NEW AMBIENT TEMP GAUGE FOR JETS

[Tom Goodrick]

NEW AMBIENT TEMPERATURE GAUGE FOR JETS
« on: Apr 22nd, 2007, 10:11pm » Quote Modify Remove

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I have noted recently that J Mac McClellan, editor of FLYING, is always talking about how much the temperature in C varies from "standard" when he tests jets. He claims this causes serious performance changes over what you would get in standard conditions. I decided to look into this. The first thing I found was thet the temperature gauge I have been using on all my panels for all my aircraft is no good for jets. It is from the Bell helicopter and shows the temp in F when you are parked. That is fine for telling you whether to zip up your jacket before leaving the plane.

I didn't know if there was a better gauge so I decided to make one. Then I tested it and found that there is indeed a big need for it in the jets. The main problem with the other gauge is that it is strongly affected by airspeed, particularly when used in jets. Here is an example of one test. This was done at 20,000 ft using a Learjet 35.

KIAS F (Bell) C (AMB)
300__25____-24.1
250__14____-24.1
200___6____-24.2
150__-1_____-24.2

The conversion from C to F is F=32+9*C/5.

Obviously these readings from the Bell temp gauge are total hogwash.

I did set both gauges in a parked aircraft in different weather files and found they agreed according to the conversion formula:

_F___C
92___33.86
75___24.36
62___16.90
56___13.76 (normal day at 630 ft msl)
32___0.00
10___-11.7
0____-17.3

Obviously a key thing to note when going up and down through clouds is when the temp drops below 0 C as that is when you pick up icing. This occurs on nearly every flight you make in a jet regardless of the season.

Now as to the performance variation of a jet when it is a few degrees warmer than normal. J Mac has proclaimed (10 C warmer than standard) as a factor in the degraded performance from book values. The actual equation for this is

The thrust ratio is:

T/T0 = (2457+9*C0) / (2457+9*C)

where the 0 denotes standard conditions at whatever altitude you are flying. The only direct consequence of the non-standard temperature is the change to the thrust level T from the thrust level T0 you would get with the same throttle setting at the same altitude in standard conditions. The equation does show a dependency on the pressure ratio but this is the same becuase you are at the same altitude as indicated by the altimeter which is showing the same pressure ratio. Working through this equation, a 10 degree C increase in temperature will cause a 4.4 percent decrease in the thrust.

Big hairy deal. Increase the throttle by 4.4% and you get the same speed with a slightly greater fuel flow.

Well at least now we can all experiment to see how the performance does change and what our options are as pilots to deal with the situation. I have put this new gauge on all my jets and will be keeping track of what changes I see in speed or fuel consumption. I have a large number of weather files to work with covering a wide range of weather conditions.

I have put a little 2k zip file on my web site tonight that you can download to get this file (AMB_TEMP_C.XML).

Here's a short list of standard temperatures for some altitudes that are often used with light jets.
________Book*__FS9 (Calm Weather)
41,000 ft -55C___-56.1C
40,000 ft -55C___-56.1C
36,000 ft -55C___-55.9C
35,000 ft -54.3C__-53.9C
29,000 ft -42.5C
28,000 ft -40.5C

*Perkins and Hage, 1965

There are many forms of "The Standard Atmosphere" with minor variations. All show a transition starting at about 35,000 ft to a constant temperature profile. How well that works in real life I don't know. It will be interesting to find out. In FS9 the temp appears to be constant at -56.1C from 37,000 ft up. This leads to the simplified equation:

T/T0 = 1952 / (2457+9*C)

This will show a variation of just under 5% in thrust ratio for a 10 degree C variation in temp from "normal" of -56.1C.

In a much more complex equation the speed that results from changes in both thrust and weight can be seen but the variation is not clear and is rather meaningless because you can either set normal thrust and take the normal fuel flow with a slight reduction in speed or dial up the thrust to keep the normal speed. You lose a little when it is warmer than normal and gain a little when it is colder than normal.
« Last Edit: Apr 23rd, 2007, 10:19am by Tom Goodrick » 216.180.4.187



Tom Goodrick
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Re: NEW AMBIENT TEMPERATURE GAUGE FOR JETS
« Reply #1 on: Apr 26th, 2007, 3:57pm » Quote Modify Remove

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I have collected a bunch of temp data and performance data in various temperature conditions. There is something important that the thrust ratio equation misses. The true airspeed will be higher if the temperature is higher at a given altitude. This can cancel the effect of the lower thrust.

I'll present the temp data when I get a chance to type it up. Meanwhile, consider this. If we are interested in the performance of a particular aircraft in various conditions, we should keep a more complete flight data log such as the one shown below. I have been working with this today and find it fairly practical. I keep it minimized on the bar under the panel and make entries as I fly. The temperature must be read without pausing the sim.

FLIGHT DATA LOG:
DATE 4/26/07 4/26/07
RW Used? Y Y
ATC Used? N N
AIRCRAFT Pip Mer Ces Must *
FROM KHSV KHSV
TO KAHN KAHN
DISTANCE 178.4 178.4
START FUEL 135 286
PAYLOAD WT 780 780
GROSS WEIGHT 5054 8048
CG POSITION 28.06 23.08
Surface Temp C 19.34 19.53
START TIME 9:26 9:28
CRUISE DATA:
CRUISE ALTITUDE 15000 25000
OAT C -6.1 -26.1
THRUST/POWER 60 48
KIAS 173.8 248.6
MACH 0.35 0.6
KTAS 219.7 361.6
KGS 249 395
Fuel Flow pph/e or gph 205 401
NMPG 7.21 3.01
FLIGHT HEADING 104 103
START WIND DIR 228 233
START WIND SPD 46 59
LATE WIND DIR 218 227
LATE WIND SPD 43 56
END DATA:
FLIGHT TIME (min) 52.1 36.24
LANDING KIAS 70.91 88.29
LANDING FPM -165 -98
Surface Temp C 21.95 22.75
END FUEL 105 179
FUEL USED 30 107
Block Speed: 205.5 295.4
Block NMPG 5.95 1.67
Notes: Broken Cu Tail Wind
Scat Storms

* Aircraft are the Piper Meridiam single-engine turboprop and the Cessna 510 Nustang twin jet.
« Last Edit: Apr 26th, 2007, 4:02pm by Tom Goodrick » 216.180.4.140



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Re: NEW AMBIENT TEMPERATURE GAUGE FOR JETS
« Reply #2 on: Apr 26th, 2007, 9:00pm » Quote Modify Remove

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Air Temperature at Various Altitudes in FS9

All Temps Deg C

Alt_ft____Clear Wx__1/16/07__4/16/07__7/16/06__10/16/05
12k_______-8.55____-15.1____-1.41_____7.96_____-0.26
16k______-16.6_____-21.1___-10.0_____-0.33_____-8.17
20k______-24.4_____-27.2___-19.1_____-9.38_____-15.8
24k______-32.2_____-34.4___-28.0_____-18.5_____-24.0
28k______-40.0_____-40.7___-37.5_____-27.9_____-34.3
32k______-47.8_____-46.1___-46.0_____-37.5_____-42.9
36k______-55.9_____-49.0___-49.4_____-45.2_____-47.9
40k______-56.1_____-50.0___-49.5_____-49.2_____-49.7
44k______-56.1_____-50.0___-49.4_____-49.0_____-49.9

The "ClearWx" condition is supposed to be a "standard" condition but I don't know what standard it might be. It does not agree exactly with the standard conditions I have in books but it is not far off. I have not checked it against some of the "standard atmosphere's" I used at NASA. (I had to include six in an orbital/aeronautical sim I wrote but those started at 20 km altitude and ended at 120 km. I only carried the density values in the data base because that is what causes aero-thermal heating.) The one thing that bothers me about these tables is that they show the constant temperature region above 35k is different for the Clear Wx compared to all the RW files. All the RW files are about 6 deg C warmer in that high altitude region than the "standard" file.

I started to run a comparison of performance using the Cessna Mustang I've been working on. I cut the study short and will show only the comparison of one condition - a full load (all seats filled with adults+overnight luggage) at 28,000 ft in the "Clear WX" compared to the July RW file. At 28,000 ft, the July air is warmer by 12.1 deg C. Here are the results:

_______Clear__July
Thrust__50%___50%
KIAS__240.8___244.1
Mach__0.61___0.61
KTAS_362.7___375.5
pph/e__389____387
NMPG_3.12___3.25

Warm air seems to improve speed while using the same thrust and getting the same fuel flow.
« Last Edit: Apr 26th, 2007, 9:01pm by Tom Goodrick » 216.180.4.173



Pages: 1

[hanspetter]

Could you please provide an explanation why high speed per se gives a wrong temperature read-out? The only physical factors that come to mind are compression/decompression, friction and evaporation. The latter will be an issue when a thermometer is wet and the moisture is evaporating -- this cools the thermometer below ambient temperature and causes a false low reading until the water is gone.

[Tom Goodrick]

Sorry, Hans, you are missing the point. I cannot provide you with an explanation of the physics because there IS NO EXPLANATION for cooling of inanimate objects.

My only explanation here is that who ever designed the Bell 206 gauge, inserted some code that gives an erroneous cooling effect. It is binary code in the .gau file so I can't read it. (I no longer have a suitable dissassembler.)

The Ambient temp gauge I wrote sees no effect of speed. It simply reads the ambient temperature.

When you reach about Mach 0.8, the real world imposes a slight discrepency on temperature gauges because of the shock wave field that develops locally on parts of the aircraft where such a probe may be located. Various ways have been worked out to get an accurate reading. The ambient temperature changes across a shock wave. Flow around a leading edge, a fairing or a nose, may be accelerated above mach 1.0 when the "freestream Mach" is 0.8 or higher. But this is nothing like what the Bell 206 gauge shows at 150 knots! That gauge is just plain WRONG.

I am sending you a copy of the ambient temp gauge so you can try it. The above article was written in April of 2007. Very few, if any, people ever asked for it. It is an msl gauge so I'll send you the text source code. Change the extension from .txt to .xml and it becomes the working gauge code in XML.

Ambient parameters of the atmosphere exist at some considerable distance from any part of the flying aircraft. "Local" parameters are in the flow field surrounding the moving aircraft. Above Mach 0.8 there may be local variations caused by shock waves. These become very significant at higher speeds. At speeds of Mach 2.5 and above there is aerodynamic heating and we begin to see changes in the chemistry of the air. At Mach 10 or above, the air near the nose or leading edges becomes hot enough to melt most materials. But nothing of this sort happens at low subsonic speds - either heating or cooling.