Innefficient Descent (going fast)

[201er]

Byron is wrong. Not by much. But he's still wrong

My premise is that high speed descents are inefficient because parasitic drag increases by the square of the speed. 150kias is a 56% increase in drag over 120kias. That extra speed in the high speed descent comes at the cost of wasting fuel that is burned off into drag. I tried to carve out some examples off the top of my head based on numbers I'm used to seeing. Granted I didn't test record these numbers from tests but I've got enough 201 time to have a good feel for what to expect. I also did not take into account the change in true airspeed but from experience in a normally aspirated plane, it's about a wash (ias goes up as tas comes down in descent). Wind can change your strategy but I will assume no wind for pure speed comparisons. Let's use a 500nm no wind xcountry cruising at 10,000ft and descending to sea level. No golden showers on this one because everyone's got the range to make this nonstop.

We will compare two descent rate strategies. One is to stay high and descend at a high rate at high speed and the other is to start a gradual descent further out at near Carson's speed. For the enroute portion, I am basing cruise on 117kias, 140ktas, 8gph. I'm ignoring the climb portion for this example because it doesn't affect the difference between the two descent strategies. I chose 10,000ft altitude so that we could draw out the descent to make it a more influential component of the flight.

Using a 300fpm descent rate, it will take 33minutes to descend 10,000ft at a continued speed of 140kts. The power is reduced to 6gph in order to maintain a descent profile at around carson's speed. This descent will cover a distance of 77nm in 33 minutes so it has to begin a ways out. The same speed as cruise is carried through the descent so the gain is only in fuel savings. This trip takes 3:34 at a fuel usage of 27.3g.

What if we target the same descent rate but skip the power reduction and maintain cruise power at 8gph? The descent from 10,000ft will still take 33 minutes at 300fpm but it will cover more ground, 85nm at a 155kts descent on 8gph.  This trip takes 3:31 but uses 28.1g.

Now we'll look at the Byron high speed descent. I don't have my own numbers for this so I will take Byron's word and use his. 200kts at 10gph in an 800fpm descent rate. The descent will only take 12.5 minutes in a distance of 42nm. You will spend more time at normal cruise and a short time at high speed descent. This trip takes 3:29 but uses 28.3g.

So what is the outcome?

3:34 on a carson's speed cruise and descent for 27.3g

3:31 for a carson's speed cruise but same ff descent for 28.1g

3:29 for a carson's speed cruise with a high speed descent for 28.3g

You get to save 5 minutes off the trip max for the cost of a gallon of gas. Hardly a golden shower but still that's $5 for 5 minutes.

 

But here's the real kicker that proves that the high speed descent is inefficient and impractical. If instead of descending faster you would just cruise a tad faster and stay with the efficient descent, you'll get there faster for the same or less fuel! Making the trip at 145ktas@8.5gph instead of 140ktas@8.0gph using the carson's speed descent profile (140kts@6gph) gets you across the same distance in 3:28 and uses 28.1g of fuel. So if you're going to waste $5 to go a little faster on your flight, you're better off cruising faster than descending steeper. This should come as no surprise because naturally the parasitic drag from going 5kts faster than 140 is lower than going 200kts.

Ok, a bunch of you don't give a damn and only "got a Mooney to go fast." But for those of you who got a Mooney to go fast efficiently, I think Carson's speed for the climb, cruise, and descent is the best way to go. Target 115-120kias for all phases of flight and let Mr Carson earn you the best speed for your buck.

Edited November 30, 2017 by 201er

[Danb]

Jeez Mike just buy a 152 or 172, plus less R/M for Cessna line of planes, if your goal is cost.

[201er]

7 minutes ago, Danb said:

Jeez Mike just buy a 152 or 172, plus less R/M for Cessna line of planes, if your goal is cost.

 

56 minutes ago, 201er said:

But for those of you who got a Mooney to go fast efficiently... let Mr Carson earn you the best speed for your buck.

It's about learning to get the most out of the plane I already have and not to be wasteful by flying it poorly.

Edited November 30, 2017 by 201er

[Raptor05121]

So lower my airspeed by another 25 knots? So I'm flying a Cherokee?

I'm a 63, so all my stuff in MPH. Vbg is 105MPH. Times Carson speed (1.32) gives me 139MPH TAS or 120 KTAS

On my cruise charts, the lowest it gives me is 144MPH TAS or 125KTAS, and that's at 10,000 feet at 53% power. So pull back to lets say ~50% power and I'll be down to 7.0gph and get ~120KTAS. 

OR I could do my normal 145KTAS/166MPH TAS, at 8.8gph @ 65% power

Over a course of 260 miles that

Carson Speed: 18.3 gals @ 2hrs 13min
Best Cruise: 18.7 gas @ 1hr 54 min

So half a gallon or 20 minutes...at $3.96 gallons.....soooooo roughly about $2 more to get there 20 minutes quicker.

My math must be wrong, that doesn't make sense. I'd gladly spend $2 and get there 20 minutes quicker

[thinwing]

I think you have too much time on your hands if you are spending it on this minutiae

[DonMuncy]

All this is interesting, and it is information one (I) should be aware of. But at my stage of life, $5 or 5 minutes is almost of no consequence, and I will probably keep doing what I do. 

[201er]

18 minutes ago, Raptor05121 said:

So lower my airspeed by another 25 knots? So I'm flying a Cherokee?

I'm a 63, so all my stuff in MPH. Vbg is 105MPH. Times Carson speed (1.32) gives me 139MPH TAS or 120 KTAS

On my cruise charts, the lowest it gives me is 144MPH TAS or 125KTAS, and that's at 10,000 feet at 53% power. So pull back to lets say ~50% power and I'll be down to 7.0gph and get ~120KTAS. 

Carson's speed is measured in IAS

[1964-M20E]

When I need to descend from 10000 feet I start my descent by lowering the nose and gaining speed.  I know you are looking at saving fuel on the descent.  However, sometimes flying IFR ATC will keep you higher for traffic as you well know.  I generally look to start descending about 50 to 60 miles out when at 10000 and going to seal level.  I let my airspeed increase in a 500fmp decent and check the MP at about 23" to 24" during the decent.  I will close the boost air at the top of the descent just so I won't forget it later.This generally gives me about 160kts IAS +/- and a decent ground speed increase.   JMHO

[ShuRugal]

what about option 4?  carb heat on, throttle back to 15", push the nose over until Vno (or faster, turbulence permitting)...

[Yetti]

Did you factor in gravity? 

[Browncbr1]

You’re forgetting byron’s indirect cost of operation of the engine, mags, and appliances.  The $5 fuel cost of flying 5 minutes less is $60/hour.   That’s much less than the typical $100+/hour operating cost most of us account for.    It’s a $5 cost realized today, but a unrealized savings of $8.33 or more later on down the road.  Since we are talking about present value vs future value, that $8.33 unrealized value today is actually $10-13 realized savings in today’s dollars.  Or something.   Blahahhaa!

[201er]

13 minutes ago, Browncbr1 said:

You’re forgetting byron’s indirect cost of operation of the engine, mags, and appliances.  The $5 fuel cost of flying 5 minutes less is $60/hour.   That’s much less than the typical $100+/hour operating cost most of us account for.    It’s a $5 cost realized today, but a unrealized savings of $8.33 or more later on down the road.  Since we are talking about present value vs future value, that $8.33 unrealized value today is actually $10-13 realized savings in today’s dollars.  Or something.   Blahahhaa!

Yeah I was waiting for Byron to bring that up. But I don't think it adds up that way because the actual wear and tear on the engine is much less from operating at lower power. Also if you reduce RPMs the tach time is lower than the actual time. Lower oil consumption at lower power, less vibration (outside the yellow arc), lower cylinder pressures. I don't quite buy into that hourly usage bit. I do understand how it can apply if you go too slow but I don't think at Carson's speed that you have anything but overall savings. Show me numbers if you disagree.

Edit: If I understand correctly how tach time works (I may not), then 2200 is 22/25 of the time of running at 2500. If this is a correct understanding, then Byron's 12.5 minutes in descent at 2500RPM will be taxed at 12.5 minutes of tach time. However, bringing the RPM back to 2200RPM as part of the 6GPH Carson's descent, that comes out to 29 minutes of tach from the 33 actual minutes. Therefore, the net gain in tach wear on the airplane is, drum roll, 1 minute! Who wants to argue that 1 minute of extra tach time is worth more than a gallon of fuel savings!?

Edited November 30, 2017 by 201er

[TTaylor]

In general it is best to descend near your normal cruise speed.  As 201er pointed out the parasitic drag goes up at higher speeds.  I usually descend at about 250 to 300 fpm to stay in a more efficient speed range as long as there is not a wind gradient that will make it less efficient. Those that like to descend up near red-line are just wasting energy most of the time, but it feels good .

[Guest]

My airplanes are time machines, I fly them as fast as practical.  I can buy more 100LL, I can’t buy more time.  If I wanted to fly slower I would have kept my Mooney.

Clarence

[Bob_Belville]

If this descent cost me $5 it was the best $5 I've spent all day.

[Hank]

Hey, Mike. @201er

How about those of us who just push to descend at the easy, reasonable rate of 500 fpm? Been doing this for almost 600 tach hours now. To make the math simple, we will ignore both initial climb and cruise and only examine the descents, since they are the only part that is different.

500 fpm will take 10,000 ÷ 500 = 20 minutes at the same 8 gph, or 8 x (20/60) = 2.7 gal to descend.

Your slow 300 fpm, it will take 10,000 ÷300 = 33.3 minutes at 6 gph, or 6 x (33.3/60) = 3.3 gal to descend.

Unless there's a mistake in the simple math? It's not really this simple due to leveling off at pattern altitude, reducing power to fly the pattern, winds, etc. 

I'll keep cruise power and keep pushing for 500 fpm. Your fuel savings above came from your slow Carson cruise, not the descent . . .  This will save me > 1/2 gallon of fuel and 13 minutes' flight time on the engine, my bladder and the delay before lunch.  

Or if you prefer to examine the whole flight, Carson's speed cruise saves 0.5 gallon on this flight, and costs 0.6 gallons in the descent. Let's call fuel a wash. What's your time worth? You lose double, the slow cruise takes much longer and you lose 13 more minutes on the descent.

Edited December 1, 2017 by Hank

[ArtVandelay]

My airplanes are time machines, I fly them as fast as practical.  I can buy more 100LL, I can’t buy more time.  If I wanted to fly slower I would have kept my Mooney.
Clarence

Some of us enjoy flying and want to prolong it.

[Hank]

Just now, teejayevans said:

Some of us enjoy flying and want to prolong it.

That's when I throttle back. But I don't often want to prolong those 500nm trips as used in Mike's example above.

[Guest]

Just now, teejayevans said:

Some of us enjoy flying and want to prolong it.

I enjoy flying too, that why I have a few planes to choose from.

Clarence

[jetdriven]

10 minutes ago, bluehighwayflyer said:

I strive for efficiency in all stages of flight except the descent. A "highspeed" (This is a relative term. You are still in the green in the J up to 201 MPH and I have never needed or wanted to descend faster) descent is my little reward to myself for the rest of my forbearance.  

That having been said, the farther you are away from best glide speed (That's right.  Not Carson's speed.) in any stage of flight the more aerodynamic efficiency you are wasting. 

FWIW. Today I flew 5.6 hours and burned exactly 32 gallons of fuel. That is a block fuel burn rate of 5.71 GPH. But I am time building right now and am not trying to get anywhere. 

Jim

Let me show you the 4.0 GPH loiter setting.  Hint it’s slightly ROP

[jetdriven]

Imagine a  15:30 block flight on stock tanks. Think of the glory, man.  All the pretty girls. 

[Gone]

53 minutes ago, Bob_Belville said:

If this descent cost me $5 it was the best $5 I've spent all day.

+1 

[jetdriven]

Around 100-105. And 2300 rpm. And the MP to park the airspeed. 

[TonyK]

4 hours ago, Danb said:

Jeez Mike just buy a 152 or 172

A 172 you have to leave the power in to descend.   If you don't you could be up there all day.

[Skates97]

25 minutes ago, bluehighwayflyer said:

I guess that would be about 1500 statute miles. It would be fun to try! 

You better have some Gatorade bottles or @Piloto's relief tube...