Adding chord to an SK rudder and fin

[facthunter]

Rudder effectiveness at slower speeds is what you're after and the extra area should achieve something there.. To load a fin and rudder you have to have some force opposing it. You still have the same fuselage keel surface area and effect and you aren't getting any greater sideslip angle. Yaw inertia can do this in big stuff but I doubt it's much of a consideration in a Jabiru. You would have made some stability changes. It might be more inclined to spiral slightly but I doubt that will bother you.. Perhaps the high speed considerations might be able to load more, but why would you be applying rudder at high speed? You will have some extra drag due to increased area. Nev

 

 

[Bruce Tuncks]

Yes Nev, at high speed ( 100knots) the only consideration I have ever had is keeping the ball centered.

 

I like your comment about the keel area being the same so the loadings have the same reaction.

 

Yaw inertia is something I have never tried, do you mean using the rudder to swing the nose as much as possible by timing the rudder pushes to give maximum effect?

 

 

[facthunter]

Kicking it from full deflection one side to the other. That took the fin off an Airbus in the USA. You couldn't replicate that effect in a Jabiru. You might get a high load reading in a dust devil where you get large rapid wind velocity changes. (That includes direction). Nev

 

 

[Oscar]

Kicking it from full deflection one side to the other. That took the fin off an Airbus in the USA. You couldn't replicate that effect in a Jabiru. You might get a high load reading in a dust devil where you get large rapid wind velocity changes. (That includes direction). Nev

I may be wrong here - Bruce will, I would hope, correct me if I am - but the actual rudder appears to me to be unchanged. That means that the CONTROL surface area and reaction loading to the fuselage remains the same (and the fin/fuselage connection loads, which for reasons I do not care to enter, need to be considered - I have discussed those concerns privately with Bruce) remain unchanged.

 

This is a good thing from a structural POV; start changing the loads into an airframe and you very, very much need an experienced Aero Engineer to tell you if you are acting wisely or putting a round into the chamber of a revolver you will be placing at your temple and pulling the trigger..

 

The increased area of the FIN will increase the horizontal stability relative to the apparent wind across its surface - much as larger feathers make an arrow fly straighter relative to the wind. That may - or may not be, depending upon the cross-wind component to be battled for desired heading maintenance - useful.

 

I would expect that for most conditions, a bit more horizontal stability would make life easier in cruise. Where there is a need for more rudder power, however - this mod. seems to me to offer little if anything. I do NOT mean that to be pejorative - about 95% or more, I suspect, of power flying is A-to-B straight line stuff where stability = comfortable, low-energy flying.

 

Glider pilots (of whom Bruce is one) enjoy the benefits of a high level of control effectiveness. Rudder power is essential for anti-spin-entry control in thermalling; serious levels of side-slip control is useful for outlandings in tight conditions.

 

I would expect that the mod. Bruce has made, will make general flying in his wee Jabby somewhat more comfortable, at the possible expense of an almost imperceptible reduction in absolute rudder authority for cross-wind conditions. Given that balancing direction in a side-slip between rudder and aileron control becomes increasingly a tap-dance as one gets closer to the ground, I personally reckon Bruce's mod. is likely to be time-effective as a 'make my trips more enjoyable'..

 

That DOES NOT mean that simply taking a trowel and a heap of foam and 'glass to your LSA55, is desirable...or necessarily, safe. A careful reading of the entirety of this thread should be your first recourse.

 

 

[Bruce Tuncks]

I reckon you are right Oscar. There is a plan "B" to increase the rudder area if needed, but since flying with the mod, I don't think it necessary.

 

The weight and balance was surprisingly improved by the exercise. There are SK Jabirus around with lead in the tails.

 

The fin mod actually puts the c of g closer to the center of the range.