Bearhawk Stall Speeds

[Battson]

Since we installed the longer wingtips (per Bob's specification) we have been consistently seeing stalls at 28 KIAS for light to moderate loads. At heavier loads stalls are around 31 to 33 KIAS.
This represents a few knots improvement over the stock wingtips.
Wind has been a problem lately so I can't confirm ground speeds, but I am expecting to see a few knots improvement there too.

[Snork]

This is awesome performance, Jonathan. I just got my new wing tips from Carbon Concepts and the should add about 2 feet of actual wingspan and even more “effective wingspan” from the Hoerner design. Hoping for similar numbers when I get this thing in the air.

[Battson]

The Carbon Concepts wingtips I have seen are not a true Hoerner design, and will not provide the "effective wingspan" increase which we discussed in some threads here.
I also caution, as I posted here, that Bob has not approved an unending increase in wingspan. This increases bending forces greatly in the spar, and needs to be done with great care, approval from the designer, and a test flying program.

[Snork]

Jon, The Carbon Concept wingtips are very close to the maximum additional wingtip span recommended by Bob. Others, I believe on some B models, are already flying the wingtip. I will have no reference for performance increase, as I will start with these wingtips for my first flight. I studied the various Hoerner design characteristics before purchasing the CC design, and while they lack some of the favorable characteristics such as an advancing tip with a perfectly sharp edge, I believe they will approximate some of the Hoerner advantage due to the similar qualities and shape. The only definitive way to know is through wind tunnel testing, and I haven't had access to that since the college days. Anyway, I didn't want to spend more time building my own, and besides, the CC shop is only a few miles down the road from where we are building and we've done business with Randy in the past. I'm sure I'll be happy with whatever performance they add after a long 5 years of build time!

[Snork]

[Battson]

They actually look a fairly good shape from that angle....
As you say, without wind tunnel testing or similar, it's hard to know what is really happening.

[zkelley2]

CC should have a set for me ready here soon. I'll be flight testing them as soon as they're on and I'll report the results.

[Bcone1381]

Battson, I am curious what needs to change on the Carbon Concept wing tip that you have seen to make it conform and perform in accordance to the Hoerner design. Some day when I fly I will experiment with this stuff and want to understand the significant nuances.

[Pbruce]

Interesting subject. From the image, one can readily see that there is a fairly large radius at the tip, and a “bump”(unused nav light mount maybe?)
?) I’m wondering if that radius was a compromise made in the interest of an easier layup and de-molding perhaps? Perhaps the way to get the sharp edge of a Horner tip is to split the mold and so that both the top and bottom plies exit the mold at the tip. Didn’t the late model C-210 have a Horner-type tip with sharp edges? I’m always interested in measurable differences with various mods, any effect on safety margins and handling, and whether they provide cumulative improvements.

[zkelley2]

Randy is using a split mold.

[Battson]

I tend to agree with Pbruce, I suspect compromises were made to allow mass-manufacturing, to the detriment of achieving a theoretically perfect Hoerner shape. This is a common compromise in aviation, perhaps one of the most common.

Wingtip design is a whole profession / science all of it's own. I am not professing to be an expert. I have only completed a few years of university study into aerodynamics, there are surely more qualified people here. I researched the theoretical benefits of Hoerner wingtips (for obvious reasons) before I invested around a hundred hours and $$$$ into making a bespoke pair. I think this link has a fair summary.

The objectives are increased lift and to a lesser extent, lower induced drag. These are achieved by projecting the vortex out beyond the end of the wing. This increases the effective wingspan, increasing lift, and increases the 'effective' aspect ratio which may reduce induced drag depending on many other variables. Per the pictures in the link above, one can assume a sharp-ended tip is most effective at projecting the wingtip vortex beyond the end of the wing. A rounded tip encourages attached flow to 'roll over' from the lower to upper surface of the wingtip, which brings the vortex closer to the wing, reducing effective wingspan and reducing the effective aspect ratio slightly.

At least, that's my understanding.

[svyolo]

I have Bob's carbon tips, which are stupid light, but fit about the same as the fiberglass ones. That is how it will fly. I might try Wittman Tailwind tips down the road. I like the concept. I think with a big low aspect ratio wing, better tips will make a difference.

[Pbruce]

Mike Silvernagel had about four sets of Horner tips made. Some were for sale a couple of years back. I don’t know if he kept any for himself to install on his Patrol, which flew first with factory fibreglass tips. If he has tried the Hoerner tips, would be interesting to hear from him. I imagine a person would have to do quite a bit of testing and use a sharp pencil to fairly document any differences, but I’d love to hear about it.

I also have factory carbon tips. I would need to see pretty compelling accounts to undertake new tips.

It could be a fun project though, if one was in the right state of mind to undertake it. I would probably build male molds out of extruded polystyrene like genuine Dow Styrofoam. I would vacuum bag the carbon and then glass onto the molds, and then scoop out the foam after curing. Any stuck bits could be dissolved with acetone. An internal support grid of ribs would be inserted as well, in order to keep the weight of the skins reasonable. I would insure that there was ample thickness of material around the outer perimeter so that a sharp edge could be formed in the finish sanding process. Whew. Lots of work. Lots of mess.

Or maybe build the whole thing out of previously laid up carbon sheets? The leading edges would have to be formed by hand out of hard foam and then glassed, but the rest would theoretically be all flat or simple curves. I’ll probably just fly what I have though, and spend my attention on honing my technique.

[zkelley2]

I'm not 100%, but I want to say these wingtips are somewhat of a collaboration between Mike and CC.

[Utah-Jay]

Geez, you guys like messing with stuff, I just want to fly. I will be using Bob’s carbon fiber wing tips.

Big wings, big engine…. It is gonna land anywhere I want to fish.

[Bcone1381]

I hope its ok to share this screen shot from a Q4 2004 The Bearhawk Patrol Newsletter. A 1/8" plywood grid is fabricated over the airfoil block, then filled with foam. The article shows a typical cross sectional shape of round. Change it for the Hoerner to a triangle shape. An afternoon with a table saw, band saw and 12" disc sander would get a lot done on the mold I would think.

When you cover it with FG cloth, If the cloth is oriented 45 degrees it will follow the shape fo the mold easier.

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[Wyo Johnson]

I asked one of the aero guys I work with about it and he sent me this link: https://aviation.stackexchange.com/q...-wingtips-work

Its a high level primer but it does a good job of explaining the basics. Basically you are getting more wing area lift with a lesser drag as compared to the standard "hersey bar" wing or with a standard rounded wingtip. The second pic in the article is actually out of the NASA paper that Hoerner documented his testing and findings.

Was around a Super Cub as a kid that had Hoerners on it...flew into some soup at the edge of an airfield (i was a PAX) and the one thing I noticed is the vortices off the wingtips. They were a small radius and started right at the trailing edge, outboard tip.

Andy
1423B Scratch

[Gerhard Rieger]

This is what I build from my research

[Gerhard Rieger]

Here are the photos