12000 ft: 1150 Hp, 44.2 "Hg MP, giving 362 mph.
(Another source states 42.0 2Hg MP.

Normal rated Power:
-------------------
at S.L. : 910 Hp, 2600 RPM, 37.8 "Hg MP.
(Another source states 880 Hp, but no MP).

10800 ft: 1000 Hp, 2600 RPM, 37.2 "Hg MP.

Critical altitude is 11200, or 12000 ft (also named military
rated altitute), depending on the source.

Landing speeds 89 mph, with flaps and gear down.

There are several cruise powers mentioned, but I´ll see to that
later on, once the basis for the .air file is established.

My thoughts as to the distribution of powers, i.e. normal
throttle lever travel and WEP (type 2 methanol-water), would
be to include the 15 minute powers in normal power, and have
the 5 minute powers as F10-WEP.

The problem here would be that 1550 Hp WEP is rather more than the
1325 Full take-off power. Were this to be included in WEP, then we´d
all take off with 1550 Hp, which doesn´t sound reasonable. But, how
to prevent prolongued use of 1325 take-off Hp would be the question.

A different matter is how accurately I´ll be able to put allo this into
the .air file, but it will be fun to try.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I am somewhat curious as to where you got the designation P-39D-D2.
Other than your posts, I have never seen this designation.
I believe that with the V-1710-63 engine that you specified, you are really referring to the P-39D-2-BE.

If this is true, then I believe you need to recalculate the Center of Gravity.
My calculations were based on an aeroplane (P-39Q-1) equipped with a 37 mm cannon.
The 20 mm Hispano that was standard on the P-39D-2 would have been quite a bit lighter and changed the balance of the aeroplane.
That 20 mm Hispano cannon was also the reason I did not choose to build the D-2 version even though it met the requirements in other respects. The 20 mm cannon was not well thought of by US forces operating the P-39.
The P-39K is basically the same as the P-39D-2 but with a 37 mm gun which is why I thought it would be a good follow-on build after the P-39D/F was done.

If you graph the data you have collected about engine power and settings, you will probably find that it will be contradictory in certain areas.
To actually build something, you will need to sort through the data to have some consistent values. I had to do the same with the V-1710-35 but there were some choices about who and what to believe.

You also need to be careful about anecdotal data. Your "20 MPH Faster" quote from the post you deleted is a good example of such a comment that I find to be not very plausible.

The table of data that you have linked is pretty good. It is actually the same place where I am finding data on other models of the P-39.
The problem as usual is that some of this data contradicts other data that I have, one has to choose what to believe.
There is also no data on the exact model that I am attempting to build.

I have actually been messing around with the EJ P-39D. Last night I finished reworking the Flaps so that they are no longer "Slotted Flaps".
I have been doing my best not to alter the shapes even though I know that they are incorrect. Proper shapes would have made things easier. My objective is to do a general cleanup and corrections without altering the general shapes and appearance of the original model.
This model uses "Retracted Flaps". I generally use "Deployed Flaps".
Both have their advantages and limitations, but I believe my standard method of Deployed Flaps is better overall if Aircraft Animator is used.
Aircraft Animator also does not remember Deployed Flaps from one execution to the next.
The problem is that the tool is so simple to use that it omits some very useful features.

I also reworked the Main Landing Gear Wells so that they would no longer disappear from certain angles.

I was also tempted to build a Transparent Canopy as well in order to test the required Assembly sequence for when I build the actual Project.
It looks to be pretty easy because of all the resources that are still remaining.

- Ivan.

[Ivan]

Hello Aleatorylamp,

You are totally correct in saying that the information isn´t terribly hard to find.
Actually, I had already seen all you posted on your Airacobra thread. That was
the reason behind my comment there:
"You have certainly put together a good summary of the different models."

Perhaps it should be "Mostly Correct".
There were two pieces of data that I had to estimate based on fragmentary evidence:
The .30 Caliber ammunition load of the P-39C and
The .50 Caliber ammunition load of he P-39Q.
I wasn't building either version yet, so it wasn't crucial that I get the numbers entirely correct but if you were actually able to find a good reference for these, I would be interested to know where. I am thinking the P-39Q information should be fairly easy to find if I look hard enough but the data for P-39C would probably be a bit harder.

I just finished the first pass at Engine Power and Level Speed tuning for the P-39D/F model.
Speed at Sea Level (500 feet)
309 MPH - Actual but this number was probably corrected to Sea Level
310 MPH - Model

12,000 feet
368 MPH - Actual
368 MPH - Model

Some sources put the altitude a bit higher for maximum speed.
Now it is time to tune some of the other bugs out of the AIR file.

- Ivan.

[aleatorylamp]

Hello Ivan,
First things first! Here are 2 links to the Allison "E"-type Engine Operations and Maintenance handbook, all 26.9 Mb of it! It takes quite a while to download, but it is something I´m sure you will enjoy. The second seems faster:

http://tradecoastcentralheritagepark...dbook_1944.pdf

http://www.avialogs.com/index.php/en...allations.html

I misinterpreted the -D2 nomenclature from somewhere, where they said the -D version had -D1, -D2, -D3 and -D4 subtypes, and mistakenly added the "D" as well, instead of only the number.

I corrected the fin/rudder and the canopy last night.
The fin was placed 0.33 ft too high and the lower part had to be extended another 0.22 ft.
The canopy, although the top and base-line seem OK, had the front and back 0.2 ft too low. These corrections will make the nose and aft fuselage slant correctly.
Then, the spinner is too small, and be about 0.28 ft fatter.
The worst is the "slim" belly - it has to go down another 0.8 ft. I still have to check the width.

These will be the reference points for the fuselage correction.
First I´ll do with the 8 sides it has, and then skillfully insert an extra 4...

I still have to do the flaps on my version. Personally I prefer putting in retracted flaps tagged with Speed below 195, because of AA missing things with deployed ones, as you said too.
Updated comment: With flaps built as retracted, I only need to delete the 4 deployed-flap parts, untag the old retracted flap parts, leaving them as normal wing parts, and repeat the undersurface of the retracted flap, tagging it with Speed Below 195.

Well, it´s coming along. Here´s a blueprint screenshot, showing the cabin and fin shape changes (which don´t fit the old fuselage properly!), as well as the centre of rotation.
The CoG is set a little aft of what I had before, but perhaps not enough. Would think it is still too far forwards? Perhaps the best place to correct the still-needing-attention CoG would be with the 2 places that offer offsets in the .air file.

Regarding the .air file, I´m not sure what to do:
I wonder if it would be better to eliminate the 1550 Hp max. 60 "Hg WEP, keeping it as the 1450 Hp standard 55 "Hg WEP. This would, however, include Take-Off power, which should really only be 1325 Hp with 51 "Hg.
I wonder if you would like to suggest something...

Well, it´s slowly progressing... It´s actually a lot of fun too!

Updated comment:
I´ve just changed the new screenshot - again! It now shows the bright-orange (thickened to be more visibile) outline of the planned, improved, more pot-bellied fuselage, but not as much as I had it a while ago! The air intake also has been moved upwards by 0.1 ft, and the spinner is fatter now too. Of course, as always, I value your views and/or opinions should you like to comment.
I´ve also checked the main landing gear. It seems to have the correct position, which is a relief.

Cheers,
Aleatorylamp

[aleatorylamp]

Hello Folks,
A lengthy part of the job, but not difficult: The fuselage, still 8-sided, to make dimensioning of all the parts in the three components easier, is now more pot-bellied.
I still have to adapt the wing roots to the new fuselage belly, which may be a bit more difficult,
and position the forward landing-gear parts accordingly. You can see in the blueprint that they are still not done yet.
Here´s 2 screenshots. The look is a bit more realistic.
I still have to get the texture mapping a bit better overall.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I had to do a couple more revisions to the flight model I was working on.
I had originally planned to do the P-39K with the V-1710-63 engine and had left a couple parameters unchanged when I switched to the P-39F.
About three hours of editing and testing and now the numbers for mine are:
314 MPH @ 500 feet and
368 MPH @ 12,500 feet.

I am a bit surprised you had such a simple time with doing the Flaps.
Mine was much more involved and took the better part of a day to finally get working the way I wanted.
First I tried the method you described, but could not get rid of the bleeds of the Wing Roots from outboard.

Thanks for the link to the V-1710 E series manual. I am not sure if I already have a copy of the one for the E series but I know I have the equivalent for the F series engines. The folks naming the PDFs don't make it obvious from the name of the file, so I can't tell without going through a bunch of files. Be very careful when using this for reference material though.
This manual is from 1944 and everything is fine if you are building a P-39Q but the earlier models may not have had the same engine power ratings and limitations. Production quality improved as time went on AND fuel quality improved so that often the same nominal model of engine would be making more power in 1944 than 1942.
My subject is the Airacobra from 1942 when it was an important fighter aircraft of the US Army.

Whose drawings are you using for your corrections? It isn't clear where you are getting the reference numbers for your changes.

Be careful about references you find. It seems there is a LOT of incorrect information even from people who should know better.
I have seen one "source" with access to an actual P-39 that didn't know that the 4 guns IN the wings were actually .30 caliber instead of .50 caliber. I have an actual printed book on my shelf that has lots of cool pictures but the assertions and conclusions in the written text are simply incorrect. I have a report that was written about the Allison engines that has aircraft performance curves that I know are incorrect because there are actual flight tests that contradict them.....

Gotta Run.
- Ivan.

[aleatorylamp]

Hello Ivan,
My version would also tie in with an early model performance, so following your recommendations, I expect the good table to go by is the one which is hand-pencilled in blue.

Yours had a stronger turbosupercharger, tuned more for "high-altitude" speed. The numbers for mine have to be slightly lower (306 and 362 mph), but I haven´t done any further adjustments to the .air file yet. At the moment they are at 334 and 380 mph, much too fast!

Then, following your original suggestions, I looked for and found a huge set of nine Paul Matt drawings at The Blueprints.com, with top, bottom, front, back, side-flying, side on ground, with nothing under-fuselage, with extra tank, and with bomb views.

They are different Q-models, with a 4-bladed prop, but that shouldn´t affect the basic shape, I suppose. The shape is the same as on the 2 before and after stretching overlay drawings you posted a while back. At least it hasn´t got the straight edged fin of the pink pinball!

Re. flaps: I hadn´t tried out my planned "easy" flap modification until just now, and I hadn´t dealt with the wing-root or any other bleeds yet, and these are quite apparent.
The results are like you said, so I´ll re-think that after wing-roots and wings are correctly placed.

I´ve dropped all wing-root parts to the new belly-line by 0.46 ft (I didn´t shift the components - this goes against my principles!), and now the wing parts will follow. However, wing-tips must drop only 0.24 ft, so that will take some doing!
All top parts of the landing-gear struts will need adjusting too.

Actually, the only components I shifted down a bit were the tailplanes, because they were already shifted, so it would have been too laborious to un-shift them first... anyway, no matter!

The model´s getting the look more and more - it had already started doing so with your main modifications a while back, and now it´s doing more so!

More later, as we´re off to the farmer´s market.
It´s finally sunny, not dull rainy and cloudy any longer, since yeasterday afternoon.
About time summer came too! They say it´s the old government´s fault - they did so many cut-backs that they also cut back on spring and summer!!
Cheers,
Aleatorylamp

"Why make it simple if you can al

[Ivan]

Hello Aleatorylamp,

I hadn't actually gotten to the point of copying down the values from the image in the link you posted.
As it stands, it is so faint that the numbers don't really stand out very well on a quick read.
I didn't see a reason for doing that because it doesn't include the version that I am interested in.

In reality, the P-39D/F that I am building didn't have a Turbocharger. Only a few of the prototype Airacobras had a Turbocharger.
In fact, the Supercharger on my version and your version were probably identical.
As with any Flight Simulator model, if you give the same specifications to two authors, the method of implementation may be a bit different because we all have slightly different priorities in what we want to show in the flight model.

If the Wing Root and Wing Tip move different amounts, it is actually pretty easy to address.
Pick one side to move the Wing Panels with (I would probably go with the Wing Tip because the joints are a bit more confusing there), and then just snap the Wing Root ends of each panel to match the corresponding Wing Root vertex.

By the way, your "Pink Pinball" manned target was actually a P-63 King Cobra and I believe it was actually painted Orange and not Pink.

- Ivan.

[aleatorylamp]

Hello Ivan,
Then the engine was similar. Of course, the first blue-pencilled column would not be the one for yours - maybe the second, the "K" version initially, with different and blurry speeds, but no "D/F".

The wings went OK - like you said: First placing the wing root and wing tip, and then the panels, lined up with the wing tip and then adjusted to the wing root. I had a little trouble with the flap parts, but it went OK at the end. For the moment they are still the way I´d thought, plus a grey panel that shows up under the wing when flaps are deployed, but there´s still some bleeds.

The textures took a little work to correctly stretch them, but they are all lined up with equal height from nose to tail, except for the rudder, so that was OK too.

What is debatable is whether one should keep the wing component distribution in the same way, top-surface and bottom-surface, although perhaps it´s OK.

Tomorrow I´ll adjust the gear parts, fight all the bleeds... and later possibly change the livery with a yellow band and a different number.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

First comment I should make, (a retraction), is that there probably wasn't a difference in supercharger between the V-1710-35 and V-1710-63. Perhaps I am correct, and perhaps I am not, but my general conclusion is not correct. Whether there was a difference in Supercharger, Intake system, efficiency, Friction, whatever, the net effect was that the P-39K was just a bit faster than the P-39D/D-1. At Sea Level, the graphs on the same page show a difference of about 2-3 MPH but altitude, the difference is 6 MPH.
There is another problem with trying to pin down what altitude we are actually discussing....
Even on that same page, the graphs contradict each other slightly and other reports add further contracdictions with the Graphs.....

I suppose I could tell you what the little Gray panel is under the Wing, but if you go poking around a while longer like I did, you will figure out what it REALLY is and you won't like it. By the way, there is a very similar Dark Green panel poking around when seen from the TOP of the Wing as well when seen slightly from the front.

The Wing as built in an upper and lower Component is pretty messy but it CAN be fixed without altering the original shapes which was my requirement. I ended up combining them into a single Component to reduce the number of bleeds. I didn't see how the original arrangement could be made bleed-free.
You and I have each worked on enough projects to be able to anticipate where the biggest problem areas are hiding. I really would not have gone after the Wing except that I needed to fix the Flaps and could not do it well without reworking the Wing.

I see you are still working with that same Table of specifications.
I suppose by now you already realise that the top half of the tables is not useful for comparisons because it is the same for every model for the useful measurements.

I can tell you that the lower half of that table isn't very useful either.
There are a few pieces of data you can pick out, but most of the table is based on assumptions (which I will get into with the "Airacobra" thread) that you may not share.
As an example, compare the P-39D-2 and the P-39K.
Both have the same engine and are nearly the same aeroplane except that the D-2 has a 20 mm cannon and the K has the 37 mm cannon.
The empty weight would not include weapons or armour, so the two aeroplanes should weigh nearly the same, which they do.
P-39D-2 Empty Weight - 5626.8
P-39K Empty Weight ---- 5664.9

Life gets really interesting when you get to the useful loads and equipment.
Fuel Capacity is the same.
Oil Capacity should be the same with the same engine.
Armament is the same except for the cannon.
The Pilot of the D-2 is 40 pounds lighter because the slightly later K probably had its weight allowance for the Pilot raised to the standard number used by other Army (and Navy) types.
The 37 mm cannon ammunition load is heavier than the 20 mm ammunition load....
The 37 mm cannon is 238.2 pounds while the 20 mm cannon is 127.2 pounds.....

....So why are the loads on the D-2 substantially heavier?
Why is the Maximum Gross Weight of the D-2 HIGHER when it should be lower or the same?
Aerodynamically they should be twins except for the Cannon.

- Ivan.

[aleatorylamp]

Hello Ivan,
Interesting! - I had noticed the weight differences too - I thought perhaps the 40 lb difference for the pilot would be a cheaper parachute and other stuff, and did wonder why all over, the plane came out heavier. I put it down to other "sundries" and discarded the thought... but now I´m still wondering. Then, the maximum gross weight was perhaps also a specification which was adjusted later for the "K" model... like perhaps the pilot?

Regarding the available graphs for the different P39 versions, I´m afraid I can´t understand them at all because of the way they are set up, but anyway, but as you say, the numbers on the different lists don´t seem to tally either.

At the end I´m going to do the wings as single components - there is really no reason (in AF99 - I don´t know about AF5), for the upper wing surface to be in Mid Wing l/r and the lower one in Wing Low l/r. Components automatically show up/dn correctly anyway, and the group Wing Low does not really behave as an independant group.

I haven´t altered the original shape of the wing either - yet... I may have a go at raising the wing tip outer edge a bit, to better fit it to the drawings.

The front grey panels on the wing root would be air intakes, but they are bent. Fine for AF5 I suppose, but strange for AF99, to say the least. Then, why are some parts built as regular on the left and some on the right ?? No consistency here.

Well, first I have to match the landing gear to the new underneath-profile. I think I saw a 3-view plan somewhere that shows a much slimmer belly with correspondingly higher wings, which was what the original author must have been using.

Well, let´s see how it continues.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

On my edits of this model, I didn't change what I didn't absolutely need to in order to modify the Flaps.... After the general cleanup and Propeller and such.
The reason why the Oil Cooler intake is bent is because it needs to be bent in order to fit onto the Wing leading edge that is a flat vertical Part. It doesn't fit all that well and isn't really shaped properly, but as I stated earlier, I didn't change it because it wasn't directly affected by the Flap modifications.

I believe the upper and lower Wing Components almost made sense with plain Flaps and no Glue. I can see the kinds of bleeds that the author was trying to avoid. The problem was that the Airacobra didn't have plain Flaps and all the business that was done to avoid bleeds for that purpose don't work well for split Flaps.
The Wing isn't really shaped correctly either, but I believe that was unavoidable with only 0.10 feet increments in AF5.

Have you actually kept track of how many things you have changed and how many you have left in their original state? Just simple moves of the entire model don't really count in my opinion because they don't require any manual editing.
I am restricting my own changes and am probably up to around 25% already and I haven't even tried for a Transparent Canopy yet.
I also know the Wing Tips are incorrectly shaped. They are more like what I did earlier on the P-40 and Corsair, but when you start there, it is just a LITTLE step to correct the Wing planform and chord and thickness and airfoil and....

The end result is still someone else's AFX and not really your aeroplane and you will probably still inherit some silly stuff because of the original design.
That is why I decided not to take that path.
Besides, I actually LIKE the Airacobra so I figured I should build one.

- Ivan.

[aleatorylamp]

Hello Ivan,
I´m changing things a bit more, without building a completely new model. It´s just a
different approach, I suppose, and will get me quite close, and for the moment, it seems
to be coming along as I had expected - i.e. altogether not too bad.

The main thing went well in its 1st phase: a) The nose-job: A rounder spinner and nose, a prop-blur, better prop blades, and a lower front landing gear. b) Overall fuselage re-shaping, which included re-positioning the null point, correcting the height of the fuselage-tail, fin and elevators,
giving the lower fuselage the correct belly-curve, moving down the wing-roots and wings,
correcting the dihedral, and improving the wing tips.

This phase will finish once I´ve corrected the upper parts of the landing gear elements, re-positioining doors and wells, fighting bleeds, of course, and I´ll post a screenshot when the
model´s presentable!

The second phase will be a 12-sided fuselage, and tail surfaces with air foil cross-sections.

Regarding armament specifications: I´ve seen mostly 37 mm cannon for all -D models including
the D-2, except one source stating that they changed to a 20 mm cannon for that model,
putting it back later for subsequent ones. Another source says the D-1 had the 20 mm cannon.
The weight difference would of course be considerable, and may account for the D-2 weight confusion.

Another problem is the power contradictions, which is making me dizzy!!
Deducing from previous e-mails then, we can discard the higher 1550Hp WEP with 60"Hg MP and 1450Hp Standard Emergency Power with 55"Hg MP, that are listed in the 1944 Allison Operations and Maintenance Manual, as they would not have been available for our earlier P39´s.

Thus, (cruise powers apart), we could have:

Sea Level:
> 5-min. Take-off power: 1325 Hp at 3000 RPM with 51.0 "Hg MP.
> 15-min Military power: 1050 Hp at 3000 RPM with 44.2 "Hg MP.
> Normal 100% max cont.power: 910 Hp at 2600 RPM with 37.8 "Hg MP.
Here there´s another source quoting 880 Hp with 37.2 "Hg MP.

Higher up:
> 11800 ft 15-min Military Power: 1050 Hp at 3000 RPM with 44.2 "Hg MP.
Another source quotes 12000 ft, 1150 Hp and 42.0 "Hg MP.

> 10800 ft Normal 100% cont.power: 1000 Hp at 2600 RPM with 37.2 "Hg MP.

Taking the S.L. 1325 Hp with 51 "Hg MP as the max. power reference point, then
Normal S.L. Max. Continuous falls a short with about 850 Hp. Maybe I´ll raise normal max. MP from 37.8 "Hg to 40 or so.
Also, powers won´t tally higher up, so I´ll have to reach some kind of compromise there too.
If numbers don´t fit, bend´em!!

Let´s see what else happens!
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I am fairly certain that the 1550 HP @ 3000 RPM was valid even with the introduction of the -63 engine.
The reason I believe this is because I was reading an article about the tests against Koga's Zero in 1942 which suggested that a regular P-39D was using 52 inches Hg during the tests.
As I commented earlier, I am not working with the V-1710-63 yet, so I haven't chased down the time lines, but you might want to check whether this was the E series equivalent of the F series engine used on the P-40K.
The V-1710-39 engine used on the P-40E was the equivalent of the V-1710-35 (I believe) and in North Africa, even higher manifold pressure setting were used. Allison even issued a memo authorizing emergency power settings that were very near what you are seeing with the V-1710-63 though I don't know offhand the exact date of that memo.
By the way, don't make your decisions based on what I am telling you. Do what your research says should be done. I have come to my own conclusions, but they may not be correct.

From what I have been able to find:
The P-39D-BE had a 37 mm cannon.
The Airacobra Mk.I had a 20 mm
The P-400 had a 20 mm
The P-39D-1-BE had a 20 mm
The P-39D-2-BE had a 20 mm cannon as well.

That is why I chose the P-39F to build.
I also would have wanted to build a hotrod D model but I don't believe such a thing was ever produced with the 37 mm cannon.
I have seen a photograph with a caption listing a P-39D-1 serial number and describing the armament as a 37 mm and with a Nose Gun that LOOKED like a 37 mm, but I have never seen the same with the P-39D-2.
Then again, just because I haven't seen it doesn't mean it didn't exist.

For what it's worth, the number "31" P-39 that we are starting with probably wasn't actually a D model either.
I seem to remember seeing a caption for a photograph describing it as a P-39J which would have put it at the end of the P-39F production run. The problem is that "31" is hardly a unique marking and the serial number on the tail seems to vary with each representation.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thanks for the information updates, and for sharing your new finds.
It´s like a busy news reporters´ office in the movies - new information coming in all the time!

So, I´ll put the 1550 Hp back in - as WEP, with MP at 60 inches of mercury, and this would then also cover Standard Emergency Power (SEP ?), 1450 Hp with 55 inches.
Thus, I´ll leave 1325 Hp Take-off (51 Hg MP) in normal power.

Updated paragraph: I was finding it hard get CA to 12000 ft, simultaneously maintaining correct power with WEP, but I had been overlooking (again) the WEP Change Rate parameter. This should help fix things, because it worked without using WEP.

I also noticed the differences in the tail numbers you mention, on photos of unit No. 31, as well as the differences in the colour scheme. I agree that not even the original .air file (P39C.air) would correspond to the modified model I´m making, and that the cannon-shape is that of a 20mm one.

Thanks to these indications, I did some more research: Having seen -D2´s with both types of cannon, I have decided to change the colour and the cannon size to fit my -D2 modifications, as such aircraft do appear to have existed.

I found colour schemes of two early Russian lend-lease units. The green one is on Wings Pallette, and the brown one on a page that shows decals of different P-39 colour schemes and markings. Finally, Wikipedia summarizes what I have also found in greater detail on a few other sites:

P-39D-2
--------
Bell Model 14A-1, production variant with a V-1710-63 (E6) engine (1,325 hp) restored the 37 mm (1.46 in) cannon, provisions for a single 145 gal (549 l) drop tank or maximum 500 lb (227 kg) bomb under the fuselage; 158 produced. Some 50 at least sent to USSR and used in combat, some 15-20 used by 16th Guards Fighter Regiment.

Consequently, I am fairly certain that you will be able to get your hotrod D model after all. How about painting it red, with flames coming out of a dragon-mouth painted at the front! That ought to be fun!

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

It seems like every time I make a statement, it gets misinterpreted and you head off in another direction.
You originally posted the link to a very good set of graphs for speed versus altitude.
They are slightly contradictory and I acknowledge that you have stated that you do not understand them, but they really are not complicated and present the information you need from a performance standpoint.

Your choice of numbers might explain why you are having such a difficult time getting the Critical Altitude to match.
Perhaps it is because you are trying to match different power settings at Sea Level and at Altitude as I am suggesting.

Regarding the P-39D-2 carrying a 37 mm cannon:
So far, I have found one site I consider reputable that makes that claim.
That would be Joe Baugher's site.
Unfortunately in this case, one of the references he used happens to be one that I also own and I can tell you that it is quite unreliable though it has pretty pictures and has value in other areas.
Aircraft Profile 165 also states that the P-39D-2 has a 37 mm cannon.

I do not consider Wikipedia a reputable source, though as I have tried to teach my Children (who are NOT ALLOWED to use Wikipedia as a reference for school work) to check the bibliography listed at the end of a Wikipedia article. Some of those sources may be pretty good.

Personally, I never believe everything I see in a profile drawing.
It may be good supporting evidence but I have followed the "research" that sometimes goes into such a painting and although they are pretty, I don't believe they are reliable.

The sources that state that the P-39D-2 were armed with a 20 mm cannon that personally have access to are the following:
America's Hundred Thousand by Francis Dean
Detail and Scale 63 by Bert Kinsey
Warbird Tech 17 by Frederick Johnsen

There may be a lot more references I can pull for either conclusion if I could read Polish.
I know what I believe, but you need to do enough research to convince yourself.

I have gone back through the book that listed the P-39D-1 with a 37 mm cannon and am fairly certain that the caption was wrong and that the aeroplane in the photograph was really a P-39C.
This particular book has numerous photographs that have incorrect captions when examined critically.

I actually do intend to build either one or two REAL Hotrod 'Cobras, but the first will be Yellow and not Red.
I may do the Red one also if I don't get bored. The Yellow one was actually a P-39Q and much more famous.

- Ivan.

[Ivan]

Hello Aleatorylamp,

I just did a quick scan of the Wikipedia article just in the variants section that you were referring to.
Here are two notable errors:

The P-39D lists only 60 aircraft produced.
The reality is that the FIRST 60 D models were converted C models.
A quick check of serial number ranges will confirm that there were around 400 more D models produced followed immediately in the serial number sequence by the P-39F.

While it is generally true that the P-39F has 12 exhaust openings per side, there were a few that only had 6 openings per side. This is confirmed by a photograph in Detail & Scale. Photograph has a serial number on the tail so it is pretty difficult to dispute.

While no P-39G were ever built, the contract is actually earlier than that of the D-2 by about a month.
Implication is that the D-2 came first and while this MAY be true, I believe the contract dates suggest otherwise.
Build dates are all over the place because the contracts were filled by different letter models.

The P-39K is listed as the first with the triangular scoop at the nose, but other sources state P-39L was the first.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thank you for your comments.

I know that in your research you are not so much dealing with speeds at the moment, and also, that you are dealing more with the -35 engine, and that you may possibly deal with the -63 at a later point.

My main question was relative to a possible distribution of the different types of power among the scarcely two available positions offered by that CFS1, i.e. WEP and NON-WEP, and every time I mention an idea, your answer seems to be concerned with the power charts, which a) as you have already said, don´t exist for the -63 engine and b) are no help in deciding where to make CFS1 separate WEP from NON-WEP.

Your answers also mentioned that a) early engines would not have had such powerful superchargers as later ones, so I supposed that one would eliminate the higher MP values, to which you commented that further research revealed that in effect, 1550 Hp were indeed possible, and higher MP values would be in order.What is true, of course, is that later engines had better high altitude power, but the D-2 was not a later engine, so that wouldn´t matter.

At the moment I have Critical Altitude at 12000 ft, where with 42 inches of mercury manifold pressure, WEP /non-WEP makes no difference. I have also reached the point where I get the same power values for a given PM pressure, be it implemented with WEP or non-WEP, so the .air file seems viable. The only thing that remains to be decided, is where to put the dividing line. It would be possibe anywhere, with such a wide range of available power types.

Any distribution is unreal anyway, but your answers have not gone into that, so I can interpret it as something not so important which can be done as one would wish.

I know Wikipedia is unreliable, but as I have said, it happens to reflect a piece of information available elsewhere, which presumably was reliable.

Now, it gets much more confusing and complicated when reputedly reliable sources no longer seem to be such, and unfortunately I have neither the insight, knowledge or patience to delve into the matter any further. Wading through so much information to decypher the puzzle isn´t getting me anywhere.

Thus, I´ll stick to what I´m doing now - the 1325 Hp -D2 version, without modifying the 20mm cannon. This cannon also had its advantages: Greater reliability and firing rate, a straighter trajectory arc, and more numerous supply of rounds, reasons for which the Russians seem to have actually preferred it as well.

Coming back to performance, I´ll get somewhere good enough, I´m sure, even if I can´t understand the performance graphs because of their different layout. They are, as you say, not so difficult, but then again, there isn´t one for the -63 engine.

Anyway, I´m more interested in improving the model itself than in the other details, so I´ll get on with it.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I am sure your flight model will turn out pretty well whether you decide to change things or not.
As I commented earlier, a lot of is a matter of choices on how to do things.

My earlier assumptions (based on a comparison of the Specific Engine Flight Charts from the P-39D and the P-39K was that there were no real differences between the military ratings between the -35 and -63.
The charts and notes on those charts state in plain English that although "Normal" power ratings are the same, the Military Ratings of the -63 are greater and we obviously know that the War Emergency Ratings are much higher for the -63 engine than for the -35.
I have already corrected my earlier assertions several posts back.

Your answers also mentioned that a) early engines would not have had such powerful superchargers as later ones, so I supposed that one would eliminate the higher MP values, to which you commented that further research revealed that in effect, 1550 Hp were indeed possible, and higher MP values would be in order.What is true, of course, is that later engines had better high altitude power, but the D-2 was not a later engine, so that wouldn´t matter.

At the moment I have Critical Altitude at 12000 ft, where with 42 inches of mercury manifold pressure, WEP /non-WEP makes no difference. I have also reached the point where I get the same power values for a given PM pressure, be it implemented with WEP or non-WEP, so the .air file seems viable. The only thing that remains to be decided, is where to put the dividing line. It would be possibe anywhere, with such a wide range of available power types.

We actually have discussed the supercharger issue many times, so I believe you are just not recalling what you already know. Let's have a go at it again and see if it seems familiar:
Critical Altitude of 12,000 feet means that the supercharger can maintain Sea Level boost pressures up to that altitude.
WEP for these engines is simply a higher Manifold Pressure setting than the normal Military Power maximum but above Critical Altitude, it doesn't matter how wide you open the throttle, the Supercharger simply cannot pump enough air to raise MP beyond Sea Level Military Power.
Now consider what this means.
BELOW Critical Altitude, the Supercharger DOES have the ability to boost higher than Military Rating.
At Sea Level, the Supercharger has enough excess capacity to boost much higher than the structural limitations of the Engine.
The V-1710-63 was a more sturdy engine than the V-1710-35 so it was allow to run up to 60 inches Hg down low while the V-1710-35 was only able to run about 52 inches Hg.

In fact, Pilots of the P-40E and P-40K with very similar engines (-39, and -73) were claiming to run 66 or 70 inches Hg for prolonged periods without engine failures. Allison's tests showed that 70 inches Hg was unlikely without a LOT of ram effect, but 66 inches Hg would be giving

[Ivan]

Didn't mean to Submit Reply.
I am unable to edit that post for some reason. Internet access here is VERY unreliable.

...66 inches Hg giving 1745 HP @ Sea Level or 1770 HP @ 2000 feet.
Allison then agreed to update the War Emergency Rating to 60 inches Hg which gave 1570 HP.
This is pretty close to the WEP setting for the V-1710-63,

There is no contradiction here. All of these Engines did not really have "Enough Supercharger" for good altitude performance, but that did not stop them from running enough boost at low altitudes to destroy the Engine. This is generally true of just about any of the supercharged combat aircraft of the time.
Understand what this means.
This is important.

Regarding Wikipedia:
Wikipedia is unreliable.
Note that one of the references on the P-39 Airacobra page is back to Joe Baugher.
If you look at Joe Baugher's site, you will see that some of the text from Wikipedia is lifted straight from his site.
Note also that Joe Baugher listed enough P-39D serial numbers to show that there were a bunch more than 60 aeroplanes. The Wikipedia folks should have looked at that section too.
Note also that among Baugher's references is a book called "Airacobra Advantage".
This book has some useful information but also has a LOT of inaccuracies. I don't happen to have the book handy at the moment but one memorable quote from the book is "The slowest P-39 was faster than the fastest P-40" which is clearly incorrect. (Think Merlin P-40 and P-40N.)
Some Wikipedia pages are pretty good. This is not one of them.

I know in general what I did so far with my P-39F, but I also don't have my test notes handy at the moment and I don't even claim that what I had was anywhere near a final version. It just happens to be as close as I can get with what information I currently have. I see no point in posting specific data that will change the next time I run a test session.
One thing I can tell you though is that at the moment the Critical Altitude for my P-39F is slightly above 12,000 feet. It was necessary to get the proper maximum speeds and the difference is fairly small.

- Ivan.

[aleatorylamp]

Hello Ivan,
At certain times, Internet access is terrible, so I always make a backup copy of my post before submitting!

I had already seen Baugher´s information before, and I already knew Wikipedia is unreliable. There are also a few other sites, probably basing themselves on Baugher´s information as well which then will also be incorrect. Anyway, I´ve gone off using the 37mm cannon, so that won´t matter.

Thanks for the information on the sturdier -63 Allison engine. That would account for the higher MP numbers in the factory specification sheet and in the test reports, so that bit of information does seem to be reliable.

The fact that Allison engines could whithstand 60 Hg MP is of course interesting, and seems not to be a mistake on the factory specification sheet, which seems to be updated for 1944 then. Also interesting is that 66 or even 70 Hg could be also be inflicted on the engine, probably leading to an early destruction, but I will not put this into the .air file for obvious reasons.

With my .air file at sea level, I´m getting 358.3 mph, and 1551 Hp with 57.35 inches of mercury,
and a T.O. Pwr of 1325 Hp with 51 Hg, giving 332.5 mph. Then, with 42 Hg, I´m getting 1105 Hp and 309.6 mph. This would indicate, as I said before, that the .air file seems viable.

From your comments I´m led to believe that the 12000 ft CA for Military Power on this sheet and the 11800 ft on the test report pencilled in blue, seem not to be so reliable. Normal Power seems to be even lower, at 10800 ft.

It seems that you prefer using 12500 ft as CA for Military Power, and incidentally, yesterday I had already started on doing trials with CA at 12500 ft.

Before, at 15000 ft I was getting 1008 Hp with 37.8 Hg giving 354 mph, and now with the new CA , I´m getting1020 Hp at 38.2 Hg, giving 358.2 mph.

The explanation as to what happens in the engine with its blower at CA is of course a good reminder, and yes, I remember, but another thing is what MP to expect at CA.

Normal rating is in one case quoted as 37.8, and in another, 37.2 for the same engine, but is seems a bit strange to only have normal power going up to only that MP in CFS, and having ALL other power types in WEP.

15-min Militaty Power is in one case quoted at 42 Hg, and in another, at 44.2. This difference also makes quite a large difference (here it is the factory spècification sheet and the blue-pencilled test report again). Other test results have 45.5 or 45.9 Hg for Military power ar Critical Altitude, depending on the type of manifold, so this makes an even larger difference.

Then, Take-off power is at 51 Hg, Standard Emergency power at 55 Hg, WEP at 60 Hg. Obviously ALL of this should go into WEP. To distribute the power types a bit more practically, I had tried out Take-Off Power as normal, but it wasnn´t very logical to have it there, so I´ve put it back into WEP.

I remember for the Baltimore, we included the 15-min power ratings into the normal throttle lever travel, and the 5-minute power ratings into WEP. The ranges of power types were not as ample as on this engine though.

So, following this criteria and placing the division between WEP and non-WEP so as to include 15-min
power as normal, still has the problem of deciding whether this is going to be at 42 Hg or at 44.2 Hg.

I´ll put it in as half way between the then, at 42.6 Hg with CA at 12500 ft, or even 45, if one were to include the extra manifold test.

Or better: I´ll foget the whole WEP/non-WEP issue, which seems not to have a decent solution anyway. There are too many contradictory pieces of information, and there are not enough possibilities offered by CFS to do it well.

Everything will go into the normal throttle lever travel: This way the pilot can do what he needs, set whatever power he needs, whenever he needs. All he has to do is wathc the Manifold Pressure gauge, and pay attention to the altitude. Of course, all kinds of abuse will also be tolerated, so that will depend on the criteria of the pilot. The engine won´t blow up anyway.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

It seems like every time I make a statement, it gets misinterpreted and you head off in another direction.

This is starting to look like a massive case of "Deja Vu". I am not sure how you just got to where you did, but it does seem a bit amusing.

You also seem to know a lot about an experimental AIR file that I have never posted.
How did you come to the conclusion that my P-39D/F AIR file has a 12,500 feet Critical Altitude?
I am the one working on it and *I* don't know that....

One thing I can tell you though is that at the moment the Critical Altitude for my P-39F is slightly above 12,000 feet. It was necessary to get the proper maximum speeds and the difference is fairly small.

Fairly small == 500 feet??? Perhaps it is 200 feet and perhaps it is 1000 feet. I don't know because for this stage of testing it was not important because it will change anyway. I actually never tested for exact critical altitude on this version of the AIR file.

You mention a bunch of other test reports and factory specifications.
What I do have is the SEFC but the data there appears to be inaccurate or incomplete which is why I haven't posted it.
I have not actually collected much information on the V-1710-63 yet because I am not working on it yet, so you certainly have more data.

- Ivan.

[aleatorylamp]

Hello Ivan,
We seem to have a case of cross purposes here, otherwise I can´t understand why it is a second time you say I am misinterpreting you, and heading off in another direction. Where you say I´m going, is obscure, but I´m glad you find it a bit amusing, because it means that at least you are enjoying this.

Your mention of CA being a little higher than the 12000ft I mentioned, led me to the assumption that you were suggesting more or less 12500. If it were any less, it would possibly not have been worth mentioning.

You yourself have just said that it could possibly be between 200ft or 1000 ft, so my assumption seems to lie within realistic possibilities.

So, no, you haven´t posted an experimental .air file, at least not as far as I can see, and I don´t have a crystal ball to look into an experimental .air file of yours.

Now I have to establish if the speeds I am getting with CA at 12500 ft are better or worse than I was getting with CA at 12000 ft. They are a little faster, but I have to see if this is correct. For this, I have to draw a set of different graphs, which will take some time.

Then, the certificate mentions 880 Hp for Normal Power, so I could be OK there

Another matter is the Manifold Pressure for Military Power. It seems to be more correct at 44.2 (from the pencilled-in-blue chart) instead of 42 (from the Allison Engins Operations and Maintenance Manual .pdf. Even though CA is now at 12500 ft, MP is still not so high there, so maybe I´ll have to push it up further.

The other performance report I mentioned as giving yet another two manifold pressure values of 45.4 and 45.9, is for a test with a manifold type with "T" Screens on a P-39D-1 with the -35 engine, reported on the Performance Test Page, where all the other graphs are on.

It is the 5th. test down from the top, in case you are interested. I´ve already posted the link before,http://www.wwiiaircraftperformance.org/P-39/P-39.html , but I know you are not working on the same aspects as I am yet, although it does have to do with the engine you have on the version you were starting out from, although it isn´t your -F version.

Updated paragraph: Incidentally, the 3rd. report down on this page also refers to a -35 engine on a P-39D model, and quotes CA for Normal Power is at 13100 ft, and Military, at 13800ft.
What fun! CA seems to be a very flexible parameter, to be put wherever looks convenient by anyone interested! It also seems to vary whether you are rating for climb or for level flight... How nice...

Anyway, what is true, is that whatever the setting is for MP, and wherever you place CA, these two things are crucial to get anywhere decent. Possibly it has to be 44.2 in this case, and maybe even higher than 12500 ft. Update: On decyphering the lines on the performance graphs, some of these also seem to point towards higher values, although not all of them do.

Quite apart from this, is where exactly to place the division between normal power and WEP for CFS1, and/or whether it should be implemented at all, given the complication of having so many different power types specified.

OK, then. As yet the two questions remain unanswered, and await further developments.

Cheers,
Aleatorylamp