Project Martin A-30 Baltimore

[aleatorylamp]

Performance adjustments

Hello Ivan,

Here are some more exact test results after finer adjustments in Zero Lift Drag and Torque Table. To have some basic reference for the MkV, I was using full throttle.

Performance aimed for:
1700 Hp
2600 RPM
284 mph at Sea-Level
308 mph at 15000 ft
Please correct me if the aimed performance is wrong.
I increased it a little from the Mitchell data because the Baltimore, being lighter and smaller, was perhaps faster, especially with the 1700 Hp engines instead of the 1600 Hp ones.

And this is what I got:
Sea-Level:
1679 Hp
2592 RPM
286.5 mph
44 Mpsi
1678 Thrst

15000 ft:
1716 Hp
2592 RPM
310 mph
40.3 Mpsi
1586 Thrst

I´m not sure exactly how to get it down to 1450 Hp at 15000 ft and keep the speed the same. I was looking in the tuning tutorial, but for the moment increasing both torque and friction tables hasn´t been working.

Engaging WEP I got a considerable increase, too much in fact, but I´m not exactly sure what I´m looking for here. I got 294.8 mph and 1853 Hp at SL and 327 mph with 2050 Hp at 15000 ft.

Anyway, I was wondering if you have any comments, which I would appreciate very much!
Thanks in advance.
Cheers,
Aleatorylamp

 

[aleatorylamp]

Hello Ivan,

I thought I´d try and get it a bit closer to what I was aiming for.
Now I got it at exactly:
1700 Hp with 309.1 mph for 15000 ft and
1672 Hp with 286.3 mph for sea-level,
both at 2590 RPM.

But, perhaps I´m under a misconception here... still...

If as you said the 15000ft Maximum speed reading of 308 mph is not at Maximum power then it is a Maximum Continuous power. Also, the most reliable source states 320 mph as Max. Speed, and this is the speed I should be aiming for with flat-out accellerator performance.

The maximum performance specified in the Mitchell engine chart for
Maximum Take-off power is 1700 Hp with 44 Mpsi at SL.
Strangely enough, their Emergency Maximum power is also 1700 Hp, but with:
42 Mpsi at 4500 ft and
41 Mpsi at 12000 ft.
BOTH these power settings are only to be enjoyed for 5 minutes!

So there is no WEP in reality, only just flat-out on the accellerator. Question: How do you select Mpsi, or does it just happen with the altitude?

Then: Maximum Continuous power appears to be completely different:
It is at 2400 RPM,
1500 Hp with 38 Mpsi at 6700 ft and
1350 Hp with 39 Mpsi at 15000 ft.

Now the thing is to get the engine revs to coincide with the power for the lower settings.

Update:
The problem with the 1500 Hp at 6700 ft and the 1350 Hp at 15000 ft is that the revs always stay the same, at 2590 and won´t go down to 2400 RPM.
Then: Getting the maximum full flat-out, 5-minute 1700 Hp speed up to 320 mph instead of 305 or 308, now brings up the Maximum Continuous speeds upto around 295 mph at low altitude and 305 mph at higher altitude. Perhaps this is correct?

Anyway, I´ll keep on trying.
Cheers,
Aleatorylamp

 

[Ivan]

Multiple Subjects

Hello Aleatorylamp,

You have brought up a LOT of subjects.
Hopefully I won't miss anything significant.

Regarding the Wright R-2600 engine being direct drive on the Martin Baltimore:
The specifications given thus far pretty much guarantee that this is not the case because....
The Propeller is 11 feet in diameter.
Maximum Engine Speed is 2600 Revolutions Per Minute.
This combination means that even with the aeroplane not moving, the Propeller Blade Tip would be travelling at 1497 feet per second or 1021 MPH.
Considering that the Speed of Sound is 762 MPH at Sea Level and only decreases with altitude, it means the Propeller Blades would be Supersonic which simply would not work well if at all.

Check your propeller specifications. If you cannot achieve maximum RPM of 2600 with 1700 HP and an 11 foot Propeller, there must be something wrong.

How are you configuring WEP?
If 1700 HP @ 2600 RPM with 44.0 inches Manifold Pressure is Take-Off and the Normal Maximum, how are you getting above that?
44.0 inches is THE HIGHEST Throttle Setting for this engine. There should be no higher.
The question is whether you allow it as normal full throttle or require that WEP be engaged to achieve this is your choice.
What is WEP is a subject I have discussed before in a thread specifically on the subject.

I do not know where you are getting Mpsi from.
The Specific Engine Flight Chart (SEFC) Table shows everything in Inches of Mercury (inches Hg) which is the same unit used in the AIR file.
With other nationalities, there may be a conversion of units required but not for the United States.

How do you select Manifold Pressure?
With the Throttle control on your Joystick. (No, it is not all that precise.)
How do you select Engine Speed?
Use <Control>Function Keys 1-4.

As I commented earlier, the 1450 HP at Altitude was probably not a maximum rating.
It probably is not a "Maximum Continuous" rating either.
My guess is that it is the "Military" rating of the engine.

Regarding the performance of the Baltimore being better than the Mitchell, I would rate them as quite similar.
The maximum speeds are pretty near identical for the Baltimore as compared to early Mitchells (such as the C).
The two aeroplanes were fairly comparable in size though the Mitchell was a bit bigger.
The A-20 Boston / Havoc was almost exactly the same size with the same engines and also considerably faster.
The Mitchell was slightly bigger and was not a particularly sleek aeroplane which would imply that the Baltimore was considerably worse from the standpoint of Form Drag.

Hopefully I addressed a few of the issues.

- Ivan.

 

[aleatorylamp]

Hello Ivan,
Thank you for your pacience. Yes, you have addressed all the issues!

Thanks about clarifying the propeller details. I´d just thought, as it was 1.6 ft smaller there might have been a chance it wasn´t geared, but my doubts are cleared.

Mpsi would be Manifold Presure per square inch, and in the Mitchel spec sheet they call it Manif.Press.(Boost). Then they also add a column with the blower speed as being High or Low. Beckwith´s guage calls it Mpsi, so I called it that... but we´re talking about inches of Hg in any case, so that will be OK.

Very well, as the Spec sheet calls it Emergency Maximum, I´ll implement WEP with 44 iches of mercury Manifold Pressure so that you have to press F10 for take-off, and leave normal Maximum Contimuous at 42 inches of mercury. That will be a very practical solution, so we can avoid going into Emergency Maximum just by giving full throttle. Excellent idea! I´ll make the necessary changes and report back (hopefully) improved results!

Interesting how the Havoc and Mitchel compare to the Baltimore!

Thanks, and
Cheers,
Aleatorylamp

 

[aleatorylamp]

Hello Ivan,


It´s unfortunately not working.
The main assumption is that the engine parameters in the Mitchell B-25C .air file would be adequate, but perhaps not the propeller tables, as the Mitchel propellers are 12.58 ft in diameter, so I´ve exchanged the propeller efficiencuy and cthrust coefficient tables for the stock Fw190a´s 11 ft propeller tables.
However, the main problem is still remains, that performance is too high at altitude when set correctly lower down, and if set correctly for at altitude, performance is poor lower down.


Perhaps it will be best to go back to square one, so I will sumarize the settings and the aims that would be used to start off with. The main thin is that these parameters should be the correct ones.


1) 1700 hp with 2600 RPM is WEP (at all altitudes). (Are the present 2592 RPM good enough?)
2) 1450 Hp is military power - presumably at 2400 RPM?
2400 RPM with 1350-1500 Hp (6700-12000 ft) is specified as continuous power,
PROBLEM: The .air file will not lower the RPM.
3) The props are geared (no problem).
4) WEP is with 44 Mpsi and normal Continuous Power is with 42 Mpsi.
4) 284 mph at SL is at Maximum Continuous speed.
5) 305 mph at SL would presumably be with WEP.
6) Specified 320 mph max. speed at 15000 ft would obviously be with WEP.
7) 305 mph (or 308 mph) at 15000 ft would be with Maximum Contimuous speed.


Cheers,
Aleatorylamp

 

[aleatorylamp]

Hello Ivan,

It´s unfortunately not working.
The main assumption is that the engine parameters in the Mitchell B-25C .air file would be adequate, but perhaps not the propeller tables, as the Mitchel propellers are 12.58 ft in diameter. So, I´ve exchanged the propeller efficiencuy and thrust coefficient tables for the ones of the stock Fw190a. However, the main problem is still remains: Performance is still too high at altitude when set correctly lower down, and too low lower down if set correctly at altitude.

Perhaps it will be best to go back to square one, so I will sumarize the settings and the aims that would be used to start off with. The main thin is that these parameters should be correct.

1) 1700 hp with 2600 RPM is WEP (at all altitudes). (Are the present 2592 RPM good enough?)
2) 1450 Hp is military power - presumably at 2400 RPM? ...but the .air file won´t lower ther RPM.
3) 2400 RPM with 1350-1500 Hp (6700-12000 ft) is specified as Max. Continuous power, but the .air file won´t lower the RPM.
4) The props are geared (no problem).
5) WEP is with 44 Mpsi
6) Max. Continuous Power is with 38-39 Mpsi. (Oops! I had 42 here!)
7) 284 mph at SL is at Maximum Continuous speed.
8) 305 mph at SL would presumably be with WEP.
9) Specified 320 mph max. speed at 15000 ft would obviously be with WEP.
10) 305 mph (or 308 mph) at 15000 ft would be with Maximum Contimuous speed.

It should get better now I´ve seen my mistake in the Max. Contimuous Power Manifold Pressure setting.

Cheers,
Aleatorylamp

 

[aleatorylamp]

Closer tuning, getting better.

Hello Ivan,
I seem to be progressing... It´s a bit difficult as this engine is very different from the old slow ones from the Great War that I´d got used to!

I finally had success after some futility trying out something you mentioned in the tuning thread, i.e. reducing the excessive surge of performance at altitude with balanced increases in the torque and friction tables. This surge has now become appreciably smaller: I increased the torque from 0.585 to 0.637 and the friction from 39 to 60.
I have the feeling that this has also caused a convenient increase in the difference in power between WEP (44 Mpsi) and the normal Maximum Continuous Power (39 Mpsi) which is at full throttle.

All in all, the performance envelope looks better now:
I´ve included some intermediate altitudes that are mentioned in the B-25C Spec. Chart.

Max RPM is always 2593 RPM... high altitude, low altidude, WEP, non-WEP... (no way to correct this for the moment).

500 ft:
Max. cont: 1460 hp, 291.4 mph
WEP: 1700 hp, 304.5 mph

6700 ft:
Max. Cont: 1543 hp, 304.5 mph
WEP: 1794 hp, 314.5 mph

12000 ft:
Max. Cont: 1624 hp, 313.5 mph
WEP: 1886 hp, 322.1 mph

15000 ft:
Max. cont: 1670 hp, 317.3 mph
WEP: 1937 hp, 325.9 mph

Where to go from here?

1) I could try and reduce the whole envelope by about 5 or 6 mph. That way the sea-level Max. Continuous setting would get down nearer the 284 mph you had mentioned, and also the high-altitude WEP power would fit into the 320 maximum specified. But we wouldn´t be getting the 305 Mph top speed with WEP for sea level (although that was only an assumption on my part, and my not be real anyway).

2) I could continue with the balanced increases in the torque and friction tables and see what happens.

Anyway, that´s the situation at the moment, just in case you have any comments!
Cheers,
Aleatorylamp.

 

[Ivan]

Hello Aleatorylamp,

I still see some serious problems in your numbers.
(Most notably the 1937 HP at 15,000 feet.)
I am guessing that your Supercharger Boost Gain is way too high still.
I would suggest swapping in the MitchellC AIR file I sent to you and see how the engine behaves.
There is something strange going on if you cannot ever hit 2600 RPM.
It is probably the Propeller Tables.

I believe the main problem is that you are tuning too many things that all affect each other.
Get the power curve right before you start tuning performance.
I am pretty sure it can be done because the Mitchell I built is just about even with the Baltimore in terms of speeds and altitudes.
If it wasn't for having to undo all of the Yaw and Roll Coupling, it would be a simple task to convert it for the Baltimore.

I believe you are still a bit confused about what the different power ratings actually mean.

The Take-Off / Emergency setting on some engines requires a power adder or anti-detonant such as MW50, or Water Injection such as on the stock FW 190A (doesn't really work) or stock Thunderbolt
On other engines it is just an increased supercharger boost setting as found on the stock Mustang.
I don't like to use WEP as found on the P-51D because it is way too short and because there is no warning before catastrophic damage.
Still other engines such as found on the late FW 190A used fuel as anti-detonant which means it simple does not run out.

Unless it is a limited supply of anti-detonant, and WEP can't be used without it, engines generally don't self destruct after 5 minutes 10 seconds of WEP use. It is an operating procedure thing which is specified by the engine manufacturer in the hopes that the engine will reach its expected service life before overhaul.

On some engines, the manufacturer may be overly conservative as I discussed in the P-40 Warhawk thread.

As you noted, the use of Emergency Power is discouraged and procedurally may be limited to 5 minutes.
The question you need to answer for yourself is whether the engine is durable enough for this procedure to be meaningless or how you go about enforcing a limitation.

Military Power or Rated Power is one of the "maximum" ratings. It may have different names depending on which military you are in.
Sometimes it might be called "Climb Power" and be limited to 10 to 30 minutes.

Maximum Continuous is just as the name implies. You are permitted to run at this power level as long as you have fuel.
This is maximum Cruise Power. Maximum "Cruising Speed" is achieved at this setting, NOT MAXIMUM SPEED.

Only WEP and "Maximum non-WEP" are important as far as the AIR file is concerned.

Now to address the points you brought up in your previous post:

1. 1700 HP should be maximum at all altitudes. You need to determine whether it is Maximum or WEP enforced and how.
The reality is that you will probably be a bit higher between Sea Level and your Critical Altitude but hopefully not too much higher.
2592 RPM indicates a problem of some kind.

2. 1450 HP is PROBABLY Military Power in my opinion, but it is Military Power at 15,000 feet.
It would be at 2600 RPM.
To adjust RPM downward if you must, Use the Propeller Pitch Control on the Throttle Panel.

3. Maximum Continuous is an interesting piece of information but really irrelevant as far as developing and AIR file.
I typically include this information in the Check List.
Use the Propeller Pitch Control to adjust RPM.

4. Information clearly indicates Reduction Gearing. You need to do some research to determine the ratio.

5. 44 inches Hg is Maximum. You determine whether this is procedural Emergency Power or whether it is enforced WEP.

6. Maximum Continuous rating specified in the SEFC is pretty much irrelevant as far as the AIR file is concerned.

7. 284 MPH at Sea Level is Maximum Level Speed at Sea Level. It is NOT Maximum Cruising Speed.
In CFS, there is no limitation on duration of maximum power but it is not a factor we can control.

8. 305 MPH at Sea Level should be impossible. 284 MPH at Sea Level is with 44 inches Hg and 2600 RPM IMO.

9. 320 MPH at 15,000 feet is with whatever maximum power level the supercharger is able to maintain at that altitude.
Without experimenting a bit, I have no idea what the Manifold Pressure should be.

10. 305 MPH or 308 MPH has no meaning at 15,000 feet if you are trying to build the Baltimore Mk.V.

- Ivan.

 

[aleatorylamp]

2593 max. RPM not a problem!

Hello Ivan,

Thank you for your time and detailed answer. I hope it isn´t putting you out too much, or putting you off!!

Directly using the MitchellC .air file certainly was a good idea:
This .air file also displays maximum revs at 2593 RPM at all times, so it must be an error caused by the computer, which is too modern for CFS1. It would be the equivalent of 2600 RPM on a "normal" computer. So, I´d suggest we ignore this, take it as 2600 RPM, discarding any propeller blade problems or further adjustments.

I saw from the .air file that no WEP is implemented, all powers throttle-controlled. OK, I´ll implement it in the same way. I could also set it as Emergency Maximum Power with Methanol alcohol instead of the short-lived WEP, but I think it´s better the other way.

Taking into account that the Mitchell´s maximum speed is lower than that of the Baltimore, the MitchellC. .air file gave the following performance results at 3 different altitudes under full throttle (Max. revs always 2593 RPM):

500 ft: 1618 hp, 262.8 mph, 44 Mpsi.
12000 ft: 1780 hp, 306 mph, 44 Mpsi.
15000 ft: 1654 hp, 305 mph, 40.3 Mpsi.

Thank you for your answers to the 10 points. I´ll go by that.

Update:
I made the necessary modifications, and have arrived at the following results:
500 ft: 2593 RPM, 1700 Hp, 44 Mpsi, 284.1 mph
15000 ft: 2593 RPM, 1735 hp, 40.2 Mpsi, 308.3 mph

Presumably now it´s looking better. I´ll see if I can manage to increase altitude performance now, which should be possible with a balanced reduction of torque and friction - inversly as I´d done before to lower it.

OK, then! Your continued assistance is excellent and I definitely appreciate it!
Cheers,
Aleatorylamp

 

[Ivan]

2600 RPM is Correct

Hello Aleatorylamp,

Your comment about the MitchellC AIR file had me a little worried.
I decided to run a quick test to see if my earlier testing was correct.

The numbers I am getting are consistent with earlier testing as far as I can remember.
I will confirm when I can find the Testing Sheet I wrote up a few years back.
I had no problem getting a consistent 2600 RPM at all altitudes from 500 feet to 15,000 feet.

I am getting slightly different numbers from your testing and I do not believe it is because of the speed of the computer because the results were the same on another faster computer.

1613 HP for 264 MPH @ Sea Level
1789 HP for 310 MPH @ 12,500 feet <-- This is Peak Horsepower reading.
1654 HP for 306 MPH @ 15,000 feet

All were achieved with 2600 RPM.
In fact, if left alone for the time it took to write down the data, the Speed would slowly climb up to 312 MPH at 12,500 feet and 308 MPH at 15,000 feet.
(My testing protocol is to record a speed as maximum if it does not change for 15-20 seconds.)

I wanted to post some screenshots so you could see what I am seeing but the only flash drive I have by the development machine does not seem to be readable on this HP laptop. (My regular Flash Drive is very near where Anna Honey is sleeping and I do NOT want to disturb her.)

As for actual numbers, You can do anything you want. It IS your project.
Implement WEP or not as you wish. What *I* chose to do may not be the same as what you choose to do.
The question is whether you want to restrict the use of 44 inches Hg Throttle setting.

- Ivan.

 

[aleatorylamp]

Interesting findings.

Hello Ivan,
Hmmmm... Interesting, thanks very much for your comments.

Possibly it´s the Beckwith Gauge programmed for a slightly different CPU - i.e. Pentium, and it reacts differently to the AMD CuadCore that I have, or maybe it´s Windows XP instead of Windows 98. I´ll try it in the Pentium 4 Laptop here with Win98 as soon as I have time to rig it up.

I found that implementing any kind of Emergency Power to use with Key F10 does have its advantages: I gives 6 more HP for Meth-Alcohol and Water Injection, and 7 for WEP. So now the speed I get at 15000 ft is 314 or 315 mph if the SL max. is at 284 mph, and 320 mph if the SL max. is 290 or 289 mph. I chose Water Injection because it doesn´t ruin the engine - the water-cooling factor seems to be beneficial.

To possibly widen the performance vs. altitude difference range, I lowered the friction and torque even more, to .39 and .585 again, but it didn´t get any better, not even going down as far as .30 with friction and 0.557 with torque.

I even tried fiddling the Induced Drag vs. Zero Lift Drag even more - i.e. ID at over 7000 and 0LD at about 60, but there was no change either. Possibly I´ll go for 290 mph at SL and 320 mph with Water Injection.

I´ll see... Thanks again!
Cheers,
Aleatorylamp

 

[Ivan]

Hello Aleatorylamp,

I hadn't wanted to get into any significant detail on the Martin Baltimore, but could not really do anything useful without doing some research myself, So....

I am pretty convinced now that the data you are using from the table on Page 112 of Monografie Lotnicze 97 is simply incorrect.
It appears to be contradicted by multiple other sources.

I have some serious doubt as to whether the engine models specified are actually correct and am pretty sure the power ratings and maximum speed at altitude for the Baltimore Mk.V are both incorrect.
A lot of the other data and drawings are pretty good, but I do not believe the engine data is good.

I suggest you do some general research with other sources and see if you come to the same conclusion.

- Ivan.

 

[aleatorylamp]

Source reliability

Hello Ivan,

Nice conclusion... What it clear is that there are several different pieces of information for the same parameters, but I don´t know enough about aircraft engines to deduce which of these pieces of information could be the most plausible ones, in order to discover which was the most reliable source.

I believe I´ve seen all the different specs I can get at, and the main problem is that the early and later models are not differentiated in some cases. The main information required for an .air file would be the maximum speeds, and here there are quite a few different ones. This is 305, 308, 320 and 329 mph, with or without some altitude reference.

Of course this is a rather lamentable state of affairs, proven by the impossibility of making speeds tally at different altitudes in CFS1.

2951 RPM max for all altitudes in CFS1 (2600 RPM on your computer):
This coincides with the criteria of a CV propeller, but the charts rather suggest Constant Horsepower and varying RPM.

I´d set critical altitude at 25000 ft, thinking that the ceiling would be the criteria, but I found out that it´s the altitude upto which the blower maintains its power, in this case the 44 Mpsi. Probably mistakenly, I´ve kept Mpsi at 44 for 15000 ft., so I´ll conduct some test correcting this.

The Specific Engine Flight Chart states an emergency maximum for 12000 ft at 41 Mpsi and maximum continuouus at 15000 ft at 41 Mpsi, so possibly the critical altitude setting for 44 Mpsi would be the 12500 in the MitchellC .air file.

Well, At the moment, with 2591 RPM on my computer (2600 on yours), and 44 Mpsi all round, I´m getting:

500 ft: 1670 hp - 294.3 mph (280.6 mph without WEP)
15000 ft: 1885 Hp - 320 mph (311.8 mph without WEP)

Looking at the performance of the MitchellC, which appears to have about 40 Mpsi for 15000 ft, so as I said before, I´ll see what happens if I reduce ctirical altitude to ther 12500 ft given there.

Cheers,
Aleatorylamp

 

[aleatorylamp]

Hello Ivan,

Setting the critical altitude to 12500 like on the MitchellC, reduced performance at altitude a bit, although the Mpsi setting still remains at 44.

I tried to get the Hp spanning from 500 ft to 15000 ft in a similar way to that on the MitchellC, arriving at following results:

500 ft: 1603 Hp with 284 mph.
15000 ft: 1830 Hp with 314 mph.

If this looks plausible, then it´s better, but unless of course it doesn´t, then it isn´t.
Now, in reality, I don´t really know what is correct any longer...

If I knew more about aircraft engines, it shouldn´t really be too difficult for me to decide which maximum speed to use - 305, 308 or 320 mph, and at what altitutude. I think the one entry I´ve seen for 329 mph can be discarded - that much, I suppose I know. But for the others... Maybe the Mk I-III units would be candidates for 305-308 mph, and the MkV, 320 mph? I really haven´t a clue.

Cheers,
Aleatorylamp

 

[aleatorylamp]

A matter of interpretation.

Hello Ivan,
Given the impossibility of getting maximum speed in the flight envelope to span 284 mph to 320 mph, possibly it is a matter of interpreting what the specifications say.

I have seen at least 4 sources that state the maximum speed of the Mk.V Baltimore as 320 mph, and some include the altitude of 15000 ft here.

I have seen 1 source stating 312 mph at 11600 ft, then, several others state max. speed at 305 or 308, without stating the altitude or model, and two more which say it´s 305 mph at 11.500 ft but don´t mention the model.

Also, a few sources describing what the Baltimore flew like and the missions it did, don´t state specification details, but do say that the plane was capable of speeds above 300 mph, and that it had the performance of a fighters, being able to outrun some of them.

This could imply that this speed was also possible at sea-level, and would consequently lead me to discard the 284 mph maximum speed at sea-level that you had initially suggested after extrapolating from the Mitchell´s performance. This speed also does not appear in any of the sources.

So, for the sake of looking for a more realistic flight envelope, my exercise this time was to see if the speed range between Sea-Level and 15000 ft could span from about 305 mph to 320 mph, and I did a test. The result IS actually possible with procedural emergency power (no WEP enforced with F10):

Aimed Power: 1700 hp
Critical altitude 12500 ft
Max. RPM all round: 2590 (2600 RPM on normal computers and yours)

Sea-level: 1690 hp - 305.7 mph 44 Mpsi
12500 ft: 1867 hp - 322.5 mph 44 Mpsi < this is the critical altitude.
15000 ft: 1718 hp - 320 mph 40.1 Mpsi

This seems to tie in with the information given by most sources. I wonder what this would be in your opinion? Useful, or useless codswallop?

Cheers,
Aleatorylamp

 

[Ivan]

Impossible???? Maybe Not....

Hello Aleatorylamp,

I was pretty sure tuning the maximum speeds to the range that you wanted wasn't very hard, so I decided to try it myself.
I started with the Mitchell C AIR file and took a few screenshots to show the numbers I am getting on a Pentium 3 866 MHz before and after some very minor tuning.

The picture of the Mitchell model has no real relevance but I thought it looked nice.

The only tuning I did was to adjust the Zero Lift Drag down to 62 using AirEd.
I did absolutely NOTHING else. No Engine Tuning was required to get this performance....

If your target performance was 284 MPH @ SL and 320 MPH @ 15,000 feet, this would be close enough I think.

The R-2600-13 on the B-25C and the R-2600-29 on the A-30A are close enough to be the same engine and you already have the AIR file containing my version of the R-2600-13.....

- Ivan.

 

[aleatorylamp]

Hello Ivan,

Good heavens, how easy that was! A nice surprise, to say the least, thank you very much.

The Germans say, "Why easy, when it can be done complicated too?".
("Warum einfach, wenn es auch komplitziert geht?").

So for a start I´ll just take the whole MitchellC .air file and do the same. Then I´ll shift over the Wing section and adjust the weights, and fuel, but nothing else just in case something breaks!

I was wondering about the 284 mph maximum speed at SL you had recommended, because it contradicts the information reflected in all the sources. As far as I can see, it comes from a calculation derived from the Mitchell, that had the same 1700 hp engines and similar performance, although it was a bit slower.

Well then, let´s see how it goes now! Thanks again.
-Yes, the picture does look nice! ...I like the slightly negative dihedral.

UPDATE:
I used the MitchellC .air file and transferred the weight, fuel and main wing parameters over.
I also had to transfer the smaller propellers diameter: 11 ft instead of 12.58 ft.
Zero Lift Drag is at 69 and induced drag is at 6800, as opposed to your 62 and 6750.
The results I got are as follows, all at 2592 RPM:

500 ft: 44 Mpsi, 1618 hp, 284.0 mph
12500 ft: 44 Mpsi, 1788 hp, 327.4 mph (speed considerably higher than yours)
15000 ft: 40.3 Mpsi, 1654 hp, 326.0 mph (speed slightly higher than yours).

Obviously the smaller propeller is giving the higher readings.
I could perhaps try reducing torque and/or increasing friction a bit.

Cheers,
Aleatorylamp

 

[aleatorylamp]

Almost bang on!

Hello Ivan, again!

Before moving the Friction or Torque Graphs, it occurred to me to lower Boost Gain from 2.54 to 2.4, which in turn lowered the Manifold Pressure slightly at altitude.

Now the results are very much closer:
500 ft: No change: 1618 hp, 284 mph, 44 Mpsi
12500 ft: 1770 Hp 326.2 hp, 43.5 Mpsi
15000 ft: 1615 hp, 322.6 mph, 39.5 Mpsi

This seems to be acceptable, I´d wager.

Cheers,
Aleatorylamp

 

[Ivan]

Hello Aleatorylamp,

Perhaps you don't care, but I think you should check out the Yaw-Roll coupling if you plan on developing from the MitchellC AIR File.
There were a lot of factors that were tuned to make that happen and offhand, I don't remember where they all were, so that is why I was recommending against it.

I get the distinct impression that you did not really understand the Engine Tuning Tutorial.
I am not saying my method is the ONLY way, but if you are intending to use it, it helps to be consistent.

Without getting into the extreme detail, here is the basic idea:

1. We need to tune THREE points on the graph of Power versus Altitude:
They are at Sea Level, Critical Altitude, and Service Ceiling.

2. Tuning the Sea Level Power should be done first.
We need to match Three Pieces of Data: Manifold Pressure, RPM, and Horsepower.
I usually do this by adjusting Torque.

3. Next is the Power at Critical Altitude.
This is done by ADJUSTING SUPERCHARGER BOOST.

4. Last is tuning the Service Ceiling / Absolute Ceiling and this is done by adjusting Torque and Friction and balancing their effects so that the Sea Level Power is not altered significantly.

.....So....

Leave the Torque / Friction alone at this point and just drop the Supercharger Boost a little bit.

Did you notice the choices I made as far as tuning the power curve for the Mitchell?
Note that the Sea Level Power and Critical Altitude Power are both a touch lower than one might expect.
This was done so that the power curve would not be too high at any point in between.

There are other ways to tune the performance without affecting the power curve but there are other side effects.

For what it's worth, earlier versions of the Mitchell were slightly faster than mine.
I built the B-25C/D or Mitchell Mk.II because I thought it was a good balance of combat capability and flight performance.
Later versions were even slower.

Perhaps it is worthwhile for you to post a little of the development and service history of the Baltimore.
Various conflicting information simply did not make sense until I read a bit about the history and put together the most likely scenario.
That history and logical developments is why I do not believe the Page 112 Table is correct.
Also worthy of discussion is how the Baltimore derived from the Maryland and perhaps a little speculation as to why the US military gave it an official designation but never used the aeroplane operationally.

- Ivan.

 

[Ivan]

Our messages crossed. I started my post around the same time you did and was interrupted by guests.
Perhaps you DO understand what I was trying to explain in the Engine Tuning Tutorial.

- Ivan.