Old 05-31-2016, 06:56 AM
  #4  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,120
Default

It's the airfoil. Flat bottom 'Clary-Y' type airfoils have a very strong nose down (or negative) pitching moment. You can often 'fudge' the problem by adding lots of reflex on the elevons but that's not a good solution even though on some models it might appear to work ok. When you get to high reflex angles you get problems with flow separation on the elevon which causes unpredictable behavior as well as being very inefficient.

For swept wing you can use lots of twist (washout) but as a rule for a tailless design you should be using a tailless airfoil that has some reflex built into it's camber line to give it a neutral or slightly positive (nose up) pitching moment. Symmetrical airfoils also work ok on swept wings providing efficiency is not your main concern..

One of the MH 6X series airfoils would be good along with a few degrees of twist: http://www.mh-aerotools.de/airfoils/mh60koo.htm

Some good reading:
http://www.mh-aerotools.de/airfoils/nf_1.htm
http://www.mh-aerotools.de/airfoils/flywing1.htm
JetPlaneFlyer is offline  
 
Page generated in 0.03740 seconds with 8 queries