alright I know there are fancy pants weather things you can buy for like $300 to calculate DA, and all the fun stuff.


Is there a cheap way of doing this effectively yourself?


Come on guys, I know their are some math genius that have worked out formulas, and can give me a hand so all I need to do is buy something like this, and bring a calculator and a pencil

http://cgi.ebay.com/Altimeter-Barome...QQcmdZViewItem

or one of these?

http://cgi.ebay.com/NEW-Weather-Cent...QQcmdZViewItem

 

I guess it just depends how serious you are about wanting good information and how accurately you plan to utilize that knowledge.

 

The first one doesn't look that bad but I wouldn't trust the accuracy of a handheld analog gauge. The second one belongs in a house and will never give you the accuracy you need at the track.

Spend the extra money on a digital barometer/altimeter. You can find a digital thermometer and humidity gauge just about anywhere for cheap.

If you want to calculate the DA with a calculator and pencil, you will probably miss a round. It's not a simple calculation.

 

there will likely a laptop invovled as well... so if I can make something in excel...


It will also be used for tuning(see results with a tune at a certain DA, not just for elimininations.)

 

well, a simple way to get PRESSURE ALT is to take the local barametric reading and subract it from 29.92 (the standard). Take that difference and mulitply by 10. Take that number and add or subract from your elevation( if the local pressure reading is higher than standard, subract--if lower then add). Using a printout Density alt chart (aviation uses them alot-google it) you can plug in your press alt and temp to get density alt. A rule of thumb is the if temp is over 15deg C then density alt will be higher than pressure (15deg C is standard)
EX:
track elevation is 110ft
local alt reading is 30.00
30.00-29.92=.08
.08 x 10= 80ft
110-80=30ft pressure alt

Hope this helps. If you talk to anyone that flys they can spit this out and explain it. Also, any local airport or general aviation place should have those density alt charts. Its required to do a performance sheet before you fly. Oh, the above method is using inches of Hg and Celcius. You can do it with milibars and F if you want. As long as you know the lapse rate.

 

I find that highly inaccurate since there's no way to account for temperature and humidity in the formula.

Pumping those numbers into my formulas and using 15c as a temperature, I get a DA of 160 feet at 50% humidity. Dropping the humidity to zero to eliminate that variable drops the DA to 50 feet. Increasing the temperature to 20c increases the DA to 637 feet and bringing the humidity back up to 50% increases the DA even more to 786 feet.

With that much variation from just a few simple number changes, I wouldn't use that formula for any kind of accuracy at the track.

I've see a perfect little hand help weather station that even calculates the DA for you. Search on Yahoo, Google or Ebay for a Kestrel 4000. I may sell off my Suunto E203 for one of those.

 

Although it does not take humidity into account, it does take Temp. By definition, thats what density alt is. DA=PA corrected for non-standard temp. Thats why I gave the method for getting PA and to use a chart to convert to DA. The chart has temps on the side and PAs on the bottom. This is the free way to do it, not the best. I use this to fly with and was just offering an alternative.

 

Poor man's, well i have a friend that is a pilot and he give me the airport info numbers for each town. So i just call the numbers and it tells you the DA but its for the airprot not the strip but its close and cheap,and poor man's

 

I cheat by using the 48 hour weather data at the National Weather Service site for the city of the track location> http://www.crh.noaa.gov/ifps/MapClic...te=MI&site=DTX

Look under the "current conditions" and theres , 3 DAY HISTORY. The time data is pretty good to where you can pull up with-in a 40 minnute window of the actual run time on your time slip.
Then I use this site to input the data for corrected da >
http://www.abc15.com/weather/index.a...yaltitude.html

I know it dosen't help at the track but it's good data to record in a run log book

 

Eliminate all the guess work and just break down and buy one of the newer weather stations that also takes into account water grains and dew point - things that are often overlooked but have an affect on performance nonetheless.

 

I use this all the time.
http://www.csgnetwork.com/densaltcalc.html

Now that I have a weatherbug on my phone, I can get the current details after each run this season and write down the temp, barometric pressure, and dew point on my slips after each respective run. Then I can come home and do my calculations.

Or, you can be like me and browse at someones weather station if you have some serious racers at your track like we do.