Wingtip tanks and G-loads

A good question. I spent about an hour reading pilot forums on this, and there's a lot of good information.

Summary seems to be, and this is for civil not fighters, that it reduces wingtip vortexes, can contribute to a thinner wing design, and can pencil out as lower power settings over the same time and distance.

They sound as though they make the aircraft more stable in rolls, which would not be conducive to fighter performance.

They are attached much the same as B-17 wings are, multiple forged fittings at the top and bottom of the airfoil surfaces, actually extensions of the multiple wing spars. They were plenty strong enough, the weight of the fuel would easily be supported by the structure. With the tanks full, the wing would have less gust stress applied, when compared with tanks being installed in the fuselage.

The early civilian Lear Jet carried either most, or all of it's fuel in tip tanks, it made great sense from an engineering standpoint.

Tip tanks themselves are designed to provide a certain amount of lift, thus supporting their own weight and then some. They are bolted to the wing spars, which are very strong. Just remember, most of the wing loading is concentrated towards the root, so the tip of the wing usually is built relatively light

Also, even if they don't generate lift, this is an example of span loading.  The same amount of fuel weight at fuselage actually creates a higher bending moment on main spar than anything mounted outboard on wing.  Torsional stresses due to turbulence and maneuvering are probably a worse problem than G-force per se- it certainly changes moment of inertia of wing about an axis along mean chord line.

patrick206

The early civilian Lear Jet carried either most, or all of it's fuel in tip tanks, it made great sense from an engineering standpoint......

...but not much sense to the "ramp rat", that would have to go back and forth to fuel em up....50gal. in one side...drag hose to the other side...100gal....back over to the other side, so on, so on. Too much in one side will tip the sucker over!

(So I was told....I believed it, and did want wish to find out the hard way!)

There's an old story about a pilot in an F-80 or F-84 in Korea. No doubt Beer Call Bu l l ***.

One tip tank drains but not the other, offsetting some vertical axis of rotation and making the jet hard to fly straight. So he opens the canopy and shoots holes in the tank with his sidearm.

And thats no sh*t!

Fermis -  Yes indeed, At Sea-Tac there was an early Lear being fueled at the FBO, ended up sitting with one tip tank on the ground, looking very sad. Even worse, the sheet metal damage required some very spendy repairs. I've heard that if a Lear crew was in need of a "quick turn,", like a medevac flight, they would get two trucks loading together. I see the newer Lears are sans tip tanks now.

Patrick

fermis

patrick206

The early civilian Lear Jet carried either most, or all of it's fuel in tip tanks, it made great sense from an engineering standpoint......

...but not much sense to the "ramp rat", that would have to go back and forth to fuel em up....50gal. in one side...drag hose to the other side...100gal....back over to the other side, so on, so on. Too much in one side will tip the sucker over!

(So I was told....I believed it, and did want wish to find out the hard way!)

This is true. Max imbalance is 125 gallons (about 800 lbs). More than than will tip it over. In flight, max imbalance is 600 lbs, and with R/X tanks, it drops to 200.

It will also tip back onto it's tail if you try to fill the rear fuselage tank without at least 1000 lbs in each tip to counterbalance it.

It's as much of a PITA for the crew as it is for the rampers. If not fuelled evenly, you'll get bubbles in the wing tanks that are hard to get out. If you really need a solid top-off, its a pain. And God forbid you should be parked on a spot that isn't completely level. And if you're filling the trunk, that pump is slooooow - only 50 lbs a minute.

About 40% of the fuel is in the tip tanks. There's about 1250 lbs in each wing, 1250 in each tip, and 1340 in the fuselage.

Someday, I will fly a jet with single-point refuelling. Someday...

MIflyer

Someday, I will fly a jet with single-point refuelling. Someday...

Single point is GREAT!!!

Hello!

There's one more thing to it - on some designs the problems with vibration - buffeting - were solved by adding mass at the wing tips. This lowers the resonance frequency and so on. In case of F-104 or F-5 this mass was a Sidewinder launcher or a tip tank. A tip tank probably also helps a little to reduce the drag caused by the vortex that is generated by the wing tip - today they solve it with the winglets.

Paweł

Roll is slower / harder to initiate with more weight around the centreline, so tip tanks can make roll easier / quicker to initiate, but also make it harder to slow / stop when started - to the extent that some aircraft are prohibited from rolling with tip tanks as it can be difficult or impossible to stop the roll.

I think that many of the early jet age tip tanks simply arose from a need to find somewhere to put additional fuel for unexpectedly thirsty engines, one that didn't interfere with underwing stores, involve the complication of release equipment & testing & didn't put them under the wing where they would be vulnerable from gear up landings.

I see I opened a can of worms here. That's what I love about these forums. So much great information. You guys are awesome. Good discussion. Thanks!

Lots of pilots, that's for sure.

Guys, maybe I am reading something wrong here, and if so I apologize.  Tip tanks will tend to slow the roll rate, not enhance it.  Think about a figure skater.  If they have their hands spread out they spin more slowly, but when they bring their hands in above their head, the rate of rotation increases.  They brought their weight closer to their centerline.  I expect that pilots were told to drain their tip tanks on the way to the combat zone, to make sure their roll rate was as high as possible and there would be the least stress on the wings during combat.  This is similar to a Mustang pilot draining his wing tanks into the fuselage before combat and draining the drop tanks, or dropping the tanks before combat.  All of these steps reduce the moment of inertia of the aircraft on its roll axis and increase the roll rate.  The aerodynamic effect of the tip  tanks would probably be to reduce drag from the tip vorticies on the wing.  I expect that tip tanks were used because they did not get in the way of carrying bombs and rockets, and because they were trying to make the wings thinner, and were looking for a place to hang a gas tank.  

regards, John

What would happen if both were near full and one tore off in a turn?

seasick

What would happen if both were near full and one tore off in a turn?

 

I was at an airshow back in the 50s.  An F-89 was doing a flyby right over the crowd.  At first it looked like the port tip tank started to peel off.  Immediately it became apparent that the wing itself was coming off.  The plane immediately snapped to port in a vicious roll, and the fuselage gave way right behind the wing.

The wing and tip tank came down in a spectator parking area, burning up a few spectator's cars, but fortunately no spectators were near.  The fuselage and starboard wing came down in- of all places- a National Guard ammunition dump.  50 cal and 20 mm ammo was going off for a long time till they got the blaze out.  Unfortunately the aircrew did not make it out.

Don Stauffer

 I was at an airshow back in the 50s.  An F-89 was doing a flyby right over the crowd.  At first it looked like the port tip tank started to peel off.  Immediately it became apparent that the wing itself was coming off.  The plane immediately snapped to port in a vicious roll, and the fuselage gave way right behind the wing.

The wing and tip tank came down in a spectator parking area, burning up a few spectator's cars, but fortunately no spectators were near.  The fuselage and starboard wing came down in- of all places- a National Guard ammunition dump.  50 cal and 20 mm ammo was going off for a long time till they got the blaze out.  Unfortunately the aircrew did not make it out.

That was such an interesting story, Don, I had to go look up the details.

http://www3.gendisasters.com/michigan/18997/detroit-mi-air-show-accident-aug-1952

I actually found a photo showing the disintegrating F-89 online.  It completely broke apart.

https://en.wikipedia.org/wiki/File:F-89_Scorpion_Crash_IAE_Detroit_1952.jpg

seasick

What would happen if both were near full and one tore off in a turn?

 

We had a guy that kept a Fouga Magister (and a Stearman) at my airport. He was on "final" at Osh Kosh...tip tank broke loose, tumbled back and took out half the tail...augered in. Tragic, he was a hell of a pilot and a great guy too...he even let me fly his Stearman.

I found a Magister rotting away in a weedy lot at Stead Airport in Reno.

Don, I didn't know about that. My dad (he's older than you) may have been there but he'd have no idea now.From the other posted links, General Vandenberg was. Didn't you later work at that AFB in California?

GAF

 

 

Don Stauffer

 I was at an airshow back in the 50s.  An F-89 was doing a flyby right over the crowd.  At first it looked like the port tip tank started to peel off.  Immediately it became apparent that the wing itself was coming off.  The plane immediately snapped to port in a vicious roll, and the fuselage gave way right behind the wing.

The wing and tip tank came down in a spectator parking area, burning up a few spectator's cars, but fortunately no spectators were near.  The fuselage and starboard wing came down in- of all places- a National Guard ammunition dump.  50 cal and 20 mm ammo was going off for a long time till they got the blaze out.  Unfortunately the aircrew did not make it out.

 

 

 

That was such an interesting story, Don, I had to go look up the details.

http://www3.gendisasters.com/michigan/18997/detroit-mi-air-show-accident-aug-1952

I actually found a photo showing the disintegrating F-89 online.  It completely broke apart.

https://en.wikipedia.org/wiki/File:F-89_Scorpion_Crash_IAE_Detroit_1952.jpg

 

Yep, that was the incident.  Showing my age!

GMorrison

I found a Magister rotting away in a weedy lot at Stead Airport in Reno.

Don, I didn't know about that. My dad (he's older than you) may have been there but he'd have no idea now.From the other posted links, General Vandenberg was. Didn't you later work at that AFB in California?

 

Yep, I was stationed at Vandenberg from 61 through 64.

Every plane, when it is designed, has a max load and max stress (g load) specification it is designed to, and the design team must design in enough stength to meet those specs.  Where the failure ultimately occurs (hopefully above the spec load/stress) depends on the exact details of the design.  If you pull too many gees, the tail may fail first, the rear fuselage, the wing root, or some other point on the wing.

  Also, if you are lucky, the failure may just be a bend of a part without breaking, and you can get her home if you are careful, but the plane is either destroyed or needs major repairs.  Pilots need to listen for certain serious sounds or slight changes in geometry.  Hard landings may bend structure too, either making plane unflyable.  Wrinkles in fuselage bottom just behind wing, or a change in dihedral,  are signs of this.

GMorrison

I found a Magister rotting away in a weedy lot at Stead Airport in Reno.

Don, I didn't know about that. My dad (he's older than you) may have been there but he'd have no idea now.From the other posted links, General Vandenberg was. Didn't you later work at that AFB in California?

 

Now that's a cool looking jet GM.

Heller and Airfix made a nice 1/72 kit. I've got two; planning for Israeli trainer/ tank attack. Funny little airplane, about the size of your hand at 1/72.