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.

 

 

 

I have the exact concerns reported by Rob Stevens in Australia.  Since the manual is vague on cruise flap settings, can some experienced L320-360 fliers help us with what is the best cruise settings?  Is it -7deg. , -10deg., or somewhere else.  Since construction variances can create different “faired-in positions”, is there an angle which can be measured from the top or bottom wing surface?  Can anyone explain the aerodynamic reasons for reflexed flaps?  Rob and I need help!

Ed Gray Dallas  L360 N77ZG XP360 Silver Hawk injection MT 3 blade prop Dual Dynon Skyview SL30