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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?

Pondlife
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Darren-Edward O'Neill
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2 Answers2

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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.

John K
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    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? – Peter Kämpf Apr 14 '20 at 04:13
  • Why did you decide to build/buy an aircraft with wingtip tanks? Did you build it yourself? – End Anti-Semitic Hate Apr 14 '20 at 05:44
  • 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. – Guy Inchbald Apr 14 '20 at 09:45
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    @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. – John K Apr 14 '20 at 12:23
  • @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. – John K Apr 14 '20 at 12:28
  • @PeterKämpf no there are no fuel limitations for aerobatics I know of other than the fact that the design aerobatic gross weight is lower than the max gross. My plane is aerobatics prohibited under local regulation so I don't have an aerobatic GW with my wb documentation, but I recall it's something like 1350 lbs and my max gross is 1500. The wing is one piece with 27ft long spar caps, custom made at one time by Alcoa for Pazmany to sell to builders. He redesigned the spar to a built-up spar the builder could make after builders complained about the 800 dollar cost(in 1970, about $4000 today). – John K Apr 14 '20 at 12:52
  • @PeterKämpf actually the most interesting aspect of this airplane is the surprisingly weak roll/yaw couple, kind of glider like. There is a normal amount of dihedral and I would expect a good skid to produce enough roll rate to make the plane reasonably controllable on rudder alone. But it just kind of goes sideways and very slowly rolls to the low wing. You wouldn't want to try to control the plane close to the ground on rudder alone. Fortunately, the aileron control linkage has a fail-safe design insofar as you can have a rod end disconnect at any one location and still have one aileron. – John K Apr 14 '20 at 16:25
  • Thank you for the extensive information! The page you linked says that the PL-1 had even less dihedral. As a low wing design it should have weak yaw-induced roll and those 5° are quite normal – the Me-109 had 5.75°. – Peter Kämpf Apr 14 '20 at 20:24
  • That makes sense. I learned to fly in the mid 70s in a Piper Cherokee, which has 7 degrees and that and the low aspect ratio would give it a bit more roll rate from skidding. PL-2 does start to roll if you skid it, eventually, but there is a big delay and I think the fuel mass is the cause of the delay. Lateral stability wise, it seems perfectly adequate and remains wings level quite nicely once in lateral trim, in smooth air. Tail wag in bumps is noticeable and it seems to lack fixed fin area. A lot of them have dorsal fins added. – John K Apr 14 '20 at 21:33
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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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L'aviateur
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