Part 5 of “Aspects of Aircraft Design and Control”

Project 5 — Wind tun­nel design
Lec­ture 5 — Scale and testing

The aim of this fifth project was to work with the main flu­id mechan­ics notions and phe­nom­e­na at play in a small, sub­son­ic wind tun­nel. The accom­pa­ny­ing lec­ture was a review of the impor­tant notions (the­o­ret­i­cal flow, the effects of fric­tion, and scal­ing tech­niques) need­ed to work on the project.

This was by far the rich­est and most ambi­tious lec­ture of the course. I set out to sweep across a very large amount of mate­r­i­al in a lec­ture that was struc­tured as though it was a review of the entire flu­id mechan­ics dis­ci­pline. In ret­ro­spect it is easy to see that this was rather fool­ish, per­haps in part because the cov­er­age can be nei­ther thor­ough nor com­plete (and indeed impor­tant parts such as com­put­er flu­id dynam­ics or com­press­ible flow are not even men­tioned), but espe­cial­ly because the lec­ture then lacked focus.
The stu­dents were not helped by this lack of clear pur­pose. I too had dif­fi­cul­ties, as I was tak­en aback by how lit­tle con­nec­tion I could make with the student’s exist­ing knowl­edge. Final­ly, because of the sheer vol­ume of mate­r­i­al, the Eng­lish lan­guage was a par­tic­u­lar­ly heavy over­draft that day.

Nev­er­the­less, I believe the lec­ture was a decent attempt at putting togeth­er the puz­zle pieces acquired over the semes­ters. Indeed, I am con­vinced that the largest short­com­ing of our flu­id mechan­ics and deriv­a­tive cours­es are that they fail to explain where and how the dif­fer­ent types of analy­sis fit togeth­er in the big pic­ture. For exam­ple, com­press­ibil­i­ty effects and bound­ary lay­er the­o­ry are always taught entire­ly sep­a­rate­ly, so one may nev­er find out in uni­ver­si­ty that they in fact may both togeth­er be use­ful for the study of a par­tic­u­lar flow. I hoped to present famil­iar con­cepts under the light of their use­ful­ness (or not) in study­ing sim­ple real-world applications.

The project focused on applied aero­dy­nam­ics, and required mix­ing ele­ments of design, scal­ing tech­niques, the­o­ret­i­cal flu­id mechan­ics, and approx­i­ma­tion tech­niques. I hoped to give stu­dents a taste of the aerodynamicist’s job by com­ing to grips with the shock­ing amount of restric­tions inher­ent to wind tun­nel test­ing, as well as the over­whelm­ing pres­ence of empir­i­cal, expe­ri­ence-based rather than the­o­ret­i­cal work.
The project pur­pose­ly left a lot for the stu­dents to decide by them­selves, and I nev­er thought they would hate this as much as they did! I could not con­vince any­one that there is no opti­mal wind tun­nel design at all, lest even that the objec­tive was to get a grasp of the main notions at play, not pro­duce cut­ting-edge data. Clear­ly the thresh­old that must be over­come to reach the most inter­est­ing parts of the project was too high.

I think the main miss­ing ingre­di­ent in this ses­sion was fun. The path through the lec­ture and project is full of dis­com­fort, as none of the beau­ti­ful the­o­ries, neat-and-unique results, and usu­al cer­tain­ties hold — real aero­dy­nam­ics is hard!
In the next edi­tion I will cut down as much as I can on the lec­ture and project con­tents so that the key aspects of the project can be han­dled ear­li­er on. I will work on get­ting more of my enthu­si­asm for the top­ic through. Stu­dents report­ed that they liked this project least of all although they felt it was inter­est­ing and impor­tant. That’s a good start­ing point for improving.