I am looking into the aerodynamic pros and cons of wingtip tanks. Let's assume they're perfectly streamlined, optimally placed and sized for a given airframe.

I am aware of the purpose of fuel storage on a wing that's too thin or otherwise inadequate for 'wet wing' fuel storage and as a means of permanent fuel storage, but how do tip tanks interact with airflow on the end of the wing?

What are the pros? Are tip tanks effective as 'winglets' or drag-reducing tips, controlling spill and cleaning up vortices? Do they contribute to useful lift and if so what percentage and what is the overall effect on the wing?

And what are the cons, aerodynamic, structural and general? Do they require heavy modifications to the spar or wing in general to attach?

Overall, if a wing design can include bladder or wet wing storage, would it be better not to employ tip tanks?

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    $\begingroup$ Related: Why don't we see business jets with wingtip fuel tanks anymore? $\endgroup$ – ymb1 Apr 13 at 15:07
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    $\begingroup$ Your question has a "2.", but no "1.". Did something accidentally get deleted, or did you just change formatting mid-stream? You can edit your question to fix things up to make your intent more clear. $\endgroup$ – FreeMan Apr 13 at 16:31
  • $\begingroup$ Pro: they look sexy. Con: the runway wants to have sex with them. $\endgroup$ – Hot Licks Apr 15 at 18:54

I happen to own a homebuilt with tip tanks for ALL of the fuel supply (Pazmany PL-2), and while they look really cool, I'd prefer the fuel was in the wing leading edges mid span.

  1. There is no aerodynamic benefit. The tuna fish shaped tank doesn't function like a winglet (it has to be a wing to work like a winglet) and is too small to have any kind of end plate effect of significance (an end plate has to extend for a full chord to do any good that exceeds its own drag). The flattened and canted tanks you see on twin engine Cessnas was intended to produce a bit of vertical lift and an enhanced dihedral effect to offset some of the inertial issues in roll that the fuel mass creates; which leads to...
  2. Inertial issues in roll. The mass of the fuel way outboard in the wings is a total pain. It deadens the aileron response. The inertia magnifies roll and yaw disturbances. My airplane is much more snappy with minimum fuel vs full fuel, and wags its tail in turbulence much less. I prefer to fly it with about 1/2 fuel if I'm just going out for an hour or so. The mass also kills a lot of the roll-yaw couple and amplifies adverse yaw. If I skid the airplane with rudder, the roll into the yaw is very mild, in spite of significant dihedral.
  3. Significant form drag. On my plane there is about a 8kt speed penalty from having the tanks taking up space in the airstream (based on the performance of other PL-2s modified to tankless wet-wing).
  4. Fuel imbalances have a huge impact because the arm is so long. If I ran one tank all the way down with the other nearly full, I'm sure it would use up an unpleasant amount of my aileron authority at low speed (I don't really want to find out how bad - at minimum, it would take probably 10lbs plus of side pressure on the stick to keep wings level). Having no trim tab or bungee, I use the fuel balance as my lateral trim, and when solo, have to achieve about a 2-3 gal right side differential to get the plane to fly hands off. Then I have to remember to switch back and forth every 10 minutes or so to keep the balance. I know if I forget because the airplane starts to wander off toward the heavy side, like a horse that suddenly remembers where the barn is while you're riding it.
  5. Structural issues are minor. You are adding mass at the opposite end of the most highly stressed part of the wing, but you need to make the outer spar structure able to handle the local bending, so some minor reinforcement would be required at the attachment.
  6. One plus is the fuel supply is far away from you, say if you flipped over. I like that.
  7. One minus is the fuel supply is far away from you, at the first point to touch the ground in any kind of landing excitement.

All that being said, the PL-2 is a delightful airplane even with the tip tanks, and dammit they look awesome. If I was building one from scratch however, I would do leading edge tanks like the Van's RVs. Just way more efficient in every way. As a long range fuel supply add-on however, they can make sense, but I think most designers will just extend the wet wing volume instead, until they run out of that.

On airplanes like the Lear, they made sense because with the super thin wings they were the most practical way to put in a large fuel supply. But you will notice that they did away with them in later models because other than a handy place to stick tanks that don't get in the way of something else, it's mostly negatives. I shudder to think about how to control a tip-tank Lear with one side nearly empty and one side nearly full, if that were to happen somehow.

A former colleague was an old F-101 Voodoo driver who also had a lot of time in T-33s and he recounted to me once that the T-33's really large tip tanks were a pain due to the flywheel effect of the fuel mass. A little bump would start it rolling, and with the big weights out there, it took more than the normal input to stop the roll and if the conditions were rough you were fighting that constantly.

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    $\begingroup$ Thank you for describing the behavior with full and empty tip tanks. While clear in theory, it is good to get it confirmed by practical experience. One question, though: Are aerobatic limits reduced with empty tanks? $\endgroup$ – Peter Kämpf Apr 14 at 4:13
  • $\begingroup$ Why did you decide to build/buy an aircraft with wingtip tanks? Did you build it yourself? $\endgroup$ – RockPaperLz- Mask it or Casket Apr 14 at 5:44
  • $\begingroup$ If the plane has the tanks designed-in from the start, it provides a degree of span loading and the root structure can be lightened. This can save more weight than strengthening the tip adds. Big tanks can also reduce wing losses, allowing a smaller area and offsetting some of the drag. But big ailerons will always be needed to give normal roll rate. $\endgroup$ – Guy Inchbald Apr 14 at 9:45
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    $\begingroup$ @RockPaperLizard my PL-2 was built in the 70s from plans by the original owner. The behaviour I describe is not bad enough to want to so something radical like re-do the fuel system; it's just not really optimal and I would do it differently if I was building it. I knew what it would be like having read pilot reports long ago of old Cessna 310s with very similar tip tanks. $\endgroup$ – John K Apr 14 at 12:23
  • $\begingroup$ @GuyInchbald yes the Paz's ailerons are quite large and a bit heavy by homebuilt standards but are very crisp and you fly the plane with just fingertip pressures. The span is 27 ft so the roll rate isn't that great even with little fuel in the tips although more responsive. $\endgroup$ – John K Apr 14 at 12:28

I just know that it forms a vertical partition which prevents the air flow from passing from the lower surface (overpressure) to the upper surface (depression). On some gliders, this partition also serves as a "shoe" to protect the fin when the wing is on the ground. can be used on some aircraft as additional tanks. Fouga-Magister, Cessna 320, Learjet 23, etc...and it seems to me that the weight at the end of the wing compensates for the upward bending effort due to the lift.


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