optimised fuel burn when in headwinds
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optimised fuel burn when in headwinds
One time I was puttering up the Murrumbidgee Valley south of canberra, over Bredbo, and I looked at the Bunyan glider strips next to me and it looked like I wasn't even moving. I checked my airspeed, it was 90 kts, and I checked my ground speed on the gps and it was 40 kts. I was enroute to Jindabyne, and while I initially had a little over an hour reserve, I thought there was a good chance I would run it dry before I ever got there if I was only making 40 kts on the ground. I thought it through, then decided that I was in a constriction of the valley that was catching a lot of wind and forcing it through a small area resulting in a locally higher wind speed. I decided to give it full throttle to get past the narrow spot to see if the wind abated. If it did, I would continue to Jindy, if not I would head Cooma for some more of the blessed av juice. About five minutes later, the headwind reduced to 30 kts, and so I continued on to Jindy, landing with exactly 45 min reserve and only a mild cramp in my puckered sphincter.
Since that day I've always wondered about fuel efficiency and headwinds. According to my POH, the more you pull the throttle back, the higher the miles per litre. However, that obviously can't be true in a strong headwind. If you're flying into a 100 kt headwind at 100 kt airspeed, you just stay in one place until you run out of fuel. If you give it a little more throttle you actually make some headway before you run out of fuel, so it's obviously more fuel efficient to open the throttle more in a headwind.
And that leads to my question: How *much* should you increase your fuel burn in a headwind to achieve maximum miles per litre?
Since that day I've always wondered about fuel efficiency and headwinds. According to my POH, the more you pull the throttle back, the higher the miles per litre. However, that obviously can't be true in a strong headwind. If you're flying into a 100 kt headwind at 100 kt airspeed, you just stay in one place until you run out of fuel. If you give it a little more throttle you actually make some headway before you run out of fuel, so it's obviously more fuel efficient to open the throttle more in a headwind.
And that leads to my question: How *much* should you increase your fuel burn in a headwind to achieve maximum miles per litre?
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i think the term you are looking for is SGR - or Specific Ground Range... which is G/S vs litres/kgs/lbs etc.
and that is the whole reason Maths is quite handy in the cockpit.
and that is the whole reason Maths is quite handy in the cockpit.
i think the term you are looking for is SGR - or Specific Ground Range... which is G/S vs litres/kgs/lbs etc.
and that is the whole reason Maths is quite handy in the cockpit.
and that is the whole reason Maths is quite handy in the cockpit.
Dr
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I concur that a fuel flow hooked up to GPS, such as 396/495 will give the results that you require. You can pick up a decent one from $300-$400 for the Rotax.
Whilst we all know that good maths is the answer, sometimes in times of stress the GPS / Fuel flow combo can be re-assuring, particularly if your passenger(s) is/are concerned, versus you just rattling off some numbers about how you will make it ok.
I like the MFD's (I think some some Avidynes etc have it) that have a "Range" line that constantly updates based on fuel burn and ground speed. Interesting to see the range decrease and increase as you adjust the power settings as the Dr. says.
Whilst we all know that good maths is the answer, sometimes in times of stress the GPS / Fuel flow combo can be re-assuring, particularly if your passenger(s) is/are concerned, versus you just rattling off some numbers about how you will make it ok.
I like the MFD's (I think some some Avidynes etc have it) that have a "Range" line that constantly updates based on fuel burn and ground speed. Interesting to see the range decrease and increase as you adjust the power settings as the Dr. says.
HEADWIND AND TAILWIND MANAGEMENT
The existence of a headwind or tailwind respectively decreases or increases range from the no wind conditions found in basic POH curves and tables. Whenever practical, speed adjustments should be used to optimize either condition. In particular, strong headwinds reduce range severely, especially when not managed correctly. The longer a headwind works on the airplane, the greater the damage. Conversely, prolonging the beneficial effects of a tailwind optimizes the advantage. Therefore, the rule is that headwinds call for increasing airspeed, and tailwinds for decreasing speed, compared to the no wind maximum range airspeed, VMR. Analysis of the POH based empirical data yielded the following practical rules:
Decrease TAS 1 knot for every two knots of tailwind component, but not below 0.8VLRC. Example: VLRC=120 KTAS and tailwind component is 36 knots. To maximize range, decrease speed to 120-.5x36=102 KTAS=.85VLRC. SR increases 3.5%, although enroute time is increased 13%.
The existence of a headwind or tailwind respectively decreases or increases range from the no wind conditions found in basic POH curves and tables. Whenever practical, speed adjustments should be used to optimize either condition. In particular, strong headwinds reduce range severely, especially when not managed correctly. The longer a headwind works on the airplane, the greater the damage. Conversely, prolonging the beneficial effects of a tailwind optimizes the advantage. Therefore, the rule is that headwinds call for increasing airspeed, and tailwinds for decreasing speed, compared to the no wind maximum range airspeed, VMR. Analysis of the POH based empirical data yielded the following practical rules:
HEADWIND RULES OF THUMB
- If cruising at or above VLRC*, do not adjust speed unless headwind component exceeds 25% of VLRC (TAS). Since VLRC=1.07VMR, minor headwind conditions receive automatic compensation.
- For each five knots that headwind exceeds the .25 VLRC threshold, increase cruise TAS 2 knots above no wind VLRC.
- Example: VLRC=120 KTAS and headwind component is 60 knots. "Excess" headwind is 60-.25x12O=30 knots. Therefore, cruise speed should be increased to 120+(6x2)=132 KTAS. Ground speed is increased from 60 to 72 knots, reducing enroute time by 17%, while fuel burned (per ground mile) is reduced 3.7%. A rare example of "having your cake and eating it too!"
TAILWIND RULE OF THUMB
Decrease TAS 1 knot for every two knots of tailwind component, but not below 0.8VLRC. Example: VLRC=120 KTAS and tailwind component is 36 knots. To maximize range, decrease speed to 120-.5x36=102 KTAS=.85VLRC. SR increases 3.5%, although enroute time is increased 13%.
(* VLRC = Long Range Cruise Speed for the type, VMRC = Max Range Cruise Speed. Long Range Cruise is a bit faster than Max Range, simply because in that area of the performance graph you can go 10% faster (saving quite a bit of time) for only 1% more fuel.)
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Checkers.........does that work for the red rockets in the garage? 
I had a former TAA/CX guy along for a couple of long legs on the weekend....ha asked with old school mental maths about the specific GS V fuel burn.....guessed about 4NM/Litre.
Bloody smart dude. The EMS did all that for me! Sure I can do it too but having good tools is so much better than guessing or worse!
I had a former TAA/CX guy along for a couple of long legs on the weekend....ha asked with old school mental maths about the specific GS V fuel burn.....guessed about 4NM/Litre.
Bloody smart dude
. The EMS did all that for me! Sure I can do it too but having good tools is so much better than guessing or worse!
Grandpa Aerotart
Hands up who flies around in a piston twin/single at LRC (50-55% power).
No one?
Then slow down into a headwind if fuel on board is a problem.
Prove it for yourselves - get your POHs and extract TAS and FF for 50/55/60/65/70/75% power. TAS/FF=anm/lt.
In a Bonanza/C210 going from 75% power to 65% power anm/lt go from circa 2.75 -> 3.2.....+15%.
Makes sense when think about it, -6% TAS for -16% FF.
Low can work if it gets you out of a strong HWC...fast never does.
No one?
Then slow down into a headwind if fuel on board is a problem.
Prove it for yourselves - get your POHs and extract TAS and FF for 50/55/60/65/70/75% power. TAS/FF=anm/lt.
In a Bonanza/C210 going from 75% power to 65% power anm/lt go from circa 2.75 -> 3.2.....+15%.
Makes sense when think about it, -6% TAS for -16% FF.
Low can work if it gets you out of a strong HWC...fast never does.
(or is/was it Checkerboard??)
There is also a "Checkerboard" but that isn't me. I did do a lot of Ayers Rock - Perth legs in the 146 for Ansett, though.
There has to be a decent headwind getting up around a third of your TAS before increasing speed and fuel flow will extend your range significantly.
The following from the Piper Lance POH, max weight, ISA, 6000ft, using BP mixture;
55% = 128TAS/52lph
65% = 143TAS/60lph
75% = 156TAS/68lph
Considering a 200nm flight (time is rounded);
55% nil wind = 1.6hr/81l, 40kt h/w = 2.3hr/118l, 60kt h/w = 2.9hr/153l
65% nil wind = 1.4hr/83l, 40kt h/w = 1.9hr/116l, 60kt h/w = 2.4hr/144l
75% nil wind = 1.2hr/87l, 40kt h/w = 1.7hr/117l, 60kt h/w = 2.1hr/142l
Converted to SGR and max range (355l max usable, no climb allowances and no reserves);
55% nil wind 0.41l/nm = 865nm range, 60kt h/w 0.77l/nm = 461nm range
65% nil wind 0.42l/nm = 845nm range, 60kt h/w 0.72l/nm = 493nm range
75% nil wind 0.45l/nm = 788nm range, 60kt h/w 0.70l/nm = 507nm range
For this aircraft from tail wind and up to about 20kts head wind 55% is good then 65% to around 50kts and then 75% above that will achieve the best range. Thats about 5KTAS for every 20kts wind.
On shorter legs with any headwind you are better off at 65%-75% as you get there faster for only very little difference in actual fuel used. When considering the 200nm leg your fixed reserve should be around 45l, even if you used the completely wrong power setting you would only eat 10l into that at worst.
Low is not always going to give you better range either, over land with local effects wind can be stronger at lower levels than high, especially in valleys or along the ranges. You also take a hit in TAS vs fuel flow at lower levels so the benefits need to be weighed up. I've seen many days with 30-40kts at 2000ft and 10kts at 10000ft. If the effect is localised due to terrain then climbing may also be an option. Once in the flight levels low into headwinds generally holds true.
The following from the Piper Lance POH, max weight, ISA, 6000ft, using BP mixture;
55% = 128TAS/52lph
65% = 143TAS/60lph
75% = 156TAS/68lph
Considering a 200nm flight (time is rounded);
55% nil wind = 1.6hr/81l, 40kt h/w = 2.3hr/118l, 60kt h/w = 2.9hr/153l
65% nil wind = 1.4hr/83l, 40kt h/w = 1.9hr/116l, 60kt h/w = 2.4hr/144l
75% nil wind = 1.2hr/87l, 40kt h/w = 1.7hr/117l, 60kt h/w = 2.1hr/142l
Converted to SGR and max range (355l max usable, no climb allowances and no reserves);
55% nil wind 0.41l/nm = 865nm range, 60kt h/w 0.77l/nm = 461nm range
65% nil wind 0.42l/nm = 845nm range, 60kt h/w 0.72l/nm = 493nm range
75% nil wind 0.45l/nm = 788nm range, 60kt h/w 0.70l/nm = 507nm range
For this aircraft from tail wind and up to about 20kts head wind 55% is good then 65% to around 50kts and then 75% above that will achieve the best range. Thats about 5KTAS for every 20kts wind.
On shorter legs with any headwind you are better off at 65%-75% as you get there faster for only very little difference in actual fuel used. When considering the 200nm leg your fixed reserve should be around 45l, even if you used the completely wrong power setting you would only eat 10l into that at worst.
Low is not always going to give you better range either, over land with local effects wind can be stronger at lower levels than high, especially in valleys or along the ranges. You also take a hit in TAS vs fuel flow at lower levels so the benefits need to be weighed up. I've seen many days with 30-40kts at 2000ft and 10kts at 10000ft. If the effect is localised due to terrain then climbing may also be an option. Once in the flight levels low into headwinds generally holds true.
Last edited by 43Inches; 14th Feb 2011 at 21:39.
If you can hire a Lance for $230/hr Dry and Fuel costs, say $2/ litre, then:
55% = 128TAS/52lph = $1.80/nm + $0.81/nm = $2.61/nm
65% = 143TAS/60lph = $1.61/nm + $0.84/nm = $2.45/nm
75% = 156TAS/68lph = $1.47/nm + $0.87/nm = $2.34/nm
So you're still saving money by flying fast. (Regardless of wind!)
55% = 128TAS/52lph = $1.80/nm + $0.81/nm = $2.61/nm
65% = 143TAS/60lph = $1.61/nm + $0.84/nm = $2.45/nm
75% = 156TAS/68lph = $1.47/nm + $0.87/nm = $2.34/nm
So you're still saving money by flying fast.
(Regardless of wind!)
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So you're still saving money by flying fast
Chimbu,
I usually agree with you but here I think you are wrong.
It is GS not TAS to look at.
Two identical aircraft flying at [say] 100kts @65% power into a 100kt headwind: One speeds up to 85% power to [say] 120kts:
Which one will make it to destination in 20 miles first and with the most fuel?
Reduce the wind and the theory still applies, computers help with the maths but basic understanding is still a good thing.
I usually agree with you but here I think you are wrong.
It is GS not TAS to look at.
Two identical aircraft flying at [say] 100kts @65% power into a 100kt headwind: One speeds up to 85% power to [say] 120kts:
Which one will make it to destination in 20 miles first and with the most fuel?
Reduce the wind and the theory still applies, computers help with the maths but basic understanding is still a good thing.
Chimbu, I usually agree with you but here I think you are wrong.
Cough, cough .......... WHAT did you say?
Hey, can someone send me one of those popcorn eating smilie things while I pull up a chair, pour myself a stiff Bundy and sit back and watch the action?
Dr
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I don't think the graph requires explanation.
For interest Louise Sacchi is a lady who made a living doing trans oceanic GA ferry all over the world. This is her analaysis
You can she recommends 58% being the sweet spot - she seemed to specialise on Beech Bonanza and Baron.
For interest Louise Sacchi is a lady who made a living doing trans oceanic GA ferry all over the world. This is her analaysis
You can she recommends 58% being the sweet spot - she seemed to specialise on Beech Bonanza and Baron.