The difference between winglets and higher a/r

[cblink.007]

Sorry, fam. Late forum participant. I have been doing some extensive research into winglets on my tailless E-AB project of late. Although the dynamic scale model flies good on its own without vertical surfaces (which it was originally designed to do), the directional stability data we have been pulling lately still leaves a little wanting. So, after testing over close to 100 iterations of differing configurations, we found out a few basic things:

1. Yes, the winglets to in fact contribute to enhanced directional stability on this aircraft. Significantly enough to strongly consider adding them to the full scale model (which would also require some significant aerodynamic redesign as a result).
2. If configured correctly, the winglets do in fact decrease drag noticeably.
3. As previously discussed, the winglets increase both effective AR and dihedral.

We got as far as getting to what looks like an optimal configuration for this particular aircraft- a split scimitar winglet...similar to what is seen on a 737. However, different airfoils/twists/sweeps/spans, etc would be different between the upper and lower surfaces.

In an effort to validate what we are seeming on simulation, we modified the model, adding a split scimitar winglet system, sized and located with respect to Re, velocities, flight envelope, etc. Here is the result, that will be tested this weekend:



The last picture was taken earlier this week at an open hangar event our local EAA chapter did. Here's to hoping the aircraft behaves on prediction. I'll let you all know the results!

 

[Tiger Tim]

Yes, this is much easier than going through the trouble of rearranging the airports for the latest 777.

Of course. My point is that they had a span constraint and were seeking a solution. These folded tips look proportionally the same size as some of the bigger winglets you see nowadays:

Yet someone’s cost/benefit analysis showed that designing a power folding tip with all its redundancies then somehow getting the thing certified was projected to be better than a winglet. To me this really props up Mr. Wainfan’s stance that when more span is an option go with more span.

 

[Jay Kempf]

All depends on how you stack your performance and structural optimization. If you just start with cookie cutter span, area, propulsion, yadda and when you add payload and put in range/endurance/speed it is possible to just blindly focus on span bigger than you can park. That drives design towards solving that problem. Winglets are proving to be a pretty simple bolt on solution for a lot of airliners with quite a bit of fuel savings data already collected and verified against theory. Winglets are tough to design but can't be hard for Boeing. That wing tip fold thing is a probably no harder than designing a flap mechanism. So also easy for Boeing. The winglet is certainly easier to get past the FAA. (I'll ask some people that probably know how the compromise was made.)

 

[cblink.007]

Winglets are tough to design but can't be hard for Boeing.

Here is a remarkable paper that served as one of the guides in our development:

 

[autoreply]

Clearly in reality this isn't feasible, so you get the classic aircraft design / analysis spiral of increasingly comprehensive analysis as the design matures. We use examples and theoretical solutions to similar problems as heuristics to bound this spiral early in the design or analysis phase. If you aren't getting paid to do the design and your company isn't riding on the result, honestly these heuristics are probably good enough for the fussy details which happen away from critical safety of flight items. As mentioned earlier in the thread, if you have a performance-related design decision which seems like it is going to be hard to clearly make, going with the low technical and implementation risk option is usually a good call -- you can probably make a bigger difference with a bit of care when you actually build the thing.

Winglets' performance are pretty significant for your typical light aircraft. If you're bound by climb (gradient), or hot&high performance, it's typically an allower to increase MTOW by a few percent for the same airframe/engine/prop combo. Design it well and there is no cruise penalty.
The sane program management approach is to design the wing for a winglet (extra bending strength) and spend the $$$ on CFD for a winglet once you have a positive cash flow from delivery of the first airframes and sell the winglet "upgrade" at a significant cost.

The sailplane guys AutoReply is talking about are racers, and racers (of just about anything) will spend immense effort and expense to gain tiny advantages or not be left behind. We may be talking very modest differences yet the top guys plus the well moneyed also-rans in all racing feel a need to have the best speed mods.

Difference in speed between #1 and #4 on a world championship is typically on the order of 0.1-0.2%.

An interesting midway for us "amateurs" is to have canted tips, say 30-50 degrees up. Design then is way less sensitive to exact toe in/out and you get a significant portion of the gains of a winglet.

Of course. My point is that they had a span constraint and were seeking a solution. These folded tips look proportionally the same size as some of the bigger winglets you see nowadays:
View attachment 127004
Yet someone’s cost/benefit analysis showed that designing a power folding tip with all its redundancies then somehow getting the thing certified was projected to be better than a winglet. To me this really props up Mr. Wainfan’s stance that when more span is an option go with more span.

The trade off for transsonic flight, especially once you get over .8Mach is very different from subsonic flight. This is clearly demonstrated in the 737/A320 vs dreamliner for example.

 

[Heliano]

Interesting thread. Very knowledgeable people here.
Five remarks:
1 - Winglets (or sharklets, or sabre lets, or any other name) also produce wing bending moments, very much like an extended wingspan;
2 - Commercial aircraft like the 777 have other constraints like fitting in a gate without interfering with the aircraft next to it;
3 - Some sailplanes have constraints too: standard-class sailplanes can not have wingspans of more than 15 meters, but can have winglets;
4 - Winglets are tuned to a certain speed When I say "tuned" I am talking about toe-in/out angle, cant angle, dihedral, span and sweepback. The added efficiency tends to drop at other speeds and may even become negative.
5 - From the construction standpoint, it is easier to extend wingspan than add winglets.

 

[BBerson]

Winglets' performance are pretty significant for your typical light aircraft. If you're bound by climb (gradient), or hot&high performance, it's typically an allower to increase MTOW by a few percent for the same airframe/engine/prop combo. Design it well and there is no cruise penalty.

How much do winglets increase empty weight? And cost?