Bob_Belville

Mountain flying in a M20E

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3 minutes ago, midlifeflyer said:

That's a great method for hitting your target EGT for leaning even at altitude (whether LOP or ROP). But the typical  leaning for high D-Alt takeoff in a normally aspirated airplane involves something richer than best power to allow for better cooling. Basically, neither our engines nor airfoils (including prop) are as powerful/efficient at higher density altitudes so they work harder - think of an extended climb at cruise power  with your wings providing less life and your prop providing less thrust.

It, as all ops anywhere anytime, require your plane to be cooling correctly, have the baffling correct, timing correct, etc. The CHT's and EGTs take this lack of air into account at high DA's. If richer ops are needed, it might mean you need to review your cowl flap settings, baffling, etc. 

Climb VY until out of the kill zone, then at 120 for better cooling, but Bob knows this.

 

 

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So, we just returned from our trip to Truckee-Tahoe yesterday evening.  While on this trip, I wasn't keeping up with the thread and am just now seeing a question about our high altitude leaning.  I lean to max RPM during run-up, then dial in (richer) two turns.... but that's just me, for starters.  I watch my JPI and richen as needed from there, but have found that seems about right for us.  Maybe 2 turns is more than needed, but I'd rather run a bit rich (here) than be too lean during take off.  I do aggressively lean during taxi and in cruise.

As far as this trip goes, we took off from Truckee-Tahoe at about 1pm yesterday and the DA was a bit over 7000' - which is the actual physical elevation of our home field.  We requested a southeast departure and, after take off, tower asked if we wanted to climb over the airport to clear the ridges.  I appreciate that they offered, but with the two of us and full fuel and about 85 pounds in baggage it wasn't needed.

 

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1 hour ago, Ross Taylor said:

So, we just returned from our trip to Truckee-Tahoe yesterday evening.  While on this trip, I wasn't keeping up with the thread and am just now seeing a question about our high altitude leaning.  I lean to max RPM during run-up, then dial in (richer) two turns.... but that's just me, for starters. 

 

No, it's not just you. Although the number of twists can vary slightly with the make/model (you learn what works for your airplane), this is the way leaning for takeoff is taught for normally-aspirated engines by instructors in Colorado (at least in 20 years of flying,  and teaching there I haven't come across one who didn't teach it this way).  

FWIW, this is a copy/paste of a write-up I did years ago.

Leaning for takeoff at high density altitude (normally aspirated).

If you are operating for the first time in a high density altitude environment, local instruction is always your best bet because it's not just about the effect on power. That said, this is the method generally taught at flight schools in Colorado.

Target leaning for max power is done at runup power. This is for two reasons other than it just works for all but a very few make/models.  One is that leaning at full power can be hard on the brakes of higher performance engines; the brakes might not even hold in some. The other is that, a constant speed prop will act like a fixed pitch one when set at runup rpm so you an easily see the changes on the tach..

When you are ready to do the run-up:

1. Enrichen the mixture (you should have leaned it for taxi, so you need to enrichen it for the run-up power demand). You don't really need to go back to full rich at this point, but there's no harm in doing so until you learn about where to set it.

2. Go to run-up power.

3. Lean. You will initially see a rise in RPM as you reach best power and then a drop. When you see the drop, enrichen back to peak. 

4. Enrichen more. On airplanes with a vernier mixture control, 3-4 twists will do it. Without a vernier, about 1/4-1/2" tends to take care of it. The enrichment is for 2 reasons:  approximate the additional power  requirements for takeoff and for engine cooling.

In most airplanes, this will be exactly what you need for takeoff. Many instructors I know stop here but bear in mind that this is an *approximation* which needs to be cross-checked, at least until you have learned  the run-up technique works for your airplane.

The cross-check should be done at full power. It does not take long a few seconds at most. If the brakes will hold, you can do it while still at the runup area, or you can wait until the runway just before beginning the takeoff roll. I usually do that final check on the takeoff roll. The exact check varies. It's often simply getting expected rpm but you might have a table of fuel flow targets for takeoff to measure against. If you know your target best power EGT, great. Personally I have never had to move the mixture more than a 1/2 twist or its equivalent. 

Finally, remember that this isn't brain surgery. Look at your POH - that instruction to lean "above 3000' is a pretty good indication of how much leeway is built in.
 

 

 

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The above is probably the best you can lean a NA aircraft for takeoff without an engine monitor, but with an engine monitor we can do far better about guessing how much more to enrich or how lean to keep it using the target EGT method described in the referenced presentation below. Keep in mind a great many aircraft are already on the lean side at takeoff at sea level, with corresponding high temps in initial climb.  So if you're possibly one of these consider leaning to what is your full rich EGTs at 1000' msl or maybe a bit higher rather than sea level; especially if you don't have low obstacles to be concerned with. But regardless, if you know how to lean by max rpm, tune your leaning using the Target EGT in the  take off run and after you have cleared low obstacles to ensure max performance without the guess work. If your just learning how to lean for high DA, you may prefer to give yourself some additional time to adjust by holding the brakes on the runway. See the Target EGT presentation here https://www.advancedpilot.com/tech.html 

With the vast majority of trainer rentals having engine monitors these days having one and knowing how to use it should be more widespread. But what I see in my area may not be representative of the entire country. 

Edited by kortopates
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