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The Bratz (Brooks – Hatz) has been flown 64 hours since certification.
The Bratz is powered by a Lycoming 0-320A, 150 hp, turning a Sensenich W74-56 wood propeller, and equipped with Cleveland wheels with chrome discs and Maule tail wheel.
The aircraft was flown to the National Biplane Fly-In at Bartlesville, Oklahoma and to the Antique Fly-In at Blakesburg, Iowa.
Before I started my project I reviewed the plans (S/N535), observed and flew other Hatz biplanes, and then decided to make several changes.
Modifications were made as follows:
- Aluminum ribs in wings and ailerons to save maintenance and reduce labor in construction.
- Modified wing tips to reduce stalling speed and reduce maintenance.
- Installed ailerons on only lower wings to reduce number of parts. Ailerons were redesigned as friese type 100% static balanced to reduce the chance of flutter and reduce adverse yaw.
- The wing spars are 1/2″ deeper due to the use of aluminum ribs and all fittings were increased from 80 thousands to 90.
- Raised center section 2 inches with Stearman-type cut out to give additional entry space for front cockpit.
- The lower wing dihedral was increased to 4 degrees for lateral stability. Due to friese-type ailerons and increase in dihedral, the Bratz in cruise flight requires little or no rudder input for turns.
- The cabane struts were modified to provide adjustment of stagger for easier rigging.
- Provisions were provided for ground adjustment of horizontal stablizer so aircraft could be rigged for cruise flight with elevator trim in neutral.
- A 15 gallon fuel tank was installed in front of front cockkpit to give a total of 34 gallons of fuel for the 150 hp Lycoming engine.
- Toe brakes were installed in both cockpits at the request of wife who holds a private pilot certificate. Cessna 140 type pedals were used in front and my own design pedals in rear.
- Landing gear was lengthened three inches to allow for slower landings.
- Aluminum panels were installed from firewall to the aft cockpit to prevent fabric damage around cockpit area. It also decreases fire hazard.
| Take-off distance | 300 feet |
| Landing distance | 400 feet |
| Stall speed | 40 mph |
| Approach speed | 60 mph |
| Rate of climb | 800 fpm @ 70 mph |
| Cruise speeds | 88 mph @ 2300 rpm 95 @ 2400 rpm 105 mph @ 2500 rpm |
Note:Cabane adjustment for stagger
Windshield – no frame
Fuselage aluminum panels
Center section cut-out
3/4″ foam with wooden strips to keep fabric off the foam
metal ribs, 2024T-3x.025
Cabane strut with stagger adjustment
Wing construction:
Root rib
Note: Fuel tank, baggage metal covering, and fin fairing
Metal fuselage formers
Uses a slip tube similar to a Piper Cub.
Aluminum ribs constructed with .025 2024 material.
Tank (19 gal.) is constructed of .050 aluminum.
Fin is removeable from fuselage
Longerons are .049 tubing
Shoulder harness tubes aft of each cockpit
Wing fittings are made of rectangular tubing
Front of fuselage modified for 15 gal. fuel tank
Controls mounted in aluminum blocks with zerk fittings for lubrication
Control stick brackets constructed of rectangular tube
Seat frames are welded tube with aluminum bottoms
Note wing-tips, ribs, and fairings
Note metal covered
Raise and lower leading edge of stabilizer using washers (AN 970) as spacers
Wing, aileron, and tip contruction:Note fueltank and fuselage formers
Note the navigation light and fin-to-fuselage fairing
| by Randy Brooks, NX24B “Bratz” CB-1 S/N535 The Hatz plans gives only the C.G. location as loaded. I established a C.G. Range for my Bratz CB-1 as follows:The Clark “Y” airfoil is used on the Hatz with 50 inch chord. The center of pressure range is 18% Forward and 35% Rearward or +9.0 inches and +17.5 inches from leading edge. Reference used is Technical Manual T-400, Theory of Flight. I prefer to use 33% or +16.5 inches for aft center of pressure.>Datum — Lower wing leading edgeForward C.G. — +9.0 inches – 12.65 inches equals -3.65 inchesRearward C.G. — +16.5 inches – 12.65 inches equals +3.85 inches Procedure to Use Calculate wing areas of upper & lower wings and add together. Use lower wing leading edge as datum. Divide upper wing area by total wing area equals 55% (approx.) of total area. Multiply the stagger (23″) by 55 % equals -12.65 inches which locates leading edge of the Mean Aerodynamic Chord (MAC) from lower wing leading edge (datum). Forward C.G. equals 18% x 50 inches equals 9 inches aft of leading edge of MAC. Rear C.G. equals 33% x 50 inches equals =16.5 inches aft of leading edge of MAC. Locate forward C.G. in reference to (datum) lower wing leading edge. 9 inches – 12.65 inches equals -3.65 inchesLocate Rearward C.G. in reference to (datum) lower wing leading edge. 16.5 inches – 12.65 inches equals +3.85 inches |
