First of all, the following information is derived from being involved in DLG from the onset and may not necessarily agree with any or all of what you have read from various noteworthy authors. This info is based on practical experience, building and flying uncountable dlgís and is based on my personal experience. The results are there, your mileage may vary and I donít intend to argue any of it with anybody.
Iíve built my new wolly whopper dlg and it still doesnít fly like I expected!! I built it according to plan and put the CG where my buddy Billy Bob said it should be. OK, whatís the problem????
First off, use the starting CG as recommended by the manufacturer. I donít care what they said on RC Groups! Flying straight and level, are you close to neutral trim on the elevator? Do the planes handling characteristics work with your flying style? (meaning general flying characteristics like, is it too twitchy to fly? Is it sluggish in response?) To twitchy would indicate one or both of two things. To much control movement or to rearward a CG. Too much control movement is easily remedied by adding dual rates for now. This will tell you pretty quickly if the control movement issue is part of the problem. The solution is pretty simple. Also controls can be left a little on the wild side by adding some expotential to the first 20 or 30% of the stick movement. This will allow you to fly softly but still have the ability to make a radical move if necessary.
So, weíve smoothed out our flying a little but things still arenít right. Take a really good look at the plane. Is the elevator trim dead neutral? It should be. Hereís where the time and attention to details comes in if you want your Wolly Whopper to kick some but. First off you can not trim the plane properly at the field in 10 mph wind. Get up an hour early and fly it in some relatively calm, neutral air. When we are done, I want to see the plane flying in these conditions with controllability that you are comfortable with and trims at neutral. This is arguable and some top pilots and designers will tell you that this doesnít matter. What you are getting is my opinion and you can do with it what you like but I will include a testimonial from the last pilot that took the time to do this.
CG is a very controversial subject and lately the trend has been to keep moving it rearward to the point of very tough controllability just because it ďindicates liftĒ better. Wrong! To rearward a CG will cause other issues that more than offset the added ability to indicate lift. I start getting questions like why does my plane drop the nose when entering a turn? Or why does every little puff of wind upset the plane? It used to penetrate well but now I have trouble getting it back to the field. Total neutral stability planes are very tough to fly and even tougher to fly well. For 99% of us we will have better results with very slightly positive stability. But Denny, the dive test says(&*%((^&^& STOP, the dive test doesnít mean squat if the plane isnít decently trimmed to start with. Lets humor old Den and shim if necessary to get that totally neutral trim on the elevator.
If way out of whack the basic starting point (on a Drela airfoil) is to place a straight edge on the flat portion of the under side of the wing in the ďas baggedĒ position meaning that the ailerons are lined up with the little stationary portion at the center of the wing. This straight line should also be the same plane that the stab is on. Another indicator of improper incidence is little or no effect from camber changes.
Alrighty then, now do your dive test. I personally like to see my planes VERY SLIGHTLY begin to pull out on their own. To be honest I sometimes fly them neutral where they do not pull out or tuck under but Iím likely defeating my own purpose. If a significant CG change was required to do this you may have to go back to incidence to neutralize the elevator again and so on. Yup, it takes some flying time to do all this but being forced to spend extra time flying and gaining performance and familiarity with the plane isnít necessarily a bad thingJ
Now we are back to item #1. Take the time to adjust control throws or expotential so that you are flying smoothly and not causing your own problems with erratic control movements.
Plain and simple, this is a cheap way to get a new plane that works better than the one you started with!