Aero shapes and their application?

[basjoos]

Cut a bunch of 3" long pieces of yarn, tape them to the surface of your car about 6" apart each way, then drive down the road at various speeds while you, a friend, or a video camera observes the behavior of the yarn in the air flow. In an attached airflow, they will all point in about the same direction with very little jumping around. In unattached or recirculating airflow, the yarn will be jumping around in all directions. Google "yarn tuft testing" to see photos and videos of it.

[Cd]

Basjoos - still waiting to see a tuft test of your car. Surely I'm not the only one on this site that would love to see that.

Whuups ... didn't mean to hijack the thread. Getting back on topic, it is actually possible to add wheel arches to a vehicle and not increase the frontal area all that much.
Looking at the wheel arches on the Ford Focus, you might notice that the arches actually don't protrude all that much from the sides of the vehicle, yet appear to do so because of the clever use of a body cavity.

[rkcarguy]

Originally Posted by KamperBob

Wind tunnels are expensive. Tufts are cheap. ABA testing is where the rubber meets the road. Full tank testing can be expensive and that kinda of data tends to be dirty (mixed inputs). A ScanGauge is your friend. Cardboard prototyping is prudent. Be scientific. Document everything. Learn from every improvement and setback. Take pix and share so others can help. A happy ending is inevitable.

Cheers
KB

I'm kinda thinking I will have to have to make a few scale shapes from wood and see if I can get a college's VRI to test them for me.
My basic understanding, is that 12* is the max angle change to prevent air separation. So , in theory, if I angled the sides of the front fenders inward no more than 12* from the front wheel opening forward, and followed that same rule all around as much as possible, it should yield good results.
My design I've been drawing on for over a year now is pretty radical. 40" tall and just under 13' long, single cam honda engine mid mounted with the radiator in the chopped boat tail. To save weight and height there is no seats, the cushions go right on the floor. ~25* layback on the seats allows occupant height up to 6'-6", and surprisingly it's pretty comforatable(tested by blocking a racing seat and measuring. Lastly, I currently have a airflow channel down the bottom center of the vehicle. At the passenger compartment it measures 1 foot wide and 18" tall and it tapers wider and slightly taller at the front, only impeded by the steering rack and shift linkage. At the engine, an upside down wing shaped oil pan will help ramp the channel air under the motor. The rear boat tail will truncate at ~10" tall, the sides consisting of the tailights and the center being radiator drawing the air into the void at the truncation(thru the radiator).
My biggest worry at this point, is that the front will be so light it could get ugly at speed. I need to carefully shape the nose to produce a slight amount of downforce as speed increases. It's my hope the air accelerated under the car and thru this "channel" will do just that while decreasing drag.

[aerohead]

Originally Posted by rkcarguy

I'm kinda thinking I will have to have to make a few scale shapes from wood and see if I can get a college's VRI to test them for me.
My basic understanding, is that 12* is the max angle change to prevent air separation. So , in theory, if I angled the sides of the front fenders inward no more than 12* from the front wheel opening forward, and followed that same rule all around as much as possible, it should yield good results.
My design I've been drawing on for over a year now is pretty radical. 40" tall and just under 13' long, single cam honda engine mid mounted with the radiator in the chopped boat tail. To save weight and height there is no seats, the cushions go right on the floor. ~25* layback on the seats allows occupant height up to 6'-6", and surprisingly it's pretty comforatable(tested by blocking a racing seat and measuring. Lastly, I currently have a airflow channel down the bottom center of the vehicle. At the passenger compartment it measures 1 foot wide and 18" tall and it tapers wider and slightly taller at the front, only impeded by the steering rack and shift linkage. At the engine, an upside down wing shaped oil pan will help ramp the channel air under the motor. The rear boat tail will truncate at ~10" tall, the sides consisting of the tailights and the center being radiator drawing the air into the void at the truncation(thru the radiator).
My biggest worry at this point, is that the front will be so light it could get ugly at speed. I need to carefully shape the nose to produce a slight amount of downforce as speed increases. It's my hope the air accelerated under the car and thru this "channel" will do just that while decreasing drag.

W.A.Mair's research concluded that body tangent angles up to 22-degrees could maintain attached flow if the form leading up to it had gently progressive curvature.
If you haven't seen the 'Aerodynamic Streamlining Template - Part C' you may want to do a Search EcoModder for it.This template was created for anyone wishing to design for attached flow and it can be used for micro as well as macro environments.
Any discussion of 'angles' should include the contextual environment in which they are to exist.
Also,if you do models,make sure the wind tunnel is fast enough to provide for scaling effects / Reynolds number.

[jcfdillon]

Why not have a nontapered air channel that has the same dimensions at front and back? That way, you are not forcing the air to be compressed as you move forward. I think having a smooth channel down the center underbody of the car would provide stability at high speed, but if you arch the top of the channel upward toward the front you are creating a lift in the front that could cause an accident at high speed and/or powerful headwind. If the channel is wider at the back and front intake is underneath the engine, you create a partial vacuum which can help hold the road at high speed. If the air channel has powerful suction using something like a 200 mph leafblower powered thru the car electrical system, you will be sucking the air out of the bow wave area which I think would provide some efficiency boost. I think the passive method may be best though. (Leafblowers will violate all sorts of noise ordinances, unless properly muffled using a vortex straight-thru muffler.)

[aerohead]

Originally Posted by jcfdillon

Why not have a nontapered air channel that has the same dimensions at front and back? That way, you are not forcing the air to be compressed as you move forward. I think having a smooth channel down the center underbody of the car would provide stability at high speed, but if you arch the top of the channel upward toward the front you are creating a lift in the front that could cause an accident at high speed and/or powerful headwind. If the channel is wider at the back and front intake is underneath the engine, you create a partial vacuum which can help hold the road at high speed. If the air channel has powerful suction using something like a 200 mph leafblower powered thru the car electrical system, you will be sucking the air out of the bow wave area which I think would provide some efficiency boost. I think the passive method may be best though. (Leafblowers will violate all sorts of noise ordinances, unless properly muffled using a vortex straight-thru muffler.)

*with a channel,you have 3-surfaces for the air to rub against instead of one,plus the sides of the channel.
*below 250-mph their is no compression of the air,only deflection and displacement.
*CAR and DRIVER Magazine attempted to lower drag by attaching a channel to the top of a Dodge van and ended up with a drag increase and lower fuel economy.
*since 1976,with the work of Professor Alberto Morelli,we have zero-lift automobile designs to enjoy, with no fear of loss of stability, unless velocities of 235 mph are approached as with Racing Beat's 3,500-lb Mazda RX-7 which flew at Bonneville during a speed attempt.

[Smurf]

Originally Posted by aerohead

*since 1976,with the work of Professor Alberto Morelli,we have zero-lift automobile designs to enjoy, with no fear of loss of stability, unless velocities of 235 mph are approached as with Racing Beat's 3,500-lb Mazda RX-7 which flew at Bonneville during a speed attempt.

Please tell me that someone, somewhere has a video of this?

[HydroJim]

Originally Posted by Smurf

Please tell me that someone, somewhere has a video of this?

http://www.youtube.com/watch?v=DOkRl4ukFyg

[aerohead]

Originally Posted by HydroJim

Notice that it isn't until the RX-7 goes sideways that it becomes an airfoil and lifts off.
Wunibald Kamm was obsessed with directional stability and included massive tail fins on his designs to present a self-correcting, weather-vaning feature to prevent a car from ever getting sideways.
A decklid extension with capping-plates (Bonneville spoiler) might have provided enough extra downforce to prevent the RX-7's wheelspin which ultimately led to the crash.
Thanks for sharing the video! (they probably don't show it at the Mazda showrooms).

[freebeard]

Lastly, I currently have a airflow channel down the bottom center of the vehicle. At the passenger compartment it measures 1 foot wide and 18" tall and it tapers wider and slightly taller at the front, only impeded by the steering rack and shift linkage.

It's not clear to me whether this air channel has 3 or 4 sides, but I agree with aerohead that the skin friction would increase the drag.

In addition, a 12x18" duct half the length of the car will take 9 cubic feet of interior volume. Won't that require increasing the overall width of the body?