Empty weight of drop tanks

Well, your "rule of the thumb" for estimating the empty weight of a drop tank might be "quick & dirty" but a lot more educated than mine. That's why I'm eating data/infos for breakfast, lunch & dinner... Mind you, I could drop a self-note to add a ratio for known items (re: national origin & material used) and see if I would end up with something meaningful. The same has been done for bombs (to illustrate the differences between GP & AP) and I must have used a similar not-so-crude one when imagining the fictional JK-200 rocket launching & bomb carrying fictional fuel tanks

Some more data points:

230 ImG / 276 USG capacity auxiliary bomb bay tank
250 lbs empty weight
Halifax Mks II/V, III, VI, VII aircraft

Computed from Airplane Characteristics & Performance publications:

58 USG capacity drop tank
387 or 388 or 392 or 394 or 398 lbs installed weight
39 or 40 or 44 or 46 or 50 lbs empty weight including all fittings, attachments, and braces
SB2C-4/-5 and SBD-5/-6 and F4F-4 and FM-2 and TBF-1C aircraft

100 USG capacity drop tank
684 lbs installed weight
84 lbs empty weight including all fittings, attachments, and braces
SB2C-5 aircraft

150 USG capacity drop tank
1,048 or 1,037 or 1,035 or 1,023 lbs installed weight
148 or 137 or 135 or 123 lbs empty weight including all fittings, attachments, and braces
F6F-3/-3N/-5N and F4U-4 and FG-3 and F2G-2 aircraft

150 USG capacity drop tank (self-sealing)
1,078 lbs installed weight
178 lbs empty weight including all fittings, attachments, and braces
FG-3 aircraft

275 USG capacity auxiliary bomb bay tank
1,883 or 1,879 lbs installed weight
233 or 229 lbs empty weight including all fittings, attachments, and braces
TBF-1C and TBM-3 aircraft

Lastly, the Pilot's Flight Operating Instructions manual for the P-40N has this to say in regards to its fuel system:

Aircraft serial numbers 42-104429 through 42-104904 = 122 USG internal capacity plus one 52, 75, or 170 USG tank under the fuselage. Aircraft serial numbers 42-104905 and subsequent = 156 USG internal capacity plus one 150 USG tank under the fuselage. The long-range fuel system effective from aircraft 42-104829 onward allowed one 170 or 225 USG tank to carried under each wing for ferrying purposes.

Good work! I already posted this some time ago but here it is again. By the way, I have read that F4F and F4U used non-metallic drop tanks.

Regarding your comment including all fittings, attachments, and braces on some aircraft

In the case of aircraft like the P-39 and P-40 the braces etc were all part of the aircraft and remained attached to the aircraft when the tanks were not carried so should not be included in the tank weight.

In the case of the P-38, P-51, etc the pylon remained on the aircraft after the tanks were dropped and were again seldom removed so should not be included in the tank weight.

In the case of the Ki-43 and A6M (and probably all other Japanese aircraft that came later) the braces and fairings were integral with the tanks so those depart with the tanks and must be included as you do.

In the case of the Spitfire the slipper tank includes all except the two hooks that supported the rear so very little was left on the aircraft after the tank was released. These hooks weigh very little but should, like on US aircraft, not be included in the tank weight.

Mega thanks to you all. The amount of "background data" will trigger a necessary "page split", but this not a complaint LOL. All the best, Pierre Deveaux, Brussels.

MiTasol said:

Regarding your comment including all fittings, attachments, and braces on some aircraft

In the case of aircraft like the P-39 and P-40 the braces etc were all part of the aircraft and remained attached to the aircraft when the tanks were not carried so should not be included in the tank weight.

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Keep in mind that statement applies to USN aircraft only as it is the weight of the entire drop tank installation that is calculated from the ACP figures.

Let's use the FG-3 as an example. The weight with 237 gallons of fuel, 13 gallons of oil, 2,400 rounds of ammunition, and no external stores, was 12,800 lbs.. Adding a 150 gallon drop tank and 7 more gallons of oil raised the weight to 13,875 lbs. Subtracting the figures: 13,875 - 12,800 - 52.5 (the weight of the additional oil) = 1,022.5 lbs. This logically would be the complete weight of the drop tank installation. Remove the weight of the fuel, 900 lbs, and that leaves 122.5 lbs, the empty weight of the drop tank and installation.

Adding a second 150 gallon drop tank, with this one being self-sealing, brought the weight up to 14,953 lbs. Subtracting from the single tank condition: 14,953 - 13,875 = 1,078 lbs. This logically would be the complete weight of the self-sealing drop tank installation. Remove the weight of the fuel, 900 lbs, and that leaves 178 lbs, the empty weight of the self-sealing drop tank and installation.

I hope this makes things clearer.

Very much so. Thank you.

I had not picked up on that.

One more:

150 USG capacity drop tank (self-sealing)
1,090 lbs installed weight
190 lbs empty weight including all fittings, attachments, and braces
F4U-1C/-1D aircraft

PierreDeveaux said:

Well, your "rule of the thumb" for estimating the empty weight of a drop tank might be "quick & dirty" but a lot more educated than mine. That's why I'm eating data/infos for breakfast, lunch & dinner... Mind you, I could drop a self-note to add a ratio for known items (re: national origin & material used) and see if I would end up with something meaningful.

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Your idea is not that bad. The 75 USG tank is 62.5 ImG, which you'd put at 62.5 lbs, not far off from the 60 lbs of one example. Similarly, the 400 ImG Lancaster bomb Bay tank you'd have at 400 lbs, reasonably close to the 428 lbs empty weight. So your system works in some instances. The advantage of the ratio is that it shows the typical range for a given capacity, allowing those tanks significantly above or below that range to stand out and be examined more closely to determine why.

In any event you've inspired me to create my own drop tank list file. Perhaps between all the interested parties in this thread we can put together one quite good reference file.

PierreDeveaux said:

The same has been done for bombs (to illustrate the differences between GP & AP) and I must have used a similar not-so-crude one when imagining the fictional JK-200 rocket launching & bomb carrying fictional fuel tanks

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If you want bomb data, I've put in a lot of time researching the topic. I started on it in response to the Gary Grigsby games War in the West and War in the East 2. These games are fantastically detailed in terms of the ground units and weapons, but the aerial ordnance is decidedly less so, especially the latter game which actually uses the same generic bombs for both the Axis and Soviet forces, which is just wrong. I resolved to correct this.

33k in the air said:

I wish I could remember what thread it was posted in so I could properly credit the person who provided it.

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Well, I can answer the above now. While searching the forum for another thread I stumbled across the source. The T.O. document was posted by MiTasol in this post.

In WW2 drop tanks were shipped assembled. The much larger drop tanks used by the USAF late in the 20th Century had to be assembled. It was expected that if a real war started all the admin types on any base in Europe would be used to assemble drop tanks; I am sure that would have gone superbly well.

After WW2 the Piper Skycycle light airplane reportedly used a non-metallic drop tank designed for the F4U as a majority of the fuselage. I guess they were cheap and available in large numbers, so that was clever, but it seems there were few such adaptations.