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Sun Angle

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Sun Angle

Postby legoboyvdlp » Mon Nov 20, 2017 1:44 pm

Hi there,
Image

I'm trying to do this, in order to simulate fuel tank temperature properly. The wing will either be in direct sun, indirect sun (basically just light, no heat), shadow ((less light, no heat), or darkness.

Could anyone explain the properties required to do this? I remember seeing a property that measured sun angle in radians. But is that clockwise or counterclockwise? Any other properties that could help? Many thanks.
Jonathan
Last edited by legoboyvdlp on Mon Nov 20, 2017 1:49 pm, edited 1 time in total.
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Re: Sun Angle

Postby Thorsten » Mon Nov 20, 2017 1:48 pm

The Shuttle thermal model

https://sourceforge.net/p/fgspaceshuttl ... alance.nas

does what you need (and more) - feel free to adapt to your use case (basically you just need the radiative balance part, not the heat transfer part).
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Re: Sun Angle

Postby legoboyvdlp » Mon Nov 20, 2017 2:17 pm

Thanks a lot!
Will certainly help. But I don't need anything as complex as that xD, I'll probably reduce the complexity a lot. Or I'll get confused and mess everything up.
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Re: Sun Angle

Postby D-EKEW » Mon Nov 20, 2017 4:12 pm

For something simple look at the extra500/Systems/extra500-system-fuel.xml, line 67

This is just a simple interaction between the air temperature and the fuel temperature. The sun radiation is not accounted for as when you are flying its influence is can be neglected. The solar power is only of importance when you park your aircraft for a long time and I don't usually do that in FG...
This is for a carbon fibre wing, so if you are modelling a metal tank/wing, you will need to increase the energy transfer rate considerably.
I also assumed the area only on the bottom as the fuel tank is usually not completely full and air is a very good isolator.

So you need this formula:
https://www.engineeringtoolbox.com/cond ... d_428.html

With dT the temp difference between the air and the fuel.
after you have calculated the energy transfer you can calculate the temperature change of the fuel with the specific heat capacity of your fuel (AVGAS of JET-A1).

Cheers,

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Re: Sun Angle

Postby legoboyvdlp » Mon Nov 20, 2017 5:00 pm

It is a metal wing. Thank you.
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Re: Sun Angle

Postby legoboyvdlp » Wed Nov 22, 2017 7:48 pm

D-EKEW wrote in Mon Nov 20, 2017 4:12 pm: The sun radiation is not accounted for as when you are flying its influence is can be neglected. The solar power is only of importance when you park your aircraft for a long time and I don't usually do that in FG...


I'm just about to get started. Thanks a lot for all your help!
Just FYI, in the Airbus, on fairly hot days, with full sunlight on the wing, the fuel heats up quite fast, enough that sometimes you have to turn off galley power to reduce generator load, thus IDG output temperature (it uses fuel to cool down the generator, and the hot fuel is returned to the tank).
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Re: Sun Angle

Postby D-EKEW » Thu Nov 23, 2017 3:06 pm

Is this because of 'fairly hot days' or 'full sunlight'?

As the generator is running, the engine must be running too. And the engine gets hotter on warm days of course. Jet and larger turboprop engines are cooled by oil and the oil by fuel (only). I know of one jet engine where 50-80% of the fuel send to the engine is not burned, but sent back to the tank typically as motive flow for jet pumps. But only after passing the oil/fuel heat exchanger. So by just running the engine you heat up the fuel in the tank. This effect gets greater when the OAT is higher and of course when there is less fuel in the tank.

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Re: Sun Angle

Postby legoboyvdlp » Thu Nov 23, 2017 3:13 pm

It's both --> the sun doesn't have as much effect, but it does have some. Thank you --> I have the FCOM of the Airbii, so I should be fine ;)
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Re: Sun Angle

Postby legoboyvdlp » Thu Nov 23, 2017 5:06 pm

D-EKEW wrote in Mon Nov 20, 2017 4:12 pm:after you have calculated the energy transfer you can calculate the temperature change of the fuel with the specific heat capacity of your fuel (AVGAS of JET-A1).



Does the specific heat capacity equal:
Code: Select all
 <sum>      <!-- Jet-A1 fuel specific energy kJ/kg-K -->
   <product>
      <value>0.00481</value>
      <property>/systems/fuel/FT-degC</property>
   </product>
   <value>1.7833</value>
</sum>

Will that equal Figure 22 on page 62 of http://www.dtic.mil/dtic/tr/fulltext/u2/a132106.pdf?

Just wondering if you could explain how, precisely you get from the energy transfer in kJ to a temperature change in degrees per time unit? Many thanks.
I have no problems in calculating the first part using that formula. I just don't understand how to get from the energy transfer to a temperature change.
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Re: Sun Angle

Postby Thorsten » Thu Nov 23, 2017 6:08 pm

Just wondering if you could explain how, precisely you get from the energy transfer in kJ to a temperature change in degrees per time unit?


Heat capacity tells you how much kJ you need in order to get a certain temperature increase in K (or degC) - in reality heat capacity is a function of temperature (and pressure,...) which makes this complicated, but in the interesting regime you can usually assume that it's just a constant - temperature increase is energy gain * heat capacity.
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Re: Sun Angle

Postby legoboyvdlp » Thu Nov 23, 2017 6:40 pm

Many thanks.
Jonathan
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