The difference between winglets and higher a/r

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speedracer

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Even if my Long EZ had a performance loss because of the winglets I wouldn't give them up because they just LOOK so **** COOL!
 

wsimpso1

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I was not making up this stuff. Barnaby Wainfan (Technical Fellow for Aerodynamic Design and Analysis at Northrup Grumman, noted author, airplane designer and member here) presented on the topic at OSH a few years back. From his presentation, I got these takeaways:
  • Putting a winglet on a wing does the same thing as adding span;
  • Adding span is way more likely to give an efficiency boost than sticking on a winglet;
  • Getting a net efficiency boost usually requires CFD and/or wind tunnel work and/or on-airplane development;
  • Structural loads go up when you add a winglet, with potential structural weight increases.
Now maybe Barnaby will step in correct me on my memory of that day, but I was paying serious attention while he was talking for us.

Then along comes AutoReply (Good to have you back!) and tells us what the sailplane end of the spectrum has figured out.

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.

The cited article is interesting. The author starts out telling us how many years of fooling around with winglets in sailplanes it took before they produced gain while thermaling slowly that did not cost you more while flying fast between thermals. Here is the first hint for us power guys: Lots of smart folks had trouble not making more drag in the operating area where the power guys cruise. here is the second hint: Then author also called the improvement possible for sailplanes from winglets "small but important". I talked about racers already.

The plots, while useful for sailplane folks, could use a bit of interpreting for us power plane design guys.
  • Figure 5 - Sink speed vs airspeed at two weights with and without winglets. It appears that sink speed vs airspeed is not changed materially by going from a plain wing tip to a winglet. Again no info on more span vs winglets;
  • Figure 6 and 7 - Increase in L/D is around 2.4% improvement over the base wing at 40 knots (where they thermal), and sinks away to about 1/4% at 120 knots. Yep, a well designed, developed, and executed winglet can be better than a plain wing tip even at 120 knots. There is no winglet vs span extension info on this plot, but I suppose somebody could paw through TOWS and tell us;
  • Figures 8 and 9 - These are from simulations of cross country flights and displays increase in total trip speed and they compare span extension to winglets! How does this relate to power planes? Days with weak thermals mean a higher percent of the day flying slowly in thermals than on a strong lift day. On strong lift days, the pilots pare off a bunch of the time spent in thermals and thus spend a bigger fraction of the flight time doing what power planes do quite a bit of - cruise. So for power plane use, the high lift end of those curves is useful. Tip extensions on the ASW-22B are slightly better than winglets when light and about the same when ballasted, while the Discus comes in with a fraction of 1% improvement in cross country speed.
  • Figure 10 - This show us how critical toe angle is to the gain or loss of cross country speed. For the ASW-22, +1 degree of toe out on the foil is better than -3 degrees or +5 degrees. In fact the other two angles are net losers in anything but light lift days.
Now we have some data points. What does the doc say to me? If we do a downtown job on designing a winglet, refine it in CFD, then, maybe some tunnel time (because sometimes CFD means Can't F'g Decide) and run a little designed experiment, so we can finally establish the design and go fly it to see how it does. Quite frankly that costs money, but in the competitive sailplane race environment that is the price of admission to the market. For us little homebuilt power plane guys, well, I can find you a lot more L/D and cross country speed by reducing baffle leaks, making it easy for air to get out of the cowling then start throttling down the inlet until CHT's get to 410 F on a 100 F day, fairing over protuberances, fixing the fit of fairings, cowlings, covers, and canopies, etc.

Oh, for perspective, if you do get 0.5% better speed on two hour trip, you save exactly 36 seconds. Maybe the racers would like that but the rest of us will play merry hell trying to even measure the difference. You could make your pattern tighter and your turns into and out of the pattern sooner and save more than that...

More later.

Billski
 

Bille Floyd

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Sep 26, 2019
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...

In the piece, he says this specifically regarding end plates (just flat wing-fences-at-the-wingtip with no airfoil shape of their own. They may extend above the wing, below it, or both):
++++++++++++++
Quote:
The end plate increases the amount of lift the wing generates at a given angle of attack and reduces the induced drag of the wing.

Typical end plates are relatively short. They are more like fences near the tip than large surfaces and rarely protrude more than about 1/4 of the cord above or below the surface of the wing.
...

1982 I was launching off Telluride Co at 12-K ASL ; it was my turn
to show what i got for the world acro champs. No wind at that altitude and I destroyed my Right tip gate on takeoff . Flying away
I had do decide the consequence for the right tip gate not functioning
properly, and had to choreograph a new routine . Stability came at the expense of drag , and the Right wing advanced forward in level flight.

OK -- all my spins were to the Right ; the drag from the
tip gate still functioning would decrease my RPM's , and the
exit would be more to what I had bin use to . OH WOW
but barrel rolls to the Left, were Way more intense than
before !!!! I got # 4 in the world in that comp ; most of
that was because of the 5-min i had to decide the new routine
with the broken tip gate .
It was a Blast ; 1000 cups of coffee , and I'm still addicted
to that feeling of uncertainty ---- !!!!!!... ... ...

Anyone , Anyone at all here ; Feel what i felt that day ???
It's like , Nothing else really matters ; my need / desire for
the drug known as adrenaline, has taken over my desire
for life . :(

Bille
 

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wsimpso1

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Even if my Long EZ had a performance loss because of the winglets I wouldn't give them up because they just LOOK so **** COOL!
The Vari-Eze and all of it derivatives need a vertical tail for yaw stability. The winglets are justified with that alone. Now if they can improve L/D over some part of the envelope too, it is a bonus.
 

TFF

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Sounds like this is an argument of scientific measurement and usage perspective. Scientifically is a longer wing. Sitting in the hangar doorway it fits or it doesn’t. Every alternate example given is trying to solve the same problem of getting rid of the vortex on the main wing from the Spitfire , Wittman style, Boeing swept ones. Essentially if you can get a 24 ft wing in a 20 ft package, it’s a win. Hershey bar wings get the most advantage. Aftermarket ones on a Cherokee or when Wittman added tips to his square wings, it was night and day. Something Boeing or Airbus is looking for a .003% gain from as perfect a design already for the salespeople to sell more planes.
 

wsimpso1

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I just moved Stan's analytical experiment with wing tip design to its own thread. I realize it is related to this one, but seemed sufficient to stand on its own.
 

aeromomentum

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There is no such thing as a "wing tip vortex". At least in regards to lift efficiency.

There is finite wing trailing edge vorticity.

In fact, if you look at the vortex some distance aft of a loaded wing the vortex is centered about 2/3 to 3/4 from the center-line. Not at the tip.

So we should stop using the misleading and incorrect term "wing tip vortex" since it leads some people to think there is some sort of magic tip that can eliminate the finite wing trailing edge vorticity. Physics is a tough taskmaster.

As a start, just keep in mind F=ma and KE=1/2mV^2. Lift is a force. To maximize lift and minimize the required input energy we must maximize m and minimize the V. m is the mass of the air that me accelerate downward to make lift. V is the down-wash velocity.

Let the fun begin...
 

Tiger Tim

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For the latest 777 Boeing has gone to the trouble of developing wing tips that fold up for ground operations instead of using winglets to shorten the span so surely that’s a worthwhile data point in the discussion.
 

Jay Kempf

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There is no such thing as a "wing tip vortex". At least in regards to lift efficiency.

And to make it less clear the vortex starts from some infinitely small place near the wing (maybe at the tip) and gets larger and larger in diameter as it forms aft of the wing and the spanwise center to center distance continues to decrease. So using a wing tip or winglet to control that whole shape and intensity is what this whole thread is about. If, and it is a big if, you can reduce the size and intensity of the vortex system you gain efficiency because any air molecule unmolested by you and your craft sliding by is energy that you didn't have to produce to go in whatever direction you are going.
 

BBerson

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A lifting wing has many “bound vortices”, shown in figure 6 of Theory Of Wing Sections.
The low aspect ratio wing has a strong tip vortex that may cover each semi-span.
High aspect ratio has weak tip vortex. Infinite aspect ratio has zero tip vortex.
 

bhooper360

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For the latest 777 Boeing has gone to the trouble of developing wing tips that fold up for ground operations(...)

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

  • Figures 8 and 9 - These are from simulations of cross country flights and displays increase in total trip speed and they compare span extension to winglets! How does this relate to power planes?

For the RedBull winglets, they have a computer program that simulates the course and finds the total time, so I guess it's like a table of load cases, except with AoA, airspeed, etc. The weight of the winglets was a big downside.
 

skewt

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I understand the desire to create simple equivalencies between options, but these simply do not exist in a general way. Even were there a nice aerodynamic equivalency, this is only helpful in setting an initial guess what you should do because mission performance is what actually matters in aircraft design. It almost physically pains me to see (to be clear, I am not speaking specifically to this thread or anything on this forum) how seriously people take the argument about which lift distribution is most efficient or whether winglets are more efficient than span. These things don't matter. Repeat that a thousand times to yourself. Once I come up with a pithier way to say it, go get that tattooed somewhere conspicuous. Rent billboards in your vicinity and get it printed.

It is not that these are not important questions, or that their answers aren't useful. Rather, when we worry too much about them we are losing the bigger picture. An aircraft is not an isolated wing tip. It is not a bound lifting line. It is not an isolated beam. It has a mission and if we aren't arguing back to its mission we are missing the plot.

To make any design decision you just need to follow a four step process:
  1. clearly define of the mission of your aircraft
    1. tattoo this somewhere else conspicuous
    2. rent billboards on the other side of the highway
    3. set hourly reminders on your phone
    4. chant it before you go to bed at night
    5. repeat it to yourself every time you get a call offering you an extended car warranty (on second thought this probably covers all of the previous instances as well)
  2. make a model which can quantitatively relate design parameters to mission performance
    1. validate that model in some way so that you can actually trust it
  3. design your aircraft with each option (note that this means optimizing the entire design around each option)
  4. use your model to see which one performs better
Easy-peasy. You just have to complete a full aircraft design every time you have to make a decision.

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.

If the continued existence of your livelihood rides on the performance of your aircraft then go see steps 1-4. If you see a winglet (or not) on a Boeing, Airbus, Schempp-Hirth, Red Bull racer, etc it came from that process. The result may or may not be relevant to you to the extent that the design drivers and performance objectives of your aircraft align with your examples. If your company isn't resting on this and you just really have to have maximum performance go see steps 1-4 and sharpen your pencil, or else start budgeting for a qualified engineering consultant who is conversant with the four step program. (A back of the envelope calculation suggests approximately the same hourly rate as a 182 which strikes me as suspiciously convenient)
 

PMD

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To pour a little bit of fuel onto this nice, warm campfire: I have followed the work of Christian Hugues for some years, and rather than quote the good Professor's work we can just refer to his website that provides a LOT of good resources to understand and appreciate wingtip devices. IIRC, we should only use the term "winglets" for devices derived from Dr. Whitcomb's work.

 
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