need help building aerodynamic billy cart

Dain

My name is Dain, and i'm wondering if someone would be able to help me, or point me in the right direction on designing a aerodynamic 3 wheeled billy cart.



Once a year at my old home town of Beechwood in NSW, Australia, is a 5.6 km 2 person billy cart race down Pappinbarra Mountain,

this year (the 28th annual billycart classic) we ran down the mountain in a time of 10minutes and 5 seconds, but the winners came down the mountain in a time of 6 minutes 57sec.. this was the 4th year in a row that "bessy" (the name of the cart) has won the billycart classic, until now we have only gone down the mountain for a bit of fun, and we didnt have a very fast cart.



Well now it time to design and build "BESSY BEATER"



I was wondering if someone could send me some simple but effective shapes and dimensions that fit the basic designs of the billycarts that are run in this event,

current cart size's are approx 800mm high x 1000mm wide x 2500/3000mm long. ( remembering that the passenger may have to get out and push up hills)

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ank_gl

thats sweet, reminds me of speed racer(http://www.tv.com/dexters-laboratory...4/summary.html).
Anyways since the cart is not having any propulsion unit & is only manually pushed, its much like a glider(conceptually, at least). Drag needs to be limited to as least value as possible. Obviously you have already got that part.
Its really hard to just suggest a design(considering that there might be many), so you first should look at something which is really don on the same line. Have you heard of super mileage vehicle contest organized by SAE, those guys built vehicle similar to yours & ride at pretty much the same speed(i guess)
http://smv.berkeley.edu/
http://images.google.com/imgres?imgu...IN287%26sa%3DN
http://dce.ac.in/research/projects/supermileage/
Last one is from my college team

Dain

thanks for the links, looking at most of these cars and all of the ones i have found googling billys carts etc,it seems that the prefered design is wide at the front and narrowing to the rear, if i was to build my cart the other way around narrow at front to wide at the rear ( to suit passenger having to get out and push) would this be defeating the areodynamics and creating to much drag, the race that we compete in seems to be the only one in the world that run 2 person carts with the passenger having to get out and push up hills, unfortunatly i have very little understanding of areodynamics and drag, as im a cabinet maker by trade but have a passion for building fast billy carts.

Dain

to date the max speed of our billy carts have been 110 kmph (with a 90deg. right hand bend at the bottom of the hill, lol), with averaging speeds of around 50/60 kmph through the corners coming down the mountain. thats not bad concidering all the carts are made in back yard sheds at minium expence (mostly scap metal and old push bike parts)

ank_gl

thats a lot, i guessed it would be around 50kmph max.

For the point made in your other post about the narrow front & wider rear, compare it to an airfoil, you ll note that it is having maximum thickness at about 20-30% down the length, & it gets narrower towards the tail.

Basically drag is the summation of viscous drag & pressure drag. You can see read about both in this link
http://selair.selkirk.bc.ca/aerodyna...rag/Page1.html

the reason being that a narrowing tail ll present lesser adverse pressure gradient, thus boundary layer separation is delayed & hence pressure drag is reduced. Since you mentioned you don't know much of fluid dynamics, you ll be better of referring to available designs for such applications. However, the above link ll be a good start to move on.

cheers!!

Dain

sweet that link was a great help, it makes my idea look good, what i have came up with so far. Do you think this is optimal design for speed and best fits the dimensions of the common billy cart used in the race ?


please see attached pics of my design

all comments welcome, good and bad..

Many thanks again..


Dain.

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ank_gl

I really doubt if any design can be judged optimal or not just by looking at it.

If you are concerned about the aerodynamics of "billy", I am afraid billy is far from being perfect, it ll generate heck of a lot of pressure drag. I assume that you have read about the types of drag in that link earlier. So to reduce pressure drag, you need to "streamline" billy.
Is there any length constraint??
If not, add a narrowing section. It ll present a smaller pressure gradient, hence pressure recovery ll be more efficient & hence lesser pressure drag. For viscous drag, it entirely depends on what the flow conditions are. I suggest you take some technical help from someone.

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Dain

No there is no lenght restrictions, but being squared off at the back makes easy acess for the passenger to get out and push from behind,but now i think about it i could prob. run it like a bob sled and push from the side and jump in over the side,to get in.(cool runnings, lol)

would having the top cut open for access would cause lots of drag ? or will the air flow direction continue over the opening, and sharpening up the tail narrow be useless ? would a plane tail shape on the end help or continue the curve, it will need a little down force but not much (just to keep the wheels on the ground), cause it only slows you down.

sorry if im bugging you with all thse questions, but i hope to start construction in the next few days, ready for the next race in a few weeks

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ank_gl

depends on the cut, I dunno how should I explain this, but here goes, consider a pickup truck, you can assume the back space as a cut, if tail gate is down, there is an adverse pressure gradient due to such geometry, now if tailgate is up, something called a separation bubble is created in the space behind the driver, which improves pressure recovery(lesser adverse pressure gradient). Got anything?? I doubt . Mythbusters did it in season 3, if you can download that episode, you ll get a better understanding. here is a link
http://en.wikipedia.org/wiki/MythBus..._Tailgate_Down

like I said, it entirely depends on airflow, if you can get a separation bubble(a separated vortex actually) & cushion the air to flow past, pressure drag will be reduced.

No, it ll always help reduce drag by improving pressure recovery.

Why do you need a cut? To jump in?? I suggest you use a canopy, you ll only create drag for a short period when it is open, you can close it lator in the drive.

I didn't get this at all.

Dain

for the tale shape, would it be better to continue the curve around , or design some type of tail fin like a plane ? ( like in attachments in previous reply), im thinking that the curved continuing around to the rear would create more downforce ? is this correct ?

ank_gl

Oh, you mean something like a rudder?? No, that serves entirely different purpose, it is used for yaw control.

Down force? oh you are not coming off the ground going down a hill. You are at 50-60 kmph at corners(a 90 deg bend). If you really want to increase your cornering speed, maybe add airfoils generating negative lift, but that would require a whole lot of analysis.

mshinavar

can you have movable aerodynamic devices?

I was thinking like a canard like drag boats have. the driver/pilot has a foot pedal that controls the angle of attack of the canard (or sorts, maybe more of like a forward flap), and when racing uses this to slightly lift the nose out of the water to reduce drag from the water (while increasing drag from the air, but air is 1000x less dense that water, thus this is quite beneficial)

maybe do something like that, where you could create a little lift to reduce force on the bearings and therefore reduce friction, but at the corners, create downforce to improve cornering speeds (it wasnt clear if you had to slow for the corners or not)

as far as a rudder is concerned, if you dont want to make the sacrifice of big tires for a couple hairy turns, maybe a rudder might be a cool thing to have. it would keep the single rear wheel from sliding out, and not add too much drag in the process. eh, maybe not. it was popular with older race cars, probably for the above reasons.

as far as overall shape, think of a tear drop, 3-d shape and all. cut it in half - thats your car. limit the rear drop off to say 15 degrees (which makes the car really long, so make it smaller in cross section)

that should keep you busy for a while

oh, and use composites to save weight if you dont already

Dain

Well i think you can have them, there is nothing to say you cant, but the rules are a bit blured, well as far as i know there isn't any, written down anyway, i have asked for them and nothing has been produced, because my first thought was to run a 2 wheel design, but i was told there is a 3 wheel minium, so thats when i asked for the cart requirements.

With braking to corner, If you have the balls (most of us looking for the win) there is no braking down the mountain, you need to carry as much speed and momentum down the mountain as possible. it gets a bit hairy at times but its all part of the rush, lol.

Now i have the painful task of trying to get the ideas out of my head and on to paper so my dad can build it, if only i had a CAD programe to draw it up properly with proper sizes and angles etc, so my old man builds what im thinking, lol.

Thanks for everyones help, and if you still have ideas please post them, as there is still time for change..

mshinavar

everyone,

would it be more advantageous to be light weight or heavy? mass always divides out of equations, so you would think that it doesn't play a significant role, however intuitively that doesnt seem the case. also mass plays a role in momentum (mV), and it seems better to have higher momentum, but at the expense of higher inertia (at start)? im assuming the higher momentum keeps your speed up as you hit flat sections (but the higher inertia retards you as you start going downhill again)

i know heavier pinewood derby cars (gravity driven) work better the heavier they are....

Dain

I was just about to ask the same question, lighter Vs Heavy ?

ank_gl

Two objects, no matter which one is heavier, ll fall to the datum simultaneously & with equal velocity, so obviously, it doesn't matter(you pointed out that already).
Like you mentioned about reducing the friction by having canard type control surfaces to reduce the normal force, lighter system ll automatically have a lesser normal force, hence lesser frictional loss. But then again, steering capabilities at higher speeds at corners is curtailed for lighter vehicle.
Then of course is momentum, it was already pointed out in this post.
Then drag, if both systems are similar in shape, heavier system ll experience lesser deceleration.
So it really is a trade off between various constraints.

Dain

well thank you to all that have made comments and given me information and ideas for my cart, as of yesterday construction started, but i hope the heavier it is the quicker it is because the tax man was not kind to me this year, so iv had to opt for cheaper materials which means heavier materials, please keep posting ideas as a new cart will probly be built for next year, if this one doesnt do the job, if anyone has some spare time and would like to design the perfect cart feel free to email it to me, my company has just purchased the AUTO CAD programe, so next year i can draw up a design proply and send it to someone to manufacture for me.

Many Thanks again and ill keep you posted by posting pics of construction and final product and how i placed in the big race (in 4weeks time).