Zenith Aircraft Company

Turbine Power
Garrett JFS 100-13A

Scott Ehni of Texas, a skilled craftsman, sent us the following photos and details about his unique powerplant for his scratch-built Zenith projects.  Scott has built a Zodiac CH 601 and also a STOL CH 701 which he recently flew with the turbine installation.  

Visit Scott's page on Zenith.Aero to view the latest updates, including video clips and photos: zenith.aero


From Scott Ehni (Texas):

"The powerplant is a Garrett JFS 100-13A Turboshaft engine.

"The BHP at an output speed of 3100 is 90 HP minimum. Fuel is jet-A. The used engines run around $3800 and probably the same to make a full top of the line conversion. We are currently working on a conversion that includes an accessory drive and most importantly a new dual exhaust to address the gearbox temperature and fuel use problems inherent to this engine. 

"The entire engine package weighs 140.lb firewall forward but what you gain you will have to carry in fuel as the stock engine burns around 12 GPH at cruise.  We expect the new exhaust to have a large impact on that figure. I am running an NSI CAP-140 three blade 68" dia.prop. 

"I will keep you updated..."


STOL CH 701 firewall-forward installation (2/06)

The initial tests went well on the new exhaust system. The efficiency was improved to a grate extent. You can look at it two ways, at a given RPM the fuel flow is less or at a given flow the thrust is more. For example we produces 390lbs of thrust at 72000.00rpm on N1 with a fuel flow of 18.1,now we get at the same rpm we get 435lbs thrust with 17.0 fuel flow. If you back down to 390 lbs thrust with the new exhaust the fuel flow drops to 15.1 that means for the same power we are using 3gph less fuel or for the same fuel we get 45 lbs more thrust. The best part is that because we are not increasing the N1 rpm we are not increasing the stress on the gas generator we are just using that same gas more efficiently. I included pictures of the mock up cowl for the 601 that we are testing but keep in mind it is just a mockup and not going on the plane. The highest thrust we saw with the cowling and full power (72000.00 rpm) was 451lbs of thrust As shown on the lode cell in the stand. I plan to do some more refining and testing over the next few months and will try to update the site with any new progress so be patient and check back once in a wile or feel free to contact me at ehpro@live.com

Scott Ehni


2/2002: A lot has happened since my last update so I will try to keep it brief as possible. We started testing within a week of my last update and within just two runs we burned up the turbine. It was not so bad so I sent it out to be repaired. Fast forward four months and we get the turbine back and burn it up again, at this point as you might expect I am discouraged! I found a new turbine and had it sent out to be checked out prier to startup. This time we started the test program with the stock turbine and we put the new parts on one at a time to try to find out what had caused the problem. In the meantime we refined the oil system and the start procedure and have run five hours of testing with no problems. 

Actually I do have one problem: I took a ride in a Zenair 701 in January and now I am looking for a second turbine to put in it because as soon as I got home I ordered a set of plans for it. Back to the JFS-100, I am supper pleased with the testing so far and it is such a smooth running engine I cant wait to fly with it . I have a lode cell on the test stand but I am not sure how accurate it is my intention was just to get a baseline to compare the new exhaust with. We recorded on a clear cold day/61deg/30.37 baro 390 lbs of thrust at 71,000.00 rpm on N1/2090rpm-prop15 deg pitch on the nsi prop/ 17.7 gph fuel flow. The fuel flow drops to 12 gph at a cruse rpm of 65,000 rpm on N1 and the thrust was 320lbs . The highest egt we got this time was1183 as apposed to the 1800+ deg we got when we burned up the last one which we now think was caused by a bad fuel controller in combination with too much oil in the gearbox at start up. The oil temps stayed around 220deg with 180deg coming out of the cooler. As you can see we have switched to the duel exhaust that we hope will lower the fuel flow .We are going to test the new exhaust this weekend and I will send in the results as soon as I have them, But as it stands even if it dies not help with the fuel flow I still plan to put this combination in my 601 and 701 and hope to be in the air next year.

Scott Ehni

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5/01: I have finished 95% of the wiring and next week I can start the first run tests with the stock exhaust. Once I establish a baseline we will switch to the new dual exhaust system and verify the changes.

   - builder Scott Ehni

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1/01: These are the latest pictures of my turbine project. I plan to put the engine on a Zenair CH 601HD.   As of this writing I have finished the prop drive that incorporates a gear drive to run the vacuum pump, alternator and a three stage oil pump to move the oil from both of the gear boxes through a filter and a cooler and back to the engine. The plumbing and the tank are complete and I am working on one of the hardest parts at this time, that being the oil cooler. The cooler is a 120 deg. Conical piece that fits behind the spinner and I am fabricating it out of 3003 aluminum and when it is finished all I have left is the wiring. When the engine is complete I am going to put it through a long test program during the construction of the plane.   The prop is an NSI-CAP 140 in flight adjustable pitch unit that can go into the beta range and that will be grate for the float operation.

   - builder Scott Ehni



The powerplant is a Garrett JFS 100-13A Turboshaft engine.

The engine has demonstrated over 200 hours of flight in a KR-2 owned by an A&P in Okalahoma and at that time he tore it down and found no appreciable wear. He has saved me hundreds of hours in development time and is a good source for info. Our test stand has a lode cell on it and we hope to start doing documented tests in early January 2001.

The entire engine package weighs 140.lb firewall forward but what you gain you will have to carry in fuel as the stock engine burns around 12 GPH at cruise.  We expect the new exhaust to have a large impact on that figure. I am running an NSI CAP-140 three blade 68" dia.prop. 

   - builder Scott Ehni


It took longer than I expected but as you can see I got the new oil cooler finished and also got the new 12" spinner fit. As you can tell from the front view I am going for a radial cooling inlet. My last task before I can start the turbine is to make my fuel control connections. I have devised a Mac servo driven throttle with a manual override as a backup. I will let you know how it goes...

   - Zodiac builder Scott Ehni

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