Constant Airspeed for maximum flight endurance

The plane in Auto or Guided mode on a long straight mission is set to have a target airspeed using TRIM_ARSPD_CM. When this is set to 1250, the airspeed fluctuates ~20 cm/s. I want to get this deviation down to <10 cm/s. How can I go about holding a constant airspeed for maximum flight endurance?

by adding weight/inertia. The cause of airspeed change is most likely turbulence.

How would I go about adding weight/inertia? Are there plane parameters for this or is it part of the xml file for jsbsim?

A bit short on details about your plane.
How much does the plane weigh?
What airspeed are trying to fly it at?
What is the wing area?

If you have a light plane flying slowly then it will be easily upset by any turbulence.
If you add weight to this plane it will need to fly faster to maintain lift and will be less affected by turbulence.

There are a lot of aerodynamic questions to be asked to optimise such things.

What is the aircraft shape?
Wing or standard fuselage and tail?
How aerodynamic is it?