Elliptical tips worth it?

[Curare]

I'm starting to design a new set of wing for my next sailplane.

I'm tempted to with a slight taper on the inboard panels and then go for an elliptical swept back tip.

Is there anything I should be looking for in this design?

[schrederman]

Lotsa work for little gain... IMHO... Triple or Quad taper will get so close...

Jack

[Curare]

I've gotta admit Jack, last time I did it it was so easy I laughed!

I did it to see if I could on this little one:
http://www.rcgroups.com/forums/attachment.php?attachmentid=1507700

and it worked really easy, I knocked the wing up in a weekend.

[atjurhs]

That plane looks awfully purty!

[WDKK]

Where the heck is Bubba when you need him :D

[Peytr]

Is there anything I should be looking for in this design?

I tend to keep the tip chord a bit wide, over 12 cm or someting in that range, depending on the size of the glider.

On an eliptical wing the tip chord is essentially nothing, so I cut the last bit (in the design that is) to keep the tip chord Re nrs in the workable range. There's quite some controversy over this issue, so be sure to get second, third, fourth opinions..... :rolleyes:

[BMatthews]

Curare, that's a sweet looking model in the link. It'll be nice when you get the new fuselage and matching tail surfaces to go with the wing.

All else being equal smooth is definetly going to be better than having any edges. So technically a smooth elliptical taper will produce less overall drag than even a 4 break wing design. Just less "ridges", minor though they are, to trip the air and produce possible separation bubbles.

But a lot of other factors come into it as well. Can you maintain the required accuracy of the airfoil shape as the elliptical shape progresses to the tips? Will the inevitable stresses in the sheeting tend to warp the wing or flex the ribs? And how will the spar be added? Would you just use a single straight spar or will you laminate or otherwise form a curved spar and how will that handle the loads? And a host of other niggling details.

Basically it comes down to any model making project is a set of compromises and solutions that are more approachable for the person doing the building. For some of us it's easier to use a series of straight and tapered segments knowing that while the small ridges will generate some drag that the accuracy of the airfoil is easier to achieve. For some others they may have found like you that forming an elliptical curve isn't that hard but may have some other compromises.

SInce you've done one wing already you're most likely in the best position to know how well it works out. If you're confident that you can maintain the accuracy of the airfoil then you stand to potentially gain some small amount of lower drag.

The big question is if you'll notice any advantage or not. After all my rambling on I'd have to say that it's not likely. The elliptical wing compromises and advantages will likely match the multi tapered compromises and advantages and any differences will be hidden by a couple of minor mismoves or gifted moments at the sticks.

HOWEVER! If the thermal gods award thermals to the more attractive models then I'd have to say that you'll be starting with a HUGE advantage if you go for another elliptical wing.... :D

[hangerdude1]

Curare - excuse me for butting in on your thread but came across this beautiful plane on another. If you haven't seen it as yet, take a look :)

http://64.233.179.104/translate_c?h...2520arcalis.htm

Thermals,

Ken

[chlee]

I would recommend against elliptical tips unless you think the good looks are enough justification.

Consider the following:
With aspect ratio 10 and a straight, single 60% taper, your induced drag is will be only about 4% greater than a wing with an elliptical lift distribution.
(Calculated from span efficiency factor from a graph in J. Anderson, Aircraft Performance and Design (1999), p.109-110.) With double or triple taper, proper sections and washout, you'll be so close that an elliptical lift distribution that it won't be worth the extra trouble.

If the elliptical wing tips require any additional weight (shaped balsa tips, etc.), then I'd recommend against them even more -- you'll pay for adding mass at the wingtips. That weight might be better spent on simply increasing your span.

That being said, if you regard an airplane as not only a machine but also a work of art to be admired for its graceful lines....

- Chung

[Robglover]

Several of the recent designs with elliptical wings have done very well indeed. I've been flying them for several years now, and I think that they are worthwhile. I also think that they are pretty.

Note that I am not the one who is building or designing them.

Note also that the wings that benefit most from elliptical planforms were designed by guys like Wurts and Drela. They have great experience, so they know what to aim for with their design tools. They also have a very well stocked design toolbox and a great knowledge of how to apply their tools. There is a lot more to their wings than the planform, everything works well together on a good design.

I'd say go for it, particularly if you like the way they look. You'll always enjoy the plane better if you think it looks sexy.

[BMatthews]

.....Note also that the wings that benefit most from elliptical planforms were designed by guys like Wurts and Drela. They have great experience, so they know what to aim for with their design tools......

These two examples are also extremely skilled in knowing which way and when and just how far to nudge the sticks.... :D

[Curare]

To be honest, there's a slight compromise occuring for me.

I love the look of the drela 'supra' tips and, well, I can do it, so while it might be a design concern, looks are pushing this too.

I havent started on the wing, but I will soon.

[schrederman]

My only worry is airfoil accuracy when sanding the thing out... which lead to my first statement. Waviness in the airfoil surface that can't even be seen can cost in drag penalties. Perhaps I'm missing something... When buying one of those perfect, molded jobs, they're usually done up in molds made by a CNC machine, and these worries are gone. I agree they're beautiful... how accurate can you make them? Maybe you should make me a set so I can be the jusge of that... ;)

Jack

[Curare]

Yeah, you're right. Getting a proper airfoil that's consistent is very hard in a built up wing.

After some serious consideration I do wonder how accurate my little wing is.

I think that maybe you're right, to gain an airfoil that's true can probably only be done easily with a triple taper.

Let me take a look at the ribs I've made on that last one and we'll see what's happened.

[Curare]

Hmm, just as I suspected, the way I've done it (the easy way) it's acutally changed the profile a little, but, only a little. But it's worth playing with to see what I get.

This is still being driven by the Oooh-aah factor, but a few more efforts to get something that is as close to the profile as i can I'm sure will pay dividends.

[schrederman]

I have to tell you... the Oooh-aah factor will get you in trouble! I'm still saying Oooh-aah after 35 years... and she's still pretty cute! Sorry, nothing to do with wings...

Oooh-aah still carries a lot of weight in my modelling, too. Otherwise, why do it? Let us know how this comes out. It's interesting. The wheels are turning... I may have to try this myself.

Jack