Forgot How To Work A Crp5!!

[BUDGIE]

Help!

I tried to answer this question using a CRP5 and keep getting -19 degrees centigrade.........

You are flying in a Standard ISA atmosphere at 30'000 at Mach 0.84 what is your ISA TAT?

Now if I use the formula TAT=SAT(1+0.2kMsqrd)

SAT = 15-(30 * 2)= -45 in Kelvin = 228(1+0.2(1)((0.84)sqrd))=260
so 260-273 (I'm assuming a perfect probe for value k=1)

I get -13 degrees centrigrade.........can someone talk me through doing it on the CRP5?

Thanks in advance,
BUDGIE

[Send Clowns]

Unfortunately my CRP-5 is at work, so I cannot confirm figures, but I can tell you how to solve the problem.

ISA at 30,000 is -44°C.
Find the Mach number index in the 'Airspeed' window of the CRP-5, and align with -44°C
Read from Mach 0.84 on the inner scale to TAS on the outer scale.
Find that TAS on the blue 'temperature rise' scale in the middle of the CRP-5 (it is calibrated TAS against temperature rise) and read off the appropriate ram rise
Add the result to -44°C

All the best
Send Clowns
Gen Nav instructor
BCFT

[BUDGIE]

Thanks Send Clowns,

I have been doing exactly what you say but my CRP-5 keeps returning an answer of -18 degrees. I have sat with two other guys trying to get this but to no avail. Whats the probability of me having a dodgy CRP-5 I wonder

-44 in the Airspeed window against Mach 0.84 gives my CRP a TAS of 494 Temp Rise for 494kts is 26degrees so -44+26=-18degrees quite a way off from the formula answer of -13 degrees.
I am sure my maths in the formula is correct?

Could someone possibly check this on their CRP5 and tell me what they get? I'd really appreciate it!

Thanks
BUDGIE

Hey all,

I really don't understand the logic of putting this question in the Private Flying Forum? No dis-respect but there isn't too many PPL's needing to calculate ram rise temps at 30'000ft etc.....
However, if there are any PPL's out there who like working out this kind of thing and are possibly doing this under the JAA then can you check it out?
Really think this is more suited to Questions - but hey I ain't a moderator!

Cheers
BUDGIE

[Cool_Hand]

I get -17 +/- 1 degrees on my CRP5. So I don't think your one is out if at all. If you vary K in the formula to between 0.8 and 0.9 you'll find you'll get an answer between -19 and -16.

[Voidhawk]

With a TAS of 494kts I get a ram rise of 32°, which with k=1 gives a TAT of -13°C on my CRP5 (ISA 30,000' -45°C).

I recon the only way of getting a RR of 26° is setting a TAS of roughly 444 kts, or your CR5 is misaligned

[Cool_Hand]

Just out of mild concern over my CRP5, voidhawk, on your temp rise part of the CRP is:

200 kts just over 4 C
400 kts just below 17 C
700 kts just over 51 C

as for a 32 C rise I need a speed of 540+ kts.

[Voidhawk]

Those figures seem to be for a temperature probe with a recovery factor of 0.8. I.e. measuring SAT + 80% RR.

So in our case the 0.8 probe would show a RAT of -19°C, sensing 80% of the RR (26°), giving a SAT of -45°C.

The 1.0 probe would show a RAT of -13°C, sensing 100% of the RR (32°), also giving us a SAT of -45°C.

Well that's my theory, anyway.

[BUDGIE]

Hey peeps,

My CRP5 agrees with Cool_Hands!
However, I am concerned.....the CRP5 assumes recovery of 1. We have to compensate if it's any less so the CRP5 should give us very close to what the formula does.
Is everyone in agreement with my figures for the formula?
Can I ask any wizzes with the CRP5 and JAA ATPL's students to do this sum on the CRP5 and give us their answers?
This seems very odd

BUDGIE

[BRL]

However, if there are any PPL's out there who like working out this kind of thing and are possibly doing this under the JAA then can you check it out?

That is why I have left it here for now as I know a few ppl's who are doing the JAA who can maybe help you.

[Voidhawk]

What I meant was that yours and Cool_Hand's CRP's seem to assume a recovery factor of 0.8 when calculating sensed RR. Using the formula with a k=0.8 you get Cool_Hand's figures.

You've used a recovery factor of 1.0 in your formula, that's why you get the difference.

My CRP assumes a recovery factor of 1.0.

[Keef]

My CRP doesn't know what you lot are talking about. Or I don't, or both.

My CRP has a Mach No arrow in one of the windows, but I never went fast enough to need it. I couldn't find anything about OAT associated with that. Maybe mine's the older, slower, lower CRP?

My Jeppie calculator does all sorts of clever things, but you have to know how to drive them, and I never cracked that bit.

[BUDGIE]

BRL,

Ah but now I know why you are a moderator and I'm not, you sent it to where it'd get responded to positively and I'd probably have put it in Rumours and News solely cause I'm biased in this particular case.

Cheers
BUDGIE

p.s. I'll buy you a pint or two if ever we meet for questioning your judgement guess I've taken MCC a stage too far

[MLS-12D]

I'm with Keef; I have no idea what this discussion is all about.

No worries though: the chance of me ever "flying at 30'000 at Mach 0.84" is remote at best.

It's questions like this that say to me: "you know, you really don't want to do an IR".