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#1 slebetman Dec 13, 2012 10:22 AM

Penut size rubber powered wing
 
3 Attachment(s)
Here's something a bit different. An indoor rubber powered wing.

I often use the online CG calculators as the starting point for designing wings. Here's a CG calculator link for the basic design parameters: http://fwcg.3dzone.dk/?wing_span=14&...ow_mac_lines=0

Based on Norm's advise I decided to start with a 5 degree twist and just build a prototype and see how it flies. I'll adjust the plans later if necessary.

Here's the "final" version of the plans for the prototype. The actual "final" plan will only be finalized once I've got it flying successfully. It's a pusher in case it's not clear from the plans.

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note: updated to include the latest plan.

#2 slebetman Dec 13, 2012 11:15 AM

3 Attachment(s)
Mounted the plan onto a building board. Tacked templates onto sheet balsa with spray mount. And cut out the ribs. That's it for tonight.

I'm using 1mm balsa for everything. That's actually roughly 1/24" but my balsa is fairly heavy. With good, light balsa you can use 1/16 or 1/20.

#3 Jon Snow Dec 13, 2012 02:11 PM

Do you think it might be worth trying without dihedral/ wing tips?I've had good results from similar wings with just the twist.
Regards Stuart

#4 slebetman Dec 13, 2012 05:45 PM

Quote:

Originally Posted by Stupot46 (Post 23518914)
Do you think it might be worth trying without dihedral/ wing tips?I've had good results from similar wings with just the twist.
Regards Stuart

In my experience, free flight wings would spiral without dihedral. It would fly straight OK but free flight planes are usually trimmed to do circles. It may be worth a try though because I just realized that putting in the dihedral risks undoing the twist.

I missed a trick there. Should have baked in the dihedral into the templates the same way I did the twist.

I think I'll try without dihedral first. Found this model of a Lippish Storch that has practically zero dihedral and it seems to fly quite well:

http://www.ffscale.co.uk/int2004/lip.jpg

There's a video of it on this page (scroll to somewhere in the middle): http://www.ffscale.co.uk/page3ii.htm

#5 Cap_n_Dave Dec 13, 2012 08:12 PM

Quote:

Originally Posted by slebetman (Post 23520561)
In my experience, free flight wings would spiral without dihedral. It would fly straight OK but free flight planes are usually trimmed to do circles. It may be worth a try though because I just realized that putting in the dihedral risks undoing the twist.

The sweep contributes a dihedral effect, reducing the need for geometric dihedral.

#6 slebetman Dec 14, 2012 01:10 PM

7 Attachment(s)
Got a lot done today. Wings built and covered. There's an error in the plans. The trailing edge of the ribs don't line up properly. I'll fix it later. For this build I just cut a few of the rib supports to get the trailing edge to line up.

Progress pics:

#7 Jon Snow Dec 14, 2012 02:51 PM

Nice work! Have you decided on rubber/prop dimensions?
:popcorn:

#8 slebetman Dec 14, 2012 11:32 PM

Quote:

Originally Posted by Stupot46 (Post 23527973)
Nice work! Have you decided on rubber/prop dimensions?
:popcorn:

Prop will be 5" with a pitch of about 33 degrees. I'll try a 12" loop of 1/16 rubber first.

#9 xlcrlee Dec 15, 2012 05:49 AM

thick & thin
 
The Wright Bros. used data they gathered from their own small wind-tunnel. As a result, they copied and merely greatly enlarged the wing-form shown to work best in their small-sized tunnel. HOWEVER, they were not aware of Reynold's work which was published a half-century earlier.

The small which worked best in their small tunnel were THIN, like a butterfly's. So on their man-carrying craft they had to use TWO thin wings with canterlever support from wires for strength.

It wasn't until Tony Fokker discovered that thick wings not only could contain the canterlever structure ... but actually WORKED much better [L/D] at fullsize speed & size.

Your wings are far too thick for best performance at "rubber-powered Peanut" size ....!!


Look at the wings of similar-sized rubber models with excellent performance:

http://marinaero.blogspot.ch/2008/08...ed-flight.html

http://www.oocities.org/capecanaveral/1245/sportrbr.htm

http://www.sciencetoymaker.org/plane/index.htm



Lee :)

#10 BMatthews Dec 16, 2012 05:25 PM

It may not be optimized but it'll fly well enough to get the idea.

Slebetman, that model of the Storch flies OK due to the wing being mounted up on struts so the 3D Center of Gravity is well below the lift point. That produces a pendulum like stability. The shoulder wing mount of this model will do this to some extent. But you may well find that you end up needing SOME dihedral to be added unless you can fly it in a rather big circle.

A rule of thumb I've read of in the past is that 30 degrees of sweep equates to about the same roll stability as 5 degrees of real dihedral.

#11 xlcrlee Dec 18, 2012 06:12 AM

Quote:

Originally Posted by BMatthews (Post 23544759)
It may not be optimized but it'll fly well enough to get the idea.

In this size, w.r.t. endurance, penetration and even stability, thick vs thin wings > BIG difference!

Lee :)

#12 Cap_n_Dave Dec 18, 2012 08:49 AM

slebetman ... keep at it, am interested to see your results!!

#13 Cap_n_Dave Dec 18, 2012 08:51 AM

Quote:

Originally Posted by xlcrlee (Post 23558123)
I strongly disagree: my 65+ years of model plane design & building tells me distinctly otherwise, esp. in this size, w.r.t. endurance, penetration and even stability > BIG difference!

Lee, perhaps you should read this site about logical fallacies. Appeal to authority = fail.

#14 birdofplay Dec 18, 2012 11:48 AM

Another fallback would be the Worlds Foremost Authority, aka Prof Irwin Corey.
http://www.irwincorey.org/

At least he was a comedian.

#15 nmasters Dec 18, 2012 11:38 PM

Quote:

Originally Posted by birdofplay (Post 23560271)
Another fallback would be the Worlds Foremost Authority, aka Prof Irwin Corey.
http://www.irwincorey.org/

At least he was a comedian.

Carlo Godel also called himself "the world's foremost authority" but he always wrote it in crayon. I wouldn't say that stating that thick airfoils won't perform as well as a flat, or slightly curved, plate at the Reynolds numbers of a 1.5 inch chord rubber model is a logical fallacy. It's simply a demonstrated fact that at Re<50,000 a plate will have lower drag than a thick section. Better pitch stability can also be expected because of separation on the lower surface. Using several thin spars instead of one thick one would also help.

--Norm


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