Posted Friday, January 5, 2007 @ 03:23 AM
My wing-tip aerobatic sighting rods (see photo of my airplane on page: http://www.cfiamerica.com/id4world.html) were inspired by those on the Extra 300L (although those protrude rearwards). They are designed to precisely indicate the Fournier’s zero-lift axis (look it up) for aerobatics.
They are each made from 4 feet of quarter-inch soft aluminum tubing. No, I don’t know the spec; it was a ten-foot length of old, corroded tube I found with a load of other junk in my hangar when I bought it. It might have been brake line, or pitot/static tubing, I don’t know. The tube has to be fairly soft, so it bends without breaking anything important when it’s hit by an inanimate object (like a spectator) but stiff enough not to bend under 6g. Mine are exactly right, but will not withstand a head-on collision with a hangar door. Ask me how I know that.
I cut off and straightened two 4-foot (ish) lengths, and painted them red and white in the hopes people would not walk into them (fat chance, I can’t count how many times I’ve had to straighten them). They are each pushed into three standard quarter-inch P-clips, which are simply screwed into the underside of the solid balsa wood wing tips.
Alignment: the rods should be aligned fore-and-aft. You can do this by judicious use of calibrated eye and thumb, or by measurement from the fuselage sides where they are parallel. That alignment is not vital.
The important alignment is in the up-and-down plane. Much experimentation over a whole year with vertical rolls, tailslides and the like eventually led to the slow realization that these rods are actually in line with the sloping forward fuselage underside just behind the firewall. That’s the line along where the underside fiberglass side cheeks screw on. Stand well back from the wing-tip, close one eye, check the alignment, then step forward and bend to adjust as appropriate.
I know the result makes them look impossibly pointing upwards, but believe me, that’s not far out. You can finalize alignment as I did by flying a succession of vertical quarter rolls and checking which wing is low after 90 degrees of rotation.
If you use this mod, you owe me a beer (or several) because it should save you a whole year of heartbreak and frustration. Ask me how I know THAT.
Good luck, have fun, and I take no responsibility for whether or not this works for you. I can only say it works for me.
Happy Fournicating, Bob
[Edit by Bob Grimstead on Saturday, January 6, 2007 @ 08:22 AM]
