Browncbr1 Posted December 1, 2017 Report Posted December 1, 2017 2 hours ago, 201er said: 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!? I was actually just joking because I thought arguing over a few dollars was a joke when we are spending tens of thousands per year on what is basically a hobby. I do admit that I conserve my tach time and am easy on my wear and tear by running lower rpms as well though. My descents are usually dictated to me by ATC anyhow. IFR descent is 500fpm minimum. So, hard to do without pulling some power. 1 Quote
201er Posted December 1, 2017 Author Report Posted December 1, 2017 1 hour ago, Hank said: 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. But you can't look at the descent while ignoring the cruise. The shortened time in high speed descent has to be made up with increased time in cruise! So you aren't comparing 20 minutes to 33 minutes. The 33 minute descent will cross 77nm while your 20 minute descent will only cover 55nm (guessing 165ktas cause you didn't provide a speed approximation). You gotta make up for the remaining 22nm and at the original cruise setting that would be another 9.4 minutes of cruise before starting the descent. Therefore you have to compare 33 minutes vs 29.4 minutes to see if it's really worth it or not. When comparing fuel, if the 33 minute descent used 3.3g, then the 500fpm descent uses 2.7g + 1.3g used during the extra cruise which actually nets 4g. So we're talking about spending an extra 0.7g of fuel to save 3.6 minutes. Might seem petty but it's more fuel than you'd use during cruise. I demonstrated earlier that it would actually be faster as well as cheaper to cruise slightly faster and descend slower (reduce fuel flow to maintain carson's speed). Quote
jaylw314 Posted December 1, 2017 Report Posted December 1, 2017 (edited) 6 hours ago, 201er said: 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. I should point out Carson speed probably isn't relevant other than as an arbitrary speed, since you're talking about fuel consumption as the outcome measure. You could just as well crunch the numbers with a 150 KIAS (or any other speed above best glide) cruise as a starting point. My guess is the outcomes will still be very similar. As an aside, there's an article from Flying Magazine that suggests actual Carson speed might be a bit higher than best glide speed, since power production becomes somewhat more efficient above best glide. They suggested "best range speed" x 1.32. The problem is, my POH does not have a "best range speed" or an easy way to figure that out from the performance charts, but if best glide is about 95 KIAS, might "best range speed" be about 110 KIAS? If so, Carson speed may be more like 145 KIAS, which is at least a tolerable speed. Edited December 1, 2017 by jaylw314 Quote
Hank Posted December 1, 2017 Report Posted December 1, 2017 26 minutes ago, 201er said: But you can't look at the descent while ignoring the cruise. The shortened time in high speed descent has to be made up with increased time in cruise! So you aren't comparing 20 minutes to 33 minutes. The 33 minute descent will cross 77nm while your 20 minute descent will only cover 55nm (guessing 165ktas cause you didn't provide a speed approximation). You gotta make up for the remaining 22nm and at the original cruise setting that would be another 9.4 minutes of cruise before starting the descent. Therefore you have to compare 33 minutes vs 29.4 minutes to see if it's really worth it or not. When comparing fuel, if the 33 minute descent used 3.3g, then the 500fpm descent uses 2.7g + 1.3g used during the extra cruise which actually nets 4g. So we're talking about spending an extra 0.7g of fuel to save 3.6 minutes. Might seem petty but it's more fuel than you'd use during cruise. I demonstrated earlier that it would actually be faster as well as cheaper to cruise slightly faster and descend slower (reduce fuel flow to maintain carson's speed). You're overthinking it. The extra 9 minutes at 8 gph instead of the same 9 minutes at 6 gph is simply: Extra fuel = (8 - 6) x (9 / 60) = 0.3 gallons which is the difference between 9 minutes at 8 gph and the same 9 minutes at your slow, 6 gph descent rate. Or are you turning your fuel valve to OFF at the top of your slow descent? There ain't no free lunch. Liberal Arts majors shouldn't often argue math with Engineers . . . . So you have halved the fuel savings from a standard, power on 500 fpm descent. But even if we both cruised at Carson's Speed (not friggin likely!), the faster 500 fpm descent will still save 0.3 gallons and 4 minutes. My C hits the same 170 mphi in a 500 fpm descent regardless of whether I start down at 3000 msl or 10,000 msl, but the IAS before descent varies appreciably. I do have to watch closely for traffic ahead of me, I once nearly ran over a friend in a Bo going to breakfast together; he took off first and flew faster, but pulled power for descent, so I passed him on the right and waited for him inside . . . . 1 Quote
EricJ Posted December 1, 2017 Report Posted December 1, 2017 I like converting altitude to speed, but I've found that flying around by myself one limitation is that I run out of down trim. Quote
Mooneymite Posted December 1, 2017 Report Posted December 1, 2017 1 hour ago, Browncbr1 said: IFR descent is 500fpm minimum. I've heard this before, but can't find it in the FARs. Can you point me to the reg? Quote
Tommy Posted December 1, 2017 Report Posted December 1, 2017 If there is ever an Ignobel prize for useless flying techniques, this surely is a contender. Asking what's the most fuel efficient way of descent is akin to asking how to best taxi in order to save fuel. Descent is a short duration low-power event that any "savings" - as calculated by yourself and others - is minimal. 0.5gallon and 1 or 2 minute? *yawn* All at the cost of complicating something simple. Much rather spend the descent on things like getting the profile right, reviewing critical airport information, AWIS/ATIS, CTAF, looking out traffic, and briefing the pax etc. Things that actually matters. Quote
thinwing Posted December 1, 2017 Report Posted December 1, 2017 It's also minimum IFR rate of climb...or that was what was taught during my IR Quote
bradp Posted December 1, 2017 Report Posted December 1, 2017 1) By the end of the trip I'm usually searching for a restroom. Maybe the hypoxia makes my kidneys unable to concentrate. 2) Mike - practically good luck doing a 77 nm descent at 300 fpm anywhere near the NY bravo - the controllers would have a fit (in fact I remember something from IFR training that if unable to maintain 500 fpm climb or descent then ATC should be notified AIM 4-4-9). 3) We need a hobby. I can't be spending my Thursday nights quoting the AIM. 4 Quote
ragedracer1977 Posted December 1, 2017 Report Posted December 1, 2017 It's not in the FAR's, IIRC. It's just in the AIM. Paragraph 270 (d) of the Airmans Information Manual directs that ..."When ATC has not used the term 'AT PILOT'S DISCRETION' nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of 500 feet per minute until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC." 2 Quote
Marauder Posted December 1, 2017 Report Posted December 1, 2017 It's not in the FAR's, IIRC. It's just in the AIM. Paragraph 270 (d) of the Airmans Information Manual directs that ..."When ATC has not used the term 'AT PILOT'S DISCRETION' nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of 500 feet per minute until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC." Another one that goes overlook but is in the AIM. “Change in the average true airspeed (at cruising altitude) when it varies by 5 percent or 10 knots (whichever is greater) from that filed in the flight plan. (AIM-5-3-3)”Sent from my iPhone using Tapatalk Pro 1 1 Quote
TTaylor Posted December 1, 2017 Report Posted December 1, 2017 29 minutes ago, Tommy said: If there is ever an Ignobel prize for useless flying techniques, this surely is a contender. Asking what's the most fuel efficient way of descent is akin to asking how to best taxi in order to save fuel. Descent is a short duration low-power event that any "savings" - as calculated by yourself and others - is minimal. 0.5gallon and 1 or 2 minute? *yawn* All at the cost of complicating something simple. Much rather spend the descent on things like getting the profile right, reviewing critical airport information, AWIS/ATIS, CTAF, looking out traffic, and briefing the pax etc. Things that actually matters. Actually we do think about best way to taxi , we are cheap Mooney pilots. If we weren't we would be flying Comanchees . Often I am descending for 80 nm or more in the western US. Dropping down from 12 to 13k across the Salt Flats and the Great Salt Lake. So yes, I do look at total time and fuel burn. I also race gliders and every second counts. Altitude is like fuel that I pay for with time. The most efficient way home is often the difference between winning and losing. Quote
201er Posted December 1, 2017 Author Report Posted December 1, 2017 37 minutes ago, Tommy said: If there is ever an Ignobel prize for useless flying techniques, this surely is a contender. Asking what's the most fuel efficient way of descent is akin to asking how to best taxi in order to save fuel. Descent is a short duration low-power event that any "savings" - as calculated by yourself and others - is minimal. 0.5gallon and 1 or 2 minute? *yawn* All at the cost of complicating something simple. Maybe if you saved a couple bucks a flight you could afford some proper tires. Quote
201er Posted December 1, 2017 Author Report Posted December 1, 2017 1 hour ago, Hank said: Liberal Arts majors shouldn't often argue math with Engineers . . . . Let me try this on a level you'll understand, you're going to have to spend more time at the self serve pumping the extra gas you burned so you really just spent extra money and barely saved any time. Quote
mooniac15u Posted December 1, 2017 Report Posted December 1, 2017 Personally, I'm more concerned about the inefficient use of an extra "n" in the title of this topic. Think of the time wasted with that extra keystroke. 1 4 1 Quote
Bob - S50 Posted December 1, 2017 Report Posted December 1, 2017 It all depends on your objective and how far you are willing to go to do that. If you want to get there quickly, push it up! Climb at 110 - 120, cruise at 75%, and descend at 75% until you have to slow down to land (ride permitting of course). If you really want to save money and you are crazy, and you can time it perfectly; pull the mixture all the way out at 10,000', and dead stick it in for a perfect landing, the descent will take 13.33 minutes at 750 fpm and 90 KIAS. Figuring an average TAS of 96 KTAS, you'll cover 21.33 miles and burn zero fuel. Personally, 4 hours in the seat is about my limit. My wife doesn't like anything over 3 hours. At my age, speed is more important that saving a few dollars. If you are trying for maximum range and minimum fuel, fly around at L/D max (about 85 KIAS) and about 2.5 or 3 gallons/hour. That could exceed my ability to stay awake. Quote
Shadrach Posted December 1, 2017 Report Posted December 1, 2017 (edited) 9 hours ago, 201er said: It's about learning to get the most out of the plane I already have and not to be wasteful by flying it poorly. Flying it poorly? Seriously? So anyone not doing it as you do is now doing it poorly? How many hours did it take for you to become that guy? Was it after the mandatory 500? Edited December 1, 2017 by Shadrach 3 Quote
kortopates Posted December 1, 2017 Report Posted December 1, 2017 Although its true that there is the implicit minimal descent/climb rate of 500 FPM in the AIM , the truth is its not a hard fast rule. It stems from: AIM 5−3−3. Additional Reports a. The following reports should be made to ATC or FSS facilities without a specific ATC request: 1. At all times: (c) When unable to climb/descend at a rate of a least 500 feet per minute But the even the controllers are told not to rely on this in their Directive Order JO 7110.65X - from page 4-5-5 4. Controllers need to aware that the descent rates in the AIM are only suggested and aircraft will not always descent at those rates. (and the above is repeated in multiple times) Consequently I target 200-300' FPM coming down from the flights levels at cruise power and rarely ever get a complaint; even flying in the some of the busiest airspace in the country (SOCAL). Of course if they're asking me to descend or climb for traffic, I know not to do that. But most of the time I can get a descent at pilots discretion and will be actively requesting that in advance so that I can descend earlier and convert that altitude to ground speed for as long as I can. 1 Quote
jetdriven Posted December 1, 2017 Report Posted December 1, 2017 (edited) Easy on Mike, he means well and sometimes comes off as badgering or abrasive. Like a New Yorker. Like me. Anyways I ran some numbers on my last trip in the jet and it looks like, its within the error of measurement, but basically its a complete wash. Like fuel and tach time. What is not a wash is arriving sooner. Please look this over, an correct me, i will revise it. So, on a 300NM flight, would you pay 2$ extra to arrive 14 minutes earlier? The bottom figure for the high speed descent, its a typo, its still 2500 RPM. I think whats happening here, is 65% power is just about the lowest absolute trip cost, given 34$/hr tach time cost. Mike is slightly slower and im slightly faster, and both end up at around the same cost, a little more than the 65% power cost. I'll take the faster. And nothing legal beats seeing 200 Knots on the groundspeed, or a 30 knot overtake on a 570HP Baron. Edited December 1, 2017 by jetdriven 1 Quote
jetdriven Posted December 1, 2017 Report Posted December 1, 2017 (edited) AT mikes behest, same climb and cruise, only difference is the slower 2200 rpm descent profile. the "A" 140NM trip only costs $3 less and is 7 minutes faster. the "B" 300 NM trip is 1$ cheaper and 6 minutes faster. So, descend at high speed, cruise power, save money, and get there sooner. Edited December 1, 2017 by jetdriven 3 Quote
carusoam Posted December 1, 2017 Report Posted December 1, 2017 Summary.... sort of. 1) Mooney pilots enjoy efficiency... from number of keystrokes used, to fuel saved in taxi and descent... with side trips that include maximizing distance between (non-mandatory Part 91) engine overhauls 2) Set-up... 201er is flying his M20J 500nm @10k’ He decides to descend using one of three different methods. (A)Byron’s (professional precision) method vs (B) 201er's (super efficient) method... (C) 201er's alter ego (constant speed descent) method... no wind and no peeing on your partner if you brought one along... Power is set using LOP gph... 8gph. 3) Three descent profiles... A: Stay High, descend to the traffic pattern at high Speed using a high descent rate, using a constant LOP FF of 8gph B: Begin the descent further out, Staying at Carson's speed minimizing effects of drag. (Wish I could reference our Carson guy here) C: Leave the throttle in until approaching TPA. 4) Cruise profile for the trip... (Stay with me, it will be worth it.....) 117 KIAS, 140 TAS, Using 8 gph 5) Descent profile... A: -300fpm, power setting LOP 8gph, 155 KIAS, same descent time as B. TT=3:31. Fuel used 28.1 B: -300fpm, for 77nm, @117 KIAS (same as cruise), power setting LOP 6gph, For 33 ... TT= 3:34. Fuel used 27.3 C: -800fpm, @200KIAS, LOP 10gph descend for 12.5 minutes, while praising the brothers Mooney, TT=3:29. Fuel used 28.3 For some reason the descent is from cruise altitude to the ground, and not TPA... a simplification..? maximum enjoyment for flight has been completely put in the back seat... Fahrfigflugen joy of flying with fig newtons... 6) There is something called the real kicker.... that would be a savings of $5, And how best to spend it... One could travel faster in cruise... Get better snacks for the ride... Buy disposable port-o-johns in place of the used Ziplocs left from the bag lunch... 7) My favorite aviation accountant said... skip the Carson's speed exercise and focus on the speed.... Carson himself, would have been speeding around in a Mooney on weekends... probably 200°F ROP... 8) My favorite method of descent was captured by a few people... Lower the nose, increase, then maintain, MP, while descending, use an IAS below Vne. Lower IAS (yellow arc limit) in bumpy air. Maximizes fun, speed, and selfies that include the GPS.... clearly not the efficiency topic 201er is describing... 9) Parasitic drag is the big item. Slowing down is the way to cut that drag way back... So going fast is going to be a drag... 9.5) tach time vs. real time vs. TBO vs. continuous monitoring the health of the engine.... If nobody OHs the engine at 2khrs, then there is no savings to be found by slowing the rpm down...? 10) Climb and descent are such small portions of the flight, their inefficiencies often get lost... If you’re mode of operation is Maximizing efficiency while flying a Mooney, Staying on Carson's speed all the time is going to make sense. 11) LOP Climb is a worthy topic. But at 2k’pm it is only five minutes to the target altitude... it may require a TN'd IO550... 201er, Have you explored LOP climbs at all? Or other high MP LOP operations? Nice work detailed in this thread. We probably could use a review of the tenets of Carson's speed. He included a lot of variables... Best regards, -a- One of my favorite 'training' flights was set up by 201er. I got to fly with Byron. One of the memorable topics that came up on that flight.... descent profiles... Thank you, gents! Quote
jetdriven Posted December 1, 2017 Report Posted December 1, 2017 (edited) Thats quite a bit of writing there, A. Excellent though provoking commentary as always. And your plane was the first Mooney, indeed perhaps the first piston powered plane I've ever seen 250 KTS groundspeed in. Low drag airframe and a huge 310HP engine does what you think it would do, whip the next competitor in its class by 40-50 knots. Truly a magic carpet. Edited December 1, 2017 by jetdriven 1 Quote
201er Posted December 1, 2017 Author Report Posted December 1, 2017 (edited) Is that what you needed a new engine!? Edited December 1, 2017 by 201er 1 Quote
Browncbr1 Posted December 1, 2017 Report Posted December 1, 2017 What a fantastic waste of time 2 1 Quote
Danb Posted December 1, 2017 Report Posted December 1, 2017 Wow, the cost savings on is only a number on those monthly bank statements, who looks at them. Quote
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