Changing Rigging ,Benefits and Drawbacks

The post on legality of rigging changes begs this question.

What are the reasons to alter the rigging?

What do you gain and what do you lose by changing washout,dihedral,wing angle of incidence?

I understand the aerodynamic principles but have not experimented with changing wing rigging.

I'm sure it's possible that not knowing what your doing could get you very dead and I have no intention of going there.
Bill

Rigging an Airplane is like tuning a suspension on a car. Just because a little is good doesn’t necessarily mean more is better.

The dihedral is for stability, if the wing is totally flat it will not be as stable as one that is rigged to spec. Would you be willing to sacrifice stability for maybe a fraction of a knot? You may pick up a knot if you set the dihedral to half of what it calls for, but it may be more unstable than you like it.

Rigging wash in the wing allows the outboard of the wing to stall later than the inboard, the reason for this is to allow for aileron control through out the stall. One reason a (properly rigged) cub stalls so nice is because of the aileron control you maintain through out the stall. I have seen cubs rigged with both extremes of wash and the cub has flown fine. Granted more wash (more then spec) means less cruise, but we are only talking fractions of knots unless you take to an extreme. Less wash on the extreme side would mean it could break to stall quicker.


Angle of incidence, is where piper located the wing attach points on the fuselage in relation to the horizontal reference line. This is not something you change when rigging the plane. It can only be changed with the welder. PA 12 have less angle of incidence than PA 18, that’s why they cruise faster but take longer to get off. So what some guys do is rig less wash (less then spec) in the PA12 to try to make it get off quicker.

Fabman,

With regard to your final statement about more washout. Don't you have that backwards? The twist negates AOI, it doesn't enhance it. To improve STOL performance one would reduce washout, I believe, which pretty much answers willyb's question.

Stewart

Thanks Fabman
It sounds like there is no real reason(no significant gain in performance) to alter rigging from the factory spec if the aircraft performs as it should.
Bill

Anbd a comment from the resident participant who cannot feel Cubs:
I am rigged to .7 degrees, and cannot discern any difference, either in stall or in cruise.

The Super D is rigged to zero - the factory says set it at zero, +/- .3 degrees. Stall is quite gentle, unless you intend a precision spin.

The only unintentional stalls I have ever done is at 6" altitude in a Micro-VG equipped Cub, and on that one, one wing quit before the other, and without warning. Net result was the landing was a few feet short of my intended spot. Only time I ever lost aileron control during slow flight/stall was with the non VG Charlie Center wing. That one was a real education, but with no real downside except to make us consider dumping flaps on touchdown.

Thanks Stewart, I'll change it so nobody gets confused like I was when I posted it.

Keeping the portion of the wing ahead of the ailerons flying to a higher AOA isn't the primary reason for wing wash.

The primary reason for wing wash is to ensure that the stall initiates and propagates in a predictable manner, and provides adequate warning of an impending stall.

The Super Decathlon is a VERY different kettle of fish--symmetrical wing, for one thing......

Eliminating the wash in the wing of a Cub tends to reduce or eliminate the characteristic aerodynamic buffet of the aircraft near the critical AOA. Note that most Super Cubs don't have stall warning horns. Back when the Cub was designed, it was assumed that the aerodynamic warning that the wing provides was sufficient and that no stall warning device was necessary.

Remove the wash entirely at your own peril. It's not dangerous in and of itself, but it does change the stall characteristics of the airplane.

Personally, I like the original Piper rigging.

MTV