5             Experimental Class Specifications

5.1          Introduction

The following rules and specifications are for the Experimental Class.  All Common Rules apply as appropriate.  The Experimental Class was created for piston-engine, propeller-driven homebuilt or experimental aircraft designs.  The Experimental Class represents aircraft of modern design, similar to the new full-scale Sport Class competition at the Reno Air Races.  The dimensions of Experimental Class aircraft are based on current USRA Aircraft Specification Sheets and the Experimental Class Rules that follow.

5.2          General

5.2.1               Subjects need not have been entered in closed-course racing events, but must have been built and flown since the beginning of 1950.

5.2.2               No reduced-scale replicas of full-size aircraft, such as the Thunder Mustang, may be modeled for this class.

5.2.3               New designs not currently approved and dimensionally specified in writing by the USRA will not be allowed to compete until such approval is granted.  Eligibility and scale documentation may be required of the applicant for use by the USRA Technical staff.

5.2.4               Aircraft that are eligible to race in the Unlimited Class may be entered in the Experimental Class under the following provisions:

5.2.4.1           Engine limits are as stated for the Unlimited Class.

5.2.4.2           All aspects of the airframe must conform to minimum specifications for the Unlimited Class.

5.3          Weight

5.3.1               Minimum aircraft weight is 25 lbs. (dry).  Maximum aircraft weight is 55 lbs. (wet).

5.3.2               Aircraft weight may be verified at any time during a race.

5.4          Wing Specifications

5.4.1               For aspect ratio (span/average chord) of 8:1 or greater, a minimum wing area of 1060 square inches is required.  Minimum thickness ratio for root and tip airfoils is 15% and 12% respectively, using not less than a straight taper from root to tip.

5.4.2               For aspect ratio (span/average chord) of less than 8:1, a minimum wing area of 1250 square inches is required.  Minimum airfoil thickness ratio is determined by the applicable USRA specification sheet for the aircraft.

5.4.3               Minimum wingspan of 76", regardless of aspect ratio.

5.5          Engine Specifications

5.5.1               Maximum allowable engine displacement is 217 cubic centimeters.

5.5.2               Aircraft must be propeller driven with piston or rotary engines.

5.5.3               Engine weight on single engine aircraft may not exceed 14 lbs.  Twin engine aircraft engines may not exceed 9.5 lbs. per engine.  Engine weight is measured with the engine out of the aircraft in a "ready to race" mode, i.e. if you need the item to race, it must be included in the engine weight.  Engine weight does not include ignition modules, batteries, wires, exhaust system, propeller, spinner, spinner back plate or prop nut.  A propeller shaft extension is included, if used as part of the engine, is included in engine weight.

5.5.4               Engines must be neatly cowled where possible with no more than 50% of the cylinder head length projecting outside the cowl (measured from the centerline of the output shaft to the top of the cylinder head).

5.5.5               Experimentals based on the NAA NA-50 are limited to an engine with a displacement not to exceed 152cc.

5.6          Exhaust Systems

5.6.1               Exhaust system components (headers, mufflers, tuned pipes, etc.) may not protrude from the airframe more than 30% of their total length.  If the exhaust system is not greater than 8 inches, any or all of it may protrude or be exposed.

5.7          Landing Gear

5.7.1               All aircraft must use scale-like retractable or fixed main landing gear as appropriate to the aircraft modeled.

5.7.2               Landing gear must be of sufficient size and strength to permit takeoff, landing and taxi in a reliable manner.

5.7.3               Tail wheels must be installed and steerable, but do not have to be retractable.

5.8          Airfoils and Planform

5.8.1               Wing and tail group airfoils suitable for model aircraft may be used.  Wing and tail group planforms should follow scale planform, chord, area, etc., of the full size subject when scaled and/or as specified in the USRA Aircraft Specification Sheets.

5.8.2               Tail area may be enlarged to insure stability as long as planform is preserved.

5.8.3               Control surface dimension may vary as long as planform is preserved.