Winglet Question from a university exam...
 

Hi guys, I need some urgent help.

I have recently sat an Aerodynamics exam at university. I went to check my script today and I think that it has not been marked correctly. Here's the problem:

For the question: "Describe performance benefits of aircraft winglets"

1 of the things I answered was something like this: "Winglets reduce the size of trailing edge vortecies, hence reducing pressure drag"

Is this not right? I scored zero points for this.


Another 2 things I said were that depending on the size and angle relative to the wing, winglets can create an adheral or dihedral effect therefore improving or decreasing the stability of the a/c.

And also, that air which is bleeding off the wingtip could be used by a winglet to create extra lift and therefore increase efficiency.

Is everything that I said incorrect?

Which university are you at?


:eek:

You'll probably get a more technical response, but since you said you need "urgent" help, I thought I'd chime in.

:)

I have a few problems with a couple of your answers, mostly your choice of words.

First of all, I've never heard of "pressure drag". There's "Parasite Drag" (form and skin friction) and "Induced Drag" (rearward component of lift).

It could be argued that winglets reduce induced drag as they effectively move the vortex off the wingtip allowing the wing to be more efficient (again, waiting for a more technical answer, I'm not an engineer).

So, the "performance benefit" is actually *lower fuel consumption*.

I do appreciate your answer regarding the additional benefits towards stability. That wouldn't have entered my mind.

But finally, your last comment about "air which is bleeding off the wingtip could be used by a winglet to create extra lift and therefore increase efficiency" is just pretty vague and BEGS for more technical language.

Air is never described as "bleeding" off the wingtip. Nobody would understand what that means.

There is a pressure differential that causes a vortex. Imagine a wing of infinite length (i.e., no wingtip exists). There would never be a vortex to interfere with this "perfect" wing. But in the real world we need wingtips and therefore the vortex is an accepted fact. The designers try to mitigate its deleterious effects by using the winglet to move it off the main lift producing surface so that the wing is more efficient.

In other words, the winglet doesn't *contribute* to overall lift (not in a vertical direction anyway, for the most part), but attempts to mitigate the inefficiency of our real world, less than perfect, limited (finite) wing.

The best analogy is a sailboat believe it or not, where the "foil" (winglet, sail) produces a FORWARD "lift" vector to help PULL the "ship" (aircraft) along.

Hope that helps.

Thats not really correct. The winglets have absolutely no effect on the vorticies. The winglets are an attempt to get the wing closer to its optimum length, without increasing the wingspan

Vorticies are not created by the tips, but the entire wing. The vorticies rollup at the tips, but that is the combination of the air flow from the bottom, combining with the airflow over the top, and of course, the end of the influence.

The fuselage and center wing also contribute, as well as the engines.

The winglets help reduce drag with the thin section, and that is likely where most of the benefit comes from

Firstly, you've spelled 'vortices' incorrectly, which may make a big difference in a university examination.

I don't think this is entirely correct.

According to a Boeing publication:

All in all, I think FE Hoppy has the most astute answer to your query.

A strong vortex accumulates on the topside of the airfoil, near the trailing edge. This stagnation causes drag. In my simplistic mind it looks like cavitation, but I am not a Comp fluid dynamics person. The winglets cause the stagnation to move out to the wingtips, reducing drag.


Sorry, I was speaking about the wake vortices. I misread the intent of his response

Winglets do not reduce the wake vortices the wing creates.

Winglets aim to reduce fuel consumption by reducing drag. They do this by minimising spanwise flow from the lower surface to the upper surface of the wing which creates a vortex and alters the induced angle of attack resulting in more drag.

By "adhedral" I assume you mean anhedral. As said before, technical terms will help in a university examination. However I'm not sure winglets affect this much as the primary function is to increase the efficiency of the wing.

"Winglets reduce the size of trailing edge vortecies, hence reducing pressure drag"

Do trailing edges create significant vorticies? You might have got a mark had you said they reduce wing tip vorticies. Basically winglets increase the effective span/aspect ratio without increasing the actual span.

Surprised the question didn't also ask about the dissadvantages of winglets.

Cant. Redirect. Captured lift. Rake. Increased aspect ratio.

Work in some of these terms.

Toe. Zero AoA. Vortex calming dispersal.

To make it simple my explanation was the vortex off the wingtip from below to above the wing struck the winglet to make it like a sail and push it forward.

Bubbers44

:D

You sure about that?!

http://b737.china-sim.cn/zl737/images/737winglets.jpg

Thanks for all your replies guys. I will dig in some books now to see exactly what the deal is with whether or not wingtips affect the size of the trailing edge vorticies. I'm sure I haven't just come up with this just out of thin air.

I believe you are right to an extent, the purpose of the 'winglet' is to reduce the magnitude of the wingtip vortices. By reducing these you reduce the 'induced' drag and that equates to less thrust required and therefore a fuel saving.

Some aircraft such as the 777 have such well designed 'raked' wingtips that there was never a need for winglets.

With your answer, they don't reduce 'trailing edge vortices' but 'wingtip vortices' and I have never heard of 'pressure' drag.

Form drag, profile drag, or pressure drag, arises because of the form of the object. The general size and shape of the body is the most important factor in form drag - bodies with a larger apparent cross-section will have a higher drag than thinner bodies. Sleek designs, or designs that are streamlined and change cross-sectional area gradually are also critical for achieving minimum form drag. Form drag follows the drag equation, meaning that it rises with the square of speed, and thus becomes more important for high speed aircraft.

But I know what you mean about "wingtip vorticies" - that's what I mean, not trailing edge vorticies. I implied, vorticies which roll off the wing tips. I guess I should have been more precise.

2 things:
- reduces induced drag
- adds a small amount to the thrust of the aircraft

reduces induced drag
--------------------
Conventional wings, as said before, create vortices at the wingtips due to the pressure differential on the top & bottom of the wing & the opposite spanwise flow top & bottom. The bigger the spanwise flow, the bigger the vortices. The biggest spanwise flow is at low speeds, which explains why wake turbulence is biggest at low speed.

These vortices create a downwash behind the wing and this has an effect on the airflow over the wing. In fact, it reduces the effective angle of attack on the wing and gives induced drag. Induced drag is the vector parrallel to drag when comparing effective lift force & theoretical lift force.

Winglets create smaller vortices & also more upwards as you can see in the picture posted by Lord Spandex Masher. This gives less downwash thus less induced drag.

Thrust
------
When looking at a winglet from above it is a small wing. The reactionary force created by the winglet is directed towards the fuselage of the aircraft with a small vector helping aircraft thrust.

http://i94.photobucket.com/albums/l1...ch/winglet.jpg

Sorry if my English is not perfect but I'm not a native speaker. Corrections and remarks always welcome ofcourse :)

rvblyky7, thanks very much, very helpful :ok:

Inspired by bubbers. Winglet extends and re-directs the point of reintroduction of divergent airflows, making the recombination more gentle, hence less drag. It allows the pressure side of the wing to act for a bit longer. There is a reason it is swept, I am thinking it mimics the sweep of all high speed transport wing design; less frontal exposure for area action.

lyman's needle: What's with the odd little sharklet? Different for different' sake?

Here is a pretty good site for background and technical explanations regarding winglets...

http://www.smartcockpit.com/data/pdf...Technology.pdf

TD

Lord,

That illustration is interesting, but studies have shown that is not correct. Using LIDAR and SODAR, we can now measure the trailing vortices, not just model them. Comp fluid dynamics just really doesnt work as well as you think, as water cannot be compressed and air can. Most models either model a thin section of the wing, or perhaps the wing itself, but very, very few model the wing, fuselage, center wing combination, let alone the significant effect of the engine on rollup.


The winglets, in reducing the vortices over the top of the wing that create the drag, actually tend to concentrate the airflow, which creates a faster velocity on the wake vortex creation. This creates what is called a small core vortex, which is very powerful, and a large core, which is not as powerful, but due to the zone of influence lasts longer.

This is why the vortex off the back of a 737-400 is a only a large core, while the 737-800 is a small and a large core. You dont want to get caught in the small core.
In board flaps and flap settings have significant effect as well on vortex creation.

Yes, at degree level I think I'd have given that zero as well.

The tips increase the effective aspect ratio through modification of the Oswald span efficiency factor (e) in the term Cd = Cdo+(1/e.pi.AR)Cl^2.

This effective increase in AR reduces the induced drag term. (NOT the profile drag term, very occasionally called "pressure drag").


Physically, it reduces the energy loss through creation of tip vortices, as admirably demonstrated by some of the pretty pictures above.

So drag is reduced, Cl/Cd is increased, wake vortices reduced thus reducing wake separation requirements, fuel burn is reduced, particularly in low speed flight where the Cl^2 term predominates, although there will still be some effect at high speed (where the Cdo term predominates).


At anything above foundation level, I'd have thought that your lecturers would be looking for an explanation along the lines I've given above, with a sketch diagram of the tip vortices and how they reduce in magnitude.

G

Sorry, but wake vortices are not reduced. They are different. With the winglets there are the two components, the small core and the large core, virtually 2 separate vortex, and they act much differently.

The small core are very high velocity, and a smaller core, which may break up earlier. The large core are slower moving, and tend to last longer.
Under the right conditions, a small core will transform into a large core. This tend to happen when the air is drier, and calmer. Under this scenario, the vortex is much longer lasting, and therefore a much higher chance of WVE.

The flaps on the ac have the largest effect on wake vortex. A 737-800 on final with flaps 20, generates a much different vortex than the same aircraft with flaps 30 and/or 40. The same aircraft.

IF you want to see some real interesting vortices, look at the A380 on final....

Can you give a reference or two for that - I'd like to read up on that, which is new to me.

G

Aerodynamics and Fluid Mechanics 3?

That exam was fun. :ok:

A real reference! Not some lecturer's course notes - I know how bad they can be, I used to be one!

G

Less wingtip vortex equal to less drag equal to less fuel consumption.

On some planes not much difference but, a nice to a put a logo on!!

The stuff about vortices, doesn't answer the question., which wants the student to describe the effect not the design of winglets.

The prformance benefits are specifically, increased take off and landing weights and lower cruise fuel burn.

Degree level education should be testing understanding, not tick-box glib answers as might be preferred for pilot TK exams.

Without seeing the module descriptor and whole exam paper of-course, it's hard to be at-all sure of the level that was expected of this student.

G

Genghis,

I assume that you are asking about the A380 wake? I have not published anything about about this aircraft yet, as the data collection is still ongoing.

There are a few significant factors that influence the creation of the wake turbulence on final. Flaps are the most significant factor in vortex creation. (as noted with the 737-400 with outboard flaps vs 737-800 without, or flap setting 25 vs 30/40). Think about this, on final in a crosswind, the AC is crabbing, exposing one wing more than another, so you have a completely different vortex off of each wing...(in all the illustrations about crosswind blowing the vortex out of the way, they are incorrect, and actually, the vortex rollup can be increased on one side and decreased on another...I have even seen them rollup and bounce along the wind like a stone skipping on water)

The GPA is another, and a shallower GPA tends to reduce the strength and velocity of the vortices.

What I will also note on the A380, is a slow FAS, and that once the wheels are down, the airflow is so scrambled, that the turbulence doesnt rollup...

My understanding of the physics of the wake is this:

The aircraft's lift force is matched by a transfer of momentum downwards to the airflow in the wake (Mr Newton), which is what drives the vortex. In level flight, the rate at which the momentum is transferred matches the aircraft's weight, and is inevitably associated with some energy imparted to the air: mostly kinetic energy in bulk flow, with some thermodynamic effect from changes in pressure, both smoothly and in shocks.

By minimizing the kinetic energy given to the wake, for a certain amount of momentum, you reduce the rate at which energy is lost to the bulk airflow (induced drag) for a certain amount of lift.

The amount of kinetic energy imparted to the air in the wake depends on the product of its density and the speed-change-squared - while the momentum transferred depends on the product of the density and the speed-change. The key goal is then to minimize the average change in speed in the airflow in the wake, especially of the fastest-moving parts of the wake - the core of the vortex, where the speed squared term is most significant.

If you can reduce the average speed of the same mass of air in the vortex, by making the extent of the fast parts of the vortex less, as suggested in the Boeing cartoon above, or otherwise, using a fence, increased span, raked tip, winglet etc., then you're making the owner happier.

It's not dissimilar to the efficiency of a jet/propellor - you want to move as large a mass of air as possible, by as small a change in speed as possible, to get the maximum force from the minimum amount of energy expended.