Are there any magic numbers?

[justpassntime]

Originally Posted by aerohead

justpassntime,if you'll go to the top of the forum page,at ecomodder search,Advanced,and type in, mods-data-% change or Cd change ( installment # 5- roofline data ),that should get you some material to chew on.

I found it, THANKS!

Lots of good info. The good thing is that I haven't wasted my time, well not yet. As I found when I was making the caps or ends for my rear window spoiler, visor, defuser, etc. (too many to chose from what would be the proper nomenclature for this item), slight bends in the curvature of the metal change the angle drastically so it is somewhat adjustable.

So if my reading is correct (and correct me if I am wrong) as long as I stay with the curvature of the roof line and not exceed 15 degrees I should be ok.

Since this particular aerodynamic addition (hopefully) just basically snaps in place it should be easy to do testing. I suppose tuft test would be the best since a powder test would stick to the primered surface of the car.

[justpassntime]

Thanks guys for the input!

I am much better with metal than anything else. Although I have made some cool things out of plexiglass. I figure I can screw this thing together so if I have to make adjustments it will be easier. Then when I find the optimum angle I'll weld it together then.

BTW...I never really welded before until I started a project doing a 73 Ranchero. It needed new quarter panels. Dad said he wasn't going to do it, didn't have time. So I bought a wire feed welder for nearly $700 bucks and went to town learning how to weld on car bodies. Not the best place to learn. I guess a good explanation of my first attempts was like soldering aluminum foil together with a propane torch. Burned holes in everything and warped the crap out of it. Oh it looks good now, ok atleast the part that didn't get damaged when I got rear ended 3 months after I finished the car. Been sitting for ten years now.

Looking back that welder has welded a lot and it's still in my dads shop. Pretty smart old man, huh! Didn't have to do the welding and has the use of a free welder.

[Concrete]

Ya know, I have a very similar story about a acetylene torch rig...
minus the 73 Rachero

and Dad still has the welder - but I am still close enough to use it

So when do you think you will get this mini Kamm back on the road?

BTW
There have been at least two other threads this week asking if such a thing like what you are working on can lower drag.
your adjustable device might help clear the air on this topic.

[aerohead]

Originally Posted by Big Dave

It’s always a tradeoff between minimizing wake area and having an excessively steep slope. My bed cover does a nice job of minimizing wake area – a 47% wake area reduction – but does so at the price of having a seventeen degree slope.

It is time for me to revise my wake cover to make it handier to use. What I am thinking of is to reduce the slope to 11 degrees (1-in-5). This will mean my wake area reduction will be only 27%.

Going to a 1-in-4 slope (14 degrees) would result in a 37% reduction in wake area.

Does Hucho have a graph showing the effects of wake area reduction on drag?

I can’t do a full boattail. That would result in a thirteen foot long “tail” if one stuck to the one-in-five slope I’m planning. The truck is enough of a bear to park as is. The other extreme is simply a flush commercial cap. Zero degree angle but zero wake area reduction.

Big Dave,I think Hucho shows Mair's boat tail chart with all the results for varying the length.Mair uses a cylinder with a stretched-out hemispherical nose.This body of revolution is out of ground effect,so you have to bear that in mind.-------------------------------------- with no aftbody,the form scores Cd0.165.Mair recommends stopping the tail at a length equal to 1.125 times the diameter of the body.Cd at this point has dropped to 0.03.Also,by this point,the body has been gently bent into a 22-degree angle.If you continue the tail all the way at the 22-degree angle,the Cd drops to 0.0225.I looked at Jaray,Klemperer,Lay,and Korff.They basically recommend that you end the tail at 50% frontal area so you don't bang stuff with the long tail.You still have half the drag leftover behind you,but to cancel that means all that additional length.Any 10% reduction in wake area cuts drag 10%.----------------------------- A note of caution.On bondo's original bedcover,he did not have enough gentle roof curvature behind the F-150's cab,and the flow separated immediately.His new cap has generous curvature ahead of the final slope angle.Kamm wouldn't go steeper than 10-degrees.The ideal teardrop ends up at about 20-degrees,but it takes a lot of length to get there.I think your 11-degrees is real good,and even if your wake is larger,it will be a a higher pressure and drag should be less.I may have Mair's graph in my photo album.I'll look.

[aerohead]

Big Dave,go to the search engine at the top of the page and type in phil Knox's aerodynamic photo albums.The third photo down is Mair's boattail graph.Hope it helps!

[Big Dave]

Thanks Phil. I thought I was on the right track. I intend to use about 25% of my bed length in a curved transition to my 1-in-5 slope.

Interesting. Mair's boat-tail shape is exactly that used by manufacturers of sniper rifle bullets. They call them "boat-tail spitzers." Of course being supersonic, they use very pointed forward sections.

All this is axial slope. How should I curve it laterally? Or should I? As I recall, aircraft canopies are rounded in both axes.

[aerohead]

Originally Posted by Big Dave

Thanks Phil. I thought I was on the right track. I intend to use about 25% of my bed length in a curved transition to my 1-in-5 slope.

Interesting. Mair's boat-tail shape is exactly that used by manufacturers of sniper rifle bullets. They call them "boat-tail spitzers." Of course being supersonic, they use very pointed forward sections.

All this is axial slope. How should I curve it laterally? Or should I? As I recall, aircraft canopies are rounded in both axes.

Big Dave,you'll want to integrate the forward edge of the cap to fit that of the trailing edge of the cab of course.And the cab has some "tumble home" angle to it,rather than just being a vertical wall.As the cap progresses rearward,you could start increasing the radius of the intersection between the top and sides and allowing the sides to kinda roll into a curved sidewall.I know pictures would be helpful.Sorry! Anyway,as you approach the end of the cap,your sides could be rolled into a continuous curvature over into the top.It would be like the boat shell on the T-100 is now.And like the "Aberdeen Projectile" boat tail you see on the supersonic bullets.It's alot harder to construct,due to the compound curves.A cedar-strip canoe is built with the compound curves and they take about 400-man hours to complete.Whew!!!!!!!!!!!! Lots of work.The air loves this stuff,but for the trouble,you may never see that much difference at the Diesel pump.You may just want to let the sides lay at the same angle but gently curve in at the same rate that the roofline is descending.On Darin's thread for his permanent boat tail we generated a ideal teardrop template.You scale your vehicle's height to where it will fit under the template at the point of max roof height,and the template defines the curve of ideal airflow.You can end it wherever you like,just like Kamm did,and have good attached flow,right up to the "chop".Check it out if you haven't seen it.You can use it for the sides as well.If you bring the sides in,you cut wake area a bit extra compared to straight sides,but is does cut down on cargo room.Everything is a compromise!

[justpassntime]

I pretty much finished the rear window spoiler (not sure what to call it). It snapped in just like I had planned, I still have to make a center support and primer it. This would also make a great place for a third brake light.

I just pop riveted it together for now.

I will also fill the gap with sealant before painting the car. After testing to make sure this mod was worthwhile, of course.

[Concrete]

justpassntime,

looks nice
a row of tuffs on the last 4-6" of the roof
& a row of tuffs on the "spoiler" will let you know what it is doing

but I'm thinking in the big picture this may not be measurable

I just tested a cab extension on my tuck that is 2 foot long
with a pre test run or two with the scan gauge & I'm guessing it is a 10% loser
so it is pretty bad - I must have hit the sweet spot for stinky aero conditions
but yours is about a 1/4 that size so a similar situation may only lead to 2.5% change
and that would be hard to find (good or bad) at the pump I think
but you never know

at least yours is cool looking, mine is obviously Not!

[justpassntime]

I think you may be right about seeing a difference without a trip computer. Then again it may be tough even then. My car is pre OBD, so have to wait for the guys doing the MPGuino.

I need to add a support of some kind to keep it from waving in the breeze. Thinking one in the middle would do it. Found an LED light strip (very small) if I want to add a higher third brake light.

Another question...

Do the taillights on the Caddys that were the the long narrow verticle lights help with aero? The reason I ask is they extended beyond the trunk area.

Concrete,

I think you are on the right track with the extension just not the right angle.