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car design , a little confused.....

Discussions about water rocket powered cars, sleds or boats.
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andicirk
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car design , a little confused.....

Post by andicirk » Mon Oct 12, 2009 4:17 pm

ive glanced (and read) my way through this section of the forum and looking at most of the designs i see a "dragster" type car , vey long + rear wing being used or thought about...
the length i can understand but the rear wing i can not , on a dragster and on fast cars the rear wing is used for 2 purposes to push the rear wheels into the ground to aid with traction and manouverability and since we need neither why have one?

i believe a front wing i.e a splitter has its benefits and also bringing the nozzle as close to ground level and infront of the rear wheels has its benefits in stability and helping with steering, and would be interested in other ppls opinions in this

andi



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thampson
Current WRA2 Dragster Speed & Distance Record Holder
Current WRA2 Dragster Speed & Distance Record Holder
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Re: car design , a little confused.....

Post by thampson » Tue Oct 13, 2009 9:37 am

Hi andicirk,

Thanks for the post ..... Im always happy to see discussion on water rocket cars .. so I will try to explain the history of the rear wing appearing on our cars.

You are right that a rear wings job is to create downforce over the rear wheels .. and this indeed is what it is being used for on our car. When we had a 4.5L - 2 bottle WRC we had twin rear vertical fins to help provide stability, they were quite big for the size of the car, much bigger than what would appear on a vertical WR of that size. They did work, but in high thrust conditions the car would be steered by the thrust and the rear fins were not able to compensate to straighten the car.

When we went up to 6.75L - 3 bottle WRC the thrust steer condition was even worse. We used the same vertical fins and they would stop the car from spinning but were not able to straighten the car, we have a few good videos of the car doing 90 degree turns at 130psi straight into the gutter .... sigh :( ...

When we looked at the problem it became obvious we would need huge wings to get enough force to straighten the car. With a vertical rocket if you balance the rocket at the CG and tap the fins with a small force with your finger, it doesnt take much to move it. When the rocket is flying if you have your CG and CP correct as soon as it moves away from vertical the airstream imparts a force on the fins, a small amount of which is a horizontal force vector (similar to tapping it above) and the rocket goes back to vertical.

Now in the rocket car scenario try tapping the rear of the car with the wheels on the ground .. it takes a lot more force to move it, as now you are fighting the friction of the rear tires on the road. Even if your CG and CP are good, its still going to take a lot more force to straighten the car this way. This will work if you can go fast enough to create a big enough force. But for the speeds we are doing at the moment, we cant generate enough force. It might be worthwhile making much bigger fins to see how much force we can generate though :)

So we flipped the problem on its head and said .. well if the rocket wants to thrust steer, lets make it harder for it to do it .. so we added a rear wing to generate downforce over the rear tires to make it require more force to thrust steer the rear of the car. This isnt a perfect solution by any means as the downforce is lowest when the thrust is highest and visa versa ... so there is more engineering to try to solve that problem ie: evening out the thrust using reduced nozzles is one way we are trying.

The design of where the nozzle should be located is an interesting one .. I too have seen suggestions for streamliner cars used at Bonneyville etc that the nozzle should be in front of the rear wheels, although they didnt elaborate, i dont know if this was done to keep the majority of the weight of the rocket/jet engine forward on the car to move the CG as far forward as possible, or to minimize the thrust steering where the torque force being generated is minimized by minimizing the distance from the nozzle to the rear axle.

For me its very interesting the engineering challenges that are thrown up as we start to push the envelope a bit, initially I thought that all the vertical WR principals would be exactly the same when I strapped a WR to 4 wheels .. put some fins on, get the CG and CP right and it would go as straight as a dart ... :) ... but the more we build, test and develop the car .. the more interesting it gets and the more we are learning about WRC's.

Just to throw a curve ball into the mix .. I have been studying the Thrust SSC car which has the WLSR and it has 2 front fixed wheels that dont steer and 2 almost inline but slightly offset rear wheels that do steer .. :shock: ... which are well behind the nozzles (well afterburners) of the jet engines ....

Id be interested in hearing more about your thoughts on a front splitter and bringing the nozzle closer to the ground .... and the effects they might have on a WRC

-todd-


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Hornsby Heights Water Rocket Space Agency
http://wrocket.hampson.net.au

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Jelo
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Re: car design , a little confused.....

Post by Jelo » Tue Oct 13, 2009 12:45 pm

i use a rear wing on my car as the bottles are amied slightly downwards to get rid of more water, when my car is at full speed, the car would just flip in the air all of a sudden... a close look at the video footage showed the two back wheels lifting off the ground, just before the crash, when this happens, only two front wheels are in contact, the are each independent (not attached together) and this results in the car been able to go anywere. so i tried to fix it with a rear wing, my problem was solved, now my cars go a good distance at high speeds in a perfect line :D


Anto'

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johnwraps
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Re: car design , a little confused.....

Post by johnwraps » Wed Jul 21, 2010 12:49 pm

It is a good thing to know about that car design. Though, it could help a bit if you could post some images of those car designs.



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thampson
Current WRA2 Dragster Speed & Distance Record Holder
Current WRA2 Dragster Speed & Distance Record Holder
Posts: 232
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Re: car design , a little confused.....

Post by thampson » Thu Jul 22, 2010 11:57 am

John,

here is a version of a rear wing water rocket car that we built.
http://wrocket.hampson.net.au/?p=1030

Also here on how I built it
http://wrocket.hampson.net.au/?p=920

hope that helps

-todd-


HHWRSA
Hornsby Heights Water Rocket Space Agency
http://wrocket.hampson.net.au

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