During the development of the AOA instruments, Jim accumulated reams of L/D data over a wide range of flight parameters which he ultimately used to write the code that makes them work. The original R&D work was done with his LA4 and then refined when he started using his LNC2 for further testing. In the process he discovered that, in general, as you lowered the flaps that the lift increased more than the drag up until you reached about 1/3 flaps down. At that point, the lift curve flattened out and the drag curve steepened. Consequently, the critical AOA remained fairly constant from about 1/3 flaps down to full flaps, so switching the instrument to the flaps database at that point resulted in the most conservative indications throughout the entire AOA range of the wing. Another way of looking at it (considering there are only 2 airfoil databases) is that the critical AOA is similar from flaps up to just about 1/3 flaps down, and the critical AOA from 1/3 to full flaps is also very similar... yes, there will be some minor inaccuracies in both databases at that 1/3 flaps position, but it's the most convenient and reasonable place to switch from one to the other and reflect as closely as possible to the actual L/D curve throughout the full range of flap travel. I hope this makes sense.

<marv>

Colyn Case <colyncase@earthlink.net> wrote:

> Marv,
>
> I asked that question to current owners of aoa pro and couldn't get a
>straight answer.  What I would really like to understand is what does the AOA
>"see" as the flaps change position, ignoring the effect of the flap switch
>for a minute.
>
> Colyn
>
> On Jan 6, 2014, at 10:13 AM, <marv@lancair.net> <marv@lancair.net> wrote:
>
> I don't know if it was mentioned in the installation instructions but the
>AOA flap switch should be setup to signal flaps down at about 1/3 of their
>down travel.  While there are an infinite number of flap positions the AOA
>only tracks 2 of them, flaps up and flaps down.  The logic of setting the
>switch to trigger flaps down when they're only 1/3 of the way down is that's
>the approximate location when the flaps' position impact drag more than lift.
> The critical AOA from 1/3 to full flaps is very similar and is the reason
>that Jim Frantz always suggested that switch position as the most appropriate
>compromise to not having multiple flap position data available for the
>instrument.  I hope this helps.
>
>            <Marv>
>
>
> John Cooper <snopercod@comporium.net> wrote:
>
>> On my L-235 I couldn't figure out a good way to mount a microswitch for the
>>AoA system which would engage in the "flaps down" position, so I used a
>>U-shaped magnetic reed switch instead. The NO contacts close when a ferrous
>>object passes between the two arms of the switch and my flap actuator arm
>>serves that function. I glued a couple phenolic "guides" onto the switch in
>>case the flap actuator and switch became mis-aligned for any reason. This
>>scheme seems to work, but I'm fiddling with the adjustment at the moment.
>>This photo (looking aft) shows the flaps in the "UP" position and the
>>microswitch not engaged.
>>
>
>
>
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