Cherokee Archer II PA28-181 RPM

For all my piper fellows. A year ago I bought a 1979 Archer II, did pre buy inspection and everything seemed ok. One time on take off I took a look at the RPM gauge and noticed the it was marking at around 2300RPM, I got a little worried thinking it should read at least 2500RPM full open throttle, so I went to my mechanic and explained the RPM reading. He proceeded to check the engine and did a compression check, all in the high 70's. He told me everything seemed ok, so I was ok with that and kept flying it like that. Performance felt very good too in climb and cruise. While in cruise it always wanted to go over to max reading power (past 25ooRPM). Now, a month a couple of weeks ago the plane got out of the annual and everything seemed great, it felt great over all, but again, I glanced at the RPM and in initial roll out it read 2100RPM this time.... As I start to climb it starts to rise little by little and in cruise it reaches high RPM. Compression readings out of the annual are again high 70's. Do you Piper owners have similar readings on take off? Thanks for the feedback.

 

Same thing, and I remember stating that to the mechanic but he found “nothing wrong” so I kept flying, but now I think 2100 is kind of low, even though it still climbs quite good in my opinion

 

I fail to mention that I got new magnetos kit because the old ones were.... well, past their service time... should be tuning better, not worse, I guess

 

Like GeorgeC said above.

Those old mechanical tacks ALWAYS read low. 200 RPM low is normal for those junkers.

 

All right, I will get into the tach to see if it is at fault

 

But low rpm on takeoff is a no-go for me. How do you know that you are making full power, or not. Fine if you are at a 6000’ runway, but my C150 wound get off the ground with 300 RPM too low.

Tach is a required instrument. If it reads that low and is known to be inaccurate, you may be flirting with an unairworthy plane. Seems like you could get hit with 91.13 if you happened to crash on takeoff for any reason.

But to check it quickly, download an app like VideoTachometer on your smartphone. It gives a very accurate rpm reading from the video and you can check it sitting in your plane. Set the App to 2000 RPM (or whatever) and set your throttle to 2000 rpm. Check to see if they match and how much it if off. When I checked my C150, it was accurate to 10 rpm, probably because we just changed the tach cable assembly with a new McFarland unit when it failed last year. Old gauge, but brand new cable.

When I set the app to a multiple of 60, my LED lights stop flickering in the video, change it by 10, and they start pulsing. You can see the 60kz frequency in the power grid.

 

Well, I thought about go no go sedition, and that is one of my parameters, I should be lifting off at no more that the 1000 runway mark, but the airplane takes off at about 900 feet from start, so it is within the POH parameters, Even with full load I took of in about 1000 feet of runway, and the climb looks good. Thats why I am asking because the RPM shows low RPM but the plane behaves diferently. What I am getting from this threat, and was my initial intend of the post, is that 21000 RPM is low for normal performance, so I should look more into it.

 

Yeah, the smartphone tachometer app will do a quick check for free in one minute. If it is off by more than 20-50 rpm, it is worth investigating. If your static RPM is less than POH figures and your tach reads correctly, you may have a slipped or out of adjustment throttle cable. But I am not an AP.

 

Wheee! Aren't typos fun! -Skip

 

App is iPhone only.
This will work well from the cockpit through the windscreen. You can use it on the ground and in flight. Create a tachometer deviation table, that is "indicated" and "true".

https://www.amazon.com/Hangar-Micro-Digital-Tachometer-HAN156/dp/B0006N72U2/

 

look up the TCDS
for my S/N the static RPM should be between 2240- 2340. unless i at least see that, i wont take off. those numbers are for ISA and at sea level. during summer, sometimes i will see max of 2100 with full Throttle, i would lean until i see at least 2240 before my take off roll. it also says "for takeoff 5 min at 2700 RPM" - never saw that on the original Tach, it does get close to it in the JPI 830

 

Thank you all for the responses. All valid and helpful.

Let me give you guys an update. Last night I downloaded the app on my phone. I went this morning to the mechanic and explained this matter to him. He is the one who did the last annual a couple weeks ago and has done two separate compression tests before, right there with me, and I personally saw the results. He is sure the engine is fine, he himself changed the mags for new ones and cleaned all filters, got new hoses, etc. He told me he thinks the plane has a "cruising prop" which gives a lower RPM reading, I heard that from another mechanic as well. With that being said, I went to the plane and test the the phone app from inside the cabin and the app reading was as follows:

Tach RPM 1000 - App RPM 1120
Tach RPM 1500. - App RPM 1650
Tach RPM 2000 - App RPM 2210
Full open 2110 - App RPM (exactly 2270)

I feel a little more confident with this results, but show that the tach may need some readjustment.

So, Im gonna test the RPM with the mechanic with his tester in the following days and I will keep you guys posted.

By the way, im based in MIAMI FL so it gets really hot, average density altitude is 1600 feet, maybe leaning the mixture instead of full mixture for take of might change the reading... just as a NOTE

 

when i bought my plane, my Tach was showing 200 RPM More. based on what you have posted, certainly looks like your Tach needs a OH. 2270 is just fine for static RPM.

 

I will certainly look the tachometer to see if it is the problem

 

I am at 1000 MSL, the only time i have to lean to get to the published static RPM is when DA is over 2500. these days i am taking off with -1500 DA and very soon it will be -3500 DA :d

 

Got it. I did my training here in C172's and always took off with full rich. In fact, all the training flights were mixture full rich. Most CFI's don't actually learn to lean. For this reason I never really gave much attention to the tachometer until I got my own airplane. Now leaning and engine parameters are primary checks!

 

there are couple of sticky on the top of this Mx bay forum by Ted about taking care of the engine on the ground and in the air, make sure you read them

 

My 1980 Archer’s tach was found to be reading 175 low at full power cruise rpm with a cheap optical tach. I had been wondering why my TAS performance numbers were so good; the reason was that I was actually flying at higher percent power to get those great numbers. Armed with that knowledge, you can fly with your old tach and make a correction factor by adding the difference at various tach readings, checking using an app or optical tach from time to time. Or you can repair that unit or replace it with a digital primary tach. My solution was to leave the original tach and use it as a trend instrument, and use the aux channel of my EI FP5L fuel flow as a digital tach option, which cannot be used as primary, technically speaking. This always gives me the accurate reading when needed for performance objectives, and a cheap solution as well.

 

Don't worry about it.
Just post a tach correction chart on your panel or commit it to memory.
Every 100 RPM from 2000 to 2700 should do it.

For example:
TACH TRUE
2000 2090
2100 2196
2200 2301
2300 2407
2400 2512
2500 2618
2600 2723

 

That is exactly what I created and used prior to getting the digital rpm input on the panel.

 

If it ain't broke, don't fix it?