Everyone in the industry knows that new and used business aircraft sales, prices and inventories generally follow the overall economy. But do the different classes of aircraft react differently to changes in the economy? Fletcher Aldredge, publisher of the Vref aircraft value reference guide (www.vrefpub.com) in Shawnee Mission, Kan., believes they do, so he is developing price indices of various aircraft groups, including large jets, smaller jets and turboprops.