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 ..
... 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-