The physics of areodynamics

[Rod Turnbull]

The initial picture I didn't feel that it closely resembled the airflow.

The initial picture didn't include the wheels at all they were just there as a reference... I was going to get back to them later and never got around to it, sorry.

I want a design that creates some serious down force on the rear of the car that is only effective on the ramp

Serious down force = serious drag... if adding down force anywhere, you would be better off putting it in the front so your could move your 'actual wight' back... moving your COM almost directly over the rear wheels and use the down force to keep the front wheels on the track. It's the 'actual wight' of the car that stores your potential energy, and the drop that converts it to energy or forward movement. See diagram:



You can see how the downforce of the aerodynamics is relative to the surface of the track and will always have a backwards push on the car regardless of the slope of the track.

how did the car that you made with the fenders and the weight on the sides come out in a race ?

Never got to race it... they closed down the only highway when we were halfway to the event and I spent 9 hours driving around in a snow storm.

I was wondering if that design also played in a little in taking a more indepth look at aero ?

In a round about way, kind of... it lead to the creation of the 5 window coupe below... which got me wondering if the large frontal area would have a serious affect on the times, which lead to wondering how it would compare to other cars, which left me pondering the drag coefficient because a lot of other cars fall into the corvette/station waggon example above... then my mind just ran amuck.

[sporty]

See, My thinking is that I can get the down force on the rear to actually mimic more weight. Not sure this can be done, But I would like to play around with it.

Even though sometimes we find math formulas or equations that say one thing and should not do what it does do.

I been lucky in proving or dis proving a few ideas over the past fews with some of the info posted on here.

My weight is pretty far back now and my front end is pretty darn light now. Which works well for us.

My thoughts have been a wing / air foil that I can adjust and play around with the down force. So I can find one set up that can improve the cars performance with mimicing more weight, with out slowing it down but rarther speed the car up.

I think it can be done on the rear. Just have not spent the time on it yet.

most of us know that the cars are faster at 6, 7,8oz. This also would mean, more weight in the rear with having alot of weight in the rear, even more than the 5oz limist of most standard races.

I am not sure the down force that can be captured and used to ones adavantage would go over 8oz and slow the car down. The guestion is what angle would best work to gain down force without slowing the car and provide a improvement t the cars performance.

Thats why the air foil / rear fin comes to mind. As I would not want to add fown force onto the front, I want to lessen the friction on the front, not increase it.

I am able to resolve most wiggle issues with alignment. The rear is already going t have more friction on it, due to the weight placement.

So, If I can add there, where there is already friction from the weight and reduce in the front even more not exceeding the friction on the rear that would begin to slow the car.

Which I am taking a guess here that the downforce and friction would not exceed what a 8 oz car would get normally. be at when racing.

So perhaps the mathatics guys here can come up with some type of base line model of what the down force would be and the range with the factor of different angles of a air foil. Then also take data from lets say a 8oz car and see what the numbers are.

Sporty

[Rod Turnbull]

To get the aerodynamic down force on the rear to actually mimic more weight that gravity will pull down on to make more power is not possible with PWD cars... if nothing more, entropy will bite you in the but... you are trying to use the forward motion of the car to generate a downward force (remember that the air is not moving, the car is moving through the air)... and with this downward force you are trying to create more forward motion. Every time energy changes form it loses quality... hence changing the downward force of the car on the track to an aerodynamic downward force on the car, you lose some energy to drag of air over the wing, if the downward force on the car created more forward energy it loses more energy in the friction of the axles... and the loop starts all over again and you constantly lose more to drag and friction each time you complete the loop... thus you actually slow the car down.

If you want to play with some things try a wing that lifts in the back rather than pushing down... this would allow you to move the COM back more and remain stable... and who knows maybe the friction of the wing will affect the speed less than that of the axles and you may gain some speed there. Then rather than adding a wing, why not use the body of the car as one big wing... that wouldn't add to the drag as much as adding an extra wing.

[Stan Pope]

My thoughts have been a wing / air foil that I can adjust and play around with the down force.

Where does the energy come from that is going to create this down force? Can that transformation of energy create more output that it uses up in the process? The on top of that, the total "down force" results in additional friction losses. It doesn't sound like a good bet. On the other hand, if you develop the simple instrumentation (a board, a couple of cheap scales, some fish line, and somebody to drive the car while you hang it out the window) to quantify the energies or forces, then it will be a good learning experience, well worth the effort!

[sporty]

Hmm,

Well I have built a many wing cars and somewhere on here is the thinnest pinewood derby car I ever made.

it's a interesting debate for sure. I am looking for you or someone else to do the math and show the variables of solutions and alternatives. After all I did not start the topic.

I say when we have some real models and working application, like Stan mentions, the testing alone would be interesting just to learn from and look at.

I can look at charts and paper all day long. but someone still has to put it into a working and functioning application.

no matter if it's trying it on the front or rear. I have seen full bodied cars win at council, did I mention out of what i think is like a comprised of 2600 kids !!

So, they were fast and they had no Aero. So we know Aero has a effect, but does it really have a effect overall to be the basis of it being a plus over a non aero dynamic car ?

I think the percentage is small compared to all the other variables that come into play, that you first have to master or attempt to master before singling on one equation.

But you guys come up with some solid data, I will take a look at it and test and see if it really makes a impact, front or back.

As thats what I was conveying in my previous posts. It always something I wondered if it could help and create a plus versus a minus.

I certainly have seen seen and proven myself, things that defied physics and all since of rational reasoning.



Sporty

[Grubartez]

Here's a body I have been working on off and on since November. I hope to have it wrapped this month or next. It was built to test out the aero theory. I've seen a couple run with a similar design and they were pretty fast. I thought it would be interesting anyways.

[sporty]

Here's a body I have been working on off and on since November. I hope to have it wrapped this month or next. It was built to test out the aero theory. I've seen a couple run with a similar design and they were pretty fast. I thought it would be interesting anyways.

I have played around with this design, or close to it. i was not able to get it fast enough for my preference. But There were also other factors at play.

As i was not happy with the weight placement or alignement. I have used stock axle slots, versus drilled and that played a huge part in the car not performing well.

i also thought I had to much weight on the front end, from the wood and to much weight of the wood overall. I have wanted to do a bulsa version and see how that performed.

But so many projects and wants, I do not get to all the things that I would like to get to and complete.

a close design to yours-







Another car-








Sporty

[Rod Turnbull]

These are some interesting designs... I particularly like the ones with the top of the fenders cut off above the wheels, this minimizes the frontal area and wouldn't cause the Cd to increase very much at all... nice. It would be interesting to see if joining the top of the front fender to the top of the back fender and filling in the section between the wheels would make the car go faster... my guess is that it would. Well time to go back to the workshop and play with the tools some more.

[Rod Turnbull]

Forgive me for the liberties I have taken with this photo, I will remove it if you like but I thought it was a good example of an aerodynamic car.

I have added in the section I was talking about between the wheels and I am assuming you took the photo of the front of the car... and by the looks of the photo it seems the the back is rounded off like the nose of a bullet... if this is the case you may want to try running it backwards... a rounded front with a pointed tail is more streamlined than a pointed front and rounded tail.



Grubartez's car modified to include fenders that stretch from the front wheels to the back.

[roosclan]

There was a post of cars made by roosclan and family before paint and wheels. Perhaps it was on another thread?

Actually, it was on another PWD forum, but I'll post it here since the body designs fit so well with the discussion:

My Webelo's car is on the left: a Warpspeed-inspired offset-weight design.
My Bear's car is in the middle. A bare frame (tungsten cubes under the rear) that was also an offset-weight design. We covered it with 1/64" plywood on the top and bottom.
My Tiger's car on the right. He wanted a car-looking pwd car, so that's what he got.

It would be interesting to see what the Cd of each one is.

[Rod Turnbull]

It would be interesting to see what the Cd of each one is.

From best to worst (not that even the worst of the 3 would be very high)... A good guess would be Bear's, Tiger's then Webelo's. To figure out the actual numbers you will need a wind tunnel... I think I will actually try to build one as the information would make the computer models more accurate.

[quadad]

Understanding the effect of wheel drag has been alluding me for a while. I don't have an intuitive feel for it however, beyond the general effort to make thin cars and to embed non-protruding weights in the bottom of the car.

My own 0.2 cent idea (if worth that) is to set up for a simple test as follows:
- create a basic car that has an easily attached and removed "front and rear fender set";
- weigh the fender set and create an equivalent weight that can be placed/removed in the same front and rear areas (total car @ 5oz);
- Test with a) front & rear weights; b) front weight and rear fender; c) front fender and rear weight; d) front & rear fender.

I would alternate between the four configurations (a-b-c-d-a-b-c-d-a-b-c-d) so that the efffect of lube is minimized (3 times each, 12 runs total). I would think some kind of pin system on the fender base would allow it to ~ 'snap in and out' of the basic car body. Obviously you could add as many "fender set" types as you want. Any representative test track with a timer would work find for this.

Better ideas or obvious flaws ???

[Rod Turnbull]

No, that is a great idea... I would however just keep the weight the same for all set ups to isolate the affect of the aerodynamics of the different fender configurations... unfortunately up here we use an old wooden track that I have to stretch packing tape over the joints to keep the cars from flying off the track and our timer is a couple of people at the end of the track going, "yup, I think the blue one was faster... what do you thing bob?" - "Well that all depends, the green one made it to the sticky tape line first but the blue one did hit the block at the end of the track pretty hard..." - I say no more.

I still think I'm going to make a small wind tunnel... but some actual testing would be a very good thing.

Any volunteers?

[priority]

I still think I'm going to make a small wind tunnel...

I ran across a wind-tunnel design that is pretty straightforward to build. There's potential that issues with friction in the hinges of the test platform that could affect the accuracy of the measurements, but it is probably more than adequate for getting comparisons between different car body designs:

http://www.make-digital.com/make/vol15/ ... &u1=friend

The subtitle says it all: Getting (or going over) the edge on Pinewood Derby day.

[Rod Turnbull]

LOL... I wonder how long it is before he realizes that the car in the tunnel will run better backwards.