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  #1  
Old 08-10-2021, 05:04 AM
ExtraKatana's Avatar
ExtraKatana ExtraKatana is offline
 
Join Date: Oct 2009
Location: Broken Arrow OK
Posts: 302
Default Replacing Hoerner Tips

Has anyone updated their wing tips from the Hoerner style? Aesthetically, Would like to get rid of the external nav/strobes and go with corner tip lenses with FlyLed's. Pros/Cons of the Bat Wing or straight trailing edge on the 8? Speed or handling difference? Has anyone priced the tips from Vans?
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  #2  
Old 08-10-2021, 05:58 AM
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Mel Mel is offline
 
Join Date: Mar 2005
Location: Dallas area
Posts: 11,172
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Quote:
Originally Posted by ExtraKatana View Post
Has anyone updated their wing tips from the Hoerner style? Aesthetically, Would like to get rid of the external nav/strobes and go with corner tip lenses with FlyLed's. Pros/Cons of the Bat Wing or straight trailing edge on the 8? Speed or handling difference? Has anyone priced the tips from Vans?
I did this on my -6 many years ago. I saw absolutely no change in flight characteristics or performance. I changed to the bat-wing tips.
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<rvmel(at)icloud.com>

Last edited by Mel : 08-10-2021 at 10:30 AM.
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  #3  
Old 08-10-2021, 09:50 AM
JDeanda JDeanda is offline
 
Join Date: Apr 2013
Location: Ventura, CA
Posts: 261
Default All Those Wingtips

Mel's experience is pretty common, except he's more honest than some owners who changed wingtips. Over the last 50 years I've seen a few scientific wingtip fads swoosh by. Hoerner, upswept, downswept subsoilers, cut off flat, flat plates, raked tips, raked the "wrong" way tips, all that. Reported results range from spectacularly life-changing to nothing at all. Usually the latter. In a lot of cases, it seemed to me that when a guy pays big $ for a mod, it dang well works, even if no change can be proven or measured. My late professional aerodynamicist buddy always said extra wingspan was the way to get the improvements those majick wingtips were striving for. He also said there was very little improvement to be had until the wing was flying at high angles of attack. In level cruise, the wing is not working very hard, hence very little wingtip vortex to try to cancel. Closed course pylon racers that constantly turn hard and airplanes flying at very high altitudes benefit the most, but in the end, span seems to matter more, leaving aside the shape of the wingtips. I remember an article by the late Ken Willard in R/C Modeler magazine back in the 70s where he tried different wingtips on a model sailplane. Trick was, he only changed one tip, then flew the model. He tried several shapes and reported that he never found much of a turning tendency with any of those shapes. I did 29 years with one of the the big gummint airframers, fairly often working with aero guys. (I built simple wind tunnel models) What I always seemed to hear was "Yeah, that's a pretty nice wingtip, but gimme more span, I gotta have more span..." Designers admitted they just rotated the top and bottom airfoil surfaces 90 degrees and blended them in CATIA to make a tip. Oh well. So here I loop back to the OP... when styling an airplane, probably the third most visually important thing is the wingtips, right after the vertical tail and the cowl. Aren't those Zip Tips spectacular?

Last edited by JDeanda : 08-10-2021 at 09:58 AM.
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  #4  
Old 08-10-2021, 10:32 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
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The 715 tips do offer a few more inches of wingspan over the 615 tips but a lot of expense/effort for miniscule aerodynamic improvement.
Biggest difference I've noticed between the different styled tips is - 715 tips tends to snag more stuff when backing the plane into tight hangar spaces... can't see the fanned out rear edge as easily.
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  #5  
Old 08-10-2021, 11:39 AM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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Jerry makes some valid observations above. There has been a lot of what I call "genetic drift" in wingtip shapes. Take a walk on the tarmac at any GA airport and look at the wide variety of tip shapes. If any one shape was obviously better, many airplanes would gravitate toward that.

That said, there are a few details about tips that do make a difference, although sometimes very small. First, you have to remember that we (most all GA) spend most all of our time at top speed, where the lift coefficient is very low, and so induced drag is very small. So tip shape doesn't matter much at all. Any benefit would be from minimum wetted area and avoiding flow separation.

During climb where lift coefficient is higher, different tip shapes can have a more significant difference. Here, you would like to achieve the greatest possible "effective" wingspan by holding the formation of the tip vortex as far outboard as possible. A unique characteristic of the Hoerner tip on a rectangular wing is that at moderate to high angles of attack, it achieves a remarkably high 'effective' wingspan by creating a more 3-dimensional wake shape that actually emulates a winglet a little bit. In contrast, the sheared side-edge tips (bat tips) achieve a lower 'effective' span because the tip vortex forms early and rolls over and inward. This is why some have observed relatively little difference in climb performance between the two tip shapes despite a substantial increase in physical wingspan.

Where the sheared side-edge tip really shines is at very high angles of attack, near stall. Here, the goal is not good performance, but rather good controllability. The positioning of the tip vortex over and inward from the tip induces a lot of downwash on the aileron, resisting stall on the wing outer panel. The sheared tip shape helps maintain good aileron control well into the stall and resists tip stall.

For high-speed cruising, the tip shape I recommend is a modified "body of revolution" tip. The tip is formed by simply making semicircle cross sections that match the local airfoil thickness. Look, for example, at the tip on a P-51D Mustang. The modification is to shift the semicircle cross sections which are aft of the airfoil maximum thickness outboard so that the side edge of the tip extends straight aft from that point. This maintains the greatest wingspan all the way to the trailing edge. This shape does have slightly more wetted area than just a flat end cap on the wing, but it avoids any flow separation over the corners/edges.

We will have a set of these tips on Bob Mills' Rocket-6 (Race #49) at Reno this year. Bob's airplane flies on the larger Sport course, where the g-loads around the pylons are modest. It is not clear where the trade-off point is where the g-loads become high enough that more wingspan would be better. It may be that airplanes on the Medallion course would be faster around the pylons with the Hoerner tips. I would love to test both on that course, if anyone is game.
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  #6  
Old 08-10-2021, 01:22 PM
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ExtraKatana ExtraKatana is offline
 
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Location: Broken Arrow OK
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Thanks for the replies folks. Starting to think there is no benefit to changing tip styles other than to add the FlyLeds, which is not worth the expense of new tips. Did I hear correctly that there is a retrofit kit that allows you to cut the corner lens on a Hoerner wing tip?
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Last edited by ExtraKatana : 08-10-2021 at 01:30 PM.
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  #7  
Old 08-10-2021, 06:08 PM
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,135
Default +1 for Steve s design

I have a set in progress with forms waxed for glass. If you want pics to observe just text 765-505-1160 or email decamplarry@gmail.com. These are really elegant and make aerodynamic sense.
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  #8  
Old 08-10-2021, 08:45 PM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
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I have tested different winglet designs in a high speed wind tunnel for business jets. A 2% drag reduction at higher lift coefficients is considered spectacular. Unlike our RVs, business jets do cruise at higher lift coefficients because they can be at 45,000 ft at the beginning of the cruise segment of their mission, so the winglet benefit is not only for climb. But these are winglets, vs simple wingtips, which have a much bigger impact on span loading and hence efficiency, than just a wing tip. And they still can’t do much better than a few %. And at higher speeds they generate a drag penalty, so the designer has to be careful.

Small induced drag reductions less than 1/2% are almost impossible to measure. One will get a much more meaningful performance improvement by fixing gaps and poor fitting fairings and fairing in linkages, or anything else that generates a wake. Also optimizing cooling drag is another low hanging fruit. They say cooling drag on a piston single can account for 15% of the overall drag of the airplane.

When people say that this type of tip does X and this other type does Y I am highly sceptical. In our business we are using very expensive, carefully calibrated wind tunnel balances to measure forces, pressure taps all over the surfaces to match computational estimates and various types of flow visualization. We do back to back tests of different configurations on the same setup to minimize experimental errors. Even then it is still sometimes challenging to fully understand what is happening. It’s always easy to come up with a speculative answer that sounds reasonable at the airport coffee shop, but getting actual solid proof is difficult and very expensive. And you won’t get it just flying around in an airplane.
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Last edited by sblack : 08-10-2021 at 08:51 PM.
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  #9  
Old 08-10-2021, 09:34 PM
JohnD.TF4 JohnD.TF4 is offline
 
Join Date: Jul 2019
Location: Portland, OR
Posts: 54
Default No Tips?

I've been wanting to ask this for a long time and this seems like the thread to put it on:

How about no wingtips? I am seriously thinking of this as an option for my -8. lighter, faster, cheaper, higher roll rate. On the downside the stall would be faster and the climb would probably be slower, but I can afford that.

I would just put a blank rib (no lightening holes) on the end. I can use the tip fastener points so if regular tips are wanted in the future it would be as easy as screwing them on.

I have asked a few mechanics and engineers and so far the word back is that it is a safe mod as long as you are OK with the stall speed increase.

Thoughts?
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  #10  
Old 08-10-2021, 10:18 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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Quote:
Originally Posted by JohnD.TF4 View Post
I've been wanting to ask this for a long time and this seems like the thread to put it on:

How about no wingtips? I am seriously thinking of this as an option for my -8. lighter, faster, cheaper, higher roll rate. On the downside the stall would be faster and the climb would probably be slower, but I can afford that.

I would just put a blank rib (no lightening holes) on the end. I can use the tip fastener points so if regular tips are wanted in the future it would be as easy as screwing them on.

I have asked a few mechanics and engineers and so far the word back is that it is a safe mod as long as you are OK with the stall speed increase.

Thoughts?
That is what I refer to as a flat tip cap. Probably faster in straight line cruise than the stock tip. Perhaps not as fast as the tip shape I describe because of areas of flow separation on the edges.
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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