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Why Hoerner tips work so well

scsmith

Well Known Member
ChiefPilot posted a nice picture of the tip vortex condensing on his RV6 as he pulled up into a loop I'm guessing. See the picture here:

https://vansairforce.net/community/showthread.php?t=185852


Here is a brief treatise on wingtip design, and why the Hoerner tip works surprisingly well on rectangular wings, and what that has to do with the great picture of the tip vortex that ChiefPilot captured.

The normal thinking on good wingtip design is that you want to achieve the greatest possible effective wingspan, by forcing the tip vortex to form as far outboard, and as far aft as possible. A second goal is to help sustain a nearly elliptical lift distribution by helping the loading to shed progressively on the outer panel. Both of these objectives are based on classical linear theory. An example of a nice wingtip add-on to an existing wing to improve performance can be seen here:

yasuo-suzuki-photo-of-bob-mccormack-l39-2.jpg
(photo credit HP Aircraft LLC, original source Yasuo-Suzuki)


But there is another path to getting good performance that is particular to rectangular wings. It involves a departure from classical linear theory, where we will consider the formation and shape of the trailing wake UPSTREAM of the trailing edge, and its influence on the wing load distribution and induced drag. Where linear theory would assume that all the vorticity is shed from the trailing edge, and that immediately downstream of the trailing edge, the wake is flat (a straight extension of the trailing edge), there can be beneficial effects from having the wake form upstream on the tip side-edge and form a dramatically non-planar shape just aft of the trailing edge.

There are two effects.

First, by shedding vorticity off of the side edge well forward on the wing, the vortex induces more downwash on the outer portion of the wing as it makes its way aft to the trailing edge. For a nice, nearly elliptically loaded wing, we would not want that - we would want to sustain the loading that was designed into the wing shape using more classical methods. BUT for a rectangular wing, there is far too much loading on the wing outer panel, more loading than the ideal elliptical loading. So getting some extra downwash from the vortex shed off of the tip side edge helps by unloading the outer panel somewhat, shifting the lift distribution to be closer to the ideal.

Second, by shedding vorticity off of the side edge well forward on the wing chord, the vortex lifts up above the plane of the wing significantly -- as shown in ChiefPilot's picture -- and the resulting wake shape just downstream of the trailing edge has a very similar non-planar shape that would be produced by a wing with a winglet. This non-planar distribution of shed vorticity results in lower induced drag, in the same way that a wing with a winglet has lower induced drag. After much digging, here is a sketch from my Thesis that illustrates this:

fig7.4.jpg

So how do Hoerner tips achieve this? First, the side edge is fairly sharp. Unlike the well-rounded side edge that we would want for a classical tip that seeks to delay the vortex formation, instead in this case we want to encourage it. Second, by making the sharp edge follow essentially the upper surface airfoil contour, it sets the side edge at a rather low angle of attack forward along the tip (the first, say, 20% of chord), then in the mid-chord area, the sharp side edge has essentially the same angle of attack as the wing itself. This curvature allows the wing circulation to be pulled out to the tip, and then shed off the side edge somewhere forward of mid chord.

The result of all this is that rectangular wings achieve higher span efficiency than would be predicted by linear theory. Still not as high as an ideal elliptically loaded wing, but much better than would be expected.
 
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Very informative Steve. Thank you.
So, which of the three different Van's factory wingtips is most efficient/ least drag?
 
Very informative Steve. Thank you.
So, which of the three different Van's factory wingtips is most efficient/ least drag?

I think that for cruise drag, and even for climb conditions, the original Hoerner tip works best on Van's wings. The later tips with the highly swept side edge add physical span (about a foot compared to the Hoerner tips) but I don't think they increase the EFFECTIVE span much) I would guess that the later tips do probably reduce stall speed a little, and may provide more solid aileron control well into the stall -- which is normally the purpose for the highly swept (sheared tip) side edge.

Adding span would normally always be good, but adding wetted area is not helpful for climb and cruise. The only way to add much of a good wingtip would be to cut some of the existing wing off, so some tip span could be added while also reducing or maintaining the existing wetted area.
 
Race Tips

Steve,

I would be interested to hear your thoughts on my self-designed and built Reno tips. My design philosophy was simply to reduce wing area and whetted surface with a low drag shape to reduce overall drag in race configuration and speed which is 230+ MPH. I expected that these tips would cost me some performance in high-weight high altitude cruise configuration, however my data indicates the opposite - that they may be worth at least 5 knots at 14,500 15,500 and 1750-1800 lb gross weight over my stock late model sheared tips.

20191Dsmaller.jpg


2018OBrienStats.jpg


Skylor
 
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Steve,

I would be interested to hear your thoughts on my self-designed and built Reno tips. My design philosophy was simply to reduce wing area and whetted surface with a low drag shape to reduce overall drag in race configuration and speed which is 230+ MPH. I expected that these tips would cost me some performance in high-weight high altitude cruise configuration, however my data indicates the opposite - that they may be worth at least 5 knots at 14,500 15,500 and 1750-1800 lb gross weight over my stock late model sheered tips.

http://www.skylor.org/images/Sport80/44088281_10217282121666456_4205563643313520640_o.jpg

http://www.skylor.org/images/Sport80/20191D.jpg

Skylor

HI Skylor,

That is very close to the tip shape I recommend to people for racing. The only difference is that from the max thickness point aft (which gives maximum span) keep the side edge straight back, constant span, and shift the semi-circular cross sections out to match that shape. This is a very slight increase in wetted area compared to putting each semi-circular cross section tangent at the outboard rib, but achieves a slightly higher effective wingspan.

At cruise speeds, this tip should be noticeably better than the Hoerner tip. I am a bit surprised, as you were, that it is still better at high+heavy conditions compared to the later sheared tips that have good side-edge radius. Must be that the hightly swept edge is still causing early formation of the vortex sheet, and a bit more separation under it.

As Skylor notes, at high speed, the angle of attack is so low and the induced drag is so low that all you really want to do is minimize separated flow. At the aft corner of the tip, there can be a small triangle of separation under the vortex core where it rolls up and over the wing surface. We minimize that by delaying the vortex sheet formation with generous side-edge radius (semi-circle cross sections) and keeping the side edge parallel to the free stream to hold the vortex as far outboard as possible.

If you'd like to visualize this, spread some used motor oil on the tip and go fly.
 
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HI Skylor,

That is very close to the tip shape I recommend to people for racing. The only difference is that from the max thickness point aft (which gives maximum span) keep the side edge straight back, constant span, and shift the semi-circular cross sections out to match that shape. This is a very slight increase in wetted area compared to putting each semi-circular cross section tangent at the outboard rib, but achieves a slightly higher effective wingspan.

Thanks!

Skylor
 
What about a hoerner tip that has the span of the newer style tip?

It would look the same from a top view.
 
What about a hoerner tip that has the span of the newer style tip?

It would look the same from a top view.

That would add a fair bit of wetted area. But more importantly, extending the span that way would eat into structural margins. It would keep the span loading of the rectangular wing with Hoerner tip, and extend it out (six inches per side). You would have to reduce the limit load factor, lower the rough-air redline (gust load factor limit) and/or reduce the weight, to maintain the same structural safety factor. So I can't recommend that.
 
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?
 
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 :)
 
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Zip Tips

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

Can anyone opine on the effect of their design?
 
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.
 
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.
 
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.
 
I love aero, but.....

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.
 
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.
 
To clarify... I have the moulds for Bob's "#2" wingtips... He said they were his "second best" for speed on the -6. They produce a mostly flat, slightly angled out, wingtip, with a 1-1.25" radius rounded top edge proud of the rib flange. At the lower edge it is almost flush, with maybe a 1/4-1/2" radius to the rib flange.

I recall he said his stall speed went up accordingly with the increase in top speed.
 
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