Prop Selection

   ~Choosing the correct propellor for your mission can be like playing a slot machine in Vegas. Each machine has it's shiny appeal, promising a big win, but when it comes to UL Power, a clean sheet design and progressive thinking is required when choosing the right prop. Let me explain. For decades, Lycoming and Continental have dominated the piston engine market, if for no other reason due to the massive cost of R&D for a new engine to hit the market. Year after year, Oshkosh brings all the progressive minds to the front lines in a constant attempt to re-invent the wheel. Every year it seems new engine manufacturers pop up and market their "revolutionary" design. However, as history has shown, very few of these manufacturers are able to weather the demands of General Aviation let alone afford them. The market is small, and the risk is high. People constantly complain about the Lycoming engines being such "outdated technology", and while I completely agree, when you look at the small number of Lycoming engines that sell every year, it makes sense as to why R&D is slow to advance. This of course is comparing the aviation engine market to say, Ford or Chevy who have millions to invest. With that said, prop manufacturers are faced with the choice to design propellors around the most commonly sold engines or take massive financial risk to do otherwise. My personal journey over the past 3 years has been real eye opening to this unfortunate reality, nevertheless, with a very positive end result, so let's get to it.
   The UL Power engine relies heavily on RPM for HP rather than displacement. These engines like to rev and NEED to rev. For example, a Honda S2000 pulls up next to Ford Mustang GT. Both claiming the same HP, but for some reason the GT always beats the S2000 off the line, reaching top speed before the S2000 can. Eventually they both end up putting out the same max rated HP but the time it takes to get there is very different. The UL power engine needs to see a whopping 3300rpm to reach full rated HP, while a Lyc and Cont reach max at well below 3000rpm. The story is the same with each engines max torque value. And speaking of torque, the UL makes significantly less torque for the HP than it's counterpart due to such a smaller displacement. What all this means is that throwing a prop made for a 160hp IO-320 onto the 160hp UL Power 390is is a huge mistake. Ask me how I know :)
   My first propeller attempt was Gary Hertzler 65x76 and was only able to get a static rpm of 1950. Way below the targeted 2500rpm. The second was a step down at 63x76. This prop only developed 2100 static rpm. Gary then sent me an even smaller 62x68 that he makes for an 0-235. This prop ended up giving me 2350 static rpm and was good enough for test flying. While I flew this prop, Craig Catto was working on his solution for me. He first sent me a 66x64 which gave the same 2350 static and similar performance to the 62x68 SB, but neither prop allowed the engine to see more than 3100rpm in any condition. I was convinced that in order to achieve best climb performance, I was in need of a prop that would allow a much higher static rpm and allow the engine to reach full power as well. Catto was shocked at the results and cut the prop down to be a 64x66, but with a thinner blade profile. STILL, the prop would not perform, and in fact got worse. As a last ditch effort, Catto made a new prop. This time a 62x68 like Hertzlers, but only giving 2200 static RPM and therefore, wasn't worth my time. Catto gave up and shared with me over the phone that they had been having a hard time making props for other UL engines as well. The assumption at the time was that the engines weren't making the rated HP they advertised. Meanwhile, a lot of conversations with Gary Hertzler continued, but Gary also didn't have much hope for the UL engine lineup, especially because he only makes props for Lycs and Conts and isn't in the custom prop making business. I then had to bring a 3rd manufacturer into the fold.
   I pursued Rupert over in England at Hercules Propellors and he gladly accepted the challenge, making a promise to build the prop right, or he'd make it right. I wire transferred the funds to Hercules and a month later had a gorgeous propellor arrive. Right off the bat, this prop was different. I still only got 2350 static rpm but after hitting 30kts the prop would come alive and unload to 2500rpm before rotation. After takeoff I could climb out at 120kts and 2800rpm which was right at my engines peak torque. This was great news. The Hercules would also Rev faster and give me 300fpm better climb. What really stood out what that it allowed my engine to rev all the way up to 3250 at 5000ft pushing me along at 182kts TAS. This was the fastest I'd ever gone at that altitude and I was in love. However, as temps began to cool off, I was seeing lower density altitudes and better performance. Down low on the deck at WOT the engine would quickly hit redline at 3300 and the rev limiter would kick in. I needed more pitch. Just one more inch would have been perfect. I contacted Rupert to see if he'd adjust the prop as promised and heard nothing. Again I sent several messages and no response. After a month or so I gave up, only to hear that another Hercules customer was having the same customer service problems with Rupert. Wine, dine, and ditch. While I was thankful for this props performance, the lack of customer service really soured the experience for me. A single prop adjustment from Hercules could have solved the problem and kept me as a customer for future orders. Unfortunately, customer service is king, and this was a deal breaker for me. The hercules did, however give great insight into the needs these UL Power engine have and how to make them come alive.
   I shared my experience with Gary who knows Rupert personally, and it gave Gary an idea. He wanted me to try one more prop of his. The 63x69 that he makes for the 0-200 was sure to see better RPM and performance. This now being the 8th propellor I've bolted to my engine and by far the sweetest surprise yet. First and foremost I must tip my hat to Gary's relentless willingness to send out prop after prop to get me where I need to be. As soon as I fired up the engine, the 63x69 revved up to 2400 static rpm which was better than any other prop so far. I experience better acceleration, great climb and a lot faster top speed. In fact, the other day I was able to Hit 203kts TAS spinning her up to 3250rpm! This was the fastest I'd ever been at that was at 2500ft Dens Alt. Finally, I had found a prop that unloaded my engine without blowing past redline! I called Gary and thanked him, letting him know that even up high, my speed was great and fuel efficiency even better.
   8 propellors later, I have learned a very important lesson about the UL Power engines, and that being that they really need to be propped according to their displacement and NOT HP. The funky relationship of HP to torque on these engines has scared off a lot of prop makers, and I've been told "no" by at least 4 of them by now. Hopefully, the more UL Power engines that sell will create a higher demand for prop manufacturers to reconsider. The UL community needs the drive and commitment that Gary Hertzler has offered and a fresh set of blades tailored to these small bore, high revving screamers.

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