View Full Version : DA and performance.
12-12-2004, 07:19 PM
Just wunder'n if there is a general 'ruel of thumb' easy calculation for figure'n wot you DA is if you know wot the alt,temp and humidity is.[and it must be VERY easy for SCG's ;) .]
Some of the conditions extreams here would provide interesting AD's I reckon.
[like wots the DA,when its 0c,next to no humidity and 1800'ASL...........compared to 50c,75% humidity and 1800'ASL.]
I know the performances in those two conditions are very different. :D
12-12-2004, 08:04 PM
Try here: aviation rules of thumb (http://www.megginson.com/Aviation/rules-of-thumb.html).
They even refer to that wierd Celcius thing you are talking about. :confused:
12-12-2004, 09:04 PM
Your still a ledgend Mike.
So,with this information
:"Airplane performance depends on density altitude. To estimate density altitude (at least at lower altitudes), start with pressure altitude and add 120 ft for every degree Celsius above ISA temperature, or subtract 120 ft for every degree Celsius below ISA.
For example, at 3,000 ft pressure altitude, the ISA temperature is 9°C. If the actual temperature is 20°C, add 1,320 ft (11 * 120) to get an approximate density altitude of 4,320 ft.:"
Thats simple enough.
:"Humidity also affects density altitude, but not enough to worry about in a rule of thumb.:"
Hmmm...........I'll argue with anyone on this point.
12-12-2004, 09:20 PM
15 degrees Celsius at sea level.
12-13-2004, 12:27 AM
If I'm read'n this right,on a 50c day,at 1800'ASL,the DA would be[50c-9c=41 .........41*120'+1800'=6720.] 6720' DA.
That don't figure,coz at the Oz Nats they reckoned it was 9000'DA,when the strip woz only 400' higher than here,and on a much cooler day,not even 30c.
This f^%$*&n SCG is stuff'n up somewhere.
12-13-2004, 04:07 AM
ISA is International Standard Atmosphere, which is a nominal set of figures for mean sea level, used throughtout the world as;
Surface temperature of +15 c
Lapse rate of 2 c per 1000 ft (up to the troposphere then constant)
Freezing level (0c) @7,500 ft
Pressure of 1013.2 hectopascals
Troposhere @ 36000ft
Temperature at and above Troposhere of -56c
Lets find out the DA for Alice Springs (elev 1789ft) on a standard day.
ISA temp @ 1789 ft = 15c - (1.789 X 2c) = 11.422c
Actual temp @ 1789 ft = +40c
Difference between actual and the ISA temp for Alice Springs = + 28.5c
Conversion to height = 28.5 x 120 = 3420ft
Since the actual temp is hotter than ISA for Alice Springs , add the new height (3420ft) to the elevation of 1789 ft = 5209ft
So your Density Altitude is 5209 ft. on a "Standard day". However Standard days are theories as Area Pressures constantly vary, Lets assume however that the area QNH is 1000 hectopascals on this day.
Difference between ISA Pressure and Area QNH = 1013-1000 = 13hpa
Difference in pressure altitude = 13hpa X 30 ft per hpa = 390ft
Therefore the Pressure altitude of Alice Springs = 1789 + 390 = 2179ft
So to get the correct figure for DA use the corrected PA figure instead of the actual elevation for Alice Springs and.... bingo its now 5599ft.
Remember this point: If Area QNH is greater than 1013.2hpa then pressure altitude is less than elevation and, if Area QNH is less than 1013.2hpa the pressure altitude is greater than elevation.
Since this is as clear as mud the first time you read it, please set down on a piece of paper the above fomula's and change the Area QNH, or Temps to find your new Density Altitude.
After a while it will fall into place.
12-13-2004, 11:28 AM
Birdy I bet ya regretting asking this DA question!!
Tee Hee, GG NZ.
12-13-2004, 05:22 PM
I may need a truck driver next summer. I need someone who can drive on the right side of the road. Are you available?! :D
12-13-2004, 09:57 PM
Keep me posted, you never know I might get a gap in my schedule next summer.
By the way, I CAN drive on the right side of the road...and that side is the left! Did you ever realise I had never driven on the right (right) before coming to your place?
All the best to you and Kris for the festive season,
12-13-2004, 11:13 PM
Hmmmmm.......................... feel'n dizzy for some reason.
Reckon I'll just stick to my general ruel o thumb,'if its over 40c,allow another 50 yards to break ground.'
[reckon if I tryed to work it out it'll have cooled down and the cattle gon to sleep anyway.]
12-14-2004, 03:47 AM
Hmmmmm.......................... feel'n dizzy for some reason.]
Birdy, mate what are you saying.... You fly at ant bruising level, chasing mindless grass eaters in heat that would make the big guy down below envious, through dust and bugs as big a beer cartons, in your 'going to see the bank manager' footware whilst putting the 'feral' through a double pike quadruple multipoint manoever and think nothing of it yet a few simple numbers has you going dizzy????
I can just imagine you and the missus talking to the bank manager. He's been rattling on for 15 mins when out of the confusion comes a question you understand. " How many head do you currently have?" You pull out an old envelope stuffed full of flying photo's like the ones you posted in this forum, throw them on his desk and say 'here you count them", and while your missus is translating the 15 mins of banker speak, the banker is quietly but surely turning green from percieved motion sickness.
I tell you what, if you have Microsoft Office, I'll set up an Excel spreadsheet for you, so that you only have to change the numbers in blue to have it work it out for you. okay?
12-14-2004, 11:19 PM
You were pretty well spot on Ted,till you said "chasing mindless grass eaters ".
They are beautiful inteligent girls and boys that pay for my lousy exsistance.
"so that you only have to change the numbers ".................... but then that means I'll have to carry round this ^%*3$ computer to put um numbers in.
12-15-2004, 07:21 AM
I made a Density Altitude table based on the Rule of Thumb above.
I think I got it right. :rolleyes:
(I didn't, so I took it down.) :(
Maybe next time.
Mike - your table doesn't take into account the fact that the standard air temperature goes down with altitude. For example - the standard air temperature at an elevation of 5000 ft is 5 deg c, not 15 (-2 deg for every 1000 ft).
This link goes to a good discussion of DA for those who want to go over the deep end... http://wahiduddin.net/calc/density_altitude.htm
Birdy - buy a cheap flight computer that you can carry in your pocket. I use my Palm Pilot with a software I downloaded called DSSCalc (http://www.deloach.com/dsscalcp2.htm).
12-15-2004, 10:33 AM
I'll see if I can fix it.
12-15-2004, 11:23 AM
When they reckoned it was closer to 9000 ft DA, it needed to be somewhere close to 880 hpa if at 1800 elevation and a temp of 30 degrees celcius. Udi has the answer to your 'puter availability.
12-15-2004, 06:53 PM
Udi,mate,if I was go'n to use one of them I'd never find any cows,I'd be play'n with the buttons all day. :mad:
Don't know the numbers Ted but thats wot they reckoned.The pressure can certanly vary a bit round ere.
I'v cum to the conclusion that DA is too technical and not a high enough priority for a SCG to be concerned bout. :p :confused: ;) :rolleyes: :D :mad:
12-17-2004, 03:35 AM
Birdy, what effect would you say that humidity has on a gyros performance ?
12-20-2004, 07:59 PM
And probably the same as temp.
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