Fuel Flow & RPM on Take Off

[Tmooney]

I am struggling a little with the take off/climb performance of my M20F.  The plane will only produce 2595-2600rpm and 15gal/hr fuel flow on take off with 30" manifold pressure at sea level.  When I level off in cruise, the RPM will peak at 2630rpm.  I have a Dynon engine monitoring system so I am assuming the tach is reading correctly but I have not verified.  I know the fuel flow computer is spot on.

Is the lower RPM on take off negatively affecting my fuel flow and climb performance?  I am typically able to keep my hottest CHT at 380 on take off, but on hot days it can be a little difficult to keep the hottest CHT under 400 during the climb.  I will try and verify that the tach is reading correctly and ask my A&P to adjust the prop governor if it is indeed only producing 2600rpm.  What are some other things/adjustments I can look at to nail down my low fuel flow issue?  I appreciate the help!

[PT20J]

During a full power static runup, the engine should turn about 2650 rpm. In this condition, the prop will be on the low pitch stops and the governor will not be controlling it. If it doesn’t make static rpm, either the tach is wrong or the low pitch stops are incorrect (unlikely unless this started after prop maintenance) or the engine is not producing rated power. When the airplane begins the takeoff roll, the airflow through the prop will want to make it speed up and the governor will move the blades off the low pitch stops to hold 2700 rpm assuming the governor is set correctly. 

According to the IO-360 Operator’s Manual, 15 gph at best power mixture and 2600 rpm is about 194 hp. According to the same graph, increasing rpm to 2700 would only increase the fuel flow by 0.7 gph. I get around 18.5 gph on sea level takeoffs in my IO-360. I would suspect that something is restricting the full power fuel flow. Maybe the mixture control is not rigged properly and the servo is not hitting full rich. The lower fuel flow would cause higher CHTs. 

[Shadrach]

3 hours ago, Tmooney said:

I am struggling a little with the take off/climb performance of my M20F.  The plane will only produce 2595-2600rpm and 15gal/hr fuel flow on take off with 30" manifold pressure at sea level.  When I level off in cruise, the RPM will peak at 2630rpm.  I have a Dynon engine monitoring system so I am assuming the tach is reading correctly but I have not verified.  I know the fuel flow computer is spot on.

Is the lower RPM on take off negatively affecting my fuel flow and climb performance?  I am typically able to keep my hottest CHT at 380 on take off, but on hot days it can be a little difficult to keep the hottest CHT under 400 during the climb.  I will try and verify that the tach is reading correctly and ask my A&P to adjust the prop governor if it is indeed only producing 2600rpm.  What are some other things/adjustments I can look at to nail down my low fuel flow issue?  I appreciate the help!

15gph is certainly on the low side for 30" of MP. What are the take off EGT numbers?  At that FF and MP, I would expect them to be in the 1400 range which is sub optimal to say the least.  Ideally the EGTs would be high 1100s to low 1200s for full rich take off. The RPM issue and the fuel flow/CHT issue are different.  I would focus on the fuel delivery issue first. 

[Tmooney]

29 minutes ago, Shadrach said:

15gph is certainly on the low side for 30" of MP. What are the take off EGT numbers?  At that FF and MP, I would expect them to be in the 1400 range which is sub optimal to say the least.  Ideally the EGTs would be high 1100s to low 1200s for full rich take off. The RPM issue and the fuel flow/CHT issue are different.  I would focus on the fuel delivery issue first. 

The EGTs on cylinder 1,3,4 are always around 1330 on take off.  Cylinder #2 is typically around 1250.  I have attached a graph below that is a typical take off at sea level.  

[Tmooney]

38 minutes ago, PT20J said:

During a full power static runup, the engine should turn about 2650 rpm. In this condition, the prop will be on the low pitch stops and the governor will not be controlling it. If it doesn’t make static rpm, either the tach is wrong or the low pitch stops are incorrect (unlikely unless this started after prop maintenance) or the engine is not producing rated power. When the airplane begins the takeoff roll, the airflow through the prop will want to make it speed up and the governor will move the blades off the low pitch stops to hold 2700 rpm assuming the governor is set correctly. 

According to the IO-360 Operator’s Manual, 15 gph at best power mixture and 2600 rpm is about 194 hp. According to the same graph, increasing rpm to 2700 would only increase the fuel flow by 0.7 gph. I get around 18.5 gph on sea level takeoffs in my IO-360. I would suspect that something is restricting the full power fuel flow. Maybe the mixture control is not rigged properly and the servo is not hitting full rich. The lower fuel flow would cause higher CHTs. 

Thank you for this information!

[PT20J]

The plot is interesting. #2 EGT is about what I would expect, but the others seem high indicating those cylinders are lean. Has anything been changed with the injector nozzels? It would seem unusual for three to be dirty and have similar EGT readings, but it couldn’t hurt to clean them. After cleaning you could do the baby bottle test.

RSA Troubleshooting.pdf

[Ragsf15e]

56 minutes ago, Tmooney said:

Thank you for this information!

You’re getting good advice here.  My F has ~18.1 gph and 2675 rpm during takeoff at 2000’ field elevation.  My egts are 1225ish at full power, full rich.  Something is amiss… the rpm is relatively easy to check with an optical handheld tach or even with a phone app.

[Shadrach]

1 hour ago, Tmooney said:

The EGTs on cylinder 1,3,4 are always around 1330 on take off.  Cylinder #2 is typically around 1250.  I have attached a graph below that is a typical take off at sea level.  

1330 is on the high side at full rich and 30". 1257 is OK, but likely could be richer. It could be an obstruction upstream of the injectors. When was the last time the fuel servo was serviced?  Baby Jar test that Skip mentioned is low risk investment.  At 30" of MP you are about 3-4gph short of spec. I am surprised that it is running as cool as it is.  Have you verified your FF reading is accurate?  If you do the baby jar test. that would be a good time to confirm.  There may  be a number of small issues contributing to the symptoms.

[Tmooney]

1 hour ago, PT20J said:

The plot is interesting. #2 EGT is about what I would expect, but the others seem high indicating those cylinders are lean. Has anything been changed with the injector nozzels? It would seem unusual for three to be dirty and have similar EGT readings, but it couldn’t hurt to clean them. After cleaning you could do the baby bottle test.

RSA Troubleshooting.pdf 783.56 kB · 0 downloads

In my short 7 months of ownership, the injector nozzles have not been touched.  Thank you for the attachment, I will start with cleaning the nozzles and performing the baby bottle test. 

[N201MKTurbo]

I think you need to adjust your governor. Your fuel flows are low because your RPMs are low not the other way around.

[Yetti]

I see 17 GPH on take off with my F and Dynon monitor.    I kept the manual tach.   

Get a RC tach or one of the apps for your phone.   

on the Dynon the Tach is not adjustable.   It pick up off the p leads that go to the key switch.

What is adjustable on Dynon EM is the fuel flow.   You set the pulses from the FF sensor in the set up.    I think mine is mostly right.

Once you verify the tach, it sounds like you might need to change the adjustment screw on the prop governor.    Which best I can see is one of those impossible task.

A baby flow test as mentioned and also when was the last time your fuel servo was rebuilt?

Not sure where you are located, but Texas Airplane Carburetor in Justin Texas did servo.   Works great.

 

 

[Yetti]

1 minute ago, N201MKTurbo said:

I think you need to adjust your governor. Your fuel flows are low because your RPMs are low not the other way around.

ha beat me by one minute.

[Tmooney]

23 minutes ago, Shadrach said:

1330 is on the high side at full rich and 30". 1257 is OK, but likely could be richer. It could be an obstruction upstream of the injectors. When was the last time the fuel servo was serviced?  Baby Jar test that Skip mentioned is low risk investment.  At 30" of MP you are about 3-4gph short of spec. I am surprised that it is running as cool as it is.  Have you verified your FF reading is accurate?  If you do the baby jar test. that would be a good time to confirm.  There may  be a number of small issues contributing to the symptoms.

After looking in the engine logbook, I see that in 2022 the fuel injection line for #2 was replaced with a new line and all of the fuel screens were cleaned.  In 2011 a new hose assembly from fuel servo to flow divider was installed.  The engine was overhauled in 2008.

I believe my fuel flow is accurate as the fuel totalizer is always within .3 gallons when I fill up the plane and I have flown a lot in the past 6 months or so. 

[Shadrach]

47 minutes ago, N201MKTurbo said:

I think you need to adjust your governor. Your fuel flows are low because your RPMs are low not the other way around.

100 rpm = 3.5gph at full rich?  I was thinking more like 1gph or less.

[PT20J]

It’s unusual to replace the hose from the servo to the flow divider or the lines to the injectors. The logbook entries probably don’t say why this was done since an explanation is not required but I wonder if someone was chasing this same issue.

[Shadrach]

56 minutes ago, Tmooney said:

After looking in the engine logbook, I see that in 2022 the fuel injection line for #2 was replaced with a new line and all of the fuel screens were cleaned.  In 2011 a new hose assembly from fuel servo to flow divider was installed.  The engine was overhauled in 2008.

I believe my fuel flow is accurate as the fuel totalizer is always within .3 gallons when I fill up the plane and I have flown a lot in the past 6 months or so. 

Well at least you know you’re getting good data. 

There’s not a lot to be done in the field. The Precision Airmotive trouble shooting chart points to injectors and the flow divider. Your Mx can ensure that the lines/injectors are clear and that the flow divider is not sticking, but most anything beyond that will require bench testing.

[PT20J]

If it is the flow divider, the good news is that it can be overhauled in the field.

[Ragsf15e]

These two issues likely seem connected to me.  One of the IAs (or Skip) will know, but is there Any chance the prop gov is governing much too early and not letting it get fully to the low pitch stop?  
 

Im kind of with @N201MKTurbo, might want to check the governor setup before messing with the fuel flow…

[MikeOH]

@PT20J

Regarding static run-up rpm of >2650.  Is that rpm independent of density altitude?  I.e., a universal check on engine power?  (Assuming the prop is correctly set-up and is on the low-pitch stops).

[Shadrach]

8 minutes ago, Ragsf15e said:

These two issues likely seem connected to me.  One of the IAs (or Skip) will know, but is there Any chance the prop gov is governing much too early and not letting it get fully to the low pitch stop?  
 

Im kind of with @N201MKTurbo, might want to check the governor setup before messing with the fuel flow…

A few things I would mention. If the engine isn’t getting to spec RPM, it’s very likely the governor that’s incorrectly set, not the prop stops, although I suppose that’s possible.

If this fuel flow issue was a matter of the engine governed to 2600rpm, why would anyone climb at redline? There’s very little penalty for reducing RPMs by 100. if I could expect to save 3-4gph for every hundred RPM reduction I’d never climb at max RPM again.
However, I just don’t think that’s the case. 
At full rich,  I don’t think the power Delta between 2600 and 2700 amounts to even 10hp. Seems unlikely that would account for 3.5gph.

In truth, I frequently reduce RPM to 2500rpm for noise abatement on climb out and have no concerns about keeping CHTs in check.  However, the OP is having a difficult time keeping things under 400 in climb at 2600.
 
 

 

[Tmooney]

I really appreciate all of the advice.  My A&P are going to look at it together on Monday.  If anyone else has any other suggestions, please let me know.

Ironically, the CHT on cylinder #2 is the one that runs the hottest and that is the cylinder with the lowest EGT on take off.

[Shadrach]

1 hour ago, Tmooney said:

I really appreciate all of the advice.  My A&P are going to look at it together on Monday.  If anyone else has any other suggestions, please let me know.

Ironically, the CHT on cylinder #2 is the one that runs the hottest and that is the cylinder with the lowest EGT on take off.

That is odd given that #3 is almost always the hottest in this application.

Which cylinder is leanest?

[PT20J]

2 hours ago, MikeOH said:

@PT20J

Regarding static run-up rpm of >2650.  Is that rpm independent of density altitude?  I.e., a universal check on engine power?  (Assuming the prop is correctly set-up and is on the low-pitch stops).

It should work at any density altitude. As the density altitude increases, the max power available from the engine decreases, but the drag on the prop also decreases, so it takes less power to turn the same rpm. 

[Tmooney]

14 hours ago, Shadrach said:

That is odd given that #3 is almost always the hottest in this application.

Which cylinder is leanest?

Cylinders #1, 3 & 4 are almost identical EGT and CHT on take off and throughout cruise. Cylinder #2 is the richest yet hottest on take off and in cruise which is very confusing. 

[EricJ]

18 minutes ago, Tmooney said:

Cylinders #1, 3 & 4 are almost identical EGT and CHT on take off and throughout cruise. Cylinder #2 is the richest yet hottest on take off and in cruise which is very confusing. 

The conditions of the baffles, baffle gaskets, cowl flaps (if you have them), etc., etc., affects airflow through the cowl and around the engine, so can make a big difference in which cylinders wind up being hot or cold.   Figuring this out is aggravated by the cylinders not all running exactly the same, and the engine monitor probes not all responding the same.   It boils down to not expecting too much accuracy from the whole thing, but keeping an eye out for deviations from norms and extremes.