Unable to achieve full power

[idahoflier]

Are you sure that it is maintaining the nearly full cross section in flight (is it possible to make it partly collapse by tilting the coils (I'm assuming it is wire reinforced))?

No, not at all. In fact I know it's not maintaining it's complete cross section by being obstructed by about 1/2". Mentioned this to my mechanic and he didn't seem to think that's an issue. That's one of the things I'm going to remedy before having the prop re-pitched. The hose is a CAT hose which is much more flexible than SCAT...

Another thing to consider - set the prop horizontally on each aircraft and measure the angle (from horizontal) on the back of the blades at a few identical radii. Are the props really the same? (I found that the two blades on ground adjustable prop on my ride did not agree when I got it). Not a likely thing, but easy and low cost to check (get an angle app for your phone?).

That's a great idea, the iphone already has the level built in, although it's only good to about 1° resolution. Not sure that will be good enough, but worth a try...

Not saying that it isn't a loss of power, or something like exhaust restriction, but it may be worth looking at the cheap and easy things...

Agree, thanks!

 

[Baron62]

Check your intake path and exhaust for restrictions then report back.

 

[charlie]

Check the muffler because it is easy to do. It takes a lot of cam wear to cause that amount of power loss. I'd take the rocker covers off and do a comparative check of rocker arm travel. There are numbers out there saying what the lift should be. I have never seen a cam failure that took out more than half the lobes. Assuming it's just one or two lobes it will become obvious which ones are bad. I have flown maybe 20 Penn Yan conversions doing the flight tests during the 4 years that I ran Penn Yan. The WOT at 7,500' TAS variance might have been 6 KTS. With just me and full fuel A good one will get to 130 KTS. Our demo would actually exceed 130.

 

[Matthew Bourguignon]

So what did you find, bud? I realize I’m reviving a zombie thread but I just picked up two Air Plains 172Ns with literally the exact same symptoms, down to the numbers. If you have any insights, I’d appreciate them!

 

[idahoflier]

So what did you find, bud? I realize I’m reviving a zombie thread but I just picked up two Air Plains 172Ns with literally the exact same symptoms, down to the numbers. If you have any insights, I’d appreciate them!

Sorry for the delay I was out of town and your interest piqued my curiosity and I wanted to go fly again before responding...

Unfortunately I'm not going to be able to provide any meaningful insight for you. In short, IDK.

Not too long after that post I left the flying club I was in and so lost access to the aircraft. Somewhere on here there's a thread I started related to my engine developing a "ticking" sound that my A&P couldn't identify. I ended up getting the engine overhauled. As part of that I had a propeller shop do everything to my prop short of an overhaul. So the engine and prop are probably as good as they're going to get.

When I went flying a couple of days ago I checked my static RPM and it was about 2295, so a tiny bit better from my original post, but effectively the same. Air Plains only publishes performance data at max gross and I was about 400lbs lighter so I can't compare my performance to book numbers with the exception of takeoff roll where they publish numbers at lower weights. Interpolating that data for the conditions I was right on the book numbers, so I think I'm getting book performance for takeoff roll.

So if I was to speculate, the only thing I can think that would be the cause of the discrepancy between my aircraft and the other Skyhawk is probably rigging. There is also the possibility that the other aircraft had the prop re-pitched. I don't remember looking in the logbooks to see if this had occurred.

Anyway, good luck with your Skyhawks, the O-360 conversion makes them a versatile aircraft. If you don't have engine monitors installed I would highly recommend them. I had an engine monitor installed at overhaul and was amazed at the high CHT's. Without it I would have likely cooked cylinders...

 

[Matthew Bourguignon]

Sorry for the delay I was out of town and your interest piqued my curiosity and I wanted to go fly again before responding...

Unfortunately I'm not going to be able to provide any meaningful insight for you. In short, IDK.

Not too long after that post I left the flying club I was in and so lost access to the aircraft. Somewhere on here there's a thread I started related to my engine developing a "ticking" sound that my A&P couldn't identify. I ended up getting the engine overhauled. As part of that I had a propeller shop do everything to my prop short of an overhaul. So the engine and prop are probably as good as they're going to get.

When I went flying a couple of days ago I checked my static RPM and it was about 2295, so a tiny bit better from my original post, but effectively the same. Air Plains only publishes performance data at max gross and I was about 400lbs lighter so I can't compare my performance to book numbers with the exception of takeoff roll where they publish numbers at lower weights. Interpolating that data for the conditions I was right on the book numbers, so I think I'm getting book performance for takeoff roll.

So if I was to speculate, the only thing I can think that would be the cause of the discrepancy between my aircraft and the other Skyhawk is probably rigging. There is also the possibility that the other aircraft had the prop re-pitched. I don't remember looking in the logbooks to see if this had occurred.

Anyway, good luck with your Skyhawks, the O-360 conversion makes them a versatile aircraft. If you don't have engine monitors installed I would highly recommend them. I had an engine monitor installed at overhaul and was amazed at the high CHT's. Without it I would have likely cooked cylinders...

I appreciate the response to this 18 year old thread!
We ferried the second Skyhawk home across the country last week and saw the exact same thing; full-throttle performance was right on the money for 62% power, regardless of what density altitude we flew at. Prop never came close to overspeeding, even in a descent, turning barely 2500 RPM in level flight at all altitudes and temps tested. Climb performance was acceptable at best; despite being more than 200 pounds below gross, she took 10-20% longer to climb to altitude than the performance charts showed for gross weight. (And yes, I corrected for temperature.) The only difference in this plane is she burned 10gph, or 75% power fuel burn, rather than the 8gph the book calls out at 62%. Hopefully that points to an ignition issue.

The first Skyhawk went right into Mx after arriving home two weeks ago. My IA took her apart and went through everything as thoroughly as possible without disassembling the whole airplane. We found no "smoking guns" regarding her under-performing nature. He put everything back in proper order, and I took the plane for a test flight, just to find the same results: under-performing climb, low RPM, 62% power despite full-throttle operation at density altitudes where 75% should have been achievable, and fuel burn to match the power (about 8gph). I'm wondering if the previous owners, a flight school, were into something sketchy to save money on fuel burn and engine wear, and now we've got to find it.

 

[Pinecone]

Let's see, you took off and flew an airplane with an unknown issue with the engine preventing full power? Really? Interesting.

 

[Lindberg]

I appreciate the response to this 18 year old thread!

I appreciate when posters visit us from the future!

 

[idahoflier]

I appreciate the response to this 18 year old thread!
We ferried the second Skyhawk home across the country last week and saw the exact same thing; full-throttle performance was right on the money for 62% power, regardless of what density altitude we flew at. Prop never came close to overspeeding, even in a descent, turning barely 2500 RPM in level flight at all altitudes and temps tested. Climb performance was acceptable at best; despite being more than 200 pounds below gross, she took 10-20% longer to climb to altitude than the performance charts showed for gross weight. (And yes, I corrected for temperature.) The only difference in this plane is she burned 10gph, or 75% power fuel burn, rather than the 8gph the book calls out at 62%. Hopefully that points to an ignition issue.

The first Skyhawk went right into Mx after arriving home two weeks ago. My IA took her apart and went through everything as thoroughly as possible without disassembling the whole airplane. We found no "smoking guns" regarding her under-performing nature. He put everything back in proper order, and I took the plane for a test flight, just to find the same results: under-performing climb, low RPM, 62% power despite full-throttle operation at density altitudes where 75% should have been achievable, and fuel burn to match the power (about 8gph). I'm wondering if the previous owners, a flight school, were into something sketchy to save money on fuel burn and engine wear, and now we've got to find it.

Sounds like you have a lot of drag somewhere. If you can get 2500 in level flight you should be able to exceed that in a descent. What TAS were you seeing in level flight and at what altitude? Not at all trying to be insulting, but are you leaning at altitude?

 

[Dan Thomas]

The prop on a fixed-pitch application on a type-certified airplane should allow the engine to reach redline at full throttle in level flight at sea level. I have found this true up to 6000' ASL.

Sounds like prop overpitching.

 

[Matthew Bourguignon]

I appreciate when posters visit us from the future!

I've always wanted to achieve time travel.

 

[Matthew Bourguignon]

Sounds like you have a lot of drag somewhere. If you can get 2500 in level flight you should be able to exceed that in a descent. What TAS were you seeing in level flight and at what altitude? Not at all trying to be insulting, but are you leaning at altitude?

I thought about drag too. But if we were being held back by drag, we would still see normal-ish fuel burns to go with the lower speeds, but that's not the case. We saw 113 KTAS at multiple altitudes tested from 3000 up to 11.5k. Mixture was always leaned to best power by max RPM method.

 

[Matthew Bourguignon]

The prop on a fixed-pitch application on a type-certified airplane should allow the engine to reach redline at full throttle in level flight at sea level. I have found this true up to 6000' ASL.

Sounds like prop overpitching.

That was my thought, but the prop matches what's called out in the Air Plains paperwork, meaning it would've had to have been re-pitched. Why would a flight school located in a high DA region intentionally overpitch their props? It's not like doing so is free.

 

[idahoflier]

I thought about drag too. But if we were being held back by drag, we would still see normal-ish fuel burns to go with the lower speeds, but that's not the case. We saw 113 KTAS at multiple altitudes tested from 3000 up to 11.5k. Mixture was always leaned to best power by max RPM method.

I disagree. 10 GPH is close to normal fuel burn for WOT. The engine and the propeller can be producing all the "normal" power and thrust, but if you have extra drag your airspeed is going to suffer. Also, are you sure your tach is accurate? Mine reads 100 RPM lower than actual. FWIW, my propeller is pitched properly and my engine, 200 hours out of overhaul, still cannot redline in level flight. Of course I start out 2,500 MSL and most of the time the DA only goes up from there...

 

[MRC01]

FWIW, I installed the Penn Yan Superhawk STC in my 1980 172N about 5 years ago, O-360-A4M. It makes full power, can pull redline in level cruise from sea level to above 5000'. For long x-country flights I aim for cruise around 10,000', plus or minus, and fly near WOT (back off WOT just enough to disengage the enrichment circuit), leaned for peak RPM, which gives around 125-130 KTAS depending on load/weight.

I wonder what differences there are between the Penn Yan and Air Plains STCs.

PS: recent flight cruising at 130 KTAS at 9500 MSL (pics or it didn't happen)

 

[idahoflier]

I wonder what differences there are between the Penn Yan and Air Plains STCs.

Perhaps the prop? Per Air Plains, redline isn't achievable until 8,000 MSL...

 

[MRC01]

Perhaps the prop? Per Air Plains, redline isn't achievable until 8,000 MSL...

That's another difference between Air Plains and Penn Yan. The Penn Yan STC doesn't have new performance tables, just a paragraph stating that performance will be better than the POH and fuel burn will be higher.

Also, the Penn Yan STC has the A&P fabricate new baffling, so engine cooling will depend on his level of skill.

I will check my prop details tonight and post back here, as it may be helpful in the comparison.

 

[idahoflier]

I will check my prop details tonight and post back here, as it may be helpful in the comparison.

I would be interested in that! The Air Plains STC covering my C172N specifies a Sensenich 76EM8S14-0-60. It may be re-pitched 56 - 62. Mine is the stock 60...

 

[MRC01]

The prop that came with the Penn Yan STC is a Sensenich 76EM8-0-60 / 76EM8SPY-0-60.
It replaced the prior prop that was on the O-320-D2G engine.
PS: pitched 60" per rotation seems about right, as 2600 RPM makes 128 knots assuming no slippage in the air. And that's about the true airspeed at that high power setting.

 

[idahoflier]

The prop that came with the Penn Yan STC is a Sensenich 76EM8-0-60 / 76EM8SPY-0-60.
It replaced the prior prop that was on the O-320-D2G engine.
PS: pitched 60" per rotation seems about right, as 2600 RPM makes 128 knots assuming no slippage in the air. And that's about the true airspeed at that high power setting.

Well, IDK? I have suspected my aircraft is out of rig which I think is responsible for a few KTS. I also fly with the speed fairings off. It's strange that the Air Plains performance data above has a max of 122 KTAS. But it doesn't list which C172 model that data is from. Anyway, it's cool that you're getting 130KTS!

 

[Matthew Bourguignon]

I disagree. 10 GPH is close to normal fuel burn for WOT. The engine and the propeller can be producing all the "normal" power and thrust, but if you have extra drag your airspeed is going to suffer. Also, are you sure your tach is accurate? Mine reads 100 RPM lower than actual. FWIW, my propeller is pitched properly and my engine, 200 hours out of overhaul, still cannot redline in level flight. Of course I start out 2,500 MSL and most of the time the DA only goes up from there...

I would agree, but we never saw 10 gallons per hour on this airplane. We were consistently burning 8.5 gallons per hour, which is what my unconverted 160 hp N model Cessna burns.

 

[Pinecone]

Perhaps the prop? Per Air Plains, redline isn't achievable until 8,000 MSL...

That is NOT what that chart is saying. That chart is saying, not until 8,000 MSL is your max CRUISE POWER at max RPM.

It does NOT say that the engine will not make redline until that altitude.

Your POH should have a information for the minimum RPM allowed for a full power static (not moving) run up. If you cannot max that static RPM, you should not fly the aircraft.

For the C-172S (factory IO-360) Section 2, page 2-6 states, "The static RPM range at full throttle is 2300 - 2400 RPM." So hold brakes, full power, the engine should make a minimum of 2300 RPM.

 

[Matthew Bourguignon]

That is NOT what that chart is saying. That chart is saying, not until 8,000 MSL is your max CRUISE POWER at max RPM.

It does NOT say that the engine will not make redline until that altitude.

Your POH should have a information for the minimum RPM allowed for a full power static (not moving) run up. If you cannot max that static RPM, you should not fly the aircraft.

For the C-172S (factory IO-360) Section 2, page 2-6 states, "The static RPM range at full throttle is 2300 - 2400 RPM." So hold brakes, full power, the engine should make a minimum of 2300 RPM.

I know you were responding to another commenter, but I’d like to add that our airplane in question DOES make the minimum static RPM.

 

[MRC01]

Well, IDK? I have suspected my aircraft is out of rig which I think is responsible for a few KTS. I also fly with the speed fairings off. It's strange that the Air Plains performance data above has a max of 122 KTAS. But it doesn't list which C172 model that data is from. Anyway, it's cool that you're getting 130KTS!

I don't have wheel pants, but at these slowish speeds they don't make much difference (which is why I don't have them).
That 130 KTAS is only when light. Near GW (2550 #) that max power cruise is about 8 kts slower.

... we never saw 10 gallons per hour on this airplane. We were consistently burning 8.5 gallons per hour, which is what my unconverted 160 hp N model Cessna burns.

At high altitude high power cruise (125-130 KTAS) it burns about 8.5 gph. The highest fuel burn I've seen is 11 gph, flying near full power sea level full rich, during break-in. If I slow down to 2300 RPM it's 7 gph.

 

[Dan Thomas]

Your POH should have a information for the minimum RPM allowed for a full power static (not moving) run up. If you cannot max that static RPM, you should not fly the aircraft.

For the C-172S (factory IO-360) Section 2, page 2-6 states, "The static RPM range at full throttle is 2300 - 2400 RPM." So hold brakes, full power, the engine should make a minimum of 2300 RPM.

Sections of the POH gets superseded if more horsepower is STC'd into the airplane. The STC should include a POH amendment. The POH would apply only to the original configuration.

For many airplanes, that static RPM also depends on the propeller installed. The TCDS is the place to find more detail.



The 172S/SP has only one OEM prop. The 172M, on the other hand, has several factory options:

 

[MRC01]

With the Penn Yan STC, the static full power sea level prop RPM didn't change much. The new prop seems to have just enough more pitch for the extra horsepower.

 

[Pinecone]

I know you were responding to another commenter, but I’d like to add that our airplane in question DOES make the minimum static RPM.

If it makes static RPM on the proper prop, then it is making full power.

 

[idahoflier]

That is NOT what that chart is saying. That chart is saying, not until 8,000 MSL is your max CRUISE POWER at max RPM.

It does NOT say that the engine will not make redline until that altitude.

Your POH should have a information for the minimum RPM allowed for a full power static (not moving) run up. If you cannot max that static RPM, you should not fly the aircraft.

For the C-172S (factory IO-360) Section 2, page 2-6 states, "The static RPM range at full throttle is 2300 - 2400 RPM." So hold brakes, full power, the engine should make a minimum of 2300 RPM.

Respectfully disagree. The O-360-A4M is has a redline of 2,700 RPM continuous. So max power = max cruise which will be achieved at 2,700 RPM. If the Aircraft/Engine/Prop was capable of 2,700 rpm at lower altitudes in cruise flight, 2,700 RPM would be in the chart.

Regarding static RPM - I know that. If you go back, you will see that covered in Post #1... Comparing a C-172S to a C172N with an Air Plains conversion is an apple to orange comparison.

 

[idahoflier]

With the Penn Yan STC, the static full power sea level prop RPM didn't change much. The new prop seems to have just enough more pitch for the extra horsepower.

If memory serves, The propeller is also 1-2" longer.

 

[idahoflier]

I don't have wheel pants, but at these slowish speeds they don't make much difference (which is why I don't have them).
That 130 KTAS is only when light. Near GW (2550 #) that max power cruise is about 8 kts slower.

Interesting. I'm very rarely at max gross and never bothered to check cruise performance. Based on what I have heard you would see the best cruise performance closer to max gross because there would be less drag from the horizontal stabilizer. I'll have to look at this the next time I'm fully loaded...

 

[MRC01]

Interesting. I'm very rarely at max gross and never bothered to check cruise performance. Based on what I have heard you would see the best cruise performance closer to max gross because there would be less drag from the horizontal stabilizer. I'll have to look at this the next time I'm fully loaded...

Being heavy makes it less efficient because the same airspeed requires more AoA which is more drag.
Cruise efficiency does improve slightly with a rearward CG, since the elevator produces less downforce.
So for ideal cruise efficiency you want to be light with rearward CG - which is possible, yet unusual.

 

[idahoflier]

If it makes static RPM on the proper prop, then it is making full power.

The static RPM limits for the C172N Air Plains conversion are 2250 - 2450. That includes O-360-A2F, A3A, A4A, A4M and A4N and four propellers Sensenich 76EM8S14-0-60, 76EM8S-0-60 and McCauley 1A170/CFA, 1A170E/CFA.

You're obviously going to see some variation due to DA. The Sensenich props are limited to 76" +/- 0". The McCauley are limited to 74.5" - 76". So I can see how it's possible the higher limit could be seen with a McCauley at 74.5".

However, in my case, I was comparing two C172N's with O-360A4M and unmodified Sensenich 76EM8S14-0-60 propellers. My aircraft has never matched the static RPM, climb or cruise performance of the other aircraft...

 

[idahoflier]

Being heavy makes it less efficient because the same airspeed requires more AoA which is more drag.
Cruise efficiency does improve slightly with a rearward CG, since the elevator produces less downforce.
So for ideal cruise efficiency you want to be light with rearward CG - which is possible, yet unusual.

Agree, there will be a sweet spot between gross weight and CG. I'm a short, fat guy, so maybe that has more to do with it than I thought... ;-)

 

[Pinecone]

Respectfully disagree. The O-360-A4M is has a redline of 2,700 RPM continuous. So max power = max cruise which will be achieved at 2,700 RPM. If the Aircraft/Engine/Prop was capable of 2,700 rpm at lower altitudes in cruise flight, 2,700 RPM would be in the chart.

Regarding static RPM - I know that. If you go back, you will see that covered in Post #1... Comparing a C-172S to a C172N with an Air Plains conversion is an apple to orange comparison.

No, even though the engine is rated for max continuous, cruise is normally quoted as 75% power or lower.

Show me a POH that lists cruise power setting of 100%

Sorry missed that. I was not saying to USE the number I was quoting, I was pointing out that it was in the POH. But fixed pitch static RPMs tend to be in the same range. If the power is increased, the prop to changed to larger diameter and/or higher pitch.

 

[idahoflier]

No, even though the engine is rated for max continuous, cruise is normally quoted as 75% power or lower.

Show me a POH that lists cruise power setting of 100%

I can't recall any so I'll concede the point...

 

[MRC01]

No, even though the engine is rated for max continuous, cruise is normally quoted as 75% power or lower.
Show me a POH that lists cruise power setting of 100%
...

I can't recall any so I'll concede the point...

In a fixed pitch airplane, it is impractical to the point of being impossible. 100% power is full throttle at 2700 RPM at sea level equivalent conditions. Most fixed pitch airplanes can't use full throttle in level cruise near sea level; that would exceed redline. So to do this, the prop pitch would be so large it would seriously impair takeoff and climb performance.

With a CSP, it should be possible to get 100% power cruising at sea level equivalent conditions (theoretically). But I've never seen 100% power cruise shown in a POH. I don't really know why, but I assume it's because the engine installation, cowling, airflow, etc. isn't able to dissipate the heat that would be produced. Of course, with a CSP you can get 100% power on takeoff and climb-out. But that's only for a few minutes.

 

[idahoflier]

In a fixed pitch airplane, it is impractical to the point of being impossible. 100% power is full throttle at 2700 RPM at sea level equivalent conditions. Most fixed pitch airplanes can't use full throttle in level cruise near sea level; that would exceed redline. So to do this, the prop pitch would be so large it would seriously impair takeoff and climb performance.

With a CSP, it should be possible to get 100% power cruising at sea level equivalent conditions (theoretically). But I've never seen 100% power cruise shown in a POH. I don't really know why, but I assume it's because the engine installation, cowling, airflow, etc. isn't able to dissipate the heat that would be produced. Of course, with a CSP you can get 100% power on takeoff and climb-out. But that's only for a few minutes.

I agree and that was the point I was trying to get at. The O-360-A4M is rated for 180 HP @ 2,700 RPM at sea level and standard conditions. Increase DA and performance decreases. Decrease RPM and performance decreases. I can't say for sure, but I don't believe it's possible for a "perfect" C172N with a "perfect" O-360A-4M and a 76" prop pitched for 60" to achieve 2,700 RPM flying WOT in standard conditions at 100' MSL.

@Pinecone was saying I couldn't use that Air Plains chart in the manner I was using it, ie redline isn't achievable until ~ 8,000 ft. I'm not sure that's correct because it shows 77% BHP at 2,500 RPM at 20°C below standard. 2550 RPM is "---" all across. Is that because Air Plains figured 2550 was well beyond 75% BHP so why include it or is it because it's not achiveable? I suspect because it's not achievable, but IDK? In any case I do concede you don't find Cruise Performance in POH's above 75%.

If I had a MP guage I could solve this because Lycoming publishes performance data for the O-360-A4M, but you need to know the MP...

In any case, for me and my original question, I think the reason I'm not seeing the performance that others, like yourself, see is because I have more drag from somewhere. Could be (probably) rigging, maybe my airframe is bent, IDK???

 

[Pinecone]

Witha fixed pitch prop, RPM is proportonal to HP. So as you climb, you need a higher throttle setting to get that same RPM.

Try this, set 2500 RPM at 1000 feet. Notice IAS and throttle position. Climb to 8000 feet and repeat.

 

[idahoflier]

Witha fixed pitch prop, RPM is proportonal to HP. So as you climb, you need a higher throttle setting to get that same RPM.

Sort of, there's a little more to it, but yes, I agree at any given DA more RPM = more HP...

Try this, set 2500 RPM at 1000 feet. Notice IAS and throttle position. Climb to 8000 feet and repeat.

Not possible for me, in the summer I'm starting out with DA's ~ 4,000 and it only goes up from there...

 

[MRC01]

Getting back to your original post:

... The static limits listed by AirPlains are 2250 – 2450 RPM. I’m about 2280 on most static run ups so within limit, but on the low side. The comparison club aircraft is about 2350.
...

... When I went flying a couple of days ago I checked my static RPM and it was about 2295, so a tiny bit better from my original post, but effectively the same. ...

The range that AirPlains provides spans different props. Being on the low end of the range doesn't necessarily mean you're down on power, it could be normal for your prop. Does the comparison club aircraft have the same prop? The difference from 2295 to 2350 is only about 2%. That's well within the margin for error when you consider density altitude, tach accuracy, and other factors.

I'm not yet sure your engine actually is below expected power. What is the maximum true airspeed that you can achieve in level flight, at high enough altitude that you don't have to throttle back to avoid overspeeding the engine; say, 10,000' altitude? Adjust for DA and total weight, and see where you are.