Hoerner wingtips, not the shoes

[wypaul 448]

just wondering if any one of you KF folks have changed from the old style tips to the Hoerner and notice any improvement.  Seems like they might have an advantage over the standard Avid wingtip which sees to offer little more than extra drag  The Hoerner style wingtips should offer shorter wing span but with greater lift and less drag.  I know of one fellow that has put them on an Avid but not sure what the results have been but he was able to get them to fit ok.  I think if I go that way I will just make some to fit the undercambered wing.

[Buckchop 479]

Not to mention the factory KF droop tips ( like the ones i took off my model1) weigh in at 3.5lbs each. 

[Willja67 144]

You might find this interesting:

http://www.n56ml.com/900hour/

Granted our birds will never be in the same category as a Kr2S but he talks about how much better his second more accurate Hoerner tips were, which might help you. It's in the bottom third of the page if you don't want to read the whole thing. 

[wypaul 448]

Thanks that was interesting. 

[Good old number 29 158]

I thought Hoerner tips were supposed to be convex on the bottom side, but the ones pictured in the article don’t appear to be. That is what makes them work, right? The convex shape speeds the air away from the wing, so the vortices don’t circle right back down and push down on the top. The vortices are sped up and out, away from the wing. Anyway, the pictures looked like a flat, or almost slightly concave, angle up to the tip. My droop tips are not removable, or I’d volunteer to try Hoerner wingtips in a fair comparison. I think the droop tips have similar benefits in reducing stall speed. The air can’t fall off (upwards) the ends of the wings, or whirl around and push down on top as easily. Holds the compressed air cushion under the wing, where it belongs. At least, that’s what the infallible internet told me.  

[lostman 198]

Years ago I read through the study NASA did on wing tips. The turn down or up tips were the most effective of the ones they tried, followed by the Hoerner style. Yes, the Kitfox tips may add weight but they also increase effective wing area making them worth their weight for slow flight. That is, if they are designed well. 
I have done my own testing with tips, although not on this tyoe of aircraft. In a high wing load, semi laminar flow wing, the rounded tips actually increased vortices, ground roll, stall speed etc. Upturned droop style tips decreased ground roll from 600 to 450 feet on takeoff, stall speed by 3 mph (from 55 to 52) and increased cruise speed by 6 knots. I will never run a rounded tip on anything again if I have a choice of it. By rounded I mean just a tip that rounds a square tip off from top to bottom, not fro leading edge to trailing edge. 
I would love to test Hoerner style tips on one of these to get a side by side comparison. 

[Willja67 144]

Here's the page from Fluid Dynamic Drag authored by Hoerner for those interested. 

Apparently there are 2 considerations, the corners need to be sharp so the air can't roll around the edges as easily from bottom to top and the planform shape is important as well.

Interestingly enough according to the diagram at the bottom a squared off wingtip actually increases the effective wing area by 0.04 or 4% and the best "hoerner" shape has a 0% increase in effective span. I guess the whole point is to look pretty without hurting performance, since most don't want to just leave the wing squared off. 

And the worst shapes even with sharp edges reduce effective span by nearly 20%!!!  That's eye watering. 

Edited 18 Jan 2020 by Willja67

[wypaul 448]

I don't believe that was Hoerner's conclusion.  He actually wrote a short paper just about wing tips, "AERODYNAMIC SHAPE OF WING TIPS" which is online if searched for.  I will refer you to that and you can draw your own conclusions. There is a nice YouTube video of a fellow making some Hoerner tips for his Sonerai.  I will try to link to that for those interested. 

   https://youtu.be/ULhZchv7ito

Edited 18 Jan 2020 by wypaul