Apparent fin incidence

[509sx]

I bought my Sonex #509 used. From the tail looking forward, it seems the fin is offset with the leading edge to port a couple of degrees. This might be appropriate if my engine was right-turning, but it's not. The plans appear to indicate zero fin incidence. Is the asymmetry only in the fin cap, with different caps for Jabiru vs. AeroVee? The fin root looks to be straight. Maybe my fin has some inadvertent twist? Is this common (difficult to avoid)? And is the only reasonable cure a ground-adjustable tab on the rudder?

On another note: My version of an in-flight canopy lock is just a 3/4" wide piece of angle aluminum that drops into the slot behind the aft sliding latch hook. I have drilled a hole in it so it stows on a short lanyard. FWIW
Art

[NWade]

Hi Art -

It’s worth noting that the fiberglass tail tips have been produced with varying levels of quality over the years, and I have seen several (including my first set) that had a twist between the root & tip of the fiberglass.

When I contacted Sonex support their response was basically “eh, it’ll fly OK anyways. You won’t notice it in the air, quit worrying”.

I bought a fresh tip after a clever idea I had went wrong and I found that the replacement tip - while still not perfectly symmetrical - has almost no twist.

So long story short: Don’t assume the whole fin is twisted based just on what you see at the fiberglass tip (either the leading edge or how the trailing edge lines up with your rudder).

Take care,

—Noel
Sonex #1339 nearing first flight

[XenosN42]

I bought my Sonex #509 used. From the tail looking forward, it seems the fin is offset with the leading edge to port a couple of degrees. This might be appropriate if my engine was right-turning, but it's not. The plans appear to indicate zero fin incidence. Is the asymmetry only in the fin cap, with different caps for Jabiru vs. AeroVee? The fin root looks to be straight. Maybe my fin has some inadvertent twist? Is this common (difficult to avoid)? And is the only reasonable cure a ground-adjustable tab on the rudder?

On another note: My version of an in-flight canopy lock is just a 3/4" wide piece of angle aluminum that drops into the slot behind the aft sliding latch hook. I have drilled a hole in it so it stows on a short lanyard. FWIW
Art

Does it fly straight at cruise RPM without undue rudder input?

[509sx]

Difficult to say, since I have been spending my flying time in the pattern trying to wire takeoffs and landings. In the delivery flight, however, it appeared to me that the airplane flew in a minor slip, with one wing down slightly. Can't remember which one, though. I just went thru an expensive, but extensive annual condition inspection, in which we corrected several rigging issues, so I'm confident the rigging is correct, or close to it. I will need to make more acute observations next time I'm in cruise mode in smooth air.

[gammaxy]

I assume you have the standard tail Sonex. Are you saying the entire metal vertical stabilizer appears to be pointed left? I'd think you could use your eye to sight along a row of rivets on the left and right skin to get a feel for whether there is significant misalignment. If the aluminum structure is twisted, pulling a string diagonally across the stabilizer might help find which side is twisted concave. I think the tail is pretty easy to build flat, though, and your problem is most likely just the fiberglass tip. I'd think it would have to be pretty obviously misaligned to make much difference since it's comparatively small and unlike a trim tab doesn't apply leverage across an entire control surface.

If built perfectly and flown single pilot from the left seat, the Sonex will be a bit out of trim. Mine has always had a little tendency to roll left that goes away if I fly with a similar sized passenger or sit near the middle. You can adjust this a little by adjusting the rigging of the flaps, but since flaps are ineffective ailerons, it seems to take a lot of adjustment to make much difference. A small trim tab on the aileron might be all you need, but you might find you don't need it with a passenger on board, so some make it removeable. Mine also requires a touch of rudder to center the ball in cruise and more pressure when climbing after takeoff.

I'm a little curious about why you needed one wing down slightly. I'd think as long as you were flying straight and had the ball centered, your wings would be level, assuming the ball is installed level with the wings (and level with the fuselage). Mine is the same way if I don't apply a touch of rudder to center the ball.

What you're describing could be fairly normal and you just have to decide whether you want to resolve it by adding trim tabs and/or tweaking the flap rigging a little from the spec.

[509sx]

Thanks, guys! I will have to take a more careful look at the fin & fin cap next time I'm out there at the airport. My Corvair engine, if I take 3000 rpm as a typical cruise value, produces somewhere around 150 ft-lbf of torque. Since the prop is left-turning, this should come close to countering the roll asymmetry due to my offset weight flying solo. The fact that the ball doesn't seem to center on its own seems strange. The previous owner tabbed the left aileron, so I have that to play with. As I said, the fin looks straight at the root. I will do some investigating as y'all have suggested.