Analysing and modifying the AFX file with QBasic.

[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.

[aleatorylamp]

Hello Ivan,
I understand that previous tests in your case would slowly be superseded by new data you are uncovering, and I now also remember the initial test results you posted.

The different Boost Gain values I am testing, give me different critical altitudes and maximum speed points (for military power) on the graph, that I am comparing to the shapes of the P-39 performance graphs shown at the bottom of the Air Coprs War Department performance test page.

For the moment, I´ve done 3 tests, and from what you are posting, I gather that you
may be interested generally in the results I am getting. If not, just ignore these figures!

Boost Gain at 2.3 and MP at 0.42 Hg for military power.
309 mph at S.L. and 361 at 12500 ft. (peak just above 11000 ft).

Boost Gain at 2.36 and MP at 0.44.2 Hg for military power.
315 mph at S.L. and 367 at 12500 ft. (peak at around 12500 ft

Boost Gain at 2.47 and MP at 0.44.2 Hg for military power.
315 mph at S.L. and 372 at 12500 ft. (peak at 13000 ft).

What is turning out to be very practical and satisfactory, is that Boost Gain increases give consistent and proportioinal power increases as altitude increases, and do not require other adjustments.

I have conveniently set Emergency Power Press Change Rate to Zero, and this prevents any manifold pressure value distortions when altering max. and WEP manifold pressure parameters.

What I´m also seeing, is that my performance at greater altitudes is on the low side, and in the middle area of the graph above and below CA, it tends to be a little greater.

If I remember correctly, this is normal for CFS1. In the case of the P-39, it will not be worrying, because this aircraft was not really designed for altitude performance anyway, so simmers will probably use it for mid- and lowaltitude dog-fighting and ground attack.

At the moment I´m using the .air file of the P51d, using the correct specifications as far as power, weights, dimensions and fuel for the P-39-D2 and the engine specs of the Allison V-1710-63 (including its gear-ratio to the propeller) are concerned, as well as the Dp files with the different rounds for the 20 mm motor cannon in the nose, the two 0.50 ca. nose guns, and the four 0.30 cal. wing-guns: 60 rounds for the cannon, 400 for each nose-gun, and 1000 for each wing gun.

The propeller is for the moment that of the P51d, but I also want to see how the propeller from your BV-141 compares. I´m not using the one from the P47d because it seems too big, but possibly the Spitfire one would also be good to test.

So this is as far as I´ve got, and it is more or less going as I had been expecting.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I am glad you are getting some satisfactory results with your AIR file.
What I still don't understand is why things seemed so upsetting over the last couple days.

If you are starting with the stock P51D AIR file, there are a couple things to watch out for.
One is that the Oswald Efficiency number there is ridiculous in comparison with other comparable aircraft.
The second is that you should probably check out the Propeller Moment of Inertia.
I don't know what the specifications are for the propeller on your P-39D-2, but if it is anything like the propellers on the other early P-39's, it is a whole lot smaller and lighter.

I actually DO have information on the Aeroproducts Propellers used on the P-39F (A632S-A1 or A632S-B1) and on the P-39K (A632S-A2 or A632S-B5) because I found a pretty good manual online on Aeroproducts Propellers.
I have not found the equivalent for the Curtiss Electric Propellers used on the version you are building.

In the Warbird Tech book, I also found one reason why the weights of the P-39D-2 and the P-39K are different.
There is of course the Cannon difference and also the Pilot Weight allowance, but apparently the armour thicknesses were different in some places.

- Ivan.

[aleatorylamp]

Hello Ivan,
The thing was that I wasn´t getting anywhere with mainly two pieces of contradictory information, CA and MP, that didn´t tally. I would have been able to deal with that better had I understood the performance graphs sooner, the same graphs that you insisted several times that I should analyse. Although you didn´t have the answers I needed, approximations could be extrapolated from the graphs, and I couldn´t see that yet. It was a simple matter of exchanging the axes and it became clear.

This way I came to the conclusion that the 1944 Factory Spec. Sheet Militaty Power at 0.42 Hg and Normal Power at 37.2 for both the -35 and the -63 engine, was probably mistaken, especially if the same data sheet gives different WEP and T.O. MP pressures for each engine. It then became clear that 44.2 and 37.8 Hg mentioned on the blue-pencilled data sheet was correct for Military Power on the -63 engine. Once that was given, all else followed!

Thank you for the information on the P51d .air file - you have more experience with adapting .air files so that is very useful. I thought of using the Spitfire one, but didn´t, as you never seemed to use it. I´ll try and adapt the P51d .air file accordingly.

I was impressed by the fact that armour was one of the main plus points on the Airacobra, so it could take a lot of punishment. I have seen that the -K had more armour and a different propeller. With the confusing amount on data on the many different models, from what you say I ended up using the wrong one! Thanks for that too.

The armour is reflected in the .air file´s dry weight, and the propeller I have still to deal with. With a lighter weight and a smaller, faster propeller on the -D2, it will be interesting to see what comes out in the tests, comparing it to the -K model.

I was testing propeller diameters, and saw diameters between 10.5 and 11.1 ft, the latter being geared 2:1. I used the second one, because it worked better, but from what you are saying, it appears that the first one would be more correct on the -D2 model, in this case with gear ratio at 1.6:1, I think. I still have to check, and that is going to be the next test, possibly also using the BV-141 propeller tables.

Update:
I did a quick test for a 10.5 ft diameter propeller, and curiously results for Military Power were as follows:
13000 ft: 370.8 mph ( - 4.2 mph)
_6000 ft: 343 mph ( + 11 mph)
__500 ft: 311.7 mph ( -3.3 mph)
This performance would seemingly still be plausible for -63 engine performance, but I have to draw up a graph and see the shape. It looks like a smaller, lighter propeller does better at mid-altitudes, and a bigger one works better at low and high altitudes.
I also tried out the BV-141 propeller with the 10.5 ft diameter instead of the 10.8 it came with, but as was probably to be expected, shortening its diameter seems not to have been so convenient. Results were -7, +5 and -16 mph. Diameter changes would need J factor adjustments, I expect.

OK, then.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I guess you are seeing why I told you that you could get quite a lot of information from those graphs.
I am glad you understand them now because they are hardly unusual. That format is probably the most typical.

I still haven't really looked at the Allison E Series Manual, but whenever there is a contradiction between a general summary and an aircraft specific document, go with the aircraft specific document.... Unless you are pretty sure you KNOW better.

From what I have been able to tell, the P-39K had a different propeller than the P-39D-2 but actually had thinner armour in certain places which meant it carried about 20-50 pounds less weight in armour overall (if I remember correctly). The difference was more than made up by the weight of the cannons though.
I am guessing that the extra armour was used as ballast to bring the CoG closer to where it should be without the heavy cannon up front.
There is a table in the Warbird Tech book that describes the armour locations and weights pretty well though the table is very hard to read because of the quality of reproduction from the original manuals.

I was testing propeller diameters, and saw diameters between 10.5 and 11.1 ft, the latter being geared 2:1. I used the second one, because it worked better, but from what you are saying, it appears that the first one would be more correct on the -D2 model, in this case with gear ratio at 1.6:1, I think. I still have to check, and that is going to be the next test, possibly also using the BV-141 propeller tables.

Update:
I did a quick test for a 10.5 ft diameter propeller, and curiously results for Military Power were as follows:
13000 ft: 370.8 mph ( - 4.2 mph)
_6000 ft: 343 mph ( + 11 mph)
__500 ft: 311.7 mph ( -3.3 mph)
This performance would seemingly still be plausible for -63 engine performance, but I have to draw up a graph and see the shape. It looks like a smaller, lighter propeller does better at mid-altitudes, and a bigger one works better at low and high altitudes.
I also tried out the BV-141 propeller with the 10.5 ft diameter instead of the 10.8 it came with, but as was probably to be expected, shortening its diameter seems not to have been so convenient. Results were -7, +5 and -16 mph. Diameter changes would need J factor adjustments, I expect.

I think you need to do better research here. I KNOW there are at least three reduction gear ratios used in the P-39 series of aeroplanes and the propeller diameters start smaller than 10.5 feet and go larger than 11.1 feet.

You are seeing now what I was commenting about a few days ago:
You need to pin some of these numbers down before fine tuning for Engine Power, Critical Altitude and Maximum Speed. If you don't do that, you just end up doing the same tuning over and over again which is a real waste of time.

You are also seeing why the data for the V-1710-35 engines isn't terribly useful for proving much about the V-1710-63 as far as maximum speed. The Propellers have different parameters and the Reduction Gear Ratio is different. According to America's Hundred Thousand (and other sources), the P-39D-2 and P-39K both used a Reduction Gear Ratio of 2:1 as compared to the 1.8:1 for the D, D-1, and F.

One amusing thing I found in the Aeroproducts Manual is that the Propeller for the P-39K wasn't really rated for the level of power that the the V-1710-63 was putting out at anything past Military Power.

Last night, I made some updates to my AIR file:
Zero Fuel Weight is now down by 9 pounds which should not make any significant difference but it is based on what I believe is a better estimate of the Oil Capacity for the P-39F. 9 pounds is the equivalent of about 1.5 Gallons of Fuel or just over 1 Gallon of Engine Oil.

Now here is the fun part: I increased the Propeller Diameter by 1/2 inch in some places and nothing in others (Those places would only take whole numbers for inches), and speed dropped by 1 MPH.
Nothing else I changed should have had an effect.
I am pretty sure then whenever I decide to work on the Propeller Tables, there will be even greater changes.
This is another reason why I am not that concerned with exact maximum speed at this point.

By the way, I was somewhat curious after all the discussions about the Critical Altitude of my P-39F AIR file so I ran a fairly quick test.
My P-39F AIR file has a 12,350 feet Critical Altitude.

I also tried out the BV-141 propeller with the 10.5 ft diameter instead of the 10.8 it came with, but as was probably to be expected, shortening its diameter seems not to have been so convenient. Results were -7, +5 and -16 mph. Diameter changes would need J factor adjustments, I expect.

Now can you explain how you would go about adjusting Advance Ratio (J) AFTER changing the Diameter????
You need to go back and look at how Advance Ratio is calculated.
I don't believe Advance Ratio is really the problem anyway. You should be looking at the Propeller Power Coefficient in my opinion.

- Ivan.

[aleatorylamp]

Hello Ivan,
Interesting findings!
So it was the other way around with the armour plating weight on the models. Sorry!
Ok, and thanks on the 2:1 prop gearing - as you say, it was the same for the -35 and the -63 models. The D1 and the C models were different, and then later the -83, but not the D2.
I believe the propeller diameter was also the same, but I can´t find the source that stated the size anymore, so I´ll have to continue checking.

Update: I found a 1939 plan of a -D model with a measurement is 10.8 ft, but I´m still looking.
Provisional tests with the 3.6 inch shorter diameter revealed the following difference
to the 11.1 ft prop:

10.8 ft diameter prop:
13000 ft: 373 mph ( - 1.9 mph compared to 11.1 ft prop)
_6000 ft: 335 mph ( + 2.5 mph)
__500 ft: 312.6 mph ( -2.4 mph)

...so the difference is not so pronounced at these altitudes.
But the interesting thing is the following, higher up, so the curve is a little more correct there:

20000 ft: 358.9 mph ( + 2.9 mph)
25000 ft: 332.5 mph ( + 4.5 mph)

Anyway, I still have to confirm whether 10.8 ft is the correct diameter for the -D2.

P.S. Then, Joe Baugher´s site mentions that on the D model, a different 10 foot 5-inch Curtiss Electric propeller was fitted. (different from the previous C model.). This would further improve high altitude performance to fit the curve even better...
At 25000 ft it´s 10.8 mph faster than with the 11.1 ft one.

P.P.S: Also, All Aero.com mentions that the Curtiss Electric propeller on early Airacobras was
10 ft 4 inches in diameter.

So much for approximate information - what´s going to be true? I wonder.
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

This is getting amusing again, but in a silly way.
Regarding Reduction Gear Ratios, you really need to read my last post more closely because your interpretation is different from my intent. Either I misstated or you misunderstood.
Whatever the cause, it is happening too often.

Do you remember my suggestion a few days ago which I quote below?

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)?

It is pretty clear from your last post that you have no reliable numbers for Item 6 in the list and the fact that you just finished posting a page of not very useful conclusions from testing that simply isn't worth the time until you have some good Propeller Specifications to use in the AIR file.

There is really no point in doing any testing and tuning until you have done your research.

- Ivan.

[aleatorylamp]

Hello Ivan,
Ha ha! How amusing. How silly. How quaint.

So you don´t think I should have posted the different performance results
given by 3 different propeller diameters quoted for the same model.

By now it should be quite clear that in view of inexact, contradictory and
sometimes useless information, experimentation to find out which information
can be used, is a requirement for any reasearch to be conducted, and comments
as to this experimentation not being very useful, are superfluous, to say the least.

I am not continuing this discussion any further. I don´t see the point.
Aleatorylamp.

[Ivan]

Hello Aleatorylamp,

You are correct, this discussion is getting pretty ridiculous.

This is what I wrote:

You are also seeing why the data for the V-1710-35 engines isn't terribly useful for proving much about the V-1710-63 as far as maximum speed. The Propellers have different parameters and the Reduction Gear Ratio is different. According to America's Hundred Thousand (and other sources), the P-39D-2 and P-39K both used a Reduction Gear Ratio of 2:1 as compared to the 1.8:1 for the D, D-1, and F.

This was your response:

Ok, and thanks on the 2:1 prop gearing - as you say, it was the same for the -35 and the -63 models. The D1 and the C models were different, and then later the -83, but not the D2.

???? Huh???? How did you get THAT interpretation from what I wrote????
WHERE did you get "same" from my statement?
This tends to happen a lot which is pretty frustrating from my point of view.

A couple posts back, we have this:

I also tried out the BV-141 propeller with the 10.5 ft diameter instead of the 10.8 it came with, but as was probably to be expected, shortening its diameter seems not to have been so convenient. Results were -7, +5 and -16 mph. Diameter changes would need J factor adjustments, I expect.

J is the notation for Advance Ratio (see Wikipedia page link below).
https://en.wikipedia.org/wiki/Advance_ratio

The formula for calculating Advance Ratio is:

J = V / (n * D)

where

V is True Air Speed
n is Propeller Rotational Speed
D is Propeller Diameter

As I interpret it, What you stated was that you would adjust the Propeller Diameter and then adjust the Advance Ratio. How would you do that?
Would you adjust the Propeller Rotation Speed by changing the Gear Ratio or Engine RPM?
or
Would you just fly slower or faster to change the Air Speed?

Either this was a typographical error or after all this time you still don't understand what Advance Ratio means.
That would be unfortunate because I have attempted to explain Propeller Tables based on the assumption that you did understand the terms being used.

So you don´t think I should have posted the different performance results
given by 3 different propeller diameters quoted for the same model.

By now it should be quite clear that in view of inexact, contradictory and
sometimes useless information, experimentation to find out which information
can be used, is a requirement for any reasearch to be conducted, and comments
as to this experimentation not being very useful, are superfluous, to say the least.

Whether you SHOULD post your data is entirely up to you.
My comments were just to indicate that I do not believe that the data has any significance.
Playing with AIR file parameters just to see what happens is not an unpleasant way to spend time though.
I probably should not have commented because sooner or later I am sure you would have found out that you were no closer to a workable solution with this method.

The reason why I believe this would not work is because the Propeller Tables you are using are not an infinite series of angles and the interpolation between angles will give strange results fairly often.

There is much more background in the Flying Swallow thread.
http://www.sim-outhouse.com/sohforum...Flying-Swallow

The effect of this is that for the 2400 RPM speed run, the Propeller Pitch is 35 degrees but at 2500 RPM, the Propeller Pitch drops to 34 degrees, so the Constant Speed mechanism is working as it should.
The problem occurs because of Interpolation.
In real life, there are an infinite number of curves between a pitch of 30 degrees and 35 degrees. On computers there is nothing between those two curves and all intermediate values are interpolated.
(See attached Screenshot)

All of the Propeller Efficiency (Record 511) Graphs will have this problem; It is only a matter of degree depending on where this happens in the graph. I was hoping to give this kind of explanation in a new propeller thread but it needs to be here as well for the discussion to make sense.
Note that this is the first Propeller manufactured by Ivan's Propeller Workshop.

Also, if you are working with the stock P51D AIR file, you will find even more odd results depending on the propeller pitch range you are using.

That is only part of the reason why conducting experiments inside the simulator and using performance results to guess at the propeller parameters will not work well.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thanks for your comments, but please, do let´s leave it. I don´t want to continue with this discussion.
I don´t have the energy for it, and I understand this back-and-forth is also frustrating for you. It´s too complicated, too confusing, too time consuming, and too tiring. I was also going to add, too time wasting, but that wouldn´t be true, as I have to admit, the present .air file is working better than the one I had 10 days ago. Thank you very much.

Inaccuracies from a different sources gave rise to my confused and confusing comments on the gear ratio. What is important is that I know it´s 2:1, and I´ve had it at that all the time. After your comments on there being smaller, lighter propellers on earlier models, I was led to believe that I also had the gear ratio wrong, which I didn´t. It was only the diameter I was using, which was too large.

Then, I supposed the reason for which your 10.8 ft BV-141 propeller wasn´t working well at 10.5 ft, was because it needed other adjustments, and I thoughtlessly and mistakenly mentioned advance ratio. You answered that adjustments would be more correct in propeller efficiency. Fine. I hadn´t acknowledged that, I´m sorry. However, I´m not about to do any of that because it makes things even more complicated.

I am conscious of the limitations of the P51d propeller I am using, but it is giving me satisfactory
results, especially now that I´ve tried out 10.4 ft diameter. As it is, it is a more workable solution. Altitude performance has improved quite nicely, and overall performance lokks to me quite fitting
for a -63 engine. It is good enough, so I´m leaving it at that for this re-worked model.

Far more important to me is now getting a 12 sided fuselage underway. I´ve done about half the length from the nose, and I´m fitting the cabin to it.

Must rush off! Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Then, I supposed the reason for which your 10.8 ft BV-141 propeller wasn´t working well at 10.5 ft, was because it needed other adjustments, and I thoughtlessly and mistakenly mentioned advance ratio. You answered that adjustments would be more correct in propeller efficiency. Fine. I hadn´t acknowledged that, I´m sorry. However, I´m not about to do any of that because it makes things even more complicated.

Actually that wasn't how I answered, but let's just leave it.

Good Luck on the Rebuild.

- Ivan.

[aleatorylamp]

Hello Ivan,
I meant to say that you had indicated the Propeller Power Coefficient - not Propeller Efficiency.
I always mix up the words of these two things.

Then, as both propeller graphs are used to set parameters depending on Advance Ratio, this is the origin of my originally mistaken comment.

Instead of saying " Diameter changes would need J factor adjustments, I expect." , I should
have said something like " Diameter changes would need changes in the propeller graphs depending on the J Factor involved ". I didn´t mean to say that the J Factors themselves had to be changed, but it came out that way.

Thanks for wishing me luck. It isn´t so easy, as you know, but it seems like the rebuild is coming
along fine. In this case I´m just doing it for myself, for the sake of enjoying the re-build process in itself, to perhaps discover different ways of doing things, the objective not being an upload.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

We don't all do things the same way. Do whatever satisfies you.
I am not about to start another argument.

I just spent a couple hours going through a Tech Manual for the Airacobra but in Russian.
My vocabulary is entirely inadequate, but I found the particular data I was looking for and some Propeller data as well.
Unfortunately, it is a Кертисс Propeller for the E4 Allison Engine which isn't useful for either of us.
So far, this is the most complete tech manual I have found for the Airacobra but reading Russian is very slow.

By the way, did you know that the P-39D had a welded Magnesium Oil Tank with a heating element mounted inside?

At the moment, I am going through the Armament section which has some information that doesn't quite agree with American manuals but may explain some of the data in the test reports.

- Ivan.

[aleatorylamp]

Hello Ivan,
Interesting. So that´s how they counteracted the increased oil viscocity/freezing problem they had during prolongued unpowered descent or dives. Prolongued dives... how long can you dive until the Earth gets in the way?

I also read that the Russians did several modifications to improve the model, which would account for differences in the manuals. I´m sure you have already read that they liked removing the 4 wing guns - some even came like this ex-factory.

With lower weight and greater rolling response, they gained maneuverability at the expense of firing-power. They seemed to find the precision of the 20 mm nose-cannon + 0.50 cal nose-guns quite enough. At least it´s what I read. They had a totally different approach to the model from the British way of looking at unusual, new things.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Interesting. So that´s how they counteracted the increased oil viscocity/freezing problem they had during prolongued unpowered descent or dives. Prolongued dives... how long can you dive until the Earth gets in the way?

I don't think so.
That is what Cooling Exhaust Flaps are for.

With lower weight and greater rolling response, they gained maneuverability at the expense of firing-power. They seemed to find the precision of the 20 mm nose-cannon + 0.50 cal nose-guns quite enough. At least it´s what I read. They had a totally different approach to the model from the British way of looking at unusual, new things.

This manual is for the Airacobra with the Allison E4 (V-1710-35) Engine which meant that it was most likely just describing the P-39D-1 that were Lend-Lease contracted by rejected by the British. The manual does list the 20 mm cannon as do some of the diagrams within.
Most of the Airacobras delivered to Russia were actually armed with the 37 mm cannon though.
For reasons beyond the scope of this project, the Russians never really seemed to like Wing mounted guns and removed them from Lend-Lease aircraft when doing so would leave a reasonable armament remaining.

- Ivan.

[aleatorylamp]

Hello Ivan,
OK, so either it doesn´t say in the manual what the heating element inside the oil tank was for, or you don´t want to say...

Anyway, I have half a mind of taking away the wing-mounted guns too!

I´ve finished the fuselage with a rounder cross-section, and fin and tailplane with airfoil cross-sections. now I´m working on the transparent canopy - although the pieces are don, they have to be distributed better to stop bleeds - and the pilot is still in the tavern.

It´s still difficult to get bleed-free screenshots. This battle is yet to come. Here are some pics!
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

The model doesn't look bad at all. Perhaps you should release it when you are done.

OK, so either it doesn´t say in the manual what the heating element inside the oil tank was for, or you don´t want to say...

I don't know if the manual indicates a purpose for the heating element or not. I don't think most technical manuals do that. The Pilot's Flight Operating Instructions manual would contain that kind of information.
I don't know if it is in the manual because I only translate what I need. It is pretty slow going with Google Translate, so I am going after very specific information. If I had know what this description was really about beforehand, I would not have kept going. I was actually expecting it to be something else. It is giving me some practice in Russian though.

Just as an example:
In the Technical Manual, you might find the schematics for the Landing Gear switches and motors and a description of the drive mechanism, but you probably won't find a set of instructions of what switches to use to operate the Landing Gear and what to do in case of an emergency. The instructions would be in the Pilot's Manual.

- Ivan.

[aleatorylamp]

Hello Ivan,
Anyway, the designers must have decided the P-39D engine liked warm oil, or at least, didn´t like it too cold - maybe to help with cold starts in winter?

Well, I´m glad you think the Airacobra upgrade is looking good. The lower belly definitely makes for a better silhouette. The supercharger scoop goes a little more forward, and the canopy shape is more correct.

The fuselage was very slightly on the thin side too, but this actually made it easier to get extra vertical side-panels in, improving and increasing the bulge on the sides seen from the front.
Then, I put in an extra panel at the top to get that less pointed. It wasn´t too difficult to do.
What was a little confusing was the metal around the aft canopy, but it turned out well.
I still have to triangulate a couple of bent panels under the nose, but that will be no problem.

I got the fin outline rounder, and used top and bottom half components for it. I couldn´t resist it!
The tailplane was even easier - I only had to slightly modify four panels, two on the top and two on the bottom, to get an airfoil cross section.

Now comes slightly more tedious work, that you had already done: To eliminate all the bleeds and disappearing pieces everywhere - propeller, front landing gear, flaps, lower-wing panels, air intakes.
Then, put in the pilot, the instrument console and maybe the seat-back. For that, I´ll split the canopy frame into left and right halves, and produce a choreography for the required glue-sequence dance. That ought to be fun, and as there´s no co-pilot, it shouldn´t be too difficult either!

Hey! The engine could be seen through the rear canopy... I wonder if that will work. It should be about as easy or difficult as it would be to get a head-rest in, I suppose.

Here´s a screenshot - I confess I re-touched the wheel-well bleeds through the front gear-door, but the stance looks nice on the model. I´ll correct the scoops on the leading edge, though.

I´m still thinking of taking away the wing guns, and producing a Russian livery. After all, that´s where the model´s capacities and virtues seem to have been exploited to the fullest.

Would you have any other interesting ideas for a livery? - even without removing wing-guns...

P.S. By the way:
I´m intrigued by a quirk this machine has on take-off: There comes a moment when it´s careening down the runway on its front wheel only, with the main gear in the air!
Very wierd-looking! The manual says this is best avoided, as the front wheel digs into the concrete too much on the last phase of take-off and slows down the plane.
I suppose one should ease back the yoke and get the nose up sooner, or trim tail-down slightly prior to take-off. Something to that effect should probably be written into the checklists.

So, on we go!
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
I thought I´d post some results of speed comparisons at Military Power, comparing the performance graph of the P-39-D2 model I am upgrading, to the P-39-D1 and P-39-K models.

I did try my hand again at clipping the propeller efficiency graphs, like on your BV-141, but even under threat of being shot at dawn, could I find a way to start anything with the Power Coefficient to try and compensate the 4 - 14 mph speed loss I was getting along the curve. Snow good for me, I´m afraid...

But no matter, because what I have managed to obtain with the P51d propeller, shortened to 10.4 ft, might perhaps not be altogether too bad.
I had also tested the propeller graphs from the P47d, the Spitfire and the Hurricane, but all fell short, and weren´t as good.

So here´s a screenshot of the comparative graphs. As we don´t really have much on the P39D-2 with its small propeller and its large engine, this approximation is the best I can do for the moment.

Military power is included in the normal throttle lever travel, and is at 44.2 Hg. At 1083 Hp at S.L. it is more than enough for take-off, but WEP can be used for Take-off with throttle at 85% to give 1325 T.O. Horsepower. Full WEP Boost of 60 Hg gives a tremendous boost with 1656 Hp. It lies just between the two specified powers I found quoted for this, which are 1550 and 1750 Hp.

Full 44.2 Hg is available upto and including 13000 ft, and then it starts declining. At 13500 ft it´s at 43.3 Hg, and then it goes down progressively to 21.5 Hg at 30000 ft.

Maybe it´s not too bad for the time being, until more exact information can be dug up.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Your Engine performance seems to be pretty much in line with the exception (in my opinion) of the WEP output.
What is the reference that is showing 1750 HP at 60 inches Hg for a Allison V-1710 and what exact model of engine was it?
The match with the performance charts is very good.

The model does not look too bad with the current paint job except perhaps changing the national insignia.
At this point, you have probably changed as much as I did for the Messerschmitt 109E Trop.
Your results are making me think that I should include a Transparent Canopy in my edits also before I start on my own version.

I have a couple suspicions about why you are trying to do a Nose Gear take-off but those are only guesses.
I don't think I have actually ever encountered such a thing on a flight model I worked on with the stock P51D as a starting point.
Remote diagnosis as you know doesn't really work all that well.

- Ivan.

[aleatorylamp]

Hello Ivan,
I´m glad you think the Military Performance is in line with what one would expect!

The too-high WEP is my mistake! Sorry about that! I committed a dyslexic error.
It wasn´t 1750 Hp, but 1570 Hp, as you mentioned in your Posts #71 and 72, also
more in accordance with the 1550 Hp at 60 inches quoted in the Allison Operations
and Maintenance Manual spec chart.

The 1750 Hp were for a much higher MP setting of 66 Hg that you mentioned in those
posts as well. So, in order to keep WEP correctly at 1550 Hp, the easiest solution would
be to limit WEP Manifold Pressure to 57.35 inches, as 60 Inches is giving me 100 Hp in excess.

Regarding national emblems - I was getting bored of the number 31 and used the Ying-Yang
from the Wingding Font, an Alladin´s lamp and a stylized star - but only provisionally, until
I decide on the definitive livery... The Russian one maybe, or an older Army Air Corps Star with
the red dot in it... I wonder.

The nose-gear take-off is not something I was trying to do, but rather trying to avoid!
It looks wierd and is bad on the nose wheel tyre and bearings. It´s the result of the definite
nose-up attitude on the ground thanks to the long nose-gear, contrasting with the different
normal flying angle - nothing to do with the P51d .air file though!
The only way to avoid it, appears to be to rotate sooner.

I thought perhaps it is a matter of trying to get a more nose-up attitude at normal flight speed,
but manipulating the CoG Offsets in the Main Wing section, the Pitch parameter offset for the
tailplane in the main Aerodynamics Section, and the angle of incidence in the Tailplane section
didn´t bring any results.

The next thing to try, is to maybe reduce change the angle of incidence in the Main Wing section for this, which will need re-adjustments of Zero Lift Drag, but OK.

Cheers,
Aleatorylamp