A WW2 fighter design for rugged areas

[HoHun]

Hi Scott,

Auto Rich is usually a pressure carb setting, Full Rich is usually a manual mixture setting.

Interesting, I didn't know that distinction!

I was in fact not remembering this perfectly right ... the terms were "AUTO RICH" on early models and, on later models, "RUN" with an override position "EMERGENCY FULL RICH".

So that sort of aligns with the common use you describe, though both carburettor types are described as "injection-type carburetors" on p. 11 of the manual. The older variant is a "two-position carburetor", which has an "AUTO LEAN" setting to complement the "AUTO RICH".

There are also two types of automatic manifold pressure regulator. The older one only works above 41" Hg boost pressure, and the newer covers everything goes to 20" Hg.

My guess was that the settings we see on the R-2600-8 versus the GR-2600-A5B-0 probably indicate a similar change in the engine-associated hardware between the two models.

Regards,

Henning (HoHun)

 

[tomo pauk]

I'm not entirely sure. The above page also provides a 2100 rpm "cruising" setting, which doesn't appear to have a special function if it's not "maximum continuous". The next page, which maybe contributes a bit to a better understanding, also has a 2000 rpm "cruising - desired" setting, as well as a 1550 rpm "cruising - long range" setting. The latter is the only one of the three that's not "full rich".

Cruising on 2100 rpm (combined with the lower boost used) has it's function - that of 'economic maximum' cruising, with power still being good. It cuts the consumption by some 40% over the max cont, and power by some 25%.
Granted, long range cruising is still more frugal on fuel, with lower rpm, boost and power, mixture on "cruising-lean".

However, I don't think that the cruising settings will make or break our brave new fighter.

Hi Tomo,



Does it fit our timeline, though? I don't really know when it was first used - my impression was pretty late in the war, but that's mainly based on the belt feed never making it into the Fw 190, which is a pretty weak "data point"

Regards,

Henning (HoHun)

Fw 190 eventually gotten the MG 151/20 instead of the MG FFMs, certainly an improvement (if one does not mind the increase in weight).
The belt-fed MG FFM is noted in the manual for the Do 217J, dated November 1942 - link.

 

[tomo pauk]

Auto Rich is usually a pressure carb setting, Full Rich is usually a manual mixture setting. Were -Ds getting some engines with standard carbs?

Standard carbs on 1st line US engines of ww2 were pressure-types, including the Packard Merlins. Float carbs were long gone on the 1st line engines by the time P-51D emerged.
British used float carbs by some time 1943/44-ish, depending on engine type (again, talking about 1st line engines).

 

[HoHun]

Hi again,

For 1942 and on, the VI is the better (and later) one, despite the nomencature that might mislead us in believing the XI was the better and later.

I just found this scan on my hard disk ... so we have power settings and ratings with their durations at least:



I also found information on the full throttle height in a table scanned from a Swiss book, which is a bit vague in not telling us for which power settings they are valid, but it's probably better than nothing: 1450 and 3600 m. That's fairly low for our purposes.

Power is given as 1355 HP at 2400 rpm, and take-off power as 1615 HP at 2900 rpm. However, the 2400 rpm weak mixture cruise with a gear switch altitude of 5000 m seems to indicate a higher full throttle height than suggested by the Swiss book, even though it's for +2 lbs/sqin boost.

There's also this table: https://www.enginehistory.org/Turbochargers/Superchargers/Table1.jpg

It shows the +6 lbs/sqin rating, and I suppose that's probably the rating also used for the two rated altitudes, 5500 ft and 13500 ft (1700 m and 4700 m respectively), though rpm and power are not given in this table.

The Beaufighter manual shows the propeller size as 12 ft 9 in, by the way ... might need a bit more ground clearance than the propellers we'd put on a Merlin or Allison.

This test shows the performance of a Beaufighter with Hercules VI engines, but unfortunately, no actual power figures are given:

Beaufighter (F) Mk.VI Testing

Regards,

Henning (HoHun)

 

[HoHun]

Hi Tomo,

However, I don't think that the cruising settings will make or break our brave new fighter.

Well, you asked!

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

Power is given as 1355 HP at 2400 rpm, and take-off power as 1615 HP at 2900 rpm.

Here is a set of power figures for the Hercules XVI ... does anyone know if that's just a Hercules VI with automatic mixture regulator, as might be possible from the scant information I've seen?

In this Lancaster data sheet, Hercules VI and XVI seem to be considered identical in their power figures:

http://www.wwiiaircraftperformance.org/Lancaster/Lancaster_II_ADS.jpg

Interestingly, this Halifax data sheet shows a somewhat different set of power figures for the Hercules XVI:

http://www.wwiiaircraftperformance.org/Halifax/Halifax_III_ADS.jpg

The Pilot's Notes for the Short Stirling do feature Hercules VI ratings for 87 octane fuel, to my surprise - to be used in training.

Regards,

Henning (HoHun)

 

[tomo pauk]

The Pilot's Notes for the Short Stirling do feature Hercules VI ratings for 87 octane fuel, to my surprise - to be used in training.

FWIW, the pilot manual for the late P-40s notes the use of 91 oct fuel for state-side use of these aircraft, both for Merlin and V-1710:

 

[HoHun]

Hi again,

Here is a set of power figures for the Hercules XVI ... does anyone know if that's just a Hercules VI with automatic mixture regulator, as might be possible from the scant information I've seen?

This thread sort of confirms this:

Comparing Merlin versus Hercules Engined Lancaster Performance

rec.aviation.military.narkive.com

It also provides climb power figures:

Climb Power (1 Hr)

1355 HP/ 2400 RPM/+6 4,750' Low (MS)
1240 HP/ 2400 RPM/+6 12,000' High Blower (FS)

Wait ... Tomo already provided these figures in post #189 ... now I feel really stupid! :-D

In any case, here the corresponding data for the R-2800-8's MIL rating:

1700 HP @ 4500 ft (2600 rpm)
1450 HP @ 12000 ft (2600 rpm)

That looks so much better that we can probably forget about the Hercules VI if the R-2800-8 is available.

Regards,

Henning (HoHun)

 

[tomo pauk]

It also provides climb power figures:

Climb Power (1 Hr)

1355 HP/ 2400 RPM/+6 4,750' Low (MS)
1240 HP/ 2400 RPM/+6 12,000' High Blower (FS)

Wait ... Tomo already provided these figures in post #189 ... now I feel really stupid! :-D

In any case, here the corresponding data for the R-2800-8's MIL rating:

1700 HP @ 4500 ft (2600 rpm)
1450 HP @ 12000 ft (2600 rpm)

That looks so much better that we can probably forget about the Hercules VI if the R-2800-8 is available.

Mil power != climb power.
By R-2800-8, I reckon you mean R-2600-8? Mil power for the R-2600-13 (Army's equivalent of the -8) was available for 5 minutes.
The R-2600 is still heavier and bulkier than the Hercules.

 

[HoHun]

Hi Tomo,

Mil power != climb power.
By R-2800-8, I reckon you mean R-2600-8? Mil power for the R-2600-13 (Army's equivalent of the -8) was available for 5 minutes.
The R-2600 is still heavier and bulkier than the Hercules.

Oh, sorry - I keep mistyping R-2800 when I mean R-2600 ... the latter is not a fighter engine, my muscle memory always defaults to the larger engine! Weird.

Yes, I meant the R-2600-8. I believe the R-2600-9 and R-2600-13 were basically the same engine. <edit: disregard the following, I got confused.>The B-25 manual page above lists the MIL power setting as available without time limit in level flight, but with a five minute limit in climb. That's indeed not directly comparable with a dedicated climb power setting that gives 30 min or even 60 min of climb, but when we're talking about combat, it'll happen somewhere between climb speed and top level speed most of the time, so to tame the complexity, I'll go with MIL for the R-2600 here.</disregard>

The Hercules is also competing with the V-1650-1, which is even light and less bulky than the Hercules. Do you happen to have Hercules VI weights, by the way?

Here's the engine comparison so far, not quite apples to apples but not entirely apples to oranges either:

V-1710-81 MIL rating (15 min): 3000 rpm/44.2" Hg, 1125 HP @ 15000 ft
V-1650-1 climb rating (60 min): 2850 rpm/48.25" Hg, 1220 HP @ 9750 ft and 1130 HP @ 17000 ft
Hercules VI climb rating (30 or 60 min?): 2400 rpm/+6 lbs/sqin, 1355 HP @ 4750 ft and 1240 HP @ 12,000 ft
R-2600-8 likely rating (have to look this up): 1500 HP @ 5900 ft (2400 rpm/37.5" Hg) and 1350 HP @ 13000 ft (2400 rpm/40.5" Hg).
R-2600-8 MIL rating (>5 min climb, unlimited level): 2600 rpm, 1700 HP @ 4500 ft and 1450 HP @ 12000 ft

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

Here's the engine comparison so far, not quite apples to apples but not entirely apples to oranges either:

Corrected version ... I got confused because the R-2800-8's "climb" rating from the B-25 manual is the same as the "continuous" rating from the USAF designation list, and I thought Tomo's comment was referring to the Hercules' climb rating.

V-1710-81 MIL rating (15 min):
- 1125 HP @ 15000 ft (3000 rpm/44.2" Hg)

V-1650-1 climb rating (60 min):
- 1220 HP @ 9750 ft (2850 rpm/48.25" Hg)
- 1130 HP @ 17000 ft (2850 rpm/48.25" Hg)

Hercules VI climb rating (30 or 60 min?):
- 1355 HP @ 4750 ft (2400 rpm/+6 lbs/sqin)
- 1240 HP @ 12,000 ft (2400 rpm/+6 lbs/sqin)

R-2600-8 climb/continuous rating (>5 min climb, unlimited level):
- 1500 HP @ 5900 ft (2400 rpm/37.5" Hg)
- 1350 HP @ 13000 ft (2400 rpm/40.5" Hg).

Hope I didn't screw up again

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

Corrected version ...

I found the Vultee Vengeance manual with the R-2600-13, and this permits 5 min of climb at MIL, followed by climb at normal settings (which previously called "climb/continuous") with no time limit. I've attached the Specific Engine Flight Chart from that manual.

Current version of our engine comparison:

V-1710-81 MIL rating (15 min):
- 1125 HP @ 15000 ft (3000 rpm/44.2" Hg)

V-1650-1 climb rating (60 min):
- 1220 HP @ 9750 ft (2850 rpm/48.25" Hg)
- 1130 HP @ 17000 ft (2850 rpm/48.25" Hg)

Hercules VI climb rating (30 or 60 min?):
- 1355 HP @ 4750 ft (2400 rpm/+6 lbs/sqin)
- 1240 HP @ 12,000 ft (2400 rpm/+6 lbs/sqin)

R-2600-13 maximum continuous rating (no time limit):
- 1500 HP @ 6700 ft (2400 rpm/37.5" Hg)
- 1350 HP @ 13000 ft (2400 rpm/41.0" Hg).

Regards,

Henning (HoHun)

 

[tomo pauk]

Current version of our engine comparison:

V-1710-81 MIL rating (15 min):
- 1125 HP @ 15000 ft (3000 rpm/44.2" Hg)

V-1650-1 climb rating (60 min):
- 1220 HP @ 9750 ft (2850 rpm/48.25" Hg)
- 1130 HP @ 17000 ft (2850 rpm/48.25" Hg)

FWIW, max cont rating for the V-1870-18 (2600 rpm, 38 in Hg):
- 1000 HP @ 14400 ft

Military rating (15 5 min) for the V-1650-1; 3000 rpm and 48.2 in Hg:
- 1240 HP @ 11500 ft
- 1120 HP @ 18500 ft

 

[Justo Miranda]

Blackburn Firebrand Mk. I and Fairey Firefly Mk.I

 

[HoHun]

Hi Tomo,

Military rating (15 min) for the V-1650-1; 3000 rpm and 48.2 in Hg:
- 1240 HP @ 11500 ft
- 1120 HP @ 18500 ft

What's the source? I only have a P-40F & L SEFC which shows 5 min MIL.

Regards,

Henning (HoHun)

 

[tomo pauk]

Hi Tomo,



What's the source? I only have a P-40F & L SEFC which shows 5 min MIL.

Regards,

Henning (HoHun)

Whoops, it is indeed a 5 min rating.
I'll edit the post.

 

[HoHun]

Hi again,

Current version of our engine comparison:

V-1710-81 MIL rating (15 min):
- 1125 HP @ 15000 ft (3000 rpm/44.2" Hg)

V-1650-1 climb rating (60 min):
- 1220 HP @ 9750 ft (2850 rpm/48.25" Hg)
- 1130 HP @ 17000 ft (2850 rpm/48.25" Hg)

Hercules VI climb rating (30 or 60 min?):
- 1355 HP @ 4750 ft (2400 rpm/+6 lbs/sqin)
- 1240 HP @ 12,000 ft (2400 rpm/+6 lbs/sqin)

R-2600-13 maximum continuous rating (no time limit):
- 1500 HP @ 6700 ft (2400 rpm/37.5" Hg)
- 1350 HP @ 13000 ft (2400 rpm/41.0" Hg).

Attached a comparison graph so all engines' power levels can be conveniently compared without the need for mental gymnastics ...



Regards,

Henning (HoHun)

 

[Scott Kenny]

FWIW, the pilot manual for the late P-40s notes the use of 91 oct fuel for state-side use of these aircraft, both for Merlin and V-1710:

View attachment 717275

Never thought I would see those numbers!

 

[HoHun]

Hi Tomo,

V-1710, as used on P-40N, or on P-39N/Q, or on the P-51A, should propel the perspective fighter beyond 600 km/h, as it did with the fighters listed.

I made a few calculcations for all four of our engines, based on the P-51A linearly scaled up to preserve the original power loading, and it seems that the most difficult criterion to meet is the 6 min to 6000 m climb requirement, at least with the engine settings that are good for 15+ minutes.

Halfway through, I developed some doubts about my idea of P-51A drag, and got sidetracked into graphing all of the Allison Mustang data from Mike's website.

Here's the result ... the F3R and F4R are pretty much the same rating-wise, the F21R is a dedicated low-altitude variant in the F20R is our V-1710-81 with increased supercharger speed. A bit like the Merlin 46, in a way.



Data quality wise, the F3R/F4R data sets group quite nicely, the F21R/F20R data has fewer sets, but more variation.

The third graph shows a single airframe, the only cannon-armed aircraft in the set, but it had three or four different individual engines for all the tests, and it was also used to measure the difference between a fresh standard paintjob and a highly polished one. Interesting stuff!

There's quite a bit of variation in the powerplants beyond the mere type number, like fishtail and standard exhaust stacks, "streamlined manifold" (which I can't place right now), manual vs. automatic boost control, different/absent backfire screens, and a "sealed radiator intake" ... I presume that must have meant sealing the gaps in the panelling, not in the actual intake, but it doesn't seem to be explained any futher. There's also talk of radiator screens, which I guess were used in cold weather to avoid overcooling. Weights of the tested aircraft varied considerably, and last but not least, propellers of different diameters were also used, and sometimes the size used for a test wasn't stated.

The take-away for our rugged fighter probably is that we can use a virtual P-51A as a basis and make changes that sacrifice speed in order to meet other requirements, but we have to take the weight issue very seriously or we'll not meet the climb rate requirement.

Regards,

Henning (HoHun)

 

[Scott Kenny]

The take-away for our rugged fighter probably is that we can use a virtual P-51A as a basis and make changes that sacrifice speed in order to meet other requirements, but we have to take the weight issue very seriously or we'll not meet the climb rate requirement.

So, probably more like the P-51F/G/H series?

 

[tomo pauk]

Here's the result ... the F3R and F4R are pretty much the same rating-wise, the F21R is a dedicated low-altitude variant in the F20R is our V-1710-81 with increased supercharger speed. A bit like the Merlin 46, in a way.

The -81 (F20) was probably in-between the Merlin XII and 45/50? Altitude power was still lacking, despite the (too) late arrival of the V-1710s with 9.6:1 S/C drive; unfortunately, it will take a V-1710 to be outfitted with the aux compressor to beat the Merlin 46, and also the 20 series and the 45 & 50.

The F4 was, basically, the high-boost version of the F3, as allowed/rated by Allison (ie. not a field mod).
Wrt. the altitude power, the F3 (eg. on the P-40E and P-51) and F4 were in between the F21 (low-alt engine, used on the A-36) and the F-20 (on P-40M/N, P-39N/Q and the P-51A).

The take-away for our rugged fighter probably is that we can use a virtual P-51A as a basis and make changes that sacrifice speed in order to meet other requirements, but we have to take the weight issue very seriously or we'll not meet the climb rate requirement.

NAA P-509 perhaps?

 

[red admiral]

The take-away for our rugged fighter probably is that we can use a virtual P-51A as a basis and make changes that sacrifice speed in order to meet other requirements, but we have to take the weight issue very seriously or we'll not meet the climb rate requirement.

Empty mass was one of my reasons for not using P-51 as a baseline. e.g. smaller, lighter Spitfire airframe size (i.e. rough cross sectional area distribution, wing area) can fit the same engine and also has scope to be cleaned up to get to a drag coefficient more like the P-51. And there's space for sufficient fuel as is.

But you could also take a P-51 like aircraft and make it lighter

 

[HoHun]

Hi Scott,

So, probably more like the P-51F/G/H series?

Good point, that's a great parallel!

In fact, that's what the USAAF actually recommended after the evaluation of the P-51 for tactial suitability ... quoting from Mike Williams' wwiiaircraftperformance.com:

Conclusions

It is concluded that:

a. The subject aircraft is the best low altitude American fighter aircraft yet developed, and should be used as the criterion for comparison of subsequent types.

b. If possible, the power loadings of this fighter aircraft should be materially reduced, without increasing the wing loading.

c. To reduce the power loadings of the aircraft, excess weight in the structure, and accessories not vital to operational use should be eliminated, and engine performance increased.

P-51 Tactical Trials

Could well be that this actually contributed to launching the P-51F/G/H project, by the time the quoted report was provided, the Allison-engined Mustang probably wasn't a good subject for a long-running improvement program any more.

Regards,

Henning (HoHun)

 

[HoHun]

Hi,

Empty mass was one of my reasons for not using P-51 as a baseline. e.g. smaller, lighter Spitfire airframe size (i.e. rough cross sectional area distribution, wing area) can fit the same engine and also has scope to be cleaned up to get to a drag coefficient more like the P-51.

Hm, the Spitfire is pretty much the same size as the P-51. Cross sectional area might not actually be such a good measure, as part of the Mustang's cross section was due to its radiator duct which actually reduced drag compared to the Spitfire's wing radiators.

The lower mass of the Spitfire makes a big difference, though. Do you happen to have empty, non-equipped figures for both types available for comparison purposes?

We could argue that we should be able to build a P-51 airframe that doesn't weigh more than a Spitfire airframe, as the two types are virtually of the same size. (Only an approximation, of course.)

Regards,

Henning (HoHun)

 

[drgondog]

The -81 (F20) was probably in-between the Merlin XII and 45/50? Altitude power was still lacking, despite the (too) late arrival of the V-1710s with 9.6:1 S/C drive; unfortunately, it will take a V-1710 to be outfitted with the aux compressor to beat the Merlin 46, and also the 20 series and the 45 & 50.

The F4 was, basically, the high-boost version of the F3, as allowed/rated by Allison (ie. not a field mod).
Wrt. the altitude power, the F3 (eg. on the P-40E and P-51) and F4 were in between the F21 (low-alt engine, used on the A-36) and the F-20 (on P-40M/N, P-39N/Q and the P-51A).


NAA P-509 perhaps?

The P-509-3 was about 10% smaller than X73 with 157gal fuel and GWmax = 7,010 pounds. It's design armament was either 2x50 nose, 4x50 wing or only 4x20mm. Top estimated speed was 412mph at 15,000 feet, 320mph @SL.

Based on NACA 2516-34 airfoil and Allison V-1710-39/F3R for the 509-1, but expected to be better with projected High Speed/LowDrag wing. The NAA 23016 was fall back.

When the mission requirements grew in late April, the wing P-509-3 morphed and grew from 28' span to 35' to accomodate 170gal, and GW Max grew to 7400pounds. Top speed estimate reduced to 385mph at 15,000 feet. But 73X grew slightly more before end of May 1940 to 8277pounds GWmax. The NA-73 emerged at 8642 pounds.

Of all the P-509/X73 variants, the first X73 iteration late April/early May 1940 projected at 7400 pounds would have been closest airframe to meet the requirements assuming the new NAA/NACA 45-100 wing. The ROC, assuming same power available vs Power required of the final NA-73 but 1200 pound slighter (7400/8642= 0.85) -----> 17% greater ROC

 

[HoHun]

Hi Tomo,

The F4 was, basically, the high-boost version of the F3, as allowed/rated by Allison (ie. not a field mod).

Ah, thanks! I've spent some time staring at the ratings in "Vee for Victory", listed on p. 434/435 as (format is HP/RPM/FT/in Hg):

F3R: MIL 1150/3000/11700/44.6, WER 1490/3000/4300/56.0
F4R: MIL 1150/3000/12000/42.0, WER 1580/3000/2500/60.0
F20R: MIL 1125/3000/15500/44.5, WER 1410/3000/9500/57.0
F21R: MIL 1325/3000/02500/47.0, WER 1500/3000/5400/52.0

On other pages, Vees for Victory tends to have variations of these, so they're not really set into stone I guess.

The one thing I can say for sure is that the F21R full throttle heights must be wrong in some way, as at the same rpm, the setting with the lower boost pressure should have the higher full throttle height, not vice versa

It's also a bit surprising that the F21R is rated at 52" Hg for WEP, while normally with the lowest supercharger speed and the supercharger rotor being identical, one would expect it to have the highest boost limit. The British actually ran a test with throttle wide open even at sea level, which gave 59" Hg boost pressure with ram. 52" Hg must have been reached at around 3500 ft, so maybe the static full throttle height was 540 ft, and because that looks unlikely, someone accidentally added another trailing zero?

Regards,

Henning (HoHun)

 

[Scott Kenny]

Hm, the Spitfire is pretty much the same size as the P-51. Cross sectional area might not actually be such a good measure, as part of the Mustang's cross section was due to its radiator duct which actually reduced drag compared to the Spitfire's wing radiators.

Crud, the Mustang's radiator ducting actually added thrust, not just reduced drag!

 

[HoHun]

Hi again,

F21R: MIL 1325/3000/02500/47.0, WER 1500/3000/5400/52.0

I just found the V-1710-87 (=F21R) SEFC ... here's the comparison:

Vees for Victory: F21R: MIL 1325/3000/2500/47.0, WER 1500/3000/5400/52.0
V-1710-87 SEFC:   F21R: MIL 1325/3000/5400/46.5, WER 1500/3000/2500/52.0 with ram

So the altitudes for the two ratings were switched, but contrary to what I assumed, these are altitudes given under the assumption that some kind of ram effect is present.

I wish I knew how that ram effect convention works! Some German charts state *their* convention pretty well, but I haven't seen anything for US charts.

The problem is, now I wonder what's with the rest of the ratings I found in Vees for Victory ... are they static, or are they meant for unkown ram parameters, too?

F3R:   MIL 1150/3000/11700/44.6, WER 1490/3000/4300/56.0 - SEFC: similar, "with ram"
F4R:   MIL 1150/3000/12000/42.0, WER 1580/3000/2500/60.0 - SEFC: similar, "with ram"
F20R: MIL 1125/3000/15500/44.5, WER 1410/3000/9500/57.0 - attached power chart shows similar figures "without ram", but SEFC again similar, "with ram"
F21R: MIL 1325/3000/02500/47.0, WER 1500/3000/5400/52.0 - I found the attached power chart ... "without ram", but 2500 ft seems to be the critical altitude for 47" Hg indeed. The critical altitude for 52" Hg is below sea level though.

What a mess! :-D

Regards,

Henning (HoHun)

 

[tomo pauk]

F21R: MIL 1325/3000/02500/47.0, WER 1500/3000/5400/52.0

On other pages, Vees for Victory tends to have variations of these, so they're not really set into stone I guess.

The one thing I can say for sure is that the F21R full throttle heights must be wrong in some way, as at the same rpm, the setting with the lower boost pressure should have the higher full throttle height, not vice versa

Good catch.
We might indeed remind ourselves that data should be always cross checked
Eg. the data sheet for the A-36 (-87 engine, or the F21R) notes the rated altitude of 2500 ft for the WER of 1500 HP, but it lists it under the 'with ram' column (the chart you've posted is for no-ram conditions).
Mil power rated altitude is listed at 5400 ft in that sheet.

Perhaps in the Vee's the two rated altitudes were indeed mistakenly swapped, and listed as no-ram values?

Crud, the Mustang's radiator ducting actually added thrust, not just reduced drag!

Please, do tell.

So the altitudes for the two ratings were switched, but contrary to what I assumed, these are altitudes given under the assumption that some kind of ram effect is present.

I wish I knew how that ram effect convention works! Some German charts state *their* convention pretty well, but I haven't seen anything for US charts.

Charts and tables should be without the ram effect, unless there is an additional set of figures/graphs that is noted as with ram.

The problem is, now I wonder what's with the rest of the ratings I found in Vees for Victory ... are they static, or are they meant for unkown ram parameters, too?


What a mess! :-D

As above - the F21R values from the table should indeed be with ram. Others should be static, or at least that is what I get after cross-checking.

 

[HoHun]

Hi Tomo,

Perhaps in the Vee's the two rated altitudes were indeed mistakenly swapped, and listed as no-ram values?

I just waded through a heap of original charts, and they're all over the place - I don't think it's just a typo in Vee's for Victory.

As it's all so muddy, I tried to approximate the static full throttle heights for the 4 engines by relying on the F20R MIL rating's critical altitude without ram, and assuming manifold pressure is proportional to supercharger speed while the pressure ratio is constant. That's not entirely true, but probably not far off.

Here the results:

F21R MIL 47/2800 (nominal: 5400) - F21R WEP 52/0 (nominal: 2500)
F3R MIL 44.6/8500 (nominal: 11700) - F3R WEP 56/2400 (nominal: 4300)
F4R MIL 42/10000 (nominal: 12000) - F4R WEP 60/500 (nominal: 2500)
F20R MIL 44.5/11300 (nominal: 15500) - F20R WEP 57/4800 (nominal: 9500)

The good thing is, the difference between approximated and nominal critical altitude is mostly in the same ballpark for both MIL and WEP of the same engine, except for the F3R where the MIL rating is farther off.

The bad thing is, the difference is the biggest for the F20R, which happens to be the engine of interest for our rugged fighter.

(I've found a chart where both "with ram" and "without ram" figures are given, and they differ by a maximum of 2500 ft for WEP, and probably 2000 ft for MIL - the scan is not clearly legible there. So for the F21R and F4R, that might be a fit, for the F3R the MIL value is borderline, and for the F20R, it's off.)

Fortunately, we have the Mustang tests, where I can try to see if the test results can be explained by the combination of engine data and a reasonable value for ram effect, so I can cross-check my approximation.

Regards,

Henning (HoHun)

 

[tomo pauk]

Here the results:

F21R MIL 47/2800 (nominal: 5400) - F21R WEP 52/0 (nominal: 2500)
F3R MIL 44.6/8500 (nominal: 11700) - F3R WEP 56/2400 (nominal: 4300)
F4R MIL 42/10000 (nominal: 12000) - F4R WEP 60/500 (nominal: 2500)
F20R MIL 44.5/11300 (nominal: 15500) - F20R WEP 57/4800 (nominal: 9500)

The good thing is, the difference between approximated and nominal critical altitude is mostly in the same ballpark for both MIL and WEP of the same engine, except for the F3R where the MIL rating is farther off.

Is there a reason why the F20R will not be able to make 44.5 in Hg at ~15000 ft without ram?

 

[HoHun]

Hi Tomo,

Is there a reason why the F20R will not be able to make 44.5 in Hg at ~15000 ft without ram?

My above figures are only for checking the consistency between the data sets. I'm haven't committed to an opinion yet.

The F20R might well be able to make that ... here's a report showing a drop to 44.5" Hg at about 16800 ft with ram:

Mustang II Performance Trial

Mustang II Performance Testing, wartime flight trials and reports of Mustang aircraft. Mustang II data.

www.wwiiaircraftperformance.org

If the P-51's intake has an efficiency of 80%, which is a good but not impossible figure, the 44.5" Hg/3000 rpm full throttle height of this particular engine would be 12300 ft under static conditions.

That 1000 ft more than my approximation would suggest, but 3200 ft less than what the rating specified.

Unfortunately, the US reports usually don't list the boost values, so we only have a handful of British reports to go by, and it will be difficult to learn something from the US reports as we don't really know at which settings they were running their engine. These changed often enough!

Regards,

Henning (HoHun)

 

[tomo pauk]

The F20R might well be able to make that ... here's a report showing a drop to 44.5" Hg at about 16800 ft with ram:

Mustang II Performance Trial

Mustang II Performance Testing, wartime flight trials and reports of Mustang aircraft. Mustang II data.

www.wwiiaircraftperformance.org

If the P-51's intake has an efficiency of 80%, which is a good but not impossible figure, the 44.5" Hg/3000 rpm full throttle height of this particular engine would be 12300 ft under static conditions.

Thank you for the feedback.

I'll still go with the book values, though.

 

[HoHun]

Hi Tomo,

I'll still go with the book values, though.

The rating value of 15500 ft would also be possible if the Mustang's ram efficiency would be 22%, which is a poor but not impossible value.

The magnitude of the ram effect can be cross-checked by looking at the full-throttle height in the climb, if we have climb data using the same rating.

Regards,

Henning (HoHun)

 

[HoHun]

Hi again,

The magnitude of the ram effect can be cross-checked by looking at the full-throttle height in the climb, if we have climb data using the same rating.

Ah, there's good data in the same report.

In climb at 3000 rpm, boost drops to 44.5" Hg at about 13700 ft. That completely rules out a static full throttle height of 15500 ft, obviously

Regards,

Henning (HoHun)

 

[drgondog]

Crud, the Mustang's radiator ducting actually added thrust, not just reduced drag!

The result of the added energy to the air flowing across the radiator tubes, emerging into a squeezing plenum through a 545 sq in radiator/intercooler face to a 118sq in opening - unquestionably generated a 'jet effect' of emerging flow of high temperature air.

Lednicer's model showed a net of 3% loss of available Hp (drag) (@.5M, 18000 feet) by assuming aft shutter exit temp of 170F degrees. Temp of coolant assumed at 212F. At 180 degree exit temp, the drag is zero. Above 180 there is net thrust.

Horkey agreed with you but all Performance calcs for high speed were assumed 'zero'.

The P-51H flight tests at 90" did achieve positive net thrust but coolant temps were in 240-250F range.

 

[drgondog]

Thank you for the feedback.

I'll still go with the book values, though.

The stated GW of 8630 is 100% internal loadout. In this example the P-51A with F20 engine time to climb to 20,000 feet was 6.9 minutes. Carving 75 gal to 105gal for fighter condition probably achieves 6 minute objectve.

 

[tomo pauk]

Hi again,



Ah, there's good data in the same report.

In climb at 3000 rpm, boost drops to 44.5" Hg at about 13700 ft. That completely rules out a static full throttle height of 15500 ft, obviously

Regards,

Henning (HoHun)

See here the P-40N with the same engine still having the boost of the 44.5 in Hg at 16400 ft in climb.
However, this report (pdf) shows that 1105 HP (~44 in Hg?) was available on juist 15000 ft.

See here the same engine, but on the P-40M, 14500 ft rated altitude (but listed under 'with ram' column - a typo?).
Here - 14600 for the P-40M.

I'll agree that 15500 might've been over-selling it a bit, 14500 or 15000 ft was probably more realistic. The data sheet for the P-40N does show 15000 ft for mil power in no-ram conditions.
15500 ft was probably the initial manufacturer's value, these were perhaps achieved once the 'madam Queen' vanturis in the intake were installed (winter of 1943/44), since these allowed for removal of the backfire screens? My understanding, FWIW.

 

[HoHun]

Hi Tomo,

15500 ft was probably the initial manufacturer's value, these were perhaps achieved once the 'madam Queen' vanturis in the intake were installed (winter of 1943/44), since these allowed for removal of the backfire screens? My understanding, FWIW.

Thanks for the charts, and you're quite right ... best to consider all sources, and possible variations with time.

I just went through all the SEFC (and equivalent) charts I had saved over the years and organized them, pulling some of them out of PDF files. Unfortunately, a few of them are borderline illegible :-(

Here are the results ... I organized them with file names like ...

Military designation/Allison designation/SEFC/YY-MM-DD/original file name (if not from PDF file)

Dates were sometimes (partially) illegible, I substituted Xs where I wouldn't even guess the digit(s). Dates are somewhat uncertain where a file is closely cropped, as apparently the original date could stay in the table when a revision was issued later, with the revision date showing outside the table.

If you have any more, I'd love to see them, but please no duplicates as I'm tired and sick of sorting and comparing now! ;-)

Graphs are still unsorted, but mercifully fewer in number.

Regards,

Henning (HoHun)

 

[tomo pauk]

best to consider all sources, and possible variations with time.

Very, very true.