Analysing and modifying the AFX file with QBasic.

[Ivan]

Hello Aleatorylamp,

First of all, a little update from my AIR file tuning:
Speed @ 500 feet - 313 MPH
Speed at 12,500 feet - 368 MPH
Climb Rate at about 2000 feet altitude - 2900 feet/minute to about 2950 feet/minute with about 55% fuel and full ammunition.
Sustained climb speed appears to be around 185 MPH IAS but the altitude changes so quickly it is hard to determine.
Speed with War Emergency Power (51,0 inches Hg for about 1340 HP) was not tested.
There are a couple places I don't like the Pitch Angles being selected so I know there is more tuning to do.

My "corrections" to compensate the drop in speed, by increasing the point on the down-slope point
(not the crests) gave the following results at S.L., also observing the propeller pitch as you recommended:

- Normal Maximum (95% power) 1014 Hp: (42.2 Hg MP), 309 mph, with 37.2 propeller pitch. This seems reasonable.
- Military Power 1083 Hp: (44.2 Hg MP), 316 mph, with 37.9 propeller pitch. 4 mph faster than before. This is better, and seems OK now.
- WEP 1551 Hp: (57.35 Hg MP), 357 mph, with 41.8 propeller pitch. This is 1 mph slower than before, and I´m really aiming at 372 mph.

Where did you get these numbers from?
My belief is that 316 MPH at 500 feet is a touch high especially with only 1083 HP, but it is close enough not to worry about too much.
357 MPH or 372 MPH seems way too high at only 500 feet altitude.
That would make it an amazingly fast fighter at low level even by late war standards.
I believe the P-39K was only getting 372 MPH at 13,000 feet as a matter of comparison.

Let´s see if I can understand the theory behind the process required:
I noted that the whole p51d efficiency table is "too efficient", i.e., as pitch angles rise, their graph-crests rise to over 91%, which is unreal.
Thus, should they really perhaps all be toned down, so that the higher pitch-angle graph-crests reach only 78% or 82%?
This would reduce speeds, though: So, to maintain them, Torque would have to be increased, which would then require reductions in the Power Coefficient graph, so that the Hp don´t go wildly out of control here.

Hmmmm.... Should I really get into this again? I won't be telling you anything new and things did not really turn out well the last time we were here.

1. You are correct. Efficiencies at their peaks are quite a bit too high in Record 511. I am no expert, but I would say 82-88% should be about right. A lot depends on other factors besides just the Propeller: What is the shape of the airframe behind the Prop Disk.
My own values for Record 511 are also in the 90-91% range at their peaks.
You should look for Propeller Efficiency Graphs on the Internet and draw your own conclusions instead of relying on just what I am doing.

2. There is a reason I suggested getting Engine Power in the proper range and then tuning other things around it.
Note that the real factors involved here are Thrust and Drag to determine Speed.
If you need to reduce Efficiency but can't adjust Power, then Thrust goes down.
If Thrust goes down, then to maintain Speed, Drag needs to go down as well.

3. As I see it, this is a bit of a cycle. Eventually even things like Engine Power may need to be tuned, but by then the tuning will be very minor.
With each cycle, you find the thing that is most wrong and fix it and then look for the next most incorrect thing to fix.
Eventually the problems are small enough that they really don't matter and you quit.

Hopefully we don't get anywhere near the place we got when I wrote Post #88.

- Ivan.

[aleatorylamp]

Hello Ivan,
Thank you very much indeed for your opinions on my current performance figures.
I think it is very interesting, and will most probably not get complicated after all,
to remain an entertaining and useful exercise without getting too heavy.

Not to worry then! From what I´ve understood in your post now, I´m sure this won´t
happen. I believe I can make some corrections without any complications.

Answering your question about the numbers I posted in my previous post:
I got them after some trials based on rough estimates, or rather guesses, as to how the more
souped-up -63 engine would behave on the D-2 model, compared to the -35 engine on the -D1 model, and also
the reportedly similar -K model, both of which I´d found some interesting Military Power performance graphs for.

Re: Your useful comments:
- You are right, of course, the 372 mph with WEP should NOT be at S.L. but at 13,000 ft! My mistake (Ufff...again,...).
I´ll try to get the S.L. WEP performance down to a little over 350 mph then, below the 358 mph I had before.
- Then, I would also prefer Military Power at S.L. to be a bit lower, and at 1100 Hp, but I think I´ll have to stick to
the 1083 Hp for simplicity´s sake, but will, however, be able to get speed down a bit without any problem - to 312 mph
perhaps, like I had before.

So, once I get that, I´ll see how it goes at 13000 ft, if I get plausible results there too.

Thanks very much again, also for the comments on the 82-88% and 91-92% peaks that you are also getting.
I had already downloaded a few efficiency curve images, some time ago, and that´s why I thought (as you also do),
the downward slopes should be corrected.
As far as your points 2) and 3) are concerned (thanks for those comments too), perhaps I won´t dive into the
swimming-pool at the deep end, and keep to water-depth with enough foothold, as it were!

More later...
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I tried a few minor edits to the Record 512 and wasn't able to get the response I wanted and still keep a nice looking set of curves.
While I certainly can get the specific effect I want, the side effects are that the curves for different Pitch Angles would be too close in some places and too far apart in others. I am better off using what I currently have which appears to do most of what I want.

I am working on custom Propeller Tables because I want to be able to fine tune performance to a greater degree than I can with other parameters and also because I can't stand the idea of weird interactions between the stock P51D Propeller Tables.

Why do you want to work on Propeller Tables? Is there a specific goal in mind?

By the way, if your AIR file has reasonable Supercharger settings, there actually isn't any WEP at all at 13,000 feet; The Supercharger has no remaining capacity at that altitude.

- Ivan.

[aleatorylamp]

Hello Ivan,
This is rather pleasing, as what I´m trying to do seems to be working out quite nicely, and only by using Table 511!

My objective here is basically to do away with the absurd looking downward slopes in the default p51d Graph Table 511,
at the same time achieving some perhaps more plausible performance results for my elusive D-2 model.

I don´t really want to work on table 512, and with any luck, most probably my goal will not require doing so.

Of course, as you say, WEP does not have any effect at 13000 ft - I adjusted the supercharger to maintain 100%
non-WEP power at that altitude, namely 44.2 Hg MP (Military Power setting, as we had agreed on some time ago).

After your comments for possible corrections, I managed to re-adjust performance as follows:
At S.L., now I get 312 mph under 44.2 Hg Military Power, and 356.4 mph with 57.35 Hg WEP.
If I go any lower here, I lose out too much at 13000 ft.

At 13000 ft I was originally getting 371 mph, but now after reducing it a bit, I´m "only" getting 362.6 mph.
Maybe this is more correct after all.

If it isn´t, I can easily get it higher again, but then performance will go up a bit at S.L. too.
I wonder... What would you say?

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I will ask you again: What exactly are you trying to accomplish by editing the Propeller Tables?
Getting rid of the long slopes on the Propeller Efficiency Graph is relatively easy.
Look for the Zero locations in the Propeller Efficiency Graph from my Ki 61 or BV 141B and use those.

I don't actually believe you will be successful by just modifying the Propeller Efficiency Graph, especially if you want to represent the points of zero efficiency. I believe the current shape of those curves is because the Power Coefficient Graph was probably non-optimal and the long flat curves made exact selection of Propeller Pitch much less critical.

Hypothetical Case:
There is a mismatch between the Power Coefficient of your Engine/Propeller and Table 512.
The mismatch causes the selected Propeller Pitch to be 5 degrees lower than it should be.
On the current stock P51D Table 511, it does not matter much because the Efficiency curves are mostly flat with very few peaks.
If an incorrect angle is picked for an Advance Ratio, everything still works even if not optimally.
With your new Table 511, the graph may drop off pretty steeply and because of mismatch in Power Coefficients, the simulator does not select a higher pitch angle and Thrust becomes too low to be useful.

Now keep in mind that although you can accomplish quite a lot with the Propeller Tables, I highly recommend that you use other methods first because the result in my opinion will be a more realistic flight model.
In other words:
If your Engine Power Curve or something else is very wrong, the flight performance can STILL be adjusted to be fairly reasonable with Propeller Graphs, but the result will most likely display odd behaviour in other places and performance may be a series of disconnected curves.
You have been warned.

What I have to say about 371 MPH dropping to 362 MPH @ 13,000 feet is that there is not enough information as to what exactly was changed to even begin to formulate a hypothesis.
What did you do to drop low altitude speed from 316 MPH to 312 MPH?
What is the Manifold Pressure at 13,000 feet?
What is the Propeller Pitch Angle at 362 MPH and 371 MPH at 13,000 feet and what does the Propeller Efficiency Graph look like in that range?

- Ivan.

[aleatorylamp]

Hello Ivan,
Excuse me, but I´m getting a bit lost again. I can´t exactly figure out what it is you don´t understand about my answers, but I´ll try and clarify.
Hopefully all this won´t get too complicated.

From our recent exchange in posts I had understood that my intention of working only with Table 511 would be possible, and probably enough, as working on table 512 would make things unnecessarily complicated for what I´m trying to achieve.

You wrote: "I will ask you again: What exactly are you trying to accomplish by editing the Propeller Tables?"

Let´s see if I can explain again:
Upto now, using the default p51d propeller graphs, I was getting what I thought was a reasonable approximation for the P39D-2:

At S.L.:
44.2 Hg Military Power, 1083 Hp, doing 312.5 mph
57.3 Hg WEP, 1551 Ho, doing 358.2 mph

... and at 13000 ft:
44.2 Hg Military Power, 1232 Hp, doing 371.5 mph.

BUT: Not having liked the shape of the default Propeller Efficiency 511 graphs for a long time now,
I wanted these to have a more correct shape, for example like those on your BV 141B.

At first I tried using those, but they caused considerable drops in performance that were too difficult for me to compensate,
so I used Table 511 of the p51d, as these graphs went up a bit higher, and cut those slopes down to zero.

That didn´t work very well either, so I left it alone for a while, until yesterday, when I had the idea of raising the point on
the down-slope just after the crest, before the graph went down to zero, to compensate the lack of performance.
At the same time, I didn´t want to get involved at all with table 512.

I tried out my idea, and it gave me the performances I posted two posts back, which at S.L. were slightly higher with Military Power,
and slightly lower with WEP, until I compensated the down-slope on the 40-degree Pitch angle graph by raising it, giving me 360 mph
with WEP at S.L. I hadn´t tested speeds at 13000 ft yet, and posted that a bit later.

Your answer to this seemed to suggest that I was on the fast side, so I reduced the speeds by reducing the height of the point
on the graph down-slope, corresponding to the pitch angles mentioned in my post, and came up with the results posted in my last post.

I dropped speed 13000 ft speed from 372 to 362 mph on purpose, using the 40-degree pitch graph, by dropping the point after the
crest down to zero, and S.L. Military speed from 316 to 312 mph with the 35-degree pitch graph (the red dot on the screenshot),
because of what you had commented in your post. With this, WEP for S.L. dropped from 358 to 356.4 mph, because the same
pitch-angle graph affected it.
This reduction in speeds didn´t just happen as an effect of something else, but because I lowered two points point to that effect
on the propeller efficiency graphs for two corresponding pitch angles.

Manifold Pressure for 13000 ft is 44.2 Hg - but I´ve said that already. What I haven´t said yet is that the
Pitch Angle for 372 mph at that height was 43.5, and for 362.2 mph, it´s 42.6.

My engine performance curve is not all that wrong, because it more or less fits in with what I expect should be the power
curve of the model compared to the P39D-1 and P-39-K models, but I´m repeating myself here, because I´ve said this before.

It seems clear that 316 mph is a bit high for Military Power at Sea level, so 312 would be more correct.
In your case you have 313 mph, but I believe your P39F was lighter, so that would tie in.

All I´d really like to know is, if at S.L. with WEP, 356 mph is better than 358 mph,
and at 13000 ft, if 372 mph, or maybe 362 mph, would be more correct.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Thanks for the explanation.
Let's see if I can present you with some useful information.
I believe the main problem you are encountering is pretty much what I suspected: Record 512 from the stock P51D is not really very good.
I believe what it is doing is pushing the Propeller Pitch for everything out to the range of about 42-44 degrees.
Just for an experiment, add about 6 Additional Cylinders to your engine and see if the Propeller Pitch Angles change to any noticeable degree.
I suspect it will not.

Let me give some numbers from my own P-39F and perhaps you can see the differences:
At Maximum Speed at 500 feet:
Power: 1153 HP
Speed: 312 MPH -- J=1.58 (approximately)
Power Coefficient: 0.110
Propeller Pitch is around 34 Degrees

At Maximum Speed at 12,500 feet:
Power: 1286 HP
Speed: 368 MPH -- J=1.87 (approximately)
Power Coefficient: 0.178 - This number is much higher partly because of 130 HP extra but mostly because the Air is thinner.
Propeller Pitch is around 40 Degrees

Sustained Climb at 2,500 feet
Power: 1175 HP - This difference from 500 feet is not significant
Speed: 180 MPH -- J=0.914
Power Coefficient: 0.119
Propeller Pitch is around 25-30 Degrees

Comparing the data for these three conditions, it becomes fairly obvious that they are far enough in the graph that they can be individually tuned without greatly affecting the other conditions. Even then, my adjustments to the Propeller Efficiency Graph to influence the Climb Rate also affected the Speed at 500 feet.
I believe that on your set of Propeller Graphs, the values are too clustered.
Using my Propeller Tables and Numbers would not help much because the Propeller Reduction Gear Ratio greatly affects the Advance Ratio which would put the tuning that I have done in the wrong place for your particular aeroplane.
Just as an example:
The P-39F is going 197 MPH at J=1.0
The P-39K is going 177 MPH at J=1.0
The Propeller Pitch Ranges are also different in that there needs to be one more column at the higher Pitch Angles and I don't even know what the parameters are for the Propeller on the P-39D-2 which is certainly different.

To state again: I believe you can get some performance changes by adjusting the efficiency values in Table 511 but if you intend to have the curves drop to zero, I believe you won't have much success unless you custom fit the Table 512 to your combination. If you don't do that, you will most likely have lots of trouble avoiding the dead spots in the Propeller Efficiency Graph.

If all you really want to know is which speeds are "Correct" in my opinion, that is pretty easy:
At 500 feet, you should be hitting about 314 MPH because that corrects down to very close to 309 MPH at Sea Level.
At 13.000 feet, you should be hitting 372 MPH because that is what the documentation states.

Hope that makes sense.

- Ivan.

[aleatorylamp]

Hello Ivan,
OK! Thank you very much for your post, and the effort that has gone into it to clarifying some
things I had started getting confused with again.

It seems not to be as complicated as I had feared after all, although a bit more than I´d
begun to expect.

Following your indications regarding the required pitch angles for each of the the different situations,
some effective adjustments to Table 512 and re-adjustments to Table 511 should be possible,
so I can get the desired results.

I hadn´t got as far as climb results yet, so that will come in very handy too.

I believe I can understand what is going on, and I´ll see what I can come up with.
Thanks again!

Update:
Small steps to keep track...
First I corrected the values to get the performances you mentioned at the end of your last post,
and secured a back-up of the .air file. Then I conducted the +6 cylinder experiment, which did
produce a Propeller Pitch-increase: +4.8 deg. at S.L. and +4.5 deg. at 13000 ft.
This was presumably because of the now goofy combination between the clipped Table 511 and default Table 512.
Now, the next step is customizing Table 512 and re-fitting Table 511, which should not be too hard, because you
conveniently indicated the reference J factors and Pitch Angles.

More later!
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
It shouldn´t be too hard, I said... but that only seemed so. In reality it is.
What to do now is the question that I still can´t figure out, I´m sorry to say.

First Part: 500 ft, Maximum speed (Military Power).
------------------------------------------------
Your P-39F Settings, as a possible reference here:

Power: 1153 Hp (Is this Take-off power at 51 Hg, or Military Power at 44.2 or 42 HG?)
Speed 312 mph -- J=1.58 (approximately)
Power Coefficient = 0.11
Prop-Pitch around 34 degrees.

Without altering anything yet, I have here:

Power: 1083 Hp (44.2 Hg Military Power) - or Wait! - Should this really be 51 Hg 1325 Take-off power?
Speed 314 mph
For J=1.6, Power Coefficient = 0.11
Simulator-chosen Prop-Pitch = 37.8 degrees.

J factor is very similar, Power Coefficient is the same, Hp is a bit low, and Pitch angle is too high.

The objective here seems to be to make the simulator choose a lower Propeller Pitch Angle.
The problem is that I haven´t a clue about what to do to achieve this.
I tried increasing Power required Coefficient for J=1.6 of the 35-degree Pitch-angle graph to 1.5, but Propeller Pitch remained the same.

So... What now, I´m afraid is the big question...

Update: I tried one thing:
Forcing the Sim to use a fixed-pitch, 34-Degree propeller at 500 ft. I got:
max. speed: 337 mph, 1306 Hp at 3616 RPM (being fixed, of course...)

Max. Military Power for the D-2 is specified at 1050 Hp at S.L. with 44.2 Hg and 3000 RPM from a test report,
so that could be the reference. Possibly 1083 Hp would be acceptable.
Take-off Power is 1325 Hp with 51 Hg, for the D-2, so that could also be the reference point for Maximum Speed as S.L.

Question: What´s the Manifold pressure for your 1153 Hp maximum S.L. power?

My assumption: This would imply that one should start by regulating Torque, Tables 511 and 512, and Zero Lift Drag,
to get 314 mph with 1050 Hp at 44.2 Hg, (or 1325 Hp at 51 Hg), at S.L. with this 34-degree fixed pitch propeller, and
then adjust nearby graphs so that a CV propeller can be installed later.

Would this be correct? ... once of course it´s clear if we are talking about Military Max. Power or Take-off Power at S.L.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

You conducted a lot of experiments overnight (my overnight anyway).
Hopefully I can address some of them.

First of all, on my own P-39F AiR file:
Service Ceiling Test gave just a bit over 31,000 feet which seems reasonable enough that I am probably not going to change anything.
It ended with slightly less than 1/2 fuel, so I may conduct the test again at some point.

You asked about the Manifold Pressure for my P-39F.
At 500 feet, it is giving 1153 HP on 45.5 inches Hg and 3000 RPM.
At 500 feet, for War Emergency Power, it is giving around 1340 HP (If I remember right) on 51.0 inches Hg and 3000 RPM.
At 12,500 feet, it is giving 1286 HP on 45.3 inches Hg and 3000 RPM.
I haven't altered the Supercharger settings, so critical altitude is probably still 12,300 feet and it might actually go slightly faster at that altitude.

As I have stated a few times, I do not have data for the Curtiss Electric Propellers, but I will make a few assumptions that they are identical in size to the AeroProp versions installed in the P-39F and P-39K for some estimates.
As for Propeller Pitch Ranges, I have no idea what they should be for a P-39D-2 but am certain they are different than or the P-39K.

Power: 1153 Hp (Is this Take-off power at 51 Hg, or Military Power at 44.2 or 42 HG?)
Speed 312 mph -- J=1.58 (approximately)
Power Coefficient = 0.11
Prop-Pitch around 34 degrees.

Without altering anything yet, I have here:

Power: 1083 Hp (44.2 Hg Military Power) - or Wait! - Should this really be 51 Hg 1325 Take-off power?
Speed 314 mph
For J=1.6, Power Coefficient = 0.11
Simulator-chosen Prop-Pitch = 37.8 degrees.

I listed what I was getting with my P-39F as an EXAMPLE of what to look for.
Your P-39D-2 is different enough that the numbers are not necessarily equivalent for your aeroplane.

At 314 MPH, your P-39D-2 is actually at Advance Ratio J=1.77.
Power Coefficient is actually 0.1422.
The difference is because you are using a different Propeller Reduction Gear Ratio.
Mine is 1.8:1
Yours is 2.0:1

I tried increasing Power required Coefficient for J=1.6 of the 35-degree Pitch-angle graph to 1.5, but Propeller Pitch remained the same.

This probably didn't work because you were going after the wrong point in the Power Coefficient Graph.

Update: I tried one thing:
Forcing the Sim to use a fixed-pitch, 34-Degree propeller at 500 ft. I got:
max. speed: 337 mph, 1306 Hp at 3616 RPM (being fixed, of course...)

All you accomplished here was to find one of the goofy "Perpetual Motion" points in the stock P51D Propeller Tables.
The results are meaningless and are the main reason I wanted to create my own versions of the Propeller Tables.
There are a bunch of these combinations and all you need to do to experience this ridiculous sleigh ride is to put the upper pitch limit in the right place.

- Ivan.

[Ivan]

Hello Aleatorylamp,

I was going to make a small addition to the post I just wrote, but I know that for some reason I can't actually edit the post if I have made quotes in it. No idea why.

The 1340 HP @ 500 feet for War Emergency Power is from memory. I am pretty certain the Manifold Pressure setting is 51.0 inches Hg.
I run this so seldom that I don't know if I have actually written it down anywhere.

This kind of power output is quite pitiful when compared to what P-40's with equivalent model engines were getting (1570 HP) but it is what the documentation supports.

From the standpoint of working on Propeller Tables, Manifold Pressure is not important.
What you really need is Propeller Specifications, Horsepower and RPM and that is about it.

When you are working with Propeller Power Coefficients, Air Density is also important.
All I did was to pull the numbers from an online table of the "Standard Atmosphere" and refer to them for calculations.
It may not be exact depending on what the data is in the internal tables for Combat Flight Simulator, but it should be close.
So far, the numbers seem to work out pretty much as expected even though in places I am using formulas for interpolation and the last couple decimal places may not be correct.

- Ivan.

[aleatorylamp]

Hello Ivan,
OK, so we are talking about Military Power, not WEP or Take-off Power. Thanks!
OK on the corrected "J" Factor and Power Coefficient numbers, which differ in my case due to the different gear ratio.
Thanks very much for confirming and clarifying!

However, I think I may have to give up, as I´m getting nowhere fast.

Just to conclude my meaningless experiment with the 34 degree fixed pitch propeller:

After lowering Torque a bit, and raising the J=1.6 position to 0.199 in the 35-degree pitch graph in Table 512,
(0.199 looks VERY high...), and then re-fitting the CV propeller, now the simulator is selecting a smaller angle,
36.6 degrees, for Military Power at S.L. instead of 37.8.

I could try and see what happens at the J=1.8 position though, after reading your comment on the
corrected "J" Factor and Power Coefficient.

Nevertheless, it is a bit meaningless to me, and consequently rather complicated for me to cope with, and I don´t know if
I´ll be able to continue. At the moment I´ve got a headache (again...) and I´ll see how it goes after the headache is gone.

Thanks very much for your patience and for your help, but the relationship between the different abstract concepts is still
rather difficult for me to see, and still escapes my comprehension.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

This IS supposed to be a Hobby: An Enjoyable way to Spend Time.
When it gets to be more stressful than that, it is time to take a break.
That is probably why I am getting nowhere with my AF99 Model of the Airacobra.
I want to own the model. It is well within my ability to create such a model,
but it is becoming more like work than an enjoyable hobby so I put it off.

Don't worry about giving up on this pass. I am sure we will visit this issue again at some point if we are both still building virtual aeroplanes.
I believe that a lot of the reason why you are not seeing the relationships is because you haven't actually done the tedious background work to support an understanding of the relationships.
You have commented that you do not work with spreadsheets which I believe are the best tool for quickly calculating fairly complex mathematical relationships.
Spreadsheets though are not absolutely necessary even though they are the ideal tool.
You can probably accomplish at least the basics with a QBasic program.

I would suggest you write two programs.
The First One is quite easy:
Write a program to Calculate Advance Ratio
Input:
Engine RPM
Propeller Diameter
Reduction Gear Ratio
Forward Speed
Output:
Advance Ratio

The Second One is a bit more difficult:
Write a program to Calculate Propeller Power Coefficient
Input:
Propeller Diameter
Reduction Gear Ratio
Engine RPM
Horsepower
Altitude
Output:
Propeller Power Coefficient

I believe if you play with these programs for a while, a lot of the relationships will make a lot more sense.
This may sound condescending, but it is not intended that way.
I notice in your last two posts that you are still thinking that your maximum Sea Level Speed of 314 MPH corresponds to a 1.6 Advance Ratio.
I tried to tell you that it does not (it is more like 1.77), but it might become more apparent if you actually do the calculations.

After lowering Torque a bit, and raising the J=1.6 position to 0.199 in the 35-degree pitch graph in Table 512,
and then re-fitting the CV propeller, now the simulator is selecting a smaller angle, 36.6 degrees, for Military Power at S.L.

This is the kind of thing that has me wondering what I should be telling you.
You have encountered an interaction of parameters in the P51D flight model that creates an unstable calculation. This result and exploiting it takes this from a discussion of simulation to speculation about pure fantasy. From a conceptual standpoint, this is basically having one sailor sitting at the stern of the USS Constitution and propelling it by blowing at the sails and outrunning a modern Nuclear Aircraft Carrier. If you really like this situation, we can even get the feedback error going seriously enough to get your virtual aeroplane to go past Mach 1.

Engine Tuning under those circumstances is meaningless because all you are doing is breaking a virtual Engine model that works well under normal circumstances.

Besides, if you really want to affect things, note that the actual Advance Ratio is 1.77 and work with the entry for J=1.8 instead of J=1.6.

- Ivan.

[aleatorylamp]

Hello Ivan,
Yes, I wrote in my last post that I acknowledged the corrections to the "J" Factor and the Power Coefficient.
Then I knew I had done the tests with the wrong values. It became clear in your post that they were meaningless.
Our posts had crossed each other, and it would probably have been better to delete my post.

I did mention, however, that "I could try and see what happens at the J=1.8 position though, after reading
your comment on the corrected "J" Factor and Power Coefficient."

Anyway, trying to correct the Propeller Tables is a very tedious exercise, and it is becoming very obvious that
even small variations in the specs, just with gear ratio, for example, require complicated modifications there.

Some time ago, I did write two QBasic programs along the lines you suggested in your post, but for the moment,
the whole issue is a bit cumbersome to handle, I´m afraid.
As you say, it is a hobby, after all, and I´m sorry that all this must also have been a bit tedious for you.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I am sorry things did not work out well.
If I had chosen to do the P-39K first, some of my work with Propeller Tables might have been more useful to you, but I know that if I build the hotter aircraft first, the lower performance version probably would never get done.
My belief also is that the lower performance version was historically more significant.

I do wonder though, how similar are the Propellers for the P-39D-2 and P-39K?
I know you are working with a diameter that you determined was optimal.
I also know that the Propeller Pitch Ranges between the AeroProducts and Curtiss Electric Propellers are different.
Did you ever find a reliable reference for the Pitch settings?

I CAN tell you that even with a semi-reliable process worked out, working on Propeller Tables is still quite tedious.
With this project, I found quite a few areas that I am still not sure of but they don't seem to affect things to any noticeable degree.

- Ivan.

[aleatorylamp]

Hello Ivan,
The Curtiss Electric Propeller used on some of the Airacobras seems to be the most elusive of all pieces of information.

Some of the different performance tests mentioned on http://www.wwiiaircraftperformance.org/P-39/P-39.html, make
reference to propeller pitch, or blade angle range, and blade design number, but if they are going to be useful to you is
another matter.

I´m looking now, and this information is mentioned for the -C, the -D, -M, -N, and -Q variants, but unfortunately there
there are no -F or -K versions mentioned, I´m afraid.

However, the -M version is mentioned as having a V-1710-83 engine and "a three-bladed Curtiss Electric constant speed
propeller, blade design No. 614-1C-1-5-21, blade angle range 26º to 56º, at 42 inch radius."

This is the only piece of information detailing numbers for a Curtiss Electric Propeller that I´ve come across. I believe the
-M version was a derivative of a more commonly named other version, but off-hand I can´t remember which.

One relationship between Tables 512 and 511 that I do realize, is something you mentioned some time ago, and is clearly
visible on your BV-141B propeller tables, namely that for any given pitch angle, the curves on both tables drop to zero
at the same "J" Factor.

Now: Were I to
1.- give, in Table 512, the 35º pitch-angle Graph (and perhaps move it to be 34º), at "J" Factor 1.77
(or 1.8, to round off), a Power Coefficient of 0.1422, then
2.- even out the rest of the graph curve shape for J=1.8, and after that
3.- put all the rest of the graphs to zero at the points coinciding with the propeller efficiency table, and finally
4.- "sculpture" the rest of the graphs to fit the surrounding shapes.

...perhaps that would not be so meaningless, and would have a good effect. Then next step would be to do
something similar for the 40º pitch graph and finally for the 25-30º one, but not at the "J" Factor positions you
mention for your P39F, but for the "J" Factor corresponding to the calculation of the P39D-2 propeller 1.4 ft
approximation with 2:1 gear ratio I´m working with.

I wonder...
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
I just did this and tested for S.L. performance, without yet adjusting Torque or anything else, just Table 512 to match the Zeros on Table 511, and smoothen the graphs after putting in Power Coefficient 0.1422 at Advance Ratio 1.8 for the 35º pitch graph.

The perhaps significant result was that at 314 mph, the simulator selected 34.8 pitch, albeit with 78% WEP to obtain the 1220 Hp it needed.
With as yet unadjusted Torque/Drag, Normal 44.2 Hg Military Power gives 1082 Hp (as before), but only does 285.6 mph, with 29.8º pitch.

Perhaps this is now becoming meaningful. Probably now would be the moment to adjust Drag and Torque to get some more power into Military power setting. As 1220 Hp would expectedly be a bit high, by reducing Drag and increasing Torque the aim would be to get 314 mph with about 1150 Hp, graduating it so that 1550 Hp would be available with WEP at S.L.

This probably sounds a bit more coherent with the objective.

Cheers,
Aleatorylamp

P.S. By the way, I recently read that later Airacobra models had a power limitation for take-off, around 1100 Hp.
I would suppose this would have been for due to the better altitude performance achieved with greater supercharger
capacity, perhaps on later -Q versions, so at S.L. the supercharger would not be used, in order to protect the engine.

[aleatorylamp]

Hello Ivan,
Instead of the two updates on my last post, I decided a separate post would be better, so the Updates are contained in this one:

I had noticed that it was better to adjust only Propeller Efficiency, also making fine adjustments in the Power Coefficient table, instead of changing Torque and Drag.

Changing Torque upset the 1550 Hp WEP setting, so I discarded that.
Then, only adjusting Drag corrected S.L. performance to 314.0 mph with 44.2 Hg Military Power 1083 Hp, and 34.3 degrees propeller pitch.
However, engaging WEP caused speed to shoot up beyond 370 mph, so the Drag adjustment didn´t seem much good either.

However, with normal military power now, 44.2 Hg, speed is only 290 mph.
Maintaining 314 mph at S.L., requires 45.2 Hg and 1120 Hp, and propeller efficiency is already very high. Nevertheless, Propeller pitch is 34.4.

Later I also regulated performance at 13000 ft by adjusting the 40 deg. pitch graphs around J=2.
The result with 44.2 Hg Military Power, is 372.7 mph with 40.0 degrees propeller pitch. This would seem quite good, I believe.

Now it is a matter of seeing how S.L. performance can be ironed out.
Perhaps the Power Required Coefficient at J=1.8, where you suggested an entry of 0.1422, was only an approximately calculated value to start off with, and could now be decreased a little?

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

The Curtiss Electric Propeller used on some of the Airacobras seems to be the most elusive of all pieces of information.

I am asking because the AIR file for your P-39D-2 needs this data and I am curious as to what you are using.
You seem to be somewhat vague on the Propeller Diameter and have not mentioned Pitch Ranges, both of which are important for what you are doing.

*I* don't need the data because the P-39F and P-39K both use propellers by Aeroproducts and not Curtiss Electric and I have all the specifications I need for the models I need. The P-39D and P-39D-1 information is also not hard to find even though I don't need it for what I am building.
Incidentally, the Aeroproducts "Aeroprop" was actually a Hydraulically controlled propeller in a similar manner to the Hamilton Standard Hydromatic but obviously without the piston in the center where the Cannon would go. I haven't actually spent the time to look for the data myself because I am not building that model of Airacobra and the propeller for the P-39D-2 was probably quite unique which is why you need it.

One relationship between Tables 512 and 511 that I do realize, is something you mentioned some time ago, and is clearly
visible on your BV-141B propeller tables, namely that for any given pitch angle, the curves on both tables drop to zero
at the same "J" Factor.

This is what prevents the feedback / Perpetual Motion business. In reality, the Efficiency would drop to zero slightly ahead of the Power Coefficient because of Frictional losses in the system.

I just did this and tested for S.L. performance, without yet adjusting Torque or anything else, just Table 512 to match the Zeros on Table 511, and smoothen the graphs after putting in Power Coefficient 0.1422 at Advance Ratio 1.8 for the 35º pitch graph.

The perhaps significant result was that at 314 mph, the simulator selected 34.8 pitch, albeit with 78% WEP to obtain the 1220 Hp it needed.
With as yet unadjusted Torque/Drag, Normal 44.2 Hg Military Power gives 1082 Hp (as before), but only does 285.6 mph, with 29.8º pitch.

Hmmm.....

Some time ago, I did write two QBasic programs along the lines you suggested in your post, but for the moment,
the whole issue is a bit cumbersome to handle, I´m afraid.

It seems to me that you should actually use those QBasic programs you wrote.
You do realise that 1220 HP instead of 1082 HP will significantly change the Propeller Power Coefficient, right?
By my calculations, this puts the new Power Coefficient at 0.162.
I can't actually help you much without actually doing all the calculations you will need and I don't have the data for that.

My advice to you if you really want to pursue this is to:
1. Build a listing of Power versus Altitude for the Engine you want to use.
2. Use your program to calculate the Propeller Power Coefficient at Each Altitude.
3. Graph Table 512 BUT along with Advance Ratios, list the corresponding Level Speeds.
4. Do a quick test of Power Coefficient at War Emergency Power just to get an idea of the extreme values but tune for Military Power only.

This is the basic background work I believe you will need to get a feel of what is going on.
I did this before ever starting work on the P-39F Propeller Tables because without it, I can't visualize the relationships.
I do this with Spreadsheets, but you can do this with any tool you want though I suggest you have at least a good pocket calculator.

My Children have the Texas Instruments Ti-89 and Ti-84, and I have a Ti-83 Plus which are all Graphing Calculators but I don't actually know how to use any of them..... (Mine actually cost me about $5 at a Thrift Store.)
One of these days I will ask them to teach me.

- Ivan.

[aleatorylamp]

Hello Ivan,
There are several as yet unknown factors which require speculation, and consequently cause problems.
I don´t know what the characteristics of the Curtiss Electric Propeller were.
It appears that the D-2 had one of these, but many sources don´t even bother to say.
The -L version was a -K version (with a -63 engine) with some modifications, which included a Curtiss Electric Propeller.
The -M version had a Curtiss Electric Propeller too, but with a -83 engine, and then for the -M1 version, the propeller
was changed to an Aeroproducts one.

So, being the 1200 Hp -83 engine, with a "a three-bladed Curtiss Electric constant speed propeller, blade
design No. 614-1C-1-5-21, blade angle range 26º to 56º, at 42 inch radius.", I wouldn´t know what to guess for
the propeller on the 1325 Hp -63 engine.

Using my general-approximation 1.4 ft propeller, I could of course incorporate the 26 to 56 degree angle range,
but I doubt that it would make much difference for the reading of 35 (or 34) and 40 degrees.

At the moment, the progress I have made so far seems to indicate that I am getting somewhere after all, and
I have identified the J-factor positions needed to regulate the Power Coefficients.

The 1220 Hp needed to get to 314 mph at S.L. were only at the beginning when I was regulating Table 512 at
the 35-degree pitch graph, with position J=1.8, without having regulated anything else, for example Propeller Efficiency
(because Torque and Drag adjustments seriously messed other things up).
Also, 1220 Hp for Military Power is totally erroneous anyway, as Military Power is 1150 Hp as per specs.
After raising Propeller Efficiency, it only required 1120 Hp to reach 314 mph.

There is a possibility that 314 mph is too fast anyway for the D-2. Some sources state "306 mph near the ground."

The problem with putting in 1150 Hp, or even 1120 Hp for Military Power, is that WEP goes out of control, and to
keep WEP at 1551 Hp, Military Power has to stay at 1083 Hp.

Now, a really strange problem:
Trying to get to 314 mph with Military power is impossible, because it won´t go beyond 290 mph if it is accellerating
from a slower speed, but if one reaches 314 mph with WEP and disengages WEP, then Military Power strangely accellerates
further, to 320 mph or more.

At 13000 ft, however, it is perfect: 372.2 mph with 1232 Hp at 44.2 Hg Military Power, with 40.0 degrees propeller pitch.

Anyway, it seems that there are still too many unknow factors to be able to do anything concrete other than speculation
and/or trial-and-error experimentation to try and regulate performance at 500 ft.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Using my general-approximation 1.4 ft propeller,

You do realise of course that a 1.4 feet Propeller would be so small it would actually fit entirely inside the Spinner, right?
I was thinking that this was a mistake when I first saw it, but it has shown up a couple times, so you must really mean it.

The 1220 Hp needed to get to 314 mph at S.L. were only at the beginning when I was regulating Table 512 at
the 35-degree pitch graph, with position J=1.8, without having regulated anything else, for example Propeller Efficiency
(because Torque and Drag adjustments seriously messed other things up).
Also, 1220 Hp for Military Power is totally erroneous anyway, as Military Power is 1150 Hp as per specs.
After raising Propeller Efficiency, it only required 1120 Hp to reach 314 mph.

Your reasoning really has me confused here.
The 1220 HP would raise the Power Coefficient well above the value you were trying to tune Table 512 for, so I don't see how that could help at all. If you change too many parameters, it is hard to tell what the interactions are (in my opinion), but if you are getting satisfactory results, then you must be following a process I do not understand.
If it works for you, keep using it. I believe I will stick with the process I understand.

Now, a really strange problem:
Trying to get to 314 mph with Military power is impossible, because it won´t go beyond 290 mph if it is accellerating
from a slower speed, but if one reaches 314 mph with WEP and disengages WEP, then Military Power strangely accellerates
further, to 320 mph or more.

I believe you are running into an interpolation problem as I described back on the "Flying Swallow" Thread last year.
http://www.sim-outhouse.com/sohforum...=1#post1082255

- Ivan.

[aleatorylamp]

Hello Ivan,
I forgot a zero... It´s a 10.4 ft propeller!

Perhaps I didn´t explain what happened with the 1220 Hp very well. I had done the corrections on Table 512 to get the same columns crossing the zero line as in Table 511, smoothening them out as well. Then I had adjusted the graph for J=1.8 to a coefficient of 0.1422, giving that graph a smooth shape as well, and I didn´t do anything else for the first test.

Just with 44.2 Hg 1083 Hp Military power I only got about 285 mph. To get to 314 mph, I had to apply WEP, and adjusting the throttle, it needed 1220 Hp to reach 314 mph. The propeller sets itself to 34.4 degree pitch at 314 mph, so that must be correct.

Then I increased propeller efficiency at J=1.8 for the second test, and it now only required 1120 Hp to get to 314 mph.
With only Military Power I´m getting 290 mph. Of course I tried regulating Torque and Drag, but that seriously messed things up elsewhere.

Thanks for the indication on the interpolation problem. I had a look at last year´s "Flying Swallow" Thread, and it seems to be a case of the sim not selecting the pitch it should, probably because it interpolates insufficient power to get there.

Update: I fiddled around with the graphs, both the 30 degree and the 35 degree ones, to give a little more power and speed in the 290-310 mph range. This made the sim select a better pitch, which now at 314 mph is a little higher than before, 34.6. Speed goes a little over 314 mph, to 316, but it is unavoidable as otherwise it will fall short again.

Not very easy, this one, but it worked.
Thanks again for pointing me in the right direction!
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

For the Kawasaki Ki 61 Flying Swallow, the Propeller Pitch Selection seemed to be working as it should.
The problem as I stated was that it was an interpolation issue.
In other words, there are only Specific entries for 30 Degrees and 35 Degrees.
If the actual pitch happens to be 32.5 Degrees, there is no direct Graph, so the computer averages the values that it has for the 30 and 35 Degree curves. Look at the Dotted Line that is plotted there and you will see what happens.

For your Military Power test, you are focusing your concern at J=1.8 (319 MPH) but the problem that you are encountering is not there.
You got stuck at J=1.6 (284 MPH).
This is why I suggested that you label your Graphs with actual MPH as well as the Advance Ratio.
Please observe that I did that with the Graphs for the Ki 61.
I don't see those numbers very well without a converter either.

The other thing worth discussing is: Why are you using 0.1422 for a Propeller Power Coefficient?
Is it because that is what I stated a few posts back? Are you sure my number is correct?
What do your own programs tell you the Power Coefficient should be?

I can tell you that I have two or three spreadsheets that all do some calculations with Propeller Power Coefficient.
They don't all agree with each other. The error is around 1% which I believe is due to differences in conversions and precision for Air Density but there might simply be an error in one or more of the spreadsheets.

By the way, messing with Engine Torque and Aerodynamic Drag is a really bad idea when you are trying to tune the Propeller Tables.
You are trying to tune specific areas of the Tables to have a very local (in speed) effect and the Engine Torque and Drag have a general systemic effect at ALL speeds.
Use it later for fine tuning if you absolutely need to but by that time, things should be pretty much under control and you are only trying to tune a couple MPH over the entire speed range.

- Ivan.

[aleatorylamp]

Hello Ivan,
Well, it worked out quite well, and I quite soon found out that Torque and Drag were best left alone!
It´s not perfect yet, but I´m tired so I´ll continue tomorrow, although 316 mph at S.L. is just about acceptable for military power.

You are right, with the 35 degree graph, I had to work on the J=1.6 column, but also bring down the J=1.8 column from your 0.1422 to 0.135
because 0.1422 was really meant for J=1.77, and mainly because it was (and still is slightly) overshooting the 314 mph mark.

Then, I not only had to regulate the 35 degree graphs, but also the 30 degree ones, because the pitch I was getting at 285 mph or 290 mph, was 31.1 and 31.6 degress. Of course, that required ironing out the graphs at J=1.4 as well. Giving a bit more power and speed with the graphs got me over the kind of "wall" there seemed to be between 290 and 312 mph.

Anyway, I found it very interesting indeed - not only fixing the interpolation problem, but the whole activity of working on this kind of propeller.
The pennies started dropping when I saw the graph tables of your BV141B, and remembered that the graphs in table 512 had to go to zero as well as those in table 511. After that, I could visualize a kind of pattern, and things started falling into place.

So, thanks very much again! Cool stuff!
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Have you thought for a moment that I, as Chief Engineer of Ivan's Workshop, give an impression that I know what I am doing but if you look at what has actually been accomplished, perhaps it is a bit overstated.

The Propeller Division of Ivan's Workshop is very new.
We have only designed 4 Propellers and perhaps provided Technical Support for another 2 Propeller projects.
Of those Propeller Designs, only TWO are in Production. (FW 200, and BV 141B)
The Kawasaki Ki 61 remains stuck for various reasons.
The Airacobra is still in the Development stage.

There is a lot of Research happening, but not a lot actually leaving the Workshop.
The main objective when creating the Propeller Division was to be able to produce Aircraft that could not be built properly with Propellers that are available, but that objective seems pretty far off at the moment.

- Ivan.
Lead Engineer,
Propeller Division.