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Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Mon Jul 12, 2010 12:35 am
by thampson
Even though it was a cold and slightly rainy winters day here, we were very keen to try out the Green Hornet with its new configuration, so we had a lunch day on Sunday afternoon.

The new rear end fins and low traction wheels were designed to make the water rocket car act more like a rocket for stability rather than a car, ie: The rear fins straighten the back of the rocket car. This means the rear fins need to overcome the friction of the rear wheels to effectively straighten the rear of the car.

Sounds good in theory .. time to try it out.
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Here is the video of the launch day. We managed only 2 launches due to the rain. We also had a father and his son on motorbikes stop to watch our second launch, that’s the noise you can hear in the background.

[youtube][/youtube]


Results
Well we had partial success in that the launches were pretty straight, especially the first one, until there was any r/c steering input, as soon as the front steered, the rear of the car had no grip to follow and just fishtailed or did a 180 degree turn. Obvious really after thinking about it, just like doing a handbrake turn in a normal car, …. On both runs we hit the gutter, we haven’t done this for a long time, even the r/c steering couldn’t control the cars direction when fishtailing.

Positives from the tests were the rear fins look to be working but we will need to put rubber back on the rear wheels, so we will see if there is enough force generated by the fins to straighten the car under thrust with the old wheels on.

The launcher updates worked great. The lightened chassis did work well as the car definitely accelerated quicker than the last revision.

We will keep working on it :)

-todd-

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Mon Jul 12, 2010 5:14 am
by RaZias
I really started to look more like a rocket car :D nice accelaration.

Don´t know if having rc-stearing in the four weels would be even better.
I just can´t wait to see one day a 300 psi launch.

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Mon Jul 12, 2010 10:06 am
by WRA2
Hi Todd,

Nice work.

Here are a couple of suggestions, Put the rubber tires on all 4 wheels. If you only put then on the rear the fronts will not have enough traction to steer the car and it will just keep going straight when you turn the wheels.

Apply gear reduction to your steering assembly so that your steering will be less sudden (like an old fashioned "fine tune"). As your car goes faster you will need smaller and smaller movement of the wheels to cause a change of direction. This will make it easier to control.

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Mon Jul 12, 2010 11:16 am
by thampson
Thanks for the suggestions LIsa.

Yes we will go back to rubber on all 4 wheels again ... was worth a try with the low grip rear wheels though ;)

The r/c has this gear reduction feature built into it, to reduce the amount of steering done by the servo based on the amount of input on the controller. It was already set to minimize response, but we will see if we can tweak it some more to produce less steering response with the higher speeds we are now getting.

-todd-

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Wed Jul 14, 2010 12:11 pm
by RaZias
If you want to reduce steering I was thinking in increasing the distance between the right front wheel and the left front wheel.

The principle is this, when you ride a bike you put your hands on the bar to guide it.
If the bar is too short a slight turn of your hands can steer too much your wheel.

In the contrary, if the bar is large your will have a better control of your the steer.

Obviously if you increase the width between the front wheels it would be better to increase also the width between the rear wheels.

In the overall you will get a better steering.

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Wed Jul 14, 2010 1:56 pm
by thampson
Thanks for the suggestion Razias.

The wheel hubs pivot where they connect to the chassis. Both wheel hubs are connected via a steering arm which in turn is connected to the steering servo. To minimize steering response the easiest way is just to move the servo less. I could possibly put the steering arm closer to the wheel hub pivot point, but this requires more force from the servo to then move the wheel hub and in turn the wheels.

I think the video is a little misleading, it looks like ive put on way too much front wheel steering, when in fact it has only taken a very small amount of front steering to get the back wheels to move a long way out of alignment due to the low grip. I think the rubber on the rear wheels will fix this problem again. We didnt have any problems like this in the last few versions of the car which had the 4 rubber wheels and the same front steering setup.

I will post updates for the next version with rear rubber wheels

cya
-todd-

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Sat Jul 17, 2010 10:02 pm
by U.S. Water Rockets1
thampson wrote:Thanks for the suggestion Razias.

The wheel hubs pivot where they connect to the chassis. Both wheel hubs are connected via a steering arm which in turn is connected to the steering servo. To minimize steering response the easiest way is just to move the servo less. I could possibly put the steering arm closer to the wheel hub pivot point, but this requires more force from the servo to then move the wheel hub and in turn the wheels.

I think the video is a little misleading, it looks like ive put on way too much front wheel steering, when in fact it has only taken a very small amount of front steering to get the back wheels to move a long way out of alignment due to the low grip. I think the rubber on the rear wheels will fix this problem again. We didnt have any problems like this in the last few versions of the car which had the 4 rubber wheels and the same front steering setup.

I will post updates for the next version with rear rubber wheels

cya
-todd-

Have you tried making custom lightweight wheels? If you made a thin wheel with a good bearing in the hub and just a thin rubber piece around the rim (a rubber band or a slice of an inner tube) you could reduce a lot of weight and reduce the rotational mass at the same time.

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Thu Jul 22, 2010 12:00 pm
by thampson
Thanks for the suggestion. US Water rockets. :)

We do have a super lightweight version on the drawing boards which Im sure we will build at some stage, always on the lookout for lightweight materials :)

Too many ideas .. not enough time to build them all :)

-todd-

Re: Water Rocket Car MkXI - The Green Hornet Launch Day !

Posted: Sat Jul 24, 2010 10:28 pm
by U.S. Water Rockets1
thampson wrote:Thanks for the suggestion. US Water rockets. :)

We do have a super lightweight version on the drawing boards which Im sure we will build at some stage, always on the lookout for lightweight materials :)

Too many ideas .. not enough time to build them all :)

-todd-
The trick is to set priorities and make sure you stick to a plan. Otherwise you will never get anything done.

Keep us all posted on your experiments.