Why Hoerner tips work so well

[Snowflake]

I wonder how this discussion dovetails with the experiments Bob Axsom did with his wingtips on his -6A. He found his fastest wingtips were the ones that were almost completely flush, if I recall. He made many different sets, including ones with half-circle profiles as described here, but I thought he said his flat ones performed better. I can't find the thread right now, maybe someone has it bookmarked?

[ChiefPilot]

This is great - thank you Steve for sharing your knowledge and expertise!

[Karetaker]

There are also the Zip Tip III Winglets shown here.

https://www.aveoengineering.com/ziptips-premiere-2/

[Bernardo]

Very interesting! Thanks for sharing. I have spent a lot of time thinking about the spanwise lift distribution, vorticity, and wingtip design - and even taught classes that touched on these subjects - but I had never put together why the Hoerner tip works so well with the Hershey-bar planform in particular, by helping to unload the tips. Thanks again!

By the way, in case anyone is curious about how Boeing thinks about wingtip design - winglets versus raked wingtips, etc. - they have published a surprisingly thorough paper about it...

http://www.smartcockpit.com/docs/Wingtip_Devices.pdf

... maybe not as thorough as Doug Mclean's "Understanding Aerodynamics", but a good place to start

[DarinFred]

I think Aveo is shipping the update Premiere 2 from the link above.

Can anyone opine on the effect of their design?

[WingsOnWheels]

Originally Posted by DanH

Anyone have a good .dwg file of the 7/8 wing's end rib?

The airfoil shape is the same as the 4/6 correct? If so, the plans for the earlier models include the airfoil dimensions. I'll have to pull out the plans and toss it into CAD really quick.

[meloosifah]

Unfortunately, all of Bob Axsom's posts are saved on another computer so I cannot readily pull up his reports, but he found SIGNIFICANT improvement in racing speed by using flat tips. He essentially just blocked off the end of the wing with a barely radiused flat plug. I have the pics on another computer. I thought about trying to build the same tips at some point. I cannot remember off hand what he gained but he went back and forth enough times to quantify the gains. Don't quote me, but I am thinking his -6A gained 3 knots over the fastest of his other designs, maybe 4 or so over the Van's tips.

[scsmith]

Originally Posted by meloosifah

Unfortunately, all of Bob Axsom's posts are saved on another computer so I cannot readily pull up his reports, but he found SIGNIFICANT improvement in racing speed by using flat tips. He essentially just blocked off the end of the wing with a barely radiused flat plug. I have the pics on another computer. I thought about trying to build the same tips at some point. I cannot remember off hand what he gained but he went back and forth enough times to quantify the gains. Don't quote me, but I am thinking his -6A gained 3 knots over the fastest of his other designs, maybe 4 or so over the Van's tips.

The flat end tip is simply a matter of the absolute minimum wetted area. That's good. The sharp corners, not so good in this case, but at high speed, low angle of attack, there is still probably very little separation. I doubt you could measure much difference in top speed between the flat tip and the semi-circular tip I described above. But you would be able to measure the difference in lap times on a pylon course, where you pull a little g.

[Freemasm]

don’t forget the design and build flexibility indirectly associated with wing tips or other fairings. Hiding antenna, smoothing streamlines with integrated lighting, etc. are all part of the equation. If you only want to go fast, in level flight, in daytime VFR, landing and stall speeds aren’t a concern......

There’s some compromise in any design. Discussing the merits of a single design point is great, just don’t forget the other variables. The OP was making a point considering a lot of boundary conditions.

[vlittle]

Originally Posted by meloosifah

Unfortunately, all of Bob Axsom's posts are saved on another computer so I cannot readily pull up his reports, but he found SIGNIFICANT improvement in racing speed by using flat tips. He essentially just blocked off the end of the wing with a barely radiused flat plug. I have the pics on another computer. I thought about trying to build the same tips at some point. I cannot remember off hand what he gained but he went back and forth enough times to quantify the gains. Don't quote me, but I am thinking his -6A gained 3 knots over the fastest of his other designs, maybe 4 or so over the Van's tips.

I had the moulds for Bob Axsom’s wingtips in my hangar for 5 years before giving them back to Rob Prior. It just wasn’t worth the hours of labor to make them to gain a couple of knots. During flight testing, I was showing 213 KTAS at 8500 DA. After paint, 208. Gained a couple of knots by sanding down paint ridges on my stripes.