Cozy

Chapter 23 - Engine - Prop

I considered making a prop per the "Nigel Field" method, or buying one. Al WIck also has a very nice web site on prop making. I'd rather make it, this way I'll be able to replace and/or modify it as needed. The cost is also a factor. On the other hand, it would be nice to take advantage of the accumulated knowledge of one of these prop guys and just take delivery of a professionally made prop that I dont have to worry about. Choices, choices....

Designing the prop

Whether I make or buy, I'm going to need some numbers to make an intelligent stab at the first prop. I need the Redrive ratio - 2.17, empty weight - say 1200 lbs, prop diameter - somewhere between 68 and 74 and engine power versus rpm (but see below on choosing the supplier). From a chart published by Mazda for the stock '90 turbo I get:
RPM    HP
1000    23
2000    66
3000   116
4000   153
5000   178
6000   198
I 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.

Choosing the supplier

I sent Craig Catto an email, he called, and I had a very good chat with him. He seemed very helpful, but he needed the numbers before being able to proceed. I tried Sensenich. They didn't reply to my email after a week and there was no reply on their phone number (on a saturday). Almost a month later they emailed back that they don't make 3 blade props.

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.

Specifications

I decided on Performance Propellers in Arizona. Tracy Crook and Nat Puffer are both running Performance props and both highly recommend them. (But see the note below about BID Wrapping). Clark sent me an order form which a bunch of questions I didnt have answers to. I called Tracy and picked his brains. A summary of our conversation follows:
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 prop arrives

trial fitting the spinner Performance were very quick. I had the prop in about 3 weeks. Clark Lydick settled on a 68 diameter, 84 pitch prop, so I'm a bit puzzelled that it measures 33 1/8 from center to blade tip. Anyway, I started fitting the spinner by copying the curves of the prop with one of those metal things with pins that move in and out. I transferred the curve to the prop and cut out the shapes with a dremel. The carbon fiber is fairly flexible. I think I could have got away with cutting both curves and bending the spinner as it went on the prop. This way I could have got a better fit. Later Clark confired that my prop is a 66 x 84 with a blade area of 312.3. cutting the spinner bulkheads and flow guide The Cozy Girrls made some glib comment about my funny hat, so I took a picture for them. Figaro, figaro, figaro.... The spinner and prop fitted

Installation

For almost two years the prop sat "propped" against the fireplace in our living room. An expensive conversation piece. During that time I learned that, perhaps, the Catto prop stands up to abuse better than the Performance prop. Apparantly one Catto prop ate a whole lower cowl without serious damage, while the Performance on Dave Domier's plane shed a blade when fed the exhaust muff. The Catto props are also a but less expensive. Ah well. This is what I've got, and this is what's going on the plane.

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.

BID Wrapping

In January '04 a there was some discussion on the Cozy list about the BID wrapping that Catto does on his props and how it saves the prop from FOD damage. I have a Performance Prop sitting in the hangar waiting to go on my plane, so I've been getting increasingly concerned by the discussions about the need for BID wrapping (which mine doesnt have) to protect the prop, and the potential for catastrophic damage. Two people I know had an exhaust muff go through the prop - Dave Domier's performance prop lost a blade. Jim Sower's Catto prop didn't. So I called Clark Lydick. The discussion helped me understand the issues, and it might help others if I try to summarize it here...

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.

Rebuffs

As always with the Cozy list there were a few who felt that Clark's stance was simply salesmanship. Perhaps that's true. We really don't have enough data to make any kind of analysis. If you threw 100 chickens at spinning props how many times would the prop suffer serious damage? Would the Catto prop survive better that the Performance prop? I don't know. I don't think anyone does, and we're not about to spend the cost of 100 props (and 100 chickens to find out). We'll have to wait for more data. All we really know for sure is that the chickens would die. Those who've read of the early jet tests done in the USA may appreciate this story. Apparantly the British inventors recommended testing the blades by throwing chickens into the running engines. The American scientists were concerned because they were getting a failure every time, while the British guys were having much better luck. When the Americans asked how they could improve the engine survival rate, the British scientist said simply "Thaw the chickens".

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 >>>