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Guest Message by DevFuse

Get you C-MAX Hybrid Registered in the official Ford Authorized Registry. More here.


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Tuft testing a C-Max

aerodynamic aids

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69 replies to this topic

#61 OFFLINE   fbov

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Posted 07 July 2014 - 02:49 PM

... Yes, 0.113 is possible - looking back about 75 years!.

No question we humans figured out low drag shapes a long time ago, in the age of lighter-than-air craft.

 

Also no question that the car you linked will be highly unstable in crosswinds, rendering it undrivable. Porsches and Zeppelins have tail fins/spoilers for the same reasons... they provide a countering torque in a side wind to prevent the car from turning away from the wind... and usually off the road.We want aeordynamically stable cars...

 

HAve fun,

Frank









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#62 OFFLINE   SnowStorm

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Posted 08 July 2014 - 03:20 PM

No question we humans figured out low drag shapes a long time ago, in the age of lighter-than-air craft.

 

Also no question that the car you linked will be highly unstable in crosswinds, rendering it undrivable. Porsches and Zeppelins have tail fins/spoilers for the same reasons... they provide a countering torque in a side wind to prevent the car from turning away from the wind... and usually off the road.We want aeordynamically stable cars...

 

HAve fun,

Frank

OK, I admit to partly linking that picture just for fun.  But is there a fundamental reason that a low drag shape, suitable for a car, has to be unstable?  Something about center of pressure offset from center of gravity?  Maybe move the battery.  Or stabilize with computer control like a fighter jet.  They are fundamentally unstable as I understand.



#63 OFFLINE   ptjones

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Posted 08 July 2014 - 04:06 PM

OK, I admit to partly linking that picture just for fun.  But is there a fundamental reason that a low drag shape, suitable for a car, has to be unstable?  Something about center of pressure offset from center of gravity?  Maybe move the battery.  Or stabilize with computer control like a fighter jet.  They are fundamentally unstable as I understand.

I would think the lower the center of gravity, the more stable the car would be. :)

 

Paul



#64 OFFLINE   fbov

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Posted 08 July 2014 - 04:13 PM

... center of pressure offset from center of gravity?  ...

Correctamundo! Give that man a kewpie doll!

 

There are two approaches described in Hucho.

- add rear side force

- design in high drag in crosswinds

 

The first is the rear fin approach, exemplified by the retractable spoiler on recent Porsche 911 variants. A sidewind hitting the front causes a yawing (spinning) force rotating the car away from the wind. That same wind, hitting the rear fin, causes an opposing force, rotating the car into the wind. Net result is a little side slip, but no change in the car's direction of travel.

 

The second is neat. It calls for a low drag shape in the absence of crosswinds, but one with increasing drag as the crosswind angle increases. As it works out, you don't change drag much until the crossing angle hits a threshold, above which angle the drag increases quickly, and rotating torque drops off. After looking at car designs for the last year or so, I think the crosswind stability feature in the C-Max is the windshield in relation to the A-pillar. Notice that the windshield is recessed a little lower than the A-pillar, which has fairly sharp edges. This clearly collects water off the hood/glass and onto the roof, rather than around the A-pillar and onto the side windows in the absence of crosswind, and in straight ahead wind, does little else.

 

As the wind direction turns increasingly off-axis, this recess starts to disturb air flow off the glass onto the side of the car, eventually causing to to separate from the surface rather than wrap around it. This separation is a transition from laminar flow to turbulent flow, the latter causing the desired drag increase as the wind's angle of attack becomes more severe.

 

I suspect all of us with grill blocks, air dams and other aero improvements owe a great deal to Ford's engineers, for giving us a design that we can safely alter, if we choose. At the same time, I can't see the rear window add-ons of the '14 and later models lowering drag, but I can see them reducing crossswind sensitivity!

 

Have fun,

Frank

 

PS Paul, you're thinking verticalheight... the instability is due to rotation about a vertical axis, in the plane of the road.


Edited by fbov, 08 July 2014 - 04:52 PM.

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#65 OFFLINE   SnowStorm

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Posted 14 July 2014 - 05:59 PM

I looked up Hucho's book - expensive and I have too many projects right now to make it worth while.  Cool stuff though.



#66 OFFLINE   fbov

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Posted 31 July 2014 - 01:12 PM

Latest update, after a month or so of driving with an air dam and side skirts...

Attached File  MPG vs Temp 140731.jpg   67.04KB   0 downloads

 

Clearly a lot of improvement over last Fall's baseline period; mods include (in order applied):

- PCM update

- hood seals above lights and across top of grill area

- VGs on the rear quarter panels, above and below the tail lights

- top grill blocked with plexiglass

- middle grill blocked by an air bladder (bicycle inner tube)

- lower grill blocked by foam pipe insulation (for easy removal in the event of heat issues)

- full width, 2"-deep air dam in front of engine acess panel

- 1.5" side skirts between front and rear wheels.

 

And the biggest mod in my estimation, better driving practices! I gave myself a month to learn to driver a hybrid. It wasn't long enough... and I started with 8K miles on the car, so vehicle break-in isn't an issue.

 

HAVe fun,

Frank


Edited by fbov, 31 July 2014 - 01:12 PM.


#67 OFFLINE   ptjones

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Posted 31 July 2014 - 04:35 PM

Frank I'm guessing you are monitoring your WT's. :)

 

Paul



#68 OFFLINE   fbov

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Posted 01 August 2014 - 09:35 AM

I have, but only through My View. I haven't seen it above center yet, but this has been a cool Summer, and I haven't done long trips up steep hills, although I will be doing a short trip up one of the steepest local hills... hosting a tour of the Mees Observatory Saturday. I haven't dug into the OBDII/Torque data either... although I did look at a phone mount that would let me see the readouts. Gets in the wife's way...

 

And yes, engine heaters are still in my future... I lose EV control even at 70F ambient; !@#$%^&* normal operation! Thinking of them as a birthday present; we took possession last Aug. 8.  Hoping I cna find a good place for a second one, as you suggest.

 

HAve fun,

Frank


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#69 OFFLINE   Jus-A-CMax

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Posted 01 August 2014 - 05:41 PM

I only have the centre grill covers for now and its been fine even with fast steep climbs. I really don't see any HUGE difference with the lower grill cover on in the mornings. Infact, I have been chalking up mega and PBs last 2 months with the 1 grill cover config. This car is INSANE MPG giver.


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#70 OFFLINE   hybridbear

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Posted 10 March 2015 - 08:47 AM

Latest update, after a month or so of driving with an air dam and side skirts...

Any more updates?








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