My Cessna 140 has a Lycoming 115hp engine attached to a Sensenich 76"X50" propeller. While this combination is admittedly a firebreather of the highest order, the hot days of summer make me yern for a bit more climb performance.

A smart man would stick a crowbar in his wallet and buy another propeller for summer flying, but I was thinking I might just have mine re-pitched. Is there any formula for figuring out the pros and cons of each pitch configuration?

Anyone out there have any suggestions?

ravi,

No, there is no formula. Every engine/prop combination is an idividual entity unto itself, mostly.

But, what sort of rpm are you seeing during a takeoff run? What sort of rpm are you getting at wide open throttle in level cruise flight?

If you are hitting the engine redline at either of those, you definitely don't want to change the pitch to anything flatter.

On the other hand, if your engine is, say, rated at max rpm of 2700, but you only make 2450 on takeoff and maybe 2600 in cruise, you could flatten it out some.

Depending on those numbers, I'd start out with an inch or two of pitch, or maybe a bit more if your engine isn't turning up much rpm at all. Try it again, and repitch it again if need be.

Contact a reputable prop shop and they'll help you out with this.

MTV

Thanks, MTV,
I'm not getting within 250 rpm of redline in level cruise, and I'm only hitting about 2200 rpm at take off. I guess I'll try taking two inches out of it and see how that works. I don't want to give up cruise speed, but a little more umph on take off would be nice. 'Course if I flatten it out too much I'll run the risk of blacking out during climb, so maybe I should add an auto-pilot to keep her straight and level until I regain consciousness.

Lycoming O-235-C1 engine - Use actual weight
Eligible on Model 120 and 140 landplanes with the following limits:
Fuel 80 min. octane aviation gasoline
Engine limits For all operations, 2600 rpm (108 hp)
Propeller Sensenich M76AM-2 with the following limits:
Static rpm at max. permissible throttle setting:
Not over 2340, not under 2240
No additional tolerance permitted
Diameter: Not over 74 in., not under 72.5 in.

You should be hitting at least 2240 static. Has your A&P checked your tach accuracy?

Looks like you can flatten that prop an inch or two and stay legal.

Reading that TCDS listing, seems to me that the Sensenich M76AM-2 is actually a 74" prop-- as in "..-2" ?
Looking in my Univair catalog, sensenich shows 110,115, & 125-horse C-152 climb props as 72-52. So a 76 (or even 74)- 50, being bigger diameter, maybe is a bit much.
I seem to recall that 2" of pitch is worth about 100 rpm.
A bit "too much" flat pitch doesn't necesarily mean you'll lose cruise speed, just turn it up a bit more. Flatter pitch means that it's not working as hard, aka lower manifold pressure, however it will run the tach time up a bit quicker.
Regarding climb versus cruise prop pitches: I'd rather be a few minutes late getting somewhere than a few feet too low at the proverbial 50' obstacle!

Eric

As usual, I'm confused. While I've never actually measured it, I sort of assumed a 76" propeller was 76" long. Shows what I know about aviation, yet again.

I'm correct in believing that the static rpm is the rpm at full throttle with no forward aircraft movement? I suppose I should check my tach before I start bending the prop too much, though bending the prop sounds like more fun.

I believe prop pitch is measured as how many inches foward you would move from one revolution if there was no slip. Of course a prop is washed out so that measurement is true only at one station of the prop. I'm sure it's not this simple, but I thought a good starting point was to just barely hit redline in level flight.
Yes, you may be money ahead by verifing the accuracy of your tach.
I'm a complete non-expert with props, but maybe this will stir some comment.
on edit, prop length is measured in inches too. second edit, repitching a prop is best done by people who know what they are doing. Find a good shop and be willing to pay for it.

Ravi, a 76" prop is indeed 76", at least to start out with. I believe the Sensenich designations use a system where the original length is shown, then "-2" or whatever to indicate it's been shortened. The prop called out in the TCDS entry posted above is a M76AM-2. I found this prop listed in my Univair catalog for a Piper PA-11,O-235 Lycoming 100hp, with the length listed as 74". BTW, standard pitch for that PA-11 airframe/engine combo is 46", no special climb or cruise pitch shown. I think Hartzell or McCauley uses a similar "minus" designation on some of their c/s props.
Is the Lyc O-235 in your 140 installed per the TCDS, or was it done by STC? If by STC, the STC paperwork should call out the approved prop(s) and static rpm limits.
(It's all about as clear as mud, I know-- a guy just about needs a Philadelphia lawyer just to figger out what propeller he needs to have. )
FWIW, a friend of mine's theory on prop pitch is that the airplane should turn redline or a bit above at sea level, wide f***in' open in level flight. I tend to agree with him. If you can't get near redline in level flight, flatten her out. If static rpm's a bit higher than spec'd, oh well- can't win 'em all.
The prop shop usually doesn't want to hear what itch (in inches) you want for a repitch-- they just want to know how much more rpm you want. Like I said before, I think 2" of twist is good for about 100 rpm.

Eric

The only prop formula I've ever heard of is this:
prop pitch (in inches) X rpm divided by 1056 = airspeed (in mph).
For example, Ravi's 140 (which he sez won't hit redline):
50 X 2500 / 1056 = 118.3
Is that close, ravi?
This is not a hard and fast thing, more like a rule of thumb, but it seems to work out for my 170:
51 X 2700 / 1056 = 130
51 X 2500 / 1056 = 120
51 X 2300 / 1056 = 111

Eric

I'm going from memory, but it seems to me that's about right. Honestly I only pay attention to my airspeed when taking off and landing...rest of the time I look at the gps for ground speed.

Just re-read an old article from the 140 Club newsletter, "A Mountain 140". Author had a 140 with a 290D Lyc, said with a 74-52 prop he cruised 115-120 at 2450, and climbed "like a (little) rocket"!

Eric