Hoerner Wingtip comparison

[Nev]

Here is the data showing a comparison of the Hoerner tips to the kitset tips. These wingtips are the same size as the kitset ones.

For consistency aircraft weight was 2265 lbs and 17" CG. I used a mid CG for the stalls, so I could be certain that it was the main wing stalling and not just the elevator losing authority. I used 1000ft density altitude (2200ft). This was repeated as exact as I could make it with both sets of wingtips.

There appears to be one outlier in the flaps up stall speed that I've highlighted in red. However overall, for normal cruise speed there was no change. There did appear to be a small speed advantage at higher power settings. This was difficult to observe, but was repeated several times with the same result.

The main gain appeared to be a reduction in stall speed. It's difficult to test for position error at these speeds and there is a chance that some of the observed IAS reduction could be attributable to a higher AOA at the stall. However a higher AOA would also imply a lower stall speed.

Overall, no negative traits were apparent.

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[Battson]

Nice, now you're seeing stall speeds more consistent with what I expected.

Nev Do you know what your overall wingspan is now?
Or I suppose, wing area would be of more interest.

[Nev]

No change to wingspan. Both sets of tips are the same size.

[svyolo]

Nice data. I have no idea what wing tip works best for what we are doing. I have Bob's CF tips, which fit up similar to the FG ones. If I try something different down the road, it might be Wittman style. Just to try something different.

I also just got lucky and I think I scored a local hangar. I have been actively looking for a year.

[Nev]

Nice, now you're seeing stall speeds more consistent with what I expected.

Nev Do you know what your overall wingspan is now?
Or I suppose, wing area would be of more interest.

The wing area is unchanged as the hoerner tips are the exact same area as the kitset ones. So the span is exactly the same and I think it gives a pretty good "apples with apples" comparison. However the upper surface of the Hoerner tips theoretically contributes to the overall lift. If my maths is correct (and it's a long shot ) I'd say a 4% increase in the theoretical upper wing area (skylight excluded).

Also worth noting is that the speeds are IAS and therefore specific just to my aircraft, and just at those specific weights and set of conditions, and I'm assuming that position error has increased at these low end speeds. But it is very useful as a "before and after" comparison. It's possible that if I now reduce the aircraft weight to say 1850lbs I might see another 1-2 kt drop in IAS at the stall (although largely unusable unless I'm by myself). But I'd expect a higher amount of position error as the speeds get lower. I'll see if there's any meaningful AOA data on the download.

[Battson]

I guess the drop in IAS at light weights will be more pronounced, because position error will creep in. I agree that TAS is likely to drop by a knot or two.

[kestrel]

Weight should not effect "position error" as it doesn't change the AOA at stall. If the new tips are improving performance, they are increasing the effective span. A higher aspect ratio wing will stall at a _lower_ AOA an so should have less "position error". I doubt that the differences in IAS error are measurable.

[Battson]

You would think that, but practically there seems to be some difference. Position error can vary with airspeed as well as AoA

[Nev]

Agree that weight won't change position error. AOA and Static port position are the main factors in my understanding.

[jaredyates]

If you become comfortable with the idea that the difference isn't measurable, perhaps the most conclusive test is to fly with one of each type of tip to see if your roll or yaw trim situation changes.

[kestrel]

Jared, if you are responding to my comment, I was specifically referring to the change in IAS error due to AoA is likely not measurable. I wasn't suggesting that the difference in lift or drag was not measurable. But, with that clarified, I think it would be very interesting to check for changes in roll or yaw trim with a different tip on each wing. One would want to be careful doing this type of test as a large difference could result in roll behavior that isn't controllable, but I would be willing to fly this particular tip pairing. Put a stock tip against a 12" extension with a winglet, and I'm a solid "no".

[kestrel]

I chickened out on doing that type of test for VG's. At the time, I just wasn't 100% certain is was a good idea. Now that I've flow with and without VG's, I'm eager to get around to doing that test. I expect little to no difference in roll and yaw trim, but probably some difference in which when drops and when.

[Utah-Jay]

kestrel what were your differences without and with the VG’s?

[kestrel]

I wasn't able to measure any difference. Back to back flights without and then with VG's at same weight showed same stall speed. I can't explain my experience vs. Battson's. Mine are further forward which shouldn't hurt except maybe a tiny hit on cruise speed. Too far back and they won't be effective. It has long been on my long list to do more experiments but haven't gotten to it. They are still on in their original locations.

[rodsmith]

I bought the fiberglass tips many years ago, was not impressed with the shape or looks. Sold them to another builder and last year finally made Hoerner tips. Will never know what they did for me aerodynamically, but I sure like the looks of them. They are the hand made parts that I am most proud of. Now I have been debating whether or not to just add VGs before first flight as I am sure I will end up with them. Think I will wait so I can see what difference they make.

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[Battson]

The wing area is unchanged as the hoerner tips are the exact same area as the kitset ones. So the span is exactly the same and I think it gives a pretty good "apples with apples" comparison. However the upper surface of the Hoerner tips theoretically contributes to the overall lift. If my maths is correct (and it's a long shot ) I'd say a 4% increase in the theoretical upper wing area (skylight excluded).

I measured my wings at 4.66m span (from the window), and 1.7m cord. The radius of the leading edge of the wingtip is 300mm.
That calculates out to 15.805m²

[Nev]

Got these little fellas painted and ready to fit yesterday. Very happy with how they turned out.



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[nborer]

TL;DR: Hoerner tips won't do much for top-end speed, but should give you increased rate of climb, a slightly slower best glide speed, and may reduce your stall speed.

*****
Hoerner tips are one of the few drag-reducing devices that won't do much to top-end performance. They reduce the strength of the wingtip vortex (just like adding span), so they reduce induced drag, not parasite drag. Induced drag trends with 1 / (V^2), whereas parasite drag grows as (V^2), where V is the equivalent airspeed (basically calibrated airspeed for our subsonic airplanes - note that this may be a far cry from indicated airspeed, particularly at lower speeds). That's why we have a best glide speed that isn't as slow as possible - best glide is where parasite drag = induced drag. So, doing things like Hoerner tips that reduce induced drag (and either keep parasite drag the same or increase it slightly due the same or a small wetted area increase, depending on how you build them) means that you should see a reduction in best glide speed, increase in rate of climb, and little effect on top speed. Since the tips may keep more 3-D lift, you could see a reduction in stall speed, which it looks like you're seeing (it's important to disambiguate how much is due to the increase increase in wing area vs. the span efficiency - the span efficiency is the impact of the Hoerner-style tip). That feature may also help you with roll control in the stall.

I'd suggest comparing rate of climb with the two wingtips, but you'll need to correct the data not just for weight & CG, which you have done (awesome!), but also density altitude, since you're using a normally-aspirated engine. Rate of climb is related to (T-D)*V/W, where T is thrust, D is drag, V is airspeed (TAS!), and W is weight - watch your units with this calculation! In normally-aspirated airplanes, T is going to be a function of power; generally speaking, 80% is a decent WAG at propeller efficiency, and thrust is generally related to power via T = P*eta/V, where P is power and eta is the propeller efficiency (again be careful with the units - metric is your friend here). P is going to drop with increase in density altitude, so you'll need to do the rate of climb comparison at similar density altitudes else your thrust won't be comparable between the Hoerner vs. round wingtip comparison. Also, V in the thrust equation case is TAS, which is density-altitude dependent.

[Nev]

Thanks Nic, enjoyed reading your explanation.

[DBeaulieu]

Question related to the topic...how does one determine the shape/dimensions when designing a set of Hoerner tips?

[whee]

My method was to read as many research publications as I could find on the topic. The US military paper from a long time ago was the most useful to me.

[rodsmith]

Question related to the topic...how does one determine the shape/dimensions when designing a set of Hoerner tips?

I used the method outlined by Harry Riblett in his book GA airfoils. Width of the Hoerner tip should be equal to the distance from the leading edge to the main spar (13.25 inches on the 4-place). Should be circular in plan view from the leading edge back to the main spar. From there straight back to the trailing edge with a square corner at the rear outside edge. Outside edge should be as thin as practical, I felt that 1/4" was about the limit using the pink foam as a mold. Top side matches the airfoil of the wing and bottom side tapers from the bottom of the airfoil to the top of the airfoil, the front circular portion is blended from the airfoil into the taper. I believe Bob said 15" max width for wingtips, so mine are total 16" width, 1" overlap for mounting on wing and 1 3/4" wing extension.

[Lee]

Thanks for this Rod. Very clear description. Not nearly as intimidating as it appears.