This page is devoted to REFLEXORS and the affects of REFLEXING an airfoil's trailing edge.

Reflexing is closely related to the  < Servo Tab  >  discussion page,   you should read that page first.

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Reflexing ( as the word is currently used in the Dragonfly world ) is the process of deflecting a portion of an airfoil's tailing edge upward to alter aerodynamic properties.  For the most part, any conversation on "reflexing" will be limited to the aft wing and will mean travel of the aileron from the neutral trailing location to slightly upward (about 1/2" reflex upward).   Expanding the scope of the term a bit more :  when the trailing edge of an airfoil is deflected upwards or downward , a new chord line is drawn in the air.  This re-defining of the "apparent AOA" will cause the airfoil to generates less or more lift and drag for the same physical Angle of Attack (AOA).  This is not magic.  This is exactly how ailerons work.  An aileron is a reflexing surface that is under the pilots control.  There just happens to be one on each side of the fuselage and they work opposite to each other.  If they worked symetrically, they would be called plane flaps.  So, to co-opt the ailerons to be auxiliary lift generating surfaces, we have to "reflex them up or down at the same time and at the same rate.   The raising of both ailerons will make the aft wing generate less overall lift and lowering of both ailerons will make the wing generate more overall lift.     

Seen right, is a sketch of the Eppler 1212 airfoil.  This airfoil was chosen because of the interest in reflexing the Dragonfly's ailerons ( both at the same time ) to reduce the aft wing's coefficient of Moment  at high speeds. 

To best explain what reflexing is and what is does for the Dragonfly, we have to create an imaginary test aircraft and hire a fearless (digital) test pilot.  

Poof. 

Now we have a state of the art Mark II (digital) Dragonfly and test pilot.  

For the sake of this discussion, our test Dragonfly aircraft has normal looking differential ailerons ( one goes up while the other goes down) that are under the pilot's control.   The ailerons on a typical Dragonfly wing are inboard, 20% chord and run about 5 feet long.   Some dragonfly ailerons have been outfitted with 18" long x 3" wide servo actuated tabs installed inboard to help power the ailerons up or down at higher speeds.   For the purposes of this discussion, we will just pretend that the ailerons are stock and have no power  assist.       

 Lets also say that our digital Dragonfly is equipped with with a "state of the art" ( TEAM RAPTOR BUILT ) aileron reflexor system that allows the pilot to move the ailerons differentially (as we need to do for flight control) and symmetrically ( that is both up or down together).  Our fearless test pilot is instructed in there use and feeding and is looking a bit less fearless.  

Now, we put our imaginary aircraft flying in flight at 3000 ft AGL, straight and level, elevators trimmed, wide open throttle at 120 mph, with the fuselage "level line"  nicely "level"  to the earth's gravity vector.  In his fantasy Fly , our fearless pilot does expert ailerons rolls by pulling the control stick from side to side.   returning to the straight and level is by the book and our pilot is looking all the better for being back into his native environment.  

After some time, we get on the radio and tell the test pilot to go faster.    

Even from the ground, we can see that he is perplexed.  The throttle is already wide open,   the gear is fixed (down) and the prop is fixed pitch.  How is he going to get this bird to go faster???   At this point, the engineering team (on telecon from the Skunk Works) tell him to reduce the drag of the aircraft and it will go faster.  The FCC will not allow a direct translation of what our fearless pilot replies with.  Our engineers tell the pilot to tuck the elevators and reduce the aircrafts drag.  That is easy enough.  So the control stick goes forward (the elevators tuck up) and the houses get bigger fast.  Our pilot immediately realizes that this aint going to work.  

After he gets the plane back to straight and level, our our fearless pilot ask the engineers what other brilliant ideas they have.  The response goes something like this:  " a great deal of the aircraft's drag is being generated by the canard's elevators.  Most likely they are slightly down into the breeze at this point, but even if they are trailing in neutral, they could be configured to make less drag.  you have to reduce the lift demands of the canard so that the elevators can be tucked and the nose will not pitch down....."

Long about the time our fearless pilot is considering quitting this project, he remembers that there is that magic switch on the control stick that allows the ailerons to be flexed / reflexed.   Nobody actually told him that reflexing the ailerons would reduce the lift requirements of the canard but he figures that it cant hurt.  

He presses the aileron reflexor button and causes both ailerons to move upwards a tiny bit (as measured at their trailing edges).  The aircraft goes into a gentle climb.  The pilot gently pushes "forward" on the control stick ( tucking the elevators up a degree or two) and the plane goes back to flying level.  Aside from re-trimming the elevators and the fuselage now flying at a degree less AOA,  nothing else seems to have changed.    Oh wait,  yea, the airspeed is 10 mph faster than before he messed with the wings.  Hummmm.  Maybe these engineers are on to something.  Same power,  more speed.   This is pilots dream.  Somehow this flying machine must be making less drag now.    

Well if a little is good, a lot must be even better.  Our fearless pilot presses and holds the aileron reflexor button again and both ailerons to move upwards 3/4" (as measured at their trailing edges).  The aircraft immediately and violently goes into a climb.  The pilot slams "forward" on the control stick (tucking the elevators up many degrees) and just barely gets the plane to level out.  After a few moments and some creative language, he gets the aircraft back under control and notices that he is now moving 60 mph faster than before he messed with the wings.  Hummmm.  What have these engineers done to him ???  Now there are very noticeable changes to the way the aircraft is flying.  The re-trimming of the elevators has used up virtually all of the forward travel of the control stick.  In fact, there is very little left for basic flight maneuvering.   The fuselage is now flying at a several degrees less AOA than before and the thrust line is actually negative.  There is something else nagging at the pilot but he cant put it into words.  It is just bothering him.  About the only good thing he can see is that he must be making far less drag to be going so much faster.    

Back on the ground, the test conductor notices the pilot has done something to make the plane go much faster and wants to know  what that is.  He orders the pilot to bring the plane down without messing with any of the buttons.  

The pilot pulls back on the throttle and the plane slows down for landing.  As the speed drops off, the pilot becomes more and more aware that the elevators do not need to be deployed.   This cant be good.   In every other flight he has ever made in this tandem wing aircraft, as the speed drops off, the canard's elevators have to be deployed to make the needed lift to keep the nose up.  For some reason, on this landing,  the elevators are not needed to keep the nose up.  In fact, as the speed bleeds off, the nose is getting alarmingly high all by itself.   The pilot has no more " forward " stick control (the elevators are already fully tucked) and he is getting very excited indeed.   Eventually, the fuselage is at 13 degrees and the shed wake of the canard destroys the lift of the aft wing.  The tail falls out from under the plane as the aft wing's lift collapses.  This is deep stall and our pilot is in deep trouble.  The digital test is terminated and our fearless pilot is put back in the hanger all safe and sound.   

So what happened ? ? ?    

Well it is complex to say the least.  The aircraft went faster as the ailerons were reflexed,  but became un-controllable on landing.  Though the two effects seem very different, they are both related to the same cause.  

                                                    The lift ratio of the aircraft's wings changed.  

In the "go faster"  on the same power case :   You reflex the aft wing's ailerons a bit and the center of lift moves backwards.  In fact,  the Center of Lift was moving away from the Center of Gravity.  This means that gravity is able to do more of the nose down moment than before.  The canard has to do less and less work to keep the forces in balance.    Less work means less drag which mean more speed.  The canard works less and the aft wing works more.  Simple enough.  Now this little trick does not work forever.  Eventually, the elevators can not be tucked up any more and you stop reducing the drag  they make.  This condition then becomes your true top speed.  

In the "cant control this flying machine as it gets slow" case,  the center of lift was already pretty close to the Center of Gravity and it moved itself even farther forwards as the plane slowed down.  Now,  the moving of the Center of Lift forwards as a plane slows down is not new,  every aircraft does it.  What is new, is that the Center of Lift  "started out" very close to the Center of Gravity.  This is a definite "DO NOT DO".    No aircraft design engineer would allow this to happen.   But excessive use of reflexors can override the designers safety precautions.   As the plane slowed down, the Center of Lift shifts forward.  Eventually the Center of Lift went in front of the Center of Gravity.  Gravity did the rest and deep stall was the result.  Simple enough.  

Now you just gota know that 99.9% of the math theory was deleted from this explanation.  There is a lot of Center of Pressure, Coefficient of Moment, static balance, dynamic stability and interregnal calculus that goes into making it all very official, but that will be left for another (way deeper) discussion.

Usually,  the aileron segments of a wing are moved opposite of each other.  In the reflexed mode, however, the surfaces are moved in consort with each other.   The image (right) is a multi part graph of " LIFT  vs Angle of Attack " for several configurations of the Eppler 1212 airfoil with 20% chord ailerons reflexed as shown. 

For any given AOA ( see the horizontal axis of the graph ) the amount of lift the airfoil will produce is a function of its Lift Coefficient (see the vertical axis of the graph ). 

As the aileron's reflex goes from neutral to negative ( the aileron's trailing edge would be moving upward ) the amount of lift generated by the entire aft wing will decrease.  This is seen in the lower most curve on graph. 

As the aileron's reflex goes from neutral to positive reflex (the aileron's trailing edge would be moving down) the amount of lift generated by the entire aft wing goes up.  This is seen in the upper most curve on graph. 

The coefficient of moment ( Cm)   is nearly unchanged by this small level of reflexing and is seen as the fuzzy line below the x-axis on the graph.

Authors note:  The Bottom line is ..... Reflexing of the aft wing ailerons (upward) gets you more speed and makes the aricraft very dangerous at slow speeds.......

revised 04/11/2002