Aerodynamics Questions???

[Halo Blu]

I've researched, and seen many internet posts, saying the "water drop/tear drop" is the "MOST" aerodynamic shape. But! Most pinewood derby cars seem backwards to that idea-being a sliver/wedge.
(Skinny front/Fat Rear- to hide the weight)
Anyone tried a bulbous front, to a pin-point rear end? Does Aerodynamics really play a part? If so, why are so many cars built exactly opposite, of what science knows to be perfectly Aerodynamic?
A falling water drop has a FAT bottom, and a skinny top...
Besides being hard to weight, why haven't Pinewood Derby cars caught on to this idea?

 

[Crash Enburn]

A falling water drop has a FAT bottom, and a skinny top...

A water droplet tends toward a spherical shape. Air resistance as it falls flattens it into a more ovoid shape.

The classic "teardrop" shape is what we see as a droplet runs down the side of something, and then our own impression of what a raindrop looks like as it streaks past.

To answer your question, though, the reason for no teardrop shape is both weight (there is a lot of material needed to make the shape), and that it is less aerodynamic than the thin wedges/planks you see here. While the air might be slightly cleaner behind the "teardrop", it has a larger frontal area, so pushes more air, and is slower.

 

[Halo Blu]

Hmmm... Now, I'm really confused.
By your own drawing, a sphere flattens out, until it splits, and becomes two- Classic Tear Drop Shapes.
Thus, doing it again, and again, infinately.
Why wouldn't a solid tear-drop shape, hold form, and act as the same, and resist air flow?
Not, PUSHING air, but rather- Shedding it to a much smaller sides, with less friction?
By your own drawing, it would seem, the faster it falls the more it becomes tear drop shaped, and less spherical.

 

[Kinser Racing]

It has been done several years ago. I can't recall the outcome, but it didn't catch on so I'm inclined to think it wasn't effective. Try it and see what your results are. /images/boards/smilies/thumb.gif

 

[Crash Enburn]

The image was just one representative one that I found. It shows the shapes relative to size; that a raindrop 4.5mm or larger would break into smaller drops.

But, like Kinser said, test it out.

 

[Halo Blu]

Once again, Thank You! To all who answer, my questions.
This might be one, for a "Wind Tunnel" test.
Fat vs. Skinny (Front to Back/ Back to front)(Built once, and ran through, both ways.)
- or-
a rocket/pencil shape?
Why, a slab "sliver"???
A wing shape, given speed, should add lift, and reduce friction.
A wing resembles a tear-drop shape from a cut away side view.
Just asking...
Why wouldn't the physics of flight aid in the building of Aerodynamic cars?

 

[KC Racing]

Halo blu, I think testing it is a great idea. Please share the results with us.

 

[Guest]

Once again, Thank You! To all who answer, my questions.
This might be one, for a "Wind Tunnel" test.
Fat vs. Skinny (Front to Back/ Back to front)(Built once, and ran through, both ways.)
- or-
a rocket/pencil shape?
Why, a slab "sliver"???
A wing shape, given speed, should add lift, and reduce friction.
A wing resembles a tear-drop shape from a cut away side view.
Just asking...
Why wouldn't the physics of flight aid in the building of Aerodynamic cars?

Hi Halo,

I always enjoy seeing questions that challenge the norm.

It seems to me like it should be faster the way you described.

As long as the bulbous area is less than .25" thick then you might be in business.
Also, it is probably important to have the shape taper in width too.
This may be even more important since it has potential for more draft.

Just think aloud.

Do you have access to a wind tunnel?

 

[The Iceman]

Your right on Halo, at least my brain has pondered the same thing. But what came to my mind it this.

• mass to mass that tear drop is better
• nothing there is faster than something.
• surface area, speed, weight placement (low to ground), mass for the front axle and the mass for the rear axle/weight housing, Will throw the tear drop shape out the window.
• So I think the front axle in the shape of a teardrop, and the rear axle/housing for weight in the shape of a teardrop connected by a round cylinder would be fastest. (minimal surface area)
• OK if I see any built like this racing in the finals next race I WON"T BE HAPPY! (I know I've seen a few out there before)

Note: Would someone post that old post that showed the importance of each part of a car for speed? (I'll see if I can find it) I think you'll be disappointed in how much aerodynamics doesn't play a part after already being 1/4 " high. (sure it will matter if your trying to take 1st if your running 2nd in the pros)

 

[down4derby]

The creation of the the wheel weights was in part to help creat smaller profile cars with less surface area and a more aerodynamic trailing edge. Look at my avatar pic, that car is all lead weight with lead wheel weights. With tungsten the design could be pushed further.

 

[LightninBoy]

I've researched, and seen many internet posts, saying the "water drop/tear drop" is the "MOST" aerodynamic shape. But! Most pinewood derby cars seem backwards to that idea-being a sliver/wedge.
(Skinny front/Fat Rear- to hide the weight)
Anyone tried a bulbous front, to a pin-point rear end? Does Aerodynamics really play a part? If so, why are so many cars built exactly opposite, of what science knows to be perfectly Aerodynamic?
A falling water drop has a FAT bottom, and a skinny top...
Besides being hard to weight, why haven't Pinewood Derby cars caught on to this idea?

The body is already so thin that it is a minor part of the aero issue. You should be thinking about how this thinking would impact the placement and shape of the fenders - particularly the front wheel fenders.

 

[Kinser Racing]

I've researched, and seen many internet posts, saying the "water drop/tear drop" is the "MOST" aerodynamic shape. But! Most pinewood derby cars seem backwards to that idea-being a sliver/wedge.
(Skinny front/Fat Rear- to hide the weight)
Anyone tried a bulbous front, to a pin-point rear end? Does Aerodynamics really play a part? If so, why are so many cars built exactly opposite, of what science knows to be perfectly Aerodynamic?
A falling water drop has a FAT bottom, and a skinny top...
Besides being hard to weight, why haven't Pinewood Derby cars caught on to this idea?

The body is already so thin that it is a minor part of the aero issue. You should be thinking about how this thinking would impact the placement and shape of the fenders - particularly the front wheel fenders.

Do you believe we should be running the fenders behind the front wheel instead of in front?

 

[The Iceman]

I've researched, and seen many internet posts, saying the "water drop/tear drop" is the "MOST" aerodynamic shape. But! Most pinewood derby cars seem backwards to that idea-being a sliver/wedge.
(Skinny front/Fat Rear- to hide the weight)
Anyone tried a bulbous front, to a pin-point rear end? Does Aerodynamics really play a part? If so, why are so many cars built exactly opposite, of what science knows to be perfectly Aerodynamic?
A falling water drop has a FAT bottom, and a skinny top...
Besides being hard to weight, why haven't Pinewood Derby cars caught on to this idea?

The body is already so thin that it is a minor part of the aero issue. You should be thinking about how this thinking would impact the placement and shape of the fenders - particularly the front wheel fenders.

Do you believe we should be running the fenders behind the front wheel instead of in front?

You know Chris that's been on my ToDo list for awhile now is testing that, but I think you still need the GEE in front.

 

[jackjaw]

A tear drop shape is aerodynamic look at solar powered and electric cars. Seems like it could be more affected by dirty air from the other cars because of the bulbous front. Now with this theory wouldn't fenders behind the front wheels create cleaner air for the back wheels?

 

[Crash Enburn]

Solar powered or ultra-economy still have to carry a person. There is no such consideration in a PWD car apart from adding weight.

 

[Guest]

Yep. A tear drop is aero dynamic, but as Crash pointed out;
There is no need for such a mass.
Fenders behind the front wheels makes sense to me, but you need something in front of the wheels to split the air before hitting the wheel.
I like the Gee fenders because they look cool instantly, no work.
Also they push the air to the outside of the car (and towards your neighbor)
Away from the wheel void.
I like to envision the car getting plunged into water.
Like a diver with arms over his head.
He splits the water and pushes it to the outside. Like a wedge car on its side.
How much is to be gained by trimming 1/16" from the thickness? A bit.
How much more room is there by streamlining it the other way?
A lot more I would think.

Basically, I am
Saying the same thing as LB
The teardrop shape makes the most sense when viewing the car from the top.

 

[LightninBoy]

The body is already so thin that it is a minor part of the aero issue. You should be thinking about how this thinking would impact the placement and shape of the fenders - particularly the front wheel fenders.

Do you believe we should be running the fenders behind the front wheel instead of in front?

As you can probably guess from my builds ... I believe both is the best.

If you just consider the basic 2D shape of a wheel + a fender side profile, then yes theory would suggest that aero is better if the fender is behind the front wheel compared to in front of the wheel. But there's so much more going on than just the basic 2D shape - you've got the rotation of the wheel, the basically flat surface for lateral displacement, the impact of the wheel cavity, etc. All of which could conspire to poop all over the theory. I haven't gotten around to experimenting with it because I'm pretty confident both front and back is ideal.

But its something to consider for folks wishing to experiment with teardrop shapes. I think it particular interesting for scout builds where super extended wheelbases are still popular but eliminate the option of putting fenders in front of the front wheels.

 

[Kinser Racing]

The body is already so thin that it is a minor part of the aero issue. You should be thinking about how this thinking would impact the placement and shape of the fenders - particularly the front wheel fenders.

Do you believe we should be running the fenders behind the front wheel instead of in front?

As you can probably guess from my builds ... I believe both is the best.

If you just consider the basic 2D shape of a wheel + a fender side profile, then yes theory would suggest that aero is better if the fender is behind the front wheel compared to in front of the wheel. But there's so much more going on than just the basic 2D shape - you've got the rotation of the wheel, the basically flat surface for lateral displacement, the impact of the wheel cavity, etc. All of which could conspire to poop all over the theory. I haven't gotten around to experimenting with it because I'm pretty confident both front and back is ideal.

But its something to consider for folks wishing to experiment with teardrop shapes. I think it particular interesting for scout builds where super extended wheelbases are still popular but eliminate the option of putting fenders in front of the front wheels.

I'm inclined to agree with you. I did some extensive research on aero early on in my pinewood derby beginnings and I recall several theories that believed the football shape was actually the most aero of them all. I hadn't given it much thought lately but it may be worth revisiting. /images/boards/smilies/smile.gif

 

[Guest]

Oh man!
This is getting good!

 

[Halo Blu]

I've never built a tear-drop shaped car. I've never built a car with fenders. I do not have a wind tunnel. Although, I'm able to do all three. (Smoke cans/bombs + a fan + a tube = cheap $ wind tunnel)

My idea, for a car... resembles a wide, low front, with wing shaped wheel attatchments.
Think- STINGRAY (The Animal, not the car.) with wheels.
-OR-
A Solid Airplane Wing shape, sloped just above the rear axel, to allow weight placement underneath with a low/rear/ weight.
-OR-
Maybe something that resembles a figure 8, or 2 teardrops, one in front of the other.(With FENDERS!)
-OR-
Who Cares! We're all going to be racing HOVER CRAFT CARS in the next 10 years anyway! /images/boards/smilies/smile.gif
I'm here to learn. I might raise a question, but unfortuately, I have no "right" answers.
Thanks again guys! If you have input, let it fly! I'm here to hear it!