Sonex Rudder Trim Tab

[skippydiesel]

I have come to the conclusion that I should not have to maintain light pressure on the right rudder peddle to keep the ball centered in cruise.

 

Having done all sorts of flap, aileron tweaks, the only option (that I can see) is to add a small trim tab to the rudder.

 

The question now are;

 

• I would like the tab to be as small as possible consistent with effect. -  What dimensions deliver the greatest effect in the smallest size?
• Where on the rudder should it be placed for best effect?

 

I am aware that a discreet triangular cross section form, mounted along the trailing edge, is all that some aircraft require to achieve the desired effect. Others have small spade like projections (usually bendable). Any others?

[Blueadventures]

I'd be asking on the Sonex forum as many experienced persons there; as well.

[skippydiesel]

2 hours ago, Blueadventures said:

I'd be asking on the Sonex forum as many experienced persons there; as well.

Done!

[facthunter]

Consider shimming your engine thrust line.... Nev

[skippydiesel]

1 hour ago, facthunter said:

Consider shimming your engine thrust line.... Nev

Much easier to stick a nice little trim tab to the rudder.

[facthunter]

Yes but why not cure the problem rather than cover it up? The rear Tab works in airflow with loss of boundary layer control.  Many engines are mounted  off axis thrust. especially U/L's. How's the trim in a glide or climb?    Nev

[skippydiesel]

This aircraft is in its early test flying stage - my son & I are slowly working through all its (mainly) small "teething" problems.

 

Changing the engine alignment, is no small fix and I am not even sure that it is possible without a major rebuild.

 

The aircraft, a Sonex Legacy (A), has been meticulously built according to Mr Monnet's plans - there has been some builder "customisation" but engine alignment was not one of them.

 

The need for right rudder, in cruise, is quite slight/subtle with the "ball" almost aligning without it - a small helper trim tab may be all that is needed.

 

I have temporarily fixed  (double sided tape) a small aluminium extrusion, to the rudder trailing edge , at the mid point - the next test flight will give me some idea of its effectiveness or not.

[Kiwi]

Get some of the 6mm door molding trim about 75mm long and stick it in the thrust line at the trailing edge on the side you are pushing the rudder. If to much try a shorter piece,  not enough make it longer. 

Edited January 14, 2023 by Kiwi

[skippydiesel]

19 minutes ago, Kiwi said:

Get some of the 6mm door molding trim about 75mm long and stick it in the thrust line on the side you are pushing the rudder. If to much try a shorter piece,  not enough make it longer. 

Neat idea!

 

Might be a bit heavy if a long 40mm strip is required.

 

The aluminium molding/extrusion I have already stuck  (double sided tape & white gap tape )in place but not tested, is curve ( . I toyed with he idea of fixing it convex out (as per your door molding) but opted in the end to try concave out .  Its 200x30x1.5mm, secured at the mid point on the rudder trailing edge.

Edited January 14, 2023 by skippydiesel

[facthunter]

An adjustable spring bias will do it. Replaces what your hand does. Jabiru and DH82 pitch are done that way (and probably many others. ). 

[skippydiesel]

1 hour ago, facthunter said:

An adjustable spring bias will do it. Replaces what your hand does. Jabiru and DH82 pitch are done that way (and probably many others. ). 

Thanks Nev - Yes I did think of that system but felt that at slow speeds (landing, etc ) the fixed trim tab would likely be not as effective, where a spring system would be constant (unless there was a a pilot trim adjust) and unhelpful at low air speed/taxying. I have always felt that these little planes should be subject to the KISS principal, unfortunately the urge to get more sophisticated/complex is overwhelming. 

[facthunter]

Once the propwash is reduced after landing the whole rudder becomes quite  ineffective and you have another thing entirely if taxiing downwind. . With springs or rubber bands it can be internal and protected from any damage by rocks etc.   Nev

[Blueadventures]

2 hours ago, facthunter said:

An adjustable spring bias will do it. Replaces what your hand does. Jabiru and DH82 pitch are done that way (and probably many others. ). 

Agree, the Nynja and the skyranger models have an internal rudder centering bungee that you adjust for feel in flight.  Ground adjustable only.

[skippydiesel]

I am not against the idea, just want to give the little trim tab a go first.

 

In fact when we have completed all the adjustments (for now) I will probably fit her with a pilot adjustable aileron bungee/spring, to counter the weight of a passenger. I had this on my last aircraft - worked a treat - its a one way system ie it will only work to "lift" the right wing.

 

[facthunter]

That is more likely to respond  to aileron trim. When you have all 3 trims it's a bit tricky to get rudder and aileron correct. The cruise. climb and reduced or power descent change too. Of course cruise is the most important because you spend the majority of your time in it. Nev

[Blueadventures]

28 minutes ago, facthunter said:

That is more likely to respond  to aileron trim. When you have all 3 trims it's a bit tricky to get rudder and aileron correct. The cruise. climb and reduced or power descent change too. Of course cruise is the most important because you spend the majority of your time in it. Nev

Agree, I find the rudder certering needs only about 3mm adjustment to get right once in the target area.  10mm out or more and need heavy rudder foot pressure.  Been thinking of a mini screw jack type adjuster for attaching the front bungee connection so I can do fine adjustments.  With recent delivery flight of a Nynja need slight left foot pressure and aileron trim was not good enough to sort it to a happy place.

[skippydiesel]

Test report - Trim worked a little too well. Have removed it , cut in 1/2. Will re  fit and try again.

[skippydiesel]

Mk2 works well BUT as you experienced builder/pilots will know, an adjustment to one control surface will impact on another;

Now have a left wing drop - did have the left flap up as far as it would go, to correct an earlier right wing drop. Have now turned left wing flap down by 1/2 turn of control rod end. Hope to test fly, in the cool of the early morning, tomorrow.