Making a more efficient, aerodynamic soapbox gravity race car

[aerohead]

Originally Posted by orengomotors

ok, so 3.92 it will be!

and the body shape?
whats the optimal body shape for this car?

the subsonic land speed record cars are a good inspiration?





why are those cars so long? the optimal shape of 3.92:1 is shorter.
i'm imagining a car like that but with open wheels. what do you think?

Bonneville cars attempt to minimize frontal area as much as possible,so the mechanical layout packaging ends up more 'linear'.Weight is not and issue,they need all the traction they can get so as to prevent wheelspin at high speed which could cause a lethal spin and takeoff.Which leads to the very long wheelbases.
The pointed nose is for transonic speeds,above 250-mph,where shockwaves can form on the body which block the boundary layer,leading to alterations in the center of pressure,which can make the car very unstable.Deadly.
None of these nose shapes would be good for you.A bulbous nose would actually be perfect for you at the speeds you'll be encountering.A piece of convex hemisphere would be fine.Here are some airship hulls,they all use bulbous noses

Here are some submarine hulls,all have bulbous nose.

Then clean up all your suspension parts.
After that,compact the frame as close as you can tolerate in the race to minimize frontal area,then do as much boat tail as you can.
If you can enclose the fuselage of the car,this Cambridge University CUER solar racer body would be an okay way to go.(Cd 0.10)

For reduced frontal area and enclosed car,do the 'red' body
For an open cockpit do the blue

[aerohead]

Originally Posted by orengomotors

well, catalonian independence is a big problem now, it is always on Tv and it is a constant battle Barcelona-Madrid. Catalonia only did a democratic votations to kwon how many catalans wanted independence and how many not. And from madrid they see that like a criminal act. But this is another subject

going back to aerodynamics, the rounded bottom causes the ground interactions?
so the triumph LSR "motorbike" isn't optimal? So whats best flat bottom with rounded edges or flat bottom with squared edges?

the bottom has to be flat? it can have a diffuser? what angle?

The Me-262 at altitude has the same Cd as the 2.5:1 streamline body of revolution in ground proximity,which suggests,from research,that the Me-262 fuselage in ground proximity would be Cd 0.16,compared to Cd 0.08 for the streamline body.So we're already in trouble with the Messerschmitt.Regardless of what the belly does.
Ive never seen a Cd published for a bellytank lakester,so I don't know the drag contribution of the exposed suspension and wheels.
Mercedes-Benz reported in the 1950s,that their all-enclosed Grand Prix racer,in spite of it's greater frontal area,had 20% less drag than their previous year's open-wheel race car.
A 1980s F-1 body was Cd 0.25.
When wheels were added,the drag went to Cd 0.54.Wings took it over Cd 1.0
The 1987 Oldsmobile AEROTECH Long-Tail was a completely-enclosed March Indycar.I've never seen a definitive Cd published for that car,but I suspect that it's drag was half of the conventional Indycar.
We could use more data.After 40-years I don't have anything definitive to offer.

[freebeard]

The Me-262 at altitude has the same Cd as the 2.5:1 streamline body of revolution in ground proximity,which suggests,from research,that the Me-262 fuselage in ground proximity would be Cd 0.16,compared to Cd 0.08 for the streamline body.So we're already in trouble with the Messerschmitt.Regardless of what the belly does.

Citation needed. Is the CD for the fuselage or the entire airframe?

The MG EX-181 is like a widened belly tank. Would squaring off the lower edge help or hinder?

[aerohead]

Originally Posted by freebeard

Citation needed. Is the CD for the fuselage or the entire airframe?

The MG EX-181 is like a widened belly tank. Would squaring off the lower edge help or hinder?

*The Cd for the Me-262 is from Hoerner's 1951 book.He worked for Messerschmitt.And the Cd is a frontal area Cd for the fuselage alone.
*The EX-181 is kinda like a flattened tank.Goro Tamai calls 'em 'flattened torpedos.' MacCready called 'pumpkin seeds'. It's similar to Jaray's half-airship body also.You can see Jaray's 1922 shape co-opted in this ALDER advertisment.

Aircraft picked up on it






Even the Skunk Works likes it

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*As to the lower edge,you'd want the edge at the bottom,not midway up.As shown at the top of Fachsenfeld's drag table

*A square edge would promote vorticity.(Funderburk & Carver)
*A 'sharp' edge has been found beneficial by the M.I.T. folks,but we're talking solar cars based upon laminar wing sections.(Goro Tamai)
*Until this has been specifically tested for in a more 'conventional' passenger car, my opinion is that we put about as much radius at the bottom edge as VW did with the front of the Gen-1 Golf.
If you look at the airdam flow of the T-100 at DARKO you'll see separation under it,kinda like this

In a crosswind gust,the little bit of radius might be beneficial.Just a tiny bit of curve.As GM has done at the back lower edge of their concept

[orengomotors]

Originally Posted by freebeard

That's the reason I mention his name. I also admire the Spanish anarchists in the 30s and think Catalonian independence might bring that back.

It isn't the added wheels, it the rounded bottom. aerohead has a chart. Starting from a hexagon section you could add curved lower skirts and get the Indigo shape going. Open front wheels and enclosed back wheels. Maybe with more boat tail.

Edit: Gaudi rant: The world-famous architect did a little-known (until 1956) project for New York city, the Hotel Attraction:


https://en.wikipedia.org/wiki/Hotel_Attraction

On the proposed site, the cabal erected a building in the shape of the number 11, with an unusual skin-and-core design almost intended for deconstruction. The site was cleared in September of 2011. The Gaudi design was proposed as an replacement, but instead 'they' chose the Freedom Tower as it more correctly propagandizes 'our freedumbs'. The Gaudi design was a masonry shell that would deflect or crush an airplane, like the Empire State building did in 1945.
[/rant]

it would be great to see this hotel built.
reminds me a soyuz rocket:



it seems that gaudis parabolic arch is very aerodynamic! the top of the hotel towers, look like the airfoils leading edge or zeppelin nose!

[orengomotors]

Originally Posted by aerohead

Bonneville cars attempt to minimize frontal area as much as possible,so the mechanical layout packaging ends up more 'linear'.Weight is not and issue,they need all the traction they can get so as to prevent wheelspin at high speed which could cause a lethal spin and takeoff.Which leads to the very long wheelbases.
The pointed nose is for transonic speeds,above 250-mph,where shockwaves can form on the body which block the boundary layer,leading to alterations in the center of pressure,which can make the car very unstable.Deadly.
None of these nose shapes would be good for you.A bulbous nose would actually be perfect for you at the speeds you'll be encountering.A piece of convex hemisphere would be fine.Here are some airship hulls,they all use bulbous noses

Here are some submarine hulls,all have bulbous nose.

ok, so pointed nose is kind of sears-haack and is for a very very fast vehicle.
so every of the zeppelin shapes would serve, and i need to find what of them fit better the chasis, this is ok?

[orengomotors]

this images are scale drawing:



what shape do you think will fit better?

[aerohead]

Originally Posted by orengomotors

ok, so pointed nose is kind of sears-haack and is for a very very fast vehicle.
so every of the zeppelin shapes would serve, and i need to find what of them fit better the chasis, this is ok?

Yes,the pointed noses are for 250-mph and above.(409 km/h+)
If you want to do a 'tank' body based upon the airships it IS okay.
Remember, that you can essentially remove any of the constant-velocity,cylindrical center section to get the length you need.
You can also take liberties with the nose.
The only thing you can't negotiate is the tail.This is where most of the drag will come from if you get it wrong.

[freebeard]

Oops! I said parabolic. He actually used a catenary curve. Not being a conic section catenaries are hard to calculate. So he used upside down models and inverted them.



It would be hard to reproduce his work elsewhere because his local craftsmen were pretty magical themselves. The attic of Casa Milá:


ibid

Would you like to know more? Here are PDFs of the geometry behind Hotel Attraction and Sagrada Familia.

Documentos de Manuel Hidalgo actualizados en Scribd [McNeel Wiki]

The Hotel document uses the phrase "Parabólicas elípticas de revolución" so maybe it is a parabolic design.

what shape do you think will fit better?

Car shaped. Seriously, on a car this small you don't have a lot of options/opportunities. It's all wheels and roll bar. You could extend the nose and tail and maybe put a curve in the rockerpanel area like the Ford Indigo. Minimizing the frontal area would have you just shrink-wrapping the frame, air-foiling the suspension and roll bar, and panting the wheels.

[aerohead]

Originally Posted by orengomotors

this images are scale drawing:



what shape do you think will fit better?

We need to include your body with helmet in the equation,including a top view if possible.
The body width is very important as well as the final roll bar height,and whether or not you can recline even lower inside the chassis.