RPM HP 1000 23 2000 66 3000 116 4000 153 5000 178 6000 198I have a '93 REW turbo engine with some street porting, high compression rotors and a single stage '90 turbo using a RWS EC2 computer. Greg Richter seems to be getting around 300 horses from the same engine (with a Turbonetics T04 turbo and low compression rotors) which is a little more than 150% of the above.
I asked in the rotary lists to see if anyone would like to take a guess at what my numbers should be. No one did. Next I needed the flat plate drag for a Cozy IV. Phil Johnson's web site helped me out with this. Based on his calculations the flat plate drag of the Cozy IV is 2.025 sq ft.
Next I called Performance (on a Saturday), and was put straight through to Clark. Again I had a great conversation, and I learned some more. Here's a synopsis of what I've learned so far.
A prop is basically defined by five parameters. Diameter, pitch, blade width, material and number of blades. Three blade seems to be the only way to go and diameter (for a Cozy IV) can be anywhere between 68 and 74 or so. Pitch is the real issue. Clarke described pitch as "the amount of Jello your plane would plough through during one revolution". Hmmm. Sounds strange, but I understand the concept. This would be an interesting practical experiment to try one day. Anyway, the more pitch, the harder the prop is to turn, so if you put in too much pitch you wont get enough static RPM to reach the top of you're engine's power curve - thus take off power would be reduced. You also might get some cavitation. Apparantly, one way to get around this is to begin the take off with reduced rpm, then increase throttle as the airspeed increases. The basic plan is to build the prop with enough blade width, diameter and pitch to account for the maximum possible power expected. Lets say 300HP in my case, then rework the prop to reduce the pitch, width and/or blade width as needed to bring the prop into range with the actual power experienced. Obviously you have to start big and whittle it down because you cant start small and add material. Makes sense. Something else I learned is that a "climb" prop is faster, but less efficient than a "cruise" prop. I'm not quite sure why. Clark says that the performance props are made out of "rock hard maple" laminated in 12 layers. They're finished in fiberglass (not carbon fiber because its too stiff). The final finish is urathane with painted tips. He's done props for a number of builders with 13B's and other high power engines (including a few pushers) and gave me a couple of references. Clark suggested that we might start with a 68/84 - i.e. 68 diameter, 84 pitch. This would be a 300HP prop that we could cut down if the engine wouldn't turn it fast enough.
All very interesting stuff. I'll carry on the research and make a decision in the next month or so.
By the way, I didnt got to Margie Warnke because I'd heard a bad firsthand account about her service. I've since read two more VERY disturbing accounts about the same vendor involving workmanship, delivery schedules and overall support. I have no first hand experience of this vendor, but all the input I've heard says pick someone (almost anyone) else.
Drive bushing dia: 5/8 Drive bushing depth: 9/16 Pilot boss Dia: 2.25 Pilot boss depth: 1/2 (I measured this at 3/16 but Tracy advised me to allow a little extra and specify 1/2. Bolt Circle Dia: 4.75 (SAE 2)We discussed the other specifications and my requirements. Tracy agreed that a 3 blade was the obvious choice. I want best possible cruise, so Tracy recommended a Max prop RPM of 2850 and a cruise rmp of 2500. This will run the engine at around 6100 and 5450 respectively. He says that above 6100 the power rises in a linier fashion while stress on the components rises logarithically. I'll be able to "dial in the power I want" by adjusting the turbo boost. The calculations of max power were interesting. Tracy asked my maximum boost and, following what I remember about Greg Richter's installation, I said 7 - 8 PSI. This is half an atmosphere and, apparantly, power rises in direct proportion to boost. So, if I have a 200HP engine boosted at 1.5 atmospheres, I should get 300HP which aligns with Gregs numbers. This assumes no power lose due to intake temp rise and exhaust back pressure. Allowing 5% or so for looses due to these issues Tracy estimated 280 HP as a reasonable target. Next came the question of speeds and altitudes. After some discussion and considering the norm for the Cozy IV we ended up at a target of 220 MPH at sea level which would give much higher TAS at optimum cruise altitude of 12,500.
I called Greg Richter and ran Tracy's numbers by him. He agreed that they seemed reasonable based on his experience with his turbo 13B Cozy III, and mentioned that he's also planning on getting a prop from performance. (He's since discarded the prop approach altogether in favor of a turbine).
Tracy added a suggestion for the first flight. Don't heat soak the engine prior to take off - begin the take off run with oil temp below 130 and water temp at or below 100 - this way there's a good chance of getting back down without frying the engine if the cooling is inadequate. Hmmmm.
The plane headed down to the hangar and, after a few weeks, the prop followed. It only just fit in the back of my car. I'm going to have to get a 2 blade for ease of shipping if this one fails.
Clark feels that BID wrapping will help a little with minor items (pebbles and the like), but with a large item like an piece of exhaust, it just comes down to dumb luck. Think of the dynamics, he said. You're trying to accellerate a 1lb piece of metal from zero (relative) to 500mph in a couple of seconds. That kind of impact will damage ANY prop. The extent of the damage depends on the timing and where it hits. A BID wrap isnt going to save you're prop if it hits the blade wrong. The main issue with minor damage is water / humidity getting into the wood afterwards, so dings need to be fixed quickly before this kind of on-going damage can occur.
So - why doenst Clark put a BID wrap on his props if it can help protect from minor damage? He says he does put epoxy and a 3 ply 7 inch * 3 inch BID WRAP around the leading edges where most minor stone and rain damage occurs. He doesnt put it around the whole prop because this affects the ability of the blade to twist and reduces performance by dampening the dynamic "constant speed" effects which are a big advantage of the wood material.
While not wanting to get into a competitive slanging match, Clark sites the sheer volume of prop business he's done over many years. He has a LOT of props out there on pushers, many of which have turned happily for 1000 hrs or more, and has had very few failures.
One other issue which came up was balancing. His advise to me was don't do it. His props are professionally balanced before they're shipped, and it's unlikely that I'll be able to improve on the balance given my limited skills and knowledge of the equipment involved.
Note that the above is my attempt at paraphasing what Clark said.
When I order a second prop I think it'll be a Catto, partly because it's cheaper, partly because there may be some truth in the strength issue and partly to see who makes the fastest cruise prop. I'm still going to safety wire everything that has any chance of coming loose. <<< Back | Index | Next >>>