The redcoats are already here. They're been at Sully Plantation since yesterday, learning how to be good eighteenth-century soldiers. And today the public is invited to observe the hundreds of troops and camp followers drill and work and play as they did two hundred years ago. This will be the biggest Sully Plantation Encampment yet, with a music performance by fifes, drums and cornets at 1:30 p.m., a major re-creation of the Battle of Brandywine at 2 p.m., tomahawk and knife competitions, and all the daily business of that era, including food preparation as it was done then. In case you want to bring along your own eighteenth century picnic, the First Virginia Regiment of the Continental Line has passed along these recipes that they will be using: Eighteenth Century Beer Bread

3 cups whole wheat flour

3 tablespoons brown sugar

12 ounces beer

Mix ingredients together. Put them in an iron pot and set in the coals, with coals covering the top until done.

Twentieth Century Beer Bread

3 cups self-rising flour

3 tablespoons white sugar

12 ounces beer

Mix ingredients together. Put them in a bread loaf pan and bake in a 350 oven for one hour.

Sully Plantation Encampment, Route 28, three-quarters of a mile north of U.S. 50 at Chantilly. Today, 10 a.m. to 4 p.m. $1.50 for adults, seventy-five cents for children. 437-8581 CAPTION: Illustration, no caption, by John [WORD ILLEGIBLE]