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Email Questions or Comments Last Updated 11/22/19
My name is Bill Hart. I created the L39 TFE731-3, and hopefully soon to be -5, conversion described here. More importantly, I created the overall lightened and balanced aircraft that is now performing like no other L39 has.
The first aircraft we built, our development platform, was named the L39GXT by Shane Phillips with Fighter Maintenance Contracting. We had great hopes for FMC. The following is an explanation of how the conversion came into being.
This web site is a way for me to share information on my projects. I often start down the path on a project and find it very hard to get useful information. Not someone's opinion but data I can mull over to form my own opinion. The information provided here is for informational purposes only and is not guaranteed to be accurate. Do not rely upon it for anything especially given we are talking about aircraft. In other words, no representations are made for accuracy or fitness for any purpose other than just plain fun.
Before I get into how it all came about, I'd like to address a question I am asked in many different ways like "what do I get with a kit of parts?" or "what is involved with doing a TFE731 conversion to an L39?" I've lived it and thus it is second nature to me; I really need to take a minute and try to accurately describe what is involved. This is what is currently included in a "Kit". In essence, there are three distinct parts of installing a Garrett TFE731 motor in an L39 and, depending upon the steps you are willing to take, many additional steps to building an UltimateL39. The three elements for basic installation (follow the links for an explanation of each) are the motor assembly/installation, the starter/generator support and engine instrumentation. The previous three sections described installing a TFE731 in place of the AI-25. Our focus is performance. The Kit to help in installing the motor is where most of the interest has come from yet I view it not as the end goal but a necessary element to achieve the end goal of the UltimateL39. My guess is that 75% of the performance of the UltimateL39 comes from removing weight and thus we strip everything we can from the plane, replace the tip tanks with low drag low weight wing tips, replace the ejection seats with very comfortable (if you have been in the rear seat of an Extra 300L you know what I mean) adjustable carbon composite seats, nose mount the hydraulic system including an electro-hydraulic pump pack (allows servicing hydraulic systems without a mule) and on and on. None of these items have been "productized" and made available. Our current plane is being built with even greater weight savings in mind and will use a custom diode/relay board based on modern aircraft relays and surface mount diodes, a microcontroller based flap control system with modern push buttons, a complete custom chassis and avionics wiring harness using all Tefzel wiring and mill spec connectors (replaces all original cockpit wiring), custom microcontroller based generator control board with all start sequencing logic and more all with the goal of removing a few hundred pounds more weight. These items are highly experimental and not "for sale" as stable sorted product. That said, anyone that wants to go the extra mile for extreme performance is welcome to join us on these development efforts provided you are willing to share in the costs, aid in the development/proving of the systems and accept the risk. Come join us in pulling off a 5800 lb L39 with 3700 lbs of TFE731 thrust. Following that, the goal is a 6000 lb L39 with as much of the -5's 4700 lbs of thrust as we can get. So, there are two very distinct parts of what we do. We provided a Kit of parts to aid you in installing a TFE731 and we are willing to work with people (by sharing development technology) who, like ourselves, believe in pushing performance boundaries and want to help raise the UltimateL39 bar.
Our current in house project came in as Ugly Duckling and is being transformed into Mallard. The original intent was to install the TFE731-5 motor but we have lost access to our development mule for doing demonstration flights and thus Mallard will be a TFE731-3 installation. We are putting everything we learned from the development aircraft into Mallard to make it better, lighter and a stronger performer. Check here to follow her progress.
The Ultimate L39 started off as an "itch" to do something jet. I took an introductory ride in a L39ZA. It was interesting to do once but nothing about the experience really screamed "you have to have one of these". It did start me thinking though.
Before I get to that I'll mention one data point from the second flight. A take off and climb out at 140 kts (slow for a 39) at the top of the green arc on ITT (max continuous power 885c for the TFE731-3). Release of brakes to lifting off was 13 seconds, 2000 ft in 53 seconds, 4000 ft in 85 seconds and 6000 ft in 115 seconds. There was a 15 kt wind coming down the runway. That is when I knew we really had something useful. Here is a plot of that take off. Here is the supporting data but note that I was still working out the fuel flow sensor K factor so flow rates are not accurate. Subsequent flights have really opened my eyes to the true performance of this combination. All have been done at top of the green arc which is maximum continuous cruise power. There is another 300 lbs of thrust available at take off power (908c) and another 250 lbs of thrust on top of that if you use APR power. Any Reno Racers out there???? I've included a lot of that data in the pages to follow.
I approached the L39 from three directions. I looked at the principal elements of weight, power plant and drag. The following sections discuss each of these areas but, before I get started, I'd like to take a minute and thank Shane Phillips of Fighter Maintenance Contracting and John Morgan of Pride Aircraft. Both were endlessly patient with me and my questions in addition to providing a ton of documentation so that I could educate myself on the L39. Shane at FMC went even further and actively participated in the project over the last eighteen months along with being involved in a lot of the decisions on how best to do things. He created the name L39GXT, is in the process of building a UltimateL39 in Gadsden for one of his customers and is an excellent source of information on the conversion. Thank you gentlemen. In addition, Alby Redick at Aviation Classics in Reno not only sold me N430J or GreyBird but was gracious enough to let his guys do a good bit of "training us up" during Grey's conditional just prior to us taking delivery of the plane.
Our development platform, N430J, is now with its new owner and in the capable hands of Shane at Fighter Maintenance Contracting. This has allowed us to shift gears and focus on the first customer motor swap assembly now just delivered. Building up this unit has brought into focus what is required to do a UltimateL39 upgrade. This is a general description of the overall process with more detailed sub-pages for different elements of the conversion process.
We have delivered the first customer motor package and are now turning our attention to "Ugly Duckling" our second UltimateL39 conversion. The original plan was to build this plane with a -5 motor but that has changed to a -3. There are some additional weight savings I want to incorporate so as to build an even lighter plane, some other improvements in implementation from what we learned with our development platform and I want to have a dedicated shop "demo" plane for showing interested parties just how much fun this thing is. We are looking for another donor airframe for our -5 installation so please contact me if have, or know of, an L39 that would be a good match for an engine swap.
One thing that has become apparent is that our approach to the L39 differs significantly from what most shops are interested in or willing to do. For us, the passion is performance. The TFE731 series of motors, engineering documentation, expanded Approved Inspection Programs and all the other details that go into an Ultimate L39 is done in the pursuit of performance. What I am learning is that most other entities are more interested in doing just engine installs. To us, the engine install is a necessary step to enable the end goal and not the goal itself but we understand the interest and have made the engine installation kit available to all interested parties (shops and owners). I'm also trying to document differnt elements of the process so, if you are like me and want to do it yourself, this web site can provide helpful information.
Here is a quick pass at a cost analysis for conversion. Current L39 stripping hours/weight (Mojave Strip)
Thoughts on quality, reproducibility and cost control.
An example of take off roll and climb out at maximum continuous cruise power (885c for the TFE731-3) from a mid-field vantage point of a 4000 foot runway. The gear is a little slower with the electric hydraulic pump pack.
N430J Grey's Flight Testing
Weight and the UltimateL39. Most of the magic happened here :)
Power Plant. Operating costs, availability and safety converge here. The savings over the AI-25 literally pays for everything else. There is also a link on this page to our TFE731-5BR efforts.
Efficiency and Going Places, some initial cross country data
Drag. Simple changes help complete the package.
Performance, yea, this is what most people want to see straight away.
Avionics Approach - the decision tree
Lessons Learned (and some the hard way). We are not perfect here and there has been, and continues to be, a learning curve. We simply try to learn from our mistakes and make the end result better with every iteration.
Turbo Cooler and Bleed Air - what you should not do Foreign Object Damage or FoD - like they say, stuff happens Not all seats are meant for standing
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