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Building a full flying stab for my SU-27

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Old 04-06-2006, 10:59 PM
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Mods-R-Me
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Default Building a full flying stab for my SU-27

I'm modifying my AKM SU-27 to have a full flying stabs by securing a 5/16" dia aluminum rod into the stab root and changing pitch with a control horn & servo on each stab; essentially making them elevons. I'm powering this plane with a Jet 90L so speed should be in excess of 100mph.

My question is, what would be the best way of securing the aluminum rod inside the stab? Also, I'm still debating on whether I should sand the stab airfoils out of solid balsa sheet or by the traditional wing construction method (i.e. ribs, spars, LE, TE, etc). I've included a sketch that I got off another thread to give an idea of what I'm talking about.

I've never attempted this so please be as descriptive as possible.

Thanks a million!

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Old 04-07-2006, 01:50 AM
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Default RE: Building a full flying stab for my SU-27

Mods, it looks good.

the critical part will be the distance betwen the pivot pt (rod) & the servo actuator (> deflection rate) - do you have an idea?

also a build up stab would give you a better "access" to beef up critical stress points
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Old 04-07-2006, 12:00 PM
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Default RE: Building a full flying stab for my SU-27

Make sure your pivot tube is at 23-25% MAC (Mean Aerodynamic Chord). That drawing looks close (it actually looks a tad forward) but if you don't have it in the right spot it can cause issues like excess servo strain or worse yet flutter or control lock out.
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Old 04-07-2006, 02:30 PM
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Default RE: Building a full flying stab for my SU-27


ORIGINAL: Troy-RCU

Make sure your pivot tube is at 23-25% MAC (Mean Aerodynamic Chord). That drawing looks close (it actually looks a tad forward) but if you don't have it in the right spot it can cause issues like excess servo strain or worse yet flutter or control lock out.
Thanks, I've just bought Profili2 and it calculates all the masurements required. Great program, but does anyone have an idea of how to secure the metal rod into the stab without it comming out?

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Old 04-07-2006, 09:52 PM
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Default RE: Building a full flying stab for my SU-27

Flying stabs are very common with sailplanes. On most, the "axle" rod and "actuator" rod both slide into a metal tubing or other materialed receiver which is epoxied into the stab itself. This allows the stabs to be easily removed for transport. A slight bend in the rod (friction) holds the stab on. Sometimes beeswax or other material is applied to the rod to "sticky" them up, and even set screws have been used. So long as your stab is aerodynamically balanced, the stab should be less likely to flutter.

Also, don't deceive yourself and think this setup will not work for a powered plane also. Most full house thermal and slope sailplanes have very wide speed envelopes (a DS sailplane has set a speed record of over 300 MPH). Take a look at a sailplane setup too, before you commit to your design.
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Old 04-07-2006, 10:23 PM
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Default RE: Building a full flying stab for my SU-27

You could glue the rod into one stab half, and hold the other half on by using a screw, threaded into a piece of hardwood or ply that's glued into the rod...? I wouldn't trust a "friction fit" on an IC engine powered plane... those vibes will get you sooner or later!

I agree with Troy though... put your pivot point a little more to the rear... your servo is going to have a hard time if you build things like in that drawing.
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