Presented by Nine Lives Racing
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Sparkydog HalfDork
8/19/20 3:49 p.m.

Wow you guys are way out of my league - and I minored in Fluid Dynamics in college! Perhaps this is a dumb question, but as you play around with the relative position of the two airfoils to each other - aren't you essentially doing what airplane wing designers haver already figured out with the spacing and position of wing flaps during take off? (High lift, low drag)


Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
8/19/20 4:34 p.m.

That's exactly what we're doing, but we have fixed (for now) profiles that aren't exceptional in any way. They are in fact eBay Megan racing wings.

So the goal is how can we optimize without changing the profile.

But to sleepy's point, fixing the trailing edge of the main foil seems like an easy big improvement here.

Sparkydog HalfDork
8/19/20 5:29 p.m.

Thanks, I also probably didn't finish my thoughts well. So my suggestion would be to find a make/model of airplane that has a takeoff speed at about what you want to optimize the car wing for, (which probably means a smaller, private aircraft) find somebody who owns one, have them set the flaps for takeoff and measure AOA and relative spacing using chord length as the unit of measure and then migrate this info to the eWings and see what the model shows. Then iterate from there.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/20/20 2:29 a.m.

In reply to Sparkydog :

so.... yes, and no

yes, this is kind of what aircraft engineers do; no we can't just 'go copy an existing wing design'.  The size of the wing, and the speeds Robbie is going to see, mean we're down in the laminar "Low Reynolds Number" range.  So, we can't just use the "similar takeoff speed" thing, because the chord of Robbie's wing is significantly less than any fullsize aircraft (and that's without expending 500 words talking about how aircraft wings have different constraints and considerations that aren't applicable here).

Another thing is... this is for GRM Challenge.  So we've got to factor in time and cost.  Which, I see how 'matching an aircraft' would try and do that.  But, we're working with an existing foils... and we need to approach maximizing those foils, and do as much as possible in the simulation so that it's 'as close as possible' when Robbie actually starts butchering up those endplates and/or turns a wheel in shakedown.

For Autocross, Robbie wants to be more in a "landing configuration"... the drag almost doesn't matter, just because the speed is so low.

And, yeah... going this way means Robbie can leverage the skills/insights/computational power of someone half a world away... while hopefully learning something about airfoils/aerodynamics at the same time.

Now, all that being said... thanks for the picture... because the extension I'm advocating for being added is similar to and will help this section function more like 'a real' aircraft wing (just without that small turning vane element):



Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
8/21/20 10:00 a.m.

Nice. Yeah I need to play around with adding a simple sheet metal extension to the bottom trailing edge of the main element. 

Also, I think I can point the exhaust at the bottom of the wing - I think this will help improve performance when on the throttle. Any way to estimate the effect of that? 600 cc motor that redlines at 15k rpm. Powerband is approximately 10k-15k. 

Thinkkker UltraDork
6/4/21 6:24 p.m.

This just leads to the conclusion I need to get my own computer to use for things.


Question, could you theoretically use airfoiljava to do calcs on diffuser design?  

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
6/4/21 10:56 p.m.

Probably need sleepy to answer more specifically, but my answer is theoretically yes but realistically not really.

Javafoil would need to look at the cross section of the entire car, and then you can play around with the diffuser shape to see what's best. But there are lots of details on the car that will be hard to model in javafoil.

Thinkkker UltraDork
6/5/21 8:57 a.m.

Yes, was curious to look at a couple of design thoughts I have and wondered if a "block" would get close enough to answer some questions.  


stafford1500 GRM+ Memberand Dork
6/6/21 7:24 p.m.

Java foil can only do a slice of the car, typically the centerline. To do a proper diffuser analysis, you need to consider the 3D influences. Some very basic analysis can get you moving toward a conservative design and help figure out placement and size, but will not give enough information to estimate the downforce very well. You really should model the shape of the car, rather than just a simple block since the shape of the front end of the car impacts the diffuser performance (this is an effect of running at subsonic speeds).

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
6/8/21 3:22 a.m.

I've also been finding some significant challenges with Javafoil of late, especially with it's response to coordinate distribution and viscous flow analysis in comparison to xfoil.  However, some of my issue is primarly in the 100,000-700,000 Re ranges, where most cars (even at 45mph) are well over 5,000,000.   The coordinate distribution thing, is a little more difficult to navigate.

I plan to get familiarized with xflr5, which is an independent GUI development of xfoil, which has 3D effects (sadly, afaik, no scripting)... and then eventually look at jumping over to the next step of that "flow5", which is subscription fee based.


stafford1500 said:

You really should model the shape of the car, rather than just a simple block since the shape of the front end of the car impacts the diffuser performance (this is an effect of running at subsonic speeds).

what about the "chamfered block" form discussed in Katz's Race Car Aerodynamics?
edit: which apparently is actually from a 1998 SAE paper #980030 by Cooper, K.R.; Bertenyi, T; Dutil, G; Syms, J; and Sovran, G. "The Aerodynamic Perofrmance of Automotive Underbody Diffusers"

I agree, though, that Javafoil would probably be best used to give you 'trendline' information for 'deflecting'/'tuning' the diffuser after it's built... and you're then working in the real world.  I'd caution against using Cl/Cd output numbers to be a 'real world indication' of the aerodynamic performance.  Also, I'd expect that packaging is probably going to be a primary driver of diffuser shape, than aerodynamic performance?

Barb_Dwyer New Reader
7/28/21 4:47 p.m.

In reply to Robbie (Forum Supporter) :

Maybe MS Paint will show you where the cursor is on an image?

Mr_Asa PowerDork
7/28/21 4:51 p.m.

Squint GIFs - Get the best GIF on GIPHY

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
7/28/21 6:33 p.m.

Yeah ^ barb is definitely an intelligent canoe. Just parrots other comments and changes the words with a thesaurus.

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