Adjusting HS incidence?

[sf3543]

As a point of interest..
I had a similar issue on an RV8 I built.
I noticed two things, the counter weights were up about a half inch in level flight and the plane felt like it was behind the power curve on approach to landing. Basically felt too tail low and I had to keep the speed up a bit.
I shimmed up the leading edge of the HS by about 3/16" and it totally changed everything.
The elevators came in perfect trail at cruise and it handled much better on approach to landing.
CG loading will affect this however, so if you make adjustments, do it to match your usual flight configuration.
I also had to rework the empennage fairing to fit the change, but during testing I just taped it on at the leading edge. The rework was minor and limited to the front of the fairing only.

[sblack]

Originally Posted by sf3543

As a point of interest..
I had a similar issue on an RV8 I built.
I noticed two things, the counter weights were up about a half inch in level flight and the plane felt like it was behind the power curve on approach to landing. Basically felt too tail low and I had to keep the speed up a bit.
I shimmed up the leading edge of the HS by about 3/16" and it totally changed everything.
.

There is no reason why stab incidence would change anything except the trim. Being behind the power curve is a function of the basic L/D of the airframe and the engine thrust vs speed. Stab incidence won't change that at all. So you must have changed something else, or you evaluated the approach after the mod at a different speed or something. The drag difference you would get would be fractions of a percent.

[merlin3]

Changing the stab incidence on my 4 gave a similar response, not only did it get the elevator closer to being in trail, but i also used to run out of nose down trim. Also after shimming the stabilizer it changed stall characteristics to where it used to break pretty rough and after it would just mush and usually wouldnt break at all.

[scsmith]

Originally Posted by sblack

There is no reason why stab incidence would change anything except the trim. Being behind the power curve is a function of the basic L/D of the airframe and the engine thrust vs speed. Stab incidence won't change that at all. So you must have changed something else, or you evaluated the approach after the mod at a different speed or something. The drag difference you would get would be fractions of a percent.

Scott: Please don't confuse basic static stability (stick fixed) with stick-free stability AKA stick-force gradient.

It is true that changing the decalage will not change the static stability.

It is false that it does not effect stick gradient. The stick-free stability is influenced by the change in elevator hinge moment with change in speed. The change in hinge moment with speed is influenced by the natural float position of the elevator when trimmed, which is strongly influenced by the trim-tab position.

As I think through it right now, I think leading-edge up on the horizontal stabilizer is the 'good' direction, that is, it is the change that should increase stick-force gradient, not reduce it. But I could easily be thinking about it incorrectly right now. In any case, the incidence DOES change the stick force gradients.

[sblack]

Originally Posted by scsmith

Scott: Please don't confuse basic static stability (stick fixed) with stick-free stability AKA stick-force gradient.

It is true that changing the decalage will not change the static stability.

It is false that it does not effect stick gradient. The stick-free stability is influenced by the change in elevator hinge moment with change in speed. The change in hinge moment with speed is influenced by the natural float position of the elevator when trimmed, which is strongly influenced by the trim-tab position.

As I think through it right now, I think leading-edge up on the horizontal stabilizer is the 'good' direction, that is, it is the change that should increase stick-force gradient, not reduce it. But I could easily be thinking about it incorrectly right now. In any case, the incidence DOES change the stick force gradients.

In the hinge moment wind tunnel testing I have seen, the Hm is a reasonably linear function of surface deflection, be it elevator or tab, and surface aoa, with limits. To change that gradient we play with beads, tabs, wedges and other toys to tune the forces. I don't recall surface aoa, which in this case would be the same as tail incidence, having an impact on gradient. On the airplanes I've worked on, you trim by moving the stab. So if changing incidence changed the gradient you would have a different force gradient everytime you retrimmed the airplane. In this case we are talking about tiny chages to the incidence, say a degree or 2.

In any case, changing incidence would certainly not stop the airplane from being on the back side of the power curve at a given speed, as was being claimed.

[scsmith]

Originally Posted by sblack

In the hinge moment wind tunnel testing I have seen, the Hm is a reasonably linear function of surface deflection, be it elevator or tab, and surface aoa, with limits. To change that gradient we play with beads, tabs, wedges and other toys to tune the forces. I don't recall surface aoa, which in this case would be the same as tail incidence, having an impact on gradient. On the airplanes I've worked on, you trim by moving the stab. So if changing incidence changed the gradient you would have a different force gradient everytime you retrimmed the airplane. In this case we are talking about tiny chages to the incidence, say a degree or 2.

In any case, changing incidence would certainly not stop the airplane from being on the back side of the power curve at a given speed, as was being claimed.

To the second point, yes of course you are right, the incidence has nothing to do with being on the back side of the power curve. I think the comment was intended to convey a subjective stick feeling. A reversed stick force gradient feels like the slower you go, the more you have to push forward - as if you are fighting a tendency to slow down more. I think you can see the parallel notion to a case where the slower you go, the more power you have to add.


In the case of trimming by moving the stab, you are correct, the stick force gradient doesn't change. The elevator deflection angle for zero moment does change (which is why it works to "trim" the airplane). The deflection angle for zero moment is affected by the airplane angle of attack and the stabilizer angle.

In the case of trimming the airplane by moving a trim tab on the elevator, the elevator deflection angle for zero moment is affected by the angle of attack, the stabilizer angle, and the trim-tab angle. But also, because of the competing contributions to the hinge moment from the elevator and the trim tab, the hinge-moment gradient also changes. Please refer to Perkins and Hage Aircraft Performance, Stability and Control, section 6.7. Adjusting c.g. and/or adjusting stabilizer angle so that nose-up trim tab deflection is required for trim causes stable stick-force gradients (see fig. 6-18 and 6-19)

The upshot of all this is that increasing stabilizer incidence (leading edge up) will increase the amount of nose-up trim-tab deflection, and will improve stick force gradient at low speed. So to the OP, if your elevator horns are sticking up a lot at high speed, I think you are going in the right direction if you want to raise the leading edge of the stabilizer a little -- it will help, not hurt, the stick-force gradients.

[snoop9erdog]

Ok so it seems that this may be something to consider if building a 9 and potentially with other models?

What are other 9 drivers seeing? Seeing the same thing and effect at cruise along with trim effects at slow speed?

If that is the case, Why don't the plans and instructions call out an increased leading edge Horizontal stab incidence or shim? Or is it just a minor nuisance/preference?

I'm at a perfect place in the build to modify this, however, I think it unnecessary to change OEM design.

Thoughts?

[rvbuilder2002]

Originally Posted by snoop9erdog

If that is the case, Why don't the plans and instructions call out an increased leading edge Horizontal stab incidence or shim? Or is it just a minor nuisance/preference?

Because the elevator horns being a large amount out of alignment is not a common issue, but as mentioned previously, a small mount is specifically designed in and expected.

These are airplanes that are the majority of the time hand built built by amateur builders. This induces the chance of numerous minor variations that can have an influence on the flight specifics of each individual airplane.

Because of this, it would be foolish to make a change in H. stab incidence while building.