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Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/7/20 9:26 p.m.

A few of us are joining together tomorrow to talk aerodynamics with respect to project F-dat, and we want to welcome anyone who is interested in listening in. There was a bit of discussion in the build thread itself, but we decided there was probably enough for a full discussion. 

Planned time is 2 central (3 eastern) tomorrow, if you would like to join send me a PM and I will forward the google hangout invite. 

Of course, I will also take notes and will post them in this thread as well.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
11/8/20 7:40 a.m.

Sunday front page bump

Stampie (FS)
Stampie (FS) GRM+ Memberand MegaDork
11/8/20 8:19 a.m.

I'd love to but working today.  Can you record the zoom?

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/8/20 12:25 p.m.
Stampie (FS) said:

I'd love to but working today.  Can you record the zoom?

I'll see what I can do. At the very least I'll post good notes.

Stampie (FS)
Stampie (FS) GRM+ Memberand MegaDork
11/9/20 8:27 a.m.

Well did you solve all the world's aerodynamic problems?

¯\_(ツ)_/¯
¯\_(ツ)_/¯ PowerDork
11/9/20 9:00 a.m.

Doing this while I was at an event was a rude choice cheeky

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/9/20 10:32 a.m.

Not sure we solved any problems, but we did develop a long list of things for me to build. I have notes in front of me but a busy day at work, so I will work on posting the notes slowly. 

stafford1500
stafford1500 GRM+ Memberand Dork
11/9/20 11:55 a.m.

We also solved that pesky world peace thing while we made a long list for Robbie, oh wait no we didn't.

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/9/20 12:05 p.m.
stafford1500 said:

We also solved that pesky world peace thing while we made a long list for Robbie, oh wait no we didn't.

The grinch (Jim carey) when he is going through his list of evil plans, one item is "solve world hunger - tell no one" my kids get a kick out of that one. 

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/9/20 6:09 p.m.

Ok, notes! We had a few GRM members (6 I think) and from where I was sitting it would appear 5 of them were much smarter than me. Feel free to chime in all if I've misrepresented something or missed something completely. 

Important offhand comment at the beginning: look at the Palatov D4 for ideas and inspiration. I did this and now need to wipe the drool from my chin. 

Goals of the conversation:

  1. What are the priorities for aero devices?
  2. What are the best ways to test effectiveness?
  3. What would work best for F-Dat?

Priorities for Aero devices:

  1. Floor, floor, floor. A floor alone can probably do 80% of all the aero you can do. It's a big surface area AND it is close to the ground. Both are important. 
    1. Many race cars would be constructed so that the body can still hinge up or come off completely leaving the floor in place. Oftentimes this means essentially a vertical flange around the outside of the floor that connects to the body. 
  2. Floor leads to diffuser
    1. USUALLY - the downforce from a floor is centered around where the diffuser starts angling up from the floor. 
    2. Key is to keep the flow attached to the top (and side) surfaces of the diffuser
      1. Add flow from other areas? (like the gap between two wings)
      2. Add flow and heat energy from exhaust?
      3. Add flow from the sides of the car by putting a hole in the side skirt?
    3. Diffuser doesnt have to go under the rear axle necessarily...
    4. Look at the rear end of something like the 919 evo - there is hardly any vertical rear panel - just large diffuser openings.
  3. Feed the floor with a tunnel in the front
    1. Compute the 'cross-sectional' area of the space below the floor and between the skirts and above the ground. Do not make the opening of the tunnel larger than 2x this area. 
    2. Radius the top edge of the entrance to the tunnel. 1/2 inch radius commonly used on the bottom of the leading edge of splitters is not enough. Bigger radius is better. 
    3. Add adjustability by being able to move the front entrance to the tunnel forward and back. 
  4. Rear wing
    1. Can be used to help pull air out from the diffuser (see 2.2 - keeps the flow attached)
    2. Aim profile design for larger CoL (or very low pressure under the wing - really)
    3. Mount with end plates if possible. Just trust us it's gooder. 
      1. A trick is to use a large fin like the 919 or LMP1 cars as an additional 'end plate'. Then you have two considerably shorter wings next to each other. Easier to build smaller wings. Active aero for turning?
    4. Consider flow over the trunk lid and tops of diffusers, wing should be placed close so that in effect the wing becomes part of the diffuser with air coming into the gap over the trunk lid is essentially adding energy to keep flow attached. 
  5. Skirts
    1. Out and down at a 45ish degree angle
    2. As aero downforce increases, they will bend down toward the ground
    3. Use semi flexible plastic, like garden edging
      1. This allows some bending
      2. Has an aerodynamically sharp edge
      3. Isn't catastrophic if it contacts the ground - angle also helps significantly with this
    4. Some gap can actually be better than sliding skirts that stay in contact with the ground
    5. See 2.2.3 - strategic holes can improve diffuser flow
  6. Think in 3d. 
    1. Keep air off of the wheels. Splitters in front (sealed away from the tunnel entrance), boat tail in rear (diffuser can go out as well as up).
    2. Boat tail behind driver and engine, allow air to come back in toward middle but also down toward trunk lid. 
    3. Reverse for the front and 'ojai-scoop' - push the air up and out. Think like a canoe. 
  7. Ducts
    1. If you want to reduce drag, you can do so by reducing the frontal area, by ducting from the front of the car to the back
    2. Use structural members as ducts - Like if you have a larger square tube for a rocker structure, leave the front and back open to let air straight through. 
    3. Ducts could be used to accelerate air and 'bleed' it off into the diffuser to keep the flow in the diffuser attached. 
    4. Don't reduce area from entrance to exit more than 50%. Also try not to have the exit larger than the entrance. 
    5. Be creative - can you close a duct entrance to get downforce for autox and then easily remove a few panels to open it up to reduce drag for the strip? (PROBABLY!)
  8. Other small stuff
    1. Dive planes - takes a lot of fiddly testing
    2. Wheel curtains - see above
    3. Vaned wheels - this can be done easily if you have spokes and can suck a lot of air out from underneath the car
    4. Biplane effect - it's not as good as 2 free air wings, but better than one wing. 
    5. Exhaust exit - use it to add energy to wing or diffuser or to fill the area behind the car

Testing for aero devices:

  1. Pressures - can be usefull but you need tons of channels to get realsitic results
  2. Data aq - look at sections, maximum G loads, beware of GPS shift, use good GPS rates
  3. Stopwatch is still a great tool. Remember you can measure a lot of stuff, but racing is about minimizing time. 
  4. If you can, a figure 8 is a great test track. Simple skidpad can be surprisingly driver dependent

Building parts:

  1. Even the epoxy for fiberglass or carbon fiber is expensive in a challenge sense. Wear safety gear (like put on 4 gloves and peel them as you go). 
    1. Practice laying up flat panels - the wrong cloth directions can produce potato chips. 
    2. Small pieces are easier
    3. Paint stuff white or grey, especially if using vinylester resins. Seriously the sun can make things really floppy. 
    4. hot wire cut foam, wrap with cloth, cut the foam in 2 where you want a spar in the wing so you can add some layers of cloth before putting the two halves together. 
    5. Trashbag and a shop vac make a surprisingly good vacuum bag solution. 
  2. Other methods of making wings include pressing ribs
    1. make a wood form
    2. hammer or press metal around the form
    3. rivet together
  3. 1/16th inch thick fiber reinforced plastic for making waterproof shower walls is available from home stores and is cheap and great for fabing up prototype parts. Also becomes a great surface to use to make a mold from. 
Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/9/20 6:12 p.m.

I'm sure there is stuff I left out or forgot about. Fill in whatever you like or remember all. 

Also, sleepy - should we move this to the aerodynamics subsection now? 

¯\_(ツ)_/¯
¯\_(ツ)_/¯ PowerDork
11/9/20 7:27 p.m.

But what about free lawnmower engine(s) powering suction fans?

barefootskater (Shaun)
barefootskater (Shaun) UberDork
11/9/20 8:21 p.m.

One day I'll build something that makes me wish I could remember what I just read. Bookmarks!

jr02518
jr02518 HalfDork
11/9/20 8:46 p.m.

Robbie,

Try to find a sliding glass window and use that to lay up the large flat section for the floor.

David 

 

NOT A TA
NOT A TA SuperDork
11/9/20 9:05 p.m.

I'm sure I'm not the only one wondering, and I tried searching in your past posts Robbie knowing I'd read something about "F-dat" somewhere here. Can you give us a model, description, or pic of F-dat so we have an idea of the vehicle being discussed.

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
11/9/20 9:12 p.m.

In reply to NOT A TA :

Totally my bad! 

I linked from the build thread to here, but not from here to the build thread 

There's a current photo at top of this page (42).

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/datsun-roadster-2020-challenge-build/161581/page42/

It's a datsun roadster body on top of an 80s formula car frame with a cbr600 Honda bike motor. Built for the $2000 challenge.

Formula datsun, F-dat for short.

NOT A TA
NOT A TA SuperDork
11/9/20 10:12 p.m.

In reply to Robbie (Forum Supporter) :

Sadly, now I know I'm getting old & senile since I'd been following that build thread and still didn't remember ! AHAHAHA    I KNEW I should know!

But at least now those who are new or didn't see the build thread for the challenge will know what F-dat is!

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
11/10/20 1:40 a.m.
Robbie (Forum Supporter) said:

Also, sleepy - should we move this to the aerodynamics subsection now? 

sure.

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
12/8/20 1:28 p.m.

Ok, lets talk some ideas! Nocones got a hold of a datsun model in sketchup, and made some pretty sweet models. I think these kinds of designs will help me achieve many of the aero elements I'm going after, but I'd like to share and get feedback from the group as well. 

 

GO!

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
12/8/20 1:29 p.m.

Also, I LOVE LOVE LOVE the look and big thanks to Nocones for the drawings!

Mr_Asa
Mr_Asa SuperDork
12/8/20 1:32 p.m.

I like that you've enclosed the engine area.   I'll be interested to see how you access it for maintenance.

Why not seal the rear wheels in?

¯\_(ツ)_/¯
¯\_(ツ)_/¯ PowerDork
12/8/20 1:33 p.m.

Anybody run it through some CFD software yet?  Looks awesome but what does some 60mph airflow have to say about it?

mazdeuce - Seth
mazdeuce - Seth Mod Squad
12/8/20 1:36 p.m.

Correct me if I'm wrong, but isn't the raised center section of a splitter a rules thing? Sort of guarantees a minimum of air under the car, but in an ideal world you wouldn't do that? 

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
12/8/20 1:38 p.m.
¯\_(ツ)_/¯ said:

Anybody run it through some CFD software yet?  Looks awesome but what does some 60mph airflow have to say about it?

If anyone CAN - that'd be awesome! However I don't think CFD would like the sharp bends in most places. in real life the build would smooth out a lot of the creases. 

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
12/8/20 1:47 p.m.
mazdeuce - Seth said:

Correct me if I'm wrong, but isn't the raised center section of a splitter a rules thing? Sort of guarantees a minimum of air under the car, but in an ideal world you wouldn't do that? 

I think this is a common misconception rooted in the fact that traditionally under cars are 'dirty' airflow-wise. Slow air is bad, and a splitter that scrapes the front pavement is better because it reduces the slow air under the car. but FAST air under the car is better than no air. That section would be intended to feed the flat bottom and then the diffusers. It's lower in front of the wheels because wheels are aerodynamically bad. So it just tries to keep the air off the wheels. At least that is my understanding. 

 

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