Making a more efficient, aerodynamic soapbox gravity race car

[orengomotors]

So, the shape has to be like shape B for ground clearance and near ground aero?


the chasis dimensions are this:

how i fit this shape here? whats the optimum fineness ratio?
The rules limit the total length, so it's better to have a longer nose and cutted rear, or short nose and complete rear?

[aerohead]

Originally Posted by orengomotors

well, let's do it! a gravity powered belly tank.
whats the optimum longitudinal shape of the belly tank? i found a lot of different belly tanks on internet.

The answer is problematic:
*belly or wing tanks are designed for flight conditions,far above the ground in thinner air.And they are optimized for frontal area and tend to be 'long' in relation to their diameter.
*In ground proximity the drag of a body doubles,so a Cd 0.05 tank might be Cd 0.10 in ground effect.
*Lay tested a 'tank' close to the ground and got Cd 0.08.
Your looking for the minimum envelope which will enclose you and your mechanical components with the smallest frontal area,but full attached flow.
It's going to be like this Soapbox Derby racer,but with a canopy/head fairing

Here are some racing bicycles

The Edison-II car body would work great for you





The 2.5:1 ratio streamline body has about the lowest drag but if you size it to the length of your rig I don't think it will work for you.
I just cut up my Lockheed T-33 wing tank for scrap.It was 610mm diameter and about 6095mm length.

[freebeard]

Google says 6095 mm is 6.665573 yards. It would have a 10:1 fineness ratio.

The graphic I reposted at Permalink #120 shows a hard-edged half-body at 0.13 and a radiused lower edge half-body at 0.09 Cd. Comments?

[Joggernot]

Only comment is that the next illustration shows a radiused edge with wheels and we're back to a cd of .15. Thanks for the link. Enjoyed the evolution of aerodynamics illustrations.

[botsapper]

Since this is not an optimum aero build but aero ideas on your existing built. No dimensions but quickly sketched out some ideas on your frame. Used an Ariel Atom as an inspiration. Your frame cladded with the shaped panels to give you better aero. Your suspension pieces need the clearance slots and allows for easier removal, maintenance and adjustments. Flat bottom panel. The side covers continue to the rear into a tapered rear cone (size depends on your overall length rules and helmet height. Might add some tubing since this will be pushed). The front end needs some thought for your safety. The nose will inevitably hit things and must be reinforced. Pinched closed or small grill front. The panels must be divided longitudinally and allow to breakaway to the sides not to pushed into to the cockpit. You will need spares.

[freebeard]

That looks like a good effort. I like the 'ablative safety panels'. They could be block styrofoam with a vinyl wrap.

Your nose mimics the Atom. Current best practice in the off-road racing builds is a forward leaning A-shaped bumper similar to the VW Cal-look baby buggy bumper. Here's an example from a more modest random build thread on thesamba.com:



I think the idea is if you hit something it's only going to go down and to one side or the other. It could fit well with the style of nose you proposed.

[orengomotors]

Hi,
i made an sketchup with the exterior dimensions of the soapbox:



you can download it here:

https://drive.google.com/file/d/0B1c...ew?usp=sharing

[orengomotors]

Originally Posted by aerohead

Hi aerohead, i like this shape, i find it's elegant, simple and functional, something like the bikes but with the open 4 wheels.

[orengomotors]

Originally Posted by botsapper

Since this is not an optimum aero build but aero ideas on your existing built. No dimensions but quickly sketched out some ideas on your frame. Used an Ariel Atom as an inspiration. Your frame cladded with the shaped panels to give you better aero. Your suspension pieces need the clearance slots and allows for easier removal, maintenance and adjustments. Flat bottom panel. The side covers continue to the rear into a tapered rear cone (size depends on your overall length rules and helmet height. Might add some tubing since this will be pushed). The front end needs some thought for your safety. The nose will inevitably hit things and must be reinforced. Pinched closed or small grill front. The panels must be divided longitudinally and allow to breakaway to the sides not to pushed into to the cockpit. You will need spares.

hi botsapper,
Thank you! your sketch is great!

[aerohead]

Originally Posted by freebeard

Google says 6095 mm is 6.665573 yards. It would have a 10:1 fineness ratio.

The graphic I reposted at Permalink #120 shows a hard-edged half-body at 0.13 and a radiused lower edge half-body at 0.09 Cd. Comments?

*Paul Jaray's pumpkin seed as a full,streamline body of revolution,in free air,is Cd 0.045.
*As a 'half-body' in ground proximity (25 cm ground clearance) the body has twice the drag,at Cd 0.09.
*With wheels added,the rounded nose,but sharp lower rear edges,the car is Cd 0.13.

*With rounded lower rear edges,Cd 0.15.
*These values were verified by Kamm and Koenig-Fachsenfeld at the FKFS.