L39 GXT and Weight

UltimateL39 and Weight

The stock L39c is the lightest of the readily available variants so that is where I started. Planes are like race cars; removing weight dramatically improves performance. They accelerate, climb, turn and land much better when lighter in addition to reducing wear and tear on the gear and brakes all while reducing fuel - increasing range. Both the project planes rolled into the hangar at about 7250 lbs no fuel, no pilot and pilot/co-pilot chutes installed in the (cold) ejection seats. In looking at these planes it was obvious that there was a ton of 60s era Eastern Block avionics and wiring that made for a lot of weight. There were many systems that simply could be removed entirely like the Ram Air Turbine (rat), the starter APU and weapons wiring. There were other basic avionics functions that could much better be served by a single piece of 10” glass from Garmin. Most of the avionics were remote mounted and are accessed through the Garmin G3X displays. Both the front and rear cockpits have access to almost all of the avionics functionality. Add onto that those ejection seats that are about as comfortable as an electric chair and weight a bunch. A seat from an Extra 300L would do nicely in the application for all the aerobatics loads along with allowing a more inclined flying position (helps with GLoC or G Induced Loss of Consciousness). Lastly, there were a few systems which could be moved from behind the Center of Gravity (CoG) to in front of the CoG. The stock L39 carries a lot of nose ballast so any pound that can be removed from behind the CoG gives you roughly a pound removed from the nose for a near net 2x weight reduction. There seemed to be a lot of room for improvement.

N430J Weight and Balance 11_7_2018 This pretty much says it all. The plane came in at just under 6000 lbs empty (note that the parachute is now pilot carried aboard so I cheated a bit). The stock L39 is a trainer and as such is very stabile and forgiving but it leaves a little be to be desired from a performance and response standpoint. Flight testing confirmed what other L39 owners have said that a CoG around 30% of Mean Aerodynamic Cord or MAC is where you want to be. The plane is still very forgiving but is much more responsive. This CoG can only be obtained with one pilot. Two pilot pulls the CoG forward.

The fan duct turned out to be one of the greatest challenges. We stated off trying to shrink/stretch and English wheel aluminum 1/4 panels. It took two weeks to do the fist panel. It was lovely but it was clear that this was no way to produce fan ducts. In addition to the shear cubic volume of time and labor, we were still faced with the fact that cold working aluminum quarter panels then welding them together (which anneals the aluminum) would cause dissimilar metal hardness in adjacent areas. In the absence of a full part heat treating, we were certainly facing fatigue cracking over time. A composite part eliminates those concerns while opening a whole different can of worms. Partially though producing the first prototypes it became clear that the rear fan duct flange was in the neighborhood of the stock AI-25 rear motor mounts. The AV L-139 (Aero Vodochody's TFE731 installation) used a different location for the rear motor mounts which meant we would have to modify each plane's engine rails to employ this approach. That seemed like unnecessary work in addition to making it harder to roll an AI-25 back in the bird if anyone ever choose to do so. To accommodate the rear mounts attaching to the fan duct, several layers of uni-directional carbon cloth were added to transfer loads from the motor mount location to the engine/fan duct interface. Several different mount brackets were tried and the stock AI-25 rear mount towers were attached to the composite fan duct. The TFE731 now rolls into the L-39 just like the AI-25.

Turbofans are very sensitive to inlet disruptions. Turbulence near the blade tips can cause different blade loading profiles as the tip completes a full revolution. Uneven loading as the blade rotates sets up blade flexing, accelerates fatigue and, well, results are predictable. Great care was taken to manage flow into the motor. You can see the way the inlet duct aligns with the motor in some of the pictures below. It worked a charm as the overall system is performing very well. Composites allowed me to keep weight off the plane while simultaneously allowing for very smooth execution of flow sensitive components.

The seat is pulled straight from the Extra 300L. I've spent many an hour in 300Ls both doing aerobatics and cross country flights. The Extra seat is comfortable for the long haul and very supportive under high G loading. The seat back is profiled perfectly for a pilot worn parachute. The original Extra seat was made from fiberglass. I chose carbon with coring to the best combination of weight and stiffness/strength. The seat pan is adjustable up and down while the seat back is adjustable in tilt. Different leg support elements can be used to adjust the front of the seat's height if needed. The whole assembly pivots forward to provide for a nice bit of storage under the seat for cross country flights. I also use this area for a pilot carried aboard pulse demand oxygen system. Best of all, gone are the electric chair ejection seats that were a real pain (literally) on cross country flights.