Mirage stability

[Mirage1]

I just finished my Mirage kit and balanced it on the aft edge of the spar. 1st launch today, 0-5 winds and immediately started porpoising. I managed to get it into a turn to control this and found I was in a thermal and climbed to 1000 feet rather quickly. I decided to headinto the wind and back to the field and porpoising again. I deployed the spoilers and wow it settled down into a very stable descent and I made a nice landing. I have flown several planes before without this problem, Oly 2 ,gentle lady etc. Too much lead?

[cosmicpossum]

too much incidence?

[slopemeno]

Probably nose heavy/ too much decalage/ too much up trim. Spoilers usually drop the nose and require some up trim. Try adding some down trim and and hand launch it; better yet have a friend launch it so you can be on the sticks if it goes up or down.

I'm not familiar with the Mirage, but the Spirit really benefits from reducing decalage and nose weight.

[kostuk]

I built a MIrage stock from Mark Miller's short kit and did not have a porposing problem. It would'nt be a bad idea to check the declage and also do a dive test. Removing nose weight will help with thermalling.
Ray

[Mirage1]

I did deepen the curve in the fuselage sides so that the curve on the bottom of the wing fit better. The laser cut around the wing mount was not exactly the rib shape. Maybe in doing that I changed the angle of incidence and thus decalage. Since the wing is bolt mounted I could shim it and see if it flys better. It weighs 38 oz and I have 5 oz of lead in the nose.

[histarter]

I did deepen the curve in the fuselage sides so that the curve on the bottom of the wing fit better. The laser cut around the wing mount was not exactly the rib shape. Maybe in doing that I changed the angle of incidence and thus decalage. Since the wing is bolt mounted I could shim it and see if it flys better. It weighs 38 oz and I have 5 oz of lead in the nose.
Kick in a bit of down trim for a fast clean glide. Re-trim for slow flight by removing 1 ounce of nose weight (at a time) until slow un-rocky flight occurs. Only if down trim elevator angle equals about 5 degrees would I consider shimming the wing. :)

I cannot fly a Mirage below 6 oz/ft, and would suggest adding about 8 oz of ballast on the CG. :D

[Mark Miller]

Sounds like a decalage issue. I have not seen porpoising with the stock CG setup.

I have flown Mirages variants (mirage wing with a pod and boom fuselage) that weighed 27 ounces. No problems flying it at all. Flown True Mirages at 36 ounces and they flew well. I have added 28 ounces of ballast to one and guess what? It flew well. Gotta watch out for the speed though.

Mark Miller
Isthmus Models

[Mirage1]

It was a 1/16 shim under the trailing edge of the wing and what a difference. It flies perfectly . What a floater. Everybody at the field was impressed especially when I told them about the kit price. I came home and made a liquid gasket that is now drying to fill in the gap caused by the shim. Thanks to all for suggestions.