Climb Prop

B

blokes

New member
I have a 1979 7ECA with the 0-235K2 engine and a Sensenich 72CK08-0-52 prop. Do any of you have the same model/engine but with a 72CK08-0-50 prop?

I was curious on what could be expected for cruise speed with the 50 inch pitch vs the 52 that I am running. Is the takeoff performance noticeably better with the climb prop?

Thanks in advance.

Bill
 
Bill
I have no experience with a Citabria with either of these props but it seems like it is possible to estimate the differences in cruise speed assuming that a fixed rpm is used with each prop. Based on my calculations a 52” prop at 2500 rpm should yield a cruise speed of 105 mph while a 50” prop at 2500 rpm will give 101 mph. Of course less power is required to turn 2500 rpm with a 50” prop so if the rpm is increased to 2600 rpm then the lost speed should be regained on about the same fuel burn or perhaps a bit more since the engine is less efficient at lower manifold pressure and high rpm compared to the 52” prop operated at 2500 rpm. Of course the prop is less efficient at 2600 rpm too so this is a very rough calculation but it does give some idea of the magnitude of the difference in cruise speed between the two props.

Here is the equation used:
MPH = prop pitch (inch) x prop speed (rev/min) x 1mile/63360 inch x 60 min/hour

For a 50 inch prop at 2500 rpm the max possible speed is 50 x 2500 x 1/63360 x 60 = 118 mph

Since no prop is 100 % efficient this number should be reduced by 15% or so 118 x 0.85 = 101 mph which seems about right. For a 56’ prop at 2500 rpm the speed works out to 113 mph which is about 5 mph less than what I see in my GCAA at 2500 rpm and a 56’ prop.

I am not quite sure how to calculate the expected difference in rate of climb. Assuming that the smaller prop will turn an extra 100 rpm in a climb (based on the above calculations) which is about 4% more power. The assumption is that at full throttle (constant manifold pressure) horse power is proportional to rpm which is probably pretty accurate for a relatively narrow rpm range. 4% is about 5 hp (for a 115hp engine) which does not seem likely to make a huge difference in rate of climb. Assuming the engine is putting out 110 hp in a climb and about 50 hp is need to drag the plane throught the air then about 60 hp is the power going towards climb. If an extra 5 hp is now available then 65 hp is now going towards climb,or an increase of 8%. Therefore the rate of climb will increase by 8%. I am not sure how valid that estimate is given the number of approximations but if the rate of climb is 500fpm then the new rate of climb would be 540fpm.

Mark
 
I also have a 79' 7ECA with the O-235-K2C and 72CK08-0-52 prop, but I had my prop repitched to 48. I like it a lot better for the improved takeoff and climb performance, and my speed is still around 104MPH at 2500RPM. I would guess the 72CK08-0-50 would be somewhere in between.
 
Thanks ak! The wife and I are touring the country this summer and we find that the 52 pitch with a full load and a warm day does not exactly leap off the ground!
We'd gladly sacrifice 10 mph for a shorter TO.

Just curious, what kind of static rpm do you turn up with the 48? Do you remember what you were getting with the 52?

Thanks!

Bill
 
Bill,

I don't remember for sure what my static RPM was before I had the prop repitched. It's been about seven years now. I do know that there was a very noticable difference. As it is now I can get about 2425 static RPM while leaning out the mix. Hope that helps some. Good luck on your trip. It sounds like it could be a lot of fun!


Cody
Palmer, Alaska
 
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