Pressure Height and Density Height

[richie-rich]

Hello all
Can I please have the formula for the mentioned topics?

A link would also do. I need to show someone how to derive it using more than one possible way.

Help would be appreciated.

Richie

[Biggles_in_Oz]

http://www.tpub.com/content/aerograp...s/14269_74.htm
http://www.4wx.com/wxcalc/formulas/densityAltitude.php
http://wahiduddin.net/calc/density_altitude.htm

[Arm out the window]

DA = PA +/- 120 (temperature difference from ISA)

or there's normally graphs in flight manuals.

Is that the sort of thing you're after?

[prop-wash]

formulas

PH = Elevation + [(1013-QNH)x30]

If QNH is 1030 & airfield elevation is 1500' then 1030-1013=17, 17x30=510, 1500 + 510= PH 2010'

Note: PH can be a negative value, however performance charts don't usually account for this so a zero PH is used.


DH = [(temperature - ISA)x120] + PH

ISA temp= 15 - twice the thousands of feet. i.e 1000' PH is 2, 2000' PH is 4, 4500' PH is 9 etc... (temp drops at a standard rate of 2deg per thousand feet, and ISA at MSL is 15deg.

If PH is 2010 and the temp is 22deg then ISA is 15-4=11, 22-11= 11, 11 x 120= 1320, 1320+1610=DH 2930'

[jetstar1]

Prop-wash, what are you trying to teach this bloke?

Your formula was correct, but then you stuffed it up trying to justify yourself.

PH = Elevation + [(1013-QNH)x30]

Using your QNH 1030 example...

PH = 1500 + [(1013-1030)x30]
= 1500 + [(-17)x30]
= 1500 + [-510]
= 1500 - 510
= 990'

If QNH is HIGH, performance in GOOD, PH is LOW

[prop-wash]

oops, thanks for that directanywhere. Going from memory is never a good thing when you don't use the formulas on a regular basis. Always good to continually go over things like this as it's easy to forget.
Cheers,
P-W

[richie-rich]

thanks guy for helping me out. however, some of those big derivations have been a bit of a pain but getting there

Richie

[jetstar1]

Richie,
When you say you need to 'derive', do you mean derive the formula or just calculate an answer using it?
Best way I can think to derive would be to draw some diagrams showing levels of QNH (MSL), 1013hPa and the airfield elevation above MSL. Is this what you needed help with?
j*1

[richie-rich]

jetstar
what you have said earlier along with pro-wash and directanywhere would suffice at this stage. that is all i needed to know .

btw, once we calculate PH, is it a simple addition of elevation to the answer? I am afraid my theory was done back in 2002 and i seem to have forgotten quite a bit!

thanks again jet*
Richie

[OZBUSDRIVER]

Originally Posted by DirectAnywhere

Note that for Density Altitude temperature variation from ISA is based on the ISA temperature at the Pressure altitude being used, NOT the physical elevation or actual aircraft altitude.

DUHDUH DUH that was why I didn't get that right in PPL written