Analysing and modifying the AFX file with QBasic.

[aleatorylamp]

Hello Ivan,
The wing doesn´t look too bad now - so I think I won´t change it too much more. On purpose... just to leave in a tiny bit of the original AFX, apart from the part-names and my goofily-named added parts.

I remember years ago extensively re-working an AF5 AFX of a twin-engined passenger liner. I upgraded the 8-sided fuselage to 12, took out some bleeds here and there and put in moving parts, and uploaded it, commenting that it was an extensive upgrade and re-work of so-and-so´s model with such-and-such improvements.

I had the feeling that the original author was really upset about this, because less than 2 weeks later, he uploaded the original aircraft with a 16-sided fuselage!! Obviously without moving parts, because parts-count didn´t allow it, but you could tell that the guy wasn´t pleased at all. I always asked myself why he had published his AFX in the first place - it just didn´t make sense.

Anyway, I have NO problem mentioning the author of the original source files for copyright reasons. I don´t mind at all. There´s a great time difference between the original P39 AFX and the present development of the model anyway, so it´s more than justified, and it is obvious that a lot of time and work goes into an extensive upgrade and re-work, and I doubt the original author will be offended.

Regarding the propeller, I thought the problem was in the down-slopes of the efficiency graphs, that don´t go to zero. But you mention the Power Coefficient graphs, as being messed up. So it´s really both graph tables!

That obviously complicates my trying to avoid working with the Power Coefficient graphs at all costs.
I have no clue if my experiment of dealing exclusively with the propeller efficiency table will work, or if there is even any point in trying.

At the moment, after clipping down the graphs, I slightly raised the point on the down-slope prior to the new zero position, to compensate the loss I was getting before. Now I´m getting 6 Hp more Military Power and 16 Hp more WEP Hp. I haven´t measured speeds yet, but will do so tomorrow.
It´s probably quite crazy, and probably won´t work - Or, if it does, it will be because the Power Coefficient Graph is messed up, like you said!

Anyway...
Cheers,
Aleatorylamp

 

[Ivan]

I remember years ago extensively re-working an AF5 AFX of a twin-engined passenger liner. I upgraded the 8-sided fuselage to 12, took out some bleeds here and there and put in moving parts, and uploaded it, commenting that it was an extensive upgrade and re-work of so-and-so´s model with such-and-such improvements.

I had the feeling that the original author was really upset about this, because less than 2 weeks later, he uploaded the original aircraft with a 16-sided fuselage!! Obviously without moving parts, because parts-count didn´t allow it, but you could tell that the guy wasn´t pleased at all. I always asked myself why he had published his AFX in the first place - it just didn´t make sense.

Hello Aleatorylamp,

You're right. With that kind of reaction, he probably never should have made his AFX available to the public.
Some people like their egos stroked. They want the world to tell them how great they are. Maybe that was this fellow.
Having someone publish a greatly improved version wasn't acknowledgment of his superiority.

Regarding the problems with the stock P51D Propeller Graphs, there are many of them.
The Propeller Efficiency Graphs have a few problems (some of which I inherited with my own graphs), but the bigger problem in my opinion is the Power Coefficients.
We have already gone through this a few times, so I won't review the process here, but it might be worthwhile to figure out what actual Power Coefficient your Drive Train has at the most interesting altitudes and see how the simulator would use those numbers to choose the Propeller Pitch Angle at each speed.
One you have that, it would be interesting to cross-reference that series against the Propeller Efficiency Table to see what that would give you.
I believe when you find the Pitch Angles, the results may be enlightening.

It is interesting that you are getting differences in Military and Emergency Power by changing the Propeller Graphs.
I don't believe that should be happening. There should be no effect on Horse Power at all.

- Ivan.

 

[aleatorylamp]

Hello Ivan,

Hello Aleatorylamp,
It is interesting that you are getting differences in Military and Emergency Power by changing the Propeller Graphs.
I don't believe that should be happening. There should be no effect on Horse Power at all.

OK, thanks! I ´ll check that I´ve got the correct .air file. Some degree of confusion is not totally ruled out, as
perhaps the latest .air file I was testing didn´t overwrite the last one I had.

Apart from that, it seems that although it would not be totally incorrect to correct the propeller efficiency table,
there isn´t much point in doing anything with them unless one were to fix the power coefficient table a bit as well.
OK, then. I´ll see, but I may not have the stamina to go through the whole thing.

Cheers,
Aleatorylamp

 

[aleatorylamp]

Propeller Progress of sorts.

Hello Ivan,
After my efforts in making the model nicer, although I´m not totally done yet,
I thought I WOULD try once more to tackle the propeller graphs,
even if it WILL be difficult, from what you have just commented!
At least I´ll be able to say that I didn´t just chicken out at the beginning after all!
... maybe I´ll chicken out half way through, but I hope not.

I repeated the operation I did at 1 o´clock this morning, making sure about the correct .air file.
You are (of course) totally right: There is NO difference whatsoever in HP, between clipping the graph
down-slopes to Zero in Table 511, or leaving them at the default p51d gentle "perpetumobil" slope style.
I don´t know what went wrong with that trial last night, but it went wrong!

Anyway, regarding the "clipped" graphs in table 511:
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.

I´ll try and check the J factor involved and increase the down-slope on the graph corresponding to the WEP speed,
and then take it from there.

Update: Fixing the down-slope of the 40 degree pitch graph, to improve WEP speed,
didn´t give more than 360.3 mph, so I´m still 11.7 mph short.

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.

Would this be the correct reasoning to follow?

I suspect I´m missing something here: The Thrust at the speeds and pitch angles mentioned,
so that thrust is the reference to go for, during the changes in Torque and Power Coefficient.
Oh dear... I don´t know what I´m letting myself in for.

Sorry about all the questions, which may seem silly or wrong to you,
but if I don´t understand the reasoning, I can do nothing with the graphs.
I hope you understand... Thanks!

Cheers!
Aleatorylamp

P.S. I was looking at your Post #88 in this thread... but I´m afraid I´m none the wiser. Maybe all this IS
too complicated and will be a nightmare for you to try and explain, if I can´t even understand Post #88.
Perhaps I should really take one step back and leave the .air file where it is - it´s not too bad anyway.
A.L.

 

[aleatorylamp]

Wing

Hello again!
Here are some screenshots showing the modified wing-root and wing, with a curved and not flat leading edge.
The shape is generally more natural, and wing textures, as well as the air intakes near the fuselage, also seem
to behave themselves better.
I haven´t lined up the mid-fuselage and nose textures yet so that the shape modifications show up better.
I should really tone down the door outline a bit - it´s too dark and contrasts too strongly.
Cheers,
Aleatorylamp

 

[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]

Table 511

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]

Perpetual Motion and the Wonders of Combat Flight Simulator

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.