Sport Cub POH Power Setting Table

[Pilawt]

I'm a bit leery of the power settings on the "Cruise speed" chart at Section 5.2.5 of the Sport Cub POH. Some of the rpm settings seem unduly low for the listed % of power. For example, it suggests that 63% power at 2000' PA would be at only 1950 rpm. I have the 72/44 wood prop, and "seat-of-the-pants" engine management feels like that would be no more than about 50% (and the quoted 96 mph TAS is certainly way off at that setting). For comparison, the power setting table for a Cessna 150G (TCM O-200-A and 69/50 cruise prop) puts 63% power at 2000' at a shade under 2400 rpm. Likewise, 2350 rpm at 5000' is 71% power according to the Sport Cub POH, but would be only 55% in a C-150G, with the same engine and an even coarser prop.

Are the power setting numbers in the Sport Cub POH indeed accurate for the wood prop? The chart in the TCM engine handbook doesn't help, because a manifold pressure value is necessary for the computation.

[Becky Teerink]

I'm a bit leery of the power settings on the "Cruise speed" chart at Section 5.2.5 of the Sport Cub POH. Some of the rpm settings seem unduly low for the listed % of power. For example, it suggests that 63% power at 2000' PA would be at only 1950 rpm. I have the 72/44 wood prop, and "seat-of-the-pants" engine management feels like that would be no more than about 50% (and the quoted 96 mph TAS is certainly way off at that setting). For comparison, the power setting table for a Cessna 150G (TCM O-200-A and 69/50 cruise prop) puts 63% power at 2000' at a shade under 2400 rpm. Likewise, 2350 rpm at 5000' is 71% power according to the Sport Cub POH, but would be only 55% in a C-150G, with the same engine and an even coarser prop.

Are the power setting numbers in the Sport Cub POH indeed accurate for the wood prop? The chart in the TCM engine handbook doesn't help, because a manifold pressure value is necessary for the computation.

Hi Jeff,
If you look on page 5-2 you can see that the data presented was tested with the McCauley metal prop and 8.50 tires. Knowing that, your experience seems right on!! Thanks for the heads up there!

[Pilawt]

Thanks, Becky. I missed the reference to the 71/41 on page 5.2.

But I'm still confused. A 71/41 is a finer-pitch (more of a climb prop) than my 72/44. It seems that if 1950 rpm at 2000' is 63% with a 71/41 as per the POH, then that same rpm would be more power with a 72/44, not less.

Or am I all wet??

[Becky Teerink]

Thanks, Becky. I missed the reference to the 71/41 on page 5.2.

But I'm still confused. A 71/41 is a finer-pitch (more of a climb prop) than my 72/44. It seems that if 1950 rpm at 2000' is 63% with a 71/41 as per the POH, then that same rpm would be more power with a 72/44, not less.

Or am I all wet??

No problem, Jeff! The wood prop is a different chord, thickness, blade style, etc. So the pitches are not comparable - meaning that the same pitch in the wood and metal prop will not perform the same. Let me see if we can get some more data...

[Pilawt]

OK, there's my misunderstanding. I didn't realize it was apples and oranges. Thanks for your help!

[Becky Teerink]

OK, there's my misunderstanding. I didn't realize it was apples and oranges. Thanks for your help!

Anytime, Jeff!

[Larry@FOK]

Hi Jeff and Becky...

Pitch is pitch. There are no apples and oranges there. The other parameters you mentioned have to do with the efficiency of one prop shape over another. That's where the apples and oranges are.

I have also felt that the numbers in the POH are way off and frankly, I ignore them for that reason. I've worked up my own numbers based on Contential's published specs that seem to work for me. For what it's worth, here's what I'm using:

75% 65% 55%
SL 2525 2400 2260
2000 2575 2450 2310
4000 2625 2500 2360
6000 2675 2550 2410
8000 2725 2600 2460

On climb out, I'm turning 2525 to 2550 which is about 75% power and that makes sense. In the winter, in straight and level flight at 1000' I have no problem going past redline so I suspect my S1 #37 with the wood prop is a little under proped. In the summer, I only get to redline, not beyond.

Comparing my performance with a Legend Cub, at the same speed, I have to turn a little higher rpm, but I burn less fuel. At the same RPMs, I'm a little slower even though my S1 is cleaner aerodynamicly.

Larry

[Steve Hamblin]

The % power information show in the cruise chart is directly from TCM. As we all know, the engine has less available power at altitude. It is also worth noting that these are charts for the engine only. The actual output will be dependant on what propeller is installed.

The reason these seem so low is that most engine horsepower is developed in the top of end of the RPM range, it's not a linear function. For example 50% full throttle RPM (1375) is not equal to 50% power, it is actually less.

Steve

[Pilawt]

75% 65% 55%
SL 2525 2400 2260
2000 2575 2450 2310
4000 2625 2500 2360
6000 2675 2550 2410
8000 2725 2600 2460

Larry, your chart coincides almost exactly with what Cessna published for the 1968 C-150H (O-200-A, 69/48 prop). I've been using the more conservative numbers from the '67 C-150G manual (69/50 prop) as a rough guide, and it seems pretty close to my actual experience with fuel consumption in my Sport Cub with the wood prop. Based on that I created the attached cheat chart.

Steve, I understand your points. But what strikes me as odd about the POH chart is that the % power numbers at lower rpms seem too high, not too low. It says 1950 rpm at 2000' is 63% (for one example), and that just doesn't seem right. Also odd is that the rpm settings for a given % of power do not increase with altitude in a linear fashion, as logic suggests they should and as they are in every Cessna and Piper fixed-pitch power chart I've seen.

[Pilawt]

Following up my last post, plotting the power chart from the Sport Cub POH in the same fashion comes out like this: