Right! Some setups will actually stumble and quit when you go to full throttle at very high DA with full rich. That usually get’s a person attention pretty quick!
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Jay Scheevel
Sent: Friday, August 28, 2020 6:20 PM
Subject: Re: [Q-List] Leaning for High DA
Pretty much did the same as you for my previous plane with the O-540, which had a constant speed prop. Still had the same "feel" approach you talk about. My DA's run between 4000 and 10000 at take off depending on the season. Bottom line: no reason to be full rich at high DA.
My Jabiru is a little different (like me), but I follow the same philosophy, even though the procedure differs slightly and will be of little interest to the group here.
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With the Marvel carbureted Lycoming’s with which I am familiar (O-320, O-360), performing the runup at 1700-1800 and leaning at that rpm is an excellent takeoff setting (due to the
“power enrichment circuit” that provides an extra-rich mixture at wide-open throttle). I can imagine other engines/carbs/fuel systems behave differently.
These days, instrumentation is so good and affordable, it is silly (financially – engines are expensive) not to have EGT and CHT on all cylinders. With four cylinder EGT, it is very easy to make adjustments during takeoff to achieve the magical 125 degree rich of peak (best power). After a handful of takeoff’s you will know what works well for your airplane. In my case – very specific to my airplane – I lean for max rpm while running up at 1700-1800. When I start the departure roll, I push the mixture slightly richer (just a nudge) and glance at the EIS a couple times to confirm an EGT of around 1250 degrees (I happen to know it’s peak EGT is around 1350 and that varies with the conditions).
Finally, you can FEEL when have the mixture about right during the takeoff roll – the difference in engine power is noticeable. Yes, making FINE adjustments during the takeoff roll. No, I would not advise that until familiar/comfortable with/in your airplane but once you are, such adjustments are completely legit. Obviously not making large/rapid adjustments. There are many times in the backcountry that we do not perform a “full runup” due to possible prop damage (dirt/stones). In that case, setting mixture during the roll is the only option.
Most of my flying at high-density altitudes has been in a turbo. When not in a turbo, I have been in a typical tricycle gear plane where I could do a full-power runup and lean for max power prior to takeoff. In our Q200's, we don't really do full power runups for risk of the tail coming up and risking a prop strike. So what is the best method to know you are leaning for max power at higher altitudes? It wouldn't always be feasible to tie down the tail and I am not comfortable leaning during the roll to find that answer out "live".