Aft CG in Tailwheel

[Richard]

I'm still wondering why we seemingly ran out of rudder on a landing the other day. Landing rwy 31, winds 330 8G12 me in the back. Would an aft CG affect capability of rudder to counter weathervaning in a tailwheel?

Post flight ground check revealed rudder is deflecting to full 20 degrees as designed and pedals were not blocked during the landing.

 

[Dave Krall CFII]

I'm still wondering why we seemingly ran out of rudder on a landing the other day. Landing rwy 31, winds 330 8G12 me in the back. Would an aft CG affect capability of rudder to counter weathervaning in a tailwheel?

It sure would. Increased moment arm for the same rudder to move weight.

 

[Ed Guthrie]

I'm still wondering why we seemingly ran out of rudder on a landing the other day. Landing rwy 31, winds 330 8G12 me in the back. Would an aft CG affect capability of rudder to counter weathervaning in a tailwheel?

It sure would. Increased moment arm for the same rudder to move weight.

First, when the CG moves aft the rudder force moment arm decreases (rudder to CG distance decreases). Second, weathervaning is a ground handling issue (rotation about a ground reference point). When on the ground the pivot point is at the main landing gear to ground contact point, not the CG. The rudder to main landing gear distance does not change as a function of CG movement. IOW, the moment arm that matters to weathervaning does not change as a function of CG.

 

[Dave Krall CFII]

I'm still wondering why we seemingly ran out of rudder on a landing the other day. Landing rwy 31, winds 330 8G12 me in the back. Would an aft CG affect capability of rudder to counter weathervaning in a tailwheel?

Post flight ground check revealed rudder is deflecting to full 20 degrees as designed and pedals were not blocked during the landing.

Assuming the tailwheel is swiveling properly ? Also, see further discussion with Ed below...

 

[Dave Krall CFII]

First, when the CG moves aft the rudder force moment arm decreases (rudder to CG distance decreases). Second, weathervaning is a ground handling issue (rotation about a ground reference point). When on the ground the pivot point is at the main landing gear to ground contact point, not the CG. The rudder to main landing gear distance does not change as a function of CG movement. IOW, the moment arm that matters to weathervaning does not change as a function of CG.

Thanks for your comments Ed.
It seems like if there's more weight towards the rudder, it's not going to be as effective in pivoting that weight around the pivot point, which with main wheels rolling would be pivoting on the mains and while airborne would be pivoting somewhere between the CG and aerodynamic center.

Either way, putting more weight closer to the rudder would decrease the lever arm distance between the rudder and the weight being moved, no ?

In any case, it seems like the wind vector was too small in this case to have put enough force on the aircraft to cause this problem. Something doesn't seem right.

 

[Greg Bockelman]

Thanks for your comments Ed.
It seems like if there's more weight towards the rudder, it's not going to be as effective in pivoting that weight around the pivot point, which with main wheels rolling would be pivoting on the mains and while airborne would be pivoting somewhere between the CG and aerodynamic center.

Either way, putting more weight closer to the rudder would decrease the lever arm distance between the rudder and the weight being moved, no ?

I think you guys are over analysing this. I think it is an inertia issue. The more weight you have at the end of the arm, the harder it is to change directions. The arm in this case being the distance from the pivot point (main gear) and the CG.

In any case, it seems like the wind vector was too small in this case to have put enough force on the aircraft to cause this problem. Something doesn't seem right.

 

[Ed Guthrie]

I think you guys are over analysing this.

Not possible--my couch isn't large enough. ;-)

I think it is an inertia issue. The more weight you have at the end of the arm, the harder it is to change directions. The arm in this case being the distance from the pivot point (main gear) and the CG.

In that case the response time would increase but the response would still happen. "Run out of rudder" to me implies it just wouldn't respond, period. But then again, I'm probably over analyzing this. ;-)

 

[Greg Bockelman]

In that case the response time would increase but the response would still happen.

That's true, but if the response was too late, or if the inertia was enough to overcome the response, you would still have basically the same result.

"Run out of rudder" to me implies it just wouldn't respond, period.

What it means to me is that the swerve or crosswind or whatever was enough to overcome the response, not that it wouldn't respond. Punch the rudder the other way and see what happens. It would get very intersting very fast. That isn't a sign of no response in my opinion.

But then again, I'm probably over analyzing this. ;-)

LOL I suppose over analyzing is what we do best on these boards.

 

[gismo]

I think you guys are over analysing this. I think it is an inertia issue. The more weight you have at the end of the arm, the harder it is to change directions. The arm in this case being the distance from the pivot point (main gear) and the CG.

The original post was about "weathervaning" and it is true that once the mains are on the ground, CG should have no bearing on the rudder's ability to counteract a crosswind. CG does. however, impact the dynamics of lateral control during a rollout, crosswind or no crosswind. The further aft the CG is, the stronger the tendency for a small swerve to develop into a large swerve and in such a case it is indeed possible to "run out of rudder" when attempting to correct, especially if the tailwheel has come out of the detent.