Conformal Fuel Tanks

"ttyymmnn" (ttyymmnn) 04/22/2014 at 17:41 • Filed to: planelopik

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Conformal fuel tanks are one of the more recent developments for the modern fighter jet. As the name suggests, they conform to the shape of the aircraft, allowing more fuel to be carried by the plane while helping to reduce the drag created by traditional external fuel stores. The configuration also frees up hardpoints under the wings to carry more ordnance.

A Eurofighter Typhoon with CFTs bulging from the fuselage.

An F-15E Strike Eagle with CFTs added beneath the wing roots.

Servicing the CFT on an F-15E.

An F-16CFT with CFTs added along the fuselage above the wings.

A French Rafale with CFTs added to the spine of the fuselage.

An F/A-18F Super Hornet with CFTs added above the wing root.

ddavidn > ttyymmnn 04/22/2014 at 17:44

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Why did it take so long to invent this? Genuine question about the perils of engineering such a solution.

ttyymmnn > ddavidn 04/22/2014 at 17:45

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I have no idea. Maybe one of Oppo's resident aero engineers will chime in.

Racescort666 > ttyymmnn 04/22/2014 at 17:46

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Hmmm, cool. They look like they accommodate laminar flow pretty well. Any word on whether they're designed to accommodate area rule or any of the other weirdness that comes with supersonic flight?

ttyymmnn > Racescort666 04/22/2014 at 17:47

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You're asking questions way above my pay grade. Hopefully one of our resident Oppo aero engineers will chime in.

Jcarr > ttyymmnn 04/22/2014 at 17:48

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The Strike Eagle's CFTs look pretty natural, but they just look awful on the F-16. Totally ruins the plane for me. The Hornet's aren't too bad. Helps that it's kind of a blocky/angular airframe to begin with.

Diesel > ttyymmnn 04/22/2014 at 17:48

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ttyymmnn > Jcarr 04/22/2014 at 17:49

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The F-16 really does look awful.

Diesel > ttyymmnn 04/22/2014 at 17:50

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Worlds scariest vagina.

Jcarr > ttyymmnn 04/22/2014 at 17:52

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Like it's wearing a backpack.

RamblinRover Luxury-Yacht > ttyymmnn 04/22/2014 at 17:53

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I'm not a resident aero engineer, but for service that's mostly subsonic to transonic, there's a lot of attractiveness to a pylon tank - fewer types needed/more universal, disposable (dump completely rather than compromise aerodynamically at all), cheap, can be jettisoned if leaking, and most of the time you're upping the weight of the airframe/reducing feasible payload anyway, so why not stick it on the pylon?

Dunnik > ddavidn 04/22/2014 at 17:59

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I'm no engineer, and this is just a guess, but it could be because as air combat is dominated by Beyond Visual Range missiles, mounting big gas tanks on the top of the fuselage is not a bad idea, as it probably would have been in WWII and Korea, or even Vietnam.

If true, I can't help but think the Israelis would have a field day against opponents using this, as the IAF famously likes to close in for the kill with guns.

HammerheadFistpunch > ttyymmnn 04/22/2014 at 18:00

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you said you wouldn't let me see those F16 conformal pictures ever again...you promised!

*weeps into keyboard*

ttyymmnn > HammerheadFistpunch 04/22/2014 at 18:01

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Sorry. I should have labeled it NSFW. Or NSFHammerheadFistpunch.

f86sabre > ttyymmnn 04/22/2014 at 18:13

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Hi, I'm an aero engineer ;), but I've never worked on conformal tanks.

As mentioned below, I imagine this style of tank on the lighter fighters is an admission that knife fights are not likely. Now, if we had a situation where close in combat was likely I wonder if these tanks would be along for the ride.

Those tanks on the EF look like they would put some interesting loads through the airframe if one were to pull high Gs with full tanks.

Racescort666 > ttyymmnn 04/22/2014 at 18:30

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Pure speculation ahead: that goofy dip in the tank as it goes over the wing could be to accommodate area rule , that's why I asked in my other post. You don't want the cross-sectional area to change drastically for better transonic and supersonic flight.

Icemanmaybeirunoutofthetalents > ttyymmnn 04/22/2014 at 19:28

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Uniformly ruining clean lean shapes to a plane that apparently had a wasp bite! Yuck! All hail greater fuel carrying capacity!

Jayhawk Jake > ddavidn 04/22/2014 at 21:11

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AE chiming in.

I have two guesses.

1: Manufacturability. Complex shapes are hard. Making a compound curve out of metal is for all intents and purposes impossible. I'd guess these are all composite, and composites weren't really practical for large structures until fairly recently.

2: Aerodynamic design. Seriously, these have to be designed carefully. Especially for a supersonic aircraft. Something like a CFT probably needed Computational Fluid Dynamics to be designed, and CFD is another fairly recent development.

Those would be my guess, another reason could also just be a simple one: they weren't necessary. I'm sure that had something to do with it anyways, but for all I know that's the only reason. These planes may not have been tasked with long range missions until recently, and the change in mission spec may have been the catalyst. As with anything else in engineering, it was likely a combination of many factors that made it take so long.

Jayhawk Jake > Racescort666 04/22/2014 at 21:12

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They HAVE to be area ruled to maintain supersonic capabilities, and looking at the pictures it's pretty clear they are. The Rafale is the best example of an area ruled shape of all these

Jayhawk Jake > RamblinRover Luxury-Yacht 04/22/2014 at 21:14

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Two words: Parasite drag.

Parasite drag is bad. Sticking it on the pylon would make lots of parasite drag. This is better. Plus these are all very fast airplanes, so they really need area ruling to be effective.

Oh, and a pylon tank takes away a mounting point for a payload. They may not have wanted to do that.

Racescort666 > Jayhawk Jake 04/22/2014 at 21:28

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I guess that's what I'm asking. When they do stuff like this, they've already area ruled the airframe, now they're sticking more stuff on it. Or is this just an extension of the area already covered by the pylon mounted tanks? I guess I had assumed that they didn't go supersonic with externally mounted fuel tanks.

user314 > ttyymmnn 04/22/2014 at 22:04

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See, I like how the F-16I looks with CTFs. Maybe it's because the Israeli F-16s have had that boxy spine for a while.

Jayhawk Jake > Racescort666 04/22/2014 at 22:11

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It's a matter of shaping the tanks to keep the area distribution smooth. Obviously it won't be the same performance, but you can minimize the impact.

Racescort666 > Jayhawk Jake 04/22/2014 at 22:16

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Cool. That's what I was wondering.

ttyymmnn > user314 04/22/2014 at 22:52

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It does balance it out a bit, though it still detracts from an otherwise clean, sleek shape. It is getting a bit meaner looking, though.

TwinCharged - Is Now UK Opponaut > ttyymmnn 04/23/2014 at 00:02

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Oh dear God this looks morbidly hilarious.

ddavidn > Jayhawk Jake 04/23/2014 at 00:55

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That all makes sense to me, thanks. What changed in the world of composites recently? My only experience is designing a composites inventory management system (gotta track in/out of freezer, etc etc etc) to maintain standards) and molding a longboard...

Jayhawk Jake > ddavidn 04/23/2014 at 07:54

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It's not that anything changed, they're just relatively new. Any new material has challenges.

Think of it this way: we've been building stuff out of metal for hundreds if not thousands of years. Fiberglass wasn't created until the mid 30's.

RamblinRover Luxury-Yacht > Jayhawk Jake 04/23/2014 at 08:56

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Yep, parasitic drag kills, but as I pointed out the old way remained popular for subsonic use because it was considered acceptable. (Why I said "subsonic" in the first place) Progressively less acceptable the faster you're operating, but other than cross-section, not drastically more drag than other pylon loads, so if you're typically loaded for bear anyway...

As to payload mounting points, it's really only useful if you have a lot of lighter payload that's not overtaxing the airframe as it is - while you can technically push the acceptable gross weight higher by removing wing load going conformal, you're still pushing gross weight up with payload + conformal - past weight you were operating at before, which isn't... optimal.

IOW, conformal makes a lot of sense for transonic and up, on frames with limited pylons or a reason to use as few as possible, missions that are stretching to the breaking point of total plane capacity for range and payload, and frames that have a large number in service so the expense for highly custom conformal setups isn't prohibitive. Mid-range milk runs with maneuvers over target a plus - why bother?