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Carson number and contiounous RPM

Mike S

Senior Curmudgeon
The new issue of AOPA has an good article by Mike Bush about how to maximize speed, range, and fuel use.

With the current cost of avgas, I am looking to try this out.

It all revolves around flying at the Carson number and leaning. For those not familiar with the Carson number, here is an older article explaining it.

https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed

One of the ideas brought out is for those with a C/S prop, you pull the prob knob back at WOT until you get to the Carson speed.

This will most likely end up having the engine running at a RPM lower than most of us usually fly at, and I suspect potentially getting into some restricted RPM zones.

Anybody know if an IO320 with a Hartzell C/S prop has any specific RPM to avoid continuous running?
 
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23/23, lean and crack on.

We are paying $12 a US gallon at the moment in the UK.

I just go flying and enjoy it ;)
 
Dave Anders flies WOT and as low as 2100 rpm. He knows more about best MPG than just about anyone through extensive flight testing and documentation. 158 knots TAS on 4.8 GPH at 17.5. Of course his RV is pretty slick.
 
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Anybody know if an IO320 with a Hartzell C/S prop has any specific RPM to avoid continuous running?

This was discussed a long time ago on VAF, but I don't think there is any limitation with an O-320, only O-360's, and it's limit continuous use between 2000-2250 RPM.

I found this document, while not having anything official in it, seems to be from Hartzell, basically saying that with EI, they found the stresses increased in that same RPM range, but this was already documented, so they didn't recommend changing the existing limitation.

Overall, in another article or book from Mike Busch, he recommends running at the lowest possible RPM anyway (ie "over square") due to other efficiencies gained from the slower rotation of parts in the engine. We had a discussion about it here.
 
One thing that I found on my data runs is for a constant fuel flow, lower RPM is not necessarily better efficiency. I can only assume the ability of a prop to transform engine power to thrust is not linear and dependent on the prop design.

This leads me to operate high at RPM between 2450 and 2500, WOT, 20-30 degree LOP. I’m using a 74” Hartzell BA CS prop on a IO-360-M1B engine. I suspect I can squeeze out 1 or 2 more MPGs at a lower RPM but for me the curve is fairly flat, and for that small gain I’d take the trade off for higher cruise speed.

I’d be interested in other builder’s data.

Carl
 
flying at the Carson number and leaning

Well, this is what I’m doing back home where fuel is kinda double price of here in the US.
Carson speed for my bird is around 116kts IAS, still gives me like 130kts TAS at 4Kft on 5.5USG. Running well lean of peak on my carbed dual electronic ignition. Many do believe injection is the key, well, ignition is the real key to LOP.
My typical settings being 19/2000, flying in a world of silence :)

Right now touring the US, I’m using higher MP settings like 23 or 24. But of course FF goes to around 7.4, and the speed is way more than Carson‘s ideal…
 
Dave Anders flies WOT and as low as 2100 rpm. He knows more about best MPG than just about anyone through extensive flight testing and documentation. 158 knots TAS on 4.8 GPH at 17.5. Of course his RV is pretty slick.

Isnt Dave running an angle valve 360?? And touched by the magic of LyCon? Different animal from mine.

As I have yet to try getting into the Carson number flight regime, I can only speculate what the engine RPM and MP will be. I expect to be flying at 8,500 or higher at WOT, and would not be surprised to see RPM at 2000 or maybe even lower. AFR leaned down to 16:1 or more.

Just thinking ahead for a cross country trip in a few months.
 
Carson's number, IMO, is just a weird number. It isn't the fastest, or most efficient, it is just a specific point on the efficiency curve. Why choose that point on the curve, as opposed to any of the others? Is it a "thing" just because there's a calculation for it?
 
Carson's number, IMO, is just a weird number. It isn't the fastest, or most efficient, it is just a specific point on the efficiency curve. Why choose that point on the curve, as opposed to any of the others? Is it a "thing" just because there's a calculation for it?

“Accepting that excess fuel is to be traded off for airspeed during normal operations, there is a method of operation which represents the ‘least wasteful way of wasting fuel,’” Carson wrote in a pioneering 1980 study.

Well, the way I take it is anything faster than best economy speed is using more fuel. There comes a place where your use of fuel increases faster than the speed increase. That is what I am shooting for.
 
Ive just finished my new RV-10 testing - with about 35h or so on it now.
Ive definitely noticed that lower RPM is more efficient, but Im yet to collect extensive data on this. Currently I cruise at at 8500' running 2200/24" which is about 70% power. LOP 11 GPH gives 168 TAS, ROP 14 GPH gets 174 TAS. I need to do some more nozzle tuning as i still have one cylinder peaking 0.75 GPH earlier than the rest. I think ill get it down to 10GPH and perhaps a few kts slower.
Ive GPS box tested 100 through 150 IAS and the PEC is <1 KCAS (actually under-reads at 150 by 0.6) so I'm confident in the numbers.

It has 9:1, CAI, showplanes cowl and a Hartzell 3 blade.
The 3 blade was a conscious choice as I intend to use some short strips as I did with my 7. Intuitively I thought that this combination should see the extra HP hopefully offset the extra blade to some degree. To my pleasant surprise it comfortably meets Vans published numbers or exceeds them. FWIW I made my own plenum as well which when combined with slightly smaller intake rings than the stock SP cowl possibly helps.

I notice almost no degradation in performance from 2400 down to 2200 RPM at constant MP. above 2600 it gets slower.

Anyway, all of this is to preface that the 3 blades are obviously much bigger than a 2 bladed aluminium (lightweight climb performance is quite something).
Ive come to operate it quite a bit differently than my IO360/Hartzell BA equipped RV7 so I think that any RPM discussion needs to consider the propeller choice. I never ran my RV7 slower than 2400. But fuel was cheaper then!
 
This would be a place where at best L/D, prop efficiency and BSFC curves come close to coinciding.

For the engine, best BSFC is a composite area where frictional losses are low and VE is high, combined with an AFR around 16 and properly advanced timing to achieve PCP at the optimal crank angle with the slower burning mixture.

You're balancing lots of factors here.

Mike, you'll have time to experiment on that long cross country...;)

Yeah, Dave Anders is running a high CR AV with a highly optimized custom intake system where he's strived to maximize the VE by timing the return waves in the runners for best cylinder filling. He's done some fascinating testing to this end. Dave is never done trying to find another knot or MPG.
 
Why choose that point on the curve, as opposed to any of the others? Is it a "thing" just because there's a calculation for it?

Kyle, now THAT'S a good question. The answer is B.H. Carson's mathematical curves (a considerable amount of good math) show that 32% above Vbg was most efficient rate of return, in the form of distance travelled, for the increase in fuel consumption as a result of the increase in speed.

Ross is on to it in terms of BSFC, however the math is less concerned about the engine, but rather the total drag of any particular airframe, hence the use of Best Glide Speed.

In simpler terms, it's not just a point on the efficiency curve, but the most efficient speed above which the increase in the rate of fuel consumption has a reduced increase in speed. It is the most efficient speed to consume fuel for distance travelled. I've tested this many times on several different airframes including a Beech Debonair, RV-12, RV-7A, Zenith 601XL and Baron B58 and found it accurate.

This paper by Prof. Rogers, Efficently Wasting Fuel , summarizes Caron's work and I highly recommend anyone flying an airplane read it.
 
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Did some testing this morning, here are some numbers. This is in a somewhat draggy airframe with big tires, consider it a 3/4 scale RV 15 :D

Altitude 7500, OAT 78.

Full throttle and prop pulled back to 1900 gave me 5.1--5.2 GPH.

Airspeed was 104--106 MPH, a bit higher that calculated Carson speed but I was planning on going +10 anyway so this is pretty close to my target. I was reluctant to pull the RPM below 1900. From prior training and testing I found prop control gets iffy when the RPM goes too low.

AFR 16.4--16.6 and timing advanced to 28*

CHT 345 or lower.
 
Carson

I play with prop/ mixture to dial in the the best MPG at speed . May play with the low rpm and see what that does to MPG .
I’m usually 25 mpg with a slight head wind …
 

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Supporting Ross's comment

This would be a place where at best L/D, prop efficiency and BSFC curves come close to coinciding.

For the engine, best BSFC is a composite area where frictional losses are low and VE is high, combined with an AFR around 16 and properly advanced timing to achieve PCP at the optimal crank angle with the slower burning mixture.

You're balancing lots of factors here.

Mike, you'll have time to experiment on that long cross country...;)

Yeah, Dave Anders is running a high CR AV with a highly optimized custom intake system where he's strived to maximize the VE by timing the return waves in the runners for best cylinder filling. He's done some fascinating testing to this end. Dave is never done trying to find another knot or MPG.

If you ignore the engine and prop then Carson's number is either correct or so close to it that it's not worth debating. But it's pure math. If I recall, he did his example in a Bonanza. Do I recall correctly?

1.However the peculiarities of an airframe can be a factor if some AOA's induce separation and localized turbulence. His numbers were based on perfect drag curves if I recall. In real life as Jack Norris demonstrates in his books, there are bumps in those curves for some aircraft. That may or may not make a lot of difference and it would be very hard to filter out that factor.

2. If my Superior IO-360 was a relevant model, the BSFC had a sweet spot and for mine it was about 8 gph (from Superior's data). That seemed to work in real life, too, for my 7A. But it had a fixed pitch prop.

3. It would make sense to begin an experiment starting at the best BSFC point, running LOP and varying the rpm/pitch if possible. For me, the sweet spot was faster than Carson's because my 7A did not like to go that slow. For me it was 8 GPH, 8K' and 160 KTAS, LOP. YMMV. At Carson's the engine would NOT run hot enough. Yes, that could be fixed, but not without cowl flaps or some such. A cold engine is less efficient.
 
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Some good stuff here. There are so many interrelated factors and unknowns that actual flight testing is the only way to find that magic figure for each airplane. I'd guess that Hartzell doesn't optimize blade twist or design for 2000ish rpm cruise for one thing.

We know the engine wants to be WOT for lowest pumping losses but can the prop be pitched enough to get the rpm low enough? Will the blade be operating efficiently at that angle?

Prop, engine, airframe will probably all be at odds with one another with regards to max efficiency of each so you'll be trading a slight loss in prop efficiency and engine BSFC at the Carson Number.

I have some data at the shop on Lycoming BSFC at the shop. If I recall, it was pretty impressive down at low rpm (much lower frictional losses there). I'll see what I can dig up.
 
Carson

Speed vs MPG vs hourly operating cost . One thing to take into consideration is the actual cost per hour to operate and engine/prop and airframe leaving the fuel out of the equation. A $40,000 engine with a TBO of 2400 probably is around $12 per hour alone ( given a $10,000 core value)
 
From a thread I stared in 2016: Carsons speed for RV's, I came up with the following rough figures - though I guess with the -14 well out and about and the -15 sneak peak I can add a couple more models to the list...

RV-1:
RV-3: 110.5 SMPH - (Avg of 73 KIAS - N223RL POH, only one so far!)
RV-4: 111.1 SMPH - (Avg of 71 KIAS N359DM POH, 82 MPH - C-GFEW POH, 80KIAS - N41RV POH)
RV-6: 139.5 SMPH - CAFE RV-6 APR
RV-7: 120.1 SMPH - (Avg of 90MPH - N2447A POH, 78KIAS - N447RV POH, 85KIAS - HB-YMT AFM, 78KIAS N585RV POH)
RV-8: 140.5 SMPH - CAFE RV-8A APR
RV-9: 125.0 SMPH - CAFE RV-9A APR
RV-10: 131.2 SMPH - (Avg of 80KIAS- N423CF POH, 90KIAS - N961M POH, 90KIAS - N42BU POH)
RV-11: - I'll ask Van when he finishes it. But how do you measure "fuel"?!?
RV-12: 95.4 SMPH - Vans Factory POH. Can't beat that!
RV-13: Classified - Believed to be in excess of 760 SMPH. If I ever feel the need to break into Area 51, I'll update this one!
RV-14: To be added "soon".
 
Prop, engine, airframe will probably all be at odds with one another with regards to max efficiency of each so you'll be trading a slight loss in prop efficiency and engine BSFC at the Carson Number.

It's interesting that nothing is optimized for maximum efficiency at the same configuration. Although we are flying "over-powered" sport planes (as if there's a such thing as "over-powered"). Most people spin the prop 2300-2500rpm, so that's where prop manufacturers optimize the blade design.

I suppose altitude could be used to reduce power. WOT, low RPM, LOP, but at an altitude that results in flying at Carson speed.
 
Science?

I dont see the science behind just adding 32% to the best L/D speed. That magic number must assume a general aircraft. I think we need an RV to go and plot airspeed vs fuel consumption and then find the real Carson number for RVs.
 
I dont see the science behind just adding 32% to the best L/D speed. That magic number must assume a general aircraft. I think we need an RV to go and plot airspeed vs fuel consumption and then find the real Carson number for RVs.

So do the NTPS GPS box method for varying airspeeds except generate true mpg - by triangulating the FF not the GS.
The G3X economy number might not much use for this as I’m pretty sure it’s GS not TAS.
 
I dont see the science behind just adding 32% to the best L/D speed. That magic number must assume a general aircraft. I think we need an RV to go and plot airspeed vs fuel consumption and then find the real Carson number for RVs.

Not magic, just pure science, which is laid out in the several documents already provided. Good math formulas, basis, development and summation. NAR and Associates website is an excellent source of good aeronautical engineering.

BTW, laminar flow analysis, which the Carson speed is based on, is indifferent to the propulsion or category of aircraft, but does require accurate development and identification of total drag and lift curves for a specific airframe. You can derive a Carson speed for the any airframe, even the SR-71.
 
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When I'm flying for fun and not actually going on a mission I rarely fly more than 50 nm between airports. I fly my IO-0360 CS RV-6 less than 2000 AGL. I typically use about 0.5-0.7 gallons to achieve cruise altitude. I cruise with less than 20" and 2000 rpm, with an IAS of about 100 kts. This yields less than 5 gph and 20+ nmpg.

This is the very low end of RV-6 performance and I may experiment with bumping up the cruise speed to 110 kts to see how things settle out.

I've embarked on building an RV-9, not just because I now have time and space to build, but also tailor my plane to my predominant mission. I think the RV-9 would be a better choice.

From a thread I stared in 2016: Carsons speed for RV's, I came up with the following rough figures - though I guess with the -14 well out and about and the -15 sneak peak I can add a couple more models to the list...

RV-1:
RV-3: 110.5 SMPH - (Avg of 73 KIAS - N223RL POH, only one so far!)
RV-4: 111.1 SMPH - (Avg of 71 KIAS N359DM POH, 82 MPH - C-GFEW POH, 80KIAS - N41RV POH)
RV-6: 139.5 SMPH - CAFE RV-6 APR
RV-7: 120.1 SMPH - (Avg of 90MPH - N2447A POH, 78KIAS - N447RV POH, 85KIAS - HB-YMT AFM, 78KIAS N585RV POH)
RV-8: 140.5 SMPH - CAFE RV-8A APR
RV-9: 125.0 SMPH - CAFE RV-9A APR
RV-10: 131.2 SMPH - (Avg of 80KIAS- N423CF POH, 90KIAS - N961M POH, 90KIAS - N42BU POH)
RV-11: - I'll ask Van when he finishes it. But how do you measure "fuel"?!?
RV-12: 95.4 SMPH - Vans Factory POH. Can't beat that!
RV-13: Classified - Believed to be in excess of 760 SMPH. If I ever feel the need to break into Area 51, I'll update this one!
RV-14: To be added "soon".
 
On local flights where I am not going anywhere, I will run 2,100 and 22 inches with a 6 GPH fuel burn and 122 KIAS.

I try not to run more than 3 or 4" over square. The Lycoming power curve chart only showed 5" over square above RPM but did not go below 2,000 RPM.

View attachment Power Chart B2B.pdf
 
I suppose altitude could be used to reduce power. WOT, low RPM, LOP, but at an altitude that results in flying at Carson speed.

Altitude should be chosen based on winds aloft. Make sure you don’t fly a particular altitude to get the small additional advantage of flying Carson speed and loose the much larger benefit of more favorable winds. Just a reminder, efficiency is based on ground speed, not TAS.
 
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Speed vs MPG vs hourly operating cost . One thing to take into consideration is the actual cost per hour to operate and engine/prop and airframe leaving the fuel out of the equation. A $40,000 engine with a TBO of 2400 probably is around $12 per hour alone ( given a $10,000 core value)

Agree that part of the “cost of ownership” is running your engine to minimize the maintenance cost and maximize engine life (TBO). Probably a small factor but engine maintenance is based on dollars per hour and maybe needs to be converted to dollars per mile to get true cost of flying slower besides just considering the reductions in fuel use.
 
Altitude should be chosen based on winds aloft. Make sure you don’t fly a particular altitude to get the small additional advantage of flying Carson speed and loose the much larger benefit of more favorable winds. Just a reminder, efficiency is based on ground speed, not TAS.

I was also thinking that Carson speed would only apply in still air, zero wind. Flying with a headwind will increase the most efficient speed. We'll spend longer flying into the headwind, so need to fly slightly faster to reduce time spent in the headwind. As an extreme example, 100kt headwind and 100kt Carson speed. Well that's 0mpg. Fly Carson speed +10 is much more efficient. 100 mile trip, 100kt Carson speed, 0 wind is 1hr at ~5gph for 5gallons. Same 100 mile trip 25kt headwind is 1.3hrs 6.7 gallons. Fly Carson speed +25, with the same 25kt headwind at, I'm guessing 6gph, 1hr at 6 gallons total. Opposite is true in a tailwind. We'll want to fly slower than Carson speed to be most efficient.
 
MPG

So do the NTPS GPS box method for varying airspeeds except generate true mpg - by triangulating the FF not the GS.
The G3X economy number might not much use for this as I’m pretty sure it’s GS not TAS.
For my GRT I'm fairly sure it was GS. But the GRT showed a quick, easy and accurate way to get GPS based TAS.

Fly directly upwind such that there is less than 5 degrees difference between heading and track. Observe GPS GS. Turn approximately 180 degrees, do the same thing, still less than 5 degrees difference. Take the average.

Yes the NTP School 3-way is more accurate but if you do both and also look at it with math you will see there is extremely little difference.

This is the way to determine MPG or NM/PG in flight if your EFIS calculates it.

I verified that the two methods produce extremely close results by trying both on the same flight in the same conditions only a few minutes apart. I always tried for no more than one degree of difference between heading and track.
 
Carson vs Laminar Flow

Not magic, just pure science, which is laid out in the several documents already provided. Good math formulas, basis, development and summation. NAR and Associates website is an excellent source of good aeronautical engineering.

BTW, laminar flow analysis, which the Carson speed is based on, is indifferent to the propulsion or category of aircraft, but does require accurate development and identification of total drag and lift curves for a specific airframe. You can derive a Carson speed for the any airframe, even the SR-71.

I think that's a typo. Carson's speed assumes that the parasite drag increases as the square of the V and that induced drag declines as the square of the V. Laminar flow has a "bucket" and does not conform to those curves.

Carson's is approximately 1.42x the point where the curves intersect which is the point of best glide (not best endurance). It is also where induced drag is 25% vs parasite drag at 75%. For best glide they are equal and for best endurance they are reversed from Carson's.

Some what-if incrementing of a few data points for a laminar flow airplane would produce a similar result (least wasteful, etc.).

I have attached a very helpful article on the RV6 by CAFE which may help understand the curves.
 

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Carson's is approximately 1.42x the point where the curves intersect which is the point of best glide (not best endurance). It is also where induced drag is 25% vs parasite drag at 75%. For best glide they are equal and for best endurance they are reversed from Carson's.

I feel like I'm having deja vu all over again...as in the topic has come up numerous times over the years with the same responses... I guess that's good we're still talking about it and learning as a group.

B.H. Carson's finding was that the most efficient use of fuel was at 1.316 times the intersection of Lift and Drag, as in Total Drag, which is the summation of parasitic drag and effective induced drag. All of this is described in detail in the previously referenced papers and I feel I would do them a disservice by trying to paraphrase them further. If you're truly interested in the relationship of flow and drag, the book by Prof. Dave Rogers, Laminar Flow Analysis, is an excellent textbook reference for this. Looking at my copy, it was printed in 1994, so hopefully it's still available at the library (a book? what's that? :D).

My hope is that folks take away a simple concept from this thread, which is that an airplane is, in it's simplest form, an object moving through fluid (air). Irrelevant of the airframe design or propulsion, the most efficient speed with regarding to fuel consumed over distance will be at 1.316 times the Best Glide Speed, which is the Lift to Drag intersection for that airframe.

For easiest calculation, the "Carson Speed" is 32% faster than Vbg, (which is in terms of indicated airspeed)

For a sidetopic, consider Affinity Laws, aka "Pump Laws" where speed is proportional to headless (drag) as squared function. A minor reduction in drag (parasitic or induced) results in improved speed of the airframe as a squared function, whereas power is cubed, which is why a reduction in drag will improve your airplane's speed much more effectively than adding power . Flow is fun...
 
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Irrelevant of the airframe design or propulsion, the most efficient speed with regarding to fuel consumed over distance will be at 1.316 times the Best Glide Speed, which is the Lift to Drag intersection for that airframe.

For easiest calculation, the "Carson Speed" is 32% faster than Vbg, (which is indicated airspeed)

In the real world world, prop and engine efficiency will enter the picture so it would be most interesting to put this to the test on various RVs with glass panels and see how theory correlates with practice. Let's do 1.1, 1.2, 1.3, 1.4 and 1.5 X Vbg to see the MPG curves. Sounds like a good excuse to go flying...

Post away folks. :)
 
Keep in mind that both Vbg (Vl/dmax) and Vcarson are speed equivalents of specific angles of attack of the wing.
So make sure to weight correct those speeds otherwise they are only valid at max gross weight.

Lenny
 
My hope is that folks take away a simple concept from this thread, which is that an airplane is, in it's simplest form, an object moving through fluid (air). Irrelevant of the airframe design or propulsion, the most efficient speed with regarding to fuel consumed over distance will be at 1.316 times the Best Glide Speed, which is the Lift to Drag intersection for that airframe.

Ron, this is not correct. The most efficient speed with regarding fuel consumed over distance is max range speed which, if not for varying prop efficiencies at low speeds (which Carson states he ignores), is the same as best glide. Carson argues that max range is not the figure of merit that should be used. Instead, he proposes that a linear combination of ‘least fuel’ AND ‘least time’ be used, as the ‘most efficient’ operating point. e.g., consider minimizing, in some combination, both fuel used and Hobbs hours. Since time is inversely proportional to (ground) speed V, he looks to maximize the ratio of (V/F), where V is the ground speed and F is the fuel used for the flight.
 
Alternative approach to Carson speed

It can be even simpler, without any worries about prop efficiency, bsfc, etc. Just need to measure in flight the fuel flow and TAS, at a fixed weight and density altitude. Can make it slightly more complex by normalising various weights and density altitudes to standard values.

We're interested in three speeds:

Best endurance - minimise fuel flow. ie, fuel burned per unit time

Best range - minimise ( fuel flow / airspeed). ie, fuel burned per unit distance

Carson speed - minimise ( fuel flow / airspeed squared), ie fuel burned per unit distance x time spent per unit distance

The mathematics behind this involves differential equations, which I'll spare you, but it's pretty straightforward.

By plotting the same test data on three graphs, it's easy to find the three speeds. I've attached an example from a hypothetical aircraft.

If you go out and try this with your own aircraft, you'll probably find that the Carson speed is somewhere close to 30% higher than the best range speed.

Hope I've reduced confusion rather than adding to it.
 

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Ron, this is not correct. The most efficient speed with regarding fuel consumed over distance is max range speed which, if not for varying prop efficiencies at low speeds (which Carson states he ignores), is the same as best glide. Carson argues that max range is not the figure of merit that should be used. Instead, he proposes that a linear combination of ‘least fuel’ AND ‘least time’ be used, as the ‘most efficient’ operating point. e.g., consider minimizing, in some combination, both fuel used and Hobbs hours. Since time is inversely proportional to (ground) speed V, he looks to maximize the ratio of (V/F), where V is the ground speed and F is the fuel used for the flight.

Bob, it appears you are talking about the definition of best glide speeds for distance versus time, but it's not clear to me what you're actually saying is incorrect, which may be due to my post being confusing, so I'll just refer you to Mr. Carson. His number "results in the best rate of return for each unit increase in speed."

While improving propeller efficiency, airframe and engine are good, they're essentially encompassed within reducing total drag and reducing per unit fuel consumption; these changes move the reference point that the Carson number is calculated from for your particular airframe.

Summarizing my previous posts, if you fly at 132% of your best glide speed, you will be able to fly to your destination with more efficiently.
 
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If you go out and try this with your own aircraft, you'll probably find that the Carson speed is somewhere close to 30% higher than the best range speed.

Good post Dan.

The only comment I have is that if you're finding the speed closer to 30%, instead of Carson's 31.6%, it could be related to calibration of equipment along with the variance of actual Best Glide Speed for your airframe from the original book or test number. Anything that changes the drag of the airframe changes the L/D data point, which then changes the realized speeds. In practical terms, you're absolutely right that "about 30%" is the number most folks will find.

Having read a lot of comments and talked with many folks about "Carson Speed" and found some confusion, I think it is important to point out that Carson's numbers are based in IAS, which means that the TAS will increase with altitude.
 
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Instead of debating the theory, let's validate reality with some flight testing and using your EFIS to do the calcs. I suggest the same altitude and WOT for all tests if possible and just adjust prop rpm to vary power. Lean to 30F LOP in all cases for consistency.

A simple plot of MPG at 1.1, 1.2. 1.3, 1.4 and 1.5 X Vbg will show the true story pretty quickly.

Might be interesting to do a sweep at 2100, 2200, 2300 and 2400 rpm and vary throttle/ MAP to see how that affects the MPG numbers too.
 
Instead of debating the theory, let's validate reality with some flight testing and using your EFIS to do the calcs. I suggest the same altitude and WOT for all tests if possible and just adjust prop rpm to vary power. Lean to 30F LOP in all cases for consistency.

Same altitude... MSL or DA?
And somehow from my testing I suspect flying same DA at wildly different temperatures won't be comparable. Just my hunch.
 
Here's some numbers I collected during phase 1, RV10, FWIW. Instead of graphing, I divided the decrease in time (minutes) by the increase in fuel - bigger is better for Carson speed. Based on imaginary 500 NM leg no wind.

==dave==
N102FM


Speeds.JPG
 
Interesting. I'd guess that best glide on an RV-10 is about 85 knots so your data would seem to indicate the Carson's Speed is very close to being right.
 
Question

Is the lift drag curve changing with air density? Meaning is Carsons IAS the same at 1000 and 12,000 ft? Or is it TAS?

The idea is to go to an ideal altitude where the engine and airframe efficiency points are the same. As a surprise, I was at 14,000 in my -7 and was surprised to find the cruise TAS a little better than 8000 and yet burning a lot less fuel. I did richen a little to match the TAS but was burning 6.4 GPH rather than 8.0 and on mags, no timing advance.

Sorry engine guy here not aero. :p
 
Thanks for the responses.

I find it very interesting how a simple question about continuous low RPM operation has generated 5 pages of response, that have almost nothing to do with the original question (which was answered by the 3rd response).

Not complaining-----just fascinated by the wide knowledge base in the VAF Brain Trust.

Keep the info coming folks, great opportunity for learning.
 
Just for the sake of being a little difficult, the drag curves that would actually apply would be for the airframe without concern for power.

It would take a whole other thread to discuss how to actually find the best glide speed for your aircraft. See Jack Norris's articles for why.

I can suggest some very close approximation techniques but I will not hijack this thread. PM for anyone interested.

Best engine-out glide is NOT the same, btw.
 
We came up with a best glide of 84 knots, with normal caveats - engine at idle, pencil drawn curve, and as accurate as we can fly. Also, as I mentioned, since we didn't produce curves for the speed/fuel flow data, the actual Carson number could be hiding in between somewhere. But it was close enough to the calculated. In actual cruise I like speed. If I'm drilling around locally to put time on I play around with slower speeds/lower fuel flow ($6.50/gallon)...

==dave==
N102FM
 
Ron

Carson's original paper (AIAA-80-1847) was worked entirely in TAS, and did not mention IAS. However, his results can be converted to IAS/CAS if desired.

In order to obtain the Vcs/Vbr = 1.316 ratio you quote, he explicitly made the following assumptions:

a. parabolic drag polar
b. constant BSFC over the airspeed range
c. constant prop efficiency
d. nil wind

Since our aircraft and flight tests probably don't exactly match these assumptions, it's unlikely the "Carson's speed" we obtain from flight test will exactly match 1.316 x the best range speed we obtain from flight test. However, for an aircraft with a constant speed prop it should be very close.

To be more general, mavbe we should just look for the speed that minimises (fuel flow / TAS^2) and call it "high speed cruise".
 
best glide

Just for the sake of being a little difficult, the drag curves that would actually apply would be for the airframe without concern for power.

It would take a whole other thread to discuss how to actually find the best glide speed for your aircraft. See Jack Norris's articles for why.

I can suggest some very close approximation techniques but I will not hijack this thread. PM for anyone interested.

Best engine-out glide is NOT the same, btw.

Typically, an idling engine will produce drag. I have been too "chicken" to test this much. But in my Moni motorglider it was like a kick from the rear when I was able to stop the prop. In an RV-12 recently it was clear that the idling engine was producing thrust so much so that the mechanic had turned the idle down a little below specs just to make it better for landings. There is no way to know without experimentation.

Best glide can best be approximated from least-sink which can be tested with the very tricky use of minimum power for level flight. The best glide is then 1.32, roughly times that speed. I have done a bit of verifying with the help of some pilots who are better at this than I am and it was within 1 knot of predictions. The way I did the predictions is beyond the scope of this.

Caution: the best glide I am describing is great for some theoretical determination of Carson's but is NOT the best glide when the engine quits. For that, see POH.
 
Summarizing my previous posts, if you fly at 132% of your best glide speed, you will be able to fly to your destination with less fuel consumed.

I’m sorry Ron, this just isn’t correct. A look in any certified aircraft’s POH will show max range speed - which is defined as flying as far as possible on a fixed amount of gas, or in other words, flying a fixed distance on as little gas as possible - is much less than Carson’s speed. Carson sought to minimize two things, that you pay for in one way or another: gas, and everything else. For example, if you had a dry lease on an airplane, you’d want to minimize fuel costs and Hobbs hours. Carson assigned equal weighting to both these factors (when Carson was writing, I think he should have weighted Hobbs time more; at today’s gas prices, maybe equal weighting is more appropriate) to come up with an operating speed that minimized both these costs, weighted equally. There’s nothing magic here. If you apply the same logic to a wet lease, you’d fly as fast as possible, since you only pay for time. And if you have a self-maintained RV that costs nothing for maintenance/depreciation (never true!), you’d fly at best range speed, to minimized fuel costs, if saving money was the goal. In the real world, I think most of us choose to spend more on gas to go faster, e.g., we value our time more than the gas cost. So I really don’t understand all the hoopla about this. iMHO it’s just an interesting calculation for one set of preferences.
 
Ron

Carson's original paper (AIAA-80-1847) was worked entirely in TAS, and did not mention IAS. However, his results can be converted to IAS/CAS if desired.
Carson states in his paper on page 2 that his study focused on the L/D ratio (and ultimately L/Dmax), which is derived from the airflow over the airframe, which is indicated airspeed and results in the Best Glide Speed, and is generally provided in units of KIAS for most certificated aircraft, labeled as Vbg. The only reference to True Airspeed in the paper is Fig. 3 where he demonstrates cruise efficiency vs Cruise Optimum Airspeed.

This is a helpful quote from Prof. Dave Rodgers paper based on B.H. Carson's (both papers referenced with previously provided links in this thread):

"First, as shown in Table 1, remember that the true airspeed for L/Dmax increases with increasing altitude. Hence, the Carson cruise true airspeed also increases with increasing altitude."


You can calculate your Vbg for each altitude in terms of TAS, and then multiply by 1.316 to get your optimum cruise speed, or you can just use the stated (or even better, tested) Vbg for your airframe and then adjust TAS accordingly for your cruising altitude, which seems simpler to me, but either way gets you there. And it's still based on L/D for the airframe.

Bob, you're right, I meant to say "more efficiently", which I did actually say earlier, proving I should stop repeating myself...

True, if you fly at Vbg the entire time, you will use less fuel per hour, but the point is to get you to your destination with the most efficiency, resulting in spinning said "Hobbs meter" less...hence the Carson number. Post #41 is a good real-world reference.
 
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