Mission performance dependency on altitude

[Basthian Galenkamp]

Hi SUAVE team,

First of all, thanks a lot for the amazing SUAVE software, it has been a pleasure working with it. I'm currently doing  a student project regarding the design of a low  average temperature response aircraft, resulting in an aircraft that  flies lower and slower than the current aircraft around, resulting in no wing sweep  and the use of propfans/open rotor engines. Furthermore, we are applying riblets on parts of the aircraft to reduced turbulent skin friction drag.  

The goal is to use SUAVE to model the aircraft over our defined mission profile, and to compare the SFC and drag breakdownj with our own models. I modified the SUAVE code to account for the open-rotors and the riblets, but my question concerns actually the following: 

I used the 737 tutorial as basis in order to define our vehicle and mission. Furthermore, I  set it up such that we can run the mission with different cruise mach numbers and cruise altitudes. Now, when looking at the fuel burnt in the misison, the program is sensitive to Mach number as it should, i.e. increasing mach number results in more drag and hence more fuel burn. However, the cruise altitude seems not to have an effect, while it should obviously be the case that increasing altitude should decrease fuel burn (for long range mission, most time spent in cruise). I thought that I might have not imlemented this flexible cruise M and h correctly, but when I run the regular 737 tutorial file it also shows no difference when the cruise altitude is changed. Do you maybe give me an insight in how this can be? Do you think I haven't installed SUAVE correctly/ a certain module/package/function is not working properly? 

Looking forward to hear from you!

Kind regards,

Basthian Galenkamp

[Basthian Galenkamp]

By the way,  I am running SUAVE Version 2.0.0 to be able to implement a few scripts I received from another SUAVE uses.

Kind regards,

Basthian

Op vrijdag 18 juni 2021 om 17:17:07 UTC+2 schreef Basthian Galenkamp:

[bdalman]

Hi Basthiang,

I am not part of the SUAVE team, but I might be able to help you troubleshooting. I can say at least that for my own work I definitely see an effect from changing cruise altitude, so the code is capable of simulating it. Here's a couple questions that might help narrow down the problem:

1) When you do vary the altitude, is the change in fuel burn you are measuring actually zero, or just much smaller then you expect? Fuel burn with altitude isn't linear, once the aircraft passes it's (L/D)_max point the drag (or specifically CD) should start increasing with altitude too, which might cause non-intuitive results

2) Have you visualized the mission results (using the plot_mission function or similar) in the tutorial script to verify that the altitude change in the script is being properly reflected each time you ?

3) What atmospheric model are you using (usually set in the base_analysis script)? And what altitudes are you trying?

4) If none of the previous questions help find the problem, I'd recommend posting the mission setup section of your code, and talking about what lines/what process you are actually using to change the altitudes

[ebot...@gmail.com]

Hi Basthian,

That sounds like a great project.

Your intuition is correct, the fuel burn should change with cruise altitude. If you take a look at one of our tutorials here, you can see how altitude affects fuel burn.

Because of software incompatibilities, I cannot go back to SUAVE 2.0. However, I checked SUAVE 2.4's 737 tutorial just now and the fuel burn in cruise does change with altitude.

My suggestion is to take a look at the regional jet tutorial since it already has the infrastructure to plot out the altitude effects for you. Then add in your riblet and open rotor analyses. I also highly suggest you update to SUAVE 2.4, the aerodynamic models are quite good and it will be easier for us to help you.

-Emilio

[Basthian Galenkamp]

Thanks for both your fast replies!  I have rebuilt my models from scratch and the altitude dependency is back, so the problem is solved! Thanks for your suggestions. I received some scripts that were not compatible with the latest version of SUAVE, therefore I'm using V2.0.  Due to time constraints I might not be able to implement my modifications to the regional je tutotial, but I hope I will! The infrastructure looks indeed very promosing. So if I understand correctly, that tutorial shows how one can sweep over a range of values for certain design parameters?  

Besides this, I have another minor question.  As far as my understanding goes, the throttle setting should always be a value between 0 and 1, and if not,  the design thrust should be changed accordingly. Can you maybe confirm this, or otherwise explain me please how this works .

Thanks a lot!

Kind regards,

Basthian Galenkamp

Op vrijdag 18 juni 2021 om 21:20:22 UTC+2 schreef ebot...@gmail.com:

[ebot...@gmail.com]

Hi,

Glad you got it fixed.

As far as throttle, yes, you're correct. The other alternative to look at when you have a throttle over 1 is whether the climb rate is reasonable. If you take a look at our 737 tutorial, it was intended to highlight that SUAVE can "over-throttle" an airplane. This allows for smooth convergence in optimization cases. If your mission is fine, than the next step as you mentioned is to change the design thrust and resize the engine.

Best of luck!

-Emilio