What's my prop

[krines]

I have a 170A with continental 145 and a McCaully 7648. The prop was repitched to 48 before I became owner. What do all the numbers (7648) mean? Why is the pitch noted in inches and not degrees? Is this considered a climb or cruise prop? My plane does 108mph at 2400rpm. What rpm should I see in which settings. I am a newby to this flying business and could probably find this info elsewhere but I appreciate the info on this site as it is 170 specific. Steve

[Bruce Fenstermacher]

Steve,

The 76 means that the prop was 76 inches in diameter when it was new. The 48 as you know means inches and stands for the theoretical distance the prop would move forward in one turn if it was 100 percent efficient. Props are not 100 percent efficient however.

For your 170A which I bet is a land plane as opposed to a sea plane a McCauley 1A170 7653 would be a standard prop. This is the prop that is used for all the performance numbers in the owners manual. A 7651 is a climb prop and a 7655 is a cruise prop for a land plane. Your prop pitched at 48" is a sea plane prop I believe.

But the pitch of the prop is not the deciding limiting factor. Static RPM is. The Type Certificate Data Sheet (TCDS) A-799 for our 170s spells out what those limits are and they are different for each model of approved propeller.

From the TCDS:

McCauley 1A170
Static r.p.m. at max. permissible throttle setting:
Landplane: Not over 2330, not under 2230
Seaplane (Models 170A and 170B): Not
over 2525, not under 2300.
No additional tolerance permitted.
Diameter: Not over 76 in., not under 74.5 in.

So assuming your plane is a land plane and you have a McCauley 1A170 model prop it should be 76" in diameter but can be as short as 74.5" and the static RPM that the engine can generate must be not over 2330 and not under 2230. As you can see the Seaplane is allowed to go to 2525 static rpm and in order to do this you would flatten the pitch of the prop and I'll bet 48" will do it.

To do a good static rpm check you should first know that your tachometer is reading correctly or use another tachometer such as an electronic one. Point your airplane into the wind on the ground. Make sure there is nothing behind your airplane that will be damaged by the prop blast. Lock the brakes and hold them. You might also have to chock your wheels or keep your tail tied to an anchor. Hold your elevator full up. Run the throttle all the way in and without your plane moving forward, note the rpm.

I'll bet with the 48" pitch prop your static RPM will be higher that the 2330 allowed again assuming a land plane and you aren't at high altitude. This would not be a legal installation without some other basis of approval like an STC. If you have the prop pitched to 51" you will find the static RPM will most likely fall with in the high allowable range, a 53" in the middle and the 55" at the low end of the RPM range.

[blueldr]

If you are presently checking within the TCDS tach limits it suggests one of these:
1. Your tach is off.
2. The pitch data is off.
3. The engine is not producing full power.

[Bruce Fenstermacher]

BL, I'm didn't following your statement at first.

Then I realized you were telling Steve that given he thinks he has a 48" pitch prop if he finds his static RPM to be within the TCDS limts then he has one of the three problems you listed. I would agree.

[W.J.Langholz]

BL, I'm didn't following your statement at first.

Then I realized you were telling Steve that given he thinks he has a 48" pitch prop if he finds his static RPM to be within the TCDS limts then he has one of the three problems you listed. I would agree.

Your a pretty quick study for a chopperhead

How we doing on the medical ...give and update on the other thread if you wouldn't mind

Thanks
W.

[krines]

That's alot of good info Bruce thanks. Currently its 10 below and there appears to be no break comming. I will do the static run when I can. Some more questions though. The prop is a 1A170/DM serial 547B 7648 now. Why is a static rpm of 2550 allowed with a seaplane prop. Is it a stronger prop? Also is there a risk for me with my engine with over rpm in this situation. How are props repitched and is there a risk to doing this too many times. Some say that if you want short field performance one should add a seaplane prop but this prop would not really be considered a climb? My home base KBIL is 3700' and with this weather STP is not anywhere close. I dont anticipated being able to static test for awhile. Steve

[n2582d]

... is there a risk for me with my engine with over rpm in this situation.

The TCDS lists 2700 rpm as the limit for the O-300. If you had a Koppers Aeromatic prop your static rpm limit would be between 2620 and 2700 rpm.

[Bruce Fenstermacher]

Steve,

I imagine that the higher RPM was allowed in order to drag the airplane off the water and then drag it around the sky. Yes with a 48" pitch I'd imagine you could easily over speed your engine but as long as you don't exceed the red line of 2700 rpm you won't hurt your engine. No your prop is not considered a climb prop for a land 170, it is beyond that. There is a point of diminishing return flattening the prop pitch to much. Cessna probably in testing found that out and that is why the prop and limits are set as they are. Cessna probably found it different for the seaplane because of the added drag and that is why the Seaplane limits are allowed.

Unfortunately the TCDS sets the legal limits we are stuck with unless another basis of approval is gained. In some special cases flying the airplane at one extreme a flatter pitch prop is desired. Many folks go with an 8048 or I think it might be an 8046 prop. They do this with an STC that allows the prop on the 170. What they get is greater climb because the prop is flatter but the RPM is kept in check trying to swing the bigger prop. We are talking a completely different prop than you have of course.

The basic method of repitching a prop is to hold the center in a fixture and put a bar on the blade and bend it to the new pitch. Of course it is more complicated than that as several bends are made and measurements taken but that is the basic premise. And there is a limit to how many times a prop can be repitched and there are probably limits how far the prop pitch can be changed as well. These questions can all be answered by a quality prop shop.

I would think that your 48' could be changed to a 51". I'd be surprised if it could be made into a 55"

[krines]

Thanks Bruce. Hey I grew up in Allentown and used to play golf a this course down near Quakertown. They had a turf landing strip in between the fairways. Couldn't concentrate on my golf game as I was mesmerized by the traffic. Thought I might ding a plane with an errant shot. There are a few par 5's in Montana I would love to land on but I might get a ding in the 1320D so I have been avoiding them. Somehow I think flying a 170 in the rolling hill of PA couldnt get much better. Steve

[N2255D]

Thanks Bruce. Hey I grew up in Allentown and used to play golf a this course down near Quakertown. They had a turf landing strip in between the fairways. Couldn't concentrate on my golf game as I was mesmerized by the traffic. Thought I might ding a plane with an errant shot. There are a few par 5's in Montana I would love to land on but I might get a ding in the 1320D so I have been avoiding them. Somehow I think flying a 170 in the rolling hill of PA couldnt get much better. Steve

Butter Valley Golf port?

[GAHorn]

It is my firm belief the static rpm limits mentioned in the aircraft TCDS are only applicable to the specific engine/propeller mentioned in the TCDS. There is no violation of the TCDS when 1) an approved propeller is installed with a different pitch or 2) the static rpm is different than that listed due to the different pitch.

The TCDS approves the 1A170 prop which MUST be of the approved length...but the pitch may be altered without violation of the aircraft TCDS....as long as the PROP TCDS is not violated. Of course, the engine limits must still be adhered.

(In other words, the 1A170DM and MDM series including the -7648 prop is legal on any installation of the Cont. 145/300 engine up to 145 hp and 2700 rpm., as is the increased rpm that pitch carries. No other basis is req'd.)

See the prop TCDS in the MX LIbrary.

[blueldr]

I have a flying acquaintance up in Idaho that used to have an early flat backed square tailed C-172 he flew with a 42" pitch prop. His flights all required lots of climb and not much cruise, so he used the radically thin prop.

[Bruce Fenstermacher]

It is my firm belief the static rpm limits mentioned in the aircraft TCDS are only applicable to the specific engine/propeller mentioned in the TCDS. There is no violation of the TCDS when 1) an approved propeller is installed with a different pitch or 2) the static rpm is different than that listed due to the different pitch.

Amazing isn't it when things seem to be in black and white yet two people see it completely different. George I completely disagree though I wish I believed as you do that the limits set in the TCDS for the McCauley or any of the other propellers with limits, can be disregarded. (There are a few other limits in the TCDS I'd like to disregard as well. )

The TCDS sets the limits of pitch not by stating a pitch but by limiting RPM to certian ranges. These ranges are different for each approved propeller though some might be the same. You can install a McCauley 1A170DM 7648 on a Land 170 as long as your static RPM are within the limits set for the Land plane and the diameter is greater that 74.5 inches.

It's clear as a bell to me.

[GAHorn]

It wouldn't be the first time I've been wrong. On reconsideration, (and re-reading the TCDS) I would say my previous response was poorly worded and inadvertently misleading.
The TCDS does not specify actual pitch of the propeller. A 7648 prop is therefore not necessarily unapproved.
But the static rpm of the specific model of prop is a TCDS specification and any other would require approval. (So I believe Bruce is correct to say that approval is required if static rpm is not met.)
Sorry for my hasty post.

[jatkins]

If my understanding is correct ( ? ) if you had a 76 46 prop, you would make more than the min RPM
and possiblly more than the max ?

If you made more tham the max , to be legal and to stay within the TCDS , would you just have to get the carb, adjusted down , to limit the max static RPMs ? Is that correct ??