Repitch prop

[MontanaBird]

I have been looking at ways to get my trusty '56 170B to levitate even faster, and am trying to educate myself on various options that would not include 180 hp and lots of $$$$$.

I have flown extensively in the Montana/Idaho backcountry for many years and so far what has worked best has been: fly when cool (mornings/evenings), keep it very light, and be conservative. This has served me well as I have flown, quite safely, into all of the Salmon River/Selways strips on the map, with the exception of The Big Four (Vines, Mile Hi, etc.). Just don't need to do it. OK, maybe I would like to, but, outside of my safety limits in my old bird.

Well, I've studied previous postings on repitching on our site, backcountry.org, etc., and have yet to find anything that really presents the difference between a prop that has been repitched to a 'climb' setting and one that is configured for cruise, as mine is. My static rpm in runup is also per book. I can believe that my blazing 119mph book cruise will be reduced to something like 100, but the climb difference is something that I remain very curious about.

Any thoughts from those that have done such very much appreciated!

[blueldr]

saandles,
What's with the skinny broad? Do you work for her?

[GAHorn]

saandles,
What's with the skinny broad? Do you work for her?

LOL...thx bl......I banned him.

[counsellj]

Montana Bird

I run a 7555 cruise and get 115KTS,

My climb prop is an 80/46. I gain 300FPM and cruise has slowed to 95KTS.

Even cruising slower, I prefer the 80/46 for X/C flying. It allows me to climb quicker to favorable winds and get higher.

Takeoff roll is obviously much better, but I don't have real numbers for you. But MAX WEIGHT takeoffs never take more than 1,000' at a DA of approx 2400'

Feel free to call or PM for more discussion

Jughead
850-723-2072

[jatkins]

I have 2 props,
I have a, C172EM 76 55, which gives me 110 kts TAS
My other prop is a C172EM 76 49 which give MUCH better climb performance, but only 100kts TAS. ( same RPM )

My Brother - inlaw has a C172EM 76 53 , freashly overhauled, (zero timed ) for sale $1,800

[GAHorn]

if one re-pitches or selects a prop with a flatter pitch, then don't forget that it's not absolutely necessary to cruise only at 2450. One can select a higher RPM (pwr setting) for cruise if you're willing to accept the higher fuel consumption.
These C145/O300 engines were later "geared" to run at 3100 RPM continuously in the C-175 airplane (with later cylinder designs, which most have installed by this date.) It does not impose undue strain on the engine to run it at 2500-2700 RPM continuously if at altitude. (See the power settings charts in the Owners Manuals. If I re-pitched my prop, and if I previously ran it WOT at 7500' which provided 2450 RPM previously...then I'd have no qualms about running WOT at the same altitudes as long as it did not exceed red-line (2700.)

[MontanaBird]

Thank you gahorn:

Your thought of " It does not impose undue strain on the engine to run it at 2500-2700 RPM continuously if at altitude. (See the power settings charts in the Owners Manuals."

I have always been a bit mystified at this information in our manual, that is, that the charts provide info up to a 2700 rpm max. Because, when I reread the manual yesterday they indicated 'cruise rpm up to 2450 is allowable'. And my tach has a green line that clearly stops at 2500 rpm. This contradicts with the charts and your (greatly appreciated) statement.

As an active CFI/ATP my greatest fear is that a coffee drinking buddy or student may ask me what the deal is; and if I don't pontificate properly and impress all with my answer it might cost me, well, some coffee.

thanks!

[GAHorn]

If one consults the cruise performance charts in the Owner's Manual one will observe that 2700 RPM at 5,000 ft is 75% power (a commonly accepted power setting for cruise.) This RPM would result in ever-reducing power settings the higher one cruises.

The 2450 RPM recommendation is only that, (useful for lower altitudes in order to meet the published cruise performance promoted in sales brochures, and will result in very low power settings at high altitude, an undesirably low one in some situations.)

I actually do use 2450 RPM as my most common cruise power setting and accept whatever performance that produces, however I also prefer 7500 (ideal) or lower for my preferred cruise altitude, and I have a fairly common fixed pitch (cruise) prop. (As an added note, 2450 @ 7500 is approximately 65% for my engine/prop combination, and results in a burn of 7.6 gph @ 104 kts.)

[bigrenna]

delete

[GAHorn]

.. jumped off the ground like a raped ape. ... Would I do it again??? In a heartbeat.

I'm calling the ASPCA.