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Undercamber characteristics question.

Cooley

Father/Son Team
#1
So, an itch I've had lately is to design a scratchbuild based on my grandfather's Stenson 108-3 (the one with the goofy huge tail fin). However, I wanted to undercamber the wings to try and make it more docile as well as give it some STOL characteristics. But my delima is, I still want it to be a 5-channel aircraft, so here's the question.

Would a heavily undercambered airfoil interfere with the control authority of ailerons and flaps?
 

earthsciteach

Moderator
Moderator
#2
I'm talking off the top of my head, here so take it with a grain of salt. I don't think you have too much to worry about in terms of ailerons and flap authority. My biggest concern would be the pitching moment of a heavily cambered foil. That could drive you to a larger than scale horizontal stab. Certainly, undercamber can add a lot of drag (depending on the thickness of the wing), but we don't typically do cross-country with RC planes, so I don't see that as a big deal!

You can cause some trouble with too much undercamber due to separation of airflow on the top of the wing at high angle of attack and separation of air from the bottom of the wing at low angle or attack/higher speeds. I don't know much more than that. This site has a huge listing of airfoil profiles and coordinates you can download and generate rib/wing templates.

http://www.ae.illinois.edu/m-selig/ads/coord_database.html

It might be helpful to pick a high-lift airfoil and use it as opposed to the trial and error method of drawing up your own. (Personally, I like the trial and error method!)

Are you considering a "Wright Flyer" type wing or a full profile airfoil with under camber?

Oh, and like Sir Mix-a-lot, I like that big ass on the Stinson 108!
 
#4
Short answer- No

It's been my experience that the ailerons on wings you describe need to have more up travel than down. Maybe because they behave more like spoilers and flaps at low airspeed? I don't know. Never really thought about it.
 

Cooley

Father/Son Team
#5
thanks for the tips guys, also the big airfoil library, any clue how to open the .dat files in the archive you showed me?

Edit: also, The idea was to make a full profile airfoil that is cut out into a wright-style at the end, like on the FT Spitfire. I figured this would prevent tip-stalls at super low speeds. The idea in my head is to be able to approach at a very low speed (with full flaps), then kick down full flaps as well as full down ailerons (using some crafty Tx mixing) and be able to stall the plane to a near-fullstop just a foot or so above the ground and drop it almost vertically onto the mains. I went with FT's idea for the wingtips to combat tipstall at those super low speeds and to make it land level in that final approach.
 
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earthsciteach

Moderator
Moderator
#6
Let me look into the .dat files. I think there is a shareware program that imports those files.

I was able to read the file with Excel by simply changing to extension from .dat to .xls. You'll have to play around with the deliminators when Excel converts it so that you get two columns of data. I didn't play around with it enough to fix that little issue.

If you go the Excel route, let me warn you that it will be a matter of scaling the x and y axes under the chart command. If you aren't familiar with that, don't torture yourself! It can be a royal pain in the rear to get the x and y scales correct!
 

Cooley

Father/Son Team
#7
You know, that trial and error method is looking better and better, lol.

So I found this, and from all appearances as well as modifying (or rather creating) the undercamber for the wings, I will also need to enlarge all of the control surfaces considerably.

Capture.JPG
 
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quorneng

Active member
#8
Let me pose a question. What do flaps do?
Answer they change the wing section to under cambered!
If you can control ok with flaps down then a proper under cambered wing will be no worse indeed rather better as for the same lift it will generate less drag.
Under camber and flaps will create even more lift so you might run out of aileron power before the wing actually stalls.

In my experience of STOL at model sizes the actual wing section is far less important than the leading edge nose radius as this has a major influence on when and how the airflow breaks away.

As Earthsciteach has pointed out the aerodynamic efficiency of the wing in its 'clean' condition is not so significant for RC flying.

This 'push/pull' has quite impressive STOL type characteristics.
Finished1.JPG
Two section flaps covering nearly 2/3 of the span and descending to 90 degrees.
You don't put these flaps full down in one go!
[video=vimeo;43270500]https://vimeo.com/43270500[/video]
They do produce a significant pitch change requiring the full trim movement.
However even at its slowest speed with full flap & close to the stall the plane's attitude remains unchanged so the wing tips are not stalled and the ailerons continue to work although at much reduced effectiveness due to the slow airspeed.