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Guys, I don't doubt your experiences or question your data. I'll add a few theoretical observations and maybe we can tease out some consistent conclusions. 1, The main effect of extending a static port above the contour of the local skin is to lower the static pressure reading (which makes the altimeter read higher and the airspeed read higher). So if you were perceiving a tailwind in every direction, and extended the static ports, that would make the erroneous tailwind go away. 2. I tried to find a Pressure distribution for the Legacy and couldn't; but because the fuselage is tapering so much, I'm sure that the pressure changes rapidly ( from below-ambient toward ambient) as you go aft, in the vicinity of the rear windows. In other words, static pressure accuracy depends a lot on putting them in the right place. If you put them too far aft, you could compensate by making them stick out. I used the recommended placement, kept them flush, and it worked. 3. Mooneys and the Cessna 172 both have protruding static ports, to compensate for off-ambient local static press. So there ARE airplanes where the ports stick out by design. 4. Pitot tubes like the basic, stock Legacy pitots, are very robust and tolerant of angle of attack changes. They HAVE to be out of the boundary layer, but if they are, pitot errors are very rare. CB from iPhone
On Jan 22, 2013, at 3:25 PM, Paul Besing <paulbesing@me.com> wrote:
This was my exact experience with my RV-6….not sure why from an engineering standpoint that it's argued when there is ready available data such as yours as to the effects of flush mounted static ports. On Jan 22, 2013, at 9:55 PM, Dennis Johnson <pinetownd@volcano.net> wrote:
I have a Legacy RG and put my static ports flush with the outer skin, located in accordance with the assembly manual. During my flight test phase, I pleasantly noticed that I always had a tailwind! I knew I was lucky to be flying such a wonderful airplane and just figured I was equally lucky to always have a tailwind.
Eventually, logic prevailed and I realized nobody could be that lucky. I flew the four course GPS airspeed check and discovered that my indicated airspeed was reading low. I added a tiny piece of machined aluminum, similar to a domed head rivet, to both static ports and it solved the problem. Other Legacy builders who installed their static ports protruding slightly tell me that their indicated airspeed is accurate.
Dennis
Legacy 580 hours --
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