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Thread: Tail winds

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    Tail winds

    If you have a 9 mph wind directly on your tail your ground speed increases 9 mph. If that same wind is 90 degrees to you then your ground speed in not increased at all. So for every 10 degrees the wind comes from your 90 degrees towards your tail does your ground speed increase 1 mph?

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    frequent_flyer's Avatar
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    Quote Originally Posted by The Kid View Post
    If that same wind is 90 degrees to you then your ground speed in not increased at all.
    A wind 90 deg to your course line results in a decrease in ground speed on course.

    Quote Originally Posted by The Kid View Post
    So for every 10 degrees the wind comes from your 90 degrees towards your tail does your ground speed increase 1 mph?
    No.

    To dig a bit deeper review "wind triangle" and "dead reckoning" in any private pilot training manual or an internet search. Engineers will think in terms of vector diagrams and trig solutions but it's more usual for student pilots to use graphical solutions.
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    stewartb's Avatar
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    It depends on your surroundings. Mechanical turbulence trumps all the flat land theories. It’s one of those “local knowledge” items.

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    Quote Originally Posted by stewartb View Post
    It depends on your surroundings. Mechanical turbulence trumps all the flat land theories. It’s one of those “local knowledge” items.
    I should have stated this is for landing, not cruising.

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    stewartb's Avatar
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    Landing and takeoff are what I was addressing. On a 1000’ strip some days a 10 mph tailwind is a bargain. It depends on the surroundings.
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    frequent_flyer's Avatar
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    Quote Originally Posted by The Kid View Post
    I should have stated this is for landing, not cruising.
    My answer is unchanged. It's not a flatland theory. It would be a simple mathematical solution except that the wind is often unknown and may change rapidly in magnitude and direction.

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    BC12D-4-85's Avatar
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    Taylorcraft have a built-in tailwind - fast cruise and float long on landing.

    Gary
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    Gordon Misch's Avatar
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    Frequent is correct. Work it out on your E6B to confirm.

    Edit: here's one way to visualize it - if you have a crosswind from the right you're pointed to the right of your direction of travel over the ground to compensate. So part of your airspeed is not pointed the direction you're going, and is "wasted".
    Last edited by Gordon Misch; 07-08-2022 at 11:36 PM.
    Gordon

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    BC12D-4-85's Avatar
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    As noted - vector analysis. Almost everything except direct behind or ahead makes us correct for wind angle most often w/o thinking.

    Gary
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    Excelent question, Excelent responce!! DENNY

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    BC12D-4-85's Avatar
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    GPS' ETA or ETE offers info providing desired course selected is followed in wind. Eats wind vectors plus GS and discharges the estimates. How did we ever survive flying without so much info? I guess it was something called pilotage, familiarity with the aircraft, and charts with desired course lines drawn on them while VFR. The Age of Thumbs and devices can replace that if needed. Never trust anything but your learned skills.

    Gary
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    frequent_flyer's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    . Never trust anything but your learned skills.
    I fly a PA-28 with stone age avionics. I flew thousands of miles cross country in gliders in the days before GPS. I now have an FX-3 with Garmin G3X Touch. Among its many feature is a real time display of current wind. I like having it and find it very useful.

    I still look out the window and compare what I see with what the avionics tells me. Recently G3X showed 30 kt headwind on close in base leg but windsock mid field showed about 5 kts cross wind. Which was right? They both were. The wind can howl down that river valley soon after sunrise but the strip is sheltered from that wind. Can be nasty turbulence on short final though.
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    cubdrvr's Avatar
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    Crawl into a bean bag chair with a bowl of ice cream..........turn the TV up loud.........and don't think about it
    "Sometimes a Cigar is just a Cigar"
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  14. #14

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    I am going to simply repeat frequent above:

    Skip the E6b and Foreflight for an hour, and sit down and learn wind triangles. Nobody does that any more, but it is the best way to visualize what is going on.

    For those with a math background some trigonometry will help dispel that rather interesting idea that you gain a knot for every ten degrees. It is non-linear - with the wind at 45 degrees you will still enjoy a .707 boost.
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    BC12D-4-85's Avatar
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    https://en.wikipedia.org/wiki/Wind_triangle

    I was taught if landmarks on the ground are recognizable (and location doesn't require periodic star or sun shots and time), draw a line for the desired course on a chart and correct heading during cruise to hold the planned course. Note any winds affecting ETA + or - at periodic checkpoints. Correct ETE, ETA, and drift during flight using actual groundspeed and heading. Often.

    Now it's simply follow the GPS' red brick road and read the data display. Or tap on the display a new waypoint and add to the flight plan.

    Gary
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    frequent_flyer's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Seems to be quite a good reference but it seem to miss the very important point that, for the vector diagram to be valid, all vectors must be True or all vectors must be Magnetic.

    Quote Originally Posted by BC12D-4-85 View Post
    Now it's simply follow the GPS' red brick road
    Usually referred to as "the magenta line" and those that blindly follow it and trust automation without thought as "Children of the magenta line". Magenta is the color used to depict the course line and other FMS derived data on air transport navigation displays.
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    CubCruiser's Avatar
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    Here is a straight ahead wind/groundspeed problem for newer pilots that may help with understanding the effects of wind on ground speed/fuel management:


    You make a round trip to a destination 100nm miles away, at 100 knots true airspeed, in no wind conditions. It takes 2 hours flight time.


    Now, put in a 10 knot tailwind going there, and the exact same wind, but now a headwind, on the return leg. Does the round trip still take 2 hours?


    100/110x60 = 54 minutes going there
    100/90x60 = 67 minutes coming back

    Of course if you’re Steve Pierce, you get a tailwind both directions.
    Daryl Hickman, ATP, CFI, XYZ, PDQ
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  18. #18

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    And that’s because the headwind has a longer period of time to act on the aircraft, and the tailwind a shorter period of time. Simple explanation from a Naval Aviator a long time ago..
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    Steve Pierce's Avatar
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    Quote Originally Posted by cubdrvr View Post
    Crawl into a bean bag chair with a bowl of ice cream..........turn the TV up loud.........and don't think about it
    Fly the airplane. Land on several gravel bars that are one way in and one way out. You start getting a feel of what you can and can't get by with depending on wind speed and direction.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
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    Quote Originally Posted by bob turner View Post
    For those with a math background some trigonometry will help dispel that rather interesting idea that you gain a knot for every ten degrees. It is non-linear - with the wind at 45 degrees you will still enjoy a .707 boost.
    I think we all can look at pictures even if the trig was a few more years ago for some. This is a plot of what Bob is talking about, actual non linear curve in red and a reference straight line in blue. 0 on the bottom axis means a wind directly at your back and 90 blowing directly to one side or the other. In reality you will always be faster than a hypothetical 1mph per 10deg.

    Click image for larger version. 

Name:	Cosx.png 
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    Attachment 61701


    In the words of a longtime friend "Tailwinds make my landings more spectacular"Attachment 61700

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    Quote Originally Posted by WayneK49 View Post
    I think we all can look at pictures even if the trig was a few more years ago for some. This is a plot of what Bob is talking about, actual non linear curve in red and a reference straight line in blue. 0 on the bottom axis means a wind directly at your back and 90 blowing directly to one side or the other. In reality you will always be faster than a hypothetical 1mph per 10deg.
    Attachment 61700

    That's a nice plot of Cos(x) which shows how the tail wind component varies. However, it doesn't show what happens to the on-course ground speed because it doesn't take account of the fact that the airplane must turn toward the wind to maintain course.

    I think a full illustration needs a plot of GSdelta for fixed TAS, fixed wind magnitude, and wind direction varying between direct cross and direct tail.

    I started but I got distracted.

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    JimParker256's Avatar
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    Quote Originally Posted by CubCruiser View Post
    Of course if you’re Steve Pierce, you get a tailwind both directions.
    And if you're me, you get the headwind in both directions...

    Why is it that the percentage of headwind flights seems to increase exponentially as the cruise speed of your airplane decreases? On my longest cross-country flight in many years, bringing home an airplane that cruised at 90 knots, I had 30-35 knot headwinds for the entire flight. And it was an Eastbound flight where you would expect to have tailwinds...
    Jim Parker
    2007 Rans S-6ES

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    Steve Pierce's Avatar
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    90 kts, mine won't go 90 mph but on a good day. Went 120 mph to Arkansas last Wednesday and came home yesterday at 110 mph. I need to buy a lottery ticket before whatever I got wears off.
    Steve Pierce

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    I remember going in to Amarillo - I had just enough fuel to make it to tradewinds, if AMA would let me through. 30 kt headwind; J-3 Cub.

    Big mistake - called them too early. I was maybe 10 miles out, which was maybe 20 minutes if I was lucky. They gave me a delaying vector for a learjet on a 12 mile final. By the time I got through with that, I had enough fuel for AMA.

    The good thing about that was - there were a pair of really attractive female Air Force F-5 pilots on the ramp, getting ready for the rest of their cross-country.

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    I don't understand the graph. But at the 45 degree angle the wind in a linear progression would be 4.5 mph of tailwind but the 7.07 trig calculation is saying more like 6.36 mph at the 45? Does it slow up then in rate as it only has 3.64 more mph to go in another 45 degrees?

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    Quote Originally Posted by The Kid View Post
    I don't understand the graph. But at the 45 degree angle the wind in a linear progression would be 4.5 mph of tailwind but the 7.07 trig calculation is saying more like 6.36 mph at the 45? Does it slow up then in rate as it only has 3.64 more mph to go in another 45 degrees?
    The slope of a parameter plot shows the rate of change of that parameter. Look at the slope of the red line at any point.

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    Quote Originally Posted by frequent_flyer View Post
    That's a nice plot of Cos(x) which shows how the tail wind component varies. However, it doesn't show what happens to the on-course ground speed because it doesn't take account of the fact that the airplane must turn toward the wind to maintain course.

    I think a full illustration needs a plot of GSdelta for fixed TAS, fixed wind magnitude, and wind direction varying between direct cross and direct tail.

    I started but I got distracted.
    I got back to it and realized the formula is just the same as the relationship between piston stroke and crank rotation - https://www.vcalc.com/wiki/EmilyB/Po...to+crank+angle

    So a bit more complicated than a simple Sine or Cosine and takes some Pythagoras too. If I got it right in the spreadsheet the result are as shown below. Note that a 90 deg crosswind reduces on course ground speed as predicted.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	GS change with wind.png 
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ID:	61733  
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    That graph really has laid it out. I can understand, I think, as I extrapolated the headwind into a tailwind for my purposes. For my application it is just slightly off, I'd say, because I don't crab into the wind when landing with a tailwind on a one way, up hill, short and no go-around strip. I drop a wing and keep aligned with the runway. What it tells me is that there is slightly more than 1 mph per 10 degrees shift in the wind. On this graph, at the 45 which should be about 4.5 miles per hour less tailwind (using the 1 mph per 10 degrees approach) it looks like there may be about a 6.5 mph tailwind for me but again, I am not crabbing into the wind so I think it would be slightly less? For safety sake I think I'll estimate the wind and figure it is all 180 degree tailwind and not land when it's more than what I think I can deal with there as wind does guest and change directions quickly.

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    RVBottomly's Avatar
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    Quote Originally Posted by bob turner View Post
    For those with a math background some trigonometry will help dispel that rather interesting idea that you gain a knot for every ten degrees. It is non-linear - with the wind at 45 degrees you will still enjoy a .707 boost.
    Hah! I still remember my trig from high school. Sines/cosines of 30, 45, 60 degrees stuck with me. Unit circles were fun.

    Not hard to remember 10 and 15 degrees, either. It was fun figuring wind components in my head quicker than my instructor could get his calculator out of his pocket. Plus or minus a knot or two is still more precise than the info we get from a windsock.




    Sent from my iPhone using SuperCub.Org mobile app

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    Cool, I wouldn't have thought of it being the same as the piston thing but it makes sense. Having a more accurate graph probably won't make my tailwind landings any less spectacular any time soon but it is interesting to have a more accurate solution.

  31. #31

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    I don't personally do sand bars - my off-airport work has always been semi-necessity. So I should stay out of this - but I routinely do quartering tailwinds right up to maybe 15 knots. I show them to my students when I can, then recommend against them. They can be deadly if you get a little crooked, and no computation can save you. Worse, with big soft tires.

    My guess is that if you have the actual wind velocity to the nearest knot when doing sand bars, you are way, way ahead of the rest of us. I fly out of the third busiest GA airport in the world, and cannot get them to install a wind sock at the approach end.

    We have three different winds - tower reported, wind sock, and touchdown zone - and in windy conditions none of those are even remotely the same.

    Good taildragger pilots feel the wind - Cherokee pilots ask for wind checks each mile or so.

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