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Message text written by Marvin Kaye
>could you post a note to the LML about the
flap position switch for the AOA, along with the reasoning for why it's ok
to simply turn it on and switch to the flaps database when you put any
flaps in at all.<
First we should point out that all AOA systems must take into account the
flap setting. When you change the flap setting you are in effect putting a
different airfoil on your aircraft with a different critical angle
(stalling angle), a different best engine out glide, maximum endurance and
etc. The AOA Professional and Sport use two databases to compute your AOA,
flaps up (the cruise database) and flaps down (the landing database).
These two data bases are built during the calibration to accurately gauge
AOA. In the flaps full up and down position your AOA is perfectably
accurate and self compensating for gross weight, center of gravity, bank
angle, temperature, altitude and everything else. To be 100% accurate for
all available flap settings, we would need an infinite number of data bases
because there is an infinite number of flap settings for our Lancairs. Of
course this would be rather time consuming to build that many data bases
into the AOA. Over many years of experimenting, we have found that two
data bases works best and building 3 or 4 or more is liken to setting the
acceptable altitude deviations during a check ride to six inches.
Nonetheless, we want to position the closure of the flap switch so that the
results are conservative. Conservative means that if the angle warning is
set to 15% above the critical angle for flaps up and down, the angle
warning should activate at the same or higher margin above stall for all
the inbetween flap settings.
For all those inbetween flap setting it would be most conservative to use
the flaps down data base. This may not seem logical at first until you
understand how lift varies as the flaps are extended. As the flaps are
extended the lift distribution along the wing span moves towards the
fuselage leaving the outboard wing more lightly loaded. Since the pressure
taps are located outboard of the flaps, for inbetween flap settings, the
flaps down data base will give the most conservative AOA warning. Even I
have to think this through logically so don't be concerned if this seems
backwards at first.
The AOA determines which database to use by constantly monitoring flap
position via a simple micro switch which is either open or closed. So when
should that switch close? Switch closure by one-third flaps will work real
well. This means that it would be proper for the switch to close anywhere
between one degree of flaps to one-third flaps. Once closed, it must
remain closed through full flaps. It would not be conservative for the
flap switch to close at the full flaps detent.
Another and perhaps easier way to install the flap switch is to
mechanically close the switch as the flaps reach their up detent.
Electrically the switch would have to be wired so that the contacts are
open with the flaps up and closed when not. In this case, the COM lug of
the microswitch would be connected to a ground. The flap sensing wire from
the AOA would be connected to the NC lug.
BTW, Kirk and Lancair have decided to install the AOA as standard equipment
in their most popular panel for obvious reasons. The Legacy builders
should know that the wing comes with the basic AOA wing kit pre-installed
and that the AOA CPU is available pre-calibrated with a copy of Carsten's
data. The AOA Pro includes a gear warning system at no additional cost.
I almost puked as I was reading the latest issue of Sport Aviation. It
featured an airspeed warning buzzer system. We have been trying over many
years now to remind pilots that stall/spins occur at nearly any airspeed!
This new product undermines some of that effort. Using airspeed or attitude
to gauge when an aircraft will stall accounts for nearly half of all sport
pilot fatalities and the Lancair is not atypical.
Contact with the ground at shallow angles (landing) is almost always
survivable, contact at steep angles (stall/spin) is not.
Jim Frantz
(952) 474-4154
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LML website: http://members.olsusa.com/mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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