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The T-18 is designed around the 290 cubic inch four cylinder Lycoming engine.   This engine is eminently reliable and at the design's inception was available in quantity at low cost from government surplus sources.   The Lycoming 0-290-G had been used in ground power units to start jet engines until the power requirements of military jets exceeded the 0-290-G's capacity.   They were then declared surplus and after minor modification back to airplane engine configuration, have found their way into many amateur built aircraft.

The 0-290-G is basically a Lycoming 0-290-D airplane engine.   It has a flange on the front of the crankcase for mounting the generator unit and uses a modified oil sump and automotive carburetion.   It also used the crankshaft from the 100 HP, 235 cubic inch Lycoming engine.  

By machining off the crankcase flange, installing the 0-290-D aircraft oil sump and carburetion, and adding a magneto, ignition harness and set of spark plugs, a very comparable engine to the Lycoming 0-290-D will result.  Further, high compression pistons from the 0-290-D-2- or D-2B may be used to increase power to as much as 145 HP at 3,000 RPM, but in doing this, it would seem wise to install the 0-290-D or 0-320 crankshaft.  

The 0-235 crankshaft of the 0-290-G engine is only 88% as strong as the 0-290-D or 0-320 crankshaft in critical sections.   However, the lighter propeller turned faster in the T-18 will reduce crankshaft operating stresses and to date all indications support the continued use of the (0-235) 0-290-G crankshaft; at least as long as the standard 6.5:1 compression ratio pistons are used.

In any case, the converted 0-290-G is an "unapproved" engine and its use in an amateur built aircraft will make the restricted operation period imposed by the FAA 40 hours, instead of 25 as it would be with a type certificated airplane engine.  

The certificated engines which may be used in the T-18 are four cylinder Lycomings from 125 to 180 HP.   A few builders will buy "new" engines for their T-18's, however most will employ "used engines".   These will come from "used up" Piper airplanes and "defunct" Brantley and Hughes helicopters.  

Although constant speed propellers have been used on T-18's, the attendant weight penalty favors the use of a fixed pitch metal propeller.  The constant speed propeller and governor weigh 47 pounds more than the fixed pitch.  It takes 13 pounds of lead in the tail to balance the 47 pounds on the nose, so the weight disadvantage is 60 pounds.

In spite of its simplified design the T-18 airframe is unusually clean.  This has been achieved by careful attention to interaction of pressure distribution patterns and to aerodynamic details.  Skin laps across flow direction have virtually been eliminated.

To fully realize the advantages of the design cleanliness, flush riveting should be employed.   However, good performance has been achieved with brazier head rivets.Initially the T-18 design was conceived with open cockpit, exposed engine.  cylinders.  etc.

Because of abundant power such relaxation from optimum design is reasonable and does not result in an airplane of marginal performance.   Reduction in building time by elimination of the "harder to build" item is significant.   However, most builders have chosen to get all that is possible from the design.   Canopy drawings are now part of the plans.

The most important design feature of the T-18 is construction simplicity.   All considerations except structural reliability been compromised in favor of building ease.  However, no serious "tradeoff" of performance features has been necessary to achieve a design which is easy to build.