High speed aerodynamics advice

[freebeard]

Drafting convention is to put the 'invisible' parts as dotted lines.

Will the pontoons be solid walls or relieved on their bottoms like Morelli's Banana car?

[aerohead]

*Honda reported the highest drag for a catamaran-type solar racer that they tested when compared to what became their 'Dream' solar racer of 1996.
*Coventry University reported that, up until the time (2012) that they tested an Audi A2, conventional wisdom dictated that decreasing ground clearance would translate to lower drag.They found that not to be the case for the Audi.
*Hucho and others caution us not to rely on rules of thumb,as there are often many exceptions to the 'rules.'

[LibertyMKiii]

Originally Posted by aerohead

*Honda reported the highest drag for a catamaran-type solar racer that they tested when compared to what became their 'Dream' solar racer of 1996.
*Coventry University reported that, up until the time (2012) that they tested an Audi A2, conventional wisdom dictated that decreasing ground clearance would translate to lower drag.They found that not to be the case for the Audi.
*Hucho and others caution us not to rely on rules of thumb,as there are often many exceptions to the 'rules.'

It certainly seems to be a black art when talking about near ground aerodynamics. My plan is to go with what looks and feels best... then make some adjustmentsfrom there.

The plan for the side pontoons is to make them just like the motorcycle land speed racers (fully enclosed)

[darcane]

Oh, I only just found this one.

When it comes to full size LSR cars, one builder really stands out to me. Jack Costella:
Jack Costella's Official Website

His Nebulous Theorem cars are all surprisingly small, low, and narrow. And Fast! His cars have over a hundred records, including setting a record over 350mph with a 1.5L engine. If you want an example to follow, this is the guy.

Originally Posted by LibertyMKiii

Some technical details for reference:
The weight is around 12-15 lbs. Width will be around 11" and length will be a fairly good size estimated around 30"
The tires / wheels are 4.15" and ill likely shave them down to 4" So I can estimate the total height being around 4.5"
the nets:
Frontal_area_m_2 0.03193542
Drag_N 34.49627467571459
Max power 330 amps at 34V
Electric motor power 11,000 - 15,000 watts

Thanks for any feedback / input you can provide.
This topic tends to be exciting for engineers since you can create the ideal package given there is no driver who needs visibility. (inside the car)

-Liberty

So... why 11" wide? Ditch the suspension and cut that in half. Figure out the largest component that you MUST design around, and fit everything else within the profile of that component. The width of Jack's cars are often determined by the width of the driver's shoulders. If he could ditch the driver, I'm sure they would be smaller.

Weight doesn't hurt you as much in LSR. Do you understand the dynamics of Cg vs Cp? Stability is strongly dependent on the Cg being forward in the car and the Cp (Center of Pressure) is more towards the rear. Adding weight to the front of the car adds to stability without adding drag like a wing would. Length also adds stability without adding much drag.

[LibertyMKiii]

Originally Posted by darcane

Oh, I only just found this one.

When it comes to full size LSR cars, one builder really stands out to me. Jack Costella:
Jack Costella's Official Website

His Nebulous Theorem cars are all surprisingly small, low, and narrow. And Fast! His cars have over a hundred records, including setting a record over 350mph with a 1.5L engine. If you want an example to follow, this is the guy.



So... why 11" wide? Ditch the suspension and cut that in half. Figure out the largest component that you MUST design around, and fit everything else within the profile of that component. The width of Jack's cars are often determined by the width of the driver's shoulders. If he could ditch the driver, I'm sure they would be smaller.

Weight doesn't hurt you as much in LSR. Do you understand the dynamics of Cg vs Cp? Stability is strongly dependent on the Cg being forward in the car and the Cp (Center of Pressure) is more towards the rear. Adding weight to the front of the car adds to stability without adding drag like a wing would. Length also adds stability without adding much drag.

The next go around I'll build a 2 wheel LSR that will be about 1.5" wide and 2" tall. My goal here on this vehicle is to have something that is very similar to a production RC car. The budget is very limited, so I do not have the resources to cnc custom axles for a narrowed setup... I am trying to work with production parts and modify the body and aero in ways that others have not tried yet.

-Liberty

[DerpWarrior]

As far as downforce, i think a diffuser is your best bet. if i recall correctly from a video i watched on the topic, the downforce from a diffuser will be centered on the the point where it starts to curve, so keep that in mind and design accordingly.

and as was already stated, keep frontal area as low as possible. also, if the area of the cross-section changes rapidly that causes drag, so if one part needs to stick out weirdly, have something else dip in at the same point in the length of the car. (it sounds weird, but that is actually why the bubble on the top of a 747-400 is longer than earlier models).

*suddenly sees more replies and no longer knows if i have anything worthwhile to say...*