What is the fat round thing on the wingtip of some small jets (see pic)?

Question

I thought they might be fuel tanks since the plane is so small but I am not sure. Here is the best pic I could find:
http://www.airliners.net/open.file?id=1177551&WxsIERv=Tngrf%20Yrnewrg%2024Q&Wm=0&WdsYXMg=Hagvgyrq&QtODMg=Fgbpxubyz%20-%20Neynaqn%20%28NEA%20%2F%20RFFN%29&ERDLTkt=Fjrqra&ktODMp=Znl%2019%2C%201980&BP=1&WNEb25u=Wbuna%20Ywhatqnuy&xsIERvdWdsY=S-OIRP&MgTUQtODMgKE=Ng%20gur%20byq%20enzc%2025.%20Ertvfgengvbaf%20hfrq%20ol%20guvf%20nvesenzr%3A%20A3818T%2C%20UO-IQX%2C%20S-OIRP%2C%20KN-FPR%20%5BPnaba%20NR-1%20%2B%20X64%20Fyvqr%5D&YXMgTUQtODMgKERD=473&NEb25uZWxs=2007-02-19%2007%3A20%3A16&ODJ9dvCE=&O89Dcjdg=24Q-271&static=yes&width=1024&height=698&sok=JURER%20%20%28nvepensg_trarevp%20%3D%20%27Tngrf%20Yrnewrg%2024%27%29%20%20BEQRE%20OL%20cubgb_vq%20QRFP&photo_nr=5&prev_id=1224073&next_id=1177550&size=L

Answer 1

Your picture is of an old 20 series Learjet. The 24, 25 and 26 all had wing tip tanks that served two purposes; they increased the amount of fuel the aircraft could carry and they work sort of like small winglets.

Old turbojets engines, such as the ones your picture is wearing, burn much more fuel than more modern turbofan engines, so the tip tanks were required, in addition to the wing and fuselage tanks, to allow them to achieve a range of something like 1500 nautical miles.

The only 20 series I know of that does not have them is the Lear 28, known in the industry as the Longhorn, which was the first to feature winglets.

Tip tanks continued on the Lear 35, which featured more efficient turbofan engines, and current production models, 31, 31a, 45 and 60 all have deleted the tip tanks, sport some large turbofans, have swept the wings while many other unique physical qualities of the Learjet have remained such as the shape of the fuselage and empennage, (tail assembly.)

If you look up the Lockheed Model 1329 Jetstar, a private jet that has four engines, mounted in tandem pairs on the fuselage, you'll see what look like huge, bullet shaped portrusions. On the -6 and -8 models, these portrusions are mid-wing, and extend above the wing. On the later Jetstar II and the 731 conversion models that feature more efficient TFE-731 engines, the tank hangs beneath the wing. They gave the Jetstar the advantage of range over the production models of Saber and Lear, putting it in competition with the much larger Gulfstreams, (the G-II had optional tip tanks,) of the time.

Best,
JT

Answer 2

Right on, they are fuel tanks. A peculiarity of these tanks is they must be filled 50-60 gallons at a time to keep the aircraft balanced. If you completely fill one side and the fuel is low in the other, the plane will tip over and if it catches your toes, you will not be able to go to the other side and fill it. If it misses your toes, the other side will be so high you cannot reach the tank.

Answer 3

I have nothing to add. I have a couple thousand hours in lears and yes, they are indeed fuel tanks. I will correct one thing though. Somebody talked about the "F" prefix indicating a US registered airpcraft. Not so. All US registered aircraft begin with "N" and must have a number immediately following the N. What you see in the picture is an aircraft with French registry.

Answer 4

As the others said they are fuel tanks. That's a Lear Jet as I'm sure you know and the wing design was borrowed from a 1960s European military aircraft. In that era, wing tip fuel tanks were very much in use at the time because the very thin wings being developed left little room for fuel storage. Tanks very much like those but mounted further in on the wings was used on one of the earliest jet powered airliners the Comet ll. Here's a picture.
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http://www.airliners.net/open.file?id=0795293&WxsIERv=Qr%20Univyynaq%20QU-106%20Pbzrg%204P&Wm=0&WdsYXMg=HX%20-%20Nve%20Sbepr&QtODMg=Yrhpunef%20%28Fg.%20Naqerjf%29%20%28NQK%20%2F%20RTDY%29&ERDLTkt=HX%20-%20Fpbgynaq&ktODMp=Frcgrzore%201990&BP=1&WNEb25u=Vna%20Eboregfba&xsIERvdWdsY=KF235&MgTUQtODMgKE=%22Pnabchf%22%20ynaqvat%20ng%20Yrhpunef&YXMgTUQtODMgKERD=2300&NEb25uZWxs=2005-03-13%2019%3A21%3A55&ODJ9dvCE=&O89Dcjdg=6473&static=yes&width=1100&height=775&sok=JURER%20%20%28nvepensg_trarevp%20%3D%20%27Qr%20Univyynaq%20QU-106%20Pbzrg%27%29%20%20beqre%20ol%20cubgb_vq%20QRFP&photo_nr=81&prev_id=0809445&next_id=0792963
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Answer 5

It is acutally a fuel tank... the wings are so small that they cannot hold very much fuel... they need the tanks on the end of the wings for max range...

Answer 6

Yeah it's a fuel tank. Extra fuel for long flights.

Answer 7

Actually they are its normally fuel tanks. They are called tip tanks. The whole aircraft only holds 847 gals (3,207L).The F prefix is it is registered to France. N prefix is US registered.

Answer 8

Auxillary or extra fuel tanks.

Answer 9

Fuel tank, you can see the RED gas cap

Answer 10

Thats what they are,but they are extended range aux tanks on that Lear jet. The F prefix in the number tells you its a US registered airplane.....George