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The most economical speed in a 30 kt headwind seems to be slow.


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As far as I can tell, my airplane (fuel injected Foxbat) has the following perfomance and econonomy figures

• 5000 rpm     17 L/hr     82 kt     0.21 L/NM

• 4600 rpm     13 L/hr     75 kt     0.17 L/NM

• 4000 rpm     10 L/hr     65 kt     0.15 L/NM

 

That means with a 30 kt headwind...

• 5000 rpm      17 L/hr      52 kt     0.33 L/NM

• 4600 rpm      13 L/hr      45 kt     0.29 (0.2889) L/NM

• 4000 rpm      10 L/hr      35 kt     0.29 (0.2857) L/NM 

 

So, unless I have messed up the maths, even for an aircraft that is not streamlined at all, the most economical speed will be slow almost irregardless of the wind. 

 

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You're forgettting that you are also paying for maintenance by the hour, so going faster and reducing the flying time may actually be more economical.

If your hiring aircraft on a wet hourly basis, fast is cheap.

 

Edited by RossK
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My concern was actually getting to my destination, with a head wind, and not running out of fuel. My understanding is that Rotaxes are happiest being flown at above 5000 rpm (although I suppose that with decreased revs there will be decreased torque.) This has nothing to do with saving money - not that you could tell from the heading for the thread. I was referring to fuel economy and therefore range. 

Edited by APenNameAndThatA
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If you like I have pushed your fixed data through my planning sheet based on what you have.  I plan only 100hrls and engine replacement and I have guessed the cost of a 912i

nil wind = 4600rpm wins on cost/nm just over the 4000rpm

image.thumb.png.b8b9b23aca86a1b57be6e5a8ac0d24c7.png

 

30kn headwind = 4600rpm wins on cost just over the 5000rpm

image.thumb.png.3c7084de9d31d742c685e1cdf2a7d8a9.png

 

Were this me I all I do is ignore the $/nm and work from cruise power setting and work out if I can get there with planned winds and available fuel.

 

For example,

My 912 trike has a std 80hp 912 and is drag limited so I am only able to run 4,300rpm and get 11lph for 60knts - any more revs and I see rapidly declining return in speed for the fuel due to drag.

So I am always looking at my distance based on 3.5hrs fuel (plus reserve) in the two seat configuration which limits me to 210nm still air planned if I take a passenger

Any more and I have to ditch the passenger and install 'George' the fuel tank in the rear seat ... adds 6hrs to my fuel so I can then plan up to 570nm still air.

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When planning I've generally chosen  a gound NM/say 10 lbs of fuel. unless you have to make up time. Generally with a headwind you go faster as you get blown by the wind for less time that way. If you have a headwind that's say 1/2 of the TAS your total flight time will double. and your range may limit you. Nev

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2 hours ago, kasper said:

If you like I have pushed your fixed data through my planning sheet based on what you have.  I plan only 100hrls and engine replacement and I have guessed the cost of a 912i

nil wind = 4600rpm wins on cost/nm just over the 4000rpm

image.thumb.png.b8b9b23aca86a1b57be6e5a8ac0d24c7.png

 

30kn headwind = 4600rpm wins on cost just over the 5000rpm

image.thumb.png.3c7084de9d31d742c685e1cdf2a7d8a9.png

 

Were this me I all I do is ignore the $/nm and work from cruise power setting and work out if I can get there with planned winds and available fuel.

 

For example,

My 912 trike has a std 80hp 912 and is drag limited so I am only able to run 4,300rpm and get 11lph for 60knts - any more revs and I see rapidly declining return in speed for the fuel due to drag.

So I am always looking at my distance based on 3.5hrs fuel (plus reserve) in the two seat configuration which limits me to 210nm still air planned if I take a passenger

Any more and I have to ditch the passenger and install 'George' the fuel tank in the rear seat ... adds 6hrs to my fuel so I can then plan up to 570nm still air.

that is next level!

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You are forgetting one of the most important things, the slower you are going the longer you are affected by the headwind so you wiil therefore use more fuel than you will going fast because you are affected by the headwind for longer

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1 hour ago, FlyBoy1960 said:

You are forgetting one of the most important things, the slower you are going the longer you are affected by the headwind so you wiil therefore use more fuel than you will going fast because you are affected by the headwind for longer

Yes and no.  The quick table I put through with fixed costs added ($20/hr) takes account of time and fuel burn.  The other way to look at it is to graph the cost per NM travelled over the ground against the headwind for the three power settings to see the difference:

image.png.bf3ca8e053c1172c5f3634c8b1a9a8b0.png

As can be seen up to around 20 knts headwind power is making bugger all different to $/nm but after 20 knts the low power setting becomes quite a lot more expensive.

But again ... who wants to be flying for 'fun' in more than 20knts of wind?

Plus its generally a plannable distance vs headwind that is more the problem for ultralights ... with a fixed fuel available of 90L in a foxbat the real importance is how far you can get on your endurance ... lets leave 9L for reserve and look at 81L ...

image.png.56ddddfbc20e80930fb0cf7ac03268a1.png

This shows that lowest power will give you longest plannable range for all headwinds up to 25knts  ... but who wants to be doing that as you are up there for longer to get there and I really prefer to run my engines in a higher cruise power setting as temps stay nicer and I have a better flight ... my bladder will not hold out comfortably for 5hrs to go 400nm at 85 let alone 5hr20 to do the same at 75 ... with more than 40L in the tank I am really looking at how many legs in total before I have to refuel on tankage and just fly legs that suit my need for fun and my requirement to be comfortable between hamburgers and toilet stops.

 

 

 

 

Edited by kasper
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On 22/10/2021 at 9:52 AM, FlyBoy1960 said:

You are forgetting one of the most important things, the slower you are going the longer you are affected by the headwind so you wiil therefore use more fuel than you will going fast because you are affected by the headwind for longer

Why on earth would you say that I forgot something when I just showed you the maths that demonstrate that it is not true? 

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