Navigation Computer (CRP1)

[AppleMacster]

I'm studying for my Nav exam and am keen to get it done. However, my CRP1 is driving me nuts! No matter how much I sharpen the pencil, and spend time setting up the wind part of the computer, I hardly ever get the answers (relating to wind) exactly right in the mock questions in the Thom book and PPL confuser. There is usually a 1-3 degree or knot error. As it's multiple-choice, it's perhaps largely irrelevant?

Am I being particularly hard on myself, or just rubbish? :ugh:

[tacpot]

the whizz wheel should get you to within a couple of knots of the right answer, but the angular answer should be within two degrees also - if you are getting three degree errors, you should check your technique. One idea might be to ask another student to work through the problem with you.

As you say, for answering multiple choice questions, your technique will get you to the correct answer reliably. More importantly, your technique will also suffice for 'real-world' VFR flying as the wind will rarely be exactly as forecast. Your technique is more accurate than the weather forecaster's!

[wobblyprop]

are you using the wind down method? Highly recommended.

Also, once you've moved the wheel round to your track, are you laying off the drift?

[Speedbird252]

Wobbly, why do you recommend wind down? Im doing this stuff now and find that wind up is an easier way of arriving at the same answer?

Or am I missing something!!??

Cheers,

Speedy

:confused:

[Foggles]

Dear Wobbly & Speedbird

Please tell us - wind up or wind down - we don't care
but which one is better ,more accurate , easy to use !

Having a choice is worse somehow !!!

Foggles

[Flying Boat]

AM

Your not the only one.

The CRP1 is OK to 2or3 knots & degrees.

I have just done my ATPL Gen Nav & Flight Planning with a CRP5.
We are OK to within 4or5 degrees & up to 8to10 knots. Now I can get within 2-3 degrees & 2-3KTS @ 450KTS TAS calculations, normally.

Alot of it depends on how well your unit is made, I had major problems with my CRP5 & had to file the edges of the sliding plate & loosen the screws on the rotating degree plate for much more free movement, speed is essential in ATPLs.

I agree with Tacpot, get another student to help you understand it. It really is helpful if you have a mental block.

Wind-down is more preferable to learn if you need to calculate wind, drift, True Track from Heading & Wind. It is the taught method for the ATPLs.
Wind-up is good enough if you are only going for your PPL & need to pass the Nav written.

Whichever way you go, don't worry, it will suddenly all make sense & be accurate. Just like turning on a lightbulb.

Enjoy your flying.

FB:ok:

[FlyingForFun]

Agree with all the other comments, especially FB's regarding wind up/down.

Wind up is fine for PPL. It's very slightly easier, but shouldn't be any more or less accurate. But it's no use for the ATPL exams, because there are lots of things you can't do with it.

My instructor taught all his studends wind down, which I was very pleased about when, over a year after first learning to use the CRP1, I decided to start the ATPL exams. If you're about to start using the CRP1, use wind down. If you've already learnt wind up, then don't bother changing if you're not planning on doing the ATPL exams.

The CRP1 is not designed to be 100% accurate. After all, the wind forecasts aren't 100% accurate anyway, even if you can fly to within 1kt and 1 degree. It's designed to give you answers that are as close as you need them to be, and any multi-choice answers should be sufficiently well spread.

Good luck!

FFF
-------------

[wobblyprop]

Speedbird,

I only recommend the wind down method because that is what i used for my atpl. It took some getting used to, but after a while it made more sense.

[Tinstaafl]

Wind up achieves the same results for the same problem as wind down, just as accurately but without the stuffing around adjusting things that wind down requires.


It's also quite easy to learn to use wind down for problems where wind down is more suited and then choose whichever method fits best.

Alternatively, get a CR type (my preference) but be warned that very few UK people seem to know anything about them. Shame really since they make a number of the ATPL questions much easier.

[Saab Dastard]

Apple Macster,

I learnt the wind down method - I posted the following in a previous thread that you may find useful.

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

[Flying Boat]

The Wind down Jiggle.

I could not get my head around the 'jiggle' until a French mathematician friend of mine helped me open my eyes in 3 minutes. He was also doing the ATPLs.

After the wind & the track move (not forgetting the correct TAS position for the blue dot) you will have your 'pencil' mark either left or right of the centre line.
If the line is on the left by, for example, 6 degrees, move the circular degrees scale 6 degrees to the left. If the pencil mark is still 6 degrees to the left that is your answer. All you need do is balance the difference in the pencil mark scale.
It works for me (passed both the Flight Planning written & the Gen Nav written). Sorry if I didn't explain it well, but it does work. Easier to explain by showing.

Hope this fumble helps.

FB:O

[AppleMacster]

Thanks everyone for all your advice. I've settled on wind down. I did my first navex today and all the calculations worked out - it was very blustery today, and yet it all worked out. Amazing!

My instructor gave one further piece of advice: instead of using a pencil, which is relatively thick, use a water-soluble fine-tip pen and just put a dot on the W/V mark. It's a lot easier to read and much more precise. Success! :ok:

[Flying Boat]

Well done!

Knew you'd do it.

FB :ok:

[FlyingForFun]

I agree re the water-soluble marker. Apart from being thinner than a pencil, it also doesn't scratch the plastic. I've got a CRP1 which has had a few years of good useage (yes, I really do still use it!), and a CRP5 which has been put through the ATPL exams, and both are as good as the day I bought them.

(Only problem with the water-soluble pen is the black mark it leaves on your hands when you wet your finger to wipe it off!)

FFF
----------

[High Wing Drifter]

Also, if you are finding the operation of the wheel a tad sticky and thus awkward to make those degree/knot fine adjustments then try covering the unit with Talcum Powder and jiggle/swivel all the moving bits. Dust of the excess and you should find the movement as smooth as a baby’s bum. Gives it that fresh, just washed feel everytime you plan too :O


I kid you not :D

[Tinstaafl]

Use a soft pencil (2B is my preference) on the whiz wheel but don't use an eraser. A wet finger works fine. The eraser eventually wears the slightly roughened surface making it less effective.

[FlyingForFun]

HWD,

Yes, I'd heard that. Unfortunately, my CRP5 is too loose, not too tight. And I couldn't find anyone who knew how to fix that. Any suggestions, people?

(Although I'm not too bothered any more - now that I've finished my exams, I just use the CRP1 - can't see that I'll ever need the CRP5 again.)

FFF
-----------

[IO540-C4D5D]

I don't know why they still teach using these slide rules in the PPL.

For general multiply/divide conversions:

1. For the price of a CRP-1 you can buy half a dozen calculators, which takes care of the "what if the batteries go flat" etc anit-anything-new argument.

For wind calculations:

2. The error in Form 214 data is, for typical GA winds aloft (20-40kt) and for typical GA planes (90-150kias), enough for a calculated heading error of a few degrees.

3. Only a robot can fly a plane to a few degrees for long periods, and you come out of it very tired indeed at the other end.

4. The slide rule can't be used during flight (unless you have another pilot or an autopilot)

5. The simple rule of thumb (max drift is half the wind (~100kt) or max drift is 1/3 the wind (~150kt) is as accurate as anybody can actually fly a plane to.

6. The DI in most GA planes drifts around so badly that holding a heading to a few degrees is a major struggle.

The time spent learning this piece of pre-WW1 technology would be better spent on other things.