RPM/MP power relationship

[Bruce Fenstermacher]

I have a McCauley Climb prop. In level flight I run at about 2525 at 20 inches of manifold pressure(MP) and obtain about 100mph and when I'm in a hurry I go to to about 2600 at 21 inches of MP and get about 105mph. I know the tach is a little off but I've corrected for that with these numbers, the MP gauge seems to be accurate.
I normally wouldn't cruise at this high of RPM but I've been told that I''m really not working the engine cause the manifold pressure is to low and although the RPM is high the engine is not working that hard.

What does the forum think about that last statement?

In a climbout at full RPM at about 75mph I can get about 2600 at 25 inches MP which I think is good. I'd like to improve my cruise buy repitching my prop a little droping the cruise RPM and increasing the MP.

Any one have any experiance how much a given inch of pitch effects RPM and or manifold pressure?

Of course we are talking general terms here. I know there are a lot of variables which go into a given Prop/Airplane/RPM/MP/MPH equation.

[N170BP]

I have a McCauley Climb prop. In level flight I run at about 2525 at 20 inches of manifold pressure(MP) and obtain about 100mph and when I'm in a hurry I go to to about 2600 at 21 inches of MP and get about 105mph. I know the tach is a little off but I've corrected for that with these numbers, the MP gauge seems to be accurate.
I normally wouldn't cruise at this high of RPM but I've been told that I''m really not working the engine cause the manifold pressure is to low and although the RPM is high the engine is not working that hard.

What does the forum think about that last statement?

In a climbout at full RPM at about 75mph I can get about 2600 at 25 inches MP which I think is good. I'd like to improve my cruise buy repitching my prop a little droping the cruise RPM and increasing the MP.

Any one have any experiance how much a given inch of pitch effects RPM and or manifold pressure?

Of course we are talking general terms here. I know there are a lot of variables which go into a given Prop/Airplane/RPM/MP/MPH equation.

What is your climb prop currently pitched to? I cruise my 51 pitch
prop at 2475 - 2500rpm and I get 110mph. I have a 48 pitch
climb prop that I can red-line in a shallow climb, and 2500+rpm
cruise only gets me about 90mph. I don't have a MP gauge in my '54,
but I used to fly a '53 B that had one with a 53 pitch prop. The
MP was always in the green no matter what you did (during climb
or cruise).

I've also heard that running the cruise rpm a little higher than what
would be considered "normal" won't hurt a thing on these engines
(the bottom ends are bullet proof). Course one could argue that
the faster you turn it, the faster it will wear out!

Bela P. Havasreti
'54 C-170B N170BP

[Dave Clark]

Bruce

If you cruise thatpower setting long enough to get an indication of fuel burn you could figure the approximate horsepower from the charts in the manual.

Alternatively try to find power charts for the engine showing the rpm/mp combination. I have that for the Lycoming but don't know if the % power would translate for the Cont. It might, I'd have to think about it. Both engines redline at 2700.

[zero.one.victor]

Bruce,I have a C-145 power-setting chart that shows percent of power at various rpm's,but it is based on a 7653 prop,so it probably isn't applicable at all.It doesn't give MP figures,just altitude-rpm-bhp-%hp-TAS-gph-mpg. It looks like I got it off the old yahoo "theclassiccessna170club" site,you could probably find it there if you surf around a bit.
When you say you have a "McCauley climb prop",which prop are you referring to? The numbers look almost like you might be talking about an 8043 seaplane prop.
The MP reading sound about right,based on having ridden in some friend's airplanes with fixed-pitch props and MP gauges.I don't have a MP gauge in mine. The rpm/mp radings always seem to be very undersquare for a fixed-pitch prop--unless it's a heck of a cruise pitch,then it might read close to square ( as in "24 squared").
I have to agree that the engine isn't working very hard at those power settings based on the MP. I run mine at anywhere between 2300 (loafing) and 2500 or a bit more (boogeying),but I feel a little uncomfortable running steady at much more than that. I don't think that it's necesarily hard on the engine,but my gut feeling seems to over-ride my head. I know a guy who owned a 145-powered Swift for years,he flew with some other Swifters who all had bigger engines. He told me that he firewalled it for takeoff,and never pulled the power back til he was entering the pattern at his destination. Ran it for hundreds & hundreds of hours like that--no problem.
I have a formula written down here that might be interesting,but I don't think it's exactly what you're looking for. I can't necesarily vouch for the accuracy of it,either.
pitch times rpm divided by 1056 equals mph

Eric

[GAHorn]

Two values missing from this question are altitude and temp. If you knew the density altitude you'd be able to determine the % hp for the mp/rpm. (See pg 36 of The 170 Book, or the Altitude Performance Charts for C145/O300 engine.)
On a standard day, at sea level 21" MP @ 2600 RPM equals 65% power (95 h.p.) It matters not which prop you have with regard to that calculation. (The prop you have will affect your true airspeed however.) The same MP/RPM at 7500' would result in 72% power (105 hp).
A "climb" prop will result in increased upper cylinder wear on a per-mile basis, but it's unlikely it will be significant over the useful life of the engine. (Theoretically, the pistons/valves/cylinders are replaced at overhaul with new anyway. Your overhaul costs are only increased if you try to re-certify your own parts.) The most significant increase in direct cost will be your fuel burn per mile.
The standard prop was selected based upon several factors regarding average optimum performance relative to both stage length and field performance, as well as MP relative to RPM (% power) at normal altitudes. Notice that the Owner's Manual Tab-data claims the standard prop results in performance closely resulting in 65% power at 7500 feet (where normally aspirated MP has dropped to all that available (22.5/2600) on a standard day. Switching to a climb prop may increase RPM, but also drops the available MP which fairly closely maintains the hp setting. (So, for increased TO/Climb performance, the trade-off is direct operating costs are also increased on a per mile basis, but it's unlikely to actually cause near-term "harm" to your engine.)

[Bruce Fenstermacher]

Thanks George, I'll look at the chart.
BTW the prop is supposed to be a 7651. It is the older blade style, I forget the model. I'm basically at sea level and rarely go over 1500ft (I'm a chopper pilot what can I say). These are general numbers I see through out the year at different temps and I should mention I may be going a little faster than indicated cause I don't think my airspeed indicator is that accurate.

Any body ever compare the Sensenich prop to the McCauley. I have one of these available.

[GAHorn]

The Sensenich prop has a slight decrease in performance compared to the McCauley, according to the AFM.

[zero.one.victor]

..............
On a standard day, at sea level 21" MP @ 2600 RPM equals 65% power (95 h.p.) It matters not which prop you have with regard to that calculation. (The prop you have will affect your true airspeed however.) The same MP/RPM at 7500' would result in 72% power (105 hp)............
Notice that the standard prop results in performance closely resulting in 65% power at 7500 feet (where normally aspirated MP has dropped to all that available (22.5/2600) on a standard day. ..............

George,I don't get it, your statements seem to conflict:

21"/2600 equals 65% at sea level
21"/2600 equals 72% at 7500'
22.5"/2600 equals 65% at 7500'
all standard day conditions

Huh? Am I missing something?

Regarding Sensenich vs Mac props,I believe the Swifter get better (cruise) performance out of the metal Sensenich,but I don't think I've ever even talked with anyone who ran anything other than a Mac on their 170. That should tell ya something.

Eric

[GAHorn]

Thanks, Eric! Good catch! I was hasty to get to another task and left out an important detail....which I corrected in Brown in the earlier msg.
I was pointing out that the Owner's Manual shows the standard prop to obtain about 65% (66% to be exact) at 2600 rpm/7500 feet.
The difference in the two charts is that the data from pg 36 of The 170 Book is a TCM chart and does not include installation differences. The Owner's Manual chart is a Cessna chart and does include installation penalties. The point being that the probable reason Cessna chose the DM7653 for the standard prop was because with installation penalties that prop came closest to providing 65% power at optimum altitude with open throttle.

[Bruce Fenstermacher]

George afer reviewing the AFM and looking at page at page 36 in The 170 book, i'm thinking that maybe tha Sencenich prop as tested might just be a bit more of a cruise prop than the McCauley. I couldn't find any evidence pro or con to this statement.

Also the chart of page 36 of The Classic 170 book, at least my copy is not that clear. I can't figure out exactly how to use it and if i knew how I might not be able to read the resulting numbers. Can a clearer copy ot this chart be found anywhere else?

[GAHorn]

The C-145/O-300 Operators Manual is available from Univair.com

[dalemed]

Does a (reasonably simple) mathematical relationship exist between %BHP, MP, RPM, and Density Altitude?