Piper Warrior low rpm?

crash7

crash7

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Crash7
Dumb question: The last couple flights I’ve done in our school’s 1977 PA-28-161 have had what I feel lower that usual RPM.

During a night flight at 2500 feet (warm evening, admittedly) FULL throttle barely got us 2500 rpm. Past experience I swear I should have seen closer to 2600.

Two nights ago a departure roll yielded around 2150, maybe 2200 rpm on the roll. I swear (again) usual is 2300-Osh rpm. Took 1700-1800 feet to hit 55kt rotate. Pilot aborted. Ran the book numbers and for that day/temp/weight should have been around 1,100 feet.

We’ve had some hot weather, but only putting DA’s around 2K to 2.5K.

Mechanic checked compressions, air filter, air intake/carb heat mechanism. No issues. Am I off base here? $chool want$ it to fly, of course.

And…. go


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Did you check if it makes Minimum Static RPM per TCDS?

Static = Standing Still and Full Throttle.

Later Archers have a little more elaborate procedure.

Not sure on Warriors.

Correct rigging of Throttle, Mixture and Carb Heat ESSENTIAL.

Cushion? ( Springback)
 
Sounds like she might be a little weak to me. Checking static rpm would be my first thought as well. I'm guessing there's no fuel flow gauge?
 
crash7 said:
Am I off base here?
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No. But anytime you're addressing performance issues vs the numbers you see its always best to ensure those numbers are accurate. Does the aircraft have a mechanical or digital tachometer? If mechanical may want to see if there's any error.
 
If you’re familiar with this plane under these conditions, and you’re noticing a performance feel or indication change, trust your intuition. Something is not right and it has not been found yet. Don’t brush it off. Not much more to add than that.
 
No fuel flow gauge.

Regarding static rpm, a couple days ago we did a full throttle parked run up and only got 2000 rpm. Didn’t lean, though. Didn’t know to.

I -think- this is the correct TCDS:
IMG_0027.jpg
I’m not sure what prop it has.

I’ll see if I can do a poor man’s rpm calibration with my phone next time. Even if the tach is off, it doesn’t explain the horrible performance vs book numbers.

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What sort of mag drops do you get in the runup? And does the RPM return when you switch back to Both?

Magman mentioned engine control rigging. If that throttle cable housing has slipped in its clamps, or the carb lever has come loose, you won't get full throttle. Was it looked at when the carb heat rigging was checked?

Does that engine run smooth? Not a weak cylinder due to a worn cam? Compression checks won't find that.

Muffler baffles not broken loose and blocking the outlet?

Prop hasn't been changed for some reason, with the wrong pitch installed?
 
As has been said before...........without a digital/optical tach.............all the above data is COMPLETELY wrong.
 
Bell206 said:
How so?
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If the cable has slipped on the tach, how would that impact take off performance? Full power is full power. The tach could be disconnected and the engine would still do its thing. Problem is, the engine doesn’t seem to be doing its thing very well.


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Dan Thomas said:
What sort of mag drops do you get in the runup? And does the RPM return when you switch back to Both?

Magman mentioned engine control rigging. If that throttle cable housing has slipped in its clamps, or the carb lever has come loose, you won't get full throttle. Was it looked at when the carb heat rigging was checked?

Does that engine run smooth? Not a weak cylinder due to a worn cam? Compression checks won't find that.

Muffler baffles not broken loose and blocking the outlet?

Prop hasn't been changed for some reason, with the wrong pitch installed?
Click to expand...

Run ups are normal. Temps seem ok. No roughness. Just seems tired. I have no idea about the cabling. I wasn’t there when the mechanic was working on it.


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Many pilots are not aware of cushion or springback.

Those that are will not fly an aircraft without it.

It is assuring the item being activa is stopped by going full travel.

The panel should never be the “ stop”.

If the engine does not “ make static” it does not conform to the

TCDS. You know what that means!
 
crash7 said:
If the cable has slipped on the tach, how would that impact take off performance? Full power is full power.
Click to expand...
Its only full power if the engine is making the necessary turns which is an unknown at the moment. Plus the accuracy of older type performance charts can cause performance calculation errors as well. So until you have a verified benchmark to compare with its only a guess which is good the engine or the chart. And as mentioned a good place to start is the engine static rpm check per the book.
 
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Magman said:
Many pilots are not aware of cushion or springback.

Those that are will not fly an aircraft without it.

It is assuring the item being activated is stopped by going full travel.

The panel should never be the “ stop”.

If the engine does not “ make static” it does not conform to the

TCDS. You know what that means!
Click to expand...
Okay, I'll bite. Perhaps I know cushion and springback by other names.

I've flown the same PA-28-161 for 20 years. The TCDS above is the correct one.
 
Bell206 said:
No. But anytime you're addressing performance issues vs the numbers you see its always best to ensure those numbers are accurate. Does the aircraft have a mechanical or digital tachometer? If mechanical may want to see if there's any error
Click to expand...
Apple Store has a "Engine RPM" app that often works well in many airplanes to verify RPM accuracy, In some airplanes it doesn't work very well as it works based the noise of the engine.

1689436341600.png
 
Domenick said:
Okay, I'll bite. Perhaps I know cushion and springback by other names.

I've flown the same PA-28-161 for 20 years. The TCDS above is the correct one.
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Cushion or springback mean that the engine controls don't bottom out against the panel or their lever stops before the engine's levers are at their stops. A carb heat or mixture or throttle or prop control should hit the panel stop when pushed full forward, but spring back about 1/8" or so, indicating that the control at the engine is at its stop.
 
Bell206 said:
Its only full power if the engine is making the necessary turns which is an unknown at the moment. Plus the accuracy of older type performance charts can cause performance calculation errors as well. So until you have a verified benchmark to compare with its only a guess which is good the engine or the chart. And as mentioned a good place to start is the engine static rpm check per the book.
Click to expand...
Tachometer accuracy is important, but what makes pilots sit up and take notice is this, from the first post:

crash7 said:
Two nights ago a departure roll yielded around 2150, maybe 2200 rpm on the roll. I swear (again) usual is 2300-Osh rpm. Took 1700-1800 feet to hit 55kt rotate. Pilot aborted. Ran the book numbers and for that day/temp/weight should have been around 1,100 feet.
Click to expand...

As instructors we used to teach the students what happened on a reduced-power takeoff. The student would go full throttle, the instructor would pull it back to get a 100-RPM reduction, and the student was expected to continue with the takeoff and climb at that power setting. The reduction in performance was spectacular, and scary for the student. He/she had to climb very cautiously so as to avoid stalling or mushing. It's one reason we taught a quick scan of the engine instruments early in the takeoff roll, to allow room for an abort if the engine wasn't producing normally.

Excess power is what is required to climb, and most lightplanes don't have a lot of it. It's expensive. Any "minor" loss of power can be a major event.

I strongly suspect there's something wrong with that airplane, beyond a tired tachometer.
 
Dom:

Dan explained it perfectly. Cushion/ Springback gives the pilot some assurance that the

engine controls are properly rigged. It is something I learned many years ago when the pilot refused

to fly the F-100 Super Sabre due to lack of the feature. Re- rigging began with removing the entire

aft section of the fuselage!


In addition to Static RPM there are a couple other rigging related things you want to be aware of :

Leaning the Mixture should always result in a slight (20) RPM Increase. if this will not take place

it could be the Mixture is already lean. Possibly from from the cable slipping and not hitting the stop.

If Carb Heat drop is less than normal it could be that it is already partially on. This would reduce

Full Power also.
 
Dan Thomas said:
Post snipped
"""""""As instructors we used to teach the students what happened on a reduced-power takeoff. The student would go full throttle, the instructor would pull it back to get a 100-RPM reduction, and the student was expected to continue with the takeoff and climb at that power setting. """""""
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Some jets are equipped with a lateral "G" meter. If the "G" meter doesn't read book value for those takeoff performance charts........T/O will be abandoned.
 
Dan Thomas said:
Excess power is what is required to climb, and most lightplanes don't have a lot of it. It's expensive. Any "minor" loss of power can be a major event.
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I have aborted more than one takeoff in an Archer because the Engine did not sound and feel proper during the initial takeoff roll.

I am not going to diagnose or second guess anything on the takeoff roll. If anything makes me immediately think something is not right than I am aborting while still on the ground and able to stop on the runway. Plenty of time to think it over on the ramp.
 
Looks like the mechanic has taken it off the schedule for the next few days to take another look at it. Will update with whatever is found…


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Skyrys62 said:
I'm not quite the Warrior I used to be either.
Don't get revved up quite as much, and my rolls have definitely increased.
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giphy.gif



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Dan Thomas said:
Cushion or springback mean that the engine controls don't bottom out against the panel or their lever stops before the engine's levers are at their stops. A carb heat or mixture or throttle or prop control should hit the panel stop when pushed full forward, but spring back about 1/8" or so, indicating that the control at the engine is at its stop.
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Thanks, Dan and Magman. My Warrior's engine controls exhibit springback. All three will hit the physical panel stop and spring back a bit.
 
If the Tech has already checked the items that were mentioned here next

for me would be would be to assure there are no fuel stains or movement

between the upper and lower sections of the carb. There is a SB on this.

Proper “Dry Lifter Clearance “ should verified also.

1. A flat lifter will cause the valve to open later and close earlier .

One bad may not be readily apparent . Folks at Penn Yan called me

to let me know that there was a bad one on a jug I had sent in.

They could tell by the wear pattern on the valve guide.


2. Bent pushrods and cam/ lifter wear will present during a DLC check.

If maximum clearance is exceeded it’s likely time for a cam!
 
Forgot to post the resolution…

Fire shield or part of it had broken off in the front muffler and would vibrate in and out partially blocking the exhaust and robbing the power.

All better now.


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Pic? Are you referring to internal baffles in the muffler?
 
That's fascinating. I just had a discussion with the mechanic at BPK about my flame shields being garbage. My home mechanic said he doesn't really worry about them. Two of them are completely gone, but the middle one is broken and rattling around. I'm thinking I should reach up in there with my long reach pliers and yank it out.
 
Just want to add you should have your standard takeoff rpm seared into your brain. Not wondering after the fact if "It's a little low". Not trying to be snarky. Maybe you fly in flatlands and density altitude has not been seen as a priority. If, on the roll, your takeoff rpm is not exactly where it should be you should immediately understand why (high DA?) or abort the takeoff. There shouldn't be any "wondering" about it.
 
I have an excuse now for when I don't think good.
Flame shields rattling around up there blocking something.

Case in point, this post..
 
Are flame shields exclusive to Pipers? Of do other's use them also. Are they inside the muffler? TIA
 
Many use them. Lyc 172’s initially had them but later versions

eliminated them.
 
They are baffles. Tubes with big holes in them. They didn't do much for muffling, and many later mufflers didn't have them. In the 172 mufflers there were two inlets at each end with these perforated tubes running across between the inlets. Two tubes. They got a lot hotter than the rest of the muffler so they'd expand more, buckle, then fracture as they cooled and contracted. Risk of outlet blockage.
 
Dan Thomas said:
They are baffles. Tubes with big holes in them. They didn't do much for muffling, and many later mufflers didn't have them. In the 172 mufflers there were two inlets at each end with these perforated tubes running across between the inlets. Two tubes. They got a lot hotter than the rest of the muffler so they'd expand more, buckle, then fracture as they cooled and contracted. Risk of outlet blockage.
Click to expand...

My suspicion has always been that the flame cones were never intended to muffle anything, but were added to improve heater performance. Regardless, they need to be inspected regularly and replaced when the flame cones are starting to distort.

It’s amazing how long these aircraft have been around and people still haven’t learned that flame cones need to be looked at now and then.
 
The flame tubes were easy to inspect with a flashlight with an extension on it. Any proctologist could do it. Every 350 hrs our 172M could be counted on to have the flame tubes so distorted and cracked that the guts of the muffler would have to be replaced. Aero Fabricators (I recall) was local so it was no big deal. I understand later model mufflers didn't require or use flame tubes.
 
It appears that the at least the end plates on “ tubeless” mufflers is of a

heavier gauge as I seldom find cracks that were present with “ tube” type.

Perhaps the tubes were also intended to provide some structure?

There is a “ Horseshoe” that is spot welded to the Pressure Can. Failure of

this weld will provide a lot of CO into the heating system yet is impossible

to visually detect. Pressure and soap/water is essential. Even the cheap

CO dots will show with a flight around the pattern.

172 mufflers are available with varyi Tailpipe lengths. While the longer

lengths might keep the belly cleaner they seem more susceptible to

cracking the pressure can. Educated choice?
 
Flames tube from my last mufflers.
The welds had broken and the flash tubes in both mufflers were loose.
27.Cyl.4.3.Muffler.jpg28.Cyl.3.Muffler.End.1024.jpg
 
Thanks for education on flame tubes. I am thinking they are like spark arresters so we don't catch dry fields on fire.
Makes me glad there is no muffler on a powerflow exhaust, just a collector.
 
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