What causes deviations from POH performance numbers?

German guy

German guy

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Oliver
In our former flying club we had two Piper Archers. Same model, same engine, same prop, similarly equipped, almost identical empty weight. One of them accelerated and climbed however so sluggish that I was close to aborting my first take off with it, because it did not feel right. Others also claimed that particularly the take off was extremely slow. The engine was checked multiple times, but nothing was found to be wrong with it. The other Archer was a great performer, despite of its much higher time engine.

I heard from other pilots that they easily do better than the POH numbers. Others claim that planes, particularly older ones, 'of course' never make the numbers mentioned in the POHs, because they were created by the marketing department, the plane was flown by a god-like test pilot and the airframe, as well as the engine were new.

As I've seem people with unmodified Cessna 170s, 172s and engines past TBO do shorter take offs than stated in the POH (OK, owner's manual back then), as well as others who came nowhere close to the POH numbers, for the given weight and density altitude, despite of a low time engine, I don't think that the common explanations make much sense.

So - what causes these deviations, to the better or worse, at a given aircraft's take off weight, assuming the maintenance is done right? Frankly, I don't see too many parameters which could vary: Static compression seems to have a negligible effect on the engine's power, the ignitions is either set correctly or not, the aerodynamic qualities of a plane won't change much and I can hardly imagine that props vary greatly in their efficiency, if model and pitch are identical. :dunno:
 
Non standard something.

Non standard pilot

Non standard temp, etc

Non standard engine performance

Non standard rigging

Etc


I'd wager pilot first, rigging second.
 
James331 said:
Non standard something.

Non standard pilot

Non standard temp, etc

Non standard engine performance

Non standard rigging

Etc


I'd wager pilot first, rigging second.
Click to expand...

The difference between the two Archers was almost shocking. I believe that my piloting skills did not vary, when I flew with one plane or the other. The difference was significant and obvious within the first few seconds off the take of roll. Given the same environmental conditions, the same Archer always performed significantly worse or the other significantly better, depending on the perspective. :wink2: As we always took off with full tanks and as it was usually just my wife and myself in the plane, the load were very constant.

The plane was a low performer right from the beginning of the take off roll, I think that most likely something was wrong with the engine. They also had the engine pulled and overhauled a few hundred hours before we joined the club. I don't recall why this was done, the engine was however nowhere close to TBO when this decision was made.

I find these variations in the actual aircraft's performance quite astonishing, and would like to understand the reasons causing them. Particularly, since aircraft engines are relatively simple, with magnetos and a carb or a simple injections system.
 
I'd use an IR tach or an acoustic tachometer app to verify that the engines are turning the advertised rpm. Are the props the same on paper, or have they been measured to be the same?
 
Old pilots and rusty airplanes, as my instructor used to say... or was it Rusty pilots and old airplanes... I can't remember.
 
Sadly, I don't have access to the two Archers anymore.

I heard so many times that engines get tired and don't make full power anymore, once they have a few hundred hours on them, that I assumed that there might be some truth behind it.

I thought that there might be a few simple answers to my question, so that I can learn something. Not sure, if this indeed the case... :rolleyes:
 
German guy said:
I heard so many times that engines get tired and don't make full power anymore, once they have a few hundred hours on them, that I assumed that there might be some truth behind it.:
Click to expand...
Having flown two Cardinal RGs with engines of the same age and HP, I've suspected there may be truth to that. At ~600 hrs, both engines see true airspeeds in cruise never much over 130 kts at 24/24, and more often between 125-130. Previous Cardinal was 10 years ago, and a much younger airframe (original first-run engine, airframe time = engine time), current Cardinal has about 4500 hours and was flown since the engine was brand new, when TAS of 135-140 at the same power setting was pretty usual. The decline in cruise TAS was gradual and insidious.

But, I haven't noticed much decline in takeoff power or climb rate, which muddies the waters somewhat. I'm having the flaps checked at annual (ongoing).
 
German guy said:
In our former flying club we had two Piper Archers. Same model, same engine, same prop, similarly equipped, almost identical empty weight. One of them accelerated and climbed however so sluggish that I was close to aborting my first take off with it, because it did not feel right. Others also claimed that particularly the take off was extremely slow. The engine was checked multiple times, but nothing was found to be wrong with it. The other Archer was a great performer, despite of its much higher time engine.

I heard from other pilots that they easily do better than the POH numbers. Others claim that planes, particularly older ones, 'of course' never make the numbers mentioned in the POHs, because they were created by the marketing department, the plane was flown by a god-like test pilot and the airframe, as well as the engine were new.

As I've seem people with unmodified Cessna 170s, 172s and engines past TBO do shorter take offs than stated in the POH (OK, owner's manual back then), as well as others who came nowhere close to the POH numbers, for the given weight and density altitude, despite of a low time engine, I don't think that the common explanations make much sense.

So - what causes these deviations, to the better or worse, at a given aircraft's take off weight, assuming the maintenance is done right? Frankly, I don't see too many parameters which could vary: Static compression seems to have a negligible effect on the engine's power, the ignitions is either set correctly or not, the aerodynamic qualities of a plane won't change much and I can hardly imagine that props vary greatly in their efficiency, if model and pitch are identical. :dunno:
Click to expand...

Being out of rig can make differences. If ailerons have fixed trim tabs and they are required to be bent to hold the wings level that will degrade performance. Ailerons sticking up or down in level cruise create drag. Same with flaps. Do they retract all the way. Small adjustments here make a difference. These things show up as degraded performance more at cruise than takeoff speeds though
You have me wondering. Are aircraft engine Dyno's very common? There are all kinds of places you can take a car or motorcycle and for a few bucks find out exactly how much horsepower your engine is producing.
One more thought. Has the prop on one of those Archers had a lot of nicks dressed out of the prop? Every horse the engine makes has to be turned into thrust by the prop before its of any use at all.
 
I'd recommend weighing both aircraft.
 
German guy said:
The difference between the two Archers was almost shocking. I believe that my piloting skills did not vary, when I flew with one plane or the other. The difference was significant and obvious within the first few seconds off the take of roll. Given the same environmental conditions, the same Archer always performed significantly worse or the other significantly better, depending on the perspective. :wink2: As we always took off with full tanks and as it was usually just my wife and myself in the plane, the load were very constant.

The plane was a low performer right from the beginning of the take off roll, I think that most likely something was wrong with the engine. They also had the engine pulled and overhauled a few hundred hours before we joined the club. I don't recall why this was done, the engine was however nowhere close to TBO when this decision was made.

I find these variations in the actual aircraft's performance quite astonishing, and would like to understand the reasons causing them. Particularly, since aircraft engines are relatively simple, with magnetos and a carb or a simple injections system.
Click to expand...

What was noticible in the first few seconds of take off roll? RPM? MAP? Do Archers have a constant speed prop? If they do that opens up a whole new discussion. Can you feel the acceleration difference in your seat? What about Sound? Could you hear the difference?
 
I have to admit that I don't remember the actual static or take off rpm anymore.

Still, though, it appears as whether your questions already provide some answers: It is not just a normal variation, if an aircraft does not make POH numbers. Particularly, when it come to the ground roll, where the engine and the prop are the main factors!?

Too bad, that I left this club. It would have been interesting to try to figure out what the difference between these two planes is. Assuming that well maintained engines makes the same power, as you seem to suggest, differing actual weights or differently pitched props are what I would look at next.
 
The pitch of the prop can make a big difference. It may be that the props were different, but nobody had the paperwork to prove it. I've seen that happen before - sometimes a prop gets repitched, but the paperwork gets lost over the years. Someone looks up the prop info based on the S/N and gets the original numbers, not the current.
 
I'm not an expert on this so I fully expect to get jumped on for this, BUT...

I highly doubt POH or AFM would show marketing numbers.
Now, if you look in the sales brochure...
 
I've noticed that newer airplanes have more realistic numbers than older ones. Both the C-206 turbo and and C-320 (1966 and 1967 models) had very unrealistic service ceiling numbers. The Lear 55 was certified to 51,000 which I doubt it could ever do except maybe at the absolute lightest weight in a zoom climb followed by an immediate descent.
 
Engines do get "tired" over time depending on many factors. Things wear, deposits form, rings dont seal as well, cam lobes get worn down, it all adds up to less performance even if the engine is still running great. You can have an engine that runs well, runs smoothly and doesn't eat oil that also doesn't make as much horsepower as it once did. Add to that a prop that may be getting worn, or has been overhauled to it's limits, a plane that might be heavier than the one you're comparing it to, etc. etc. There are a bunch of factors in a plane not making book numbers.

Remember those book numbers are for a brand new plane on a standard day (usually). The book for my Mooney has numbers for 3 different temps at several altitudes on the takeoff performance page and the numbers vary greatly.
 
German guy said:
Assuming that well maintained engines makes the same power, as you seem to suggest, differing actual weights or differently pitched props are what I would look at next.
Click to expand...


Using that assumption, if the slower climber is the faster one in cruise then the props are most likely pitched differently. One that favors climb and one that favors cruise.
 
Ignition timing AND valve train timing. Sometimes engines get put together with the cam timing off by one tooth. Or the marks on the crankshaft flange are not correct.

Of course if this has developed slowly since overhaul, it could be a cam lobe has worn round. It is amazing how little valve lift it takes for an engine to still run smooth - at idle.
 
German guy said:
Sadly, I don't have access to the two Archers anymore.

I heard so many times that engines get tired and don't make full power anymore, once they have a few hundred hours on them, that I assumed that there might be some truth behind it.

I thought that there might be a few simple answers to my question, so that I can learn something. Not sure, if this indeed the case... :rolleyes:
Click to expand...

Engines don't get tired, if you keep an engine properly tuned, it will make the most power right before it spins a bearing and blows up.

There's multiple factors that it could be as to why an engine doesn't make proper power, in planes, if the timing and mixture are correctly set, it's mostly caused by people running too rich for too long and the valve seats and faces carbon up and leak. Lots of times though it's as simple as the points need to be reset correctly, or the points cam in the mag are worn beyond limits and proper dwell and timing cannot be established. Then there are induction leaks, carb tuning, bad spark plugs.... There is a small litany of little things that when combined add up to losing significant power.
 
Henning said:
[...] the points cam in the mag are worn beyond limits and proper dwell and timing cannot be established. [...]
Click to expand...

This is actually something I was thinking about just the other day. In the next days, I will conduct some preventive maintenance on our 172 with its O-300, now that we have already flown close to 25h with. More specifically, I will replace the engine oil, the oil- and air-filters and will also check the timing of the ignition, for what I ordered an ignition timing kit.

Anyway, the maintenance manual and all the information I could find on the internet, only discuss the ignition timing of cylinder #1. Once this is set, the magnetos take internally care of the timing of the other cylinders. Even though our engine start just great (I guess thanks to the SlickStart) and runs very nice, I would still like to be able to check the timing and dwell of all cylinders.
I was therefore wondering, whether there is a way to check the internal timing of the magnetos, without removing them?
 
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