Jim Smith has been doing some TAS testing of his -6 with the new wingtips. When he was doing the testing with his new prop a couple of years ago I recommended that he do two-way GPS runs using forecast winds and temperatures aloft. The reason I made this recommendation is that with testing of my aircraft I've always found that the forecast winds and temps, at least in my region, were very accurate, whereas I've found that most OAT installations on planes I've tested give erroneous temperatures which throw off density altitude calculations, and thus give IAS and TAS errors.
I've also noted in writings in the past that you can have a direct crosswind of 20 mph on a 200 mph plane and it will only give a 1 mph error, which is well within most people's ability to fly the two opposite runs without an autopilot, altitude hold, and no convective activity. So if you are at least within 30* of the wind direction, your error will be almost imperceptible, even with strong winds.
So Jim, for his flights at 4000', 6000', 8000' and 10,000' did something I had recommended in the past if you didn't have forecast winds, fly 360* turns and note the highest and lowest GPS groundspeeds along with the headings where these occurred. He did these and noted that his determination of the wind speed and direction from this was very close to those forecast, which were in excess of 20 mph. He also noted that when you come around to the headings where the wind is on the nose or tail, that you will go through 40* to 60* where the groundspeed hardly varies, which verifies what I had noted earlier, that if you are even close to pointing into or with the wind on two-way GPS runs that your overall error is minimal.
I won't yet divulge the speed increase he has seen with these tips until he has performed enough testing to get very good averages which he feels he can stand behind, but they have been even better than the 4 mph I had predicted, and not only that, but the increase has even been below 5000' dalt, which I didn't think would occur.
I'm starting to think that these tips, along with the expected reduction in induced drag, have somehow reduced the overall parasite drag, either through a flatter body angle, i.e. less nose-up attitude, a better Oswald efficiency factor for the tips, or a combination of both. This has been very exciting for both of us!
I've also noted in writings in the past that you can have a direct crosswind of 20 mph on a 200 mph plane and it will only give a 1 mph error, which is well within most people's ability to fly the two opposite runs without an autopilot, altitude hold, and no convective activity. So if you are at least within 30* of the wind direction, your error will be almost imperceptible, even with strong winds.
So Jim, for his flights at 4000', 6000', 8000' and 10,000' did something I had recommended in the past if you didn't have forecast winds, fly 360* turns and note the highest and lowest GPS groundspeeds along with the headings where these occurred. He did these and noted that his determination of the wind speed and direction from this was very close to those forecast, which were in excess of 20 mph. He also noted that when you come around to the headings where the wind is on the nose or tail, that you will go through 40* to 60* where the groundspeed hardly varies, which verifies what I had noted earlier, that if you are even close to pointing into or with the wind on two-way GPS runs that your overall error is minimal.
I won't yet divulge the speed increase he has seen with these tips until he has performed enough testing to get very good averages which he feels he can stand behind, but they have been even better than the 4 mph I had predicted, and not only that, but the increase has even been below 5000' dalt, which I didn't think would occur.
I'm starting to think that these tips, along with the expected reduction in induced drag, have somehow reduced the overall parasite drag, either through a flatter body angle, i.e. less nose-up attitude, a better Oswald efficiency factor for the tips, or a combination of both. This has been very exciting for both of us!
