F1 in Schools Help - basizeland's project

[tok-tokkie]

I noticed how first the front wheels lifted then the back. The guide wire really saved the day.

[basizeland]

tok-tokkie I have seen a car that has been shot down the track that wasn't attached to the track and it ended up spinning off the track and smashed to pieces, so the wire does really help and is important.

I have been busy the last couple of days remodeling the car from scratch using a new technique that I was taught which will get us more points in the engineering section, I have finally made the change that flynfrog suggested with tapering the rear section behind the rear wheels to the center and only doing it in one direction so here is the pics of the change. Im not sure whether it should taper in a straight line like it is or curve like the front wing.

[flynfrog]

that looks pretty good maybe a slight taper on the rear tails but not to a point.

Id try you cfd with that to see what it spits out.

[tok-tokkie]

I have now read the rules and skimmed this entire thread again. I find it a very interesting topic.

So the wire guide works. You are looking for very small improvements now to move from 1.046 secs to <1 sec. How about the friction on that wire? http://en.wikipedia.org/wiki/UHMWPE ?

Somewhere you quoted the wheel RPM. F=M*A so the lighter the wheel the better. I see that their size is specified but construction is free. I also see that it has to have line contact with the surface. I had thought it would be nice to angle them down slightly and run on just the outer edge; but illegal I see.

The wheels have to be fully visible when viewed from above. Your current design has the sidepods conforming to the wheel shape below the axle line. The closer the better I would have thought. Flyinfrog?

Lubricating the wheel bearings with WD20 before each run?

[basizeland]

tok-tokkie I find the whole program really interesting and its a heap of fun even tho the competition only lasts 3 days its such an adrenaline rush especially when your car races for the first time and it beats the best time so for.

With the wire guide they use some type of fishing line so we have no influence over what they use but we are using the rings off a fishing rod as they are made to be really low friction with the fishing line.

The wheels are made out of acetal because it is reasonably light but has a low coefficient of friction which we have found to be better, they are machined as thin as possible and the middle section that spins only weighs 1.5g for each wheel and 1.9g with the bearing. Here is a pic of a section cut of our wheel.



The yellow section is the bearing which is a hybrid ceramic ball bearing which we have washed all the lubricant out and we spray some silicon spray on the bearings before we hand the car into the judges at the start of the day. We aren't allowed to touch our car between runs although once the automatic firing time trials are finished we have an hour to work on our car so we usually just fix anything that has broken and reapply the silicon spray.

With the sidepods conforming the bottom section of the wheels it currently only has a 1mm gap which might be too close to the wheels because they have a bit of movement side to side because it is only running a single bearing in the centre of the wheel which allows it to move and this is why we have to have the gap between the covers of the wheel and the centre section that spins.

[TheMinister]

I can't help but wondering whether or not you'd be better off lifting from the ground altogether. If you've got the guide line to keep you going straight, maybe using a bit of ground effect under the car to lift it might not be such bad thing. I'm thinking only a few mm, obviously the optimum height would depend on how high the guide wire is; no point wasting energy pulling the wire upwards.

The way to achieve this would be with a flat bottomed car, maybe a slight skirt, that simply pulls air under itself at the front. Think of the way a piece of paper glides across a flat surface (or a protractor when thrown just right, though I doubt anybody else ever had that boring math lessons at school).

Of course, if you could somehow vector the thrust from the canister downwards, you could generate a bit of lift that way. Though thats probably going to be harder to make work within my vague approximation of the rules.

The savings would be numerous; much smoother ride, so no energy lost on any bumps. Less problems with skidding sideways (no idea if this is an issue anyway though). If the car got airborne before hitting max speed, then you no longer have to waste energy in spinning up the wheels (they look fairly heavy to me). And of course the friction is gone.

Problems would be keeping the car straight, not wasting too much energy on staying afloat, getting it to hold a good pitch (not going nose down/up).

I have no idea how to quantify any of those savings or problems; that's for someone with CFD and a car to toy with. Just a few ideas.

[tok-tokkie]

That wheel is gorgeous. Acetal is about the hardest plastic so there will be no deflection giving lowest rolling resistance. It is also the nicest to machine of all plastics in my experience. No way to reduce the rotational mass or the air resistance with that design that I can see. Congratulations.

[basizeland]

We have always wondered whether the car would be better to lift off the ground as it heads down the track and other teams have thought the same, I have never seen or heard of a car that has done it properly but most of the cars leave the track surface a few times during the run but I think that it is from the joins in the track that causes the car to lift.

I could ad a flat section on the bottom to the front section of the car but it will be hard to do it underneath the co2 canister housing because we are only able to machine from the two sides and the top and bottom, so if we were sure that it would work we could try and get somewhere else to machine it for us.

We could easily angle the canister down to the ground a bit so that wont be a problem.

Thanks tok-tokkie I have spent alot of time revising the disgn of the wheels to get them to what I thought was the best, some other teams have done the same thing by having the sides of the wheels stationary but have made them really complex with a heap of parts to get the same effect that we have with only three parts. We originally made our wheels out of alluminum but we tried acetal as it was suggested to us and it was .1 of a second quicker just from the material change.

[syguy]

I'm sure an airborne car will have less frictional resistance than a car in contact with the ground. However, if you are trying to fly then you have to worry about roll, yaw, and pitch stability.

Pitch
I would guess the weight bias is towards the rear of the car due to the mass of the CO2 canister, so if you want pitch stability (nose down tendency) while flying you'll need the center of pressure (lift point) further back than the center of gravity - not easy for a tail heavy car.

Roll
For roll I guess you can rely on the wheels touching the ground and leveling the flight.

Yaw
For yaw - are the sides of the track close enough to help? If not, you'll need a fin (vertical stablizer).

Thrust Vectoring
I think using the thrust of the CO2 jet to get airborne would be costly in terms of energy. It's better to spend the energy budget by converting it into horizontal velocity.

Safety
I'd question how much force the car can exert normal to the guide wire before the wire breaks? I'm also guessing that the judges would ban a car that has a tendency to get airborne due to safety concerns - a 30-40 mph flying projectile could do serious damage to something.

Conclusion
I'd suggest sticking to minimizing weight, friction, and drag of a car not a flying car. I'd think that a touch of downforce at the front of the car might help keep it rolling straight to avoid contact (friction) with the sides of the track.

[Giblet]

You know, I have long ignored this thread, and that is a shame, because I think you guys would have like the Rocket Powered Two Stage Batmobile we ran, just for kicks.
[youtube]http://www.youtube.com/watch?v=Y3FhsvvJytg[/youtube]
[youtube]http://www.youtube.com/watch?v=oj-2xPIrVYk&NR=1[/youtube]

[basizeland]

Yeah thats a good point Syguy I hadnt really thought about that, so I think that I will stick to leaving it on the track.

Giblet that is awesome and we have always thought of getting a model rocket engine and putting it in our car just for some fun and see what time it does it in.

[flynfrog]

lifting off the ground is not that good of an idea. your rolling resistance will be less but the new gain in aero drag and loss of thrust that could be pushing the car down the track.
tok=tokkie is right the more wheel you can cover with the side pod the better.
also the WD-20 is the super secret prototype to wd40. you can only get it on the black market.

[basizeland]

I would like to try WD-20 so does anyone know where I cloud get it from because i had a look on the net and couldnt find anything about it.

[tok-tokkie]

I would like to try WD-20 so does anyone know where I cloud get it from because i had a look on the net and couldnt find anything about it.

He is just teasing me for my typo. WD-40 is magic. You use silicon. I use it on the cables on my motorcycles & can't say it has lived up to expectations. It is magical on the sliding parts of old fashioned wooden drawers though. I intend switching to WD-40 for my motorcycle cables (I thought silicon would be better through being less sticky so dust adhesion would be less)

Notice Giblets rocket was angled down to hold the car on the ground. Notice how accurately it tracked straight without guidance.

I have had another thought about the wheel bearings. Need to read the rules before posting.

[TheMinister]

I would like to try WD-20 so does anyone know where I cloud get it from because i had a look on the net and couldnt find anything about it.

It's rubbish, just watered down WD40. Half as strong.