Aging Engines and Power Output

hish747

hish747

Pre-takeoff checklist
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Hish747
My engines have, thankfully, gone well beyond TBO on my 1978 Grumman Cougar and still have good compressions and oil usage. However, I have seen quite a gap between real world and book performance. For example, the book says that at 4500 ft, 23 squared, I should be seeing about 140-145 knots TAS on a "standard day."

In real life, I get closer to 130. What would cause such a gap? If the engines turn the propellers at the specified RPM, what else in the power equation could result in decreased power output? Could it be the propellers? Am I looking at the wrong factors, and it's actually the airframe?

This is something I wondered about for some time as I hear folks talk about decreasing power with older engines. Now, however, it's personal.
Thanks,
Hish
 
Many reasons.

But since you have constant speed props, you cannot tell power by RPM. If you can read the actual pitch in flight, that would tell you power.
 
Books on older planes sometimes are a wee but optimistic. Like the C-150 manual’s claims of climb rate at gross. Big LOL on that one.
 
Pinecone said:
Many reasons.

But since you have constant speed props, you cannot tell power by RPM. If you can read the actual pitch in flight, that would tell you power.
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Sure, but I really meant RPM and MP are set according to the book, in my example above, 2300 RPM and 23" MP.
 
Rigging, trimming correctly, inaccurate ASI, dirt on the airframe, bumps or dents on the airframe causing drag... I'd suspect any of these first before the engine.
 
Checkout_my_Six said:
If climb rate were a problem I’d look to the engine…but top speed is not greatly affected by HP.
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Precisely this. Power is proportional to velocity CUBED. 130kt vs. 140kt would be a 20% power reduction! Given climb ability is proportional to excess power you would expect a massive reduction in climb performance from this.

Does the engine make static RPM? This isn't perfect because the fine pitch stop could be off but it'd be helpful to know.

A recent episode of Ask the A&Ps had a discussion on identifying the cause of not hitting book airspeed: https://megaphone.link/AOPA3099906028 (around 15min in).
 
@hish747 Are you basing your performance number on IAS or True Air Speed? The POH is usually TAS. Older POH's are wildly optimistic as @texasclouds mentioned. Also @Half Fast and @Racerx suggestion of rigging is more likely.

If you have good compressions and the cam is healthy, the engine will make rated power regardless of age or tach time.

Climb performance tests engine strength, level flight performance is a test of drag which is commonly rigging related.
 
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Could it be the propellers?
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That's what I would ask. Is there something different about them, compared to new? Diameter maybe?
 
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I had a 182-RG. Book said 155 TAS. I always got between 140 and 145. I figured the book was optimistic. Then I got an overhaul (Penn Yan). I was hoping that I would see some improvement. Nope.

Then about 500SMOH, I got some metal in the oil filter. All signs pointed to the cam. So the engine came off and went to Poplar Grove. It was the cam. So that and the lifters got replaced. After the engine was installed, I started seeing 155 TAS. That made the sting of cost of the cam replacement easier to take.

I guess when the engine was overhauled, the cam was barely within limits.

So the answer is: It depends.
 
NordicDave said:
Older POH's are wildly optimistic as @texasclouds mentioned.
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Even if the POH isn’t a blatant lie, you can expect the numbers may have been obtained under nearly perfect conditions. Minimum fuel, skinny runt pilot, just enough ballast to put the CG at the aft limit, ideal altitude (probably 7000-8000 ft), plane freshly waxed, rigging perfect, favorable weather, etc., etc.
 
hish747 said:
Sure, but I really meant RPM and MP are set according to the book, in my example above, 2300 RPM and 23" MP.
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Yes, but you don't know the pitch. With a new engine at 23/23 maybe the pitch is 6 degrees, but with a worn out engine you set 23/23 but the pitch is only 5 degrees to allow the RPM to come to the set level.
 
donjohnston said:
I had a 182-RG. Book said 155 TAS. I always got between 140 and 145. I figured the book was optimistic. Then I got an overhaul (Penn Yan). I was hoping that I would see some improvement. Nope.

Then about 500SMOH, I got some metal in the oil filter. All signs pointed to the cam. So the engine came off and went to Poplar Grove. It was the cam. So that and the lifters got replaced. After the engine was installed, I started seeing 155 TAS. That made the sting of cost of the cam replacement easier to take.

I guess when the engine was overhauled, the cam was barely within limits.

So the answer is: It depends.
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Are you sure you didn't do anything else at the same time? 145 vs. 155 TAS in the same scenario would imply only 82% (-ish) of book power. Did you notice matching climb improvement?
 
I have always wondered how to determine if a an engine with a constant speed prop was developing the appropriate power or not.

Like mentioned RPM is constant and not related to power output.

Manifold pressure is based on Throttle position and RPM. Since RPM is constant it is a good indicator or throttle position, but not power output.

We sort of infer that we are getting the appropriate power based on RPM and throttle position, but it really isn't telling us that directly.

Like mentioned, propeller blade angle could really help, but I have never seen any constant speed props that gave in indication current blade angle. but even that would be somewhat dependent on propeller condition.

Aircraft performance is an indicator as well, but also as mentioned rigging, modification, and loading can easily affect performance also.

Brian
 
brcase said:
I have always wondered how to determine if a an engine with a constant speed prop was developing the appropriate power or not.

Like mentioned RPM is constant and not related to power output.

Manifold pressure is based on Throttle position and RPM. Since RPM is constant it is a good indicator or throttle position, but not power output.

We sort of infer that we are getting the appropriate power based on RPM and throttle position, but it really isn't telling us that directly.

Like mentioned, propeller blade angle could really help, but I have never seen any constant speed props that gave in indication current blade angle. but even that would be somewhat dependent on propeller condition.

Aircraft performance is an indicator as well, but also as mentioned rigging, modification, and loading can easily affect performance also.

Brian
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I am just spitballing and have zero data to back this up, but it seems like in theory you could take multiple book-relative performance figures in cruise and climb. The performance deficit is either attributable to airframe or powerplant/prop, and the relative contribution of these would differ in each case. Of course this relationship is complicated (parasitic vs. induced drag, etc.) but given the two values and two unknowns you ought to be able to solve for the contribution of each.
 
Eric Pauley said:
Does the engine make static RPM? This isn't perfect because the fine pitch stop could be off but it'd be helpful to know.
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This should be the FIRST stop in any troubleshooting. The Service Manual, or maybe even the POH/AFM, might have that. With CS props, MP would also figure into it along with RPM.
 
hish747 said:
My engines have, thankfully, gone well beyond TBO on my 1978 Grumman Cougar and still have good compressions and oil usage. However, I have seen quite a gap between real world and book performance. For example, the book says that at 4500 ft, 23 squared, I should be seeing about 140-145 knots TAS on a "standard day."
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IF... the engine has a good cam shaft?

Engines over 20 years since last overhaul are usually in need of fresh intake system gasket and seals, Ignition harness, magneto and carburetor overhaul. The calendar age of gasket, seals and rubber is a big problem. Most all engine manufacturers declare there engine's TBO at 12 years. My experience has been by 20 years and it's time to do a thorough gasket replacement and accessories inspection.

Many times the old ignition harness is the biggest power loss issue. The insulating properties between the center wire and the surrounding ground break down and the high voltage spark jumps across the insulation instead of the spark plug electrodes. Especially at higher power settings and altitude. If you replace the Ignition Harness "do not squeeze the wires" with adel clamps and/or ty-wraps/cable ties/zip-ties or whatever you want to call them.

P.S. the Ignition harness manufactures recommend replacement of 7 year intervals. Realistically a 10 year harness is probably due...
 
Eric Pauley said:
Are you sure you didn't do anything else at the same time? 145 vs. 155 TAS in the same scenario would imply only 82% (-ish) of book power. Did you notice matching climb improvement?
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AFAIK, the cam and lifters are all that were replaced. My mechanic said that a worn out cam (but within specs) could account for the performance change. And yes, there was an improvement in climb performance as well. I was VERY happy with the results.
 
donjohnston said:
My mechanic said that a worn out cam (but within specs) could account for the performance change.
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That would be a stretch in my opinion and experience. My current airplane performed no different before and after cam and lifter replacement as have many others that I have flown both before and after I overhauled the engine. In fact, my Cub still outperformed most of the other Cubs I flew with while I had a sick engine.

The engine would have to be REALLY sick before I’d expect to see a real performance reduction.
 
To see if it is airframe related, compare the performance to the book at a low airspeed and a high one. Drag increases as the 4th power of the speed.
 
mondtster said:
The engine would have to be REALLY sick before I’d expect to see a real performance reduction.
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Power loss shows up in climb far more than in cruise.

I wonder if the OP has flown north, east, south and west legs, averaging the GPS or map readings, to confirm that the ASI isn't off? The tiniest pitot line leak will drop the airspeed indication. A damaged/distorted/contaminated static port, or a static line leaking in the cabin, can cause ASI errors as well. The whole system might need a leak check.
 
Isn’t airspeed indicator IAS and to get TAS you need to add 2% per 1000’ so at 5000’ that would be 10%?

IAS 130
TAS 130 + 13 = 143

I also have some questions on airspeed numbers as they drastically reduce at higher altitudes (7000’+) but I run it a little slower at lower altitudes (130 v 135) as fuel consumption drops quite a bit (12 v 10). But the gauges are old analog so who knows what the real number is.
 
mondtster said:
That would be a stretch in my opinion and experience. My current airplane performed no different before and after cam and lifter replacement as have many others that I have flown both before and after I overhauled the engine. In fact, my Cub still outperformed most of the other Cubs I flew with while I had a sick engine.

The engine would have to be REALLY sick before I’d expect to see a real performance reduction.
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Engine was running fine before (and while) the engine started making metal. And one of the things I did occasionally (before and after the engine repair) was to fly a three point course and plug the groundspeed into a spreadsheet. Numbers don't lie.
 
donjohnston said:
Engine was running fine before (and while) the engine started making metal. And one of the things I did occasionally (before and after the engine repair) was to fly a three point course and plug the groundspeed into a spreadsheet. Numbers don't lie.
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Numbers don't necessarily tell the whole story either. See some of the above posts. It takes a significant power increase to see a 10 knot cruise increase. I suspect there was either a lot more wrong with the engine than just a slightly worn cam or there were other changes that were made that might not have been recognized.

I just did a test flight on a Cub that went from 135hp to 180hp today. I only saw a little over a 10mph increase in cruise. Climb is another story. Everything was the same before and after the swap, except for engine and the baffling.
 
mondtster said:
Numbers don't necessarily tell the whole story either. See some of the above posts. It takes a significant power increase to see a 10 knot cruise increase. I suspect there was either a lot more wrong with the engine than just a slightly worn cam or there were other changes that were made that might not have been recognized.
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I doubt that Poplar Grove would have done work without charging for it. Cam and associated parts were all that were replaced.
 
donjohnston said:
I doubt that Poplar Grove would have done work without charging for it. Cam and associated parts were all that were replaced.
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I don't think you're picking up what I'm implying. I don't think your engine was what caused the speed increase.
 
Check tachometer calibration. If you face directly into a fluorescent light you should see the prop shadow apparently stop at 1800 rpm and at 2400 rpm. 2100 rpm may be difficult but it might also be apparent.
 
Just to add a data point. Going from a 230hp o470 to a 280hp O520 in my 182, I gained around 5 or 6 knots. But the difference in climb is very noticeable!
 
AlphaMike said:
Just to add a data point. Going from a 230hp o470 to a 280hp O520 in my 182, I gained around 5 or 6 knots.
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At the same altitude? Or do you find that you need to fly higher to get the extra speed?
 
asicer said:
At the same altitude? Or do you find that you need to fly higher to get the extra speed?
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Not that I have noticed. But there are a lot of little factors that can change your speed by a couple / few knots so its hard to say for sure. But in general I get 5 or so knots better with the O520. It definitely climbs up faster but it burns more fuel and can be a little more challenging to keep the temperatures where I like them in a longer climb.
 
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