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Interesting the inboard .30cal MGs removed and drop tank/bomb racks installed.
C models were the first 20 of an order for 80. They didn't have self sealing tanks and armor that were included on the remaining 60 planes designated P-39D. The fuel tanks weighed an additional 240# and the armor plate and glass weighed another 240#. Delete the 100# nose armor and those two additions (tanks and armor) would have added an additional 380#."THREE-BELL-P-39-AIRACOBRA-FIGHTER-PLANES-IN-FORMATION"
Those are P-39C's. Note the bulges above the nose and the lack of wing guns. The C had four machine guns ( two .30 cal, two .50 cal) and at least provisions for the cannon in the nose, an arrangement that did not last long; they only built 20. I think all the C's built were modified into D's. The D was over 1000 lb heavier than the C's.
The P-39Q had the option to remove the external .50 cal pods and install bomb racks instead.
C models were the first 20 of an order for 80. They didn't have self sealing tanks and armor that were included on the remaining 60 planes designated P-39D. The fuel tanks weighed an additional 240# and the armor plate and glass weighed another 240#. Delete the 100# nose armor and those two additions (tanks and armor) would have added an additional 380#.
7075# P-39C gross weight
+240# self sealing tanks
+140# armor plate/glass
+30# additional 15 rounds of 37mm ammo brings total to 60 rounds
-100# delete twin .30cal MGs in nose to make room for the 15 rounds of 37mm ammo
-300# reduced 50gal gas by self sealing tanks (from 170gal to 120gal)
7055# P-39C with armor plate/glass, self sealing tanks, 37mm cannon with 60 rounds and 2 x .50cal MGs and full 120gal internal fuel.
Compare performance of the P-39C (now fully equipped) at 7055# with the P-39D at 7500#. 10mph faster and 1000feet/minute better climb.
Ivan,Hello P-39 Expert,
In trying to gather some information about the P-39C, I am coming up with some different conclusions.
From what I have been able to find, the Gross Weight of P-39C was 7180 pounds. Maximum Take-Off Weight was 7300 pounds.
There was no provision for external fuel or stores.
The interesting thing about Bell's "Gross Weight" is that they had a tendency not to list full fuel loads there. Full internal fuel loads were considered "Overload". In this case, it seems to fit with the 120 pound difference being explained by 20 Gallons less fuel.
Your addition of 15 rounds of 37 mm ammunition would only bring the total ammunition load to 30 rounds, not 60.
The P-39C only carried 15 rounds and the P-39D only increased that to 30 rounds total.
The big problem comes with the other changes you are proposing.
The changes in fuel load would not affect CG by much, but they are located a couple inches ahead of the empty CG of the aircraft, so it WOULD have some effect.
All the other major deletions are at the nose very far ahead of the CG.
The two .30 cal MG with their ammunition is probably a bit over 100 pounds but about 50 pounds was permanent.
The guns would not have been removed in flight.
The additional 15 rounds of 37 mm ammunition adds back 30 pounds of weight but that was likely to be expended in flight.
The 95 pound gear box armor was there probably as permanent ballast and removing that without something compensating would have been "interesting".
So, although the modified P-39C you are suggesting would certainly be light, it would also be pretty badly out of balance.
The idea that nose armour was permanent ballast is supported by the fact that the weight of this armour was reduced to 70 pounds in some models of Airacobra. I believe armour weights were used to balance the aircraft which explains all the different thicknesses for the same basic pieces of armour plate.
From a flying and handling standpoint, the best piece of armour to delete would be the 29 pound plate behind the oil tank.
The Soviets definitely did that in testing but I am not quite sure if they did that on their operational Airacobra. They also increased the ammunition load for their P-39D up to 270 rounds per gun to push the CG even further forward.
- Ivan.
If you read five different P-39 weight tables you will get five different weights.Hello P-39 Expert,
The comment about "the guns would not have been removed in flight" was just that in the P-39C even after the ammunition was expended, there was still about 50 pounds of .30 caliber machine guns in the nose.
Regarding armour for the oil tank: The Russians probably did as much or more experimentation with the P-39 as Americans did.
I came across a report recently about Spin Tests that were done on the P-39Q in various load conditions.
From what I can tell, of the 5 distinct configurations, most kept their wing armament (!) but 4 of 5 test aircraft had the oil tank armour removed.
A comment was made earlier about the weight difference between P-39C and P-39D being about 1000 pounds.
I believe the actual number to be around 400 pounds with a fully loaded P-39C to be 7300 pounds and a fully loaded P-39D to be around 7700 pounds. One could of course get a lot lighter by carrying only partial fuel and ammunition.
Note that the "Empty Weight" in my spreadsheet isn't really an "empty" weight because obviously items such as coolant, gear oil, and other little essentials are not broken out individually.
- Ivan.
My point exactly, most all the references are a little different. I read for years that a P-39D weighed 5462# empty. The load should have been:The weights for the P-39Q came from the weight and balance chart in the aircraft manual which would be fairly accurate.
I also have a few pages of weight and balance charts for other models of P-39 but have not entered them into the spreadsheet yet.
So what do you believe is most correct for an operational P-39D? (Without deleted equipment of course.) Keep in mind that some of the numbers listed in that spreadsheet are "official" but not likely to be accurate such as pilot weight at 160 pounds.
The weight for P-400 seems particularly high considering that it is carrying a much lighter 20 mm cannon as compared to the 37 mm cannon, but when one looks at the locations of armour, it starts to make a bit more sense because the P-400 had armour over the oxygen bottles in the nose and other aircraft did not. I am assuming the extra weight is in the "miscellaneous" section because it is not accounted for in the armour section.
As for the P-39C, the word "operational" isn't really appropriate. and many of the aircraft did not even fly with armament when they were first put into service as can be see in the photographs. I don't know for sure that my 7180 and 7300 pounds numbers are correct but they do seem to be in the proper range. They came from the "Detail & Scale" book but you may have better sources.
160 Pilot <---- Although this is official, it is not realistic for a pilot + parachute. 200 pounds was the accepted figure for later aircraft.
300 37mm w/ammo
267 .50x2 w/ammo <---- Should be 300 pounds - 161 pounds for guns, 129 pounds for 400 rounds for the two guns.
71 Oil <---- This is about 20 pounds under full oil capacity.
4 Gunsight
8 Oxygen
720 Gas 120gal
130 Armr plate & Glass <---- Subject for more detailed discussion.
1650-1700# total load
5462 Empty weight
7112-7162# Gross weight No nose armor, no .30MGs. Compare with 7710# on your schedule. Would have drastically improved climb and ceiling, and added about 10mph.
Didn't want to bore the internet to death, but regarding the weight of armor there were also a couple of small armor panels in the turnover structure around the rear armor glass that I would have deleted that weighed about 15# each to get to approximately 130# remaining armor.Hello P-39 Expert,
The big question is what exactly is accounted for in the "Empty Weight". Is it really an empty weight or something more like what was commenting about with fluids and some non-disposable loads already in place? Without knowing what else had to go into the aircraft to make it operational, the "empty weight" may not make sense. That is why I don't believe the empty weight as stated in Detail & Scale for P-39C at 5070 pounds is in the same context as other empty weights in the spreadsheet.
Does your 5462 pounds account for coolant, reduction gear oil, and other minor equipment not itemized or is 5523 pounds which does account for these items a better number?
Either way, the difference is only 61 pounds, so we will leave that for later discussion.
The armour weight from my spreadsheet was 262.2 pounds.
From adding up all the weights of each piece of armour that was listed for the P-39D, the total was 261.44 pounds.
I figure the differences are rounding because the numbers in the table were listed to 0.01 pounds.
The gear box armour for the P-39D was 96.08 pounds, so without it, that still leaves 165.36 pounds.
The items you want to delete were
wing .30 cal MG - 92.8 pounds
wing .30 ammunition - 260 pounds (Note that this is max ammunition of 1000 rounds per gun instead of "normal" load of 300 rounds per gun.
Gear Box armour - 96.08 pounds
Deducting these weights from the 7710 pound gross weight of P-39D gives 7261 pounds.
Now we didn't agree on the empty weight or the weight of engine oil, so with these differences accounted for, we now get around 7160 pounds which is in pretty good agreement with what you arrived at.
As I commented before, I don't know that my numbers are any better than your numbers. I just trust them more because I know where they came from, so it seems like there really isn't a great deal of argument about weight.
As for aircraft balance, I am still not convinced that moving the radio would be sufficient to offset the removal of the gearbox armour.
- Ivan.
Didn't want to bore the internet to death, but regarding the weight of armor there were also a couple of small armor panels in the turnover structure around the rear armor glass that I would have deleted that weighed about 15# each to get to approximately 130# remaining armor.
Regarding balance after the nose armor was removed, Bell was able to balance P-39s with 37mm or 20mm cannons and their weight difference was 140#.
The 61# difference in P-39D empty weight may have been the communication equipment (radio). May have been included in load instead of empty weight in the early planes. Just a guess.