Having one servo control BOTH the elevator a aileron at the same time is very unrealistic and un-natural. It makes a BIG difference!

With only the two servos the plane flies like a very strange delta wing plane, and it is even more difficult to trim out properly.


That is not to say it is unflyable... however it is more difficult.

Adding the two servos makes a big difference, makes it easier to fly, and will help assure that you will be able to land it safely on a runway ( the plane is unsuitable for grass landings... ).