|Thorp T-18 in flight|
|Unit cost||approximately $3670 to build in 1971|
The aircraft was originally designed as an open cockpit aircraft, powered by a military surplus Lycoming O-290G ground power unit engine, but evolved into a fully bubble canopied aircraft powered by engines of up to 200 hp (149 kW).
The T-18 was designed to be easily constructed from sheets of aluminum, and use the modified O-290G powerplant. It was originally designed with an open cockpit and with the cylinder heads protruding through the engine cowling in the interests of simplicity. Even as originally designed, the cruising speed was quite high. The design showed great potential for higher performance and so modifications were created to install larger, cowled Lycoming engines and a bubble canopy. These modifications allow a T-18 with 125 hp (93 kW) to cruise at 160 mph (257 km/h) and higher powered examples to cruise in excess of 200 mph (322 km/h). Some aircraft have been constructed with retractable landing gear.
The T-18 was designed to use the 125 hp (93 kW) Lycoming O-290G Ground Power Unit. At the time the T-18 was developed these engines were inexpensive and widely available as military surplus generator motors. When converted for aircraft use they are virtually identical to the O-290D or O-290D2 aircraft engines.
 Kits and plans
T-18 plans are currently available to builders from Eklund Engineering, which is also developing a laser-cut kit version and as of 2009 had aileron, flap and empennage kits. Classic Sport Aircraft supplies plans, parts, and kits for the S-18 and the S-18T tricycle gear version.[5