Aerodynamic Effects of Tip Tanks
 

Hello,

I was wondering what the aerodynamic effects of tip tanks would be? Tip tanks being the wing tip mounted tanks that you would see on a plane like the Lear45.

My hypothesis is that tip tanks would not have much effect at low speed and would have negligible lift effect at any speed and high drag effects at high speeds.

What are your thoughts?

They can affect your Vmca if you have emptied one. Admittedly, that would only be if issue in a go around after a long asymmetric diversion.

Big weights out there affects roll authority, which affects a number of things...Lears are known to have all the aftermarket slots, slats, fences, vgs, anything to help the wing. Maybe hit wiki on Lear Wing upgrades.

In theory they should reduce induced drag - by inhibiting the formation of wing-tip vortices. Just like winglets or other wingtip devices, they would tend to block spanwise flow of high-pressure air beneath the airfoil around the wingtip into the low-pressure region on top.

http://history.nasa.gov/SP-367/f59.htm

You'd have to do calculations for a given size and shape of tank to determine if the reduction in induced drag (if any) would exceed the additional form drag from the tank itself.

Additionally, tip tanks allowed the use of a thinner and more efficient airfoil in the case of the Cessna 310 series, since no fuel volume needed to be accomodated within the wings themselves.

- from Classic Cessna 310 Ariticles

The same applied to the F-104 - the knife-edge wing needed for supersonic flight had no space for fuel (or landing gear, for that matter), leaving fuselage or external tanks as the options.

http://www.generationv.co.uk/ejcgall.../f104_2_3v.jpg

You'll note that tip tanks tend to appear on planes with short wings (note the mention of limited wingspan in the Cessna quote) - and short wings benefit more from tip devices (and require less structural strengthening to carry fuel weight at the tips - less lever arm).

The B-47 had a thin wing like the F-104, but much higher aspect ratio. Thus wing tip tanks (and the suspended weight) would have caused structural problems, while producing less benefit from vortex reduction (already improved through the long aspect ratio of the B-47 wing). So the B-47 design used mid-wing external tanks and fuselage tanks.

http://www.aviastar.org/pictures/usa/boeing_b-47.gif

The Lear 45 does not have tip tanks. No Learjet model subsequent to the 35/36 does. They were necessary to give these models additional range but at the cost of increased drag and variable roll response/lateral stability characteristics according to the quantity of fuel carried in the tip tanks. Nor are these models as tolerant of fuel imbalance conditions during takeoff and landing as they apparently need to be... (if the accidents attributable to same are any indication)

Pilots who operate these airplanes in a professional manner have fewer such problems of course.

I flew an old, straight-tail 310 in the 60's. It wallowed on final with a bit of turbulence and partial fuel in those old-design, straight tanks. Not an issue, really. Just seemed odd at the time.

The Aztec, with the thicker "mattress wing" having all the fuel in the wing and no tip tanks, was much more stable, although a bit slower. It was a good, stable platform.

Fly anything offered, know the limitations, fly the airplane, adapt, have fun.

I flew JP5s for a while. Some were fitted with tip tanks, others not. The tip tank equipped aircraft went further on the same fuel than the "bare winged" ones, or in the job I was trained for, the tip tanked aircraft flew the same speed at lower engine power. In other words, the tips provided an aerodynamic benefit.

But spinning with fuel in the tips wasn't allowed. It upset the B/A ratios and all that stuff.

If the effect would be neglectable, de Havilland would not have bothered to install a fixed slat on the inboard side of the "external" Comet IV wing tanks. Those tanks have a significant influence on the wing region inboard of it. This is the same for tip tanks, those are very low aspect ratio very thick airfoil wings. They produce additional local AoA in their vicinity (their "tip vortices"), roughly they double local AoA. It definitely increases lift in the tip region, especially at high AoA. How much that matters for the complete aircraft is hard to determine.

Or a Twin Comanche !!.

As in Jet Provost?

IIRC in about 1976 a Strikemaster (developed from the Jet Provost) of SOAF got in a flat spin. The pilot ejected near the ground and landed just beside the aircraft. I saw some photo's taken from the helicopter sent to rescue him.
Not sure if there was fuel in the tip tanks but no post impact fire.