Quote:
Originally Posted by Tesla
One of the primary contradictions are the semi truck boat tails, they are consistantly setting their ideal angles at a straight 12-15°, this does not appear to be an arbitrary guess that contradicts boattail theory, they are not doing 22° or 5° as the template theory would suggest as an ideal start.
So something else is happening to suggest that with longer vehicles or shorter boat tails that the optimum diverges from the template, my thoughtsm are that it rests with the boundary layer thickness and its ability to maintain attachment in proportion to it's thickness.
It may well be likely that if the truck boattails extended in length beyond 4' or continued to increase angle that the limit of attachment may well be 4' at 12-15°, but for many vehicles this may be an important distinction as there are very few cases where a full boattail is possible and length seems to be the overwhelming compromise.
The difference been that with a short boattail using the template you get a 20% wake reduction or with a more aggressive short tail you get a 50% wake reduction, that makes for a big difference in FE if the theory holds true, what does it matter if we get seperation just before the end of the steeper angle tail, the vehicle is seperating from the air at that point anyway and we would be leaving a smaller wake to hold the vehicle back.
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*tails on commercial vehicles are 'optimized' for their arbitrary length restriction,overall weight penalty to payload,considerations for manufacturing cost,on-road durability,as well as 'pain' to any driver who must deal with the tail when loading/unloading cargo.
*No thought is given to what a rig would experience with an 'ideal' tail.
*If you're interested in a limited-length tail,I would recommend the GM' 'Optimum' tail,as you will find in the Aero Lab at Texas Tech Universities' College of Mechanical Engineering.It possesses all the features you're interested in.
*Outside of the context of legal length restrictions we can do better.