M

Monacam44

I'm New Here
Hello,

I've ordered a new three blade prop from Craig for my RV6 and before he starts making it I want to check I'm going the best route regarding pitch.

My 6 is 180hp 0360 A4A and until now it had the Sensenich 72FM8S9-1 (85)

I want good take off performance but would like to improve my cruise speed too. Craig has suggested 74 degrees pitch. Who else is using a Catto on a similar aircraft and what pitch are you running?

Kind regards,

Mona
 
Craig has changed his prop design over the years, so the pitch that works for one engine/airframe may be different due to the design he is using.

Call Craig and tell him about your airplane, engine, and engine accessories. ie: Engine size & HP, pistons, injected vs. carbureted, dual magnetos/one magneto & one EI/dual EI?s, ported heads, etc. With that information he can cut you the perfect prop for your setup.
 
My 9 just got a new 74" pitch Catto. Will be doing more test flying today/tomorrow (I hope) but I agree with Bill. Craig knows what he is doing with design and it is continually evolving. My hangar mate has a 6 (O-360) with (originally) a 74" pitch. He was worried about takeoff performance (note we are at 5000 elev) so had it slightly repitched downward and lost 10 mph in cruise (but picked up some takeoff RPM). In taking off yesterday afternoon with the 74 pitch prop, it was clearly better than the old Sensenich (exactly the same model as yours, also on IO-360) Takeoff performance in most any RV would be fine even with more than 74 pitch (IMO).

Given that most of my time (like 80%) is spent flying cross-country, my preference is to have the coarsest pitch reasonable and still be able to turn ~2700 rpm when necessary. That gives the most economical cruise. Unless you are limited to a very short field with trees at the end plus high density altitude and therefore need the climb performance, that end of the spectrum is not an issue for most any RV (again, IMO).

http://www.vansairforce.com/community/showthread.php?t=92485

Greg
 
How can a Catto prop with a 74" pitch, pull an RV through the air as fast as an 85" pitch Sensenich at the same rpm?

If the pitches are measured different, why?
 
Pitch is apparently a weird thing and measured differently by different folks. Maybe one of the propeller gurus or engineers on here can explain. I certainly don't understand it all either. Maybe has to do with real vs. ideal traction.

Greg
 
With one Revolution of the prop you will theoretically travel that far in distance number of inches, but there is more to it than that. I'm sure someone will chime in with all the cool complex fluid dynamics numbers and formulas....
 
gasman said:
How can a Catto prop with a 74" pitch, pull an RV through the air as fast as an 85" pitch Sensenich at the same rpm?

If the pitches are measured different, why?
Click to expand...

A few differences... three blade vs two blade, the design loading & flex differences in the composite vs aluminum blades, tip design and diameters. :)
 
The pitch numbers generally referenced by the manufacturer are geometric pitch. As mentioned before, geometric pitch is the distance the prop would travel in one rotation with no slip (think through Jello). However, the geometric pitch does not account for the dynamic behavior of the prop. Different airfoil shapes, loading, and twist will significantly impact the Effective pitch of the prop. The 74" catto and 85" Sensenich props probably have a very similar effective pitch.

I's also quite possible that Craig uses Effective pitch in his designation. The 85" Sensenich has a geometric pitch that would produce 217mph @ 2700 rpm (obiously high, but probably pretty close when you account for slip 200/217mph = 92% efficient), while the Catto is 189mph (low for a geometric pitch). It is also possible that the Catto twists to higher pitch under power.
 
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Thanks for all the replies. Craig definitely seems to know what he's talking about. And further to the last reply, he did mention to me on the phone that the pitch does change under load.

Kind regards,

Mona
 
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