Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail

[AntiochOG]

Nice! Looking forward to your results.

[freebeard]

So I need to either rebuild the structure in a better material, or paint the whole box white.

How far are you willing to go with that? There's a big opportunity there.

Remove the top and sides of you box and keep everything else—front and rear bulkheads and the steel frame. Fab new sides and top similar to what MetroMPG did on his Insight in cardboard., but maybe in Polymetal. The front edges could have ~3/8" gap with vinyl edging.

v1.5

[aerohead]

Originally Posted by christofoo

I'm a sucker for flow charts.

(... but don't forget, this is the wife's car. )

...

When I looked at this I thought:
Only 0.02? So a well-designed gap in an air-foil can be ignored for vehicle design purposes.

That doesn't seem like the right analysis?

This was always one of my favorite posts, BTW. gap vs drag coefficient

Ignoring the bottom configuration,we can see that the gap will add from 8% to 30% drag depending on its location.And that's a best-case scenario,considering that there are no additional 'missing' pieces of the wing and perfect onset/source flow towards the sink.
If you can do the baffles,they will really help sequester the turbulence and hinder its ability to telegraph its effects around the interface of the box.

[christofoo]

Trip report in brief (details in the log):
Total 1719 mi
Total 43.7 gallons
Average: 39.3 MPG

Sounds decent, maybe, considering hot weather, air conditioning, and the heavy load? Actually I don't think so. For one thing I did 100% of the driving and my P&G / EOC, etc. is pretty well honed.

I need more coast-down runs to verify, and I'm still crunching numbers, but the conclusion I think I'm heading toward is that my Cd is currently higher than stock, and higher than it was with the cargo rack-and-bag.

The most interesting road-trip segment was with minimal wind, minimal elevation change (from end-to-end, with a reasonably shallow hill climb and descent in the middle, no braking or engine braking), minimal AC, where I cruised at 73 MPH for a few hundred mi and the SGII reported an average of 36 MPG.

Speaking of which, I've noticed my engine is going open loop under conditions that surprise me. From a little earlier in my log:
WOT thresholds (LOD where engine goes open-loop on SGII) at various RPM and speeds (from my fuzzy memory):
75 MPH - 3200 RPM - 58 LOD
70 MPH - 3000 RPM - 65 LOD
65 MPH - 2800 RPM - 72 LOD
60 MPH - 2600 RPM - 80 LOD
50 MPH - 2100 RPM - 90 LOD
I actually cease to P&G when I cruise at 73 MPH on flat ground with the AC on because I'm hovering on the WOT threshold at that speed, so P&G becomes a super-long pulse and brief glide - a.k.a. a waste of effort.

Anyone want to run a similar test? My mechanic told me not to worry about my O2 sensor, essentially because the computer isn't complaining about it... and yet this worries me...

[California98Civic]

Originally Posted by christofoo

... the conclusion I think I'm heading toward is that my Cd is currently higher than stock

Which might be what is predicted in the graphic quoted above in post #153

Originally Posted by christofoo

... and higher than it was with the cargo rack-and-bag.

Did the cargo rack and bag offer as much storage volume?

[christofoo]

Originally Posted by California98Civic

Which might be what is predicted in the graphic quoted above in post #153

...

Hmm, I was looking at the other graphic, i.e. #146, and thinking I should have gone from .315 stock to .235 total, or 25% reduction... it would be interesting if the roof-to-rear-window-curvature turned out to be the wild-card.

To be clear, it looks like my Cd with the gap filled may be around 0.4 (still preliminary, yes it has been revised upwards since my previous coast-down report - I owe you details).

Originally Posted by California98Civic

...

Did the cargo rack and bag offer as much storage volume?

The Aero Hitch Box has way more storage volume, and on this last trip it was 100% used (to my dismay ).

[aerohead]

Originally Posted by christofoo

Hmm, I was looking at the other graphic, i.e. #146, and thinking I should have gone from .315 stock to .235 total, or 25% reduction... it would be interesting if the roof-to-rear-window-curvature turned out to be the wild-card.

To be clear, it looks like my Cd with the gap filled may be around 0.4 (still preliminary, yes it has been revised upwards since my previous coast-down report - I owe you details).



The Aero Hitch Box has way more storage volume, and on this last trip it was 100% used (to my dismay ).

I believe that when you can do an airtight integration between car and box,that the performance you're looking for will rear it's pretty head.
You've got the equivalent of Walter Lay's Cd 0.12 'pumpkin seed',but with the bum windshield out front which wouldn't allow below Cd 0.24.
The tail can only be effective it it has a really clean 'source' or 'onset' flow as Hucho describes it in his chapter on CFD.
The kinetic energy which would otherwise be converted to pressure regain for the wake is being converted to useless heat as the air tumbles into turbulence between the roof top and front of box.Sides as well.
When these areas are sealed,only then can the box perform as tunnel studies would predict.
You're very close!
And 39+,at up to 73 mph sounds pretty tasty in its own right.

[christofoo]

I was taking the box off and noticed the skid pad had popped off of one of the retaining screws.



The weakness around the counterbored screw went through my mind during design but it's not an easy issue to side-step. I could double the number of screws... But I think it's time to upgrade to the nylon wheel version. (Custom cut nylon wheels on a pair of idler pulleys.)

(No picture), I've also noticed the caster I put on the bottom of the box has taken some marring. My guess is that it has grounded over more than one dip, and since it was unexpected it ended up taking a little impact. I'm glad the caster is there, and glad the frame is hinged. For having a boattail at 12-13 degree departure angle, I'm pleased with the practicality of this design. (Give or take pending aero details...)

==============================

Still slowly, slowly chugging away on coast-down... maybe doing the next Kammback AB test this week or next...

[aerohead]

I've had great success with inline-skate wheels.
I find 'em at yard sales at giveaway prices.
A local welding supply has steel fence hinge elements,die-cut with hole,and they weld up easy after fitting to support the axle.
Nylock nuts come with the skates and recycle nicely with new steel support.

[aerohead]

Looking back over Hoerner's book,'Aerodynamic Drag' I ferreted out some tidbits.
The following info is for streamlining 2-D flow around aircraft machine gun barrels,conducted by the German DVL,and also Junkers (aircraft company).
----------------------------------------------------------------------------
The 1934 DVL research was for super-critical Reynolds number:
*The un-faired barrel had------------------------------------------- Cd 0.4
*A simple prismatic fairing,with gap rendering L/D=3 gave------------- Cd 0.29
*The same fairing,but with sealed gap yielded------------------------ Cd 0.16
*A classical 'teardrop' fairing of same L/D ratio yielded --------------- Cd 0.13
--------------------------------------------------------------------------
The 1943 Junkers research was measured at critical Mach number:
*The un-faired barrel measured-------------------------------------- Cd 0.50
*A L/D=3 teardrop fairing gave-------------------------------------- Cd 0.08
*An L/D=3 teardrop,but with 20% of the tail chopped off =------------Cd 0.17
*An L/D= 4.2 teardrop,but with 10% of the tail chopped off = ---------Cd 0.20
-------------------------------------------------------------------------- I'll do the graphics when time allows,but you can get a sense of how sensitive the boat tail is to either gap or truncation.