once upon a time, the French company, Panhard, made among other things, armoured cars, one of which, the M3, had a flat four, aircooled, 3-litre giving 85 hp, which powered a 13-ton vehicle. This engine was known as the 4HD. Smaller versions ended up in civilian cars of the 1960s.Has anyone ever used one for an aircraft? The Citroen flat twin was used for microlights in Europe. I wonder if the 4HD was ever used?!
