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@NineLivesJohnny

Anyone have Aero questions?

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On 4/11/2020 at 3:23 PM, Max said:

Newb aero question: does that 45 degree air dam have an aero advantage over a vertical air dam? 

yes and no. 
Less drag but more air over top of car so more lift. 

 

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1589825954_results_mmthumb.jpg

WE present the following things we found as data. 
The answer to what roof is best is YES.
The corvette C5 
FRC/ZO6 made more drag.
The Base "fastback" roof made more lift.
The story:
During our design work of the new c5 Corvette aero packages, we decided to see what effects the two c5 roofs made. All numbers provided are of 100% stock cars, the only change is the roof. Yes, the pictures show our aero packages but those are not finished and we didn't want to share the numbers on those until it was done.

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22 minutes ago, @NineLivesJohnny said:

yes and no. 
Less drag but more air over top of car so more lift. 

So pushing air over the car causes lift? Geez I better get rid of my splitter, didn’t know that it caused lift.

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5 hours ago, enginerd said:

So pushing air over the car causes lift? Geez I better get rid of my splitter, didn’t know that it caused lift.

I'm n expert but I do work for the Air Force. the basic design of an automobile looks like a wing. It has more surface area on top thus creates lift. You can run a splitter (and we do) but that doesn't mean you still do not have "lift". 

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I am avoiding scanning the previous 13 pages of posts out of laziness.  I respect @NineLivesJohnny's expertise and opinion a lot.  For those who like falling asleep reading books instead of iPad screens, and are looking for other resources, there is a British author names Simon McBeath who authored a book called Competition Car Aerodynamics (it's not cheap) now in its Third Edition and updated in  2015. It does a pretty deep dive into the effects of increasing/decreasing downforce at the front and rear through various measures (splitters, diffusers, wings), along with the effects of splitter and wing heights.  

 

I for one, will be ordering one of NineLivesRacing's Wangs (maybe two different chords) for my next build, and look forward to balancing high and low downforce packages with front splitters, S ducts, and diffusers. 

Competition Car Aerodynamics.jpg

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I'm a big fan of the Nine Lives wing we installed. We picked up 8 seconds a lap at VIR with it...YMMV. The increased stability under braking was huge for us and we could run the climbing esses with 15 more mph. Being a MR drivetrain the car was super unstable under heavy braking and high speed cornering before. Now the car has a slight push in sweepers around 95-100+ mph, but we want that. Makes the car much easier to drive. It's loose enough in the slow corners and then slight push in the high speed stuff. Super fun and confidence inspiring. Now we get to concentrate on reducing drag with the rest of the car to help our fuel mileage. We've got a pretty flat floor using a front under tray and the stock floor pan. We have another cover for the rear 3rd of the car, but we haven't installed it yet. I think the first thing we need to address is the tires being out in the airflow. We need to modify the front bumper sides and rockers to cover the bottom half of the tires better. I would love to take this thing to Johnny and CFD it someday. Aerodynamics is fascinating and it's amazing from a driver's perspective when you start to understand some of the basics. 

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On 5/18/2020 at 11:54 PM, Voodoo Child said:

 "...(it's not cheap)..."

 

 

 

You don't say!, I thought that might mean like $50... NOPE

 

174078936_aerohandbook.jpg.fe6d4a805b7757a2e1e2f0ce79302677.jpg

 

The second edition is like 2011... and is 50 bucks used.  Could it have changed that much?  Given the pricing, I assume this is used as a textbook somewhere?

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I have that book, 3rd edition, and paid $50 for a hard cover back in 2015. I see it listed for $188 hard cover, and $46 paperback.

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2 minutes ago, TiredBirds said:

what is CoG?

 

Center of gravity. Needs something to add downward force on the nose. 

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Try changing the angle of the engine a little so that the back of the engine is slightly below level. Once the front starts to rise with a rocket motor it becomes a self-worsening problem. 

 

In this particular case moving the CoG forward probably won't be enough unless you use something really heavy relative to the rest of the vehicle. The proportion of thrust to the overall vehicle weight is just way overboard which is why a slight adjustment to the angle of attack (AoA) of the motor is likely the better fix. 

 

Edited by XelderX
engine/motor tomato/tomahto

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15 minutes ago, XelderX said:

Try changing the angle of the engine a little so that the back of the engine is slightly below level. Once the front starts to rise with a rocket motor it becomes a self-worsening problem. 

 

In this particular case moving the CoG forward probably won't be enough unless you use something really heavy relative to the rest of the vehicle. The proportion of thrust to the overall vehicle weight is just way overboard which is why a slight adjustment to the angle of attack (AoA) of the motor is likely the better fix. 

 

well my bro and I have a challenge so I will be using this advice for the new build....I can add weights to the nose if need be.

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Just now, TiredBirds said:

well my bro and I have a challenge so I will be using this advice for the new build....I can add weights to the nose if need be.

 

I was thinking about it. The engineering solution would be to drill through the bottom of the "chassis" below the front of the engine. Then thread in a set screw to have more precise and easily changed AoA for the engine. 

 

Just know that these things are really hard to control. Most of the races with them that I've seen they have little eye screws on the bottom that ride on a wire to control trajectory. 

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1 hour ago, XelderX said:

 

I was thinking about it. The engineering solution would be to drill through the bottom of the "chassis" below the front of the engine. Then thread in a set screw to have more precise and easily changed AoA for the engine. 

 

Just know that these things are really hard to control. Most of the races with them that I've seen they have little eye screws on the bottom that ride on a wire to control trajectory. 

ahh wire ride is cheating

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13 minutes ago, enginerd said:

Wheelbase is where I would start.. it's too short.

yes the new car will be longer... those were the stock derby holes. 

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39 minutes ago, TiredBirds said:

yes the new car will be longer... those were the stock derby holes. 

 

 

That's a good idea too if legal. I'm assuming at this point we are talking about no rules. Get the back wheels as far back as possible. The rear axle is the fulcrum by which the vehicle will rotate on the long axis. Treat it like a jet dragster with the wheels at the very back of the chassis and if possible move the engine farther forward.

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Also, weld the diff. You want to make it as hard as possible for this thing to turn. 

And grippy tires for the same reason.

And a zero toe alignment with all wheels pointed forward and aligned with the direction of thrust.

Edited by enginerd
  • Haha 1

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Technically, that would be considered a motor swap.  What did you use as the starting weight and HP in the calculator?

 

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1 hour ago, Racer28173 said:

Technically, that would be considered a motor swap.  What did you use as the starting weight and HP in the calculator?

AFAIK, jet engines are measured and rated in units of thrust force (lbs), not horsepower... we’ll have to have tech weigh in on a conversion factor. 

Edited by enginerd
  • Haha 1

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