Wing Shape Question
 

I am building a plane with Depron wings and carbon fiber spar. I noticed that my wing shape has the high point mid span vs. 25% to 30% back from the leading edge. Attached are a pic of the plan profile and my actual wing. Is this flyable and if so, how will CG be affected?

Use this to calculate the CG. Put it at the 25 to 30 percent point. The actual airfoil will have some effect but you will be able to survive the first flight to refine it. Dan.
http://www.geistware.com/rcmodeling/cg_calc.htm#mac

Sure it is flyable, the center of pressure is further back so I suspect that your C/G will be a little further aft as well, but if this is a really lightweight foamy then I don't see it posing much of a problem anyway.

Bob

Dan, the app you suggest is good when the model is going to have a "regular tail". What that is is anyone's guess. To deal with a model that might have a smaller, or larger, or longer, or shorter tail than "regular", the best app to use is one that also considers the details. It only takes a minute more to measure how far back the tail is and what it's measurements are. This app http://www.geistware.com/rcmodeling/cg_super_calc.htm gives a CG location that takes everything important to the CG location into consideration. And it doesn't take any more effort than the app that assumes your model is "regular".

Good point Rock. I've used that one also. Dan.

The shape of the airfoil does not affect the CG location. That's been mentioned already. The desired CG location is set by the tail area and distance it is located behind the wing and the wing's area and chord.

What will happen is that even after sanding the leading edge to shape the curve will transition quickly to a rather flat rise to the high point. That will tend to make the airfoil stall at a lower angle than it would if you had used the proper thin foam for the top skin. Or if you had pre-shaped the top skin.

Something else you should have done is to sand the hinge line filler to a more wedge like shape with the upper surface tangent to the curve of the airfoil. That way the top skin would be glued down along the whole upper side instead of only at the rear corner. The way you have it now when you cut through the control surfaces free the top skin won't be glued to the filler any more. By shaping it to a wedge like profile that fits the airfoil curve you would have both gotten a better glue joint and you would have pulled the foam down to the rear which would have pushed the high point further forward. This is now a serious enough mistake that I don't think you have any option but to cut away the top skin and re-do the filler. So while you're at it try to use a bigger diameter spar tube and curl the to skin over the edge of a table to give the shape more curl along the first 1/3 of the skin. It'll then hold its shape when you re-glue.

To get a feel for all this work with a one or two inch wide scrap of the Depron. While pushing it down firmly against the rounded edge of a table or counter pull it back and see how much pressure and what technique you need to get the foam to curl smoothly. And you can use this scrap as a test guide for shaping the wedge angle of the filler and finding a better diameter and position for the carbon tube spar.

BP so you tell the guy to not build it by the plans a totally redesign it???

OP the airfoil looks fine to me and will fly ok. It isnt the best but for a foamy it will be ok as the fly with flat wings.

Thanks guys for the help. I will continue with the build a set up CG per plans. I realize that I may need to move the cg aft a bit after first flight tests. Thanks again.

Well, consider that he didn't correctly follow the plans in the first place.

The drawing clearly shows a thicker 6mm foam for the lower surface, a foam filler and tube spar and a THINNER 3mm top surface. The use of the thinner foam would have generated an airfoil that was closer to the original intent than the arc like shape he got by using the same thickness for both top and bottom.

Just out of curiosity. Why didn't you build it according to the plans?

The image looks like the high point is at 50% of the chord.

Looking at the plans in the picture on the left, it shows the desired construction with beveling into the thicker lower sheet, the high point is obviously desired to be much farther toward the LE. The expected location looks to be @ the spar and roughly about 20%.

I was building a 125% sized plane from original plans. The plane designer suggested 6mm depron both upper and lower surfaces to effect the scaled up size. However, the 6mm foam upper surface didn't bend as easily causing the further aft high point. Airraptor hit the nail on the head. The wing flew just fine. See post #60 and then a few posts later for flight report in this thread, http://www.rcgroups.com/forums/showthread.php?t=2160082

It may sound obvious but a simple solution to achieving a reasonable aerofoil section is to simply add Depron ribs. As you no longer need the stiffness of the lower skin to maintain the shape both skins can be thin.

It obviously is a bit more complex but the principle remains the same. Bottom skin, add ribs (and spar) then apply the top skin.
In this 40" plank wing built in 3mm Depron there wasn't even a spar and it relied for its strength on the skin alone.
http://www.rcuniverse.com/forum/atta...mentid=2004608
The technique can be taken to extreme as in this 78" span high aspect ratio glider wing using just 2mm Depron but it did have a proper spar.
http://www.rcuniverse.com/forum/atta...mentid=2004607
But simple to build it was not!