Decalage and YOU
What usually is asked:
- What is the proper relationship between decalage of the tail and the CG?
- What is the best way to adjust them to operate at optimum levels simultaneously?
- How does a dive test, and the manufacturer's recommended CG location, etc. relate to these variables?
Some very helpful information is from Dick Proseus of Wilson, NC:
The main problem is your stab is glued on with too much negative incidence (up trim). To reduce this you may heat the tail boom of the fuselage with a heat gun used to shrink Monokote [or other heat shrinkable coverings]. It will become plyable and you can bend it slightly in a downward direction and hold until cool. It will help to have a second pair of hands to hold the plane and also apply a cold wet cloth to the heated area to cool it faster. I would try to adjust by getting about 1/8" of droop the first try. BE CAREFUL, WORK SLOWLY, YOU COULD MESS UP THE PLANE.
To answer your main question about decalage vs. CG. If your plane is trimmed correctly and you increase the decalage, to maintain the same flight characteristics the CG would have to be moved forward and/or down trim added. From experience at least two things will happen. At some point as you fly faster the decalage will overcome the moved CG and the plane will nose up unless you add more down. This also would happen at the old setting, but increasing the decalage amplifies the situation. Also, at some point of increased decalage you will have moved the CG so far forward that normal flight is hard to achieve.
HLG's use a small amount of decalage normally because of the wide speed range. It also allows the CG to be moved aft for a more sensitive aircraft that will signal lift more easily-stall, wing wiggle, etc.
A better understanding of the aerodynamic principles at work and advice on adjusting the center of gravity came from Dave Register:
The scalloping [or 'porpoising'] type flight you're experiencing is probably because you've got a bit too much up elevator for the CG location your using. The first thing I'd suggest is to dial in some down trim (a little at a time) until you've got this under a little better control. I'd also guess that your CG is a bit far forward... But when you move it back some, you'll need a bit more down trim.
What's happening here is that the CG and center of pressure are pretty far away from each other. The CG is the balance point for your plane. If all of the lift balanced at the CG, you'd have straight and level flight. However, the center of lift (often referred to as the center of pressure - CP) on an airfoil moves around. At low angles of attack it tends to move back on the wing. At high angles of attack it tends to move forward.
Let's say you've got the CG pretty far ahead of the CP for a given flight attitude. That tends to tuck the wing down. To compensate, you put in some up elevator. Things balance out but now the wing and the elevator are fighting each other. A little gust picks the nose up. The increase in angle of attack moves the CP forward. The elevator setting is trying to tip the wing up and when the CP moves forward, that's easier to do. When they gang up like this, the wing stalls and then dives. Now the angle of attack is lowered and the CP moves back. The elevator isn't working too well because there isn't much speed. The CP behind the CG tends to steepen the dive and the plane picks up speed. Eventually the air speed is sufficient for the up-trim in the elevator to take over and the cycle repeats.
The dive test attempts to address this by providing a tool to assess when you're close to having the CG and incidence right. In this case, you put the plane into a shallow dive (about 30 degrees or so), and hold it like that for a second or two. Then let go of the stick and see what happens. If it continues to tuck under, the elevator has too much down trim for the CG position. If it pulls out very quickly, you probably have too much up trim. What you want is a fairly gradual pull out (some people like it neutral but that doesn't leave any margin).
A way to approach this is to start with the manufacturer's recommended CG (usually, generally, conservative and too far forward). Dive it as above and adjust elevator trim until it barely pulls out. Fly it for a bit and see how you like it. Then move the CG back maybe 1/8 inch and repeat the exercise. You'll eventually reach a point where it handles OK in this test but the CG is far enough back that it may be a bit too 'goosey' for normal flying. Move the CG forward a bit from that point, re-trim and go have a good time.
As you move the CG back, the plane usually becomes more responsive which may require you cut down on your elevator throw some to keep it from over-controlling.
The solution to my particular problem came when I sighted across the wing and found 2-3 degrees of decalage. I decided to heat the tail boom to soften the epoxy and push the stabs down to reduce the decalage to about 1/2 degree (suggested by John Roe). The heating was done with an ordinary hair dryer on the "hot" setting and the boom was held in place while it cooled - it worked great.
Upon flying the model it was evident from a dive test that the center of gravity was a little forward of where it should be. I adjusted the radio gear (using the velcro method) to move the CG aft in 1/16" increments until I found the position that allowed the plane to fly hands off without stalling. Now it just floats around the sky signaling the lightest lift.