First Electric van boat tail

[JohnForde]

Building all day today. New 1/4" Plywood 'cuff' @ 11 degrees.
Tomorrow new doors with top at 11 degrees.
Extensions seen in photo will be replaced with polycarbonate twin wall.

[freebeard]

Here's why I think the cuff replacement should be curved. Mair is one of the Aerodynamic subforum's foundational documents:

[Piwoslaw]

@John, if you have tested a lengthy boattail (even if it was only partial) at least twice, with no measureable improvement, then maybe you are doing it right, but something wrong with your measurements?

I mean, just the naked side panels at 11-14 deg would be a ping on the butt-o-meter, let alone if you have numbers from A-B testing

[aerohead]

Originally Posted by JohnForde

Thank you JSH, Aerohead & Freebeard.

I have purchased 1/4" plywood with which to do the cuff properly and build doors with an 11 deg downslope.

My measurements are approximate. I have been using a "rise over run" webpage to plan.

I will get the cuff, doors and top all between 11 deg ( 5 over 1) & 12.5 deg (4.5 over 1).

I will test at 11 deg before removing the VGs.

Aerohead - the exposed down facing winglet is what allows me to adjust door angle while in development. I think with doors at about 11 degrees the winglet is all swallowed up. It will be sawed off when I am done.

I knew that it was a 'necessary evil', and I'd have done the same thing.

[aerohead]

Originally Posted by freebeard

Here's why I think the cuff replacement should be curved. Mair is one of the Aerodynamic subforum's foundational documents:

From Lawrence Livermore's boat tail research, the tearing edge of the 'straight', 11-degree tail, is essentially in the exact same location of NASA's 'curved' boat tail.
There 'WILL' be immediate flow separation, but the tearing edge, positioned at the 'reversal point', will reattach the flow, capture a locked vortex, with the local streamline following a 'phantom' pathway, as if 'curved', with almost the same quality of pressure recovery as the 'ideal' curvilinear surface provides.
Goro Tamai of MIT refers to the process as 'mending the flow.'
It's exactly what Dr. Hermann Burst did for the Porsche 911 Carrera RS 2.7 'duckbill' spoiler.
Fabricating the 'curved' panels would magnify the man-hours by 1000%.

[freebeard]

Fabricating the 'curved' panels would magnify the man-hours by 1000%.

I do not think that percentage means what you think it does (that's three orders of magnitude). Or maybe it's the conjunction [by] vs [to].

I'd say to 125%. The case of a Porsche ducktail is different to a box van.

Remember your Morelli's 'Fluid Tail' thread?

*The circular,or elliptical shape of the transverse separation line is crucial to the performance of the fluid tail.

Blown rear wheelwells FTW.

[freebeard]

I'd say to 125%.

Soak the 1/4" plywood on one side until it swells. glue and screw curved ribs on the concave side. Cut the ends to 45 degrees. Vaporize the moisture with a blowtorch to set the curvature and marbelise the wood grain.

Done.

[JohnForde]

"cuff" as seen here is 16" W. It is set at 11 degrees with the doors planned at 12.5 degrees.
https://photos.google.com/photo/AF1Q...dpwZ4VOuv_0BqC

[JohnForde]

Originally Posted by Piwoslaw

@John, if you have tested a lengthy boattail (even if it was only partial) at least twice, with no measureable improvement, then maybe you are doing it right, but something wrong with your measurements?

I mean, just the naked side panels at 11-14 deg would be a ping on the butt-o-meter, let alone if you have numbers from A-B testing

I have been too ambitious. First test at 28 degrees, second at 22 deg, third at 16 deg. I think now, at 11 deg with a quality cuff I will see numbers. Probably Monday.

[JohnForde]

Originally Posted by Piwoslaw

@John, if you have tested a lengthy boattail (even if it was only partial) at least twice, with no measureable improvement, then maybe you are doing it right, but something wrong with your measurements?

I mean, just the naked side panels at 11-14 deg would be a ping on the butt-o-meter, let alone if you have numbers from A-B testing

Here is how I measure. The instantaneous readout on the dash of power in Kw. It is very volatile and hard to read, varying wildly up & down.
The best I can hope for with no WT is 34 Kw @ 63 mph under ideal conditions: room temp, no wind no hill.
If the WT can reduce my power need by 15% I can cruise at 63 mph on only 29 Kw, pushing usable range from 300 to 350 miles. I think I can do better (-30%?) - pushing power consumption in to the mid 20s and range to 400. I will know soon.