BRIEF:

The Zorbin government needed to retire their light and heavy attach aircraft and wanted one aircraft that they could build in numbers to replace the two different airframes. At the time, studies where showing that twin boom aircraft had more flexibility, maneuverability and survivability in ground attach and hostile environments. Also, the ability for Short Takeoff and Landing (STOL) on unimproved runways was a must for forward theater operations. The government issued contract ZAF 37755/94 to various contractors to develop an attack aircraft that had the following specifications:

• Mid range (750 mile), sub sonic, highly maneuverable twin boom design
• Two seat aircraft that with duplicate controls for A/C Commander and Pilot
• Low radar profile
• STOL ability
• Munitions load:
  • Minimum of 4000 lbs of internal stores
  • Four Mk-21 “Paralysis” air-to-surface missiles
  • Two Mk-18 “Furball” air-to-air missiles
  • One Mk-44 “Vengeance” pulse laser cannon
• High survivability for crew and aircraft
• Simple to maintain and support in forward operations

 

The contract specified that the aircraft should be simple enough to build in large numbers using existing materials and manufacturing techniques. Two engine types were specified, the Rolums Air Ram air induction engine or the Wratt & Pritney J96 axial flow turbine.

 

One of the major stipulations of the contract was that the contractors would be responsible for funding the design and tooling for the prototype aircraft. Only after the prototypes were built and initial airworthiness proven would the government start paying for further development.

 

The contract was let to eight aircraft manufactures for initial design concepts and cost analysis. After review, Universal-Motion (U-M), Block-Durum and The Aircraft Consortium where chosen to build prototype aircraft. Universal-Motion would have the designator XA-71, Block-Durum XA-72 and The Aircraft Consortium XA-73.

 

An advantage the U-M had over its competitors is its patented radar absorbing paint which was proven to absorb 98% of all radiation striking a painted surface. Using this paint meant that U-M did not need to do special tooling or use special materials to get an aircraft shape that didn't reflect radar. The other manufacturers would have to spend significant time and cost in engineering and tooling to achieve the same results. After initial studies, it was obvious to Block-Durum and The Aircraft Consortium that Universal-Motion, was far more capable of meeting the production expectations and pulled out of the competition, leaving the XA-71 as the sole design.

 

The XA-71 design allowed for two Mk-44 pulse cannon instead of just one stipulated by the contract. It also allowed for an extra 1000lbs of fuel to be carried in externally mounted, jettisonable tanks along with a full weapons load, giving the aircraft a significantly extended range. The engine selected was the Wratt & Pritney J96 for its compact size, fuel economy and thrust ratios.

 

For survivability, the design included triple redundant hydraulic and fuel delivery systems, self-sealing fuel tanks, and redundant flight systems.

 

Crew survival was improved by the use of a crew ejection pods instead of ejection seats. The pods are made up of high strength, light weight, ballistic material composited in layers. Construction of the pod is one of the only components of the aircraft that goes outside the realm of normal metallurgy and machining techniques. Since the pod is only used in an emergency and requires no normal maintenance, it was argued by U-M that the spirit of the contract, that the aircraft be cheap to make and easy to maintain, was still enforced.

 

The unusual “backward” design of the aircraft was based on the stipulated mounting of the Mk-44 Pulse Laser. This weapon, although not extremely heavy is very bulky, requiring a full 1/3 of the fuselage. The amount of heat produced by the weapon, when rapidly fired is so significant that it was felt that putting the flight crew above the laser would cause overheating of the crew compartments. The crew stations were moved rearward, which significantly changed the center of balance. To counteract, the wing was moved to the back. There were concerns by government officials that the fuselage and horizontal stabilizer would block the view of the crew for target acquisition and during landing. Initial flight testing proved that this was not the case and the flight test crew did not feel that the extended nose was a problem.

 

An added bonus of the configuration is that it provides better high AOA flight characteristics which makes the STOL requirements of the specifications easier to achieve. Leading edge slats, Fowler type flaps and partially vectorable trust of the JP95 engines combine to improve slow speed flight as well as take-off and landing.

 

The initial XA-71 (serial number 96-0001) went through 20 government prescribe flight test sorties to prove initial air worthiness. The government was suitably impressed enough to order another five prototypes (96-0002 through 96-0006) to be built under contract funding to prove out construction techniques and to be used for full scale flight testing. A two year flight test plan was developed for flight testing of the five new prototypes while testing continued on the initial airframe. The first three airframes (96-0002, 96-0003 and 96-0005) were deliver within five months of contract start date and flight testing commenced. Flight testing was proceeding well until a wing strut in 96-0003 failed during a 5 g pull up from a 30 degree diving attach simulation. The aircraft was destroyed, but both crew members ejected and survived. The incident proved to be a good test of the crew escape pod system. All aircraft were grounded for inspection and determination of the failure, which was found to be metal fatigue occurring in the struts. New strengtheners were designed and retrofitted into the existing airframes and incorporated into the build of the last two prototypes.

 

As of this writing, the XA-71 has met or exceeded the expectations of the government contract and expectations are that a full 3,500 airframes will be contracted from U-M over the next 10 years.

 

WORKFLOW:

The XA-71 spun out of some ideas I was playing with for a model to print out on my 3D printer. After a couple of nights of drawing ideas, I came up with the design for the aircraft and drew up a finished “two-view” drawing.

 

Now that I had a “finalized” drawing, I spent about six hours creating a rough model in Blender, which I then cut up and prepared for printing. I wanted the physical model to be pretty large (over 12” in length) and since my printer will only print 5”x5”x5” I had to cut the model up quite a bit to print it. Each fuse is cut into four segments. The center pod/engines/wing was cut into four segments (left front, left rear, right front, right rear), the horizontal stabilizer into three sections (inner left, inner right and outer) and each of the outboard wings as two sections each. All segments were printed in ABS and can be seen in various states of assemblage and sanding in the side pictures.

 

While printing was going on, I worked on several different aspects of the model. I am looking at various different camo schemes. I really like the new digital camouflage that is starting to appear on modern fighter aircraft so I used a camo generator I found here (http://www.happyponyland.net/camogen.php) to generate some schemes similar to those I found used on Russian and Slovakian Air force planes. After generating the camo, I applied them to the two-view drawings along with some insignia. If I decide to do the more pixelated design, I will create the camo in Photoshop and print it out on water-slide decal paper. The larger camo scheme can be done with individual masks and painted directly on the model.

 

As part of the physical model building process, I need to define accurate panel lines and access hatches. Unlike purchased plastic models that have these panel lines inscribed as part of the molding, my 3D prints are smooth, so I will need to scribe the panel lines into the plastic myself. That makes it very important to understand how the panels work together, so I’ve spent some time defining the panel lines using the outlines of the aircraft from the initial drawing. These drawings will be transfered to the model and small saws, scribes and knives will be used to cut the lines into the plastic.