LOP

[George Mohr]

You aren't really at the same % power at a constant MP and RPM if you change the mixture. See the chart posted above.
...

It should be simpler for a fixed pitch prop. Not sure its simple enough for me to figure out though!

 

[Tom-D]

It should be simpler for a fixed pitch prop. Not sure its simple enough for me to figure out though!

Any time the power or the resistance is changed, the governor will compensate to hold RPM. it will not compensate for airspeed, in fact airspeed will be sacrificed to hold RPM.

 

[Checkout_my_Six]

You aren't really at the same % power at a constant MP and RPM if you change the mixture. See the chart posted above.


If you are ROP(for purposes of this discussion lets call that +75* of peak EGT) then you can use the power charts in the POH and get a good approximation of % power. When lop(and normally aspirated), multiply fuel flow by 14.9 to get HP and divide that by rated to get % power. For example the POH on the 310 calls 2500 RPM and 23" MAP at 5000 ft on a standard day 70.9% power or approximately 203 HP per side. It calls for a fuel flow of around 30 GPH to do this. Empirically I know this is going to put me around 75* ROP and CHT's will be...high. However, I can push the throttles up, pull mixture back to 26.2 GPH total and get the same 203 HP per side and have CHT's that will be 360 or below. IIRC, this would put me at about 20-30 LOP assuming I'm WOT.

In other words, I could set my MP and RPM per the POH and voila I'm at 75% power. However, because I've got the plane dialed in I can pull the mixture back until the engines are 100* LOP. One will give me 195 KTAS and the other will give me 165 KTAS. What is my % power? Go by the POH when ROP and multiply by fuel flow when LOP. Now, what happens when I bring the throttles back a bit and leave everything else alone?

this is a three variable input problem........throttle (air), prop (rpm), .....and mixture (fuel). Change any one....the HP changes. The easiest way to see this is to hold altitude constant and make airspeed adjustments using the mixture. Hence the recommendation to lose 3-4 kts by leaning which is roughly 30 degrees lean.

 

[FastEddieB]

When I fly the 310 the throttles only come back from WOT when landing.

Deakin et al have compared partial-throttle cruise with operating with a dirty air filter - why would you ever voluntarily want to obstruct the flow of air to your engine at cruise, which is exactly wat a partially closed throttle plate does.

In my Cirrus, I was not quite as radical as James, but "WOTLOP*" at cruise was my SOP most of the time at cruise.

Here's an article by Gordon Feingold that through repeated readings helped me understand LOP operations in the Cirrus: http://www.sysdyn.com/xyz/LOPops.pdf


*Wide Open Throttle Lean Of Peak

 

[coloradobluesky]

You need balanced fuel injection and CHT's and EGT's on every cylinder to do it. Fuel flow gauge really helps too. And it is most useful on turbocharged engines. Doesnt work at all with single EGT or carbureted engines (they have too much uneven flow).

How do you tell if you are lean of peak? Answer is, if you lean it and it gets cooler, you are lean of peak. And of course, if you lean it and it gets hotter, you are rich of peak.

 

[azure]

You need balanced fuel injection and CHT's and EGT's on every cylinder to do it. Fuel flow gauge really helps too. And it is most useful on turbocharged engines. Doesnt work at all with single EGT or carbureted engines (they have too much uneven flow).

How do you tell if you are lean of peak? Answer is, if you lean it and it gets cooler, you are lean of peak. And of course, if you lean it and it gets richer, you are rich of peak.

Umm... if you lean it and it gets richer, your mixture control needs a looking-at.

 

[flyingron]

The problem with that chart is SFC is not really a useful curve. SFC is HP/Fuel Burned. What you really want likely is airpseed/fuel burned. HP goes up with the cube of the airspeed in the simple case.

 

[coloradobluesky]

Well I did notice that slip up. I changed it. But not before you immortalized it.

Point is, pilots wonder if they are rich or lean of peak. Lean it and find out by watching the temps. There IS a scientific way to find out if you have balanced injectors and EGT's on every cylinder.

 

[Tom-D]

Well I did notice that slip up. I changed it. But not before you immortalized it.

Point is, pilots wonder if they are rich or lean of peak. Lean it and find out by watching the temps. There IS a scientific way to find out if you have balanced injectors and EGT's on every cylinder.

Pretty much ,,, simply read your spark plugs.