LMP360 Aero Design Discussion Part 1: Front Element

the Clark Y isn't an inherently bad foil.

In a modern context it mainly has the issues of being relatively thin (11.7%), somewhat low camber (3.7%, which is especially low in a motorsport context), it's thickness is a bit forward (28%) for promoting Natural Laminar Flow (that's more a modern aircraft thing)... but a bit back for promoting Clmax at LowerReynolds numbers {many caveats here about LeadingEdge shape and TrailingEdge shape to manage flow deceleration; but closer to 20% seems to be the better location), and the nature of the bottom surface means it can have issues with laminar separation bubbles at low Cl's (especially at 'low Reynolds Numbers' flows) which impacts L/D for "cruise"... and/or it can't take advantage of "aft loading".

a lot of these things disappear in a motorsport context, other than the limited camber, and aft loading.

as it happens, I tried out a "double Clark Y" setup when I was banging around at solutions, with the addition that I blew it up to 15.5%  and made the 2nd Clark Y element be 33%... so about 5inches.  One issue with the approach I took is that the bottom isn't quite flat... so this might be over-predicting a tad; but it was a quick/easy way to test out a thicker section. 

Again, "Free Air, 2D Estimate".  So, it's generating 10% more downforce than the "circular nose" 'refinement option #1' above, for a 65% drag increase.  So, I think maybe something in between is maybe what we should look for (for the front; I think your Rear Main Foil is "good for now", and this is in-part to show that the rear 2nd element foil being 'Clark Y' is good too.)

since you're open to rebuilding the whole front aero setup... which would probably also help reduce its weight, and improve the lower surface of the main front element... which might be contributing to some of its downforce impairment.  I think doing that, and drooping the front edge to ~3deg would have some benefits.  I'll work today about refining a "main" foil design; as well as pull a "clark y" version of the "tube and taper" 2nd element I proposed on the last page.  I think the design would have a lot of benefit in ease of mounting the second element with a consistent gap while also being about to adjust the back edge to tweak the 2nd element to tune front/rear balance.

The concern about aft loading on the main-plane is reduced with the second element slot gap jet helping draw the main-plane trailing edge. Your models with the extreme Cp shifts are local stall/turbulence that are not handeled well by the code. Large mesh count/reduced cell size may help...

stafford1500 said:

The concern about aft loading on the main-plane is reduced with the second element slot gap jet helping draw the main-plane trailing edge. Your models with the extreme Cp shifts are local stall/turbulence that are not handeled well by the code. Large mesh count/reduced cell size may help...

Another side benefit to the Clark Y "negative pressure side flat TE" shape, in the context of using it for the main element of a double element wing, is that once the main element in inclined w.r.t. the flow... then the flat surface into double element gap "probably" means that the 2nd element (with a sizeable front nose radius) will have more consistent reaction to changes in the 2nd element's angle (and maybe also more leeway to gap size/location?).  whereas the convex shape of an "aft load" is going to make angle and gap more intensive to optimize?

stafford1500 said:

Your models with the extreme Cp shifts are local stall/turbulence that are not handeled well by the code. Large mesh count/reduced cell size may help...

I think, honestly, I'm going to try and just move away from "using"/"suggesting" foils in the future that have harsh/90deg corners.  plus, I've tracked down a better web plot digitizer... so I think I can generate "physical" models and suck them into JavaFoil/XFoil/etc better in the future.

In reply to sleepyhead the buffalo :

I have had reasonable luck with Javafoil and multi-element shapes in ground effect. I t did take some significant computational space/time but ended up with pressure plots and flows that "looked" right and gave reasonable results based on my experience.

Yes, highly loaded/cambered foils will tend to get picky about slot gap between elements. I adjust them in 0.1% chord increments in Z and X when optimizing.

alright, I haven't been idle... I've got an initial design on the front main foil,

and am fighting through trying a new digitization scheme which should produce better results (fingers crossed)

but, I've got some sleepyfamily things to go take care of.  then i'll try to get some tweaks to those coordinates done, and a first cut run in JavaFoil.  Then, repeat for the second element.

Some initial results, compared to the 15% Clark Y modification.  I'll have some more work to do, making a second version of the "new" foil with a "striaghter" trailing edge, per nocones' request...

also, pretty clearly these coordinates need some smoothing around the nose to the maximum thickness.  Dunno how much that's going to change the results, which are slightly better than the ClarkY modification (not too surprising, since this has a tad more camber.

I've managed to smooth out the coordinates for the main foil, and have focused in on the "pointy" version of it since it will be easier for nocones to build based on his current method.

this has a modest Cl increase over the previous 15% thick Clark Y modification, and a modest reduction in drag over the old foil.

here's the concept for the 2nd Element foil:

as mentioned, it's based on a 1" dowel or tube front, a 'flat top' (which I expect to be the bottom during the build), and a straight tapered surface.  this is the orientation I expect on the car, with the 'tapering'/'cambered' side 'down', and the 'dead flat' side up to be used to tweak the second element deflection angle.

pop the two of them together, reduce the chord of the second element to 6" (40% the 15inch main element chord), crank the 2nd element to the 'easy first angle' of -15deg, Re of 1,000,000... set a conservative gap of main.TrailingEdge (TE) lined with 2ndElement.LeadingEdge (LE) and 6 %of main chord gap (i.e. 1") and take a look...

compared to the initial proposed "just change the 2nd element" configuration with the $ChallengeFoil main element setup, this is showing 20% more downforce, although at a bit more drag... but, still much better than before.

now for a new wrinkle, let's crank the whole think down to -3deg (nocones and I have talked about how much pitch the car has, and setting the front to -3 means even if the car squats the front elements should stay at 0deg relative at most)

that is tantilizingly close to the ~3.0 Cl I'm looking for from JavaFoil, to have some confidence in the outcome; and drag is staying down in the 'low 0.042s' (compared to the 0.055s of most of the other setups on the last page).

there's a subtext to stafford's comment further up this page.... call it a good-natured aero-ribbing.  He can see, just from the flow fields, that I'm being very conservative with my gaps.  I'll be a bit more explicit than I have been in the past two pages... I've been keeping these gaps fairly large (and deferring to lining the TE and LE up 'horizontally') out of a combination of expediance, and simplification.  6% vertical gap pops out to around a 1" gap... which should be dead simple to measure and install (hopefully);  and hopefully it's dead obvious how aligning the TE to the LE would simplify fore-aft positioning of these two elements.

well, "we" don't have a whole lot of time to mess around with optimizing the gap... and who knows how much nocones want's to mess with that.  but, after a "quick and dirty" (i'm a buffalo, afterall) 15min optimization of the gap, coming down to 2%chord vertically and having a 3%chord overlap (this is pretty close to lining up the 'tangential low' of the rod/dowel/tube with the main foil's TE.

and there's another 5% of downforce gained, as well as a 5% reduction in drag.

and (I don't have picture of it, but I did a quick 'velocity tab'/Cp sweep of 0 to -5 deg AoA... and a -4deg this setup his "the magic" 3.0.

I reckon, though, it's about time to let nocones build some things... see how they come out (is easier to mount this stuff at 3,4,5deg?)... and do some quick tweaks to the second element to try and narrow the gap of adjustments required when the car offloads at NCM.

Looking good. Simple is always the right direction for initial studies but optimizing always introduces lots of factors to track. The slot gap variability is powerful as you saw in both down force and drag. Don't 'optimize' too close to the edge for this case since some of the extra factors will not show up in your simulation (crosswind, other cars, and even the car tire combination the wing is attached to).

The parts do look simple enough to build quickly and accurately. Keep it up.

got this from nocones today.

I've pull a curve, put it into webplot, scaled to dimensionless units, got the foil about where it should be "in space", and can do a inviscid solution at 2deg....

which shows I've got a bunch of clean up to do... especially around the nose (you can't see it here, but there's a bunch of 'wiggling' going on there that xfoil is chocking on.  plus, I need to tweak the TrailingEdge so it's a bit closer to 'closed'.  Maybe-hopefully will get that done tomorrow night.  Then, a big push Tuesday to have a thing for nocones to stare at maybe make informed decisions about "how do I make an informed adjustment before the next session"?

alright, I've managed to fix the nose on the 2nd element; and then found that the trailing edge also has some issues and manually tweaked the points there a bit; and then I work through filtering/cleaning up the foil so the xfoil output isn't as jagged.

next was the process of I pulling those coordinates into javafoil and renumbering the points to the dispersion it favors... which created more issues with the trailing edge; so I had another round of fixing the trailing edge manually in xfoil, before having a section that worked in javafoil.  then I sized it down to 0.4c (i.e. 40% the size of the 15inch main front foil), and worked on the 'base' angle to measure the 2nd element from (compared to the 'traditional' aerodyanmic angle based on a chordline that passes through the LeadingEdge and the TrailingEdge {this meant tilting the trailing edge of the 2ndElement foil up 9deg; so the main 'flatish' portion aft of the rounded leading edge and the crank {that adds camber and aid forming the trailing edge} is flat.)

now we've got to position the 2nd element so the gap is where nocones said it was in the build thread {3/4inch overlap, 1" gap.... which is 95% chord, and 6% vertical gap {this is a 14%chord vertical movement from this 'base' positioning}).

I did a first check at what I assume is the 'rigged' setup based on the simulations run before things were built (2nd element at 15deg, and it and the main foil inclined together at -3deg w.r.t. 'wind'/'ground' outside of ground effect...

I suspect this might be a tad too much (although, I need to go back through the aero balance sheet I've built and verify that).

Because it'd be easy (and JavaFoil wouldn't crash on me because of the flowfield streamlines)... I decided to iterate the 2nd element from -5deg {green}, -10deg {red}, -15deg {purple}, -20deg {blue}, each time with the whole setup swept from 0 to -5deg:

based on that sweep of sweeps, I think the initial setup should be with the main wing around -3deg, and the second element at -10deg.  Downforce "looks" good there (~3Cl I think is required), and drag is actually better than the foils I designed by a smidge (0.045 compared to ~0.055).

Dunno if I'll get around to it tonight; but next I need to pull this polar data into my aero balance sheet and spit out a "make this change to get this % aero balance change"... table? widget?  thing.

I built the table I was thinking about, although not as simplified as I was originally thinking... in part because bulding it up, I'm realizing that "I" might have overshot the mark on what kind of downforce/balance the front can make...

which means I've made more work for myself... I reckon I should work up a couple of more of these tables with different deflections of the rear 2nd element in case the front is actually making a much as the simulation indicates (contrary to the first design).

considering I was off the first design, y'all might understand why we erred on the side of "more front is good"