Using the ARC 2 Flight Computer

[Cron]

Being a bit thick I'm bu**ered if I can work out how to do Heading and Groundspeed using rhe High Speed side of my ARC 2.

Manual no good at all.

Would be very grateful for a rough explanation.

[mckrll]

Is it not exactly the same as the Low Speed side? Don't have an ARC-2 to hand so apologies if this doesn't work.

Mark the wind, turn to set the track against the index mark. Slide to get the TAS under the wind mark, read off the G/S under the centre dot. Check where the wind mark lies on the drift lines (=Wind Correction Angle) and subtract from track if left of centre (or add to track if right of centre) to give HDG.

There's also an interactive CD here:

http://www.afeonline.com/shop/produc...roducts_id=785

Andrew

[Cron]

Thanks Andrew, but that's the problem, the high speed side has no wind grid..or so it seems.

[Pub User]

Sorry to state the obvious, but you do know you have to take out the slide and turn it over, don't you?

[Cron]

Actually there was a time when I didn't know PU!

Can I give more info?

Here goes: Low speed side: lots of curved lines (a/s) beginning 1/3rd of way up the slide. Lower 1/3rd delineated by horizontal line with more Horizontal lines below it. (wind speed).

Ok, flip over the slide to high speed side. Now nothing but curved lines (a/s?) but no delineating horizontal line and lower 1/3rd for wind speed.

Is it because I'm from Birmingham?

[mckrll]

Aha!

The "lower 1/3" as you describe it on the Low Speed side is only used to find Head- and Crosswind components. It's not used when solving wind triangles.

The High Speed side doesn't have the HW/XW section - you need to use the section on the Low Speed side if you want to compute them.

Other than that there is no difference between the use of the two sides except for the speed ranges provided (up to 230 on Low and 150-900 on High?).

Andrew

[Cron]

Thanks for the reply M, even more confused now.

Say I want to calculate Heading and Groundspeed given w/v 240/20, true track 300 and TAS 300kts?

Just how do I go about it on the High Speed side?

Many thanks.

[mckrll]

Well, as I understand it, this is what you do:

(BTW I'm using a Pooley's CRP-1 here)

1. Set the the wind dir: Turn the disk so that 240 is opposite the Index.

2. Mark the wind speed (I use an X rather than a dot):

centre the zero "dot" (which is actually the hole in the rivet) over an accessible number (I'd use, say, 500 as it's in the middle of the slide);

mark the wind 20kts up from there - so put an X at the 520kt point.

3. Turn the disk to align the TRK (300) opposite the Index.

4. Slide the rule so that the wind mark (X) lies on the TAS 300 line.

5. Read the G/S under the centre "dot" - 289kt

6. Read the Wind Correction Angle under the wind mark (X) where it meets the drift lines - 3 deg left.

You need to be careful here because, at least on my CRP-1, 300kts is where the drift lines change from 2kt to 1kt presentation.

7. Subtract the WCA from the TRK to get HDG - 300-3 = 297 degrees.

Left of centre is a -, right of centre is a + correction.

I actually prefer to use the Jeppesen CR series. It's the same thing but only it is completely circular - I use the CR-3 which is about 15cm across (there is a smaller one that fits in a shirt pocket but it's too small for my eyes!).

It's a circular slide rule, so the front is exactly the same as you'd find on the ARC-2 but there is no vertical slide and the wind presentation is very slightly different, but far easier to use - only need to mark and rotate (in fact you can do it without even marking a wind X).

It also "automatically" displays the crosswind and headwind components (which in our example are HW 10kt, Left XW 17kt).

My reason for using this series is that there is no slide to get in the way, so you can use it comfortably in one hand in the cockpit to work out speed/time calcs for enroute navigation as you fly.

Reading your profile I see you already have a CPL(H). Do you not use an electronic version for flight planning at this stage?

I have both the Jeppesen Tech Star and FlightPal on my Palm - which I honestly only use to cross check my manual results (well, so far anyway!).

Hope this helps, though I have to admit to wondering why you need to do it this way - was PPL training a long time ago and you're now returning to doing things manually?

Andrew

[Cron]

Now I have it. The bit I was missing was 'over an accessible number' -didn't realise you could do that (having, somewhat mechnically, on the low speed side always put wind dot on the top of the wind grid).

No, don't use an electronic version - I try and stay away from all things electronic and I rather like the 'feel of the wheel' - I'm in computing and therefore hate all things electronic.

Yes PPL training was some time ago but I amuse myself by reading CPL nav stuff and dreaming. The faster airspeeds are appearing in problems thus the high speed side Q.

Thanks for you for your help and interest. I predict you'll be FO in 12 months.

All the best.

[mckrll]

Great that you have it!

You can use exactly the same procedure on the Low Speed side as well.

The point about placing the wind X is to get the wind direction against the Index and the wind speed x kts up from the centre "dot".

As the whole thing is a slide rule, 20kt = 20kt wherever you start from. Hence finding a convenient starting point (where you chose is entirely irrelevant as what matters is where you end up).

If you imagine a problem where you set the wind at say 20kt and then turned the disk thro 90 degrees - you'll see that the G/S = TAS (because the slide hasn't moved) and you have the full 20kts as a XWind.

In a normal scenario, the act of turning the disk from wind dir to TRK plus sliding to get the wind X on the TAS draws the (in)famous wind triangle - though admittedly the movement of the wind X sideways is really drawing an arc (it's probably easier to see it as drawing three points - one where the wind X starts, one where the wind X ends up and the third where the centre dot ends up).

I actually have the CRP-1W. This has a rotating arm instead of a full circular window. The arm has the knots shown on a vertical line from the centre dot - Low one way and High the other.

This means that you don't actually have to mark anything at all - just do the alignments. It's miles easier.

As for

I predict you'll be FO in 12 months.

that's very kind, but not the direction I'm going at all. As a born teacher (I've taught everything from primary to post-grad) I'm aiming to combine CFI with some charter work and a part-time church postion (I'm a liturgical musician or a musical liturgist depending on where you're coming from).

Unfortunately plans are not quite on track at the moment, but I'm hoping they'll sort themselves out via an emmigration later in the year.

Best wishes,

Andrew