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convexengineering / SPaircraft

Models for commercial aircraft design
http://spaircraft.readthedocs.org
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Pi-tail HT structural model not up to snuff #88

Closed 1ozturkbe closed 7 years ago

1ozturkbe commented 7 years ago

I did some digging (and drew some moment and shear diagrams) and realized that we may be getting wonky HT weight results. I will be making modifications to the structural model soon.

1ozturkbe commented 7 years ago

As a resource, I have been using this to generate my diagrams. It is awesome.

1ozturkbe commented 7 years ago

Sample load distribution, assuming that the lift per unit span is proportional to local chord:

samplefbd sampleshear samplebending
1ozturkbe commented 7 years ago

I will need to add a few variables to the model to be able to more accurately model this. It is very straightforward to model max shear, but the moment will require some work.

1ozturkbe commented 7 years ago

It turns out that these calculations are quite involved, and it's not clear how to split the models between conventional and pi-tail HTs. Will be working on it over the next few days. My current plan is to use the moment at the pin joint as well as the outboard span to calculate the weight of a conventional HT, and then use a linearization of the shear and moment distributions to come up with a good, conservative estimate for the actual HT weight using pi-tail structural factors.

1ozturkbe commented 7 years ago

Will implement issue 89 before I proceed further on this thread... I have already added some extra variables that will help in the better structural model, but they all blow up since they aren't being pressured atm.

1ozturkbe commented 7 years ago

This is evolving in the HTstruct branch, but I am getting some underpredictions for the weight with the new model. Investigating.

1ozturkbe commented 7 years ago

At least the low HT weight is uniform across the board, so I think it just has to do with some scaling factor.

1ozturkbe commented 7 years ago

Hmmm, I think we were underestimating the HT weights to begin with. I compared the results of the new model and the old model for the b737 (expecting there to be no difference), and the difference really was due to numerical precision. It turns out we were always low on HT. Is that possible @mayork and we just didn't notice?

mayork commented 7 years ago

yes I think we were

1ozturkbe commented 7 years ago

Darn, this is problematic... The new pi HT structural model I wrote comes within -30% of D8 predictions, and I'm pretty certain it is conservative. But it is more than 80% off on 737... And that bit of the model has not changed.

1ozturkbe commented 7 years ago

I will do my best to figure out why. It may be we are under on D8 because we aren't accounting for panel weights, and that would equivalently help with 73 a little. but there is something fundamentally wrong with the 73 sizing atm.

1ozturkbe commented 7 years ago

Hmm some weird numerical things going on, variables not being pushed properly (untight constraints). Thinking of adding more surrogate variables, and hoping to figure this out soon.

1ozturkbe commented 7 years ago

As of commit 9c7e9472 the new piHT structural model is good. However,we are still under on conventional HT weights. No clue why. Will look into this in another issue.