Unable to achieve full power

idahoflier

idahoflier

Pattern Altitude
Joined
Jul 21, 2017
Messages
1,731
Display Name
Display name:
idahoflier
Long post, so thanks for reading!

I purchased my first aircraft last summer, a Skyhawk with an AirPlains 180HP conversion (O-360-A4M). Since I have owned it I have felt that I’m not getting 100% power. I also belong to a club that recently purchased a Skyhawk with the same conversion and that aircraft definitely has a bit more power than my aircraft. I’m close, but I would guess I probably have 10% - 15% less performance which is more pronounced at altitude. 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.

So, I have two questions:

1) The aircraft recently went through an annual inspection and compressions were found to be 70/80, 74/80, 76/80 and 78/80. Mags were found to be properly timed. I had four sparkplugs that failed the resistance check so I replaced all of the sparkplugs. Visual inspection of the induction system looks good. Verified full throttle hits the stop on the carb. The engine has approx. 1730 TT SNEW. I did notice the hose from the air filter to the carb was a bit too long and when the air filter housing is mounted to the cowl the hose collapses in a manner that obstructs the diameter of the hose by about ½”. My A&P didn’t feel this would cause any issue. Reviewing info from Lycoming suggests the issue is either ignition or induction. Since performance seems to get worse at altitude this would seem to me to point to an induction issue? Based on this info, any ideas where I should look next or anything I may have missed?



2) If it is as good as it’s going to get, I’m considering having the propeller re-pitched. I have never been able to achieve redline (2700 RPM) in cruise flight with 2600 being as high as I can get. A 2” reduction in pitch should give me an 80 RPM increase. I have previously asked a question in the forum regarding a prop re-pitch and the limited responses were mixed. Based on the above situation I don’t see a downside with doing this. Thoughts?

Thanks!
 
Not an AP (disclaimer), so my thoughts are worth what you paid...

1) possible mag/wire (would say plugs, but you replaced those) issue that goes beyond timing and/or something that shows on runup.

2) cam lobe(s) rounding off, causing less valve lift and this less power (would think this would’ve shown as metal in the filter, but you never know).

3) do you know for sure that the prop is the one you think it is (in other words, could the PO have swapped for a cruise prop and you’re basing your standard RPM readings on a prop that’s not installed?)

4) have you verified the condition of the exhaust (ie no internal baffles coming apart and blocking flow)?
 
Omalley1537 said:
Not an AP (disclaimer), so my thoughts are worth what you paid...
Click to expand...

Absolutely! I appreciate your thoughts though!

Omalley1537 said:
1) possible mag/wire (would say plugs, but you replaced those) issue that goes beyond timing and/or something that shows on runup.
Click to expand...

I have considered this. The mags are about 230 hrs and 10 years since they had their last 500 hr inspection. I need to do a cruise flight mag check, but keep forgetting to try this...

Omalley1537 said:
2) cam lobe(s) rounding off, causing less valve lift and this less power (would think this would’ve shown as metal in the filter, but you never know).
Click to expand...

Didn't think about this. Certainly possible. I have only changed the oil once and the filter was clean, but...

Omalley1537 said:
3) do you know for sure that the prop is the one you think it is (in other words, could the PO have swapped for a cruise prop and you’re basing your standard RPM readings on a prop that’s not installed?)
Click to expand...

It's possible. I guess I wouldn't know for sure unless the prop goes to the shop.

Omalley1537 said:
4) have you verified the condition of the exhaust (ie no internal baffles coming apart and blocking flow)?
Click to expand...

I have not. I watched the A&P inspect the exhaust, but maybe there is an obstruction internal? This is a good excuse to buy one of those cheap bore scopes!
 
Daniel T said:
Have you checked fuel mix? Maybe the jet fuel was bad as this may be common due to overheating esp. if in a truck
Click to expand...

Apologies, I'm not sure I follow you. Are you thinking maybe fuel contamination? If so, I don't think that's the issue as this situation has been very consistent through about 500 Gal of fuel from different sources... Thanks!
 
idahoflier said:
that aircraft definitely has a bit more power than my aircraft. I’m close, but I would guess I probably have 10% - 15% less performance which is more pronounced at altitude.
Click to expand...
Based on flying side by side, or by looking at gauges?

Same prop?

How accurate are the two tachs?
 
When was the last time your tach was calibrated?
 
idahoflier said:
Since I have owned it I have felt that I’m not getting 100% power
Click to expand...
Don't quite follow your issue. How are you determining 100% power? Have you thought of using the AFM performance charts to establish a baseline? As for comparing one similar aircraft to another performance wise doesn't always give you identical results. If your aircraft is performing to established charts based on environmental conditions then that might be what "100%" is for your aircraft. If not, then there are steps to take fix that problem. But first you have to quantify the 100% mark.
 
A motor provides the torque, a prop provides thrust. I'd look at the prop every time when investigating two similar airplanes' performance. Before you re-pitch make sure the prop is worthy of getting it done, but your assertion about 80 rpm for two inches is consistent with my experience.
 
Capt. Geoffrey Thorpe said:
Based on flying side by side, or by looking at gauges?
Click to expand...

Based on flying the same climbing profile with approximately the same fuel and weight. Also comparing takeoff distances (which is admittedly subject to some error) Both aircraft are C172N.

Capt. Geoffrey Thorpe said:
Same prop?
Click to expand...

Yes.

Capt. Geoffrey Thorpe said:
How accurate are the two tachs?
Click to expand...

I should have mentioned that I was using my TruTach II in both aircraft to compare the RPM. The TruTach is supposed to be +/- 1 RPM and has been calibrated by florescent light.
 
Bell206 said:
Don't quite follow your issue. How are you determining 100% power? Have you thought of using the AFM performance charts to establish a baseline? As for comparing one similar aircraft to another performance wise doesn't always give you identical results. If your aircraft is performing to established charts based on environmental conditions then that might be what "100%" is for your aircraft. If not, then there are steps to take fix that problem. But first you have to quantify the 100% mark.
Click to expand...

Yes, I have used the performance charts and I'm not getting the expected performance in the charts. It's a little more difficult because the charts provided are based on weights higher than I usually fly at, but I fail to meet the higher weight performance at my lighter weight. I should have mentioned that too. Thanks!
 
Last edited:
Stewartb said:
A motor provides the torque, a prop provides thrust. I'd look at the prop every time when investigating two similar airplanes' performance. Before you re-pitch make sure the prop is worthy of getting it done, but your assertion about 80 rpm for two inches is consistent with my experience.
Click to expand...

I will double check to make sure the prop on the club aircraft hasn't been re-pitched. My prop is pitched at 60" (stock) and AirPlains/Sensinich allow a pitch of 56" - 62". The local propeller shop told me that Sensinich allows a lifetime repitch of up to 8" on my model. I would re-pitch to 58" and if I ever went back to 60" then that would be 4" total so I should be OK. Thanks!
 
idahoflier said:
I would re-pitch to 58"
Click to expand...
Before you go the prop path I would verify the accuracy of all the instruments you use to determine performance. As mentioned above a faulty tach can skew your results. Might even contact AirPlains to see if they have any performance figures to compare to. Other things like off-spec flight control/flap rigging can cause performance issues. Once you determine all the surrounding issues are good then look at your prop. But tweaking prop pitch can be a slippery slope where you gain in one direction and lose in another.
 
It looks like things are actually worse than I thought... I compared the two flights with data from flightaware. The profile is a climb from 3,600 to 11,500. My "control" aircraft was the club aircraft. It was able to achieve the climb in 687 seconds with an average rate of climb of 680.72 FPM. My aircraft did the same profile in 960 seconds with an average of 504.33 FPM. Conditions, OAT and pressure were about the same. So it looks like I'm only about 75% of where I should be...
 
What's the club plane's static RPM? What's its level flight full power rpm?

Check your muffler for partial obstruction.
 
Bell206 said:
Before you go the prop path I would verify the accuracy of all the instruments you use to determine performance. As mentioned above a faulty tach can skew your results. Might even contact AirPlains to see if they have any performance figures to compare to. Other things like off-spec flight control/flap rigging can cause performance issues. Once you determine all the surrounding issues are good then look at your prop. But tweaking prop pitch can be a slippery slope where you gain in one direction and lose in another.
Click to expand...

I was using the same portable electronic tach. Both aircraft are equipped with ADSB so I think I'm comparing apples to apples.
 
Stewartb said:
What's the club plane's static RPM? What's its level flight full power rpm?

Check your muffler for partial obstruction.
Click to expand...

Club: 2350 RPM
Mine: 2280 RPM

I'll check the muffler. Thanks!
 
idahoflier said:
I was using the same portable electronic tach. Both aircraft are equipped with ADSB so I think I'm comparing apples to apples.
Click to expand...
Not quite. You are using external data. And FlightAware/ADS-B are not performance tools. There are specific methods to determine aircraft and engine performance, both static and dynamic. Using any other methods may not give you the whole picture, or worse, overlook a potential issue. But it's your nickel.
 
Bell206 said:
Not quite. You are using external data. And FlightAware/ADS-B are not performance tools. There are specific methods to determine aircraft and engine performance, both static and dynamic. Using any other methods may not give you the whole picture, or worse, overlook a potential issue. But it's your nickel.
Click to expand...

OK, well, those are the tools available to me. What action would you recommend? How should I compare performance? Would you be able to direct me to reference material for education? Thanks!
 
idahoflier said:
Close, within 7 - 10 kts.
Click to expand...

7-10 knots for a 172 is not close. Remember that power is proportional to the cube of velocity. It takes lots of power to go a little faster. But that also means that going a little slower means a loss of a lot of power.

P=(1/2)*ρ*Cd*A*v^3

I must admit that I doubted that you were really down on power after the first couple posts, but you now have me convinced. Something is definitely wrong with your powertrain somewhere.
 
Daniel T said:
Have you checked fuel mix? Maybe the jet fuel was bad as this may be common due to overheating esp. if in a truck
Click to expand...
Fuel mix? 172’s don’t burn Jet fuel......................
 
idahoflier said:
OK, well, those are the tools available to me. What action would you recommend? How should I compare performance? Would you be able to direct me to reference material for education? Thanks!
Click to expand...
First, forget comparing your aircraft to another. No two aircraft are the same even new ones with sequential S/Ns.

To give you an idea on engine performance charts look online for a Lycoming O-360 Operators Manual. Section 3 usually has the power curve charts. As for where to start troubleshooting will depend on a few things and whether you plan to work with a mechanic --which I recommend.

But a few things you can do is verify accuracy of tach, manifold pressure indicator, and OAT gauge. You could also talk to Air Plains once you read up on the ops manual and look for input. Engine/prop issues could be carb settings off, carb heat rigging off, mag wear, prop blades not in track, and other items mentioned above. If the engine and prop end up being in spec look to the airframe. If a plane doesn't fly straight it doesn't climb fast.

So it could depend on a number of things.
 
Hey @idahoflier it sounds like you indeed aren't getting full power. Sure every plane is different but those are pretty apparent items you've noted.. the 7-10 knots is a pretty big hit, the slower climb, and lower RPM, are 3 different items that suggest the plane is not making full power. This sounds like more than just airframe differences

Good luck with ehat you find, I'll be curious as well
 
arnoha said:
7-10 knots for a 172 is not close. Remember that power is proportional to the cube of velocity. It takes lots of power to go a little faster. But that also means that going a little slower means a loss of a lot of power.

P=(1/2)*ρ*Cd*A*v^3

I must admit that I doubted that you were really down on power after the first couple posts, but you now have me convinced. Something is definitely wrong with your powertrain somewhere.
Click to expand...

I apologize but I think I overstated how far out I am. I have reviewed a couple of recent flights and one flight where I flew a square with two minute legs specifically to calculate what I was getting for airspeed. On average I am within 5 KTS of the cruise performance data provided by AirPlains. Also I don't have my speed fairings installed so that's probably responsible for 2 KTS. However, the AirPlains performance data is for a gross weight of 2,550 lbs. and most of my flights are at 2,150 or less. The AirPlains performance chart also lists 2600 RPM and I can only make 2500 RPM.
 
Bell206 said:
First, forget comparing your aircraft to another. No two aircraft are the same even new ones with sequential S/Ns.

To give you an idea on engine performance charts look online for a Lycoming O-360 Operators Manual. Section 3 usually has the power curve charts. As for where to start troubleshooting will depend on a few things and whether you plan to work with a mechanic --which I recommend.

But a few things you can do is verify accuracy of tach, manifold pressure indicator, and OAT gauge. You could also talk to Air Plains once you read up on the ops manual and look for input. Engine/prop issues could be carb settings off, carb heat rigging off, mag wear, prop blades not in track, and other items mentioned above. If the engine and prop end up being in spec look to the airframe. If a plane doesn't fly straight it doesn't climb fast.

So it could depend on a number of things.
Click to expand...

Good advice, Thank you.
 
Tantalum said:
Hey @idahoflier it sounds like you indeed aren't getting full power. Sure every plane is different but those are pretty apparent items you've noted.. the 7-10 knots is a pretty big hit, the slower climb, and lower RPM, are 3 different items that suggest the plane is not making full power. This sounds like more than just airframe differences

Good luck with ehat you find, I'll be curious as well
Click to expand...

Thanks! I believe I overstated the cruise performance discrepancy, however the climb performance and lower RPM are definitely there.
 
Heavy will definitely reduce climb but generally speaking a Cessna is faster when heavy than light. At least mine have been.
 
Stewartb said:
Heavy will definitely reduce climb but generally speaking a Cessna is faster when heavy than light. At least mine have been.
Click to expand...

This. I was thinking the same thing...weight isn't all that important to top speed. Acceleration and climb, yeah, but not top speed.

172 get faster with weight in general because it's added to the back. That means a center of mass that moves back, reducing elevator deflection, improving drag.
 
Keep in mind as you figure out if yours is accurate, the club airplane may be out of whack too. If it has a tach that’s reading low, they may be flogging the crap out of it and not know it.

Easiest way to check the tach without any gear at all is to park at night facing away from a sodium vapor style light on the ramp. The 60 Hz flicker will cause the prop to stop apparent motion on exact divisible by 60 numbers as you slowly adjust the prop speed.
 
idahoflier said:
What action would you recommend? How should I compare performance?
Click to expand...
Fly side by side - both climb and WOT level flight.
Then, if there is a consistent difference, the question becomes: is it engine power or aerodynamic issues?
 
Geez, guys, he already stated that he has a TruTach. That's as accurate a tach as there is.

Go put a clothes pin on your throttle to keep it from going full in. Go fly with that slight reduction in power (RPM with a fixed pitch) and see if you recognize a difference in takeoff and climb. You will. The OP's question is whether the engine can't push the prop or the prop is too course for the engine. Both are true but the remedy needs to address them in order. Verify engine health and adjust the pitch to maximize power.

My Lycoming life centers around Cubs. If my static is under 2400rpm I'm giving away performance. I want 2400+ for static and 2700 in full power level cruise at low altitudes. Some guys have slightly different target numbers.
 
Another option would be to trade propellers between the two aircraft and repeat the comparative flights.
 
Capt. Geoffrey Thorpe said:
Fly side by side - both climb and WOT level flight.
Then, if there is a consistent difference, the question becomes: is it engine power or aerodynamic issues?
Click to expand...

That's pretty much what I have already done. The other aircraft is consistently a better climber by about 300 fpm. I don't believe it's aerodynamic as I can get close to book numbers in cruise. Thanks!
 
Stewartb said:
Geez, guys, he already stated that he has a TruTach. That's as accurate a tach as there is.

Go put a clothes pin on your throttle to keep it from going full in. Go fly with that slight reduction in power (RPM with a fixed pitch) and see if you recognize a difference in takeoff and climb. You will. The OP's question is whether the engine can't push the prop or the prop is too course for the engine. Both are true but the remedy needs to address them in order. Verify engine health and adjust the pitch to maximize power.

My Lycoming life centers around Cubs. If my static is under 2400rpm I'm giving away performance. I want 2400+ for static and 2700 in full power level cruise at low altitudes. Some guys have slightly different target numbers.
Click to expand...

Your post gave me another idea. I can fly the club aircraft with the same climbing profile but adjusting throttle to match the RPM that I consistently see in my aircraft. If the club aircraft then matches my performance I think that more or less proves that my engine isn't putting out full power and I'm probably not suffering any increased drag due to being out of rig, etc. Thanks!
 
Thanks for all of the feedback provided! I appreciate the discussion and I believe I have a plan. If everything checks out with the engine, checking for exhaust and intake obstructions, then I'm going to have the pro re-pitched. I have spoken with a propeller shop and a re-pitch should only run about $200. This seems like the lowest cost solution.

Again, thanks for the feedback!
 
I would look at the carb and all of its attachments and venturi
 
idahoflier said:
I did notice the hose from the air filter to the carb was a bit too long
Click to expand...
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))?

Some years ago, when I was doing combustion analysis for an auto company - we had a test car come in that had "high speed detonation" according to the engineers running it. Instrumented the car with the pressure taps in the cylinders and all the other stuff (which takes about a week), put it on the dyno, ran it. No indication of any detonation in the combustion data. Got the engineer in charge of the vehicle to come in - demonstrate please. He gets in, fires it up, starts up, I'm standing outside the car and the hood is open.
As he gets into the power, I notice that the intake tube (which is a bit too long) collapses and the engineer driving calls out that "it's doing it - loosing power and detonating". I have him repeat a couple times, and yep. I see the problem. Send a tech off for some dikes, trim the hose to the correct length, re-attach. 10 minutes, car is fixed.

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?).

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...
 
You must log in or register to reply here.
Back
Top