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High altitude airport Ops

 
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Coop85



Joined: 11 Aug 2020
Posts: 15

PostPosted: Mon Jan 24, 2022 8:50 am    Post subject: High altitude airport Ops Reply with quote

I just relocated to KAPA, Centennial in Denver, CO. I learned about high altitude takeoffs long ago, but was wondering if anyone has specific techniques for the RV-10 with an IO-540?

Thanks,
Marcus


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Bob Turner



Joined: 03 Jan 2009
Posts: 869
Location: Castro Valley, CA

PostPosted: Tue Jan 25, 2022 8:40 am    Post subject: Re: High altitude airport Ops Reply with quote

The usual techniques. Above 5000’ DA I lean for takeoff, but richer than best power. With no cowl flaps my 10’s CHTs will go over 420 F unless some extra fuel is used to keep them down. Flaps in trail.

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PostPosted: Tue Jan 25, 2022 8:53 am    Post subject: High altitude airport Ops Reply with quote

I fly my RV-10 routinely between KLHM in the Sacramento valley to South Lake Tahoe KTVL (elev. 6269, 8000 ft runway but situated in a shallow box canyon) and Truckee KTRK (elev 5904, two runways each a tad less than 6000 ft.).  In the summer the density altitude of both airports routinely is in the 8500-10000 ft range.  Getting there from the valley in the mornings is usually pretty easy except when winds across the ridges surrounding the Tahoe basin are over 30kts.  The trick is getting back home in the afternoons.  Here is the recipe I have found that so far has worked safely.  It is benefited by having a GRT EFIS that shows percent max power.
1. In flight planning, pay careful attention to the combination of winds and density altitude.  Even for a lightly loaded RV-10, KTVL becomes a one way in, one way out airport (land to the south, take off to the north toward the lake).  Prevailing winds are from the southwest in the afternoons, meaning there will be a quartering tailwind on departure.  The FAA guidance is to expect that each knot of tailwind component lengthens the takeoff run by 3-5 percent, so the multiplier effect of high density altitude plus tailwind gets very large very quickly, even for an 8000 ft runway.  The common rule of thumb is to abort the takeoff if one doesn't have 80 percent of rotation speed at 50 percent of runway length, and most every mountain airport has a midfield marker to use.  But if you hit that decision point it gets pretty scary because your groundspeed is quite a bit higher than your indicated airspeed by then.
2. If the planning numbers give you a safety margin that works for you, then optimizing takeoff power goes like this for me:
  
  A. First, you will have had to lean the mixture to even get the IO-540 to start and run smoothly, and taxi to the runway at high density altitudes.  At the runup area, do a wide-open-throttle runup with the mixture unchanged from the leaning you did for startup and taxi.  If the max percent power is below about 70 percent, then adjust mixture to get max EGT prior to initiating takeoff.
 B. On takeoff roll, once you get airborne remember that best rate (Vy) will be a higher IAS than at sea level, and you will have to drop the nose below your normal pitch angle or you will most likely see your CHTs hit your alarm limit (mine is set at 400F).  Don't attempt to climb at Vx (best angle) if you can avoid that since it will quickly drive your CHTs up due to the design of the -10s cowling.  In practical terms, sea level best rate in my -10 is 90 KIAS, and I usually have to climb out at about 110-115 KIAS to get an acceptable combination of climb rate + CHTs at high DA.
 C. Once you are airborne and running at max power, you can drop your CHTs by slowly enriching the mixture,  The IO-540 seems to tolerate this well.  I have not experienced a stumble caused by going richer as long as you are still running at 2600 rpm or higher, and it is very effective at cooling the cylinders down.  Have never needed to go full rich to get the cooling effect, so I don't know where the limit to that strategy is, but it definitely helps in my experience in the -10.  Upon leveling off for cruise, just do what you normally would with respect to finding optimal mixture.
3. Give yourself some performance margins.  Whenever I can, I treat the -10 as at most a 2.5 place aircraft for high DA ops, and don't fill the tanks to more than you need with usual reserves.  There is no benefit to topping it off in most high DA situations.
This is the ultimate YMMV situation, and others may have a different recipe, but with 1240 PIC hours in the -10 this has worked well for me.
-Dan Masys
N104LD
Time: 08:50:47 AM PST US
From: Marcus Cooper <cooprv7(at)yahoo.com (cooprv7(at)yahoo.com)>
Subject: RV10-List: High altitude airport Ops

I just relocated to KAPA, Centennial in Denver, CO. I learned about high altitude
takeoffs long ago, but was wondering if anyone has specific techniques for
the RV-10 with an IO-540?

Thanks,
Marcus


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Bob Turner



Joined: 03 Jan 2009
Posts: 869
Location: Castro Valley, CA

PostPosted: Tue Jan 25, 2022 1:35 pm    Post subject: Re: High altitude airport Ops Reply with quote

Dan, I respectfully disagree with some of your post.
First, be very careful interpreting the GRT power reading. It just works off a look up table that you entered, and, in particular, does not know the actual mixture ratio wrt peak egt.
Second, lean to best power, not peak egt, if there is any question about having sufficient power for takeoff. Best power is about 125 F rich of peak EGT. And with a long runway, go even richer, to help keep CHTs down.
As you note, caution is needed with downwind takeoffs. I won’t accept more than a 5 knot tailwind component. And while some airports really are one-way, KTVL isn’t one of them. It’s a bit tight, but a -10 can takeoff to the south and execute a left downwind departure. And if it won’t climb fast enough, there is a golf course a bit further south over which you can circle.
Vy (in IAS) decreases, not increases, as density altitude goes up. But you do need to watch CHTs.
I recommend flaps in trail for these takeoffs.


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PostPosted: Wed Jan 26, 2022 6:39 am    Post subject: High altitude airport Ops Reply with quote

Bob's right.  Although peak EGT is stoichiometric fuel+air and is the most complete combustion, best power occurs about 100-125 rich of peak.  Thanks for reminding me so there is more power available for the next Tahoe run. 
And looking it up Vy does decrease with altitude while Vx increases.
So you can pretty much ignore my entire post, other than somehow I have been able to survive the mountains so far in my RV-10.  : )
-Dan Masys


Time: 01:36:06 PM PST US
Subject: Re: High altitude airport Ops
From: "Bob Turner" <bobturner(at)alum.rpi.edu (bobturner(at)alum.rpi.edu)>

Dan, I respectfully disagree with some of your post.
First, be very careful interpreting the GRT power reading. It just works off a
look up table that you entered, and, in particular, does not know the actual mixture ratio wrt peak egt.
Second, lean to best power, not peak egt, if there is any question about having
sufficient power for takeoff. Best power is about 125 F rich of peak EGT. And
with a long runway, go even richer, to help keep CHTs down.
As you note, caution is needed with downwind takeoffs. I wont accept more than
a 5 knot tailwind component. And while some airports really are one-way, KTVL
isnt one of them. Its a bit tight, but a -10 can takeoff to the south and execute
a left downwind departure. And if it wont climb fast enough, there is a golf
course a bit further south over which you can circle. 
Vy (in IAS) decreases, not increases, as density altitude goes up. But you do need to watch CHTs.
I recommend flaps in trail for these takeoffs.

--------
Bob Turner
RV-10 QB


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