During the past couple of years I have been building fiberglass fairings for both ends of the wing struts, the landing gear shock struts, and wheel pants. This covered all of the areas that budd davisson said could use some aerodynamic cleanup. Many of you saw the finished versions at Oshkosh 2013.

Of course, the first question that everyone asks is "How much speed did you gain?"

Being blessed or cursed to be an aeronautical engineer and a professional flight tester, I knew that most claims of airspeed gain are wildly exaggerated. I would love to tell you "Yep, I gained about 30 to 40 knots of free airspeed. In fact, it was so good I have to pull the throttle back to keep it out of the yellow arc." I'd love to tell you that, but it certainly wouldn't be true.

Based on nothing more scientific than looking at the true airspeed on my Dynon display and trying to remember what the values used to be a couple of years earlier, I have told many of you "about 5 knots." Not a very scientific approach.

The problem is that cruise speed is affected by so many things, such as pressure altitude, temperature, power setting, gross weight, turbulence, and pilot's shoe size. Okay, maybe not the last one. Additionally, since airspeed varies with the square root of the drag coefficient and the cube root of the power, most changes don't make a big difference. Statistically it is tough to determine a 5 knot change while watching airspeed vary either direction by 5 knots. Noise in the data of similar magnitude to the change makes it hard to find the magnitude of the change with any accuracy.

I recorded a bunch of cruise data during the Oshkosh 2011 trip with no fairings, and more cruise data during the Oshkosh 2013 trip with the fairings. I finally finished converting my data reduction software that I had written on the work computer (in Matlab) so that it would run on my home computer (in Excel).

I haven't done much analysis yet, but the first look shows a reduction in drag coefficient with the fairings of 35 drag counts (delta coefficient of 0.0035).

I know what you're thinking...WOW!! That's great! I have no idea what that means!

Well, for a flight condition of 7500 feet pressure altitude, standard day, 2440 lb gross weight, that equates to an airspeed increase of ...4.5 knots.

4.5 knots. Not quite 30, but surprisingly close to my initial guess of 5 knots. Not a whole lot, but it's free! A little more weight, but it doesn't take any additional fuel flow. I think it also makes the airplane look better, or at least more complete.

Keep in mind that a given reduction in drag coefficient has a bigger effect on airspeed as the original airspeed increases. Thus, I added wheel pants and gained about 4.5 knots. A friend added wheel pants to his Glasair II and gained 10 knots. The difference was that his initial airspeed was 160 knots, not 125 knots.

There's a lot more flight testing and analysis yet to be done, but I knew many of you were sitting in front of the computer thinking "when is Erbman ever going to figure out how much speed he gained with his fairings." Well, now you know.

Of course, the first question that everyone asks is "How much speed did you gain?"

Being blessed or cursed to be an aeronautical engineer and a professional flight tester, I knew that most claims of airspeed gain are wildly exaggerated. I would love to tell you "Yep, I gained about 30 to 40 knots of free airspeed. In fact, it was so good I have to pull the throttle back to keep it out of the yellow arc." I'd love to tell you that, but it certainly wouldn't be true.

Based on nothing more scientific than looking at the true airspeed on my Dynon display and trying to remember what the values used to be a couple of years earlier, I have told many of you "about 5 knots." Not a very scientific approach.

The problem is that cruise speed is affected by so many things, such as pressure altitude, temperature, power setting, gross weight, turbulence, and pilot's shoe size. Okay, maybe not the last one. Additionally, since airspeed varies with the square root of the drag coefficient and the cube root of the power, most changes don't make a big difference. Statistically it is tough to determine a 5 knot change while watching airspeed vary either direction by 5 knots. Noise in the data of similar magnitude to the change makes it hard to find the magnitude of the change with any accuracy.

I recorded a bunch of cruise data during the Oshkosh 2011 trip with no fairings, and more cruise data during the Oshkosh 2013 trip with the fairings. I finally finished converting my data reduction software that I had written on the work computer (in Matlab) so that it would run on my home computer (in Excel).

I haven't done much analysis yet, but the first look shows a reduction in drag coefficient with the fairings of 35 drag counts (delta coefficient of 0.0035).

I know what you're thinking...WOW!! That's great! I have no idea what that means!

Well, for a flight condition of 7500 feet pressure altitude, standard day, 2440 lb gross weight, that equates to an airspeed increase of ...4.5 knots.

4.5 knots. Not quite 30, but surprisingly close to my initial guess of 5 knots. Not a whole lot, but it's free! A little more weight, but it doesn't take any additional fuel flow. I think it also makes the airplane look better, or at least more complete.

Keep in mind that a given reduction in drag coefficient has a bigger effect on airspeed as the original airspeed increases. Thus, I added wheel pants and gained about 4.5 knots. A friend added wheel pants to his Glasair II and gained 10 knots. The difference was that his initial airspeed was 160 knots, not 125 knots.

There's a lot more flight testing and analysis yet to be done, but I knew many of you were sitting in front of the computer thinking "when is Erbman ever going to figure out how much speed he gained with his fairings." Well, now you know.

## Comment