NEPTUNEBILL

title saids it, is there an online weather site or what have you that post DA for areas of the country--how is DA determined is there a formula like heat index factor or wind chill factor etc--thanks guys

SFVetteman

To calculate DA, you need to know the temperature, the barometric pressure and the dew point.

Here is a calculator and a link to the National Weather Service: http://wahiduddin.net/calc/calc_da.htm


[Modified by SFVetteman, 9:31 PM 3/11/2004]

kumar75150

I use this one:
http://www.csgnetwork.com/densityaltcalc.html

EHS

The problem with using that one is there is no input for your pressure (current) altitude. So it's meaningless.

Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. In other words, you are at 1,000 feet pressure altitude and it's a hot day. The car thinks it's really at 2,500 feet, depending on the temperature, barometric pressure, and air moisture content.

An accurate rule of thumb (usually any error will be less than 300 feet) for determining the density altitude is easy to remember. For each 10-degrees Fahrenheit above standard temperature at any particular elevation, add 600 feet to the field elevation. (And, conversely for each 10-degrees F below standard temperature, subtract 600 feet.)

So, use the first caluclator, not the second.

Allthrottleandsomebottle

I use this to see the specific info for a time,date and place.
http://www.wunderground.com/

I use these to figure my DA based on the above inf.
http://www.wahiduddin.net/calc/calc_da_m.htm
http://www.csgnetwork.com/densityaltcalc.html
The naval one I use is at home..........I have bounced this info against local racer weather stations and its pretty damn close!

NEPTUNEBILL

EHS-- Great info thanks for the rule of thumb--is there a chart somewhere that shows a standard or mean temp for various elevations--

What I am trying to do is have our on HP dyno readings--I don't think SAE is very accurate at all and would like to take the uncorrected dyno numbers and adj them by a current DA--so first I have to find out how to get my current DA which you guys are providing info on (thanks all) and then determine what a typical HP gain or loss is per 500 or 1000 diff in DA

We then can adjust the uncorrected numbers by DA and not some sliding scale (SAE) that places everyone in a bowl that lies between Pikes Peak and Death Valley or whatever--

as an example SAE penalizes FL because it is basically at sealevel--what is doesn't consider that our dyno in Orlando is typically at a DA of 3500ft because of dewpoint and humidity--thats right down the hill from Denver elevation guys--that is why when you bring your 500hp cars and dyno them here you loose a good 10% of your hp--SAE does not consider enough factors for this area--another area I suspect would be Atlanta GA it is a humid sob there in the summer and I bet Marco's car will dyno considerably less than 528rwhp in Atlanta because of inacurate SAE corrections--

So does anyone have a rule of thumb for typical HP loss or gain per 1000ft or whatever diff in DA--those of us on the forum could start applying our on DA to the uncorrected numbers and then we will have a very good basis of comparison even if only used by us and screw SAE--

Allthrottleandsomebottle

Glad you asked.....here are the links to 2 dyno runs on different days with me calculating the DAs. 3% difference.
http://forums.corvetteforum.com/zerothread?id=697571
http://forums.corvetteforum.com/zerothread?id=692028

Screw SAE.............. :boxing :lol:

EHS

NeptuneBill:

As a pilot, we deal with this all the time. Otherwise sooner or later you die. Got this off the web, hope it helps:

"Standard temperature at sea level is 59-degree Fahrenheit. For elevations above sea level, subtract 3.5 degrees per thousand feet of elevation from the sea level temperature of 59 degrees. For example, at Jackson, Wyoming the elevation is 6,444. Multiply 6.444 times 3.5 for 22.55. Subtract this from 59 (59-22.55) for 36.45. The standard temperature at Jackson is 36.5 degrees. If the existing temperature is 80 degrees, subtract (80-36.5 = 43.5). Divide this difference by 10 degrees (for each 10-degrees F above standard), and multiply 4.35 times 600 (600 feet per 10 degrees) equals 2,610. Add 2,610 to the field elevation (6,444) for a density altitude of 9,054. Under the existing conditions (of our example), the airplane will perform as it would on a standard day at 9,054 feet elevation.

The point is 59 degrees at sea level and 29.92 pressure is considered a "standard day."

Here's the "rule of thumb" on horsepower that pilots use:

A normally aspirated aircraft engine loses approximately 3.5% hp per 1000 feet increase in DA from sea level.

Example: 230 hp airplane at the 5000 ft airport where the DA = 7600 feet

HP reduction= 3.5% x 7.6, which = 27% reduction (approximately 73% available), which in turn is 230hp x (73%)= 168 hp available at 7600 ft DA.

You're correct, the high and cold folks are often better off than the low and hot.

If you wouldn't mind, I'd be curious to see how this all caluculates outfor particular situation. Because that's a lot of loss.

NEPTUNEBILL

EH: I will have some examples we can discuss by monday on some new pulls, I will compute the DA and see what the dyno sheet gives me for a correction factor--a week ago I computed the DA at just under 2200' and the dyno penalized me by adjusting my hp to 99%--before the FAST was on and with stock MAF small with screen the tq jumped to 530 at 4K when the tires started spinning, then it ran out of air at 5700 and dropped like a rock--the bigger MAF and the 90mm setup are being tuned in so I will have a better run monday--outside dyno no fans hood down just like in traffic--the car was climbing fast when it ran out of air and though my rev is set for 7K at 5700we were at about 482rwhp the dyno corrected to 478--the DA was 2200'--based on your rule of thumb I should be adding apprx 7.2% to the 482hp which would correct to about 517rwhp at 5700 and hoping for another 50 by 7K--so by SAE corrections I was already at 478 and by my DA calcs I was already at 517hp--I will gather all the info on the pulls monday and get back with you and maybe we can put together some interesting numbers--great info BTW and much appreciated--coming from a family of pilots and a one time private pilot myself you would think I would know more about DA but I flew over 40 years ago and maybe things were a little diff or I just don't remember--I had a hard time getting into my Cessna as a kid when I had a motorcycle that could go faster in the 1/4 then my old 170 could go top end :lol:

EHS

Look forward to seeing the numbers. Good luck.

Oh, BTW -- that ol' Cessna will go a lot faster than your motorcycle or Corvette, it just depends which direction it's pointed...... :rofl: :rofl: :lolg: :lolg:

Mean Green

:D :reddevil

http://www.biondoracing.com/WeatherStations.shtml

427CPE

Actually not true. The 172 is near slower even in a dive. I have pointed a T41 (172, EFI 210hp) almost at the ground during stall testing. I accidentally hit 165 kph, slight faster than a Vette a guess. Would like to see a Vette's speed nose down on a mountainside!! :jester

NEPTUNEBILL

Well guys I learned to fly in the old 170, tail dragger, if that sob got to 165 the wings would have come off :lol: So far in the last 24hrs, thanks to all you guys, where to find the info and calc DA--a rule of thumb for loss per 1000' of DA, now I am going to double check the dyno's and see how good the software really is, I have already found descrepencies and now wonder how much they vary from machine to machine--at least we know now how to compute our on DA, convert it to HP gain or loss and add that to our uncorrected numbers--this is good I will report on the dyno I use monday and if time permits I will do another pull on Mike Norris dyno less than 10 miles from that one and compare that as well--thanks all

EHS

Sorry, disagree.

When did you pull out, and why, if you were only at Vne? Was it because you were still accelerating (power on)?

Without going back to the POH, Vne on a 172 is approximately 160 knots, from what I remember. That's about 185 mph without calculating it. Vne, as you know, isn't the maximum speed the airplane can achieve, nor is it drag limited terminal velocity. It's the "Velocity/Never Exceed" for "normal" operations. Make it clear air at that, i.e., distinct from Vno, which again, as you know, is maximum structural cruising speed in clear air.

Having flown a little aerobatics (about 800 hours, i.e., not total hours, but actual aerobatics -- not many of those in a 172 though ;) ), and knowing what happens in even dirtier airplanes than the Cessna (like an S2C), I can assure you that at full power, pointed straight down, your 172 will accelerate well past Vne, and be faster than your C5 Corvette, right until it starts shedding the wings and empenage parts. Ask JFK junior sometime (I know, the Saratoga is a cleaner airplane) -- not that he was going fast enough when he hit to actually be missing pieces of the airplane -- but give him a few more feet of altitude.

I'd be worried if you wanted to try that full power dive again, straight down. Use the wings against the horizon to make sure you're exactly vertical on the way down. You'll probably need some forward stick as you keep accelerating, so be sure to trim it out on the way down. I assume with all of this there's no danger at all involved, since you won't be able to get the 172 past Vne. Or did I misunderstand?

All the best.

:cheers:

427CPE

No, you misunderstood and I sort of mis-stated a few things. There was no intention of exceeding Vne in this particular plane and yes, as I recall 160 is about right. This was actually the result of a poor spin recovery, was near idle, and dark out and obviously a person would have a death wish that intentionally pointed this plane nose down, but you already know that.

Yeah, the cessna might be faster in a dive if maintained, but the point was how stable the Vette is at high speed. Would you really want to zip around in a 172 at 180?? No telling what JFK did. He was probably more of a pilot than his mistake lead most to believe though. 99.999% of my flight hours were spent straight and level.

Nice to see others on here that use da for other than quoting 1/4 miles passes.
:cheers:

EHS

427cpe:

No problem, I did misunderstand. I originally posted that I was looking forward to those dyno pulls to see how much DA will come into play this weekend, and then I guess I was kidding NeptuneBill a little about how to get a 172 going faster than the Corvette, hence the :rofl: :rofl: :lolg: :lolg:. Now, what would be interesting to see would be a Corvette coming straight down next to the 172 (say out of a C130); I bet it's clean enough to beat it.

And, oh, nothing like a little test flight at night with an incipient spin recovery. :eek: Only a pilot would know how interesting that really feels. Closest I came to that was a stuck mixture control on a Stinson L5 WWII taildragger at night -- thankfully near an airport. Strictly one way operation, it came out, but wouldn't go back in. Not necessarily dangerous, but amusing nevertheless.

Take care.:cheers: