A thought on retractable spoilers

[aerohead]

Originally Posted by freebeard

I'm not sure what you're saying here. Holes to the upstream exterior, or to the interior?

Open box-cavities OK?

The base pressure of the entire wake is determined by pressure existing at the line of separation nearest the lowest pressure point on the body.Typically just ahead of the windshield/A-Pillars.
If a box cavity isn't airtight around it's attachment point interface and periphery,the base pressure of the wake behind it will be governed by the pressure of the separation line ahead of the trailing edge where any 'hole' exists,telegraphing this low pressure into the rest of the wake.So it is an upstream exterior effect.
This is why air will flow backwards from an open hatchback,bringing exhaust fumes into the cabin when a front window is rolled down,rather than channel air rearwards into the wake.It's all about pressure equalization.

[freebeard]

Thanks. So, holes to the upstream exterior, through the interior.

The good news is I don't have a '67 Lincoln convertible with the roll-down back window, like Neil Young, anyways. Assuming taped seams all the way back, the box cavity faces that are not facing rearward (the inner sides, top and bottom) don't produce drag, do they?

[aerohead]

Originally Posted by freebeard

Thanks. So, holes to the upstream exterior, through the interior.

The good news is I don't have a '67 Lincoln convertible with the roll-down back window, like Neil Young, anyways. Assuming taped seams all the way back, the box cavity faces that are not facing rearward (the inner sides, top and bottom) don't produce drag, do they?

Those inner faces are in dead air pretty much so they're, so to speak,inert.The box (if properly configured) creates locked-vortices,all around, of which the outer flow skip over, re-attaching at the rear.Since the rear bulkhead is smaller in cross-sectional area,the wake is smaller and at a higher pressure since the air is decelerated,cutting the pressure drag.

[bobdbilder]

I am developing a fixed spoiler with adjustable angle of attacks. The surface is actuated by an RC servo connected to a microcontroller (Arduino UNO). I will be doing some tests to find out its viability. I've got an add-on CAN BUS card that connects to the OBDII port but have not developed it further. The idea is to have several modes including an auto mode where the OBDII connection is utilized.

[bobdbilder]

Some changes to the design. Currently manual adjustment without remote control. I need a bigger servo than what I have. I have calculated at the max speed of 200km/h (124mph) there would be 10kg of force on the spoiler. I was doing average 5.3l/100km (44.38mpg) before this. First test today resulted with 4.3l/100km (54.7mpg). Both driven values at average 80km/h (49.7mph)





Did not have the guts to go any faster than 140km/h with this thing being new. This is only my first try and I know I should be doing more. But its 18% savings was too much to keep it in a closet.

[bobdbilder]

I finally had a rocker switch installed on one of the blank pads on the dash. Set Zero Position (meant for eco at -25degrees) and Set Span Position (meant for high drag at +30degrees). Did 2 x 22km average 80km/h (sorry wont be doing conversions this time around) runs on same route on 21 June. First run 4.3l/100km (indicated). On the second run one of the pins was accidentally dislocated 1/3 of the route. So the servo lost control of the flap. I could hear the metal link hitting the rear window. But amazingly had a recording of 4.0l/100km. Effectively that flap/spoiler was just hanging by the shafts and it moved according to the air momentum. The flap weighs approximately 120g. Its approximately 45mm from center of the shaft axis to the edge. I could only imagine that the angle would change with different speeds/mass flow. Interesting. It would seem you don't need control. Just weigh it down to force air to move over the hatch.

However, I really doubt anybody would buy a flappy spoiler no matter how good it works

[aerohead]

Originally Posted by serialk11r

Yea that is obviously better, but it looks kind of ugly and I'd be concerned about damage from other people's poorly executed parking maneuvers since that would probably be implemented in cloth. If diffusers and spoilers had some seriously versatile hydraulic arms and could move all the way back you'd get a pretty good approximation with something that folds right into the body. Maybe it could be done with rear hatch/trunks that have multiple piece panels that fold outward as well, I imagine it's possible to make that look stylish.

Here is the image Hucho published from early German engineering.It's the extensible tail on an Omnibus,which is drawn completely inside the bus body when not on the open road.There's nothing to be damaged.
Aerodynamically,it's the best add-on drag reduction system I've seen.

[ecomodded]

Originally Posted by aerohead

It doesn't matter how the elongation and taper is executed,but the trailing edge must to be continuous and airtight,wrapping all the way around as Kamm has done,or as with a sealed box-cavity.Any 'hole',anywhere, will allow lower pressure are to bleed into the space and adulterate the pressure increase you're trying for,netting low to zero gain.It's not negotiable.

This piece of advice is useful to me as have thought I could add a "virtual" boat tail by extending the roof only with a void underneath it..

If I ever was to mod my beetle I would cut it down to the desired boat tail shape. My roof is exceptionally high I would cut it down to blend it into the rear hatch similar to the karmann Ghia's roof line.

[freebeard]

Here is the image Hucho published from early German engineering.It's the extensible tail on an Omnibus,which is drawn completely inside the bus body when not on the open road.

For some reason your post has no Thanks button. Thanks.

Was this ever built?

[aerohead]

Originally Posted by ecomodded

This piece of advice is useful to me as have thought I could add a "virtual" boat tail by extending the roof only with a void underneath it..

If I ever was to mod my beetle I would cut it down to the desired boat tail shape. My roof is exceptionally high I would cut it down to blend it into the rear hatch similar to the karmann Ghia's roof line.

You can also leave the roof as is,and just add a false K-form structure to slow the contour.
You'd be lofting the curvature up to the 'Template' as VW does with the 1-liter car.And it could be open all inside,with a horizontal shelf extending back kinda like Ernie Rogers did with his 'Bug Wing.'
The whole attachment would effectively be a box cavity.On the passenger side C-Pillar buttress extension you'd want to cut in a window for the new blindspot.