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Catto & Prince Props (long)
Why write this? First, there is very little direct comparison data on FP props on the same airplane in similar conditions. Second, I'm not aware of any data comparing these two well reputed props. Third, I hope to learn from the other members of the forum by presenting this information.
I started with a Prince P-Tip and recently bought a Catto. The P-Tip is a 2-blade and the Catto is a 3-blade. I did some testing under almost identical climate conditions. In both cases, a mild high with haze layer below and clear above. On warmer days I've had higher speeds from the P-Tip by about 6 kts and I don't know why the difference, but at least these two runs were under very similar conditions. Both props were pitched by their makers after thorough discussion about my engine, cowl, fairings, etc. but still ended up too finely pitched. This may be evidence that my engine is stronger than expected. However, the airplane is not going as fast as it should, so that's an unsolved mystery for now. The Catto is on its way back to Craig for adjustment. Both prop makers are excellent people to work with and I'd have no hesitation in recommending either one. The instrument is a GRT Horizon & EIS and the EFIS is calibrated for TAS as per manufacturer's directions. Based on how it computes hw/xw and on the GPS speeds, I believe the TAS is accurate to within 1 knot. The OAT is below the wing in the first inspection panel and seems to agree with the ASOS once it stabilizes after leaving the heated hangar.The EFIS altimeter is IFR certified and tested to a max error of 20' all the way to 20K. The MAP was calibrated when the IFR check was done. Cruise Data P-Tip Start with full tanks OAT -6 deg C at altitude Density Alt 9130 Power (per EFIS) 76% WOT, best power mixture by ear 10.2 gph (maybe higher) 2750 rpm 168 KTAS Catto Start with -19 gallons = 114 pounds approx. OAT -3 deg C at altitude Density Alt 9472 Power (per EFIS) 78% WOT, best power mixture by ear 10.8 gph 2810 rpm 173 KTAS Climb Observations Catto climbs better but at lower RPM: 2200 vs 2400, roughly, with about >200 fpm difference for Catto. Takeoff quicker by a few feet and accelleration subjectively better. The P-Tip climbed at about 1500 FPM using 16.4 GPH at 2440 rpm. I did not write down the climb numbers for the Catto, but I remember seeing 1800 or more on the VSI and lower GPH.Test was done about 100 pounds lighter but this seems to me a difference beyond the weight difference. In other flights I thought the Catto was doing this, even with a passenger and the FPM was higher. Noise observations If there's a difference, the P-Tip is quieter, but the difference could be how I inserted my HALO earplugs that day. Other factors The Catto is significantly out of balance and thus could be expected to perform a little better if balanced better. Side Note #1 Prince says his prop self adjusts by as much as 3" of pitch based on load and rpm and until now I could not test that. The RPM range of the P-tip is about 200-250 less than that of the Catto and this is evidence that Prince is correct. Range here means from climb/static to top power at altitude. It's interesting that while the prop does seem to self adjust, the 3-blade Catto is climbing better and going faster anyhow. While the difference in cruise is not great and can be attributed to higher RPM (higher power), the climb difference is clear. Conventional wisdom says the 3-blade will climb better and cruise slower. Side Note #2 Power percentage is, as I've noted before, a slippery concept. These numbers show that. In both cases I moved the red knob to where the airplane flew the fastest but I don't know how much ROP that was. The BSFC's should have been very close to equal. The Density Altitudes were quite close and should not matter anyhow. Superior says that at peak EGT BSFC is .43 and at 75 degrees ROP it's .50. So best power is likely at something between .43 and .50. (a quick Google of "best power EGT" gets answers from 25 to 125 ROP.) If we arbitrarily use an optimistic .45, then 10.2 gph is about 75% but if we use .50, then 10.2 gph is 68%. For 10.8 gph, it's 80% and 72%. The ratio of 10.2 to 10.8 does not equal that of 76 to 78. It's possible my 10.2 number is wrong. It's also interesting to note that the traditional 8000 Density Altitude would, in my airplane, give well above 75%. On the Catto flight, I tried it and WOT at 8000 yielded 82%. I think that's the result of the ram air in the intake snout, but I have no direct proof other than in other tests with this airplane, the MAP values were higher than the DA would account for and the MAP has been calibrated. |
More info on the Catto please.
What pitch is your 3 blade Catto? Is it the newer style? I know that Craig has changes the blade a bit making hte newer 74 euqal to the older 76 pitch. How old is this prop.
Also, can you comment on how speed varies as you drop RPM. What speed did you get at 2500RPM, ect. At a particluar altitide, is your speed linear with RPM? |
responding to whifof100ll
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New Data
Catto Prop 3-blade, new blade shape, re-pitched by Craig to keep the RPM's under redline. First flight. Smooth air, but cold.
NTPS 3-way run using TT autopilot with altitude hold. Flew GPS tracks 360, 240 and 120 degrees. WOT MAP 22.85" -4 Centigrade RPM 2660 78% power per GRT Pressure Altitude 8480 +/- 10' Density Altitude 8000-8100 360=176, 240=169, 120=181, TAS=175.4 kts. Full rich (a little more speed might be available with best power mixture) |
Too rich
Good news....your speed is very close to ours but even in cold weather, I see a definite increase in RPMs at the higher altitudes as I lean. Our best so far was 204, solo, 7500' warm spring day, leaned aggressively, WOT..2700 RPMs.
Regards, |
Catto Prop
I Have a 3 blade Catto for my Lancair 360. Everyone has been telling me not to use it and get a Hartzell CS. They all claim its too hard to slow the airplane down with a fixed pitch. Have you guys been having any problems with this on your RV's?
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Yep its true
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A fixed is just that and has a bigger pitch or bite in the patten so it makes more thrust at idle. Some of the real light two blade all wood props need higher idle RPM for smooth idle on the ground, so that aggravates it further. The metal blades have more inertial so you can idle smoother at low RPM with the flywheel affect. Is it a problem? No you learn to live with it. You can assume a fixed pitch prop will get a little more engine out glide. For a constant speed prop, it would be nice if you could increase the pitch with the engine windmilling; there is probably not enough oil pressure to do that. I have not tested it but as a CFI I have never seen it written down in and AFM. Some claim they pull the blue knob back and feel the difference with their constant speed prop? The Aerobatic prop will go to high pitch. It's not going to feather but will go to high pitch with out oil pressure. The MT three blades I hear are down right scary engine out, windmilling. They just have more flat plate drag than a two blade prop. The down side with acro props is they are counter weighted, so they are heavier. To me this is a second or third strike why I thing two blades are better than three. Bottom line slowing down is probably the last consideration you want to consider when deciding on a prop. There are other more prominent factors. Now RV'ers are into formation flying and if you are flying with a bunch of constant speed prop RV's and you have fixed, it can get sporty. You just can't slow down like you can with the c/s prop. |
Dos Blades Catto Por Favor...
Craig made the first 2 blade RV prop for my RV4 back in 99' when I lived near his shop in CA. I stipulated 69" diameter for my short gear legs and frequent use of gravel runways in the Idaho mountains. Craig used 72 pitch and with my 150HP Narrow Deck 0-320 it was a perfect combination. Craigs desire for customer satisfaction was exemplary. With the exception of rain protection, the Catto prop is a home run in value, performance and efficiency. Applying leading edge tape from Tennessee Propellers cures rain damage but reduces performance about 1-2%.
One of my buds owned a Catto 3 blade 160HP RV4 and was slower and only an even match in climb with my 2 blade, suprisingly even with slightly more HP. At equal RPM my 150HP 2 blade was 5-7 knots faster at higher power settings (above 2500). We even switched props and the roles reversed. As stated previously, 2 blades are more efficient than 3. Less drag=More speed and efficiency. Having tried many props on my RV4, the Catto 2 blade was the most efficient across the board, but not the fastest. That honor went to my Hertzler composite and Margie Warnke "Claw"...As far as Fixed Pitch props being hard to slow down, "technique is everything". Turning my RV4 sideways in the air slowed it down very well...:) Rob Ray RV4/HR2 [i]In the Rocket world John Harmon has shown the big 2-blade "Paddle Hartzell J-twist" as the most efficient prop on the Rocket. My 2 Blade Hartzell Rocket is no exception having flown alongside 3 blade MT's and Hartzells and been faster across the board. In my ANG squadron of our nine RV4's the fastest and most efficient was a 190HP C/S 2 blade Hartzell. |
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Tom |
What's that last 50 rpm worth
My speed at 2650 is 175.3. Divide by 2650 and multiply by 2700. You get 178.6 for a difference of 3.3. Yours is similar.
In reality, it doesn't work this way and you'd likely get less than 3.3. To turn the engine faster you have to reduce the pitch, so each revolution is worth less in forward motion. To go faster you need more HP in the percentage equal to the cube of the speed difference. This is true regardless of RPM. By turning the engine faster (assuming flat torque) you would increase HP a little. You'd get 1.00624 more speed (0.624%) from the additional HP. (2700/2650)^(1/3). If you are going 200 mph, you would get 1.2 mph. At least that's what my simplistic math tells me. You are probably right not to send it back to Craig. Your RPM and mine are almost identical. I'm happy to have a little margin to keep me from redlining it. |
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