Analysing and modifying the AFX file with QBasic.

[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]

Power compromises

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]

Wikipedia Article Errors

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

 

[Ivan]

Hello Aleatorylamp,

Here is why I think when I post a reply, you interpret it strangely and you go off on yet another weird direction:

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.

I am really not sure how you arrived at that solution but one day later you are back to doing something else.

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.

This is also a strange sequence to choose to do things.
How did you select 12,500 feet as the Critical Altitude?
I am not saying this is correct or incorrect because I don't really know, but if you are finding the Manifold Pressure to be too low at 12,500 feet and need to raise it, then either you really raising the Sea Level Manifold Pressure.... Or you are raising the Critical Altitude so the 12,500 feet value you have listed is just a particular test altitude and not really the Critical Altitude.

You mention a certificate which I do not know about. If it is a modern Type Certificate, just keep in mind that modern operating conditions are not the same as they were during war time.

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.

Why did you choose to reference a report on a P-39D-1 which does not have the same version engine as the one you are building? Why did you pick a report on an engine with an experimental intake manifold? Power outputs, Manifold Pressures and even Critical Altitudes would be pretty useless information for a standard production aircraft. You are wasting your time with this report unless you are just trying to get some extra historical perspective and you would have to come to your own conclusion as to how this report is useful.
I believe it is entertaining but not useful.

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...

This is a more useful report but still not representative of a service configuration P-39D.
From your summary of a summary, it appears things do not make sense, but if you read the actual report, the numbers make perfect sense.
Read the last line of the "Purpose" section of this report.
It says "Individual intake port backfire screens not installed in engine."
Now you would have to ask: What does this mean and does it alter the data I am looking for?
Simple answer is yes. It significantly changes the numbers and was probably the reason for conducting this test.

If it is specifically listed then it probably was not the normal configuration of this engine.
It would also be the removal of an obstruction from the intake system and without the obstruction, it makes sense that there would be better air flow and Critical Altitude would increase.

Now if you look at the table for Critical Altitude for Normal Power and Military Power, you will see that Normal Power is 2600 RPM while Military Power is achieved with 3000 RPM.

THAT was the difference, so it all makes sense if you read the information in the report.

- Ivan.

 

[aleatorylamp]

Hello Ivan,
It´s interesting when things are clarified as to why they are stated when they are stated.
Thank you very much. Now I can ignore the other details for their experimental nature, and hang on to the seemingly more pertinent ones. Given the circumstances, I suppose it could be questionable how much use my participation in this discussion can have...

As for the engine certificate I mentioned, it´s in the Allison Operations and Servicing Manual which I have given the link to a few times.

The graph with the performance comparison with/without Belly-tank indicates at the top in red ink, that there is little difference between the D and the K models. Consequently, using a D model in the way of a guide line, for want of anything else, I´d say is not too far fetched.

Raising the Boost Gain setting, thereby increasing critical altitude to try and match the general layout of the performance graph (because at the moment, the altitudes for points in the top two thirds of the graph are maybe not high enough), has the drawback of also increasing power elsewhere, so it is not really that easy.

Updated paragraph:
After comparing my graphs to the existing Military Power Graph mentioned above, it looks like Critical Altitude is better slightly higher - thanks to a small increase (0.06) in Boost Gain. I thought it would be negligible, but it wasn´t. Now I´ll check if changes elsewhere need correcting.
Regarding Manifold Pressure, 42 Hg seems more fitting for the -35 engine, and 44.2 Hg looks better for the -63 engine. So, all this hasn´t been totally futile, and the contribution is not as bad as I´d begun to fear.

I was going to post a picture of the graph, but the shape is very much reminiscent of what you would be able to find inside the top part of a bikini, so I have refrained from doing so.

Regarding the powers that go into WEP or non-WEP: The fact that I mentioned one idea, and then another one after that, does not mean I must necessarily discard the first and only work on the second. It just means that for the moment, either possibilities exist.

The fact that I keep mentioning WEP and how or whether to implement it, has one clear reason, which obviously you don´t think is important, otherwise you would have probably commented on the subject.

What kind of T.O. Power will be supplied conveniently by the .air file? ..."Posted by been there before?"... Been there before? Of course, but not in the same way: The Baltimore´s T.O. Power was close enough to WEP so as to be included with it, but here it is considerably further apart.

We can conveniently place military power within the normal throttle lever travel, to control it easily, and WEP can be separated equally conveniently, simplified into one instead of two emergency powers. However, Take-off Power is in a limbo between the two.

- Putting it into the normal throttle lever travel will distort normal (incl. military) operation.
- Putting it into WEP will be distorting T.O. Power.
- Limiting WEP to T.O. Power will be distorting WEP.

Eliminating F10 WEP setting altogether and having everything in the normal throttle lever travel has its drawbacks too, and complicates things because everything will have to be done manually, having to look at the Manifold pressure gauge each time a different power is needed.

Actually, I am very tempted to withdraw from the discussion at this point. It is becoming increasingly clear that I am quite out of my depth in some of the pertaining issues, and can´t contribute as much as I´d have liked to. At any rate, I´ll try to avoid further tedious to´s and fro´s between the pro´s and con´s of doing things this way or that.

So, it could be a matter of cobbler, to your shoes! I had better put my efforts more into building, and less into speculations on power parameters that I´m not so good at.

Cheers,
Aleatorylamp

 

[Ivan]

Hello Aleatorylamp,

It seems to me that you are looking for a set of recommendations and answers from me or at least an approval for what you are doing. You have built plenty of aeroplanes so why is this one any different?
I am treating my project a little differently because I am seriously interested in the peculiarities of the P-39 Airacobra and want to understand it better.

You are asking for information that I simply don't have at the moment. I am not really working with the same engine as you are although some of the information I am coming across while trying to figure out what you are doing will probably end up helping my P-39K AIR file whenever I start it.
It might even help the P-39F AIR file....
The problem is that at the moment, they are just ideas and I don't know whether they are workable or not.

If you are wondering what I might be working on, I can tell you that last night I was reading through the test reports you referred to and then hunting for information about the exact Propeller that was used on the P-39F and the P-39K and trying to figure out how I would address those issues when they came around.
A few days ago, I was reading through another descriptive report on the handling characteristics of a P-39D and thinking of how to adjust my flight model to match.
A day before that, I was reading through a short magazine article describing how great the NEW P-39 Airacobra was. The article in general was garbage but it gave a couple pieces of information I have not found elsewhere.
An excerpt from Monografie Lotnicze gave me a correction to my assumption about the ammunition supply of the P-39C. It took Google Translate to be understandable though.

A few of the items I have come across in the last week are very likely to affect the maximum speed and have slight effects on the engine power as well, so if I had given you some test results and specifics from a week ago, they would not agree with what I will have in a few days.
I don't like to post something and then post a retraction in a few days and that is for something I am working on.
You are asking for something that I am NOT working on. I have a bunch of guesses, and am pretty sure what is NOT correct, but it is harder to pin down what ACTUALLY IS correct.

I made a comment earlier that EXACT power and maximum speed were unimportant at this stage and I meant it.
I believe you like working on Engine Power and Maximum Speeds because they are relatively easy to tune, so you go there first. I do that as well, but I don't get hung up on exact numbers early in the design process because I KNOW that other things will affect them.

If you believe that it is time to tune for precise Power and Speed, then perhaps you can tell me a few other things about your current flight model:
1. What is your Zero Fuel Weight?
What is the Fuel and Ammunition load and what is your current Loaded Weight without Bombs?
2. Where is your Center of Gravity Empty and Loaded?
3. What is the Oswald Efficiency value you are currently using? How did you get it?
4. Where is the Center of Lift of your Wing?
5. What is the maximum Coefficient of Lift and what does your CL Graph look like?
6. What is the Diameter, Pitch Range, and Gear Ratio of your Propeller?
7. Where did you get your Propeller Tables (Records 511 and 512)?

I have only reasonable guesses for a few of these numbers in my own AIR file and some very reliable numbers for some of the others. There are also a few other numbers that may also affect your level speed.
I am still trying to chase down some of these numbers that I know I will need.

There is a LOT of information out there about the Airacobra.
This is actually a great opportunity to learn about the aeroplane and to try to understand how the information is presented in official test reports. I also am learning as I go though this isn't the first time I have seen reports of this type.

- Ivan.

 

[aleatorylamp]

Hello Ivan,
Thank you for your post, and the reasoning explained therein. Obviously your project is of a different nature, and requires a completely different approach, which I understand, involving considerably more detailed research to start off with.

I am just trying to achieve a reasonably usable .air file without so much complication. There is a lot of information available to be sifted through, as you say, and a lot of it is contradictory, especially in important areas like CA and MP. Hence my experimentation with different values here, and my comments on the thread. As it is, I´ve progressed a little further, and it´s looking better.

I hadn´t expected that commenting on these factors would be so complicated, and that it would be difficult or impossible to deal with in a simpler fashion, and I don´t want to interfere with your project, which is going along rather different lines.

I have almost all the answers to list of questions you posted, but I´m sure it will involve a lot of time and effort on your part to go through, and it will definitely interfere with your project, so I prefer not to do this.

It would mean hijaking too much of your time for something that is not so important.
I don´t even know yet if the result of the model upgrade will be good enough to upload.

Thank you very much anyway!
Cheers,
Aleatorylamp

 

[Ivan]

Hello Aleatorylamp,

I just figured out why you thought I had told you the Critical Altitude of my P-39F was 12,500 feet.
Back on June 24, I posted:

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.

This just happened to be my standard test altitudes rather than any special test at 12,500 feet.
As you probably already know, I test at 2,500 feet intervals except that the lowest altitude test is normally at 500 feet.
I remember that on that particular day, I also tested at 75 feet altitude and at 12,000 feet altitude which I just did not comment on.
I actually don't know if 12,500 feet was best speed on this day. It might have been higher, it might have been lower.
I didn't try to find out because I was just going for something in the correct range rather than something specific.
It doesn't actually matter any more anyway because with the changes I need to make, those results are certainly no longer valid without another test and edit session.

- Ivan.