Ed, Rob, et al,
The purpose of reflexed flaps (wing TE) is for reduction of drag
at cruise speeds. After certain minimum speeds, induced drag is lessened
by reducing lift through a reflexed TE (also reducing anti
lift at the horiz stab). Parasitic drag in a laminar flow
environment may also be reduced by reattaching the airflow at the
reflexed flap. The basic fat wing design helps when the leading edge
gets dirty (disrupts laminar flow) because lift is only slightly reduced but
draq increases (ice, rain, bugs - see note 1). This is as opposed to a
thin laminar wing which may lose lift when dirty. The fat wings allows our
grocery cart wheels to be stowed within.
When the flap is reflexed, the point of lift moves forward where higher
speeds require more nose down trim (less negative AOA at the horiz
stab) and lower speeds require a higher angle of attack to achieve adequate
lift. In general, below a max Vf (160 to 140 KIAS), the flaps may be
taken out of reflex (to 0 degrees) where the attitude of the airplane can
be changed by up to 6 degrees nose down (as measured digitally at
the longeron) and this will require substantial nose up trim for level flight
while the speed drops (about 15 KIAS) from increased drag. For this wing,
further reflexing beyond -7 has little effect on speed in cruise (Note
2).
This form of wing is not unique - reflexed trailing edges are used in all
tailless (and non-canard) aircraft. In such aircraft, changes in CG are
compensated for by changing the angle of reflex (changes fwd/aft point of
lift). Remember that the same is true with the 200/300 series wings.
(hint: aft CG wallow may be reduced by taking a bit of reflex out of the
wing). Remember also that slow speed operation with flaps
fully reflexed requires a high AOA to compensate for the reduction in lift
from the reflex. Be careful out there.
Max lift, min drag at 70-100 KIAS is with the flaps at TO position (about
+10 degrees down from full reflex). After about +17 degrees down from
full reflex, the flaps seem to merely add drag that is useful in higher than 3
degree descent angles (preservation of kinetic energy) - keeping the speed
controlled (below 100 KIAS) at low power settings and also keeping the
nose low.
Maybe next time I will record AOA digital display data in some of these
configurations.
Scott Krueger
N92EX
Note 1: In an Air Venture x-ctry race, KARR was a turning checkpoint that
required I descend over the monitor. In that descent I ran through a
mass of little black bugs (so did others) and my return to level flight as the
same race power resulted in speeds 6-7 KIAS less than before the
encounter. Later, I noted that the splattered bugs were very close
together (less than a 1/4") and had completely messed up the laminar flow at the
leading edge of the wings. Slow downs have also occurred in
flight through rain.
Note 2: Certain 320/360 racers experimented with more reflex with the
conclusion that there was no significant improvement in speed.