Question stuck with on Nav in Air Pilots manual

[liam548]

It asks the question but gives example on how to work it out..

"if you wish to maintain a groundspeed of 80kts on track calculate the true heading and TAS required if wind velocity is 090/20 true and track is 180 degress true?"
how do I work this out on the old whizz wheel?

[whoswho]

Use the wind side of the Whizz-wheel.

(the following is the wind-down method)

1. Spin the rotating part (inner scale) so index points to 90 (wind direction)
2. Set the centre blue dot (wind index) over the 100 speed arc.
3. Mark (down the way) the wind speed in at 80 (80 because 100-20(wind speed) = 80)
4. Rotate the inner scale to set the true track, 180 (180 under index)
5. Move the vertical rectangle part down (in this case) until the mark you made sits on the 80 line. (80 because that's the groundspeed that you need to maintain)
6. Now you need to apply the drift, in this case it is right drift of 15 so rotate inner scale right by 15. Index now is 165. (you could've just done 180-15 also). The drift is shown as that's where your put the mark (step 3)
7. True heading is 165
8. Blue dot (not the one you made) is over the TAS which is 78kts.

Pretty sure this is the way to do it - I'm sitting my Nav exam on Sunday - if I'm wrong....oh b****cks.

I've have made a tutorial on how to do various calculations with the whizz-wheel and also complete a flight plan using it. PM me your email address if you want a copy.

Do you have the PPL Confuser? Its' got really good explanations on how to do various things with the whizz-wheel. Worth it.

Cheers,

[liam548]

thats great thank you. why does the book not explain it!! Ive got confuser but wanted to work through the book itself before using it.

Sitting nav very shortly too.
Ive used the OAT whizz wheel cd rom up to now and it does not have any questions like this, so will they be in the exam or not, thought the cd roms covered everything?



Liam

p.s you have PM

[Deeday]

Originally Posted by whoswho

Pretty sure this is the way to do it - I'm sitting my Nav exam on Sunday - if I'm wrong....oh b****cks.

Sorry to pee on your fire, but yes, you are wrong. To me this shows two things:

• Using the wind-down method for problems like this one (at least at PPL level) means asking for more complication.
• Using any method without having first a clear picture of the problem in mind means potentially getting it wrong without even realising it.

Think of the three vectors and draw them on paper, before doing anything on the wind computer. If you are tracking 180° with a wind from 090°, your heading will be a few degrees to the left of track, and in this particular case, with track and wind at a right angle, you are always going to have a (small) headwind component: how can your TAS be less than your GS It has to be slightly higher!
So, of the three vectors, here we have TR/GS and W/V, and need to find HDG/TAS. The way I'd do it is:

1. Lay down the TR/GS vector on the wheel's centre line. i.e. set 180° on the index and 80 kt under the centre dot. That's it, one down.
2. Next, the W/V vector will always blow us from the HDG/TAS towards the TR/GS vector, i.e. it will come from the left, from 90°, and point to the centre dot. To draw it, turn for a moment the wheel to get 90° under the index and mark 20 kt up from the centre dot. Put back the wheel to index 180°: two down.
3. That's it, you have your triangle of velocities: the HDG/TAS vector goes from the base of the wheel to the wind mark, which tells you that your Wind Correction Angle is 14° to the left (i.e. HDG = 166°, which you can read off the drift scale) and your TAS is 83 kt.

whoswho, what you got wrong is that you stopped too early: at point 8, where your read 78 kt of TAS, your wind mark indicates 75 kt, while it should always stay at 80 kt, because that mark represents the arrowhead of the TR/GS vector (which is 80 kt-long of course). Basically you have to go back to step 5 and repeat the steps until the solution doesn't change (have I already said that the wind-up method is quicker and easier? )
Hope that helps.

Deeday

[IO540]

No wonder most pilots flying for real go out and buy a GPS

[Deeday]

Just to clarify a point, in my previous post I mentioned a "small headwind component". Strictly speaking, given that wind and track are at right angle, there is neither head- nor tailwind, in this case. What I meant is that, given that wind and track are at right angle, the remaining side of the triangle, HDG/TAS, will always be longer than theTR/GS side, i.e. the True Air Speed will always be higher than the Ground Speed.

Is anybody still awake there? Well, I've finished now .

Deeday

[BOAC]

'Scuse em fer butting in, but unless HEADING (not track) and WIND are at 90 degrees there will always be a head/tail component.

If you are not convinced, apply a 'crosswind' of, say, 100kts and see how fast you go along track.

[Deeday]

unless HEADING (not track) and WIND are at 90 degrees there will always be a head/tail component.

I'm afraid you are getting confused more than I am...
I'm not sure that the head/tail component of the wind, relative to the heading, is of any interest. If you want to fly straight from A to B, what pushes you, or slows you down, is the head/tail wind component relative to your trajectory - i.e. track.
In other words: the HDG/TAS vector represents the motion of the aircraft relative to the air mass, while the W/V vector represents the motion of the air mass relative to the ground. The two vectors are completely independent of each other.
The aircraft, happily 'floating' in the air mass, doesn't give a toss where the air mass is going, and what the component of the W/V vector is, relative to its heading. The pilot, on the other hand, does give a toss, because he wants to navigate with relation to the ground, not to the air mass, so what the air mass is doing relative to the ground (= wind component along track) becomes important.

If you are not convinced, apply a 'crosswind' of, say, 100kts and see how fast you go along track.

Not sure what you mean mate, if you can expand it.

Deeday

[BOAC]

Head/tail is the difference between TAS and GS. Where I think you are getting confused is that the head/tail ON THAT TRACK is zero. It does not mean YOU have zero in your a/c. If you were in a car it would. You have, of course, the right answer anyway.

[IO540]

'Scuse em fer butting in, but unless HEADING (not track) and WIND are at 90 degrees there will always be a head/tail component.

If you are not convinced, apply a 'crosswind' of, say, 100kts and see how fast you go along track

I must confess to writing this without having thought about it in detail, but I think the case of a crosswind having a zero effect on one's speed along the desired track is when the wind is perpendicular to the vector sum of one's heading and the wind (or maybe the vector sum of one's track and the wind).

That's why using the circular slide rule (the wind calc side of it) in a particular way requires iteration, and the number of iterative steps increases as the wind becomes a higher % of one's speed.

[Deeday]

IO540, I think you are referring to this sort of situation:

where HDG/TAS and TR/GS have got the same length (or the triangle is isosceles, but I'm getting pedantic now) so that ground speed and airspeed are the same. In this case, the wind is perpendicular to the bisector - or equivalently, to the vector sum of heading and track.
Anyway, this is all to say that the only way to never get it wrong is to have always well clear in mind the vectors' triangle.

Cheers!

Deeday

[Jumbo Driver]

... or look at BOAC's point another way ...

Fly from A to B, then back from B to A, where the wind is 90° across track ... and you will have a headwind in both directions ...


JD

[MarkerInbound]

Think of it this way, if you're going 100 kts in a no wind situation you travel 100 nm. If the wind is 90 degrees off your course some of your 100 kts gets used compensating for the crosswind and you may only travel 97 nm on course. Even if the wind is 95 or 100 degrees off the nose your correction is going to cost you more than what you gain from the very slight tailwind. Therefore you can prove mathematically that you will have a headwind component more than 50 percent of time while you fly.

[Deeday]

Guys, if I figured out the way you are reasoning, you are saying that if TAS is greater than GS, we have headwind, if it's smaller, we have tailwind. Is that what you mean? I'm not sure that it is the correct definition.
I'm looking at this problem in the same way as a crosswind landing: if we are landing on RWY 18 and the wind is 150°/20 kt, then we say that we have a headwind component of 17 kt and a crosswind component of 10 kt, and those components are referred to the ground track, not a/c heading. En-route it's exactly the same thing, is it not?

Fly from A to B, then back from B to A, where the wind is 90° across track ... and you will have a headwind in both directions

No, I will have a TAS higher than the GS, to compensate for the crosswind, which makes me travel farther in the air mass than I would on the ground, but that doesn't make the wind vector a headwind, it just stays a crosswind. "To head into wind" does not coincide with having a headwind, as I see it.
Is there any book that defines head/tailwind as "the component of the wind vector relative to the aircraft heading"? I would be interested in seeing one.

Deeday

[IO540]

Fly from A to B, then back from B to A, where the wind is 90° across track ... and you will have a headwind in both directions ...

Which is why, on average, flying random tracks, you are worse off when there is wind than when there is no wind....

I know a lot of people try to simplify these things in various ways, but basically the whole subject of correcting for wind comes down to the following:

- if there is no wind, you just head to the destination - simple!

- if there is wind, you have to pretend to be flying to a different spot, which could be before the destination, behind it, to one side of it, etc. And, because the wind is just a large scale movement of the air (a movement which the plane is totally unaware of) by the time you have reached the "fake destination" you will actually end up over the real destination because the wind has been moving you along. If you draw some diagrams, it is really obvious that more than 50% of the time the "fake destination" will be further away (in terms of ground distance) than the real destination.

[Jumbo Driver]

Originally Posted by Deeday

Fly from A to B, then back from B to A, where the wind is 90° across track ... and you will have a headwind in both directions ...

No, I will have a TAS higher than the GS, to compensate for the crosswind, which makes me travel farther in the air mass than I would on the ground, but that doesn't make the wind vector a headwind, it just stays a crosswind. "To head into wind" does not coincide with having a headwind, as I see it.

Deeday, I think you will find this statement is correct.

OK - perhaps I should have said "headwind component" to make myself absolutely clear ...

JD

[Deeday]

We are at a dead-end here, because we are applying different definitions of what headwind and headwind component are.
I would be interested to see if there is an 'official' one.

Deeday out (for now )

[BOAC]

Deeday - never mind -'definitions' - what JD, IO, me and others are saying, having spent many bum-numbing hours flogging to and fro in a jet, is there is always an overall loss of GS on outandbacks UNLESS the wind shifts, for which one spends hours praying........................Absolute still air (if only!) would be the only case to disprove this You never get back what you lose. (There's always the cost index, of course....shh!)

For the OP - just stick to DD's whizzwheel brief and worry not about all these esoterics.

[DaveD]

Studying NAV at the moment and im taking my test tomorrow. This is how you do it.

"if you wish to maintain a groundspeed of 80kts on track calculate the true heading and TAS required if wind velocity is 090/20 true and track is 180 degress true?"

Put differently. - Given a W/V of 09020kt and a track of 180T, what heading and TAS should be flown to maintain a ground speed of 80kts on track.

On your whizzwheel first you need to set your black dot to 090 and 20 knots on a spot of best convenience (100 is a good spot). Mark 20 knots from the black dot. 100 - 20 = 80 so make a mark at 80 knots. Now we need to rotate it to a track of 180T. So we want to maintain a speed of 80kts so we need to slide our wheel till our mark is on 80kts. (the mark we made)
Next we need to correct for drift... it's 12 knots to the starboard side, so we need to correct it by spinning to the right, it sits at 12 drift, giving us a new heading and velocity.

165T and 75kts

This is how the test will want you to do it, and it will get you the mark, theres no reason to go too far into things, at the end of the day, if it gets you a mark then that's what you need to pass.

[Islander2]

165T and 75kts

This is how the test will want you to do it, and it will get you the mark

Not with the wrong answer it won't. But it will make you a lot of money, because you've discovered the secret to perpetual motion! Having headed partially into wind to compensate for drift, it must be the case you have a head-wind component. Yet you are telling us that you are only going to need a TAS of 75 kts to maintain a GS of 80kts! Amazing.

Garbage in, garbage out. Is it the case that students are no longer taught to think about what might be roughly the correct answer before using the wizz-wheel?

I think you'll find the answer is 166T Hdg, 82.5kts TAS.