Most efficient decent profile?

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docmirror

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I'm wondering what would be the best settings for a long descent from cruise. I'll postulate that the speed should be best glide or just above. I'm wondering if at 14,500' and the runway is at 500'. Assume we want the engine to keep running and we have a CS prop.

Right now, I just tip the trim down a bit and coarsen the prop a bit to go down around 400FPM and maintain the mixture and throttle in the same place. This gives more speed in the decent but not the best economy.

Any ideas?
 
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If I'm not in a hurry I'll pull one inch per minute for 5 minutes. After the 5th pull I'll be descending at cruise speed and 500fpm. Then I just monitor the manifold pressure on the way down and keep it constant. I also slowly enrichen to keep the egts the same as they were in cruise.

If I'm in a hurry I just trim down to attain 500fpm, again monitoring the manifold pressure on the way down and keeping it constant and slowly enrichening to keep the egts the same as they were in cruise. This technique is for smooth air only and requires a close eye on the airspeed.
 
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The efficient descent is conducted at the best range speed, computed per Byington's method. Since max range is the same at all altitudes, it doesn't matter how far out you start down, as long as you do so at best range speed for that altitude.
 
docmirror said:
I'm wondering what would be the best settings for a long descent from cruise. I'll postulate that the speed should be best glide or just above. I'm wondering if at 14,500' and the runway is at 500'. Assume we want the engine to keep running and we have a CS prop.

Right now, I just tip the trim down a bit and coarsen the prop a bit to go down around 400FPM and maintain the mixture and throttle in the same place. This gives more speed in the decent but not the best economy.

Any ideas?
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The red line - It's not a limit, it's a goal. ;) :rofl:

Seriously, usually I just give the trim a swipe and leave power the same. If I'm not gonna speed up in the descent, I'll just pull to 20" MP.

It's not clear if you're looking for the descent that burns the least gas per mile, or per hour, or whether you want the MOST efficient possible, or the best balance between fuel burn and speed?
 
Well, I know the speed to go, I just don't know the most efficient for the engine and prop. One idea has me staying high as long as possible, then bringing the power back to idle, prop to coarse(min drag), and set up best glide(or range as Ron says). That will burn very little fuel but the decent in FPM would be fairly high. Alternately, I could start down much earlier and do a gradual decent with coarse prop power at same speed.

Part of me thinks the parabolic(slow climb, slow decent) from cruise is the best, but OTOH, with the throttle at idle, and the prop full forward I'm staying higher longer, and coming down steeper to my runway. Hmmmmm.
 
Ron Levy said:
Since max range is the same at all altitudes, it doesn't matter how far out you start down, as long as you do so at best range speed for that altitude.
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Assuming no wind changes with altitude.
 
docmirror said:
Well, I know the speed to go, I just don't know the most efficient for the engine and prop. One idea has me staying high as long as possible, then bringing the power back to idle, prop to coarse(min drag), and set up best glide(or range as Ron says). That will burn very little fuel but the decent in FPM would be fairly high. Alternately, I could start down much earlier and do a gradual decent with coarse prop power at same speed.

Part of me thinks the parabolic(slow climb, slow decent) from cruise is the best, but OTOH, with the throttle at idle, and the prop full forward I'm staying higher longer, and coming down steeper to my runway. Hmmmmm.
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Ignoring any wind issues, the most efficient climb would probably use an engine configuration that minimizes BFSC (probably around 65% power and 20-30F LOP) and RPM set for cruising. Then climb at whatever rate you get with best range speed. For the descent I suspect the most efficient method would be to shut the engine down, especially if you could feather the prop and descend at best range airspeed. OTOH, if you'd prefer to keep the engine running, a shallow descent that allows a reasonable amount of power (still LOP) at best range airspeed would seem most efficient.
 
docmirror said:
Part of me thinks the parabolic(slow climb, slow decent) from cruise is the best, but OTOH, with the throttle at idle, and the prop full forward I'm staying higher longer, and coming down steeper to my runway. Hmmmmm.
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Why would you want the prop full forward? All that does is add drag and increase engine wear. :dunno:

I never push the prop forward from the cruise setting until my final power reduction, when I'm coming out of the governing range. The only time the prop is at full RPM is on the takeoff roll and initial climb.
 
I like to take advantage of descent speed to make up for time lost in the climb. I leave cruise power/prop in and trim for 500fpm on the descent. This give speed at the very top of the green arc, and in a no wind situation gives me 185kts +/- 5 over the ground. I leave full throttle in until I pass thru 5500 or so, then pull back to 25", then pull back 1" per thousand as I descend. I usually have to start to enrichen below 5000 as well. Mixture goes full rich after I get below 3000. 15" and prop forward on downwind.
 
Bill Jennings said:
I like to take advantage of descent speed to make up for time lost in the climb. I leave cruise power/prop in and trim for 500fpm on the descent. This give speed at the very top of the green arc, and in a no wind situation gives me 185kts +/- 5 over the ground. I leave full throttle in until I pass thru 5500 or so, then pull back to 25", then pull back 1" per thousand as I descend. I usually have to start to enrichen below 5000 as well. Mixture goes full rich after I get below 3000. 15" and prop forward on downwind.
Click to expand...

That's faster but certainly not the most efficient operation (and pretty much what I do when I'm in any kind of a hurry).
 
Ron Levy said:
The efficient descent is conducted at the best range speed, computed per Byington's method. Since max range is the same at all altitudes, it doesn't matter how far out you start down, as long as you do so at best range speed for that altitude.
Click to expand...

Ouch. 1 out of 5. I need to read the article. Thanks for the post, Ron!
 
flyingcheesehead said:
Why would you want the prop full forward? All that does is add drag and increase engine wear. :dunno:

I never push the prop forward from the cruise setting until my final power reduction, when I'm coming out of the governing range. The only time the prop is at full RPM is on the takeoff roll and initial climb.
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High or coarse pitch. I have an elec prop, so to me forward is a large bite of air, not flat pitched. Sorry.

What I was trying to find out is if a short but steep decent at idle is more efficient than a longer shallower decent. Still don't know......
 
docmirror said:
High or coarse pitch. I have an elec prop, so to me forward is a large bite of air, not flat pitched. Sorry.
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Aha! So for you, "full forward" is the opposite of what it is for the rest of us. :yes: Interesting. And I'm guessing you control prop pitch directly, not RPM? Or is it an electric constant speed prop?

What I was trying to find out is if a short but steep decent at idle is more efficient than a longer shallower decent. Still don't know......
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Depends. What speed are you using in cruise? I think if you're using the constant AoA, maximum range profile, it probably doesn't matter as long as you keep that AoA in both cruise and descent. I'm guessing, however, that you probably do like the rest of us and cruise a bit faster at the cost of a bit more fuel, in which case a longer, shallower, slower descent is probably better.

(Speculating, for the sake of discussion. But my speculations are usually correct! :D :yes:)
 
Bill Jennings said:
I like to take advantage of descent speed to make up for time lost in the climb. I leave cruise power/prop in and trim for 500fpm on the descent. This give speed at the very top of the green arc, and in a no wind situation gives me 185kts +/- 5 over the ground. I leave full throttle in until I pass thru 5500 or so, then pull back to 25", then pull back 1" per thousand as I descend. I usually have to start to enrichen below 5000 as well. Mixture goes full rich after I get below 3000. 15" and prop forward on downwind.
Click to expand...

It must be a Mooney thing, as I fly the M20F pretty much the same way. Although in the Archer I flew it similarly, I just had to pull the throttle back to prevent from overspeeding.

Main difference is I don't do full rich below 3000, I do full rich on late downwind when I push the prop full forward. I modulate the mixture knob to keep EGTs about the same as in cruise.
 
docmirror said:
Well, I know the speed to go, I just don't know the most efficient for the engine and prop. One idea has me staying high as long as possible, then bringing the power back to idle, prop to coarse(min drag), and set up best glide(or range as Ron says). That will burn very little fuel but the decent in FPM would be fairly high. Alternately, I could start down much earlier and do a gradual decent with coarse prop power at same speed.
Click to expand...
Byington's work tells us that you'll see the same burn either way, assuming you fly at your best range speed for each altitude as you pass it. The only question is what prop/throttle combination to use to get max fuel efficiency from the engine at the particular power percentage you choose, and that will come from the curves in the engine manual.
 
Ron Levy said:
Of course. But if you start trying to figure in wind, you need a much more complicated model to compute the right answer. I think Doc's just looking for a general rule of thumb.
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Agreed. But IME the wind often has more impact than the "optimal" no wind profile. Any general rule of thumb that ignores the wind is likely to be ineffective.
 
Thanks Ron, I looked at the link and sure enough, it's a wash. So, I just operate the engine at best BSFC and trim for the appropriate speed at weight and altitude.
 
Mixture settings make a big difference.

If you want the most efficient descent profile, you'll have to leave the mixture leaned until touchdown. Going full rich is not only completely unnecessary and, if anything, harmful to your engine, it will also reduce efficiency.

If descending from a ROP cruise, I normalize the EGT display and adjust the mixture to keep EGTs constant. If LOP, I don't need to do anything. Saves work and fuel. That was with a NA engine. With the turbo'd 520s, I don't touch the mixture or throttle until I need to slow to 170 or so indicated to put the flaps down.

Felix
 
I use VDP and ground speed X's 5 for descent rate. You can use any power setting you like. It doesn't always work flying IFR if you get slam dunked.
 
i usually just wait for the 396 to tell me to start down, leave cruise power set, and set up at 500 fpm. where we fly IFR, its usually not a problem. only time you get hosed is coming over the edge of chicago approach, they have to keep you above 10K. then you get dunked if you are going into NW Indiana anywhere.
 
If you keep speed up, your efficiency will remain about the same. You would do better to slightly reduce speed and adjust pitch for cruise speed during a gradual descent.

In playing with a spreadsheet for a Skyhawk, it came out to a headwind being nearly half of TAS in order for a range of power settings to be close to equal in total fuel burn.
 
By the way, doc,you should keep your pants on. No mile high club.

Otherwise it's indecent.
 
2 mile high club.....

Riiiiiiiiiight.... :)
 
tonycondon said:
i usually just wait for the 396 to tell me to start down, leave cruise power set, and set up at 500 fpm. where we fly IFR, its usually not a problem. only time you get hosed is coming over the edge of chicago approach, they have to keep you above 10K. then you get dunked if you are going into NW Indiana anywhere.
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I think the problem vs. not depends a lot on where you are. Here in the east, the slam dunk approaches are pretty common, and I've gotten the words "expedite" and "no delay" a number of times on both climb and descent. Of course on climb there's not a lot I can do in the planes I fly other than leave the knobs firewalled and pedal as fast as I can, but on descent I can hurt some ears, and have.

Once my instructor got a "climb expedite" order in the Navajo, so he demonstrated a 2000 fpm climb to me. That was cool. No comments from you jet jockeys who see that every day. ;)
 
Ted DuPuis said:
Once my instructor got a "climb expedite" order in the Navajo, so he demonstrated a 2000 fpm climb to me. That was cool. No comments from you jet jockeys who see that every day. ;)
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Bah - I can come close to that in the 182 this time of year! I've gotten 1900fpm sustained climbs, but the top peg on the VSI still eludes me...
 
flyingcheesehead said:
Bah - I can come close to that in the 182 this time of year! I've gotten 1900fpm sustained climbs, but the top peg on the VSI still eludes me...
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Yeah, but the Navajo was doing it at cruise power. ;)

To be fair, We were cruising along at about 170-180 kts, got told climb to whatever expedite (it was 2000 ft above our present altitude) and he said to me (knowing I would enjoy this) "Watch this, I'm going to hurt your ears." A smooth pull back on the yoke and watch the 2000 fpm climb. Speed dropped to 120 kts, but sure enough, we were up that 2000 ft in a minute. Expedite enough? :D

To the point of climbing or descending, though - if ATC allows me to do as I please, it's generally about 500-1000 fpm climbs (whatever I get when going 120 mph IAS, which keeps the cylinders happy) and 500 fpm descents. The faster your forward speed is during climb and descent the faster you get where you're going. There are, of course, certain exceptions, but it seems to work for me and also keep the cylinders happy.
 
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