Eagerly awaiting the replies on this one... Mine is pitched at 53, not sure whether to go down to where Jr is, or leave it.

Zane, mine's pitched at 51 & for safety's sake I wouldn't have it any coarser. Sometimes the climb ain't any too good even with this "climb prop". I still see 120-ish cruise speeds so I don't feel the higher cruise possible with a 53 or 55 to be worth the trade-off for less TO/climb performance.

Eric

I hate generlaziations because someone always comes up with an example that shows your wrong. but generally smaller diameter, higher pitched props are for speed because they have less drag. Look at say a pre-war supermarine to see what I mean. Longer props will develop higher static thrust, extreme example is helicopter blades, there long because they have to be to develop enough lift.
Your case is a perfect sales pitch for a constant speed prop, but those cost money and add weight.
Oh yeah I forgot, generally speaking three blade props will out climb two blade (more bite just like a longer prop), but are slower in cruise (more drag, just like a long prop) are are heavier to boot.
I'm putting a Hartzell scimitar on my M-6/235, I'm hoping that the scimitar blade design will help keep me from losing too much in cruise, but I feel that I will have to lose some, just so long as I get a little better short field I won't mind.
I guess that any prop is some sort of compromise, you just have to determine what you want it to do best or pick a prop that does everything OK, but nothing well.

Mine is a 51 pitch (was 53 when I bought it). I also have a 48
pitch prop that is too flat (and too short). It will get mounted
over the fireplace here one day.

I get about 113-114 mph cruise with the 51 pitch prop (this is with
850 tires). It did about 117-118 mph with the 600x6s that were on
it when I bought it.

I also think for a B model 170, 51 pitch is the "sweet spot". You
can still get OK cruise speeds, and get good static rpm (mine
turns 2425 or so when you cob it, which comes up to 2475 or so
50 feet into the takeoff roll).

I flew a 53 B for awhile that had a "cruise" prop on it and it would
barely make static rpm per the TCS....

When you think about it, with the 51 pitch prop, I'm making about
133hp 50 feet into the takeoff roll. WIth the 53 pitch (at 2200
rpm or so static), you're only making 118 or so hp. Not a great deal
of difference, but in a stock-engined C-170B, I need all the power
I can get!

(still thinking about that $9K MT controllable pitch propeller upgrade...)

I better point out that I'm turning it up pretty good for that 120 cruise speed- somewhere around 2550 or so. Lotsa people like to cruise at 2300 or 2400, and would therefore see a correspondingly lower IAS. My airspeed indicator seems pretty close based on calm-wind GPS readings.
FWIW, I ran across this formula when looking for prop rpm info. RPM times pitch divided by 1056 equals mph. Seems to be pretty close: in my case, 2550 X 51 / 1056 = 123. How about another: 2300 X 51 / 1056 = 111.
I was actually looking for information on how much pitch change (in incjhes) equals how much rpm change, but I couldn't find it. Did the prop man tell you what the pitch on your prop is now, JR?

Eric

You can get an optical tachometer from a hobby store like tower hobbies that is extremly accurate and will read 2,3 or 4 bladed props. They are inexpensive, like 25 bucks or so and accurate enough so that when you point it at a flourscent light it will come up 3600 rpm for a 2 bladed prop. 60 cycles per sec is 3600 per min. It will tell you how much your installed tach is off.

3 deg = about 100 rpm usually.

I have a 90" pitched to 32 and do about 96 mph @2500 rpm

I pull static 805 lbs of thrust and turn 2450

If you want better short field put a longer prop on, I don't know if there is a longer approved prop but if you don't care about that bigger is better.

also for high and hot you are still going to be better off with a larger diameter. I only have real experience no book talk but I can tell you without a doubt my 90/32 will blow the 84/42 out of the water. Probably around 30 percent shorter ground roll and climb will be amazing.

Greg

Our field elevations range from 5500-7500ft here. I know the Borer prop on a Super Cub sucks at these altitudes if your using the recommended pitch to Hp.

The closer you are to sea level the better the Borer performs. This info also is not published in book form but comes from many years of flying the Rockies.

Byron Root (of Sherpa) has done a lot of testing with fix pitch props and this 1A200FA90 is the one that worked the best from sea level to high elevations. They ran them on 160, 180, 235 hp cubs. I do not do a lot above 6000 ft (Idaho in the summer, Eastern Oregon) but I can tell you that if you saw my airplane take off at 6000 ft with the 84" and then saw my performance with a 90" you would be sold.

Greg

Mauleguy wrote,

I have a 90" pitched to 32 and do about 96 mph @2500 rpm.

With Zero.one.Victor formula, this would yield a speed of 76mph, do you mean "pitched 42"? Then the math works with your #'s

NOPE it is pitched to 32, as for the formula it is wrong I would guess.

There is a formula for hydroplanning developed by the military that says the square root of your tire pressure X 9 will give the minimum speed that you can hydroplane at. With this formula and my tire pressure of 2.5 it means I should be able to drive on the water at 14.23 kts. and I know that is not the case. It may work just fine with 70 psi !

Greg

As to length, I agree with Mauleguy 100%. My 170, with a 180 engine reduced takeoff run on floats by a good (no joke) 30% when I exchanged the 74 inch diameter prop for the 80 inch prop that's on it now. That is close to sea level, but I have to believe the ratio would be significant at altitude as well.

A friend recently converted his Husky from the standard 76 inch prop to an 83 inch MT prop. This fellow is based at over 6000 feet elevation, and his response was simply that, in some 6,000 hours of flying this airplane, going to that prop was simply the best thing he ever did.

Now, as to pitch on a fixed pitch prop, of course you'll need a steeper pitch when you are at altitude. That's why an engine equipped with a constant speed prop is referred to as an "altitude" engine.

It is worthy of note that almost EVERY mechanical tachometer has significant (in my experience between 50 and 100 rpm) errors. If you haven't already done so, get a tach checker and find out what your engine is REALLY turning.

Also, pitching fixed pitch props is a VERY imprecise science. You will find a lot of these things that have one prop at one pitch and the other an inch or two off--on the same prop!!

Again, that's why we have constant speed props.

MTV

Jr.CubBuilder wrote:Supercubber what are you running on your SC, and what's your HP if you don't mind my asking?

For a long time we run a 150 hp SC with a standard prop and pitch. Switched to a Borer 82-41 and it was like driving a car with the transmission going out. Lots of rev but no go.

The prop had been checked for balance and correct pitch.

Had the same opportunity to fly the borer at 1000 ft elevation in Nebraska and it was a totally different animal.

If flying at altitudes of 5000 ft or above I would not go with the recommmended prop pitch on the Borer as it simply does not work. A 42 or even 43 would work much better on a 150.

According to Eric's testimony, the 76/51 is the way to go IF you are operating at the elevations he is operating at. But, given that I operate from 1200 MSL sometimes, and on up to airports at 6000 MSL here in the Sierras, the 76/53 I have on the 170 would actually do me better since it is going to slap more air per revolution, and in the higher elevations, you need that?

I think that choice of prop/pitch for the 170 is a bit more delicate, given its power/weight ratio. It needs to be as efficient as possible, since it's not exactly bulging at the seams with ponies. Jr, do you feel you've got enough takeoff performance to sacrifice a little for a better cruise?

There's actually some good info on this over at tic170a, but it's a slalom trying to read in between the sincere opinions and the verbal high horse of the moderator.

zane wrote:.............There's actually some good info on this over at tic170a, but it's a slalom trying to read in between the sincere opinions and the verbal high horse of the moderator.

A verbal high horse on the "170 World According to George" website--come on now, who's gonna believe that?

I don't operate a lot up high, but "slapping more air" aside, I'm betting that you'd need the flatter pitch prop for TO/climb performance even more up in the mountains than you would at sea level. Zane, do you know anyone with a suitable prop you could borrow? Only way to know for sure yourself is personal experience with your own airplane.

Eric

I went through the prop thing just like you guys with my Pacer. What I came away with, is that the factory pitch is usually the best compromise. Everything said on this thread is correct regarding changes and how they affect performance. The inescapable fact remains that with out increasing the horsepower there is no way to have both short field and good cruise performance. Even a constant speed may not help much due to the increased weight and change in CG.