Forum: F-16 Design & Construction

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

Why isn't the F-16 designed to carry its 370gal tanks on stations 3 and 7? They are rated at 2500lbs, aren't they?

That would free the 4500lbs stations (4 and 6) to carry about twice as much ordinance.

If they do that to reduce wing fatigue, the 4 and 6 could be used for peacetime and the 3 and 7 for wartime with twice the payoad on the 4 and 6..

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My guess would be that stations 3 and 7of the wing isn't as thick as station 4 and 6, the attach point's are also reinforced with a steel sleeve commonly referred to as the "coke bottle" to provide the support to hold full 370's on each wing, the tank has a single post attach point that is about 6 to 8 inches in length that is bolted in side the pylon attach point and a pivot bolt at the rear of the tank attached externally, at least that's what I see each time the tanks are mounted. Somebody esle may have a better explaniation.

Plus the plumbing isn't set up for the tank on 3 and 7.

The hard point at 3/7 doesn't have the required strength for 370 tank. The 2500 lb nominal load is for 5.5g weapons, but the tank is required to go to 6.5g. The biggest strength problem is pitching moment, the load component that tries to twist the wing. Highest pitch moment is from stopping a 5.2g roll with the tank 1/3 full. All the fuel in the tank is in the forward cell. That condition would tear the pylon off the wing. Another serious problem is flutter, which more than likely would rear its ugly head very quickly if a tank was mounted at 3/7. In addition to all those problems, the aft end of the tank pylon would be under the flaperon (no place to attach the pivot, no way to move the flaperon down. Also, tank jettison would likely be a problem due to the wing flexibility farther outboard.

The reason for developing the conformal fuel tank was to do do exactly as you suggest, make 4/6 available for heavy weapon carriage.

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

All these could have been adapted to the other station.. or all the plumbing wiring and pylons would have been inverted between the stations.

Isn't there a more obvious explantion, like aerodynamics, drop tank release or whatever?

If 3&7 are truly rated at 2500lbs then full 370s would exceed the capacity since the tanks empty weigh somewhere in the neighboorhood of 300 lbs empty.

Yes, most of those problems could have been addressed in the original design, but at a significant weight and cost penalty. I already mentioned the flutter and jettison problem. I'm no expert on stability and control, but I suspect flying qualities would be degraded with tanks that far outboard.

Here's another consideration - center of gravity. With the tank farther outboard it is also farther aft. That could lead to the cg being too far aft.

Bottom line is, yes it may have worked, but it would not have been worth it. Heavier tanks, heavier pylon, heavier wing, probable flutter, jettison, and flying qualities prblems, higher cost, etc.

Block 15 hard points were strengthened, so that heavier weapon loads could be carried. That was a much simpler solution. Then the CFT allowed 4/6 to be used for weapons.

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

johnwill wrote:

The hard point at 3/7 doesn't have the required strength for 370 tank. The 2500 lb nominal load is for 5.5g weapons, but the tank is required to go to 6.5g. The biggest strength problem is pitching moment, the load component that tries to twist the wing. Highest pitch moment is from stopping a 5.2g roll with the tank 1/3 full. All the fuel in the tank is in the forward cell. That condition would tear the pylon off the wing. Another serious problem is flutter, which more than likely would rear its ugly head very quickly if a tank was mounted at 3/7. In addition to all those problems, the aft end of the tank pylon would be under the flaperon (no place to attach the pivot, no way to move the flaperon down. Also, tank jettison would likely be a problem due to the wing flexibility farther outboard.

The reason for developing the conformal fuel tank was to do do exactly as you suggest, make 4/6 available for heavy weapon carriage.

Ah ok Johnwill, we posted at the exactly the same time I didn't your answer.

Now it makes sense.

But couldn't they have designed a tank that could withstand 6.5Gs. I guess it would carry about 20% less fuel than for 5.5Gs, so about 300gal. Possibly it could have remained compatible with stations 4 and 6 since the requirement for those stations are less strict.

The difference in terms of range would have been maybe what 5% I guess..

With a lighter tank with a shape optimized for that station wouldn't it have been possible? The F-18E for instance can carry 4 tanks on his wings.

For the issue of clearance with the flaperon, wouldn't it simply have been possible to add an extention, like they did to carry the HARM I believe?

Also a smaller tank would probably give a better clearance with the pylon next to it. Which sometimes apparently is a problem to carry bombs on TERs. So at the same time it would have increased the loadout by 50% for certain weapons. 2 TERs with 3 weapons mounted in a raw on station 4 would carry 3 times as many weapons as 1 TER with 2 weapons on station 3.

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

johnwill wrote:

Yes, most of those problems could have been addressed in the original design, but at a significant weight and cost penalty. I already mentioned the flutter and jettison problem. I'm no expert on stability and control, but I suspect flying qualities would be degraded with tanks that far outboard.

Here's another consideration - center of gravity. With the tank farther outboard it is also farther aft. That could lead to the cg being too far aft.

Bottom line is, yes it may have worked, but it would not have been worth it. Heavier tanks, heavier pylon, heavier wing, probable flutter, jettison, and flying qualities prblems, higher cost, etc.

Block 15 hard points were strengthened, so that heavier weapon loads could be carried. That was a much simpler solution. Then the CFT allowed 4/6 to be used for weapons.

Why absolutely 370gal? A lighter 300gal tank or so would have given the a/c almost the same range and the lighter weight and size would have diminished most of the problems, including the COG issue. The tank would have been optimized for that station. Using it on station 4 would probably not have been a problem.

The a/c would even have add the ablility to carry 4 tanks for ferry flights or for CAP sorties with 4 a/a missiles.

All your questions and points are reasonable, Viperalltheway. In fact, 4/6 were cleared to carry a 6-bomb rack called MER (multiple ejector rack) which is essentially two TERs in tandem as you suggest. It could haul 6 Mk-82 or 4-CBUxx on 4/6. The AF never used it in service on the F-16, as the rack was too flexible and the AF wanted more fuel anyway. The flexibility caused some unwanted vibration problems and more scatter in the bomb pattern.

I mentioned 6.5g as a maneuver requirement for the tank, but as I said the most critical maneuver is stopping a roll at 5.2g. That means the airplane is at 5.2g and performs a full stick roll. Roll acceleration adds more g to wing mounted stores. On the F-16, when starting a roll, the acceleration is much less than stopping the roll. The acceleration at the tank from stopping the roll is greater at 3/7 then at 4/6. So imagine you are a tank at station 3. While stopping the left roll you feel 5,2g from the airplane and an extra 4g from stopping the roll. That's 9.2g at the tank. At sta 4 the acceleration is about 7.7g

One of the reasons for using a 370 tank was it already existed from use on another airplane. The F-16 centerline tank is custom fit to clear the ground, the landing gear and the tail hook attachment. It has a very odd shape accordingly.

One of the principles of wing design is to design the wing to provide the required lift and strength to fly the airplane. Then add in the secondary requirements (weapon carriage, flutter resistance, flight control effectiveness, internal fuel, etc) and and minimize additional weight to the wing. Usually they don't, but I feel pretty sure carrying tanks at 3/7 would have added significantly to the weight and cost of the F-16 wing.

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

Ok it would probably have been a bit complicated, but look I found this:

http://www.defense-update.com/products/f/f-16-fuel.htm

It seems that it's already done. Shocked

I'd like to see a picture of that LOL..

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

http://www.fartechnologies.com/

looks like they modified the tank.

Good links, interesting. Looks like they are trying to do something similar to what you said. However, note the 4/6 tanks must still be installed, eliminating 4/6 as heavy weapon stations.

I would like to see the mods they are doing to the 4/6 tank pylon, as it will not mate with the 3/7 hardpoint as they propose. If they modify the pylon to mate to the wing, then the tank cannot be jettisoned. At 4/6, fuel is transferred out of the tank by air pressure only - no pumps in the tank. So in addition to the external fuel line, they will need an external air line to the tank. Maybe they will combine those into one package. As I mentioned earlier, there will be significant problems with carriage at 3/7. Flutter will be a problem, so expect severe speed limits, perhaps only 350 kt. There will be severe maneuver limits, perhaps only 4g symmetric/3g roll and limited to half stick rolls. There will be severe jettison limits, perhaps only 300 kt., if they can figure out a way to jettison it. The USAF would never buy a tank which could not be jettisoned in an emergency. Any loading with tanks at 3/7 will be classed as Category III flight control. All these limits must be developed as a result of extensive (expensive) analysis and flight test.

It's really easy to develop a new concept to improve capability, but actually doing it and clearing it is an entirely different story.

Forum Veteran Joined: Apr 16, 2005 - 03:16 PM Posts: 800

I don't know if they have really tested their stuff but if they have a lot of potential customers especially if they can free the 4/6 to carry ordnance and if the racks exists to exploit the capacity of those stations.

Here's a picture of the rafale with its supersonic tanks on the outboard stations..
http://www.globalaircraft.org/photos/pl ... fale_2.jpg

Maybe they could have done the same thing originaly on the F-16..

BTW have a look at their air rearming system lol..

"if the racks exists to exploit the capacity of those stations"

The weapon pylon and rack used at 3/7 are fully compatible with 4/6.

Forum: F-16 Design & Construction

370gal tanks on stations 3 and 7..