Watching the animated discussion about density altitude over on SSW makes me want to ask a question: Assuming I don't want to bring my barometer, hygrometer and thermometer, along with an E6-B or a calculator to the contest, what should I get to determine it?  I have seen Randy playing with a small handheld device at Marietta one time, and I have also seen Kent Tysor with some sort of drag racing device that expressed air density as a %, normalized to a standard day I assume.  What do you guys use to determine density alt?

The Kestrel 4250 has density ratio, too, but the 4000 has heat stress index, which I may need more than the other parameters. 

Ouch, I see why Brett borrowed the 4000.  Those things are not cheap(!)

http://store.bruntonoutdoor.com/instruments/altimeters-wind/pro/

Available from amazon for $111.00

http://www.amazon.com/Brunton-ADC-Altimeter-Barometer-2010/dp/B002IAOM04

The simplest and cheapest computer for density altitude is the old E6-B military style computer pilots carried in their flight bags.  You can still buy one at any airport/flight school, or on amazon for $15-$20. Used for navigation, performance and speed calculations to name a few, it is basically a circular slide rule, comes with complete instructions, has no batteries, led's, watts, amps, volts, and never needs oil. Do a google for "E6-B" and read up.  Hope this is helpful.

Steve Thornton

If the actual altitude is high and so is the temperature, you will have a loss in performance.  That's about all you really need to know, unless your airplane & handle have some calibrated gizmos to crank the actual density altitude into.  

I have a wrist watch that gives me altimeter, barometer, and temperature readings, as well as the usual date & time.  But I can't see that it really tells me anything I can't just feel about temperature & humidity.

When I was flying the KC-135, we spent a lot of time getting data from the Weather Service and computing the precise density altitude.  It boiled down to a few simple facts:

Up to 90 degrees you can take off at full throttle.
Below 40, you need half throttle or less.
Above 90, you might have to reduce fuel load.

Turbine engines lose a lot of thrust at high density altitude, piston engines lose some, electric motors aren't engines & don't care.

A CL model airplane that can do a 90 degree vertical eight in Wichita will do a 120 degree vertical eight in Amarillo.

<snip> When I as flying the KC-135, we spent a lot of time getting data from the Weather Service and computing the precise density altitude.  It boiled down to a few simple facts:

Up to 90 degrees you can take off at full throttle.
Below 40, you need half throttle or less.
Above 90, you might have to reduce fuel load.

Turbine engines lose a lot of thrust at high density altitude, piston engines lose some, electric motors aren't engines & don't care.

* I was a Crew Chief on USAF KC-135A/Q models from 1984-1988. We were taught that the water injection on the J-57's increased thrust by about 13% (if I remember right) I work with guys who crewed the original config. back in the late 50's/ early 60's - The water inj. was paired LH/RH instead of the later Inbd./Outbd. Losing water on takeoff meant you were in a world of s%#*! The current "R" models (CFM-56) were just being modded the last year I was in. Some of those planes are older than me and I'm 51!!

** I still have my E-6B from 1983. Learned to fly on a C 150M. GPS? huh?

* I was a Crew Chief on USAF KC-135A/Q models from 1984-1988. We were taught that the water injection on the J-57's increased thrust by about 13% (if I remember right) I work with guys who crewed the original config. back in the late 50's/ early 60's - The water inj. was paired LH/RH instead of the later Inbd./Outbd. Losing water on takeoff meant you were in a world of s%#*! The current "R" models (CFM-56) were just being modded the last year I was in. Some of those planes are older than me and I'm 51!!

** I still have my E-6B from 1983. Learned to fly on a C 150M. GPS? huh?

With those old water-injected J-57's, we had a 3-engine peacetime takeoff weight of 256,000 pound and a 4-engine war weight of 306,000 pounds.
The "new" re-engined airplances take off at 330,000 peace-or-war and forget about the water.
We didn't even think about humidity when were were blowing three tons of water into the engines in the first two minutes.

Incidentally, those "new" Air Force engines are surplus obsolete third-generation airline engines that were removed when the airlines went up to the fourth level.