Tip tanks - pro's and con's

[GoneWest]

I've just been trying to do some homework on the effect of tip tanks on drag.

Struck me whilst struggling to find something definitive - that there seems to be very little about them.

Anybody know of any good reference sites that can give me some factual clues - if not, want to list your own suggestions (with reasons) as to the general benefits (or otherwise) of tip tanks?

[power lever]

PRO---tip tanks can reduce wing bending.

[Genghis the Engineer]

(1) They reduce wing bending moments due to moving mass out from the centre of the aircraft. Consequently they should extend the fatigue life of the upper wing skins, mainspar and D-box.

(2) They tend to prevent swirl around the wingtip, which reduces induced drag.

(3) They tend to increase profile drag, primarily through interference drag at the attachment point. This however is generally less than the reduction in induced drag.

(4) ESDU data sheets are the best place for such wisdom, the abstract for the sheet you need is at http://www.esdu.com/cgi-bin/ps.pl?sess=unlicensed_thqmmcypbt&t=di&p=97017 although I'm afraid that you'll need to either buy the sheet you need (they're a mercenary bunch at ESDU) or get it through a good technical library. If you do a search on the ESDU website you'll probably find some other relevant data sheets.

G

[lunkenheimer]

Try this link:
http://naca.larc.nasa.gov/reports/1951/naca-rm-a50k15/

for a report on tunnel testing of several tank geometries.

They also increase the moment of inertia around the roll axis more than fuselage and wing-integral tanks of the same capacity.

[Genghis the Engineer]

Good point that, altering the lateral rotational inertia by fitting tip tanks will alter the A:B ratio. The consequence is likely to be modified spinning characteristics (aside from the alteration due to aerodynamic effects), an increased roll mode time constant, and possibly an altered Dutch Roll characteristic.

G

[Sheep Guts]

But they sure wobble out there when in your turbulence in a C402B. 190 Litres in each one thats a 44gallon, 55gallon drum( what ever side of the pond youron) on each wing, yikes!Great bus the C402 , roomy Cadillac of the GA Skys.....:p

[compressor stall]

Hey SheepGuts,

You should see the old 402B you used to fly - had a pretty impressive tart up.

Still a 402B though....

[john_tullamarine]

.. apart from other considerations .. they do look rather nice .... I always like the look of the 300/400 series with tiptanks .. ....

[mcdhu]

Re spinning characteristics with tip tanks, my basic training was in the Jet Provost Mk4 which had tips, but spinning was forbidden with any fuel in the tips.
Cheers
mcdhu

[Stan Evil]

Tip tanks are a mixed blessing with regard to wing bending moments. Full they can give bending relief (although inertia in turbulence can undo this) as, for the same amount of fuel carried, less of it may be in the fuselage (the Canberra is a case in point). However, empty they increase wing bending stress as they move the centre of pressure outwards (reduction of induced drag and increase of lift at the tips because of reduced spillage from lower surface to upper surface). Canberra 'g' limits were +3g (or was it +3.5g?) with tips and +4.5g without.Their effect on B:A ratio with regard to spinning is very significant. Full tips will make the spin very stable, so much so that recovery may become impossible. Don't spin a JP or Strikemaster with fuel in the tips!

[aztruck]

Mr Cessna reckons they keep the gas away from the pax in the event of a crash. They also look really pretty and I vote for the 421B as hands down winner over the C model. Who's idea was it to put the tip tank scavenge pump CB's on the landing light CB though? I always found that somewhat bizarre.

[compressor stall]

And even more bizarrely not tell anyone about it in pilot accessible literature.

[pigboat]

Saw a GII once with tip tanks. Had French registration, but I believe it was based in Zurich. The crew told me they used to get down to central Africa with it, and fuel was sometimes scarce. I think the tips carried about a thousand pounds apiece.
I thought it wasn't a pretty mod. Sure made the airplane look different.

[411A]

aztruck

The answer is cost...it is the nearest available power source, ie: no additional wiring required.