LOP and altitude

[PaulB]

I've been struggling with figuring out LOP operations recently.  My question pertains to LOP and fuel flow setting with respect to altitude.  I understand that the APS guys state LOP FF = %HP / 14.9 (for the IO-360) but doesn't this change with altitude?  I would think that as we ascend the air gets thinner and the corresponding fuel flow should be lower to remain the same number of degrees LOP.  I wonder this because sometimes I like to putt around at around 3k and I'm never sure where to set the FF.  When I'm going somewhere I am usually 7-10k and use the above formula which puts me around 8.7 gph for 65% power.  Am I missing something here?

[clh]

I am sure someone who can explain it better will come around soon, but basically. the FF is proportional to the HP output IF it is all burned...  So if you burn a pound of avgas, you will produce so much energy.   energy/time = HP.  

 

It takes so many lbs of air to burn so much fuel regardless of the altitude.  Because our normally aspirated engines can only suck in a finite volume of air and at altitude air becomes thinner (fewer cubic feet/lb), you lean the engine to restrict the amount of excess fuel.  (excess fuel will result in the fuel going unburned out the exhaust.  

 

So at the end of the day, if you are feeding  8.7 gph at 1000 ft, you are running at 65% of 200hp,  or 135hp.  If you are running at 10000 ft and BURNING 8.7 gph, you are producing the same 135 hp.   The key here is burning.  If you are running rich or lean, you will be producing less power due either dumping fuel (ROP) or heating Air (LOP).   

 

Now we either run rich or lean of peak depending on your belief as to which is better for your engine..  Neither are from a combustion efficiency perspective optimum.

 

So i believe that if you are a setting the engine to a specific fuel flow and leaning to the same point rich or lean of peak, you will be producing the same HP if wherever you are.

 

Does this help?

[carusoam]

LOP means....

You are using excess air.

Fuel is being completely burned.

If you are decreasing altitude, you will go more LOP...

If you increase altitude, you will go richer towards peak....

Solution...

Level off, accelerate, set mixture LOP.

If you change altitude significantly, re-lean.

Know that there is a lot of air down low, going LOP comes with a lot more power there...

The 65% hp safety limit goes out the window, until much higher up...

How's that sound?

-a-

[PaulB]

Ok starting to make sense.  I was thinking for some reason that the formula was calculating the ideal FF to make your perfectly 25 degrees LOP.  Just a brain fart.  Thanks guys.  I think I should take the APS class as I really am curious about this stuff and figure it would iron out the details better for me.

[carusoam]

APS is the source of truth and logic...

We have their Australian guy check in here every once in a while...

Best regards,

-a-

[Bob - S50]

I agree with what has been said but I look at it this way.  Regardless of the altitude, if you are LOP, then HP is directly related to fuel burn.  8.7 GPH LOP is 65% regardless of altitude.

 

Level off, set your throttle (full forward at higher cruise altitudes) and RPM then lean to LOP.  If the FF is below 8.7 you are getting less than 65%.  You can either increase throttle (if available) or RPM (if available) then lean again.  An increase of 100 RPM but the same number of degrees LOP should make a difference of about 4% power or about 0.4 GPH.

 

If you set your power and lean but you find the FF higher than the target you want (8.7 for 65%) you can either lean more or reduce MP/RPM and lean again.

 

Just yesterday I found I could get 65% at 2400 RPM at 9500' but at 10500' on the way home I needed to use 2500 RPM.

 

Have fun,

 

Bob

[PaulB]

I agree with what has been said but I look at it this way.  Regardless of the altitude, if you are LOP, then HP is directly related to fuel burn.  8.7 GPH LOP is 65% regardless of altitude.

 

Level off, set your throttle (full forward at higher cruise altitudes) and RPM then lean to LOP.  If the FF is below 8.7 you are getting less than 65%.  You can either increase throttle (if available) or RPM (if available) then lean again.  An increase of 100 RPM but the same number of degrees LOP should make a difference of about 4% power or about 0.4 GPH.

 

If you set your power and lean but you find the FF higher than the target you want (8.7 for 65%) you can either lean more or reduce MP/RPM and lean again.

 

Just yesterday I found I could get 65% at 2400 RPM at 9500' but at 10500' on the way home I needed to use 2500 RPM.

 

Have fun,

 

Bob

It's starting to make more sense to me.  Another question, do you ever actually check to see how lean of peak you actually are?  I've read John Deakin suggests just pulling back until you feel the plane decelerate.  If you do do you use the Lean find function of your engine monitor or just do it manually by watching the EGTs?

[Bob - S50]

It's starting to make more sense to me.  Another question, do you ever actually check to see how lean of peak you actually are?  I've read John Deakin suggests just pulling back until you feel the plane decelerate.  If you do do you use the Lean find function of your engine monitor or just do it manually by watching the EGTs?

For me personally it depends.  If I'm flying a relatively long leg I use the LOP function.  However, after I think I have it set I was maybe 15 seconds because invariably it changes.  I try to get all cylinders at least 10F LOP.  To do that, my leanest cylinder is usually about 40 LOP.  However, if it is running a bit rough I'll richen just a bit.

 

If I'm just flying a local short flight or only leveling off temporarily, I just watch one of the EGT's.  I'll watch it rise and when I see it start to fall I'll stop.  Close enough for such a short period of time.

 

Of course, in all cases I watch the CHT's and if they start to get close to 400F I'll lean a bit more or reduce power and lean again.

 

Bob