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Navigation flying and the Triangle of Velocities

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Navigation flying and the Triangle of Velocities

Old 4th Mar 2013, 21:59
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Navigation flying and the Triangle of Velocities

I am very close to my Navigation exam and the one thing I am unable to get to grips with is the Triangle of Velocities and working them out.

I have the "Microlight Pilot Hand Book", but the way it is explained and set out just increases my confusion.

If any of you in the same boat as me, but have figured them out, or if you are a mechanical genius, I would really appreciate some help in understanding these.

C

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Old 4th Mar 2013, 22:07
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There are 3 vectors in the wind calculation:

Wind speed and direction
Heading & IAS (or more accurately TAS)
Track and ground speed

If you know 2 of these you can calculate the third, but remember that a vector has both magnitude and direction.

In the usual flight planning problem, the wind velocity vector is known (from forecast), but the other two are actually unknown, insofar as you will know your desired track but not the ground speed and you know your IAS but not your heading.

This is why the problem requires an iterative 2-step (or more) approach to solution - the "jiggle".

An approximation is made, based on the assumption that heading and track are initially the same. It is then necessary to enter the revised heading to refine / confirm the calculation. In strong winds or large angles between track and wind direction it may be necessary to do this more than once.

In effect you are solving the problem: given wind velocity, heading and IAS, what is your track and ground speed? You are assuming a value for heading, and then checking that the assumption is correct by solving the problem and ensuring that the answer is your desired track, and iterating as required until it does.

Also remember that you are always blown from heading to track, so if you are ever unsure about whether to add or subtract the drift, just look at the direction of the wind on the map and then see how it relates to your track line - which way will it blow your aeroplane, and will you need to increase or decrease your heading to compensate?!

Stick with it, it gets easier!

SD
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Old 5th Mar 2013, 08:18
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The easiest way to deal with this is to draw it out on a sheet of paper.

The problem as stated is that you have the wind vector which is a best guess and two more vectors where in one you know the direction (your desired track) and in the other your speed (TAS) so you are trying to find the two missing numbers Heading and Ground speed.

Start by drawing your track A-B on a piece of paper. The direction represents your track measured with a protractor but you don't know how long it should be. Start with a guess and use the TAS to be proportional to the line length. From the destination B you can draw the wind vector, remembering that you are interested in where the wind comes from not where its going to i.e the reciprocal. Mark C as one hours worth of wind up that vector.

Now with a pair of compasses scaled to represent the true airspeed draw an arc from C back towards A. Where it cuts the track line AB you have a new position A1

The distance A1- B is proportional to Groundspeed and the direction A1 -C is the Heading. Remember to use the same scale for all measurements.

The advantage of drawing it is that it makes it easier to comprehend. Once you understand it you will find it quite easy to calculate the missing numbers on the Wizz Wheel which does nothing more than your compasses did on the paper.

Amended to simplify explanation

Last edited by Whopity; 6th Mar 2013 at 10:05.
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Old 5th Mar 2013, 10:43
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This link may help Conor.

276 (Chelmsford) Squadron Air Training Corps

Scroll down to the bottom 'Subjects and Resources' -> Air Navigation

Click on Air Navigation Part 2, it's a powerpoint presentation.

The other Air Navigation parts may be of interest, I didn't check those.
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Old 5th Mar 2013, 16:08
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b2vulcan

May I beg to differ about the usefulness of that link.

It makes the subject sound difficult when it should be kept simple, it wastes times with unnecessary analogies, it uses terminology that could create the wrong mental image (an aircraft being "blown" off course), and the introductory illustration of the triangle of velocities is wrong!

2 s
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Old 6th Mar 2013, 15:03
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The easiest way to deal with this is to draw it out on a sheet of paper.
No it isn't.

A navigation computer is faster.

Otherwise, and better and faster, just use pure experience and you will be within a gnats' cock of the solution.
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Old 6th Mar 2013, 15:12
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A navigation computer is faster.
Not the talking kind I take it?!
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Old 6th Mar 2013, 15:12
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We weren't talking about speed, the issue was one of understanding. The triangle of velocities is an easy enough concept, but if as the first poster stated:
or if you are a mechanical genius, I would really appreciate some help in understanding these.
he is clearly having a problem with the navigation computer. If he can learn to work it out without one, then he can learn to speed up with one. I would suggest that someone who is trying to get to grips with the Microlight Handbook is not in a position to rely on "pure experience" either.
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Old 6th Mar 2013, 15:35
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Not the talking kind I take it?!
Nah - I would talk to myself though:

"half a degree left and up five knots to 425 should do it".

Last edited by Lightning Mate; 6th Mar 2013 at 15:35.
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Old 6th Mar 2013, 16:03
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Conor,

I thought Saab Dastard's post was a particularly good explanation, but the best way to learn this is to have an instructor or another experienced pilot show you.

It is a difficult concept to learn by reading, but becomes very easy, when you are shown and practice it.
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Old 6th Mar 2013, 17:19
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up five knots to 425 should do it".
At 425 you max drift is only 1/7th the wind speed so you can largely forget about the triangle of V!
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Old 7th Mar 2013, 07:48
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At 425 you max drift is only 1/7th the wind speed so you can largely forget
about the triangle of V!
Not when you're trying to hit a target +/- 5 seconds.

Ask Wholigan - I'm sure he'll be along shortly......
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Old 7th Mar 2013, 15:23
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The thread was started by a student pilot who asked for some help with some very basic understanding. With the exception of Saab and Whopity who gave sound advice, it seems to have been hijacked by some willy-wavers.

2 s
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Old 7th Mar 2013, 17:01
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I found the Cosgrove book really made a bit of a meal of the whole thing, but drawing them out helped. The most important thing is to make sure you keep to the same scale, whatever scale you choose in order to fit the triangle onto the page.

Reading another book (I went with the big green 'Air Navigation' one) helped more, the Cossie just doesn't go into enough detail for my taste.
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Old 7th Mar 2013, 21:20
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I agree with 2 sheds, that ATC link has the diagram the wrong way around!

SD
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Old 8th Mar 2013, 08:19
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Some of the other material in that link is rather poor also - has all the hallmarks of keen amateurs. If there are specified training outcomes, how is it that the material has not been properly checked?

Apart from spelling errors - and apostrophes! - the illustration of the 4-stroke cycle is also wrong. The whole lot suffers from typical PowerPoint Problem, that the originators do not have clearly in their minds whether it is supposed to be a set of visual aids to teaching, or a set of DIY learning material.

2 s
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Old 9th Mar 2013, 09:15
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The easiest way to deal with this is to draw it out on a sheet of paper.
Even easier is to draw it on a topographical chart as any good instructor would show you during navigation training.
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Old 9th Mar 2013, 22:23
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Thanks you SD
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Old 10th Mar 2013, 09:16
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Conor, it's worth mentioning that Saab Dastard's and Whopity's techniques are both valid, and both use the triangle of velocities, but solve the problem on the wind computer in different ways. (They are called "wind up" and "wind down" if I remember correctly).

The SD method requires a jiggle (typically you have to do the calculation twice) and Whopity's gives it to you in a single step.

You probably don't want to learn both at this stage (in fact I know ATPLs who refuse to learn the 'other' one) - your best bet is to get an instructor to show you one way and then stick to that.

Good luck!
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Old 10th Mar 2013, 17:13
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Thanks Whopity,

For fun & self education - it's been far tooooooo long since PPL - I drew your diagram.

CourseampWindTriangleCorrection100313.jpg Photo by mikehallam | Photobucket

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