Yet Another Prop Question(s)

[juredd1]

So I have searched and read for hours on propellers and I think I have an increased knowledge level but I have to admit I am not the sharpest knife in the drawer.
I currently have a Sensenich 74DR-0-56 according to my paperwork. I have not verified any stamps. I have an O-300 engine.

I have had not issues thus far with the grass scripts that I currently use but want to venture out to some other grass strips in the surrounding area and would like to increase my safety margin by putting a climb prop on my 170.

If I understand correctly the Sensenich version of that would be the 7454. I don't completely get the whole pitch thing. According to a post I read that the pitch indicates how much travel forward you get for each rotation. So a 7458 I can see where that gets you a better cruise prop for the a fore mentioned reasons. However, how does a 54 pitch prop get you off the ground faster with less forward travel?

Is 54 the maximum pitch that can be put into that prop without putting the engine in some situation no recommended?

Also 99% of my travel is within 60 miles of my home base so the reduction of speed at cruise is not that big of an issue but on the off chance I get to attend a convention or visit Oshkosh that 10-15 mph drop in cruse speed could cost you a couple hours on a 1000 mile trip and an extra $100 bucks in fuel. So the thought to purchase a second prop came to mind as I could swap it out for the climb prop if a long travel opportunity came up. Is that a viable option as I am not sure if an IA has to be involved for that prop swap.

There was some mention of doing a static run up to check max RPM values. Sorry to say I am not sure how to perform that test.

Last but no least I'm in North Central, AR....any recommendations for shops in my state or surrounding states?

I do apologize for the lack of knowledge on my part but unless I ask I won't learn.

Thanks for your time and any knowledge you could pass on to me would be most appreciated.
Justin

[GAHorn]

Hello, Justin!
The lower pitch (54) prop you suggest will be similar to starting your car off from a standing start in a lower gear than usual. In other words, your engine will turn a slightly faster RPM and will therefore generate a bit more horsepower and your airplane will accelerate more rapidly, using less runway for the takeoff and provide slightly better climb rate.
The compromise will be similar to cruising your car in that lower gear..... the top speed will be slightly less than with the 76 pitch prop, and you will experience slightly less mpg fuel burns. The exact amount of change in performances is difficult to predict exactly, but I suspect you will not find the reduction in cruise speed that unpleasant, and you will enjoy a better takeoff and climb from short strips.
The max/min pitch will be limited by the RPM limits of your engine, i.e., you don't want a pitch so flat that it over-revs the engine during takeoff or during cruise. The engine is restricted to 2700 RPM max.
The "static rpm" test of your engine is dictated by the Type certificate data sheet. This refers to the maximum RPM achieved on a standard day at sea level with the brakes held and the throttle at maximum. (It's important when checking that matter that your tach is determined to be accurate. You can use a hand-held tach-check to do that, your mechanic likely has one. IF you plan to come to convention let me know and I'll bring mine.)
Yes, you may change the prop back and forth if you have a spare prop... but that will require an A&P logbook entry (Not necessarily an IA). You should be aware that frequent changing back and forth will place addt'l wear on your prop bolts, spinner (if any) etc. and you will likely want to replace the prop bolts/fasteners more regularly.
If you decide to purchase a second prop I"d suggest you look for a McCauley as they are slightly better performers, according to the charts on this airplane.
Hope this helps.

[juredd1]

Thank you George for your patient reply. I am sure I have a couple more questions based on your reply but will digest this a bit here in hopes of asking the right questions.

Thanks again,
Justin

[Bruce Fenstermacher]

George covered it pretty well.

One little detail he left out of the static PRM test should be to perform the test with a clear path in front and behind the aircraft such as a runway environment would provide. You would not want to do this with your hanger or some other object restricting airflow in front or behind the aircraft. The restricted air flow could make a subtle difference in achievable RPM.

I would also look for a McCauley only on principle the book says it is a better prop. My partner and I have 3 props, a McCauley standard 53 pitch, a McCauley climb 51 pitch and a Sensenich 74DR-0-56.

For the longest time the plane had the MC climb prop installed as the plane was based at a one way 1300 (more like 1200) foot grass strip. When the plane was based at a longer field I had to talk my partner into switching out the prop to the standard. He was trained to and flies at minimal power settings and I could keep up to him in my Cub. He was always last to breakfast and the extra 5 mph meant the rest of us didn't have to wait for him so long to get our breakfast on our morning flights.

For about 3 months I installed the Sensenich 74DR-0-56 to compare it to the MC standard 53 pitch. I could not really tell any difference though I new the book said there was and so there must be and I swapped out for the MC.

I'm here to tell you. It is nice to have two props while you decide which you like better. But once you decide, you will likely never change out the prop. If you decide a second prop is the way to go I'd look for a MC but I wouldn't pass up a deal on a Sensenich.

[Pdogace]

I also have read a lot of posts on climb props versus cruise props. My question is this... I totally get the aerodynamics and theory of all the discussions but what are the real world takeoff numbers between the two props. I have read a lot that the cruise prop give you 5 to 10 mph in cruise but how much takeoff roll are you trading? Is it like an extra 200 feet or are we talking 800 to 1000 feet extra takeoff roll. Since Bruce has used both props, would you share your estimate on the takeoff roll for the 2 props? I am more interested in real world experience than book numbers. Thanks for your help in advance.

[bagarre]

My dad just swapped props on his 65HP Luscombe from a climb to cruise and back to a climb.
His difference (same density altitudes for both props) was about 5mph in cruise at 3,000 msl BUT he lost 100FPM in climb.
The difference from 85 to 90MPH is no where near as noticeable as 200FPM and 300FPM.

I saw the same when I went from a 53" to 51" in the 170. I lost 5 - maybe 8 or so MPH cruise at 3,000MSL but the rate of climb was 100FPM higher on the VSI.
I don't know about takeoff distance - I never measured it but with the climb prop it 'feels' like the tail comes up a little sooner.

On a 150 mile flight - a cruise prop will get you there 5 minutes sooner but a climb prop will let you get over the trees with more room.
I personally don't like trees and have never been punctual anyway.

Hope this helps.

[Bruce Fenstermacher]

I've actually experienced flying the MC '51, '53 and a '54 I had twisted special. There is more to it than just pitch so it is pretty hard to give numbers. For example these three pitches were on 3 different MC props and two different engines and even two different 170s. Each prop will have slightly different characteristics and the given engine will have different ability to spin each one.

What has been described already is the best. A climb prop will make it a little more comfortable on departure than the '53 given the same departure. The climb rate is better.

I had a prop twisted to '54 from '53 because after lots of digital tac readings and thought I felt I could get away with a slightly courser pitch on that prop and that engine and take advantage of a mile or two more speed. Basically I thought the prop was acting a little flatter than it was. I asked to have another degree put in it. I didn't notice appreciable takeoff difference and the plane seemed to go just a bit faster.

[ghostflyer]

Each brand of prop has different characteristics . They can be the same pitch but operate differently . I am a fan of Sensenich props. They are made of a harder aluminium which resists stone dings and do not flex like the other brands and plus they have a different profile . The only way to find what suits is test out a range of props. Sensenich have a thinner profile which allows them to spin quicker but allow a courser pitch .

[juredd1]

I have never performed a static rpm test with my 7456 so I do not know what the rpm value of that should be or will be. I could have this in reverse but it would seem a climb prop would be taking a bigger bite of air if it's going to get me off the ground faster thus the engine will not be able to reach the rpm range of say the standard or cruise prop. I expect the cruise prop does not get as big a bite (flatter pitch?) thus the longer take off and thus a higher rpm reading?

Ok, so I re-read George's post and looks like I have this backwards. I would expect to see higher RPM readings with the climb prop so if it is turned to much that could push me over the 2700 max rpm range. Close?

I need some direction on how to start this. Can I get some recommendations on shops that I can trust?

Thanks to all that have provided input.

Justin

[canav8]

I have never performed a static rpm test with my 7456 so I do not know what the rpm value of that should be or will be. I could have this in reverse but it would seem a climb prop would be taking a bigger bite of air if it's going to get me off the ground faster thus the engine will not be able to reach the rpm range of say the standard or cruise prop. I expect the cruise prop does not get as big a bite (flatter pitch?) thus the longer take off and thus a higher rpm reading?

Ok, so I re-read George's post and looks like I have this backwards. I would expect to see higher RPM readings with the climb prop so if it is turned to much that could push me over the 2700 max rpm range. Close?

I need some direction on how to start this. Can I get some recommendations on shops that I can trust?

Thanks to all that have provided input.

Justin

Justin. Hold on a second. Propeller max RPM occurs in two different states. Static and Dynamic. Static is done when the propeller is loaded. The static RPM of propeller will always be less then the dynamic rpm of the same propeller because of the lack of air passing over the blades. The test that we check is to make sure the engine is developing full rpm for takeoff. It is the static state. The type certificate Data sheets have a specific RPM based on the engine propeller combination you have. Your job is to make sure you get that RPM only. If you are getting less then the required RPM on takeoff you need to find out why. If your engine is healthy then the propeller may be the wrong size or has been re-pitched to coarse. If the RPM is above the maximum range then the propeller may be wrong or it has been re-pitched and is to fine of a pitch.
So now, how does a 54 pitch get off the ground faster then a 58? Because the engine can turn faster there by making more horsepower with the 54 pitch. It will shorten takeoff distance but you will loose in cruise speed. Vice versa with the 58 pitch but you will gain speed in cruise speed. In either case your propeller choice is governed by the same document. The TCDS. you can only put certain type props on your engine. It is the same thing for repitching props. You cannot go and just have a 54 pitch prop repitched to a 59 and be legal. It must fall into the max static rpm range for takeoff.
I also point out that your engine rpm will go higher in the takeoff roll as the propeller leaves the static state and goes into the dynamic state.
In flight it is easy to get the max redline of the engine and that is good but that is what a throttle is for (to stay below redline) It is not important that you can exceed the redline other then it is a limitation and you shall not do it. Also you can fly full throttle and load the propeller more by increasing your climb. It will also keep you below redline.
The bottom line is correct propeller pitch is when the engine propeller combination fall into the max throttle static rpm range for your configuration only! Do not start hunting for a shop, there is probably nothing wrong!

[bat443]

I'm going to muddy the waters a little more here. A common perception is that a "climb" prop will have a slower cruise speed than a "cruise" prop. That is only true if they are both run at the same cruising rpm. Typically for fixed pitch propeller equipped aircraft including the Cessna 170 engine rpm is used as an indication of power. For any given rpm the "climb" prop will be using a lower percentage of power than the "cruise" prop as it takes less power move a lower pitched prop blade through the air than a higher pitched one. Running the "climb" prop at a higher cruising rpm will bring the percentage of power back to that of the "cruise" prop and restore most of the higher cruising speed minus slight losses do to less propeller efficiency at higher rpm and higher engine internal friction at higher rpm. It is important to note that the cruise power settings in the Owners Manual are for a specific propeller, for example for the 1955 170B the cruise charts are for a McCauley 7653. Fuel burn is a factor of percent of power not engine rpm, this is not exactly true but adequate for this discussion. Here is an interesting observation you can make with a fixed pitch propeller aircraft during takeoff. If you takeoff tail low and accelerate in ground effect to best rate of climb speed you will note that the full throttle rpm increases, as you reach best rate of climb speed you will feel the aircraft begin to climb out of ground effect, part of that increase in rate of climb is due to the greater lift from the higher speed and part is from the increase in percentage of power indicated by the increase in rpm. This is caused by the effective reduction in the angle of the relative wind of the propeller blade due to the forward movement of the aircraft. By now you should be thoroughly confused until you have a chance to think it over.

Tim

[Pdogace]

This discussion on props has not answered my question. So I will try to ask it again from a different angle. I understand all the aerodynamics and fully understand all the variables that go into takeoff performance. So my question is this: If I could wave my magic wand and put two identical 170s with o 300 engines, on a standard day, with one having a cruise prop and the other having a climb prop, what would be a good guess of the difference for each of those two airplanes to takeoff at max gross weight and clear a 50 foot obstacle? Or to ask it another way is the performance difference really noticeable or is it entirely personal preference? Does it allow the climb prop to get out of a 1500' strip versus say a 2000' strip. Or is it 100 fpm more on the vsi versus 5 mph in cruise and takeoff performance not really affected. Everyone is talking theory but I would like some real world experience that can say I have used both and there is a big difference or say you know what my ground roll was 100 ft more with the cruise prop and that was all. Thanks for all the info in advance.

[juredd1]

Doug and Tim,

Thank you for your valuable information.

Doug,

I am not trying to suggest that I think I have a problem. I need to perform the static rpm test to verify first off if that is in line with where it should be I guess. I placed this post in hopes I could gain some knowledge about propellers and the process to determine what type of prop I can safely put on my 170. I was just hoping to decide if I could have my prop re-pitched to get a little more takeoff/climb performance out of my plane. Thus the reason that I was asking about a good shop to call upon if I decide to go that route.

[C170U2]

Don't trust your tach when doing the static test. Get an optical tester to make sure are spot on. I was really worried about not making static until I tested it with the optical tach and found my mechanical tach was 150 rpm off.

[cverdoljak]

If you want a cruise prop stick with the short one, if you want a climb prop get an 80", no comparison to the short props. I will never bolt on the short prop again. I get about 800-900fpm climb at gross wt. I am rumming an 8040. it's a little thin but performs good.