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RV8R999

Well Known Member
I posted this in another thread a few days ago and don't think anyone really saw it. Thought this was revealing info you might find useful - if not disregard.



Previously I've conducted many, many hours of CHT testing with various cowl configurations including my current exit nozzle/variable exit configuration so I have solid repeatable data.

I have 7 hours on my new Catto prop and have collected 10 runs worth of CHT data during full rich, WOT, Vy (110 mph) climb from SL to 8000 ft in the same conditions as was conducted with the Sensenich prop (OAT within 3 degrees at SL and 1 deg at altitude) under similar GW (within 50 lbs).

I averaged all 4 CHTs at 2000ft, 4000ft, 6000ft and 8000ft and plotted vs Pressure altitude for both prop configurations with variable exit full open (my normal climb configuration).

The Sensenich prop cord varies from 6" to 6.25" for the length of the cowl inlets, while the Catto varies from 3.5" to 5" over the same inlet location.

The sensenich prop completely covers the both inlets for a short duration twice each revolution (a function of being a two blade and wide cord). While the Catto prop never completely blocks an inlet (narrower cord) and only 1 inlet is ever partially covered at a time (3 blades @ 120 deg phase).

The blade thickness of the sensenich throughout the inlet span location is almost twice as thick as the Catto.

Here are the results:

2hhd1zn.png


It would be interesting to collect CHT data for those with 2 blade props versus those with 3 blade props.
 
Ken, I doubt the observed CHT reduction has anything to do with 2-blade vs 3-blade, or any sort of inlet blocking. Obviously you have an increase in mass flow. I'd suggest it is because the inboard blade sections of the Catto are more efficient.

Many props have very poor airfoils inboard. The shape can be driven by the designer's structural concerns; some have conservative, massive shanks. Materials can make a difference. For example, your Sensenich is yellow birch, while the Catto is maple with a glass shell. Shear strength parallel to grain is about 11% better for the maple, plus the glass has some value.

From an aero standpoint, some folks don't think the sections hear the hub matter very much, while others do. Paul Lipps, for example, was very much a believer is efficient root sections. I think Paul and Craig spoke a lot. I've had a new Catto parked in my hangar all week (on the RV-1), and it has very good roots indeed.

As you know, at low speed propeller outflow velocity is high in proportion to freestream, and much less so at cruise or max speed. So, a simple test. Measure upper plenum pressure in climb and cruise with the new prop, and compare to previous data. If climb pressure is higher than previous but cruise pressure is not (or only slightly so), then the pressure increase is a function of higher inlet velocity due to an efficient propeller airfoil. If both climb and cruise pressures are significantly higher, you might attribute them to lack of inlet blocking.
 
I totaly agree...I calculated the blockage time with the sensenich based upon 3 deg of blockage per inlet, twice each revolution at a nominal 2400 RPM during climb and the total time the inlets are blocked is about .6 seconds per minute. This isn't it.

Exchanged emails with Craig Catto about it and he did in fact tweak the root airfoil to provide more efficient inflow during climb and has reports from owners of other aircraft types with similar climb CHT reductions.

The pressure test is on my list for sure and agree with your theory about pressure differences between climb and cruise relative to prop flow work which I have ample data with the previous configuration.

I suspect most of the modern props will show an improvement over the Sensenich in this area - I just didn't expect it.
 
I noticed it ...

... and was astounded. After pondering it awhile, I came to the same conclusion discussed above ... it's about the shape of the blade roots, not 3-blade vs 2-blade. Thanks much for your original post about prop performance ...VERY enlightening. It was especially enjoyed by those of us who decided on a Catto prop. :D
 
My 3 blade Catto never went above CHT's of 390 or so, more often 385 max. After conversion to MT constant speed, I find I can easily exceed 400. Is this due to better cooling with the catto or higher revving during climb with a constant speed? Not sure of the answer but the catto was better at keeping the CHT's cooler.
 
root thickess = wake loss

The thick root sections produce relatively little loss of prop efficiency, although it is measurable. The big effect here is the much bigger viscous wakes that are shed, and those wakes are a substantial loss of cooling flow intermittantly.
 
scsmith said:
The thick root sections produce relatively little loss of prop efficiency, although it is measurable. The big effect here is the much bigger viscous wakes that are shed, and those wakes are a substantial loss of cooling flow intermittantly.
Click to expand...

"Please, sir, I want some more." ;)

11hzdia.jpg
 
Is this an isolated experience or have other observed the same.

Numerous other people have switched to the Catto props, is this improvement in CHTs more generally observed or is it particular to the situations quoted here?

I have the Sensenich metal prop on my RV-6 and the CHTs stay below the 400s only when I climb out at below 110-120kts. It sounds like a new Catto might be the ticket.
 
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