CG for Zagi Wing

[Arkitexas]

My plane is a stock Zagi XS. The build instructions call for the CG to be 8 inches back from the nose of the wing. I have balanced the wing at that point but after trimming out at 2/3 throttle, the plane lands with 5 to 7 degrees of up elevator. This must be creating a lot of unnecessary drag. I am thinking that the nose must be heavy and the CG needs to move rearwards. I have read of taking the trimmed plane into a dive and then cutting the throttle. If the plane rises, it is nose heavy. If it maintains the dive, the CG is good. If the dive steepens, the nose is light.

My questions are:
1) Is the above correct?
2) What is the correct angle of dive?
3) What speed do I reach before cutting throttle?
4) Does this method provide an exact CG or is it a generalized test?

Thanks
Rick

[Sparky Paul]

There is a natural difference between the elevator position used for high speed, and that needed for low speed.
The dive test you describe -should- work. But don't push too far.
It is usually performed -gliding-, power off.

[Ollie]

Please read the article:
http://polecataero.com/articles/cg-location

Cut the throttle just before the test. The CG location should mainly depend on the wing planform. In a Zagi wing, forget the tail. The CG location is about a range of position locations. It is about "too much" stability to "too little" stability. It depends on the fliers style more than a "correct position." With "too much" stability and needs pitch up elevator trim for the Zagi. With "too little" stability and needs pitch down elevator trim for the Zagi. Adjust the elevator trim or your flying style for too much airspeed or too little air speed after the CG test.

Be happy with adjustment to your flying style not with any CG or elevator trim "correct position"

[Julez]

Try this:

www.maltemedia.de/swp

[Joe Minton]

Arkitexas:

Use the dive test as illustrated on the http://polecataero.com/articles/cg-location page (Thanks Ollie). Move the CG back until your Zagi will just pull out of the dive. You'll find that the CG will need to go back quite a bit from the recommended 8".

As you move the CG aft, you will need less total elevator travel and might prefer some exponential as well. The elevator becomes more effective as the CG move back.

Sparky Paul is correct about elevator trim varying with speed. However, when the CG is near the 'neutral point' (i.e. -- the CG position where it will not pull out of the dive) the trim-with-speed change is much less and the model is generally easier to fly.

I prefer to "CG" my planes so that very little down-elevator is required for level inverted flight. I don't remember where my Zagis' (three of them) CGs wound up but they sure fly better in the aft position. The tendency to tip stall is greatly reduced. They fly better at all speeds. Drag is indeed reduced and that is was most obvious with the 5C on the local slope which often has marginal lift.

Control and maneuverability will improve as the CG moves aft towards the neutral point, especially on a flying wng. Remember to reduce elevator travel and sensitivity as you move the CG. Move it back, fly until you get the elevator travel/sensitivity trimmed and get used to it. Then, move it back some more, re-trim and fly. At some point you'll find the CG that suits.

Fiddling with the CG is the best thing I've ever done to improve the performance of my flying wings (a dozen so far).

Joe

[Arkitexas]

Thanks guys, I'm going to experiment with moving the CG back this weekend. Currently I use moderate down elevator when flying inverted. Also, when half-way through rolls I add down elevator to prevent losing too much altitude. I'll do the dive test as well as take close note of changes in control response.

Rick