Induced drag and winglets
Thread Starter
Join Date: Jul 2006
Location: Hong Kong
Age: 75
Posts: 71
Likes: 0
Received 0 Likes
on
0 Posts
Induced drag and winglets
All wings produce induced drag and as a result a number of manufacturers have added winglets to their wings to reduce the effect of induced drag therefore leading to greater efficiency. We have all seen them, ranging from the Airbus fleet to certain models of the Boeing family, as well as others. Why is it then that Boeing continue to manufacture their 777 without winglets, which if added would surely improve that wing's efficiency?
Join Date: Oct 2006
Location: AMS
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
The Boeing 777 has got so called 'raked wingtips' where the wingtips have a larger angle angle of sweep then the rest of the wing. This increases wing span and thus aspect ratio. According to tests from Boeing and NASA this gives a bigger drag reduction compared to winglets. On the other hand an increase in wingspan can result in ground handling difficulties, that's why on the most medium-haul aircraft winglets are installed.
Correct me if I'm wrong.
Correct me if I'm wrong.
Join Date: Aug 2007
Location: Nottingham
Posts: 56
Likes: 0
Received 0 Likes
on
0 Posts
Raked Wingtips
The raked wingtips will also be a feature of Boeing's new airliners, on the 787 when viewed from above is marked.
It will also feature on the 747-8i, it's noticable on the artists impressions, although how this will transition to the real aircraft is still up for debate!
Spru!
It will also feature on the 747-8i, it's noticable on the artists impressions, although how this will transition to the real aircraft is still up for debate!
Spru!
Join Date: Jan 2000
Location: sussex
Posts: 347
Likes: 0
Received 0 Likes
on
0 Posts
My understanding is that the gain you make in induced drag is opposed by the increase in profile drag.
On Bert Rutan's "round-the-world" take-off, didn't both winglets break off as the wingtips hit the runway.................they thought they'd blown their chances of the record but found that the fuel consumption was better than predicted.....ie TOTAL drag had reduced.
On Bert Rutan's "round-the-world" take-off, didn't both winglets break off as the wingtips hit the runway.................they thought they'd blown their chances of the record but found that the fuel consumption was better than predicted.....ie TOTAL drag had reduced.
Join Date: Dec 2001
Location: England
Posts: 1,389
Likes: 0
Received 0 Likes
on
0 Posts
As I understand it...
You can't just bolt on winglets and expect them to work well, the whole wing may have to be redesigned for them. They increase lift as well as reducing drag so the angle of attack for the whole wing may have to change. They also work best at one speed. Perhaps that means they also work best at one height or over a narrow height hange? They add weight and move the lift distribution outboard. All sorts of issues.
You can't just bolt on winglets and expect them to work well, the whole wing may have to be redesigned for them. They increase lift as well as reducing drag so the angle of attack for the whole wing may have to change. They also work best at one speed. Perhaps that means they also work best at one height or over a narrow height hange? They add weight and move the lift distribution outboard. All sorts of issues.
Join Date: May 2006
Location: 2 m South of Radstock VRP
Posts: 2,042
Likes: 0
Received 0 Likes
on
0 Posts
An end plate's an end plate. Pure induced drag is a what one is stuck with; supressing the the vortex is the clever bit without increasing parasite drag.
Join Date: Oct 2007
Location: north lincs
Posts: 2
Likes: 0
Received 0 Likes
on
0 Posts
on winglets and raked tips
Winglets need to be designed-in from the start, as the optimum twist distribution along the wing span will be different with/without winglets. In general, you need more tip-loading for a wing-winglet configuration to work. Take a look at the 747 version with winglets. You can clearly see that they had to re-twist the outer wing section as well as adding winglets to get them to work.
With regards to raked tips they work in a similar fashion to winglets. It is, infact the shape of the trailing edge of the wing that matters, as viewed from behind (the shape in the Trefftz-plane if we want to get a bit technical). As the wing is pitched nose-up at cruise, the trailing edge line curves down at the tip a bit like a downwards-pointing winglet.
As the aim is to spread the vorticity over as long a length as possible in a non-planar fashion, it makes no difference if the winglet or trailing edge shape at the tip is pointing up or down.
Of course the natural world evolved this way before Boeing did....although that hasn't stopped them trying to claim a patent on it. They are also called 'lunate' or 'crescent moon' configurations and you see them in the wing planform shape of the european swift (if you stay in the air for a whole year without landing, you can be sure your wing planform will evolve to be optimum...that fact regarding swifts still takes my breath away). Also check out the tail fins of tuna, sword fish, dolphins and whales....maybe dolphins really are the most intelligent life form on the planet......bye and thanks for all the fish.
A cresent-moon planform on an aircraft will be very expensive to manufacture, so just adding the feature to the tip area makes more sense - the raked tip.
Raked-tips on aircraft also have one aerodynamic advantage over winglets - with winglets the main design problem is the interference you get in the wing/winglet junction. Profile drag and wave drag penalties can occur here. You don't get this penalty with a raked tip.
With larger aircraft, wing weight becomes more important, in which case it is better to have a span loading with a more inboard centre of pressure to reduce wing root bending moment and so wing weight. In this case, winglets dont work so well. This is why A380 has relatively small winglets/end-plates (relative to the wing span that is!).
Hope this helps.
With regards to raked tips they work in a similar fashion to winglets. It is, infact the shape of the trailing edge of the wing that matters, as viewed from behind (the shape in the Trefftz-plane if we want to get a bit technical). As the wing is pitched nose-up at cruise, the trailing edge line curves down at the tip a bit like a downwards-pointing winglet.
As the aim is to spread the vorticity over as long a length as possible in a non-planar fashion, it makes no difference if the winglet or trailing edge shape at the tip is pointing up or down.
Of course the natural world evolved this way before Boeing did....although that hasn't stopped them trying to claim a patent on it. They are also called 'lunate' or 'crescent moon' configurations and you see them in the wing planform shape of the european swift (if you stay in the air for a whole year without landing, you can be sure your wing planform will evolve to be optimum...that fact regarding swifts still takes my breath away). Also check out the tail fins of tuna, sword fish, dolphins and whales....maybe dolphins really are the most intelligent life form on the planet......bye and thanks for all the fish.
A cresent-moon planform on an aircraft will be very expensive to manufacture, so just adding the feature to the tip area makes more sense - the raked tip.
Raked-tips on aircraft also have one aerodynamic advantage over winglets - with winglets the main design problem is the interference you get in the wing/winglet junction. Profile drag and wave drag penalties can occur here. You don't get this penalty with a raked tip.
With larger aircraft, wing weight becomes more important, in which case it is better to have a span loading with a more inboard centre of pressure to reduce wing root bending moment and so wing weight. In this case, winglets dont work so well. This is why A380 has relatively small winglets/end-plates (relative to the wing span that is!).
Hope this helps.
Join Date: Oct 2007
Location: Millington
Age: 59
Posts: 54
Likes: 0
Received 0 Likes
on
0 Posts
With regards to raked tips they work in a similar fashion to winglets.
Join Date: Nov 2001
Location: Earth (unfortunately)
Posts: 74
Likes: 0
Received 0 Likes
on
0 Posts
the design is intended to use the vortex to generate thrust in the forward direction
Can we turn off the engines and fly using the "thrust" produced by the winglets? Would those winglets be producing any force if the engines weren't propelling the aircraft through the air?
In aerodynamics you can't just look at a single component and say it produces thrust. You have to look at the interaction with the neighboring components. If you look at a wing with a winglet installed it's obvious it does not produce any more thrust than a wing without winglets, or a landing gear or an antenna sticking out of the fuselage for that matter.
I'd say to leave the thrust part to the engines
The short and sweet answer to the original question is that the net effect of winglets can be approximated by hypothetically increasing the wingspan in the amount equal to the height of the winglets. What the winglets accomplish is basically "fake increased aspect ratio". If instead of building the winglet you were to increase the wingspan by the same length, you would achieve approximately the same reduction in induced drag.
And even this effect requires a very carefully engineered winglet... otherwise the net effect could be a performance loss (or, more often, a smaller reduction in induced drag than what could have been achieved by an equivalent increase in wingspan).
Depending on the design, a manufacturer can opt for one option or the other. As you can imagine, there are advantages/disadvantages (ie. costs) associated with either option. With the increase in air travel and subsequent saturation of most airport hubs, the reduction in wingspan allowed by winglets can be very useful in fitting more aircraft side-to-side in a given terminal. For example a CRJ-200 has a wingspan on 21 meters, with each winglet measuring approx. 1 meter. That's almost a 10% reduction in wingspan thanks to winglets (ie. every 10 aircraft you can fit an extra one at the terminal.) Do not underestimate the importance of this aspect.
What always amazes me is how the majority of aviation enthusiasts (including many professional pilots) think of these winglets as the latest-and-greatest invention and that just slapping an end-plate (or winglet) will dramatically reduce induce drag (some ATPL books even show a winglet "stopping" any vortex generating at the wingtip). Its not this simple nor this effective. If that were the case all aircraft would have them
Lastly, let's not forget that marketing plays a role in every product's R&D, including aircraft. It's no secret manufacturers have been trying to find every possible gimmick to differentiate their product...
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
To say "thrust" (or, if you prefer, a force in the forward direction, if "thrust" is somehow associated with engines) is correct. The winglet doesn't just, if designed correctly, "reduce drag". In a manner similar to the sail on a yacht while tacking, the careful correspondence of the winglet angle and the direction of flow of the local tip vortex flow is such that the "lift" generated by the airfoil that is the winglet is - very slightly - oriented in the forward direction. Thus there is indeed a forward force component.
No, there would be no force if there were not air flowing and a vortex - but that applies to any airfoil in any orientation. We do not say that the engines produce "lift", even though the answer to "Would [those winglets] the wing be producing any force if the engines weren't propelling the aircraft through the air?" is also "no". So why deny that the force generated by the winglet acts forwards?
(And yes, even with the induced drag accompanying the winglet lift accounted for, the force vector is still forwards, at the winglet design conditions. Off-design, it's a different matter, which is why winglets are so sensitive to the mission and design point of the aircraft.)
Oh and to save a lot of typing .... we've been here before
No, there would be no force if there were not air flowing and a vortex - but that applies to any airfoil in any orientation. We do not say that the engines produce "lift", even though the answer to "Would [those winglets] the wing be producing any force if the engines weren't propelling the aircraft through the air?" is also "no". So why deny that the force generated by the winglet acts forwards?
(And yes, even with the induced drag accompanying the winglet lift accounted for, the force vector is still forwards, at the winglet design conditions. Off-design, it's a different matter, which is why winglets are so sensitive to the mission and design point of the aircraft.)
Oh and to save a lot of typing .... we've been here before
Join Date: Oct 2007
Location: Millington
Age: 59
Posts: 54
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by palgia
Can we turn off the engines and fly using the "thrust" produced by the winglets? Would those winglets be producing any force if the engines weren't propelling the aircraft through the air?
The short and sweet answer to the original question is that the net effect of winglets can be approximated by hypothetically increasing the wingspan in the amount equal to the height of the winglets.
Join Date: Oct 2007
Location: Millington
Age: 59
Posts: 54
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Mad (Flt) Scientist
In a manner similar to the sail on a yacht while tacking, the careful correspondence of the winglet angle and the direction of flow of the local tip vortex flow is such that the "lift" generated by the airfoil that is the winglet is - very slightly - oriented in the forward direction. Thus there is indeed a forward force component.
However, I suspect that once the winglets start generating lift, they change the flow field, so that intensity of the vortex is reduced. There would be a high pressure area outboard of the winglets which would tend to inhibit the flow from the bottom of the wing.
