Location of Axis of Rotation for Full Flying Stab

[Greywing]

What is the preferred location of the rotation axis when using a full flying stabilizer and how does the selected airfoil play into the determination? I am in the process of modifying a Kestrel 100" to include a flying stab and have used the TLAR method which is placing my axis at about 36% of root chord on an SD8020 foil. Is there some standard method that I can apply to this?

Thanks,
GW

[unabob]

for what it's worth, Andy Lennon's book on aircraft design recommends putting the pivot location at 25% of the mean aerodynamic chord. I just pulled it for quick reference but I didn't see any equations or reasoning behind this placement. I will dig a little deeper and see if it mentions anything else.

[jkettu]

If the pivot line is in front of aerodynamic center of pressure, the stab will tend to be blown to neutral, which is good to resist flutter. More distance between pivot line and aerodynamic center will require more servo torque to deflect, so it is a compromise.
Another flutter avoidance trick is to put the center of mass at or in front of pivot line. Usually this will require mass balance weights at leading edge and is seldom used in models.
Pivot line at 25% mac is a good rule of thumb. Depending on the geometry this could be close to 36% root chord of a tapered stab.

[gyrocptr]

for what it's worth, Andy Lennon's book on aircraft design recommends putting the pivot location at 25% of the mean aerodynamic chord. I just pulled it for quick reference but I didn't see any equations or reasoning behind this placement. I will dig a little deeper and see if it mentions anything else.
I understand that, if the pivot is along the line located at 25% of the mean aerodynamic chord of the full flying stab, the pitching moment (the torque required to move the stab) remains constant and does not change with angle of attack (deflection angle).

[HerkS]

In reality it's usually a little more complicated than just considering the pivot point. Most stab setups use two rods - you might call them the pivot and the drive. Together they form the elastic connection of the all moving stab to the airframe.

As a way to visualize this --- if the stab was conscious, it would have no way to tell which is the pivot rod.

It's really their combined effect that should be aimed at the neutral point of the surface. So if the pivot rod is aimed at the NP and the drive rod is forward of the pivot rod, the elastic connection to the airframe is ahead of the NP. Likewise if the drive rod is aft of the pivot rod then the effect is behind the NP.

When the elastic connection to the airframe is too far forward, flutter is the danger. Also the aerodynamic forces on the servo and linkage are somewhat higher. If the axis is behind the NP then divergence is the issue, the forces are reversed and unstable neutral is usually the problem --- unless there is to be very high speed. Then the stab can twist off (divergence)

The best possible arrangement is for the elastic combination of the rods to be aimed just a bit forward of the NP. So if the rods are the same size the line midway between them might be aimed at say 22 or 23% of the MAC. The reason to stay a bit forward is that in the real world the NP moves around a bit and you really don't want to get behind it. By connecting just ahead of it, the stab is essentially "stable."

Keep in mind that for most planes this is not much of an issue. The linkage stiffness and servo power is such that the phenomena are not usually noticeable. Only if there is a very flexible structure or control setup, and/or very high speed involved will you likely need to be concerned about this.

However I have seen both kinds of failure on sailplanes that have been pushed to high speed - flutter and divergence. It's ugly.

[Greywing]

Thanks for all the feedback. The axis I have chosen using the TLAR method turns out to be fairly close to the suggested 36% MAC, perhaps a little forward of that point. Fortunately for me any necessary adjustment should be fairly small.

Your responses have given me an appreciation for the dynamics of the pivot axis location. I suspect that in my application (100" RES woody) that the criticality of pivot location is somewhat diminished, albeit certainly not eliminated.

GW

Edit: Meant to say 25% MAC but 36% of root chord as suggested by jkettu.