Seems performance is worse after prop repitch.

[Bruce Fenstermacher]

Folks, Justin emailed me directly prior to starting this thread which he then did as I suggested it.

He was looking for a hand held tach he could buy to help him figure this out. I thought no problem I'll just search for the one I have I bought years ago. No dice. In fact I was not able to find a single hand held tach suitable for full scale airplanes for under $200. I know I payed in the range of $30 for mine years ago. There are several available in the $30 price range for RC aircraft but those first don't have a range fine enough for full scale purpose and second may not have the range to see the prop from the cockpit.

Ok, I just did one more check and found this: https://www.amazon.com/dp/B000I5LDVC?psc=1. This has a lazer detector not a photo detector. It requires reflective tape. May actually be better and more versatile for other projects (I may have to buy one) than the old photo type but you have to put on a piece of reflective tape to use it. (woops just read this only has a range of 2 to 20 inches)

So first lets help Justin out getting a tool he can use. What hand held tachs have worked for others that are reasonably priced?

[Bruce Fenstermacher]

I'd think this would work great:

https://www.amazon.com/TOOGOO-RCD3063-H ... S08AQSHJDB

Here is a video of it being used to measure the rpm of a model helicopter at several feet.

https://www.youtube.com/watch?v=cAGBEM3x1hE

What do others think? Maybe someone here has one or one like it. Ryan, Aryana what do you know?

[sfarringer]

I have been very satisfied with the TruTach II from Aircraft Tool Supply. Just under $200, which I guess is out of the price range.

[Bruce Fenstermacher]

Justin, static rpm is taken standing still. Assuming the tach you used is accurate your findings of 2200 or 2250 best is on the low end of the allowable range of 2220 - 2320. This would indicate, assuming your engine is developing it's rated power, that the prop is pitched course for this plane. This is the opposite what you are trying to achieve.

One problem of many here is you don't know really where you started far as I can tell. I understand you have a feeling for how much rpm your prop use to make, but not a good static test, and you have a feeling for how far down the runway your takeoff roll was. But our starting point is really to vague to compare to where you are now.

Another thing to understand is just because a prop is marked something i.e. 56 pitch, does not mean it is really 56 pitch. Even from the factory. No two props are really ever the same. Add wear and tear and the occasional twist at a prop shop and who knows what you have or had. Maybe your prop as it was, was not allowing your engine to make rated static rpm? In fact I'd say, assuming your prop shop twisted the prop the correct way to make it a 54, that your prop as it was was not allowing your engine to make rated static rpm before it was repitched.

So what do you do. First I would call the prop shop and ask if they decreased the prop pitch and if there was any chance they twisted it the wrong way increasing the pitch. They will probably say they did it correctly but who knows.

Bottom line this engine and this prop turns low static rpm and you want it higher. Lets assume this prop allows 2250 static. I'd think you want it 2300 static. Rule of thumb is 1 inch of pitch is equal to 25 rpm so I'd have the prop shop repitch the prop 2" less from what ever it currently is no matter what they measure it at.

Of course I wouldn't do anything till I had a more accurate and consistent way to measure rpm and I was sure my engine was operating correctly.

You ask how to measure what pitch your prop is. You need to follow the manufactures instructions. Basically the angle of the prop is measured at a station or station(s) along the prop blade and compared to the hub face. If you had the manufactures reference and the tools, you could measure it.

Ever wonder how a metal prop is pitched. They clamp the hub, measure and graph the current prop pitch then take a big bar clamped on the blade at the appropriate station(s) and twist the blade by hand. They do this on both blades and when done adjust the tip path plane to insure each blade will turn in the same plane. This is not rocket science and there are variables and it is only as good as the artist performing the repitch.

[gfeher]

I have a TruTach II as well. Got my from Spruce for about $190. I shied away from buying one not made for (full size) aircraft and just bit the bullet on the price.

[juredd1]

One problem of many here is you don't know really where you started far as I can tell. I understand you have a feeling for how much rpm your prop use to make, but not a good static test, and you have a feeling for how far down the runway your takeoff roll was. But our starting point is really to vague to compare to where you are now.

I understand that I failed to get this before having the prop repitched. You know this and I am kicking myself at this point for being in this situation. So the takeoff roll and just the feeling of how the plane performs now compared to how I think it did before is all I have to go on right now. Although maybe I just need to toss that out of my head if that is possible.

Maybe your prop as it was, was not allowing your engine to make rated static rpm? In fact I'd say, assuming your prop shop twisted the prop the correct way to make it a 54, that your prop as it was was not allowing your engine to make rated static rpm before it was repitched.

That may be true but even the guy that has flown with me more than anyone thinks the plane is not performing as well as it did before the repitch. If indeed the pitch needs to go down two more inches it would seem the plane would have been even worse when at 56 if that is where it actually was.

So what do you do. First I would call the prop shop and ask if they decreased the prop pitch and if there was any chance they twisted it the wrong way increasing the pitch. They will probably say they did it correctly but who knows.
Of course I wouldn't do anything till I had a more accurate and consistent way to measure rpm and I was sure my engine was operating correctly.

I have contacted the shop and I need to find my invoice number so they can look up what was done. I am out of town now so can't get that info until Wednesday night so hopefully by the end of the week I'll know something there. The last link you since Bruce does look like it might work for what I am after without having to put out the $200 for probably very few uses by me. I did locate a friend with one, it will be a few days before I can get my hands on it.


Thanks to sfarringer and Gene for providing the info on the optical tach being used.

[canav8]

Any optical tach will work. It is a great idea to get one. I have used my handheld one since I was a teenager playing with RC planes. The same device is now available for in excess of $100. If I was Justin, I would find an A&P that does prop balancing or contact a local EAA chapter to see if there is someone that has dynamic prop balancing equipment. They will be able to dial in his static RPM and also balance his prop drive train while at it and he would have a super smooth running O-300. I would offer to help but my equipment is in Las Vegas. Justin, if you want to fly it to Las Vegas and I can help. Doug

[juredd1]

If I was Justin, I would find an A&P that does prop balancing or contact a local EAA chapter to see if there is someone that has dynamic prop balancing equipment. They will be able to dial in his static RPM and also balance his prop drive train while at it and he would have a super smooth running O-300. I would offer to help but my equipment is in Las Vegas. Justin, if you want to fly it to Las Vegas and I can help.

Doug,

It's only 1232 statue miles. I think I could make it by Christmas if I start now.
I appreciate the advice. My A&P did suggest at my annual a couple weeks ago that I should find someone that does prop balancing to have a look...just wasn't sure where to start with that as he didn't offer much direciton. Maybe the local EAA chapter can get me pointed in the right direction. I appreciate the kind offer Doug.

Justin

[edbooth]

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.

I'm with Richard on this, after reading all the comments, looks to me like it was re-pitched in the wrong direction.

[GAHorn]

I don't have the sensenich data, but the prop shop should have the approved data... Which should specify the exact blade angles at specific stations along the blades for each pitch desired. This not INexact...it is SPECIFIED!
I agree that it sounds as if your prop was repitched into a cruise (coarse) pitch rather than what you expected.
If your instructions to the shop were correct/written/provable.... Then repitching another time beyond the original towards the previously-requested is bending the paper clip more times than was necessary, and it might be better to insist upon a replacement prop at the shops expense, if possible.

[juredd1]

I got my hands on a friends TruTach...and the optical and mechanical tach are very close. With the TruTach the static bounced between 2213 - 2232. Not sure if that is normal or if I just couldn't hold it still enough. With full throttle in level flight it was right around 2550 as previously stated with the mechanical tach.

I spoke with Jerry from Fosters Propeller Service Inc, he pulled the data for the repitch and said it was check when it came in with a pitch of 56 and was repitched to a 54. Read off some stuff that I didn't follow as far as stations and such. I don't have enough knowledge in this area to question what he was telling me. He said it was right on where it should be for a sensenich climb prop. So other than my engine not producing power as it should I really don't know where to go next. "If" I understood Bruce right it sounds like I need to go down a couple more inches in pitch based upon the static RPM results. I could have completely misunderstood. Several of you think it was repitched the wrong way maybe. I am not sure I have a leg to stand on with the prop shop unless I get that checked out by them or some other shop to prove that out.

I take there is no real data on what static RPM to expect at certain pitches on certain props. Thus the static test is required. Does anyone else out there have a Sensenich pitched at 54 to compare?

Since Doug is over 1200 miles away from me. Anyone else know of a trusted shop within a decent range of Jasper, AR that can assist me in some solution? I am also not sure what that would entail.

Again thank you all for your input, advice, and patience.

Justin

[hilltop170]

Justin-
You didn't mention what your aircraft tach was reading when you checked static with the TruTach or what the TruTach was reading when your aircraft tach was showing 2550 at full power cruise. "Close" is not a very good data point. Those would be good numbers to know to see if there is really a problem or a tach error.

Not trying to be a smart**s, I'm an engineer. I can't help it.

If you are going to make decisions that costs money, based on data, the data has to be accurate and complete to get good results for your money. The weather needs to be close to the same, the aircraft wt&bal needs to be the same, tach data needs to be accurate, etc. My O-300D with a McCauley -EM 76-55 has low static but it will make 2700rpm at level full power cruise using a digital tach. Cruise is what I want to optimize so low static doesn't bother me. I realize it is not a short field airplane. The engine had 1750SMOH before it was overhauled in 2006 and it made the same rpm numbers before and after overhaul.

I think you could probably go down in pitch another few inches, to 51" or 52" depending on what the Sensenich specs say. That should bring rpm up some more in static and level full power cruise. But if it was mine, I would fly it like it is for awhile and see how it performs for your type of flying.

[GAHorn]

Justin, it takes a steady hand to utilize a handheld tach check tool
Your readings appear to be within the range allowed by the type certificate for this propeller: 2220-2320. Your readings are at the low end of this scale, and therefore you either have a very low density altitude when you took the readings, or you have a coarsely pitched prop....or both.

The differences in take off distances and ROC measurements you made at the beginning of this thread are suspect. As you've already noted, it takes accurate measurements, accurate instruments, and standard atmospheric conditions...to make such comparisons valid, and you've had none of these.

So, what is your bottom line? Do you want to spend a lot of money to achieve identical results that Cessna printed 60 years ago? or do you want to relax and go fly your airplane?

I have to say I don't agree with Bruce's comment about a flatter pitch prop producing longer takeoff runs than a coarser pitch prop. (This assumes similar conditions and accurate measurements.) The intended result of a climb prop versus a cruise prop is exactly that: Reduced takeoff distance and improved rate of climb at specified climb airspeed. (Certainly, attempting to climb at a higher speed with a climb prop will have a disappointing result,...as will a lift off at higher speed than before the prop change. Again, the results in this original post do not reflect identical conditions, or consistent measurements with calibrated equipment.)
A climb prop (finer/flatter pitch resulting in a higher static rpm than with a cruise prop) will result in quicker acceleration to flying speed.
But if you use a cruise prop and lift off at 60 IAS...then attempt the same with a climb prop and lift off at 70 IAS... the result will be a longer takeoff roll, because it's not the same conditions and technique.

Props are like standard 3-speed manual transmissions. Climb prop is like First Gear- it will demonstrate more rpm and accelerate "off the line" faster, but it will run out of acceleration at cruise speeds, and will be over-revving at cruise with poor fuel economy. A cruise prop will be like starting out in 3rd gear- it will demonstrate less rpm and be slow to start, but will result in a higher cruise speed and have better fuel economy on a per-mile basis.

[hilltop170]

Well I think George and I just said the same thing at the same time. But, as usual, George's statements are much more refined and eloquent.