drift formula

[jlfd26]

Hi everyone,

I need a little help from you guys: I've been searching for the last 2 and a half hour the formula to insert in an excel spreadsheet to calculate the drift angle and the groundspeed.

I had it last year but i lost my spreadsheet and i can't seem to find the formula again on the net

thanks to all

[Beefitarian]

There might be one here.
http://williams.best.vwh.net/avform.htm

[jlfd26]

This is not the king of drift i need

[Beefitarian]

Are you not trying to find values in a wind triangle?

[FlyGy]

Perhaps?

http://answers.yahoo.com/question/index ... 143AATUFKy

[jlfd26]

tried with the links posted and still no conclusive results

if someone wants to give it a try PM me I can't post it here becasue .xlsx is not allowed

[FlyGy]

Maybe these guys can help, they were working on it 5 years ago.

http://www.aircraftpilots.com/threads/w ... ions.1724/

otherwise I can offer no other help.

[AuxBatOn]

Hi everyone,

I need a little help from you guys: I've been searching for the last 2 and a half hour the formula to insert in an excel spreadsheet to calculate the drift angle and the groundspeed.

I had it last year but i lost my spreadsheet and i can't seem to find the formula again on the net

thanks to all

Basic Trigonometry dude.... Come on....

[TTJJ]

Actually it is not basic trigonometry, especially for the ground speed.
eg. With a 90 degree cross wind you would expect the ground speed to be the same as the TAS. That is not the case. There is a loss in ground speed due to you having to correct into the wind for drift. TAS 120 and 90 degree X-wind of 50 = GS 109
Adding Head or Tail wind components to the TAS almost never adds up to the GS.

As such:
TC = True Course
TAS= True Airspeed
GS=Ground Speed
Wd= Wind Direction (True)
Ws=Wind Speed (Must use the same units as the TAS)
WCA= Wind Correction Angle
^ = Raised to the power of…

WCA= ASIN(Ws/TAS*SIN((Wd-TC)/180*PI()))/PI()*180
Positive WCA is to the right and Negative WCA is to the left

GS= SQRT(TAS^2+Ws^2-2*TAS*Ws*COS((Wd-TC-WCA)/180*PI()))


For your spreadsheet you will need to have cells something like this starting at the top left corner of the spreadsheet:

TC, Dist, TAS, Wd, Ws, Mag Var, E or W, WCA, GS, HD True, HD Mag, ETE, ETD, ETA

So: WCA=ASIN(E2/C2*SIN((D2-A2)/180*PI()))/PI()*180 This formula goes into H2
GS= SQRT(C2^2+E2^2-2*C2*E2*COS((D2-A2-H2)/180*PI())) This formula goes into I2

You can check your answers for the above calculations with this E6B computer simulator:
http://www.vandeenensupport.com/projects/rs/index.html

During an extremely slow sex week I made a spreadsheet (actually 5 of them in 1) that does this.
You can download it here.
http://www.fileden.com/files/2009/6/10/ ... 201.0.xlsx

It is more Lat/Long orientated, but if you scroll down to the number “5 sheet”, it is for a typical small aircraft log.

An interesting thing you can do with the spreadsheet is see how small a difference there is between Rhumb line and a Great Circle distances in a flight from Vancouver to Toronto for example.

If you connect any two points you use for the spread sheet and plot them on Google Earth, you will se that the graph values for the Great Circle Lat/Longs are correct.

You copy the airport Lat and Long from the pages at the bottom and paste them in the top left hand corner of the spreadsheet. They are in alphabetical order per country.

note. There is one known error using the database for distance calculations. If a destination is within 1 degree South of the Equator the spreadsheet assigns a positive value to the "minutes". If someone wants to correct this, have at it.
There are probably some other ones as well, but what the hell, it is free.

I really have to get a life...

[AuxBatOn]

Actually it is not basic trigonometry, especially for the ground speed.
WCA= ASIN(Ws/TAS*SIN((Wd-TC)/180*PI()))/PI()*180
Positive WCA is to the right and Negative WCA is to the left
GS= SQRT(TAS^2+Ws^2-2*TAS*Ws*COS((Wd-TC-WCA)/180*PI()))

So: WCA=ASIN(E2/C2*SIN((D2-A2)/180*PI()))/PI()*180 This formula goes into H2
GS= SQRT(C2^2+E2^2-2*C2*E2*COS((D2-A2-H2)/180*PI())) This formula goes into I2

Which is basic Trigonometry...

[Colonel Sanders]

This isn't going to work where you're flying in a gale wind, but ...

Break the wind down into relative headwind/tailwind and crosswind components. You don't need trig for that, just use the chart in the CFS.

Apply the headwind/tailwind using subtraction or addition to get your groundspeed.

Use the 1-in-60 rule for the crab angle. For small values of theta, adjacent is the same as the hypotenuse, so you don't need to worry about the effect on your groundspeed.

And that's it.

Now obviously, if you're flying a 172 in 100 knot winds this isn't going to work very well, but if you try something that dumb, frankly navigation is going to be the least of your problems.

[Colonel Sanders]

Let's try a real-world example, using my simple estimation method.

You're travelling north at 120 knots TAS and the wind is 25 knots out of the northwest.

You can use the CFS chart or either remember that the sin and cos of 45 degrees is 0.7 so the headwind component is (25 x .7) = 17 knots and the crosswind component is also 17 knots.

Your groundspeed is therefore 120 - 17 = 103 knots.

Using the 1/60 rule, you need to crab left into the wind 17/2 = 8 degrees, so a heading of 360-8 = 352.

There. I did that in a matter of seconds in my head. Someone want to crank the numbers through an electronic E6B to 10 digits and see how far off I was?

[FlyGy]

I come up with a GS of 101 and a heading of 352. Thanks Colonel, I'd never heard that before.

[RenegadeAV8R]

You're travelling north at 120 knots TAS and the wind is 25 knots out of the northwest.

You can use the CFS chart or either remember that the sin and cos of 45 degrees is 0.7 so the headwind component is (25 x .7) = 17 knots and the crosswind component is also 17 knots.

Your groundspeed is therefore 120 - 17 = 103 knots.

Using the 1/60 rule, you need to crab left into the wind 17/2 = 8 degrees, so a heading of 360-8 = 352.

There. I did that in a matter of seconds in my head. Someone want to crank the numbers through an electronic E6B to 10 digits and see how far off I was?

Using a battery powered CX-2:
Heading = 351.5 deg
Ground Speed = 101.0 knots

[Colonel Sanders]

I can't set or hold a heading to a half a degree, so
that's a wash, and I had 2 knots of error on 101
which is two percent so that's a score of 100% and
98% for a combined score of 99% accuracy

Learning how to precisely estimate is a very valuable
skill for a pilot. It allows you to notice when the fancy
stuff goes off the rails.

[Big Pistons Forever]

Or you could just turn until the GPS track is the same as the bearing to the station, and the read your ground speed......

Says the guy with a Biology degree.......

[Colonel Sanders]

I hear ya. These days, my primary nav is often my cell
phone on top of the dashboard.

[RenegadeAV8R]

I can't set or hold a heading to a half a degree, so
that's a wash, and I had 2 knots of error on 101
which is two percent so that's a score of 100% and
98% for a combined score of 99% accuracy

Learning how to precisely estimate is a very valuable
skill for a pilot. It allows you to notice when the fancy
stuff goes off the rails.

Your values were spot on (who cares about 2 knots?). You asked for "10 digits", but my CX-2 only gave me 1 digit after the decimal point

[Meatservo]

This isn't going to work where you're flying in a gale wind, but ...

Break the wind down into relative headwind/tailwind and crosswind components. You don't need trig for that, just use the chart in the CFS.

Apply the headwind/tailwind using subtraction or addition to get your groundspeed.

Use the 1-in-60 rule for the crab angle. For small values of theta, adjacent is the same as the hypotenuse, so you don't need to worry about the effect on your groundspeed.

And that's it.

Now obviously, if you're flying a 172 in 100 knot winds this isn't going to work very well, but if you try something that dumb, frankly navigation is going to be the least of your problems.

I agree with this. The real answer is, in fact, basic trigonometry, but these "rules of thumb" are even better. It shows a basic knowledge of one's environment and is more than acceptably accurate. Some people try too hard and others don't try at all. Rules of thumb are there for everyone. Too few people are aware of them.

[TTJJ]

All very interesting indeed, and yet the OP (with a total of 7 posts I might add) poised a simple question as to how he/she might acquire a WCA and GS formula for a spreadsheet. Perhaps it is a school project or possibly a mental exercise. Who cares? A direct question was posted by a fellow AvCanadian.

Whatever the reason for seeking an exact number, would not eloquence require they not be dissed, nor that we be dismissive of their question?

My fear is that newbies and elder pilots alike with aviation related questions will avoid posting them here to avoid ridicule thereby perpetuating doubts, myths and errors.

I believe we owe them more, don't you?