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# Whizz wheel calculations for CAS

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# Whizz wheel calculations for CAS

30th Nov 2017, 12:29

Join Date: Mar 2017
Location: Scotland
Posts: 5
Whizz wheel calculations for CAS

Hello all!

I am currently studying the Pooleys Navigation book. I have just finished the airspeed section and onto the practice questions.

I'm stumped with the following:
"Your destination is 77nm away; you must arrive in not less than 42 minutes. If you are flying at FL50 and the OAT is +10 degrees Celsius, what is the minimum CAS required to arrive at the desired time? Assume no head or tail wind component."

So far, i have had no issues with using the whizz wheel, but the annoying thing is, there were no examples for this type of question in the airspeed chapter!

The furthest i have gotten is moving the wheel to FL50 and +10 degrees, but i dont know how to use the distance and time to find the CAS!

Any help would be much appreciated. :-)

Thanks,
Fran
30th Nov 2017, 14:46

Join Date: Feb 2000
Location: UK
Posts: 14,006
I must admit, I'd usually all of this with a calculator and look-up table, but let's use the CRP-1 in my flight bag.

First G/S, you need to know your minimum groundspeed. That is

77*60/42 = 110 kts Much easier on a calculator, but just using the outer rings as circular slide rule is easy enough in two stages.

77 x 60 = 4620, rotate the cursor around to that value. Now rotate 42 on the inner ring to line up with the cursor, look at the 10 on the inner ring, and it lines up with 11 on the outer ring. There's your 110kts.

If I turn the Airspeed window so that +10deg.C lines up with 5,000ft I can see a density altitude of 6,000ft-ish in that window. That's intuitively correct as ISA for 5,000ft would be 15deg.C minus (5 * 2) = 5 deg.C, so we're above ISA, and therefore the air is less dense, increasing the density altitude.

Now looking to the outer ring, against 11 on the outer ring, I can see 10.15 on the next ring in. That maps G/S in zero wind to CAS - so 101.5 kts.

So, I need 101.5kts CAS to reach my destination in 42 minutes. I'd call it 102kt to play safe.

G
30th Nov 2017, 18:29

Join Date: Mar 2015
Location: Off the map
Posts: 12
http://www.mypilotstore.com/mypilots...E6B_Manual.pdf
Assuming your whizz wheel is the E6B model. If you have the CR-3 model then this is the relevant one:
http://fer3.com/arc/imgx/crinstructions.pdf
30th Nov 2017, 18:43

Join Date: Oct 2017
Location: Bressuire
Posts: 657
Whizz wheel question

Bit complicated G

Using the CRP 1 or 2 first always set that which you know.

Set 77 (the distance to travel) on the outer scale over 42 (time in minutes) on the inner scale.

Read off over 60 (minutes) on the inner (time is always found on the inner) 11 (that being = to a distance of 110) so the true airspeed (TAS) in still air requires to be 110kts.

So what should you expect to see on your ASI in the aircraft, which needs correcting for air density?

Set in the AIR SPEED window +10 degrees over pressure altitude 5000ft (5)
On the outer scale find your 110 (11) and below on the inner scale read off 101 (and a little bit) so your indicated air speed (IAS), now known as the corrected airspeed (CAS) is 101kts (forget the little bit).

You are not required in the question to identify the actual density altitude.

G is absolutely correct of course but this is an exercise in wholly using the whizz wheel using only basic arithmetic.

F
30th Nov 2017, 19:18

Join Date: Mar 2017
Location: Scotland
Posts: 5
Thanks guys!

I use a CRP-1 computer.

Fl1ingfrog I find your method suits me as I'm letting the whizz wheel do all the work for me haha.

Thanks everyone for your suggestions and help though! :-)
30th Nov 2017, 20:03

Join Date: Sep 2011
Location: Mare Imbrium
Posts: 638
The advice above is all good.
But let me just say, understand what you are doing with the whizz wheel - it makes it easier to get things right when the question is a new one!!
30th Nov 2017, 22:32

Join Date: Jan 2005
Location: Hotel this week, hotel next week, home whenever...
Posts: 1,492
Sorry, but when I want to know the CAS I look in the POH Correction tables.

If I'm calculating TAS or resolving IAS or applying compressibility then perhaps I might get the Wizzwheel out.

Maybe I'm just thick...
1st Dec 2017, 07:11

Join Date: Feb 2000
Location: UK
Posts: 14,006
TAS to CAS you use the whizz wheel.

CAS to IAS, or the other way - yes, you need to use the corrections in the POH as these are type specific.

G
1st Dec 2017, 09:13

Join Date: Oct 2017
Location: Bressuire
Posts: 657
Correction v Calibration

Duchess_Driver your confusing "correction" for "calibration"

Having found the Corrected Air Speed (CAS) which is the question: then by reference to the actual aircraft manual Calibration Table or graph if applicable: you then interpolate to discover any "position error" but that does not change CAS.

The question does not include a calibration table/graph so cannot be included in calculating the final IAS for the answer.

We all learn in building blocks and I'm sure calibration will be covered in a later unit.

F
1st Dec 2017, 09:29

Join Date: Feb 2000
Location: UK
Posts: 14,006
For the record DD instructed me for my instructors course, so I can vouch for him being quite bright with a grasp of the technical aspects of aviation). Calibrated Airspeed (CAS), Corrected Airspeed (CAS) and Rectified Airspeed (RAS) are all different names for the same thing. So DD isn't particularly wrong, just perhaps could have phrased it a bit better.

It goes...

Groundspeed <--> (correct for wind) <--> True Airspeed

TAS <--> (correct for density) <--> Equivalent Airspeed

EAS <--> (correct for compressibility) <--> Calibrated Airspeed

CAS <--> (correct for position errors) <--> Indicated Airspeed (IAS)

At PPL level you are unlikely to be flying anything where compressibility is an issue, so you can assume that EAS=CAS, although technically that's untrue, and becomes an issue in an airliner or combat aircraft.

For most real-world operational purposes, you can also assume that IAS=CAS, although technically they can be as much as the greater of 5kts or 5% different.

G
1st Dec 2017, 09:59

Join Date: Feb 2016
Location: S.E.Asia
Posts: 1,841
Originally Posted by Fl1ingfrog
Bit complicated G

Using the CRP 1 or 2 first always set that which you know.

Set 77 (the distance to travel) on the outer scale over 42 (time in minutes) on the inner scale.

Read off over 60 (minutes) on the inner (time is always found on the inner) 11 (that being = to a distance of 110) so the true airspeed (TAS) in still air requires to be 110kts.

So what should you expect to see on your ASI in the aircraft, which needs correcting for air density?

Set in the AIR SPEED window +10 degrees over pressure altitude 5000ft (5)
On the outer scale find your 110 (11) and below on the inner scale read off 101 (and a little bit) so your indicated air speed (IAS), now known as the corrected airspeed (CAS) is 101kts (forget the little bit).

You are not required in the question to identify the actual density altitude.

G is absolutely correct of course but this is an exercise in wholly using the whizz wheel using only basic arithmetic.

F
Forgive me for saying this but the Crap 1 is on a par with one of these.

Signal squares,morse,ADF etc all belong to the last century along with whizz wheels.
1st Dec 2017, 11:10

Join Date: Feb 2000
Location: UK
Posts: 14,006
Unless you have a radio failure or there just is nobody to operate the radio at a small strip.

You are identing a beacon

You want a really basic approach aid that requires minimal infrastructure on the ground.

Or like anachronistic ways of doing sums.

The whizz-wheel is anachronistic, but it does work. Using it does no harm and does help somebody develop an intuitive feel for what all the numbers mean which putting it into an electronic device doesn't so well.

I don't use my whizz-wheel much, but it's there and occasionally is handy. NDB approaches and signal squares on the other hand are certainly a regular feature of my life (albeit probably not in the same aeroplane at the same time!).

G
1st Dec 2017, 15:34

Join Date: Mar 2015
Location: Off the map
Posts: 12
The whizz-wheel is anachronistic, but it does work. Using it does no harm and does help somebody develop an intuitive feel for what all the numbers mean which putting it into an electronic device doesn't so well.
Well stated.
Also let's not forget that EASA, in its infinite wisdom, is expecting the ATPL students to use the whizzy thing during the exams.
Electronic flight computers are verboten.
3rd Dec 2017, 15:10

Join Date: Jan 2005
Location: Hotel this week, hotel next week, home whenever...
Posts: 1,492
Also let's not forget that EASA, in its infinite wisdom, is expecting the ATPL students to use the whizzy thing during the exams.
Electronic flight computers are verboten.
Whilst I am no big fan of doing things 'for the sake of it' I think this statement sums up most of what is wrong with modern flight training. When I was a lad I was taught to UNDERSTAND THE BASICS and to be able to work from first principles. Yes, electronic flight computers are not allowed and IMHO for a very good reason - people get used to putting in the figures the wrong way round and get the wrong answer and don't go back to basics of 'roughing-it-out' first or sense checking things. Computer says it is this therefore....

How many times have we seen GIGO* on perf-calcs feature in accident/incident reports?

CAS to me is Calibrated Airspeed - always has, always will. Other forms of airspeed have been CORRECTED to Indicated, True, Rectified or whatever...

GTE - too kind

* = garbage in, garbage out
3rd Dec 2017, 16:06

Join Date: Feb 2000
Location: UK
Posts: 14,006
To really get back to basics mind you, drawing triangles of velocities would deliver that understanding far better than use of the good old flight computer.

G
3rd Dec 2017, 19:08

Join Date: Sep 2003
Location: UK,Twighlight Zone
Posts: 0
Only the backwards European civilian world still uses that CRaP.......
3rd Dec 2017, 19:28

Join Date: Apr 2008
Location: Hampshire
Posts: 0
Do those who are knowledgeable in such matters think it desirable or even essential to carry a spare triangle (of velocities) or indeed a spare whizz wheel should the first breakdown ?

Is this at all likely or, am I being too observant of current safety issues and liabilities ?
4th Dec 2017, 08:12
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 6,848
The old slide rule certainly is a bit historical, as is the whizz wheel version.

However, it does have the advantage of no batteries to go flat at an inconvenient time.

people get used to putting in the figures the wrong way round and get the wrong answer and don't go back to basics of 'roughing-it-out' first or sense checking things

That's the crux of the matter, though. Far too many kids and not so young adults who were brought up with calculators instead of times tables, have no idea of where the answer ought to lie in the scheme of things. If engineers etc., do it this way .. bridges fall down, etc. If pilots do it .. aeroplanes run out of gas, waypoints don't turn up, and so forth.

Bit like raw data and using the automatics .. best to be competent with both ..

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