john491

My Feedback: (13)

I have searched extensively and not found the answer to the preferred location of the pivot point for a flying stab. It seems that everything is an ARF and that decision has already been made by someone else.
I have assumed that the pivot point should be forward of the 50% location on the stab. But by how much? I went with a TLAR location of 42% which is 25mm forward of the 50% location, but I can still move it.
What are everyone's thoughts on the matter?
John

Halcyon66

Flying stab pivot point

https://www.rcgroups.com/forums/show...ll-flying-stab

Some forum posts to review.

Regards,

speedracerntrixie

My Feedback: (29)

I’ve only designed sailplanes with a flying stab and did the same as you where I went slightly forward of where the stab would statically balance. My thought was that I didn’t want it perfectly balanced as that could induce hunting. I had no issues but these were slow flying. I’m interested in what other guys have to say on this as well.

Halcyon66

There has been a lot of conversations about this for years, you are not just dealing with the aerodynamic centre you are also dealing with the centre of pressure, most RC stabs are symmetrical so the lift feedback is not involved.

The AC point is fixed and the CP moves along the surface towards the trailing edge as the lift increases on a non symmetrical airfoil.

This topic also brings in mass balance of the stab and again this is based on where the actual pivot point is. And again that topic has been talked to death for years.

There is normally a lot of math's involved with all the above and yet going slight forward of the MAC is normally sufficient for most RC applications.

Regards,

Ron S

My Feedback: (2)

Go to the ama website, pdf documents, and pull up the document for over 55 lb models. For LMA paperwork… in there is a great explanation on how to set up the pivot point.

id pull up the document for you, but I’m driving 🤓

Halcyon66

Academy of Model Aeronautics (modelaircraft.org)

Page 23-24.

Regards,

john491

My Feedback: (13)

Thanks for the assistance. Now to do a little math.
John

john491

My Feedback: (13)

Turns out after doing the math three times and drawing it all out on a large tile that my TLAR was within parameters. The actual pivot point of my stab calculates as 20% so I should be good to go.

Thanks again for the help.
John

Joseph Frost

On some of my full moving stab jets I add on some lead weight where ever needed to balance them on the axis pivot point to take some extra load of the servos. Simple fix from inside cord of the stabs.

David Gladwin

[QUOTE=Halcyon66

The AC point is fixed and the CP moves along the surface towards the trailing edge as the lift increases on a non symmetrical airfoil.



Regards,[/QUOTE]
No it does not, it moves forward towards the LEADING edge ! ( at least at subsonic speeds !! )