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Fuel efficiency in headwinds


jaylw314

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I see the point...

Burning 8gph...

Taking whatever speed you get...

Near 140kts...

LOP...

Go from NJ to FL with few fuel used calculations...

You can do that with the O...

But mpg are really going to hurt when you put the throttle to the IP...

165kts, using 12gph LOP...14nmpg

175kts, using 15gph ROP... 12nmpg

And you can really use the blue knob ROP to see how much fuel can be flared.... to go 185 kts... @2700rpm

The Best part of this discussion is on how do you get decent wind info at various altitudes before and during your flight.

Best regards,

-a-

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19 hours ago, Andy95W said:

With stiff headwinds I worry more about if I'm going to make it to my destination before I have to pee... :rolleyes:

A reasonable aeromedical concern.  I myself rarely drink anything before a flight, especially one where I might have these sorts of concerns.  And of course that last thing before I spool up is always a trip to the loo.

Of course, if you take all these precautions and still have the need to relieve yourself before you get to your destination, either you need to plan shorter legs or go see your doctor  for that fun little bit of intimacy.

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With stiff headwinds I worry more about if I'm going to make it to my destination before I have to pee... :rolleyes:


I follow the 1 hour rule. Nothing to drink 1 hour before the flight and only drink something 1 hour before landing.

Of course, this timeline is dependent on the condition of your prostrate and whether or not you are a subscriber to the technologies of Depends, Travel Johns or the infamous “Piloto Pee Venturi”.


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16 hours ago, 201er said:

But now compare 145ktas@8.5gph vs 155ktas@10.0gph as a headwind is added on a 1000nm trip:

0kts headwind - 20nmpg, 145ktas, 145kgs, 6.9hrs or 15.5nmpg, 155ktas, 155kgs, 6.5hrs

10kts headwind - 15.9nmpg, 145ktas, 135kgs, 7.4hrs or 14.5nmpg, 155ktas, 145kgs, 6.9hrs

20kts headwind - 14.7nmpg, 145ktas, 125kgs, 8.0hrs or 13.5nmpg, 155ktas, 135kgs, 7.4hrs

50kts headwind - 11.2nmpg, 145ktas, 95kgs, 10.5hrs or 10.5nmpg, 155ktas, 105kgs, 9.5hrs

 

 

 

That's what I did for my analysis.  But looking at it from the OP's initial question, you can see that even with a 50 knot headwind, you still get better MPG at the slower speed.  So if you are really concerned about saving gas, not time, slow down.

Here's another set for you right out of my POH.  Although that's not the way I fly my airplane, and the numbers are probably not quite real world, it should be a pretty good evaluation.

8000' MSL, 2700 RPM, Economy Cruise Mixture, max gross weight:

75% power, 10.8 GPH, 169 KTAS

65% power, 9.7 GPH, 157 KTAS

55% power, 8.6 GPH, 145 KTAS

44% power, 7.4 GPH, 127 KTAS

No wind: 75% - 15.6 MPG; 65% - 16.2 MPG; 55% - 16.8 MPG; 44% - 17.2 MPG

10 Kt HW: 75% - 14.7 MPG; 65% - 15.2 MPG; 55% - 15.7 MPG; 44% - 15.8 MPG

20 Kt HW: 75% - 13.8 MPG; 65% - 14.1 MPG; 55% - 14.5 MPG; 44% - 14.5 MPG

30 Kt HW: 75% - 12.9 MPG; 65% - 13.1 MPG; 55% - 13.4 MPG; 44% - 13.1 MPG

40 Kt HW: 75% - 11.9 MPG; 65% - 12.1 MPG; 55% - 12.2 MPG; 44% - 11.8 MPG

50 Kt HW: 75% - 11.0 MPG; 65% - 11.0 MPG; 55% - 11.0 MPG; 44% - 10.4 MPG

So you can see that it doesn't pay, fuel consumption wise, to use higher power setting until you get a lot of headwind.  Interestingly though, I noticed a pattern for a rough gouge for my J.  In all cases, if I look at the highest power setting that gives me the best MPG, my ground speeds would be 127, 135, 125, 115, 117, and 119 KGS.  With the exception of 10 knots of headwind, they all cluster right around 120 knots ground speed.  So as a rule of thumb, I might opt to use whatever power setting it takes to get about 120 knots ground speed... if I can do that using 75% power or less.

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So after some research and time spent in a place to be unnamed, I've come to the following conclusions:

  • Power required IS in fact proportional to the cube of airspeed.  I was incorrect, I forgot to factor in propeller efficiency, which decreases with increasing airspeed.
  • I did some research on theoretical models of power required (induced plus parasite drag) to get a more complete picture of power required, and it seems that induced drag is inversely proportional to airspeed, and parasite drag is proportional to the cube of airspeed, so I added those together.
  • Carson speed is where the property of  "power (or fuel flow) per unit airspeed" is minimized.  It's already established this is 1.32 x best glide or maximum range airspeed.   Double checking my derivates, I confirmed that the best power per airspeed is at the 4th root of 3 times best glide, or 1.32
  • When tried to figure what where the property "power per unit GROUNDspeeed" is minimized, I got...okay, it all fell apart.  I did the derivative but the groundspeed term made the solution impossible to solve for airspeed, so I cheated and used a graphing calculator, but still no go.  There does not appear to be an equivalent Carson's for groundspeed that has a simple relationship with best glide or headwind.

Hmm, disappointment.  I suppose that shouldn't be surprising, or else someone would have already figured it out.  I know I can do it by brute force using the numbers from the POH, but I was hoping there'd be a theoretical answer.

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If your normal cruise speed is 90, roughly best glide...

your Carson (sp?) speed 1.3x 90, is roughly 120...

This allows you some room to speed up to fight a 30kt head wind... 120 + 30, 150kts airspeed, with 120kts ground speed...

Realistically, most of us NA mooniacs are flying around at WOT, battling the clock...

This leaves nothing left to better battle a head wind...

Use your technology (radio or onboard weather), find where the wind is least and fly at that altitude... keep you eyes on the FF... and out the window for trees... :)

Speaking of technology, Do you have a FF gauge and totalizer? Important for this type of flight, unless there are a lot of airports available on your path...

I think what made this discussion interesting decades ago.... is they were really flying slowly to begin with... a 50kt headwind took up 50% of their ground speed...  using their logic made sense back in the day...

Best regards, 

-a-

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I do it the okd fashioned way--battle headwinds at known power settings, check ground speed; if too low, check wind against Winds Aloft forecast, vary altitude looking for improved ground speed . . . . My worst day was 105 knots at 4000 msl, dropping to 68 knots at 10,000 clearing terrain, back up to 95 knots at 6000 before turning and look leaping past 150 knots while slowing for the approach. 

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I have never been successful in finding an altitude that improved my ground speed.

First up, my TAS drops as I descend. A few times I have found that while my headwind dropped my TAS dropped more.

Next, the winds are variable. So you are never sure it was the altitude that made the difference. Maybe it is just where you are?

Third, lower is bumpier. And I don’t like that.

Finally, I find the winds aloft forecast quite inaccurate. I am generally happy as long as they are not 20 kt worse than forecast.

So now I pretty much just pick the altitude I want (high) and get the speed I get.

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Larry do you have a preferred cruising altitude for your Encore?

I like the... ‘higher is better’ value that is available to Mooneys with compressors... (TCs and TNs)

Flying high where the air is thin enough, do they give an ‘equivalent’ wind speed? As in the headwind isn’t as strong because the air isn’t as dense...? :)

It has got to be cool selecting a high power setting in a low resistance environment.

Best regards,

-a-

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I cruise at 11/12k for a short 1 hour trip. 16/17 for 2+ hour trip. Adjust for IFR/VFR and direction of course. Lots of mountains in the west where I live so you have to fly pretty high anyway. But even on our cross country trip in the east this year I flew the same altitudes. On just about every leg on that trip as we climb through 10k the bumps smooth out. And every time descending through 10k it gets bumpy.

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As I look at the POH and see the 50 year old now-some-what-theoretical air speeds, I wonder: If my real word airspeeds don’t match my POH airspeeds, how sure can I be that my best glide speed is accurate (and am I sure that is the number to multiply by 1.32).

I really like the Carson speed theory, but I bumped upon the possible problem of the accuracy of best glide as I calculated my numbers this morning.

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Best glide is going to be based on total weight...there is a chart in the POH and the speed varies by 10kias from fully loaded to fully empty. From 90 to 80 or so...

Find the thread where somebody compared glide speed to distance in their Mooney... under various conditions... including pulling the prop vs. stopping the prop...

He had a blast gathering data...

Was that puddles, Dallas Ashley ??? (Old memory, kind of fuzzy)

Best regards,

-a-

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7 minutes ago, 211º said:

As I look at the POH and see the 50 year old now-some-what-theoretical air speeds, I wonder: If my real word airspeeds don’t match my POH airspeeds, how sure can I be that my best glide speed is accurate (and am I sure that is the number to multiply by 1.32).

I really like the Carson speed theory, but I bumped upon the possible problem of the accuracy of best glide as I calculated my numbers this morning.

You can always do flight testing for your airplane.  Develop your own power settings and speeds.  Determine your own best glide speed.  Set the power at idle and maintain different speeds.  Write down the sink rate for every speed you try.  Minimum sink rate will give you max endurance speed but best glide will be a bit higher.  Use the sink rate, TAS, and trigonometry to determine which speed gives you the shallowest descent angle.

And remember, best glide speed, just like stall speed will vary with weight.

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So, a little spreadsheet math and:

HeadwindFuelComparison.thumb.jpg.193c6eeef8faf8e9825ab6e03b6ca238.jpg

For a 250 nm flight with a 30 knot headwind there's less than $2 in 100LL A $5/gal between 75% and 44% power.  At 44% power the 250 nm would take 47 min longer.   If you want more hours in your logbook that's fine.  Personally, I'd pay the $1.76 more in 100LL and get there sooner.  :D

HeadwindFuelComparison.xlsx

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So, a little spreadsheet math and:
HeadwindFuelComparison.thumb.jpg.193c6eeef8faf8e9825ab6e03b6ca238.jpg
For a 250 nm flight with a 30 knot headwind there's less than $2 in 100LL A $5/gal between 75% and 44% power.  At 44% power the 250 nm would take 47 min longer.   If you want more hours in your logbook that's fine.  Personally, I'd pay the $1.76 more in 100LL and get there sooner.  
HeadwindFuelComparison.xlsx

I assume you meant GS, not TAS. TAS remains the same no matter the wind direction.
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22 minutes ago, m20kmooney said:

Carson's speed gives the best speed to fuel flow ratio. Not to be confused with best mpg speed. So if the goal is to get there without overly wasting fuel fly Carson's speed.

"I didn't buy a Mooney to fly slow." A frequent refrain whenever Carson's Speed comes up. Few if any Mooneys fly at such low power very often. 

The discussion here is what to do when facing strong headwinds. Wayne has some good numbers, I'm curious how well they hold up in the Real World. 

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30 minutes ago, Hank said:

"I didn't buy a Mooney to fly slow." A frequent refrain whenever Carson's Speed comes up. Few if any Mooneys fly at such low power very often. 

The discussion here is what to do when facing strong headwinds. Wayne has some good numbers, I'm curious how well they hold up in the Real World. 

In a normally aspirated Mooney, assuming little/no wind, on a long cross country the goal is to fly as high as it takes to accomplish Carson’s speed at full power! That will be the maximum TAS for the buck you can get. It will end up something like 120kias at 60% power at like 12000ft for a TAS of about 150ktas on 8gph.

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11 hours ago, 211º said:

As I look at the POH and see the 50 year old now-some-what-theoretical air speeds, I wonder: If my real word airspeeds don’t match my POH airspeeds, how sure can I be that my best glide speed is accurate (and am I sure that is the number to multiply by 1.32).

I really like the Carson speed theory, but I bumped upon the possible problem of the accuracy of best glide as I calculated my numbers this morning.

I can see cruise speed not meeting POH speeds after 50 years, but I HOPE the shape of the airframe hasn't changed even after 50 years to affect best glide speed!;)

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2 hours ago, Wayne Cease said:

So, a little spreadsheet math and:

HeadwindFuelComparison.thumb.jpg.193c6eeef8faf8e9825ab6e03b6ca238.jpg

For a 250 nm flight with a 30 knot headwind there's less than $2 in 100LL A $5/gal between 75% and 44% power.  At 44% power the 250 nm would take 47 min longer.   If you want more hours in your logbook that's fine.  Personally, I'd pay the $1.76 more in 100LL and get there sooner.  :D

HeadwindFuelComparison.xlsx

Nice!  I have to say, though, the main reason I started the idea in the first place was to maximize the amount of fuel reserve I had left after a long trip in headwinds rather than the cost savings.  Fuel is cheap, but fuel remaining is priceless...

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As mentioned winds aloft are suspect. On my trip this week to Texas from Delaware the headwinds were nasty and inconsistent. My spread sheet or notes are in the plane, trying to make Tx nonstop and before darkness. My Bravo like higher sky’s, at 8000 ft,low for Bravo’s the winds were say 20+, going to just 10000 the winds increased by a whopping 17 knots my true airspeed was up 6-8 knots, 12000 ft 30+ more increase versus true increase by another 6-8, ended up at an inefficient 6000 ft with headwind component, of 20 knots headwind. Point being winds are differing many times it becomes trial and error to find the best place. Now I’m basing this on turbo planes which results in differing ideas.

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