winglets?

[mosquito]

OK, I've been wondering this for awhile now... perhaps someone knows the answer? Winglets seem to be the rage on airliners of late. I see them on all the new RJs and I think 737-800's come stock with 'em. They're credited with increasing efficiency of the airframe, and 'generating' thrust by eliminating drag... sounds ideal for a race plane, right? how come they haven't appeared?

Here's a link to a winglet article if you have no idea what I'm talking about.

-mosquito

[spacegrrrl]

Re: winglets?

Ah, the topic of wingtips..... now there is some great hangar racing! :-) Actually, if you want to look into an even better idea for wing tips the NASA research on elipitical wing tips is very interesting. Bill Rogers was seriously considering those for MAII.

If you look at the unlimiteds today they all have lower than optimal aspect ratios. That would be one of the first things you could improve if you built a true ground up racer. Winglets might be a considertion on a next generation unlimited.

Michele

[FlyKidChris]

Re: winglets?

While popular, and getting more popular with the increase in fuel prices, the new 737s do not have winglets as standard equipement (except for the BBJs which do come with winglets). Southwest has winglets on all of their 200+ 737-700s, for example.
I'm not an aerodynamicist, however, my understanding of winglets is that they effectively add wing area (equal to approximately 5/8s of the height of the winglet) without increasing span (an equal (horizontal) amount)(I'm not sure of the structural implications). This increased wing area allows the airplane, like the 737, to operate at higher altitudes or to carry greater weights, or both.
However, the surface of the winglet is "wetted area" and as such contributes to drag.
Those winglet equipped airliners are not faster that winglet-less ones. So for racing, speed is still a higher priority than improved altitude or weight carrying ability.
I'd welcome any criticism from any aerodynamic engineers. I see this as a learning opportunity for me.

[FlyKidChris]

Re: winglets?

First, I'm not an aerodynamicist, so I'm hoping one will add to, or correct, my comments.
As I understand it, winglets - like those on the 737s - effectively add to the wing area, without increasing the horizontal span the same amount. As it's been explained to me, on the 737, the winglets (about 8 feet tall) add the equivalent of 5 feet (span) to each wing. This larger wing allows the airplane to fly higher (with the same load as a non-winglet 737) or carry a greater load, or a bit of both.
But the additional "wetted" surface area also adds drag. The higher priority in racing is going fast, not lifting a greater load or flying at a higher altitude.
Michele's point about racers with non-optimal clipped wings bares consideration, though. The racers are considerably lighter than a stock military fighter and don't likely need all that wing area either, but a wing clip results in a lower aspect ratio and that's rarely a good thing, also.
Finally, except for the BBJs, the 737s don't come standard with the winglets, but with the high cost of fuel, they're increasingly popular (all of Southwest's 200+ 737-700s have are winglet equipped).

[Apteryx]

Re: winglets?

... how come they haven't appeared?-mosquito

There are & have been some on varies aircraft, a few Bipes I can think of, a few on some Unlimiteds... A.J.Smith (I believe) did a University study on variations used on a Cassutt. Worth it if you get them just right, but without wind tunnel access, most guy's don't bother.
G-loading, I think, negates some of the benefits you'd see in straight line speeds.

Good place for Eric (Bluefoam) to chime in...

Paul

[Blue Foam]

Re: winglets?

WARNING!!! DANGER, WILL ROBINSON!!!

Winglets come up for discussion every few years and are quite a controversial subject. It does not help that both proponents and detractors use what I call "partial equations" to justify the use or elimination of them. The web encyclopedia article quoted is unfortunately both false and incomplete:

Winglets work by increasing the effective aspect ratio of a wing without adding greatly to the structural stress and hence necessary weight of its structure - an extension of wing span would also permit lowering of induced drag, though it would cause parasitic drag and would require boosting the strength of the wing and hence its weight - there would come a point at which no overall useful saving would be made. A winglet helps to solve this by effectively increasing the aspect ratio without adding to the span.

Whooooaaaaaaa..... Who wrote this? It wouldn't pass a freshman Aero student with all those contradictions.

First, let's define drag of a subsonic, streamlined aircraft in steady-state, low alpha (angle of attack) flight as being parasitic (skin friction) plus induced (drag due to lift) drag. Neglecting separation, stagnation, and internal (cooling) drag, aircraft drag is dominated by parasitic, or viscous skin friction, losses. This is simply proportional to wetted surface area. So a bigger aircraft has more drag, all other things being equal.

Induced drag can be approximated as:

CL2/(pi*A*e)

Where
CL = wing 3 dimensional averaged lift coefficient vs. reference area
Pi = ratio of a circle's circumference to its diameter
A = Wing aspect ratio: span squared divided by reference area
e = Oswald's span efficiency factor (An "official" fudge to balance the equation)

For a given wing we are going to summarize a "trade study" on winglets. A trade study is an objective evaluation of multiple alternatives held to the same rules and examined for all rational advantages and disadvantages. Without objective trade studies, an engineer is simply an opinionated geek.

First, we understand that both increasing wing span and adding a winglet will increase wetted area, and with it, parasitic drag. So lets simplify the study by normalizing for identical area increases and see what falls out for induced drag, weight, stall speed, etc.

First, if we use the increase in area to increase aspect ratio, we will decrease the span loading and with it CL. 'A' has increased and 'e' will slightly decrease. Therefore, we are reducing induced drag in our equation by three orders. This is VERY powerful. Wing weight increases due to the increased span, however area is increasing as well, so the weight increase is proportional to span2/area. If we haven't built the wing yet, an increase in span allows moving the ailerons further out and an increase in flap span. The resulting reduction in stall speed or increase in gross weight for a given stall speed is significant. Increasing wing area and aspect ratio requires bigger tail surfaces, so there is a secondary penalty added. At some point, the increased weight costs us as much as we gain and we find the optimum span and area for a given design.

Winglets work differently that a simple area and span increase. They are highly cambered airfoil sections that are aligned to fly through the wingtip flow and create lift that has a forward component to it. This counts as "negative drag". They allow a decrease in CL and an increase in 'e'. Contrary to the encyclopedia, they do NOT increase 'A', and they DO increase wing weight. For an equal area increase, they are worse for the flutter margin of the wing. This is caused by the concentration of the weight increase at one span location. Again contrary to the encyclopedia, this increases wing weight, unless the wing was already built with excess flutter margin (like the B737 NG). The winglet does decrease roll rate more than an optimum span increase. Winglets do not require an increase in tail sizing, and in fact can increase yaw stability when located behind the CG as they are on a swept wing.

Secondary advantages of winglets are significant. If a wing has already been optimized at a given aspect ratio, taper, twist, etc. then a span addition may not be aerodynamically practical and may create an outboard stall problem. For the already completed wing, the advantages of flap and aileron span increase would not exist and increasing tail size can be difficult. Next, winglets allow an increase in span efficiency for aircraft with non-aerodynamic restrictions on the span. These include gate spacing at airports, hanger and door size, and ramp clearance. This last was a secondary limit for the C-17.

Another advantage to winglets exists if they can be used for an additional purpose that allows the reduction of other wetted area, controls, etc. This is the case of a canard or flying wing aircraft where the winglets double as rudders.

Summary:
1. For a new conventional design, Boeing believes optimum aspect ratio without winglets will beat equivalent area lower aspect ratio with winglets. Airbus disagrees. (Boeing typically beats its drag predictions and Airbus typically misses them ) Hundreds of millions of dollars have gone into studying this and the jury is still out. Given the variety of winglets in use, a consensus has not been reached.

2. Given an already built wing, it may be easier and better to add winglets than span.

3. Given a non-conventional design that allows winglets to serve multiple functions, they are an advantage over increasing span alone.

All of this said, I hate the damn things. <LOL> As admitted in the business jet industry, they are as much a fashion statement on many aircraft as they are functional. My next new design does use them for reason 3 above.

[First time Juke]

Re: downpointing winglets?

I recall chatting about winglets few years ago with Eric A. One thing still bothers me: why does A-10 and why did " RIVETS " have sorta downpointing winglets.

I would guess they are to help preventing an early stall at landing, but I would like to know for sure.

[spacegrrrl]

Re: winglets?

I don't remember Rivetts having any sort of winglet. It had a highly tapered almost delta like wing planform but I don't remember anything like a winglet.

Michele

[FlyKidChris]

Re: winglets?

Eric,
First, and foremost, thanks for the reply. It was quite educational on just the first reading (yes, I'll read it several times).
Second, do you have suggestions for web-sites or books for further education (for the "layman" professional pilot).
Third, I regularly get to fly both winglet-less and wingletted 737NGs and don't notice any difference other than an improved altitude capability (and a need for additional caution when taxiing in congested areas).
Thanks again.
Chris

[First time Juke]

Re: winglets?

I don't remember Rivetts having any sort of winglet. It had a highly tapered almost delta like wing planform but I don't remember anything like a winglet.

Michele,

I could be wrong..I recall I saw in 1970 something taken pic winglets...but I could be wrong.

It was like this:

http://img75.imageshack.us/img75/9919/rivets4gq.jpg

And it had a Naca M6 modified foil...here is a M6 root and M5 tip:

http://img387.imageshack.us/img387/7046/nacam6nacam51ok.jpg

[Blue Foam]

Re: winglets?

Eric,
First, and foremost, thanks for the reply. It was quite educational on just the first reading (yes, I'll read it several times).
Second, do you have suggestions for web-sites or books for further education (for the "layman" professional pilot).
Third, I regularly get to fly both winglet-less and wingletted 737NGs and don't notice any difference other than an improved altitude capability (and a need for additional caution when taxiing in congested areas).
Thanks again.
Chris

Good questions, here we go:

1. It was only a summary, I expect some of the more experienced aero readers on this board to expand on certain facets. It does require several reads to appreciate the inter-relationship of all the factors. The biggest myths that I wanted to bust were the weight penalty and the "effective aspect ratio increase" nonsense. Winglets increase "span efficiency", not aspect ratio.

2. There is so much poor and out of date information out there that its not possible to go on the web and find anything useful. I know its an expensive book, but if you want to get real info in a readable form, buy Raymer's "Aircraft Design: A Conceptual Approach". Its about 90% accurate. IMHO, web articles run 10 to 50% correct.

3. There is a control boost schedule change with the Aviation Partners winglet installation. Interestingly, there is also a BIG stuctural beef up in the outboard wing section to support those monstrosities.

A question for you: the winglets only add a couple of feet to each side, is the extra caution taxiing due to a depth perception issue vs. regular tips?

(a side note: my Father who is now retired was the professor who granted the PhD to the chief designer at AP)

[First time Juke]

Re: winglets?

Actually, if you want to look into an even better idea for wing tips the NASA research on elipitical wing tips is very interesting. Bill Rogers was seriously considering those for MAII.

Hasn't elliptical wing always been the optimal form...it is just not as cost effective to make'em.

I also liked the wing form in Eric Ahltroms Dart racer. Certainly a superb and well studied wing, if anything.

[spacegrrrl]

Re: winglets?

Hasn't elliptical wing always been the optimal form...it is just not as cost effective to make'em.

I also liked the wing form in Eric Ahltroms Dart racer. Certainly a superb and well studied wing, if anything.

What you see now sometimes (I've seen the planform on some turboprop commuter airliners for example) is the kind of progressively tapered planform that can approximate elliptical. I can't for the life of me remember the name for that planform. The Lancair 235 has something like what I'm talking about. I think the planform was first seen in sailplanes.

Michele

[G. van Dyk]

Re: winglets?

Blue Foam, thanks a bunch for the discussion on winglets. I always suspected Boeing had the philosophy to avoid them, although it looks like they're going that way on the Dreamliner / 787.

I wonder if you could give us some thoughts on the upturned tips that look like inverted Hoerner tips. I've used something similar on a couple of R/C model racers and heard some chuckles saying they were high drag. I figure if they're on Dago Red, Strega, and about all the fast and semi-fast mustangs, they're plenty good for me.

What effect do they have on lift / drag / airflow.

Thanks a bunch

Gerry

[G. van Dyk]

Re: winglets?

What you see now sometimes (I've seen the planform on some turboprop commuter airliners for example) is the kind of progressively tapered planform that can approximate elliptical. I can't for the life of me remember the name for that planform. The Lancair 235 has something like what I'm talking about. I think the planform was first seen in sailplanes.

Michele

If I'm not mistaken, this is the "Schumann" wing planform. All the high performance R/C gliders use it nowadays.

Gerry