Elevon Polar Pitch Effect & Thrust Vectoring Ass Wagging

Early on in the development of the v3M3e3, Scott and I had two goals with modifying the elevons.  Our first goal was to give the Mig a more scale appearance.  Over the course of development of the RCPs from v1 to v3, the planes underwent “surface creep” as coined by Scott Lott (designer of the RCP park jets).  As I’ve mentioned before in modifying these planes, there are aerodynamic consequences to any change.  

The oversized elevons on the v3 had two major negative effects on the flight characteristics.  In high alpha, the oversized elevons made the plane prone to thrust vectoring stalls, or ass wagging as Scott calls it (TVAW).  When you are flying in high alpha with too much pitch, the wings will start to rock and the plane loses lift.  The natural response is to give more throttle and up elevator.  The up elevator then induces the ass wagging as it essentially acts like an air brake to the thrust coming off of the prop.  The counterintuitive yet proper way to get out of TVAW is to drop the nose with down elevator and give it throttle.  Practice this at altitude and hopefully it will save you from lawn darting your plane, speaking from experience!

The second negative effect of the oversized v3 elevons is what I call elevon polar pitch effect (EPPE) and Scott calls wind vaning or tail washout.  If you fly on a windy day, you surely have experienced EPPE when you roll the plane on its side 90 degrees to make a tight turn.  The wind hits the elevons and creates adverse pitch/wind vaning/tail washout.  Additionally, EPPE also disturbs lift and the plane typically loses altitude.  Combining altitude loss and EPPE has tightened my sphincter several times!  No low tight turns on a windy day.  With the M3e3, we markedly reduced EPPE with downsizing of the elevons.  When I saw the downsized elevons on the v4, I thought the EPPE would be similar.  It wasn’t.

Even with downsized elevons (still larger than M3e3) the v4 suffers from EPPE about as much as the v3 with the oversized elevons.  There are three obvious reasons for this.  The tail plate of the v4 is longer and wider than the v3.  This increased surface area causes more EPPE.  Secondly, the v4 has a lower polar moment than the v4.  On the v4 the motor has been moved forward which allows the battery to moved rearward.  Scott and I also mount our servos more centrally to further decrease the polar moment.  The lower the polar moment, the more precise the handling—the best handling production and race cars are mid-engine.  However once again, there are consequences.  A lower polar moment actually worsens EPPE.  Think about it, for example take a broom stick with tennis balls at the ends and one with bowling balls.  Which is easier to start to twirl like a baton and which is easier to slow down?  With the lower polar moment on the v4, it takes less force to change the pitch.  This is great when there is no wind, but in the wind you have more EPPE.  Lastly, the v4 is longer than the v3 which changes the polar moment also and increases EPPE.  

In the pdf file I have overlaid the v3 and v4 tail plates.  The elevons on both the v4 and the v3 are M3e3 downsized elevons.  The purple line is where Scott and I cut the tail plate on the v3 to increase flow off of the prop with added benefit of decreasing surface area and EPPE.  In modifying the v4 tail plate, I am going to have to move the tab further back and change the insert hole on the side plate.  I am also considering shortening the overall length of the plane.  

On the v4 you will notice the front portion of the elevon is fixed.  I refer to the front piece as the elevator and the moveable piece as the elevon.  This is to me the single greatest positive design change by Scott Lott.  This a whole another discussion in aerodynamics for another day…