M20J prop vibration?

[aaronk25]

Anyone on their "j" ever been around 25 inches of M.P. and pulled the prop slowly back from lets say 2500rpm to 2100rpm and around 2250rpm start to notice the vibration the feels like a out of balance prop? I have the updated round tip propeller, 200smoh and thought it was the power strokes showing up because of only having 4 cylinders, which were putting out a greater "power stroke" at the slower rpm but I believe it's a function of the 2 blade prop as its apparent the vibration happens with a revolution of the prop. Maybe there is 2 vibrations per stroke? It becomes more pronounced around 2100rpm and continually gets more dramatic as prop speed is slowed even down to 1750rpm. Could it be that at a higher blade angle of attack creates some turbulence and coupled with the power stroke of a two blade prop creates a out of balance type vibration?

I tried doing it at 20" of MP and the vibration was still there below 2250rpm, but not as pronounced.

The reason I found this was trying to get some loiter power settings.

I really like the idea of high MP with low prop rpm however it just doesn't seen smooth.

Thoughts? I'm also dynamically balanced, well not me that is, but the prop ,

[Cody Stallings]

I'm not sure a dynamic balance is gonna help ya much there. I'm sure your engine is counterweighted, but I have never seen a 4 cyld that is very smooth below 2300. What is the model# of your propeller?

[aaronk25]

I think its a c212. Round tip. I upgraded it from the old square tip.

[Cody Stallings]

Well that is a good prop...... I wonder if it could just be the lugging of the low rpm, like when driving a car around in high gear when you are only doing 25mph.... I have been told MP higher than RPM is never good, I wonder if vibration is why..... I'm sure some folks with more mooney smarts than me will chim in shortly...

[OR75]

maybe obvious but just in case....

your tach must have a yellow arc that stops around 2100

but are you sure your tach is calibrated correctly.

the tach cable that goes from the magneto to the tach play games sometimes and could lead to easily 100 rpm off

[PTK]

I think its a c212. Round tip. I upgraded it from the old square tip.

Aaronk25, you're sure it's the C212? or C214 prop you have? It doesn't matter, just curious because you say it's round tip.

On the vibration, I would think that by maintaining high mp and decreasing rpm, the mp is exceeded at some low rpm. This would overstress the engine by increasing cylinder pressures. iow it may not be your prop but the engine. Also if you want to go slow you don't need that high mp. Any thoughts?

[aaronk25]

Tach is spot on as I double check it with the JPI one that takes the single straight from the mags.

Ill check today exactly which prop it is.

[bd32322]

Yeah I have felt the same vibration going to 2100 rpm and 25 mp. I didnt think much of it and just didnt operate there.

[jetdriven]

Ours is the round tip prop and it is a C214.  Interestingly, I use 2050 or 2100 RPM as a loiter power setting with ~15-17" of manifold pressure, leaned to peak EGT or slight rich of peak.  The blue knob pulled back all the way on ours is only 2000 RPM, and it has been noted that our prop doesn't behave like many other J's on here.

 

2100 RPM sounds like its thrashing around, but if you go to 2300 RPM, then 2200 RPM for a couple minutes, then 2100 RPM after a few more, it seems normal. I haven't run more than 21" like this, since it does increase cylinder pressure, but if you are just trying to get minimum fuel flow, you can do it with low RPM and MP, around 3.9 GPH.  It does kind of chug like an old Ford tractor, but I think thats the dynamic of 4 cylinders and 2 blades.

[aaronk25]

Byron I think your right being the dynamics of 2 blades and 4 cylinders. Ill use some lower manifold pressure and try bringing it back over a span of a few min.

I've read that the 6 cylinder guys can lug them down and there still smooth but it must be the 4 cylinder harmonics.

[duke]

not sure about j, but my f has a yellow band from about 2100 to 2350. I have run 18.5 squared and it runs nicely. Good for just toolin around etc. 

at that setting you get 150 fuel burn and close to 172 speed.

[Hank]

The max range settings shown in my Book is 20"/1950. When loitering with a pack of 172's [Young Eagles, Airport Plane Ride day, etc.], I generally run 2300 and whatever MP keeps me from running through them, generally somewhere in the 16" range. Flew 7 rides last fall, 4.6 hours on the clock, 2.7 on the tach, used about 14 gallons. But I have a Red Zone 2000-2250 on my tach.

 

Start with your IAP power settings for 90 knots and adjust from there. If loitering, why keep MP high? Lower RPM is less power & fuel, back off on the fuel lever, too. Pay attention to any MP restrictions--I have a placard about less than 15" at some RPM range that I don't recall right now.

 

Gotta love digital photos! Found one with the placard readable when zoomed in:

 

Avoid continuous operation below 15" between 1950 and 2350 RPM.

 

I am rarely below 15" except when in the pattern to land; sometimes I pull back to slow down after descent, but am generally still running cruise RPM [mostly 2500].

[N201MKTurbo]

What you are experiencing is torsional vibration. It is very destructive and should be avoided. Dynamic prop balancing can only take care of radial vibration. Torsional vibration can only be measured with a torque sensor between the prop and the crankshaft.

http://www.enginehistory.org/NoShortDays/TV.pdf

[aaronk25]

What you are experiencing is torsional vibration. It is very destructive and should be avoided. Dynamic prop balancing can only take care of radial vibration. Torsional vibration can only be measured with a torque sensor between the prop and the crankshaft.

http://www.enginehistory.org/NoShortDays/TV.pdf

Very interesting! My prop is model G-90DHB-16E. I bet that is exactly what is happening. It's hard to describe almost a ,woof,woof vibration sort of like one of the blades adjusts to a deeper pitch than the other one and its felt every rotation. I've been avoiding high MP low rpm cruise and haven't been operating under 2300 except at low power settings. Funny thing is my old square tip propeller never did it.

[OR75]

Just out of curiosity, why do you operate at low rpm with high MP?

For me:

Decrease MP then RPM

Increase RPM then MP

Basic of "complex" training.

[aaronk25]

Just out of curiosity, why do you operate at low rpm with high MP?

For me:

Decrease MP then RPM

Increase RPM then MP

Basic of "complex" training.

I don't operate at those settings continuously, I was more curious on why there was a vibration at theses power settings as the 6 cylinder engines are smooth at those settings.

Lower rpm and higher manifold pressure yield better mileage as long as the prop isn't operated out of its efficiency curve. Cylinder pressures are higher, but given the engine and prop are turning at a slower speed, there is less parasitic drag. I was more curious if the example is taken to the extreme, why this vibration was showing up.

I will if not in a hurry cruise at 25-27" of MP (if I can make that much depending on altitude) all day long at 2300rpm peak egt-50lop if I'm trying to get best sorta fast economy mileage. Really smooth there. Or 2400-2500 peak, or go fast up high at 2700 peak egt to 80rop. for go fast

[MB65E]

If you feel it pulse on the rudder pedals your engine is not happy. The load on the counter weights is extreme and they are slapping the crank itself. That what's it sounds like is happening. The ware will show up on the counter weight pins and faces. All lycomings seem to like HI RPM. 2375-2400 would be as low as I'd go. As for the 540's their not a flat crank like on the 360's. The loads are spread out another 60 degrees on the crank. Thus vibrations are less. However, the low rpm pulses will still occure but tend to happen at even lower RPM

I too agree with 201mkturbo.

[jetdriven]

I don't know if 2000 rpm will detune a counterweight or else there would be an AD or service letter about it. How many Lycoming cranks fail in a decade, three? Jamming the throttle forward on takeoff or cycling the prop below 1500 rpm might, but again where is the evidence of this? Those counterweights are suspended by pins and float on the crank. It is their job to dampen torsional vibration, that's why the IO-360A1A and other have that huge redline on the tach. That's a limitation. The A1B6, A3B6 and the -D variants have no such limit.

There's a lot of kickback in the aviation community if you suggest operating anywhere other than "24 square" and 50 ROP.

[bd32322]

Just out of curiosity, why do you operate at low rpm with high MP?

For me:

Decrease MP then RPM

Increase RPM then MP

Basic of "complex" training.

You can use rpm to slow down and keep MP and CHT up in a descent. As long as the POH does not restrict it.

The training you mentioned is very black and white Search for oversquare operations on this forum and you will ind plenty of material

[Alan Fox]

The vibration is more than likely harmonic , which can not be balanced out dynamically , best not to operate in these ranges as the energy is being transmitted through the prop blades.... 4 Cyl lycomings have the highest incidence of prop tip failures, which is why they limit the rpms from certain ranges.... Counterbalanced engines have loose weights on the cranks that float on pins and change the dynamic properties when harmonics occur , without the counterweights the harmonic forces act on the prop blades....

[jetdriven]

Some interesting information from the Lycoming O-360 Operators Manual. There is a limiting MP for a given RPM, but its higher MP than you think, and this is based on ROP cylinder pressure, peak EGT or LOP operation is going to be lower cylinder pressure, so presumably you have 1-2" more MP for LOP power setting. I don't operate anywhere near these limits, for example, you are allowed 25" of manifold pressure running at 1800 RPM, and 26" at 2000 RPM.