Finished up some testing on my flat wingtips this week and last, in preps for the Taylor 150 race. Thought I'd share the preliminary results, which may show a slight increase in speed over my normal tips. I say "preliminary" and "may" because my three test runs are really just scratching the surface, showed mixed resluts, and I have done a few other items during the tip build that may impact the overall picture. Some explanation of that follows, so the test results can be viewed in the proper light (Bob Ax, I know you're watching!

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Here's a couple pics on the table in the hangar. We went with flat tips with fairly sharp edges, per Paul's posts earlier (and thanks for the diagrams with Oswald Factor notes Paul...someday maybe I'll even figure out what it all means!

). We made ribs to strengthen the trailing edges, but did not cover them (another project for later!). The inner flange is 3 layers of glass with an AL backplate to help hold the nutplates.
I do admit that my testing purity (from a one-mod-at-a-time perspective) was not superb this time. Mostly due to schedule constraints (working between trips and with the race looming). I've been working on sealing all plenum leaks and re-routing my heater air source, in preparation for some cowl inlet/exit experimentation, but the inlet/exit work is stll pending, so the plenum work is unlikely to effect speed...might make things run cooler (I hope), but no speed gains expected till the cowl work begins...and even then its a long shot!
The biggest additional aerodynamic change I made during the tip build was to rebuild my right flap, which we discovered had a nearly 3/4" twist in the trailing edge. After installing the new flap, the flap, aileron and tip on the right wing were in great alignment, and the ailerons were in good rig (outer rib tooling holes lined up). However, with the right wing looking good, we then discovered the left flap had a more insideous twist, of about 3/8". No time to fix that, so we extended the flap push rod a couple turns on the rod-end bearing to make the flap-aileron disparity a bit smaller (by about 1/2, or about 3/16' to 1/4". I didn't really like that idea, as extending the flap at all would, to my simple mind, invite more drag. But we figured we'd test it and then perhaps raise it and retest...which is how it played out.
I learned more about testing through the next few flights, as in the devil really is in the details, and no one flight does a result truly show.
I flew a test flight with the new flap and the new tips, as the old tips had been originally matched to the twisted flap, but I know the new tips are true, so I elected to test in that configuration first. From Wayne Hadath's tests of similar tips, I expected about a 4-5 knot increase in stall speed, and hoped for the same 4-5 knot increase in top speed that he saw.
To my surprise, the stall speeds I saw were identical to my previous stall testing, when calibrating my Dynon AOA pitot. 58 clean, 53 full flaps. Hmmm, that's odd, I thunk.
Then I ran a 4 way GPS run down as low as I could go over the Black Rock Desert, in this case at 4500' MSL. Plugged the numbers in, and the result was...no change! This run was 207.6 kts, and was nearly identical to my last run (+0.1 knot...so insignificant), before flaps and wingtips. That has been a pretty consistent speed in the 8500-9500 msl range that I do testing at here in Reno. I've moved out over the desert and found a great test range, and now do the runs down as low as I can get...similar to race conditions. Even down low (4500') the runs have been consistently in the 207kt range. I ran 208+ in Taylor last year at sea level (wind/turns included), so it seems like a good baseline.
Between the flap and the tips, I was really hoping to see a some change, but these mods can be a mystery, as Tom Martin and Bob Ax and others have said many times. But the local gang was really scratching its head on this one over the weekend. The stall speed similarity was more surprising than the top-speed similarity, until I realized that the early testing/calibrating was done at full fuel, and once with a pax (that was at 550+ hours TTAF, not in phase I) to give good max gross stall testing, and the recent test was done at my (now) standard test fuel load of 24 gallons.
So on Tuesday, I shortened that flap push rod-end bearing to put the flap flush at the fueselage, and accept the slight reflex next to the aileron. I flew back to back runs with flat tips and then normal tips (the only difference between the two runs was the tips). Two identical profiles...stalls then high speed runs, in very similar conditions, an hour apart.
The stalls with the flat tips were again 58 kts clean, 53 dirty, while the stalls with the normal tips (and thus longer span) were 54 clean, and 48 dirty. OK, there is the 5 or so knots Wayne saw, and the 48 is the original Vso from the builder's Phase I testing. This makes more sense!
On the 4-way GPS runs, I saw some interesting results. I flew the runs very carefully, and although it was a bit bumpy, it was the same on each run. This time the runs showed the flat tips faster by 1.7 knots. Perhaps a positive result, but no claim yet, as it needs more testing...and no place better to test it than at the Taylor 150 race this weekend!
The real interesting result was the actual speed. These were the fastest runs I've ever had, and were 212.8 (flat tips) to 211.1 (normal tips).
In reviewing the data, and trying to poke holes in it or determine what might add that speed, I came up with the following thoughts:
- The left flap droop on last week's test may have masked improvements from the right flap rebuild and the flat tips. I hope that is the case!!

- The temps at 4500' were within two degrees, but the altimeter setting during the tests was 29.88 last week, and 30.33 this week. I saw more than a 1" increase in MP during the test this week over that of last week.
- The bumps could be a factor as well, and the std dev from the NTPS spreadsheets were less than 1 last week, and just over 1 this week.
So did I get a 1.7 kt increase from flat tips, or a 4-5 kt overall increase from all the recent work...I dunno (I'm not betting on it yet!). We'll see what shows in Taylor (man, I guess I better fly that course well!)
But its fun work, and I'll keep playing and testing to see what might be there.
And Bob, the flat tips and aileron plugs you mentioned in your previous post sound very interesting. They soud a lot like the EVO Rocket wingtip, so it may be a very productive project.
As Tom has said...the proof is in the races, so let the season begin!
Cheers,
Bob