I'm concerned about the wingtips of this new Hobby-Lobby plane.

[Miami Mike]

This new electric-powered hotliner-glider is scheduled to be available from Hobby-Lobby in a few days:

http://www.hobby-lobby.com/images/hc007.jpg

http://www.hobby-lobby.com/freebird.htm

There's a thread about it in the Electric Sailplanes forum here:

http://www.rcgroups.com/forums/showthread.php?t=705298

Notice how the wingtips have a chopped off look and the ailerons go all the way to the ends. I was wondering if that's a good design for a hotliner, where high speed from vertical dives and energy retention are considered important, as well as a good glide ratio. If it's not a good design, what bad effect might those wingtips have on performance, and what might be done about it? Perhaps some sort of add-on tips could be made and attached, but would the design be critical? Would it take extensive testing to get the tips right, or would just about anything that looks good improve the performance?

Thanks,
Mike

[Julez]

Hi!

http://www.allstar.fiu.edu/aero/images/Image18.gif

Choose #1, but with a straight trailing edge.
Should look like this afterwards:

http://www.thelawlers.com/GF_Images/VAGF%20L%20wingtip.jpg

Cheers,

Julez

[markdrela]

Depends on how you define "performance".

Any sort of tip addition which extends the span will
1. (+) Reduce the induced drag, which will give better energy retention in sharp turns.
2. (+) Increase the glide L/D and reduce the sink rate.
3. (-) Increase the wetted area and the profile drag, which will reduce the top-end speed. If the model normally climbs vertically, this will also reduce the climb rate.
4. (-) Effectively weaken the wing, so it will fold up a lower G's than before.

If the tip addition is fixed and does not also extend the aileron, then it will also
5. (-) Reduce the max roll rate and increase induced and profile drag during fast axial rolls.

Pick your cure/poison.

Just looking at it, it doesn't look so bad. The wing taper, better seen at the Hobby Lobby page, is about right for a straight taper. The photo above makes it look like it has less taper than it really does.

[Miami Mike]

A big thanks for the response, Mr. Drela! :)

Do you mean this photo?

http://www.hobby-lobby.com/images/hc007s.jpg

Since you're here, what I'd really like to know is this: If you had one of these planes, would you modify the wing tips? If so, how?

[JetPlaneFlyer]

It should be easy enough to add some balsa block tips to give a more elegant rounded tip, but the effect on performance will be none or too small to measure.

One thing I have seen with ailerons that go right to the tip is that on high speed models the ailerons can be prone to 'flutter'... I guess they get 'excited' by the tip vortex. The effect is worse on long strip ailerons that have the horn or torque rod placed at the inboard end. I've seen a model loose an aileron due to flutter. Given a choice I'd stop the aileron slightly short of the tip.

Steve

[Ollie]

Be careful of adding mass to the wing tips. Adding mass on the tips slows the roll rate and slows yaw rate. I would use foam with ~2 pounds per cubic foot rather than best ~6 pounds per cubic foot balsa.

Looks vs. function?

"Pick your cure/poison."

[chaush]

How about making a chordwise cut on each aileron, about 90% outboard, and then gluing in the last segment to the wing (and fairing with light filler, of course)...

[Miami Mike]

That's an idea, but I still feel (for what that's worth) that the leading edges of the wingtips should be rounded. Adding foam add-on tips could attempt to address both issues, and would be easy to install and remove for evaluation.

I have one of these on order from Hobby-Lobby, but apparently they haven't shipped any yet.

[JetPlaneFlyer]

Be careful of adding mass to the wing tips. Adding mass on the tips slows the roll rate and slows yaw rate. I would use foam with ~2 pounds per cubic foot rather than best ~6 pounds per cubic foot balsa.

Looks vs. function?

"Pick your cure/poison."

I agree with the principal Ollie but I've got numerous gliders all with balsa block tips, all of which fly just fine. Balsa tips can always be hollowed out if your really concerned about a few grams. Balsa is more 'ding' resistant than foam too ;) I like balsa :D

[MarkusN]

That's an idea, but I still feel (for what that's worth) that the leading edges of the wingtips should be rounded.

I remember reading a report of a modeller who tested several wing tip designs on an hand launch plane (jevelin style; it was before discus launch).

He tested various exotic winglet designs and among others the square cut tip.

I don't know which design won, but I distinctly remember that the square cut tip came in second. It definitely performed better than the rounded tip.

Now measuring methods were somewhat crude, but he repeated the tests several times and they appeared to be quite consistent.

Also, when I entered this hobby (70ies) the skived out from the bottom sharp wing tip was all the rage. Planes flew with that, too.

[Ralph Walton]

I use tips like this. Just make some out of balsa amd stick them on. The drag reduction is noticable. Downsides like reduced roll rate etc are not noticable.

[Ralph Walton]

Oops. missed the photos in the last post.
The one from the front of the wing is a bit blurry, but I'm trying to show that its sharp. My theory is that there's high pressure air under the wing, trying to get to low pressure air at the top of the wing. the easiest way for it to get there is via a rounded tip, helping to induce a tip vortex. With a sharp tip its more difficult for the air to move from bottom to top. That should = less tip vortex. Less vortex = less drag.

[Miami Mike]

Beautiful!

There's no word yet when Hobby-Lobby will start shipping the Free Bird, but when I get mine I'll trace the existing wingtip and see what I can come up with.