canard for bluff bodied box

[bluebunny]

Hi Folks,
I do not have a box-in-the-wind, so have not tested this mod, but wish to present it for discussion.

It could be considered a canard, designed to improve aero around a square leading edge, like on a trailer or box top. Notice crude canards running around on top of some of those armored cars?

This one is also crude, but can be built robustly for under $100. I believe it to be an improvement over simple rounding of a leading edge. I would have liked to shown one with double the diameters presented, but believe any larger would exceed the limits of the material strength. If any were to show the benefits of this little one, I am confident a good aluminum welder could safely fab it larger to good effect...

I speculate that bursts of 1 psi could hit a device like this, so calc the pressure times the area of the canard and you can realize how big the forces get! - hundreds of pounds on this little device - so build these stuuUURRDY!

Please take a look and give me your thoughts?

I am hoping one of you smart people will be inspired to build, AB test, and report back to us?

[Frank Lee]

I don't understand the theory behind the proposal- how it would be better than just the rounded leading edge(s). It would be fun to see it tested though.

[bluebunny]

Originally Posted by Frank Lee

I don't understand the theory behind the proposal- how it would be better than just the rounded leading edge(s). It would be fun to see it tested though.

Hi Frank, thanks for asking...

I have to be careful how I 'splain it, so as not to lauch into incorrectness...

There is "a lot" of data regarding slots and their effect on wings, and I only know enough to be very very dangerous if we were flying this thing

Roughly speaking, the underside of the canard's "rounded edge" captures a high pressure zone. This high pressure zone is gently and smoothly squeezed down to a rather sharp "nozzle", where the air velocity increases and pressure subsequently goes down. This small jet of air blows across the surface of the top of the box at its leading edge (where all of the aero trouble usually is), which in wing terms is far back on the top side of the wing.

The whole attempt is to provide extra lift (actually downth) to the air and to delay it from stalling. A simple small radius like 4" on a 90 degree turn stalls very easily and then has lots of drag. The 4" radius with slot (calling it a canard) may also stall very easily and then have lots of drag, but if successful, the 90 degree turn will stall less and later!

One unknown is the location of the canard. Testing would tell us if lifting it up so say a 2" gap is better (at a cost of a bigger windage). My money is on "keep the windage low, and let the little slot do what it can". Stuffing the slot closed with foam could be an interesting test as well! Just think of all the fun we can have!

For say box on a semi, I would expect the rounded edge (slotted or not slotted) should be 20" or even 30" radius to be right-sized. If the radius is large enough, the slot may not help at all? But 20"-30" is not a practical size for your pickup topper. In any case, the sharper the bend, the more we need lift-cheating devices like flaps and slots and other devilry!

Speak up you aeronautical experts, and kick me in the pants! Somebuddy throw this easy model into your software program, and compare with a simple radiused leading edge!

Otherwise, if you happen to have a square boxed do-hickey that goes down the road, I believe that tacking this ON will be easier than sawing the box DOWN, and might even work betterer? A-B-A-B KNOWS FOR SURE!

[Frank Lee]

Wings are subjected to a wide range of angles of attack where road vehicles aren't so much.

[bluebunny]

Originally Posted by Frank Lee

Wings are subjected to a wide range of angles of attack where road vehicles aren't so much.

Correct! The protruding box shape is "locked in" at a very high angle of attack regardless of speed, always in stall. This is where slots can be most effective. Just wondering if the high lift to bend the flow tight to the turn provides more benefit than the drag of the canard...but this high drag vector on the canard points verily perpendicular to the free stream flow, and may somewhat cancel on the sides, and lift along the top?

[freebeard]

I don't get that last bit, at all. The closest analog I can think of is the Air Curtain BMW, Ford and other use on the front wheel wells.

I suspect you will just move the separation from the top behind the edge to the throat of your converging duct. Suppose you replace the quarter-circular canard with a flat, horizontal plate to maximize the air ingested. Or just trim the curved one back. It need only protrude forward a little more than the half-round.

Edit: OTOH there're also the 'turning vanes' on the front corners of European trucks. They accept the air at right angles to the ambient flow.

With 13 posts in 3 years I'm not sure how much you've seen of the subforum.

http://ecomodder.com/forum/showthread.php/virtual-boat-tailing-30594.html"]http://ecomodder.com/forum/showthrea...ing-30594.html

Check out the PDF in the first post, figure 8, page 12. To get the effect you're hoping for they were adding extra engines. Not saying it can't be done passively, but the scale may be different than what you propose. Ford's air curtain slots are pretty small.

What about this?



Here's is MB using what I would characterize as a 'perforated Gurney flap' to reduce buffeting in an open convertible. It seems to work. What might that do at the leading edge of a long body. It would slow the air down instead of speeding it up.

Also, Australian sun visors.


http://blog.bbt4vw.com/category/stock-parts/page/9/

I like perforated plates. Flowing air will see a surface, but stagnant air can pass right thorough.

[bluebunny]

Originally Posted by freebeard

The closest analog I can think of is the Air Curtain BMW, Ford and other use on the front wheel wells.

I suspect you will just move the separation from the top behind the edge to the throat of your converging duct. Suppose you replace the quarter-circular canard with a flat, horizontal plate to maximize the air ingested. Or just trim the curved one back. It need only protrude forward a little more than the half-round.

Edit: OTOH there're also the 'turning vanes' on the front corners of European trucks. They accept the air at right angles to the ambient flow.

Yes, the extreme is the flat-plate canard in use today by one builder of those big armored trucks! They are likely very low drag canards, and may actually outperform the one I propose! But I suspect even that one may benefit from a curved leading edge, and its greater height will also tend to increase the cross-sectional pressure area?...

The sketch shows both designs for comparison...

The curved slotted canard design takes greater care to accelerate the flow through a true nozzle, but they both are intended to control the boundary layer along the sides of the box...

leading edge slots and slotted flaps are explained in this ancient film from the 30's. the canard slot follows the principle. starts 7:30 into the film...

https://www.youtube.com/watch?v=q_eMQvDoDWk

[niky]

Something like this?

[freebeard]

That's it. If they're so good, why aren't they four times as tall?

Nice truck; I like the mirrors out front, and the ladders at both ends of the bed. How does it open up, as a dump?

[Frank Lee]

The roof spoiler is a whole 'nother animal.