The International Cessna^® 170 Association

First question - Recommendations on a hand held optical tach. I need to verify the accuracy of the one installed in the plane.

Now back in late May before I had my Sensenich prop repitched from 74DR-0-56 to 74DR-0-54 I did some take off distance and climb tests. I realize that the pilot has a lot to do with the accuracy of these test but I just did the best I could with it.

Prior to repitch

63 degrees and 2000 ft. density altitude reported at the local airport.

1st take off - 503ft
2nd take off - 533ft
3rd take off - 542 ft

850 ft. per minute climb at 60mph

After the repitch

57 degrees and 1316 ft. density altitude

1st take off - 594ft
2nd take off - 576ft
3rd take off - 564ft
4th take off - 587ft

Somewhere around 750 ftm climb at 60mph

I can't promise you the weight of the plane was exactly the same. I failed to document but from what I remember I'm pretty close. One thing I did not document was the wind speed and direction. I know on both days it was such a small factor I ignored it.

What I also failed to do is the static RPM test that was recommended. Not much else to say there I just didn't get it done.

From the first time I tried the plane out after the repitch it just didn't seem right. Seemed like a longer takeoff roll and just didn't seem to climb out as fast. Just a big disappointment for me.

Is it actually possible to loose takeoff and climb performance when you have a prop repitched to be a climb prop?

Be gentle....I am still learning.

Thanks for your valuable time,

Justin

juredd1 wrote: 1st take off - 503ft
2nd take off - 533ft
3rd take off - 542 ft
Be gentle....I am still learning.

I'm learning too...how do you measure your take off roll down to the exact foot?

rusbac wrote:I'm learning too...how do you measure your take off roll down to the exact foot?

Hello Christine,

Well I am sure it is not as perfect as I have listed but we use the little wire stakes with the colored flags on the top. We put one at the starting point and my uncle goes down the runway to where we think will be close to my takeoff point and he just watches for my mains to leave the ground and does his best guess of where that flag should be placed. The first couple are usually off and are thrown out until he gets closer to the spot. If I am consistent then the rest he can eyeball pretty closely. Then after all the test takeoffs are done we take a 100 ft. tape measure and measure the distance between the starting point and the other flags.

Hope this helps.

So the answer is yes it is possible to lose takeoff performance by flattening the prop. Reason is the plane does not accelerate as fast to take off speed and in really bad mismatches the plane may never accelerate to takeoff speed till way down the field. It might get rolling faster but like your car stuck in a really low gear, acceleration will slow down.

I had a friend with a Maule M4 with a 0-300. The pitch on his props was so flat my Cherokee 140 would out perform his Maule in a short field takeoff and climb competition. His would quickly get moving but just about stop accelerating as he'd get light on the wheels. He'd have to level the nose to continue acceleration and since he was now hitting redline, pull the power back. He complained about it for years and sold the plane to a fellow who immediately changed the prop.

Rule of thumb is for every inch you change your fixed pitch propeller pitch you will effect your static RPM by 25 RPM and your cruise RPM by 50 RPM. So you should be seeing about 50 RPM more than you did as you start your takeoff roll.

Bruce,

So more of what may seem like silly questions to you more knowledgeable people. So my Sensenich 74DR-0-54 may perform differently on my plane/engine then on other planes with the same engine, excluding any other performance addons? I know keep reading the static RPM testing thing and making sure that fits into what the allowed RPM range. However if this prop and pitch are allowed per the TCDS then it should be ok, right? It has been tested before on my type of engine by someone or it wouldn't be allowed. I suspect I am wrong. I guess my point is if two people have the same Dodge RAM 3500 truck and one is using a 4.10 gear ratio differential than the other guy or gal should be able to as well and expect the same results. It's gears, they are going to turn the same speed on both trucks.

Would it be helpful to get an accurate RPM reading to see where I am there but we start guessing?

I suppose I failed by thinking that if a 54 pitch on that prop works for someone else it will work for me.

Thanks,
Justin

Sounds strange. You should know the static RPM and the RPM you are climbing at. Kind of hard to do, I know, as it's hard to control everything.
My 170A has a McCauley 51 in. pitch, which was the standard climb prop of the day. So, your 54 in. pitch isn't really that flat. Of course, the shape and airfoil of the prop makes a big difference too. It would be hard to compare a McCauley and Sensenich just by the pitch.
What is your cruse speed? Maybe they pitched it the wrong way. Seems unlikely, but strange things do happen.
Would be nice to have both props separate so you could switch them back and forth and see the difference. Maybe someone has a prop like yours was and you could try that.

Justin the pitch in the TCDS is not the limiting factor. Your prop is limited in diameter and your static rpm must be in the limits. Those are the only limits. I suspect your repitched prop will be in limits static RPM wise. Doesn't mean it will perform better for the purpose you were trying to achieve. It only means it is legal and the plane will perform to the minimum standard it was certified.

I don't know much about fixed pitch props, but this seems strange to me, too. No offense to Justin's test-flying technique, but is two inches pitch change enough to be clear in a before/after test?

A couple things that are red flags with this evaluation. The first thing I notice is the omission of static rpm. On a normally aspirated motor, HP is made by higher RPM. if the OP doesn't know what the static RPM is, he should not have even taken off. It is an important first step when any prop changes are made. The minimum rpm for static is there for a reason. Without knowing this info all other data is moot. D

When an engine with a fixed pitch prop will just make redline rpm in level flight at full throttle in standard conditions, it has the correct pitch in my book.

hilltop170 wrote:When an engine with a fixed pitch prop will just make redline rpm in level flight at full throttle in standard conditions, it has the correct pitch in my book.

Richard, I wish it was that easy. First thing comes to mind is engine health.

Doug, what I was referring to is I don't want a prop that is pitched so low it will allow an overspeed or pitched so high it will prevent the engine from making redline rpm in level flight. That is a very easy test to perform with nothing more than an accurate tach. I would hope nobody would fly with an unhealthy engine.

Everyone I apologize for the delay in responses. I am on a computer 5 days a week at work and on the weekends I stray away from it for the great outdoors at times.

Bruce Fenstermacher wrote:Justin the pitch in the TCDS is not the limiting factor. Your prop is limited in diameter and your static rpm must be in the limits. Those are the only limits. I suspect your repitched prop will be in limits static RPM wise.

I went out today and hopefully performed the static RPM test correctly. I let the plane warm up for a bit, oil temp had not registered but felt luke warm air in the defrost vent. With breaks held and no obstacles behind me the engine was turning 2200 RPMs (After about a 5 minute flight to another strip it was showing 2250 on for the static RPM, maybe it warmed up a little?). On take off roll just before liftoff it was turning 2300 RPMs. At my best effort at level flight the engine was turning 2550 RPMs about 150 from the 2700 set at redline.

bsdunek wrote:What is your cruse speed?

I went out today and with my best effort to keep things level the ASI showed 120mph.

DaveF wrote:No offense to Justin's test-flying technique, but is two inches pitch change enough to be clear in a before/after test?

No offense taken. I had just read on multiple posts where it seems changing the pitch from 53 to 51 on a McCauley had made a noticeable difference for some folks. I was in the hunt for a McCauley from another member but he recommended just getting my Sensenich repitched. That was the cheapest route but was really wanting another prop so I wouldn't mess with what I currently had but the cost savings ultimately changed my mind about just getting my prop worked on. I am not at all suggesting his recommendation was wrong.

canav8 wrote:A couple things that are red flags with this evaluation. The first thing I notice is the omission of static rpm. On a normally aspirated motor, HP is made by higher RPM. if the OP doesn't know what the static RPM is, he should not have even taken off. It is an important first step when any prop changes are made. The minimum rpm for static is there for a reason. Without knowing this info all other data is moot. D

Thank you for your input and points made. There is a lot I am still learning and was never taught to be looking at the RPMs being made. If the engine sounds good then I just went with it. I do now understand the importance of the static RPM now.


Also still waiting on someone to give me some suggestions on a hand held optical tach tester.

Thanks to everyone.

Justin

This is starting to sound to me like the prop was re-pitched in the wrong direction. Takeoff and climb are worse, cruise is high, and it won't make redline rpm at full throttle in level flight all indicate a higher pitch, not lower.

hilltop170 wrote:This is starting to sound to me like the prop was re-pitched in the wrong direction.

Thanks Richard. I'm going to call the prop shop tomorrow and discuss my findings with them. Not sure what to say to them but here's what I got. Is a prop shop going to be the only folks that have the tools to verify the current pitch?

Thanks again.