[kentb]

When doing my flight testing (performance), I am setting my pressure to 29.92 which will give me pressure alt. How do I factor in temp?

If I fly at 8000 ft pressure alt. at a temp of 53F degrees, what is the true alt for testing purposes?

Kent

 

[dan]

Here is some data that my buddy Laird "Lucky" Owens (RV-6) gave me a while back...

For 8000' DA		
Temp C	Temp F	Fly Altitude
4	39	7538
5	41	7443
6	43	7349
7	45	7255
8	46	7161
9	48	7068
10	50	6975
11	52	6881
12	54	6789
13	55	6697
14	57	6605
15	59	6512
16	61	6421
17	63	6330
18	64	6238
19	66	6148
20	68	6057
21	70	5967
22	72	5877
23	73	5787
24	75	5697
25	77	5607
26	79	5518
27	81	5430
28	82	5340
29	84	5252
30	86	5165

For 10000' DA		
Temp C	Temp F	Fly Altitude
4	39	9165
5	41	9071
6	43	8979
7	45	8885
8	46	8793
9	48	8700
10	50	8608
11	52	8517
12	54	8425
13	55	8334
14	57	8243
15	59	8153
16	61	8062
17	63	7972
18	64	7882
19	66	7792
20	68	7702
21	70	7613
22	72	7524
23	73	7435
24	75	7347
25	77	7258
26	79	7170
27	81	7082
28	82	6995
29	84	6907
30	86	6820

For 12000' DA		
Temp C	Temp F	Fly Altitude
4	39	10796
5	41	10705
6	43	10613
7	45	10521
8	46	10429
9	48	10338
10	50	10247
11	52	10157
12	54	10066
13	55	9976
14	57	9886
15	59	9796
16	61	9706
17	63	9618
18	64	9529
19	66	9440
20	68	9351
21	70	9263
22	72	9175
23	73	9088
24	75	9000
25	77	8913
26	79	8825
27	81	8739
28	82	8652
29	84	8566
30	86	8480

 

[kentb]

Dan, the alt to fly is...

The altimeter reading after you set the colsman to 29.92, correct?

Kent

 

[dan]

The altimeter reading after you set the colsman to 29.92, correct?

Yes, I believe so.

If you google "density altitude calculator" you'll find lots of javascript based tools to use, i.e. http://wahiduddin.net/calc/calc_da.htm

 

[gmcjetpilot]

Errors

Getting or measuring accurate temp and static pressure (altitude) is difficult sometimes due to instrument or probe error.

To get accurate static pressure (altitude) use a trailing cone. You take off it land with it dragging. Don't laugh that is what Boeing does. Actually they have it on a reel. The idea is measure static pressure away from plane. Makes sense and really needed if you want to get a grip on static error (altitude and airspeed). Just don't snag it when landing. Probably have a ground crew at side of runway to disconnect it so you can taxi.



Here is more on pitot static error. Of course static error affects airspeed indicator. This is pre GPS but its a golden oldie.




Temp is hard to measure, especially if your temp gauge/probe is in the slip stream of the engine heat. Calibrate, compare (other planes, weather reports, lapse rate, atis, on the ground) and determine error. There is some RAM RISE in temp at due to airspeed (friction) but its fairly small but not Nil. If you want to be supper accurate you should correct IAT - indicated air temp for airspeed to get TAT - True Air Temp.

Just saying Junior scientist, if you want to be accurate. None of this needed for most SWAG close enough pilots. Also GPS has really helped in airspeed calibration. If airspeed is accurate than you can be pretty sure your static source is close enough for accurate pressure altitude.

 

[kentb]

Dew points make a difference.

After playing with some of the online tools for doing density altitude calculation, I am now wondering how I am going to do this.

I want to test my plane at 8000 ft (need some point to compare against other and my self after make changes. So this is what I was thinking.

Take off from the airport and set one of my altimeters to 29.92. When I reach alt, use the chart that Dan posted to to adjust alt based off of OAT.

I can get the due point from the airport AWOS, but how do I factor that in?

I guess if I had the exact formula I could compute it in flight after I found the OAT at alt.

Kent

 

[kevinsky18]

maybe what we need is a nice little computer program. Climb up to 8000, do your test flights, record your numbers and return home.

Just plug in the altimeter setting that you had, air temp at altitude and all other numbers then have the program do all the corrections.

It's all basic formulas and could run on something as simple as a PDA.

 

[kentb]

Kevin, the problem is...

If you want to compare two flights, you need to do them under the same conditions.

Lets say 8000 as a comparable alt. That means that one flight might be done at 6700 ft and the next at 7200 ft for the engine and plane to be working in the same air condition during the test. This would be true if the temp or dew point were different when the test were done. Oh yes and the current pressure, but that is easy to compensate for by changing the colsman value in the alt.

You need to use all the adjustments (alt, dew point, temp) to determine to corrected alt to fly at during the flight.

Kent

 

[ScottSchmidt]

When doing my flight testing (performance), I am setting my pressure to 29.92 which will give me pressure alt. How do I factor in temp?

If I fly at 8000 ft pressure alt. at a temp of 53F degrees, what is the true alt for testing purposes?

Kent

I would recommend using a sextant.

 

[ScottSchmidt]

I now you are being a wisenheimer, but tell me how to use a sextant for altitude?

All you have to do is attach a Garmin 496 to your sextant and your sextant will be within +- 50 feet.

I'm sure there are some MIT guys out there who have found a way to do it.

 

[hevansrv7a]

Sextant requires known altitude?

In "Alone Over the Tasman Sea" Sir Chichester writes that
http://www.gileschichestermep.org.uk/francis-chichester_cv.htm
in 1931 he used a sextant to navigate in his open Gypsy Moth. He had to fly at wave top level and manage the stick with his knees while taking sightings. He notes that a known altitude is necessary to correctly use the tool. He was first to fly the Tazman Sea E-W. On the way he had to do a major rebuild of the wreck with local unskilled assistance and materials.

 

[kentb]

It is not true alt. that I need

My GPS with give me true alt and my altimeter is calibrated accurately. It is the equivalent air altitude that I need for speed testing.

Kent

 

[gmcjetpilot]

Good one

All you have to do is attach a Garmin 496 to your sextant and your sextant will be within +- 50 feet.

I'm sure there are some MIT guys out there who have found a way to do it.

Sorry I deleted my message, ha ha ha, good one.

Actually, a side note the SR-71 had star nav, an automated celestial nav system (sextant) that took star shots and was its main navigation system at altitude and over hostile territory. This was way before GPS of course. I think it could calculate elevation to some extent. Any early SR71 drivers, engineers out there?

 

[Ron Lee]

I never worried about dewpoint.

Set the altimeter to 29.92 then go to the right pressure altitude needed based upon the temp. Note that you have to adjust the target pressure altitude as you climb/descend because of temp changes.