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DmdSeeker
09-04-2006, 03:50 PM
Most sources agree that the 190 series; especially in the later inline engined versions, was one of the absolute better fighters of WWII.

And most sources put roll rate as one of it's best points.

Why then was there such a vogue for wing tip tanks in the first and second generations of jet fighters?

I would have thought that placing so much mass so far away from the roll axis would have drasticaly slowed roll response; if not absolute roll rate; and have made recovery from roll (i.e. stopping the thing) even harder.

Any ideas?

ColoradoBBQ
09-04-2006, 03:56 PM
http://www.aerodyn.org/Drag/tip_devices.html

Pretty mcuh explains everything about tip tanks and simular devices.

SkyChimp
09-04-2006, 04:04 PM
Originally posted by DmdSeeker:
Most sources agree that the 190 series; especially in the later inline engined versions, was one of the absolute better fighters of WWII.

And most sources put roll rate as one of it's best points.

Why then was there such a vogue for wing tip tanks in the first and second generations of jet fighters?

I would have thought that placing so much mass so far away from the roll axis would have drasticaly slowed roll response; if not absolute roll rate; and have made recovery from roll (i.e. stopping the thing) even harder.

Any ideas?

It didn't slow the roll as much as you might think.

The P-80 was a fast rolling airplane. At 35,000 feet at 250mph IAS it could roll 165 degrees per second without wingtip tanks. At the same altitude and speed, but with tanks, it could roll at 145 degrees per second.

At 5,000 feet, about 300 mph IAS, and without tanks it rolled at 135 degrees per second. Add tanks and it rolled at 125 degrees per second.

So there was some degradation, but not that much.

Treetop64
09-04-2006, 04:18 PM
Originally posted by DmdSeeker:

Why then was there such a vogue for wing tip tanks in the first and second generations of jet fighters?

Any ideas?

Beacuse early jet engines were so inefficient and fuel thirsty that fuel storage was created anywhere storage space could be found (or created), without geting in the way of existing internal aircraft systems. This is especially the case for the F-80. The F-80 had cripplingly short range without the tanks, and the designers felt that the tip tanks were the best resolution for that airframe.

tigertalon
09-04-2006, 04:22 PM
Originally posted by SkyChimp:
It didn't slow the roll as much as you might think.

The P-80 was a fast rolling airplane. At 35,000 feet at 250mph IAS it could roll 165 degrees per second without wingtip tanks. At the same altitude and speed, but with tanks, it could roll at 145 degrees per second.

At 5,000 feet, about 300 mph IAS, and without tanks it rolled at 135 degrees per second. Add tanks and it rolled at 125 degrees per second.

So there was some degradation, but not that much.

I can imagine roll rate was not affected that much, what about time to gain it?

How fast could it reverse from full speed left rolling into right one with and without tanks?

DmdSeeker
09-04-2006, 04:27 PM
Thanks for the link; but unless I'm misunderstanding it; the document says what they are; and what they're for; but not how they work.

As to Skychimp's point that they didn't slow roll much; I can certaqinly believe they didn't have much of an effect once the roll was commenced; however I'd have thought they would have slowed the start and stop of the roll due to the mass. I seem to recall "roll lag" (is there such a term? I don't mean a lag roll!) being a hot topic of the P38.

But why suffer any degredation at all of such an important charecteristic as roll? Why not either design in more tankage; or carry a conventional drop tank (which would, after all, be closer to the roll axis); or adopt the over wing tank such as was used on the BAE Lightning?

Treetop64
09-04-2006, 04:32 PM
Originally posted by DmdSeeker:
Thanks for the link; but unless I'm misunderstanding it; the document says what they are; and what they're for; but not how they work.

As to Skychimp's point that they didn't slow roll much; I can certaqinly believe they didn't have much of an effect once the roll was commenced; however I'd have thought they would have slowed the start and stop of the roll due to the mass. I seem to recall "roll lag" (is there such a term? I don't mean a lag roll!) being a hot topic of the P38.

But why suffer any degredation at all of such an important charecteristic as roll? Why not either design in more tankage; or carry a conventional drop tank (which would, after all, be closer to the roll axis); or adopt the over wing tank such as was used on the BAE Lightning?

Dood, you're making a lot of cross-comparisons with fighters of different eras, missions, and completely different design philosophies. Apples and oranges, man. Apples and oranges... Why do they taste so different? Ask the experts who designed them.

Same goes for the planes... http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

berg417448
09-04-2006, 04:34 PM
http://naca.larc.nasa.gov/reports/1947/naca-tn-1317/

From this report it looks like the negative effects were less if the tanks were attached under the wing tips rather than directly onto the tips.

SkyChimp
09-04-2006, 04:54 PM
Rolling moment probably was effected. Maximum roll rate was not probably effected a great deal.

One of the reasons the P-80 used tip tanks is because there were minimum hardpoints and they had to be put somewhere.

SkyChimp
09-04-2006, 05:00 PM
Oops, just saw the report above. should make for some good bathroom reading.

DuxCorvan
09-04-2006, 05:11 PM
I guess that a lot of mass and weight in the tips of the wings would increase inertia significantly. Roll rate would be more or less the same, but the response to aileron movement would be laggy: the same roll rate but slower in initiating and in stopping the roll movement.

On the other side it would be much less draggy, more aerodinamically efficient, improving speed, flap autority, energy retaining and fuel saving over under-wing tanks.

Viper2005_
09-04-2006, 05:35 PM
Once you've finished an aircraft design, the easiest way to add to its range is to add tip tanks.

Tip tanks reduce the wing root bending moment, which means that the rest of the aerostructure is less concerned about the extra weight than it would be if you added tanks to the fuselage.

At jet speeds roll rate is rarely a problem because even small helix angles give high roll rates.

Rate of change of roll rate is more of a problem, but again, not perhaps so large as might be imagined at high speed (aeroelasticity is more worrying, but that's a whole different can of worms!). At low speed, the chances are you're dead anyway unless you're landing, in which case, what do you need roll rate for? Or at least, that seems to have been the official line in some cases.

However, on the "design mission" the aeroplane will generally engages the enemy with empty tip tanks, suffering very little roll rate penalty as a result.

In addition, you'll tend to find that whilst handling is of importance in the initial stages of aircraft design since it is to an extent subjective it tends to be sacrificed for the attainment of easily testable performance goals such as speed and range.

Of course, in reality you don't gain a lot from tip tanks in many early jets because often the extra drag means that you've burnt much of the extra fuel by the time operational altitude is reached. But if you've got tankers on hand to top up at altitude they can start to become very attractive because the extra range/endurance potentially reduces the number of tankers required for a given force projection.

In general, tip tanks are the result either of the airframe designer being optimistic with his sizing calculations, the engine designer being optimistic with his sfc predictions or the customer asking for a stretch of some sort (F-104 being a classic jet example, whilst the A-26 shows that piston engined aeroplanes are not immune to such modifications). They're the simplest fix in many circumstances after the initial design freeze, and you'll find them affixed to all manner of aeroplanes from the period, ranging from fighters, through biz jets and airliners (Comet 4) through to GA twins...

BfHeFwMe
09-04-2006, 07:07 PM
Besides they're going to be empty after climbout. http://forums.ubi.com/images/smilies/winky.gif