Hoerner Wingtip Build

[Battson]

To get the best performance, pls review this... http://www.dtic.mil/dtic/tr/fulltext/u2/a800374.pdf

The sharp edge of the wing tip (sharp corner at the outmost point of the wingtip) seems to be very important in all cases, based on that experiment.
This is the main problem with the stock Bearhawk wing tips. The blunt, rounded end to the wing is very inefficient (low lift generation, high drag creation).
They state the induced drag penalty is in the order of 11% to 20% for a blunt rounded tip, like the stock design. Induced drag being one of the largest contributions to total drag.

[Battson]

I've started into my new wing tips. The photos show the first plug, made of XPS foam.

My focus is on maximising STOL performance, for obvious reasons. So as well as the hoerner design to improve the effective span and CL, I wanted larger wingtips to increase wing area and thus lift. After discussion with Bob, the largest size he was prepared to approve was 6" for a hoerner design. That is what you see pictured here.

I think I still need to rasp some more material off the leading edge. At you see it, the air has to move downwards then upwards as it passes under the wingtip. I think that will be draggy. But then again this shape might produce more lift at high AOA... so I am in two minds. Let me know if you have some knowledge or advice!

On a similar line of thinking, I may also change the planform circular radius at the front of the wingtip to a parabola. I want to get the widest point (spanwise) coincident with the thickest point of the wing, which is the highest point of the wingtip in-flight. Again, this is just to stop the air from travelling down then up again.

I want to fly these in the next couple of months.

[rodsmith]

Battson, your tips look very much like what Harry Ribblett recommends in his book. Some key parameters he advised were width equal to leading edge to main spar distance, circular plan form back to the main spar, top surface same as wing airfoil, bottom surface angling up to top surface, square corner at rear outside edge. I could easily make them from aluminum sheet from the main spar back, the front I would have to do from composites. That is what I am considering.

[whee]

Nice work Battson. They look quite similar to what we (mostly my dad) did. These are in progress photos. Wish I had better/current pics.

[Battson]

How many layers of fiberglass did you decide to use Jon?

[whee]

How many layers of fiberglass did you decide to use Jon?

Wish I had an answer for you. I put a pause on the tips for now so we haven't laid them up and know the plan works. My dad was planning on using one layer of 9oz cloth and one layer of 2oz mat. That's was the standard layup for the fiberglass whitewater kayaks he used to build and he is confident it will be about right for the tips. Since he built a clamshell mold it will be easy to add more cloth to strengthen them so he isn't all the worried about getting it right the first time.

[Battson]

I've used four layers of 6oz mat, and they are SUPER flexible. I bet they are strong enough, but they are going to flex upwards in the middle.

I might need to put a small square block of foam inside the tip and fiberglass over it, to stiffen everything up.

[whee]

Bummer. We had trouble with our molds being super flexible which is part of the reason we put a hold on the tips.

[Bdflies]

Batson; For larger unsupported areas, if you can bond a piece of 1/4" foam sheet on the inside, then laminate a couple layers of light cloth over the foam. That creates a composite sanndwich that will be VERY STIFF. I like the 7781 cloth, because it's very flexible and compliant.
Whee; same composite sandwich on the outside of your molds will stiffen them up tremendously.
If you've never bonded foam to a fiberglass panel, I could explain the process. This is an area that's best done by vacuum bagging, but not absolutely necessary.

Bill

[Battson]

After removing the plugs the bottom surface of the wingtip proved to be rigid, but the top surface of the wingtip was downright floppy.

The top surface effectively a flat plane, particularly near the trailing edge, and so gets no strength from it's shape. Extra glass in the layup would do little to help prevent that floppiness. Even when we attached it to the wing, which provides a lot of strength and fixes the shape, that upper surface was still flexible in the mid-span position.

We tried using foam to stiffen it, and it works well. But foam proved very heavy for the extra stiffness it provided. A lot of foam is required to stiffen such a large part. We were using high density XPS gold foam.

Instead, we decided to put an aluminium rib inside each wingtip. The ribs are halfway into the wingtip, or midway along their span in other words. This way, the wingtip's shape is fixed at the wing, and the curved outside edge (which is very stiff), and in the middle by the rib. We riveted the ribs in place just to hold them in the right spot, then fibre-glassed them in for a strong bond.

This solution proved to be a great deal lighter and more rigid than the foam option, given the materials and wingtip size we are working with. I can post some photos later.

[Battson]

IMG_20171219_192924.jpg I am pretty happy with how they turned out, given they are totally hand crafted.
The finish is almost as good as a gel-coated part, once the paint has covered up the minor imperfections.

[whee]

Looks great Battson! I look forward to your eventual flight report.

[Battson]

I've found that the internal rib has stiffened them up a LOT..... but if I really lean on them, the glass still flexes a little at the flange where the wing joins on. I think this is because there is very little structure "normal" to the wing surfaces. Anyone else following this design should be aware of that.

I will report back once they are flying.

[Battson]

A couple more photos for the record, now everything is painted and installed.

IMG_20180107_202801.jpg


IMG_20180107_202810.jpg

IMG_20180107_205108.jpg

[whee]

Looks fantastic Battson!