catto vs constant speed

[Reheat]

A lot of you have expressed interest in the Catto prop vs a constant speed propeller. There are many of us flying behind a Catto. In fact the endorsements from this group are what convinced me to try the propeller in the first place. It is simple, LIGHT WEIGHT, smooth, relatively inexpensive, has no harmonic issues and provides GOOD performance. Does it perform as well as a CS prop? No, it does not. The constant speed prop will give a higher rate of acceleration as well as a higher rate of climb. How much so depends on how well the builder kept the weight down.

Allow me to attempt to illustrate with the following graphs (My RV-8).

Graph 1 shows power available vs power req?d for fixed pitch and C/S AT THE SAME WEIGHT. The vertical distance between pwr req?d and pwr avail at a given airspeed is EXCESS POWER. This stuff is addictive. It is crack-for-pilots. The more you?ve got, the more you want.





Graph 2 shows rate of climb comparison, again AT THE SAME WEIGHT





Since MOST C/S machines are a little heavier than their fixed-pitched cousins,
Graph 3 shows rate of climb comparisons with the C/S ship weighing 100# more than the Fixed




The bottom line is that I love my Catto. For an old fighter-pilot it provides me with a very lively airplane with minimum weight and complexity. If you are an EXCESS POWER junkie, I understand... 10:1 pistons, C/S prop, and shoot the nitro to it! Just gimme a ride in it sometime.

I hope that maybe this sheds a little more illumination... more food for thought for those of you who are still contiplating a propeller.
Cheers!

[pierre smith]

.....now do a max cruise speed graph of the two, with the Catto at 7500' and 2700 RPMs

Thanks,

[Reheat]

Hi Pierre. If you look at the top graph you'll see that the two pwr avail lines come together where they cross the pwr req''d line at about 180ktas. The data point for my airplane at 75%/7500' is about 175ktas, and I would assume that the C/S would run close to that as well, given that the two prop efficiencies were similar. Since this data is derived from flight testing, cafe reports, and a Hartzell computer program, and good ole curve fitting, don't pay as much attention to the absolute numbers (they're as close as I could get em) as the comparison of the two types of props. Everyone's aircraft will have a slightly different drag polar and Installed Power number and results will vary accordingly. I should add that the fixed pitch curves are for a GENERIC prop with a max efficiency of .84, but feel it represents my Catto pretty well.

[jonbakerok]

I'm probably missing the point, but to me the charts just make me wish I could afford a Hartzell. I thought you were going tell us about Catto props. Do you have one that shows Sensenich vs. Cato vs. Hartzell?

[Reheat]

Sorry Jon, didn't mean to be misleading. No intension of trying to sell you on the Catto. I just thought some graphic illustrations between Fixed and C/S props might be helpful to some. I do not have the where-for-all to give you a comparison between the Sensenich and the Catto. Both are good propellers, I'm sure, and I expect their performance to be similar. The Sensenich has some harmonic issues, weighs more, but is impervious to rain.

The basic physics (and the whole point, I suppose) is that there is no magic in the Catto that would allow it to perform like a C/S prop. I really like mine for its smoothness, light weight, and lack of moving parts and am willing to give up a little acceleration and rate of climb. Craig nailed the data point I gave him which was 75%/8000'/2700 rpm/200mph(175k). I did not ask for more speed so that TO/Climb could be as good as possible given the pitch of the prop.

The graphs definitely show the superior performance of a C/S prop. It's the "how much difference" I wanted to illustrate. My apologies if there was any confusion.

[Steve Brown]

I've put about 100 hours on mine so far and I really like it. Smooth, quiet, light weight, NO MAINTENANCE, and the performance is good enough.

I have completely lost the urge to fly behind a CS prop. For me, the extra performance is not worth the hassle.

Also, the drag is much lower at low airspeeds. This was a pain in the pattern until I got used to it. During an engine failure, however, it becomes a life saving feature.

When I want to come down fast I keep the TAS up around 160-to-170 and pull the power. It comes down really fast.

[Caveman]

Chuck,
I certainly appreciate your time and effort in putting this info together. Even though I went with a Hartzell it is a bit eye opening to see the bottom graph which highlights the effect of the weight diffence . It would be interesting to know the actual weight of my installation when you include gov., cable control, oil lines, added oil, and brackets. I'd be surprised if it was a 100# but 60 wouldn't surprise me at all. In any case it certainly has its penalties and evens the playing field some. More important to many, would be the hard hit on the wallet & cost of maintaining the CS. Thanks again for posting this.

[L.Adamson]

Originally Posted by Steve Brown

I have completely lost the urge to fly behind a CS prop. For me, the extra performance is not worth the hassle.

All depends where you live and fly, I suppose.

Out here in mountain country, we try to convince all that are still "sitting on the fence" to go with a C/S; even with the difference in cost.

Why......... because I don't know any RV owner around here, who wouldn't prefer a C/S over their fixed pitch prop, if money and engine limitations weren't of concern; especially if they've flown both for comparison. With high density altitudes, and quickly rising topography at various airfields, the C/S is amazingly different, performance wise. And that includes getting out of the airport, as well as getting in.

L.Adamson

[elippse]

It looks like from your plot that the CS peaks in efficiency at 150k/173mph and the Catto at 172k/198mph, right? Seems to me your CS needs to be re-twisted to get it to peak at a higher speed. Notice that those two efficiency peaks are reasonably flat over 10k-20k, so the prop could be designed to deliver its peak at the higher speed and still deliver good performance if the drag was later reduced.

[elippse]

I would suggest that you people at the higher fields have your props designed to deliver rated rpm at summer density altitudes of 14,000' to 15,000', baro altitudes of 11,500' to 12,500'. That way you'll have plenty of power for your high-hot takeoffs, plus you can cruise reasonably fast at the higher altitudes where the traffic and turbulence is reduced. I've flown with a friend out of Casper, Wy, about 6000', 25C, for a density altitude of about 8500' at 1550 lb. My Lancair has 77 sq.ft. area, and SL 125HP at 2800rpm. I designed the prop to give me 2800 rpm at 10,000' dalt, so it gives me 2230rpm static and 2410rpm in a climb at 110mph IAS for a ROC of 1450-1550 at 1000' dalt, 1350lb.