optimised fuel burn when in headwinds

[Ultralights]

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?

[apache]

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.

[ForkTailedDrKiller]

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.With digital fuel flow and GPS (two of the best things since sliced bread!), you can skip the maths. A little bit of experimentation with power settings will answer your question and give you the confidence to either continue to your destination or head for a tech stop.

Dr :8

[VH-XXX]

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.

[cficare]

Without a GPS the general rule would be...go for a higher power setting (increased speed) resulting in less time suffering the headwind!!

[VH-XXX]

That sounds like the same concept for when your car is running out of fuel. Drive faster and you will get there quickly. :ugh:

[Checkboard]

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:

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%.
MAXRNG (http://www.db.erau.edu/research/cruise/piston.frame.html)

(* 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.)

[mcgrath50]

Thanks Checkboard, a really good explanation!

[Jabawocky]

Checkers.........does that work for the red rockets in the garage? :ok:

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 :ok:. 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!

[boocs]

Checkboard (or is/was it Checkerboard??)

As only you and I know from those Ayers Rock -Perth flights of a previous life, many many moons ago....

Rgds.,
b.

[boocs]

Go low and fast seemed to always work... does that compute??
b.

[Chimbu chuckles]

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.

[Checkboard]

(or is/was it Checkerboard??)
Nope - this is the only name I have ever used on PPRuNe, in over 10 years.

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.

[43Inches]

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.

[Checkboard]

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. :ok: (Regardless of wind!)

[glekichi]

Low can work if it gets you out of a strong HWC...fast never does.

Jeez Chimbu thats a big statement! Forgotten about gliding after an engine failure? :E

[VH-XXX]

So you're still saving money by flying fast

That point is quite valid. Also comes into consideration with the cost of fuel versus engine rebuild. A mate has always said in his RV7 that he can bang along at 120 knots at 18 litres an hour but the extra hours cost more at rebuild time.

[Tankengine]

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?:E

Reduce the wind and the theory still applies, computers help with the maths but basic understanding is still a good thing.:ok:

[ForkTailedDrKiller]

Chimbu, I usually agree with you but here I think you are wrong.

Cough, cough .......... WHAT did you say? :eek:

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 :8

[Brian Abraham]

I don't think the graph requires explanation.

http://i101.photobucket.com/albums/m56/babraham227/q0004.jpg

For interest Louise Sacchi is a lady who made a living doing trans oceanic GA ferry all over the world. This is her analaysis

http://i101.photobucket.com/albums/m56/babraham227/q0005.jpg

You can she recommends 58% being the sweet spot - she seemed to specialise on Beech Bonanza and Baron.

[desmotronic]

http://www.nar-associates.com/technical-flying/range/wind_wide_screen.pdf

or see page 161 Aerodynamics for naval aviators.

[Chimbu chuckles]

Tank engine I think you need to reread my post-I assumed people had the brains to be able to apply a head wind to my anm/lt figures and come up with a gnm/lt figure by themselves.

Yes, if your starting point is LRC, then going a LITTLE faster into a headwind gives you better gnm/lt. if your starting point is highspeed cruise you need to slow down.

In my example the anm/lt increased from 2.75anm/lt to 3.2anm/lt as power was decreased from 75% to 65%-apply the same HWC to each power setting and if you were hurting for FOB at destination which power setting would you chose?

If you were already at 65% and looking a like eating into you FR would you be better served increasing power to 75% or deceasing it back to 60%?

[Tankengine]

No worries Chimbu, I think we all sometimes post knowing what we mean but not what people read into it.;)
The strength of the wind verses speed of aircraft is the key,
remember the first poster was talking about an ultralight!:E

[motzartmerv]

Yes, and we all know that ultralights are subject to a totally different set of aerodynamic laws and principles.:ugh:

[Jabawocky]

Merv
Yes, and we all know that ultralights are subject to a totally different set of aerodynamic laws and principles.:ugh:

Stop banging your head mate.

A 20 knot headwind is a big deal in a Savannah or most UL aircraft, but to a 16o-180 knot GA machine its more annoying than anything else.

Do the numbers and think before you post a comment like that one, because around here you will be flamed out pretty quick!:}

[Brian Abraham]

Jaba, I think merv was serving a dose of irony.

[motzartmerv]

Yea, chill out Jaba.:ok:

[Tankengine]

Wot Jaba said.:rolleyes:

I WAS referring to an ultralight's relative lack of speed.

A 20kt headwind affects my glider more than at work in an A330! ;)

[Ultralights]

a 20 kt headwind is significant in a Savannah, but so is a 60 kt headwind in a 172/210/pa28.... whats relevent is the headwind as a percentage of cruise speed.

[cficare]

VH-XXX seems to have gone quiet on the subject...having said (as his input to the discussion).."That sounds like the same concept for when your car is running out of fuel. Drive faster and you will get there quickly.".........

[motzartmerv]

Don't forget the.. :ugh:

[Checkboard]

VH-XXX also posted #17, pay attention!
Drive faster and you will get there quickly.
What's your problem with the statement? Drive faster and you will get there quickly. ;)

[motzartmerv]

Backpeddling gets you there quickly too... :E

[Brian Abraham]

Backpeddling gets you there quickly too...But only if you're pointed in the right direction.

[aussie027]

Checkboard and desmotronics's articles have some interesting general rules of thumb.
A few people have spent time applying the theory to the AFM figures to extract accurate data. Some AFMs will have detailed range,endurance charts that factor in power, weights and winds and they can then be used to extract the data.
All good if done in advance in spare time if you think a flight may specifically need such exact figures and aircraft performance.
In the real world of normal ops other factors such as time to work the numbers, company policy and cost usually end up trumping any finer theoretical aspect of operating technique. Usually some reasonable trade off of all the factors or a predominant factor is the way to go.
Years ago a pilot told me a first hand account of a guy he flew with in United who decided when planning one day after looking at all the winds aloft reports etc to fly a Boeing airliner from San Francisco to Seattle at 18-20,000' as it resulted in no HWC or a TWC for the whole trip compared to the horrendous HWC in the usual 30-40K altitude range. He worked out from AFM that speed, ETI and burn off was far better so he and the dispatcher worked a plan, it was accepted and away they went. Flight worked out exactly as he had expected and was perfectly legal in every way.
Later when the FAA and senior United Flt Ops staff realized what he had done there was an initial over reaction and Q to be answered as they didn't consider operating a heavy jet airliner down at those altitudes "prudent".

Nowadays with the bean counters running everything and trying to make operational and planning policy you would think he might have been commended for actually saving time, fuel and therefore money on the trip.:hmm: