Max drift calculation
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Max drift calculation
Hi all,
Just a quick question, does anyone know or have a trusted method or rule of thumb to find out the max drift of the wind velocity when working out crosswind components. Say W/V 280/25 what would be the max drift?? and how do you work it out?
Many thanks
Just a quick question, does anyone know or have a trusted method or rule of thumb to find out the max drift of the wind velocity when working out crosswind components. Say W/V 280/25 what would be the max drift?? and how do you work it out?
Many thanks
Why not draw the wind on your Prayer Wheel, move the slide up and down, rotate the bezel and you will find that you can work it out for yourself! That way you will understand the relationship between drift and airspeed.
calculating max drift
learnt this one off my cfi and it works a treat
windspeed divided by tas in miles per minute
tas 120 knots i.e 2 miles per minute wind 360/20
20/2 = 10degrees max drift
now this is a bit confusing but great if i can explain it well enough without a picture
you takeoff off on a flight with a pre calculated max drift of 10 degrees in the above example
you turn onto a track of 330 for example on the dg. look to where the wind is coming from and drop the line straight up or down as applicable. if it falls half way along the horizontal from the centre of the dg you have half of 10 as your drift, if it falls 2/3 the way along you have 2/3 of the 10 as your drift etc. try it, i guarantee it will be accurate to within 1 degree of what you get off your whizz wheel. cheers glen
windspeed divided by tas in miles per minute
tas 120 knots i.e 2 miles per minute wind 360/20
20/2 = 10degrees max drift
now this is a bit confusing but great if i can explain it well enough without a picture
you takeoff off on a flight with a pre calculated max drift of 10 degrees in the above example
you turn onto a track of 330 for example on the dg. look to where the wind is coming from and drop the line straight up or down as applicable. if it falls half way along the horizontal from the centre of the dg you have half of 10 as your drift, if it falls 2/3 the way along you have 2/3 of the 10 as your drift etc. try it, i guarantee it will be accurate to within 1 degree of what you get off your whizz wheel. cheers glen
glenb.
I agree, it's an excellent technique but (unfortunately) not widely taught. If you go horizontally from the wind vector until you hit the vertical centre line you can do the same thing for head/tail wind component as a fraction of windspeed.
Then divide the groundspeed by 10 and you have the distance flown in 6 minutes.
Sooooooo easy. All solved and not a wizz wheel in sight.
HFD
I agree, it's an excellent technique but (unfortunately) not widely taught. If you go horizontally from the wind vector until you hit the vertical centre line you can do the same thing for head/tail wind component as a fraction of windspeed.
Then divide the groundspeed by 10 and you have the distance flown in 6 minutes.
Sooooooo easy. All solved and not a wizz wheel in sight.
HFD
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I've posted a reply to this question somewhere else on a forum but cannot think where.
I have an idiots guide, that has proven useful to new PPL guys, to simplified navigation using max drift and clock codes. Email me and I'll look it out and forward it to you.
I have an idiots guide, that has proven useful to new PPL guys, to simplified navigation using max drift and clock codes. Email me and I'll look it out and forward it to you.
See: http://www.pprune.org/forums/showthread.php?t=208745
Max drift = Wind velocity / (TAS in miles per minute)
Then apply the relevant 'clock factor' - i.e. if the angle between the wind and track is 15 deg use 1/4 max drift, 30 deg use 1/2 Max drift, if it's 45 deg, use 3/4, 60 or more use all of it.
Unfortunately you will read nonsense such as '3 deg per 5 kt' - please ignore that and use the correct method as described above.
Max drift = Wind velocity / (TAS in miles per minute)
Then apply the relevant 'clock factor' - i.e. if the angle between the wind and track is 15 deg use 1/4 max drift, 30 deg use 1/2 Max drift, if it's 45 deg, use 3/4, 60 or more use all of it.
Unfortunately you will read nonsense such as '3 deg per 5 kt' - please ignore that and use the correct method as described above.
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Correct. We are touching on vector "arithmetic" here but because such a mathematical title will scare the majority of people away unless they have a grounding in Mathematics, Physics or Engineering, all sorts of devices have been invented to prevent people from realising what they are really doing but enabling them to get the answer that they require.
Basically, for those who don't already know there are two types of quantity: scalar quantities which only have magnitude, like mass or volume or time and so on and vector quantities which have both quantity and direction. The most important one for pilots is velocity, because we need to know in what direction our aircraft will travel and at what speed over the ground - a vector quantity. The answer is the sum of two other vector quantities: these are the direction that the aircraft is pointing plus the speed at which it is travelling through the air and the speed of the wind and the direction that the air is travelling. When two vector quantities are added we need to take into account the directions of these vectors as well as their magnitudes.
Think of it another way: the aircraft is moving through the air in a certain direction at a certain speed. The air itself is also moving at a different speed and in a different direction, and it is the combination of these quantities (the result when we add them) which gives us our heading and groundspeed.
Now pity our poor groundborne bretherin, the boating fraternity. When they do similar sums they have to take into account the speed and direction of their boat, the speed and direction of the wind and the speed and direction of the tide! No wonder our forebears got frequently "uncertain of their position" before Mr. Harrisson invented his seagoing clock to enable them to work out their longitude!
P.P.
Basically, for those who don't already know there are two types of quantity: scalar quantities which only have magnitude, like mass or volume or time and so on and vector quantities which have both quantity and direction. The most important one for pilots is velocity, because we need to know in what direction our aircraft will travel and at what speed over the ground - a vector quantity. The answer is the sum of two other vector quantities: these are the direction that the aircraft is pointing plus the speed at which it is travelling through the air and the speed of the wind and the direction that the air is travelling. When two vector quantities are added we need to take into account the directions of these vectors as well as their magnitudes.
Think of it another way: the aircraft is moving through the air in a certain direction at a certain speed. The air itself is also moving at a different speed and in a different direction, and it is the combination of these quantities (the result when we add them) which gives us our heading and groundspeed.
Now pity our poor groundborne bretherin, the boating fraternity. When they do similar sums they have to take into account the speed and direction of their boat, the speed and direction of the wind and the speed and direction of the tide! No wonder our forebears got frequently "uncertain of their position" before Mr. Harrisson invented his seagoing clock to enable them to work out their longitude!
P.P.
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BEagle is correct in that nav solutions start with TAS. The confuser goes from plotting TAS against W/V and course. It then resolves groundspeed and drift.
If using max drift - actual drift using clock method for MDR you have to divide W/V by groundspeed (nm/minute) to get Max Drift.
If using max drift - actual drift using clock method for MDR you have to divide W/V by groundspeed (nm/minute) to get Max Drift.
Not so.
Max drift occurs when the wind direction is at 90 deg to the track and is calculated from the ratio of TAS and wind speed - all the 'clock factor' for the proportion of max drift to use for the actual difference between wind direction and track is doing is approximating the front face of the computer by using 1in60 values rather than exact trigonometrical values.
Perhaps you're thinking of SCA correction?
Max drift occurs when the wind direction is at 90 deg to the track and is calculated from the ratio of TAS and wind speed - all the 'clock factor' for the proportion of max drift to use for the actual difference between wind direction and track is doing is approximating the front face of the computer by using 1in60 values rather than exact trigonometrical values.
Perhaps you're thinking of SCA correction?
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Max drift
Surely to avoid a circular arguement we need to agree the purpose of 'max drift'.
Based on W/V forcasts we calculate the heading to steer and groundspeed (time). We therefore minimise off track error. A major concern is that we do not infringe Controlled airspace, Danger/prohibited areas and other places nor unwittingly hit anything. We therefore consider an assumed off track worst case scenario in deciding on our intended track.
Radians devised by the ancient Babylonians (the marsh people of Iraq) worked this out for us thousands of years ago. In aviation we call it the 1:60 rule. BEagles 'clock code' as he saids is simply that, Wv v TAS, The change in heading by the pilot that then changes ground speed and will prevent drift has nothing to do with worse case scenario calculations.
You must decide the worst case scenario to use, be it; 15 degrees (1/4), 30 degrees (1/2), 45 degrees (3/4) and 60 degrees (the lot) but applied simply. Max drift needs to be no more complicated than that.
Based on W/V forcasts we calculate the heading to steer and groundspeed (time). We therefore minimise off track error. A major concern is that we do not infringe Controlled airspace, Danger/prohibited areas and other places nor unwittingly hit anything. We therefore consider an assumed off track worst case scenario in deciding on our intended track.
Radians devised by the ancient Babylonians (the marsh people of Iraq) worked this out for us thousands of years ago. In aviation we call it the 1:60 rule. BEagles 'clock code' as he saids is simply that, Wv v TAS, The change in heading by the pilot that then changes ground speed and will prevent drift has nothing to do with worse case scenario calculations.
You must decide the worst case scenario to use, be it; 15 degrees (1/4), 30 degrees (1/2), 45 degrees (3/4) and 60 degrees (the lot) but applied simply. Max drift needs to be no more complicated than that.
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This is how I was taught by the RAF on my EFT flying course,
before you take off calculate your max drift, by your miles per minute divided by wind speed, eg 120 knots 2 miles per min/20 knot wind.
Then for your heading look at your wristwatch. wind is up to 15 degrees off your heading take a 1/4 of max drift. if it is up to 30 degrees off take 1/2 max drift. if it is 30-45 degrees of use 3/4 of max drift, and any thing more use all max drift.
but remember to apply it in the corretion in the correct direction!
If the aircraft is equipped with a HSI you can set the heading bug on your course and turn the bar to the wind direction and at a glance it maked the above method so easy.
Wizz wheels are not needed, I was never taught the use of one in the RAF and was never off a target/waypoint by more than 20 seconds, passed my nav test by being 4 seconds fast..oh hapy days!
before you take off calculate your max drift, by your miles per minute divided by wind speed, eg 120 knots 2 miles per min/20 knot wind.
Then for your heading look at your wristwatch. wind is up to 15 degrees off your heading take a 1/4 of max drift. if it is up to 30 degrees off take 1/2 max drift. if it is 30-45 degrees of use 3/4 of max drift, and any thing more use all max drift.
but remember to apply it in the corretion in the correct direction!
If the aircraft is equipped with a HSI you can set the heading bug on your course and turn the bar to the wind direction and at a glance it maked the above method so easy.
Wizz wheels are not needed, I was never taught the use of one in the RAF and was never off a target/waypoint by more than 20 seconds, passed my nav test by being 4 seconds fast..oh hapy days!
What? The RAF doesn't even teach knowledge which even a 17 year old PPL student is expected to know?
Ridiculous. Of course you should know how to use a navigation computer!
Perhaps it's time to re-examine the credit given to EFT graduates for the PPL navigation exam.....
Ridiculous. Of course you should know how to use a navigation computer!
Perhaps it's time to re-examine the credit given to EFT graduates for the PPL navigation exam.....
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seriously they do not, I completed EFt about 2 years ago and there is not computer at all on it, the nav is much more indepth at PPL level, on EFt a lot more is taught so that you can react quicker to changes in flight and spend less time with your head down with a wheel. But then again it is geared to put pilots in the cockpits of a fast jet and how cares about drift at 450 knots!
You do not use a wheel at all, not for fuel/time/headings/it is all done in your head. which i feel is enough personally. everyone develops there own nav methods and i never use a wheel now.
You do not use a wheel at all, not for fuel/time/headings/it is all done in your head. which i feel is enough personally. everyone develops there own nav methods and i never use a wheel now.
In flight - of course head-out mental work is needed at low level.
But for pre-flight planning, you MUST know how to use a nav computer!
I understand that the Royal Navy and Army do still teach such basic aviaition skills - why doesn't the RAF?
No wonder some service pilots have had such difficulty learning how to navigate a PA28 accurately at 2500ft.....
But for pre-flight planning, you MUST know how to use a nav computer!
I understand that the Royal Navy and Army do still teach such basic aviaition skills - why doesn't the RAF?
No wonder some service pilots have had such difficulty learning how to navigate a PA28 accurately at 2500ft.....
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but why? when we was taught we start off doing medium level navs, about 3000 ft, then work down to 500ft (used to be 250 in the days of the dog) we use regular check points spaced no more than 5-8 mins apart, use HAT checks before and after each checkpoint (Heading, Altitude, Time), you know the wind before you take off and the clock method of max drift is accurate enough. When I completed my PPL the wheel was the part of the course I was dreading most, when I used the wheel on the ground I then re worked using the clock method and the results were nearly always the same. I can honestly say that when I was on the squadron I know of not one pilot getting lost on any nav, on my ppl completion it was a regular having a stude lost!! You don't even do the route with the instructor then fly it yourself you just plan and go on your own.
In my flight test for my ppl i did the nav just like i was taught in the RAF and at the end the instructor told me " I can see you have flown before because none of my instructors know how to do what you did in the air, and you did it better than them!" proof of the pie!
In my flight test for my ppl i did the nav just like i was taught in the RAF and at the end the instructor told me " I can see you have flown before because none of my instructors know how to do what you did in the air, and you did it better than them!" proof of the pie!
What pre-flight method (using MDR) are you taught in order to calculate your groundspeed of at an IAS of 120 kts at 3000 ft with an OAT of, say +5 deg C and a QNH of 1013?
Track 245 deg M, W/V 285/20 - what GS would you use?
Track 245 deg M, W/V 285/20 - what GS would you use?
Last edited by BEagle; 7th Feb 2006 at 15:15.