12
$\begingroup$

Look at a 737, A320, A330, etc. All of them have upturned winglets. Why are they all upturned instead of downturned? Is there a reason for this or is it just how things ended up?

I was looking around and I found that the A-10 has downturned winglets, kind of (meaning not all winglets are upturned).


enter image description here


One of the reasons is probably due to the manufacturing complexity of the wings. Not sure though, as turning them upward probably causes the same amount of manufacturing complexity.

$\endgroup$
4
  • 6
    $\begingroup$ Newer 737NGs, 737 MAX, A300-600/A310, older A320s and the A380 have both up and down wingtip devices. $\endgroup$
    – user71659
    Commented May 6 at 0:48
  • 3
    $\begingroup$ These are no winglets! Better use the absurdities which Boeing puts on its latest 737 models as an example - one part of them points a bit downwards. $\endgroup$ Commented May 6 at 10:47
  • 1
    $\begingroup$ One reason might be that upward winglets look better. Don't they? $\endgroup$
    – Florian F
    Commented May 6 at 16:16
  • 1
    $\begingroup$ These wingtips are called Hoerner wing tips (they are not winglets). $\endgroup$
    – nonDucor
    Commented May 7 at 13:04

3 Answers 3

25
$\begingroup$

Up winglets are less likely to hit the ground.

The clearance from the wingtip to the ground in a worst case situation is important. Sometimes there is a requirement on the wingtip clearance when one main gear is at maximum extension and the other is at maximum squish with a flat tire.

$\endgroup$
5
  • 11
    $\begingroup$ Or a crosswind landing with an inconveniently timed wing dip that gets the aircraft rolling for some length of time before the pilot can counteract it. $\endgroup$
    – Ralph J
    Commented May 6 at 2:07
  • $\begingroup$ Yes -- that is the sort of thing that would cause you to derive a requirement like this. $\endgroup$ Commented May 6 at 3:51
  • $\begingroup$ but they're not bigger than jet engines under the wing. It must have some aerodynamic benefit $\endgroup$ Commented May 6 at 12:23
  • 20
    $\begingroup$ @user3413723 They're much further out than the engines... $\endgroup$
    – jcaron
    Commented May 6 at 13:13
  • 2
    $\begingroup$ Don't forget the servicing trucks. $\endgroup$
    – mins
    Commented May 7 at 13:23
18
$\begingroup$

Just to complement the other answers a bit.

I was looking around and I found that the A-10 has downturned winglets

Those wingtip devices on the A-10 are not really winglets but much simpler "dropped wingtips".

Even if they might improve the aerodynamic characteristics of the wing (especially maybe in ground effect), their main job is much easier i.e. protecting the ailerons and the chaff & flares from ground contact during low-level manoeuvring. The same trick is widely used in gliders. For the same reason also the vertical tailplanes protrude a bit downward in order to protect the elevators, especially during belly-landings (picture source):

enter image description here

And their use actually points us toward the answer to your question: virtually all winglets are turned upward (except the split-tip type) to avoid a dangerous contact with the ground.

$\endgroup$
0
4
$\begingroup$

It's important to separate winglets, which are thrust generators, from flow inhibiting fence-like devices like droop tips.

Winglets make thrust from the circulating flow, like a sail boat close hauled (Whitcombe originally called them "tip sails"). There is a lift force at 90 degrees to the spiraling flow mostly pushing the winglet toward the fuselage (or in the case of a sail boat, heeling it over), but angled a few degrees forward so there is a forward thrust vector. As a bonus, you have the downwash made by the winglet's airfoil which is redirecting the flow off its trailing edge outward, weakening the circulation.

You can theoretically extend it down, angle it slightly inboard instead of outboard so the lift force is out and forward instead of in and forward, and get a similar result, but you give up ground clearance if you want the winglet to be big enough to do any good.

The droop tip is a different device and is not making lift, just blocking some of the core circulation of the tip vortice. A winglet uses an airfoil to exploit the energy in the circulation flow to create to a counteracting force though lift generation, like any other wing, just horizontally. Hence, wing-let.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .