Innefficient Descent (going fast)

[201er]

Just now, bluehighwayflyer said:

Agreed.  Another consideration, though, is that rather than focusing entirely on phase of flight you might want to instead spend your extra $4 at the point in your flight profile in which you encounter either the greatest headwind or the least tailwind in order to minimize the amount of time you spend in this space. In other words, although certainly a factor, I think that the impact of winds aloft will exceed that of parasitic drag.  Along these same lines, in the absence of a big tailwind at altitude, it is darn hard to make up for a high power ROP climb.  

Can't get to wind until there's an understanding of how it works no wind. But for a guy like you who appreciates a slow and steady efficient cruise (without the $72 piss stop), I think (if I'm right and I'm still trying to figure out if I am or not), then this discussion could be quite enlightening.

What I am coming up with is that if you generally fly slow/efficient, then the high speed descent is disproportionately wasteful compared to the efficient cruise profile. Here are some numbers I came up with going back to the initial example of a 500nm distance starting from the top of climb.

If you use my 6gph descent profile from 10,000ft which will cross 77nm, that leaves 423nm for the enroute portion. With the demonstrated 1g of gas saving by using this descent profile, what if instead of pocketing the $5 ($4 when you factor in cost of additional tach time) you decided to try to spend it most effectively on the same flight. We already know that if you do the high speed descent, you will save 5 minutes at the cost of 1g of gas.

If we take the economy descent profile for granted and spend that extra gallon entirely during the cruise, I estimate that we can make 148ktas@8.8gph. That will take 2.858 hours instead of the 3.021 spent at 140ktas@8.0gph. That saves 9.8 minutes compared to 140@8 cruise with economy descent.

 

Spending the same $5 (1g) - that most guys would blow in a high speed descent so they could piss 5 minutes sooner - during cruise instead ends up being a 9.8 minute savings. That gallon of gas saved in the descent does you better spent in a faster cruise than in a faster descent. For the same cost you arrive 4.8 minutes quicker (9.8 - 5) than the guy who thought he was getting there sooner by diving it. At least with this kind of distance/altitude no wind combination. Gotta think about how it factors into shorter, longer, and lower flights.

[Bartman]

Interesting discussion and obviously a lot of thought and calculations put into this so Bravo to you guys (pun intended).  From a practical standpoint I'm in the Appalachian mountains and even though I am usually on IFR flight plan if I ask for a descent like that I'm pretty sure they would give it to me until I was in terminal environment.  However, I'm flying in the Appalachian mountains and would not wish to extend a slow descent at low altitude because... I'm flying in the Appalachian mountains.  I find that more often I am cruising higher than really necessary for the flight just for glide distance, and requesting to delay my descent when ATC requested and ATC almost always grants that.  So strictly from a practical standpoint I would rather remain higher longer and descend at or slightly above cruise speed, and if I don't have to maneuver much then I'll remain LOP to the runway.

[mooniac15u]

One advantage of long, slow descents is that you don't have to do steep turns in the pattern to bleed off excess energy.

[jetdriven]

The airplane will slow from 190 knots to 130 in about 2-2.5 miles.  If you target pattern altitude 3 miles from the center of the field on the 45 entry, and set it to 20" then 18" once it slows, you are well below gear speed abeam the numbers.

[mooniac15u]

10 minutes ago, jetdriven said:

The airplane will slow from 190 knots to 130 in about 2-2.5 miles.  If you target pattern altitude 3 miles from the center of the field on the 45 entry, and set it to 20" then 18" once it slows, you are well below gear speed abeam the numbers.

 

[markejackson02]

I ignore the costs and descend at 500' a minute to minimize headaches.  Unless IFR in which case I usually have to either throw out the dive brakes or cruise for 80 miles at 2000' around KIAH