Propellers

[Richgj3]

My new to me 170B does not quite make static rpm. Maybe 30-40 rpm low. I used a tach checker to verify my tach is correct. The prop is a Mc Cauly 76 54 at Least that’s how it’s marked. It is 76 inches long. In reading I thought 53 inch pitch was cruise and 51 was climb.

So, anybody running a 54 inch pitch? I am thinking of having it re-pitched to 52. What do you guys think?

Rich

[Bruce Fenstermacher]

I assume we are talking a 8 bolt DM 1A170 not a MDM 1A170 prop. 54 pitch is semi cruise as a 55 pitch is cruise. Do you need a climb prop? A 52 would be semi climb. If you want more climb why not go all the way to 51 pitch?

[Richgj3]

It’s marked DES 1A170
Then DM 7654.
I’ll try to attach a picture. I guess I’d prefer climb over cruise. Climb is good once off the ground but acceleration at the beginning of the take off is doggy. With this prop I’m seeing 120 mph IAS at 2400-2450 rpm.

I didn’t realize the pitch could go up to 55.

[rnealon1]

Hi,

My prop is a McCauley DM7554; I have the same issue with static RPM, also had it checked, best RPM was 11 short.

Not sure what the solution is, I get good performance in climb and cruise (120 mph at 2450) but I have nothing to compare it to.

Bob

[Bruce Fenstermacher]

It’s marked DES 1A170
Then DM 7654.
....

I didn’t realize the pitch could go up to 55.

You have a DM 1A170. On a 170, 7651 is climb, 7653 standard, 7655 cruise. Seaplane is 7649.

Your acceleration is doggy? Compared to what? A 180hp constant speed propped aircraft maybe? Do you realize that at SL at 21-2200 rpm you are only developing 70-80hp, while any aircraft with a constant speed is developing nearly it's rated hp on takeoff. Big difference.

What you need to do is get into the performance charts for takeoff.

Landing & takeoff perf.jpg

This chart data is for a 170 at 2200 gross, DM1A170 7653, full throttle, no wind, hard surface. There is a wind correction stated at the bottom of the chart and if your on grass I remember a rule of thumb being 10% longer.

Using flaps up at SL at 60°, to clear a 50ft obs. is 1820 ft. Ground run is approx 40% of that or 728 ft. Do a calculation for your takeoff conditions and measure how long on average it takes to break ground. Then you will know if your plane is "doggy" compared to what the book says. Rich, in your case, I'd expect your performance to be slightly longer but not that much longer. Pilot technique, quite frankly, will come into play here.

[Bruce Fenstermacher]

Hi,

My prop is a McCauley DM7554; I have the same issue with static RPM, also had it checked, best RPM was 11 short.

Not sure what the solution is, I get good performance in climb and cruise (120 mph at 2450) but I have nothing to compare it to.

Bob

Really Bob, you can read 11 rpm difference on your tach? Your eyes are better than most.

[rnealon1]

No I can't! But the mechanic could with an electronic tach gizmo. I just adjust my parallax until I see 2230.

Bob

[Richgj3]

Bruce

Your thoughts parallel mine this morning. It’s been 30 or more years since I’ve flown an airplane with a fixed pitch prop other than my Legend Cub, which essentially was a 100 hp J3. So my expectations may be colored by that.

I have all those tables. I should spend more time studying them. My static rpm is more like 30-40 rpm low. All cylinders 78 or above on a 250 SMOH engine.

Thanks for the info

Rich

[GAHorn]

Bruce has been drinking too much water... the prescribed static rpm does not result in “70-80 hp”. It results in about 118-120 hp.

Besides confirming your tach is accurate, you should also confirm your magneto timing is correct (28 left, 26 right, BTDC) and that your intake/induction system is not blocked and your carb throttle plate is opening fully, among other things, ...before jumping to the conclusion your prop is wrong. IMO