Who was the primary author of federalist papers writing 51 85 articles published to increase support for ratification U.S. constitution?

The primary author of the Federalist Papers was Alexander Hamilton, who wrote 51 of the 85 essays.

While James Madison wrote 29 and John Jay contributed 5, Hamilton's involvement was significantly greater. He was the driving force behind the project, coordinating the effort and contributing the bulk of the content.

The Federalist Papers were indeed published to increase support for the ratification of the United States Constitution.