Conformal fuel tanks for ww2 planes?

[tomo pauk]

The thread about the Bf 110's belly thank got me thinking - how useful would be to have conformal fuel tanks on combat aircraft of ww2 era? The Spitfire used slipper tanks (my idea being to install the s-s lining on the inside of 30 or/and 45 gal tank, so they will not be a fire hazard any more than the other internal tanks), while the P-47 was tested with the conformal tank of 70 US gals. Lancaster was tested with dorsal 'saddle' tank, for use in Far East.
Obvously, those tanks can't replace the regular drop-tanks, and some aircraft can't have them installed under belly (P-51 P-38 that don't need them that much, Hurricane), and it would be a problem if the aircraft is already a low performer (like the said Bf 110).

 

[stona]

PR Spitfires had them under the wings, can't remember which Marks so I'm not guessing. They tried something similar on the Fw 190. I'm sure there were others.
Cheers
Steve

 

[FLYBOYJ]

Aerodynamically it's obvious that a conformal external fuel tank would be a better choice, but I think from an engineering standpoint the pylon/ tank configuration is probably the easiest to design, build and deploy. I've put underwing tanks on L29s and 39s and although these aircraft were flown decades after WW2, the design and principal were the same. I was able to hang 2 tanks on an aircraft within 30 minutes, and that was working by myself.

 

[stona]

And of course jettisonable external tanks can be ...errr...jettisoned which incurs no (or minimal due to pylons/racks) performance penalty for the clean configuration.
Cheers
Steve

 

[tomo pauk]

The proposal does not cancel the drop tank facility, but it is trying to improve the 'fixed' fuel tankage, while imposing the drag/speed penalty as low as possible. Eg. the 90 gal slipper tank on the Spitfire V was 'stealing' 17 mph. So I'd not go above the converted 45 gal tank (cost of ~10 mph?), where we could expect maybe 35-40 extra IMP gals once the tank received self-sealing lining. Might be mostly useful in SE Asia, in concert with wing drop tanks of course. Also on the ETO, on the Spit VII/VIII/IX and especially XIV, that have had plenty of engine power to help out. Another recipient of a similar tank, but longer (75 IMP gals?) might be Typhoon/Tempest - so it can carry bombs and extra fuel; plus for the Spit XIV again. Same size (but not shape) for the P-47, obviously it too needs wing drop tanks to matter.

 

[Shortround6]

Part of the trouble with this idea ( and some Spitfires did carry 30 and 45 gallon tanks during combat so it is not that far afield) is that to make them self sealing makes them heavy. To make the tank low drag means keeping the tank thin. Unfortunately a thin tank has a lot of surface area in relation to volume. As a simple illustration imagine two rectangular tanks. #1 is 0.5 feet high, 2 feet wide and 6 feet long. it holds 6 cu ft of liquid and has a surface area of 32 square ft. #2 tank is 1.0 ft high, 2 ft wide and 3 ft long. It too holds 6 cubic ft but has a surface area of 20 sq ft. and so needs only 63% of the self sealing material. A cylindrical tank that is 1.274 ft in diameter and 3 feet long will hold the same 6 cu ft but have a surface area of 16 sq ft. and be roughly 1/2 the weight of the #1 rectangular tank empty.
Tapering the front and back of a conformal tank just makes things worse.

Conforming plain metal tanks may have some use in allowing higher cruise speeds due to lower drag than the higher drag drop tanks (but better drop tanks might help to cancel that out) without a large weight penalty.

Unfortunately for the P-47 the best place for a conformal fuel tank under the belly is right at the fattest part of the fuselage.

According to the caption that is a 75 us gallon drop tank and 75 gallons requires 10 cu ft of space.

 

[RCAFson]

The thread about the Bf 110's belly thank got me thinking - how useful would be to have conformal fuel tanks on combat aircraft of ww2 era? The Spitfire used slipper tanks (my idea being to install the s-s lining on the inside of 30 or/and 45 gal tank, so they will not be a fire hazard any more than the other internal tanks), while the P-47 was tested with the conformal tank of 70 US gals. Lancaster was tested with dorsal 'saddle' tank, for use in Far East.
Obvously, those tanks can't replace the regular drop-tanks, and some aircraft can't have them installed under belly (P-51 P-38 that don't need them that much, Hurricane), and it would be a problem if the aircraft is already a low performer (like the said Bf 110).

The Spitfire's 30 and 45IG slipper tanks were self-sealing from the start as they were designed to be retained in combat. The 90IG tank was not S-S, IIRC.

Other UK aircraft that used slipper tanks were the Fairey Fulmar (60IG) and the Blackburn Roc (70IG). IIRC, the Roc tank was a true conformal tank.

 

[Conslaw]

The teardrop shape of drop tanks is the ideal (or close to the ideal) aerodynamic shape. That's why postwar land speed record cars were made out of surplus drop tanks. This makes it hard for conformal tanks to do better aerodynamically than conventional drop tanks. Where conformal tanks have an advantage is that the weight of the tank can be spread across a larger area of the plane during high-g maneuvers. There is no such thing as a free lunch. Even if you add fuel by lengthening the fuselage, adding fuel without increasing the frontal area, the added weight has to be made up with a bigger wing or higher wing loading. You can expect higher induced drag, that is drag that is created by adding lift.

The relevant factors to consider are different for supersonic jets than subsonic WWII fighters. On the supersonic planes, you have to look at propagation of shock waves in addition to traditional drag minimization. Secondly, Mach 2 fighters like the F15 don't just have to worry about drag as a a maximum speed limitation. Heat from the friction of supersonic airflows can be a limiting factor. It is therefore sometimes possible on a jet to add a substantial conformal fuel tank /weapons pod without exceeding the heat limitations of the vehicle or hurting its supersonic performance. Of course, with extra weight, climb rate and maneuverability might be adversely affected.

 

[Shortround6]

Part of the reasoning behind conformal tanks on modern jets is that they help the jet "conform" to the "area rule". With tanks and a few bits of external ordinance in place the jet may have higher drag than when clean but lower drag than when using conventional drop tanks. Of course if you are not flying at transonic or supersonic speeds then the area rule doesn't apply or is a lot less important.