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GRV-1 Turbotug Project 1/1/05

Posted on January 1, 2005

       After some “thorough” testing of the GR-5A on the GRV-1 Turbotug, I am able to report the engines performance. Multiple test runs were performed in the last two weeks including test runs on my street :0? I had very good luck with the engines reliability until I got carried away, but more on that later. The GR-5A had a couple items to refine as the testing process indicated.

       The engine had a problem with the RPM sensor system. The model aircraft paint that I had painted the rotor nut for the RPM sensor was coming off. I had opted to use a mild acid to “roughen” the steel of the nut so the paint would stick and it seemed to do the trick. After recalibrating the Omron sensor, the RPM circuit was back in business. With an additional adjustment to the RPM switch, the ECU operated flawlessly.
       The next issue I had to deal with was the oil accumulator on the gearbox. The oil accumulators job is to reclaim oil from the gearbox and remove air bubbles created by the gears. The oil flow was being impeded by improper vent tube routing, causing the tank to fill with air. I added a new vent tube to the top of the oil accumulator and voilla, no more problems. (I promise not to use voilla again :0)
       My last problem was caused by my inability to resist pushing the engine to failure!!! During the last few test runs with the GR-5A, I gradually increased the combustor pressure with each run. I eventually was pegging the 15 PSI gauge regularly. My guess is that I was pushing 25 PSI combustor pressure. The temperature was looking good at 845 deg Fahr. so I wasn’t worried about overheating the engine at the time. The Turbotug was really moving as I was clocking speeds of 31 MPH and getting faster.
       The GR-5A was really screaming! I cant tell you how cool the sound of the turbine was as it whined it’s beautiful song, and then it happened...................  Without warning, a loud buzz vibrated throughout the Turbotug. The RPM gauge immediately fell from 20,000+ to 8,000 RPM. I let off the throttle instantly and looked back to see if there was a visible problem. I could not see anything obvious as the tug coasted to a stop from 32 MPH. The engine was still idling so I assumed it was a power turbine problem. I shut down the GR-5A in “cool down” mode and inspected it for damage.

       My assumptions were correct as I could see a missing power turbine blade through the exhaust diverter. The blade had let loose while the turbine wheel was spinning around 24,000 RPM! The turbine blade tip speed was approaching 427 feet per second (291 MPH!) when it spiraled out of the turbine housing. The upturned exhaust housing threw the blade upward and away from me! Further investigation ensued after the disassembly of the power turbine housing. The turbine wheel was removed from the shaft and inspected.

       After examining the wheel, I concluded that the tensile strength of the blades had been challenged by the centrifugal force and temperature conditions. As the wheel approached 24,000 RPM, the blades started to stretch inside of the housing. There is only a .030” clearance between the housing and the blades so contact was immanent. One blade had stretched to contact the housing and it is my theory that the initial contact tore the blade at its weakest point, above the weld.
       The missing blade then threw the wheel into an unbalanced wobble (the buzz that shook the Turbotug) which caused the remaining blades to scrape the turbine housing. You can see the flattened edges of the remaining blades below.

       The “liberated” blade spiraled around the turbine housing and out the exhaust diverter never to be found. You can see the path of the blade in the photo below. The paint is scratched where the blade peeled off the wheel.

       Luckily there was no damage to the turbine housing or gearbox/shaft assembly (and me of course). My integral “scatter shield” worked flawlessly as there was no distortion damage to the housing. The turbine housing has a double wall thickness at the turbine’s axial plane which should “contain” any shrapnel. A separate hardened steel plate was my second line of defense against flying metal and I am glad it was not used!
       The turbine wheel was originally designed to operate roughly at 1000 deg. Fahr. and at no more than 17,500 RPM. Although the temperature was a bit less, the turbine wheel still performed up to 23,000 RPM repeatedly with no problems. That is roughly 30% over the red line limit for the wheel. The HP output of the GR-5A was approaching 10 HP @ 4,600 shaft output RPM. That does not seem too shabby for the scrap pipe turboshaft engine.
       All of this business left me with a problem, no turbine wheel :0{ I had to build a new one so back to the “bat cave” I went. I figured that while I was at it, I would replace the turbine shaft bearing and seal.

       The turbine bearing was still in excellent condition which was good to see after what it had been through. I still wanted to replace it so I removed the bearing tube from the gearbox and pushed out the bearing.

       Over 10 hours of engine operation left some discoloration where the cooling oil hits the shaft as you can see on the top right. I used my dial indicator to look for any wobble in the shaft and found none. The hardened steel shaft I chose was perfect for the job. The bearing itself had some discoloration on the inner race but otherwise felt good as new. The fiberglass rope seal on the bearing cap was also in excellent condition.

       I installed a new bearing and rope seal to the bearing tube. After I bolted it up to the gearbox I gave the assembly a new coat of paint in preparation for reassembly.

I had also repainted the expansion housing and turbine housing, allowing me to cure the high temp paint in the oven. While the paint was curing I got started on preparing the wheel for a new blade collar. The blade collar is the round ring that the blades are welded onto and conveniently designed to be removed with a lathe. After carefully cutting of the old blade collar I prepared a new 2” EMT collar to be welded on. I followed the same procedure that I used while making the GR-5 turbine earlier.

       This was an opportunity to try modifying the blade design a bit in hopes to create more power. Because the engine was running very cool it made me think that some additional back pressure could be afforded by the GR-5A. By making the blades a little wider, maybe I could capture more exhaust gas pressure. I cut the blades from a piece of 1-1/4” EMT that was slightly thicker than the old EMT stock (about .004”). The finished blades were .125” wider than the old blades which I hoped would improve power. I also rotated the blades about 3 deg. flatter than the old turbine wheel design.
       The finished wheel was sand blasted and mounted onto a balancing shaft. After some careful balancing (2 hours later) I was ready to paint and mount the new wheel. The wheel had cost me $0 dollars to rebuild so “I had that going for me” (Caddyshack......nevermind :0)

       I mounted the turbine housing to the gearbox for turbine fitting. I adjusted the blast seal and tightened it to the turbine shaft at .015” clearance. The turbine wheel was next so I slid it into the housing while checking to see that I had .030” clearance all the way around.

       The final job of reinstalling the power turbine assembly was quick and painless. I had a lot of fun doing it actually as this stuff is very relaxing to me. The GR-5A was ready for testing so once again I rolled out my “steerable engine stand” and prepared for ignition.

       The neighbors, now accustomed to my shenanigans, did not even pop their heads out the door this time as I spooled up the turboshaft. All seemed normal as the wheel began to spin. The temp was normal and the power turbine was spinning at 5,000 RPM. I turned on the generator and noticed less of a dip in RPM as before. I throttled up the GR-5A till the clutch engaged and felt a small difference in low end torque. It seemed more responsive in the low end, engaging with less throttle. I kept checking the temp and was pleased to see 845 deg. Fahr, right on the mark.
       The engine was ready for a road test so I pulled her onto the street for a go at it. The engine had no problem pushing the tug to 25 MPH in short order. The turbine wheel was right at home inside of the engine as it happily whined away. Several engine starts were performed and it seemed to be back in the game. Knowing the limitations of the turbine wheel will hopefully prevent me from getting into trouble again. An additional RPM switch coupled to the fuel supply solenoid would safeguard this from happening again and maybe I will install one for this purpose in the future.
       As for now, I can once again drive the kart down the street, imagining it is a Chrysler turbine convertible, grinning ear to ear :0) Thanks once again for looking a my crazy project!

Don Giandomenico

 

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