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Author Topic: Sig Twister rudder offset?  (Read 1239 times)

Offline James Holford

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Sig Twister rudder offset?
« on: April 14, 2018, 10:56:12 AM »
Plans shows rudder offset for the twister.

 Should I do as plans and just offset rudder?

Or

Should I offest entire vertical Stab with a tiny offset??

Or

Shall I just put it square and just a degree of engine offset?



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Offline Tim Wescott

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Re: Sig Twister rudder offset?
« Reply #1 on: April 14, 2018, 11:59:21 AM »
If you're going with adjustable leadouts, put in just enough offset that you're sure it's not pointing in, and then spend some time trimming.  On a profile I kind of alternate between doing my best to get them dead straight forward, and putting in about 1/4 to 1/2 a degree offset (that's about a 1/32 kick-out at the back), When I mount them straight it's because the upside of a whippy profile fuselage is that with a heat gun you can correct for mild warps in the rear fuselage.

If you're not going with adjustable leadouts I have neither experience or advice to offer (except that it's pretty easy to add adjustable leadouts before the covering goes on.
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Offline Bootlegger

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Re: Sig Twister rudder offset?
« Reply #2 on: April 14, 2018, 12:20:30 PM »

  I am with Tim, just enough to know that there is no inset.
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Offline James Holford

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Re: Sig Twister rudder offset?
« Reply #3 on: April 14, 2018, 01:00:19 PM »
Thanks.

 It does have adjustable leadouts installed.

 Wasnt sure wether to set entire stab or just the rudder itself.

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Offline wwwarbird

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Re: Sig Twister rudder offset?
« Reply #4 on: April 14, 2018, 03:37:04 PM »

 James,

 Just wondering, when you say "stab" are you referring to your horizontal stabilizer or the vertical fin (ahead of the rudder)? "Stab" is typically a short nickname for the horizontal stabilizer, and you would never want any left/right offset when mounting it.
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Offline Dane Martin

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Re: Sig Twister rudder offset?
« Reply #5 on: April 14, 2018, 06:57:26 PM »
Agreed. Glue it all on straight. No offset anywhere.

Offline Brett Buck

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Re: Sig Twister rudder offset?
« Reply #6 on: April 14, 2018, 07:18:30 PM »
Shall I just put it square and just a degree of engine offset?

   I think you should make it (ground) adjustable, and then adjust it to be straight ahead or *just barely to the right* (maybe 1/32" at the trailing edge) to start with, mostly just to cover the possibility that something else is a little off. Then adjust from there, my suggested procedure is in the archives.

    Brett

Offline Glen Wearden

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Re: Sig Twister rudder offset?
« Reply #7 on: July 17, 2018, 11:16:25 AM »
On my first Twister, I got the idea of shaping the fin and rudder into an airfoil, like the Nobler.  I removed the stick up thing on top of the rudder, first.  No offset.  It seemed to work quite well, but the plane met its demise due to an up command to come out of inverted flight.  That'll ruin a nice flight every time.  Live and learn.
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Offline James Holford

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Re: Sig Twister rudder offset?
« Reply #8 on: July 17, 2018, 09:12:06 PM »
There is an ever so slight touch of offset...wasnt even going to put any but just so happened when I glued it on.

 

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Offline Fredvon4

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Re: Sig Twister rudder offset?
« Reply #9 on: July 18, 2018, 12:00:40 PM »
For profile like this I like to form up the VERTICAL FIN and Rudder in line...set the assy on the Fuselage zero zero and scribe a line around the mating parts...then glue it down slightly offset to the outside ...I personally believe perfectly (perpendicular) aligned Up and down is more important while ensuring no INBOARD bias.....

Also if you only glue down the vertical fin and leave the RUDDER free,,,you can add a bit of outside rudder (or subtract some) Then later glue it solid...I, when trimming, have used small Map PINS to force some extra rudder off set and also fussed with engine offset until I found a lead out rake, Rudder, engine alignment that felt right

But what do I know, many of my planes have trim tabs....grin

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Offline Brett Buck

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Re: Sig Twister rudder offset?
« Reply #10 on: July 20, 2018, 08:44:59 AM »
I cannot emphasize enough that you need to be able to adjust it, in extremely fine increments. Particularly so on a Twister, because the fin/rudder alignment has a very powerful effect. We have made adjustments as small as cardboard glow plug package, bent in half, taped to the fin, and offset 1/8", that had remarkable effect on the way the airplane flew (positively, as it turns out). That corresponds to, eyeballed, a few thousands of an inch of movement of the entire "rudder" section.

      The design of the airplane makes it a very critical adjustment. The fat square wing, with no taper and flat-plate wingtips, is extremely sensitive to the yaw angle. The yaw axis of the airplane is extremely stable, with a pretty skinny fuselage with a large relatively high aspect ratio  fin/rudder, so it's like a dart. That alone is going to make it fly at a yaw angle set by the fin/rudder, and react very poorly to trying to force it out of that angle. You need that because the wing isn't going to tolerate yawing around, either.

     As an aside, airfoiling one side of the fin/rudder is completely indistinguishable and has exactly the same effect as offsetting the rudder. Given that it's (as I recall) 1/8" thick makes it a pretty small effect, but much larger than I think the tolerance of an equivalent adjustment would be. It's just another form of rudder offset, but again, *unadjustable*. It is a lot better than using the specified offset of something like 3/4" but still far larger than what I think you can tolerate.

    To maybe visualise what I mean, compare the Twister to its stablemate, the Banshee. Almost the same airplane, with two crucial differences - the yaw stability of the Banshee is very weak, close to neutral, as opposed to extremely stable in yaw like the Twister. And the banshee has pointed wingtips that reduce the effects of yaw angle on the roll axis by tapering off the lift towards the tips, as opposed to the Twister, , which just ends, and has a disproportionate amount of lift right around the tips. Even slight yaw angle alters the flow around the Twister tips radically, causing it to roll with even the slightest yaw offset.   The Twister is known to be touchy to yaw/roll trim, the Banshee is much less touchy, it almost doesn't care at what yaw angle it flies. Compare the Banshee to a Max Bee; same effect, although it's almost certainly accidental on the Banshee  - and is definitely NOT accidental on the Max Bee.

  For this type of airplane (which in this respect is pretty close to my real airplane that I have been trimming on for 25 years in various forms), the rudder adjustment is extremely critical and get it even a little off, and you will have great difficulty getting it to fly cleanly with consistent line tension.

    BTW, if you have a built-in offset with no adjuster, here's a tip (used by the current and many-time national champ, and invented by the first ever 4-time Hurl champion buddy and trim advisor) - everything is adjustable as long as you have a saw and some glue. Take a Zona saw, cut a saw kerf along where the rudder hinge line would be, on the side you want , and most of the way through to the other side. Stop short of cutting it all the way off. Bend it open, put some medium Hot Stuff in the crack, then bend it closed. Hold until set, rudder is adjusted. IF you take a look at David's 1997 NATs winner, you will see a series (as I recall, at least 3 and possibly 4) of saw cuts with glue lines on the inboard side of the "unadjustable" fin/rudder that have the effect of removing most of, if not all, of the rudder offset. This was done on successive test flights, and tested by how well it got through the overhead 8s.

     Brett


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