I have a Cap 232 73" wingspan 12.5 lbs. I just test flew it the other day and it flys great, but now I have to dial it in. I was at the forward end on the CG and when I flew straight and level at 3/4 throttle, then cut back to idle, the plane continued straight and level with a gradual nose down attitude after about 5 seconds. Ok, now I decided to move the CG back a bit and really try to find a good spot to leave it. I did the 3/4 throttle, level flight test again, and reduced the power to idle. Now the plane starts a gradual climb, does this mean I have a degree or so of too much downthrust with the CG back a little bit farther?

I have always found the easiest test for CofG is to go inverted. The amount of down needed to hold level flight indicates how nose heavy the plane is. Add a little weight to the tail until it holds the same horizontal line inverted as it does right way up.
From there it might be easier to see what effect your thrustline is having.

Flyboy,
Hold on...

MBoland,
With all due respect, there are a LOT of factors that influence flight, not just thrustline.

Flyboy,
Do this first, before making any changes...
( I am assuming that your plane is latterally stable, (tracks straight left and right- or yaw).
Gain altitude, about 300 feet.
Put the plane in a straight down dive at idle.
The plane should continue straight down until you pull it out.
If it pulls up or starts to go inverted, trim the elevator accordingly.

Next, gain altitude again.
This time dive under full power while being careful not to overspeed your engine.
This gives a good indication of thrustline.
Make changes accordingly.

Now, after that is done, fly straight and level right side up at cruise speed.
Trim your plane.
Go inverted straight and level at cruise speed. If the plane climbs, you are tail heavy. If the plane dives, you are nose heavy.
Adjust acordingly.

Please note that the last tests should be performed with 1/2 tank of fuel, and a slight drop in the nose should be expected to prevent a phenmenon called tail heavy [:@]

Bob

Oh, I am also assuming that you CG is close to being correct...

And is the take-off climb straight out, or does it climb to the left while pointing straight up?
This is a definate sign that your thrust angle is off laterally.

Flyboy,

Please note that different trim levels are required for different speeds too.
Slower flight requires more up elevator than faster flight. This is due to the higher lift created by the main wing at higher speeds.
If you had the stick to the firewall and it flew great, then you backed off to idle and the nose dropped, that is normal...

I hope this helps.

Bob

As you move the CG back, the elevator becomes more effective. One result is that the previous pitch trim is going to be wrong. Needless to say, it's going to be wrong because the CG move usually makes it wrong, but the amount of deflection is not only wrong for the new CG, but it's even more wrong since it's also more effective. Minor details, but the point is................

As you move the CG back, as the elevator gets more effective, you need less and less. If you don't keep that in mind, you might see things you didn't want to see. Caps are quick to snap with too much elevator.

Also, as you move the CG aft, the effect of speed on trim is lessened. The CG is heading for the neutral point after all.

And a CG becomes a single location only when you get it where the airplane flies the way you want it to. The CG actually has a range of places it can be and the airplane is perfectly safely controlled in pitch. It's positive pitch stability is changed by CG moves, but that's just the recovery aspect of pitch stability. The elevator will simply become more effective or less. And as you move the CG toward the NP the airplane responds to these pitch tests with less and less recovery. But still reacts. It's simply going more and more neutral in pitch recovery. Just as it would go toward neutral roll recovery as the wing is moved up from low wing or down from high wing and dihedral removed. And we do that on purpose, don't we.

Thanks, I will try to get the CG where I think it should be before I move on with any other test. Right now in level flight I need two clicks of up elevator to fly straight and level. When I roll inverted it takes a just a tad of down elevator. I haven't tryed the inverted 45 degree up line yet. I'll have to do that one. Thanks for the advice, after I get the CG set I'll work on the thrust lines.

That test you mention is a good one. It's not so speed sensitive, nor takes really any setup technique. And seems pretty reliable.

Just keep in mind that as you move the CG aft, the elevator gains effectiveness. It'll do whatever it could do before, but with less deflection. And it will also be able to do more than it could before. So simply re-rig when it starts feeling too quick.

On an aerobatic machine like the Cap 232, the best thing would be to have 0 degrees of down thrust on the engine. I would rather look at my CG.
Down thrust is an event of keeping (mostly on high-wing planes and full size stuff) the nose straight and level during increased throttle.
The basics of down thrust on these machines is simple: as throttle is increased, the aircraft speeds up, the wings produce more lift and eventually it will at the same time produce more drag. This will make the plane do a nose up movement. To avoid this engineers of the very beginning of aircraft construction started messing around with down thrust to avoid this.
However on an aerobatic radiocontrolled airplane like yours, with almost no difference on the upper side and the underside of the wing, it has no significant effect.

Try starting with a 0 degree thrustline. But keep the 2 degrees or so Right thrustline. This is for coping with the left-drift a right-turning engine produces. (basic: it's for keeping the nose straigt instead of having the airplane tending to yaw left).

The CG is a matter of keeping the plane in a level or slightly nose down position in the event of engine cut. This is really only a way to keep the plane flying and keeping enough speed to generate the needed lift and not stall out.
But do not go more than about 33 percent back from the leading edge OR the point of the wing where it has the thickest part. CG is often the thin red line between life and death of your model.
A great CG is the point where you can pull of the throttle from full to nothing, and the plane keeps flying in the attitude you put it in, or lowers its nose a little bit. I prefer the little bit of nose down, and rather compensate with a little down elevator when upside down.

Good luck!

Hans-Kaare

OK, I took out a degree or so of downthrust I had on the engine and now I think it is flying or acting like it should. Under a full power fly by, wings level, when I cut the power to idle, the plane does not pitch up or down. I just stays level and starts to descend. What I did notice though, is that it starts a shallow turn the the left as it starts a shallow descent. Is this just an indication that the left wing is a little heavy? I do have an ounce and a quarter of weight in the left wing tip.

It might be the extra weight that you have in your wing. It may also be too one or two degrees to little of sidethrust on your engine. I would start off with removing the extra weigt.

Good luck again! Nice to hear that you got the "nose down" problem solved.

That's what I'll do first, remove 1/4 oz. and see if I get the same results. That leaves me with 3/4 of an oz. in the left wing. I had the 1 oz. in the wing in the first place due to my latteral test on the bench. I'm hoping that will do it, because on the upline, I do have to start feeding in right rudder after 100 feet or so. It initially starts the upline straight, but then starts to need right rudder to continue, so I would like to keep the small amount of right thrust in there that I already have. If the wind dies down today, I'll check it out.