Power

The power comes from two custom made elastic bands weighing 5g, each cut from Latex rubber sheet (which can extend up to 8 times its rest length and store loads of energy in the process).

 

They are too strong to be tensioned by hand and so a lever system is used to stretch them to 4 times their initial length with a force of 28 kgf (4.5 stone).

 

Of course, the more tension in the rubber bands, the stronger the framework needs to be to hold them stretched.

 

About 43% of the energy in the stretched rubber bands is used in achieving its jump, the rest being wasted in the elastic bands getting hot, unwanted flexing, frictional losses, aerodynamic drag etc.

 

Weight

About 43% of The competition frog was made from available bits of wood from the garage, and initially weighed 135g. After dismantling it and weighing each part, the weight was reduced by a third to 101g just by attention to detail such as changing steel bolts to plastic.

 

Legs

Initial trials showed the wooden legs bent a lot and so absorbed much of the launch energy. Development then consisted of making these legs as rigid (but as light) as possible.

The competition legs are made of 6mm carbon fibre tube from a model shop (one snapped the day before the competition).

 

One leg needs a foot sticking out sideways to hold it vertical during the launch.

Various materials were tried from aluminium brass to cocktail sticks, but again the deformation sapped considerable energy from the launch.

Also the foot is very vulnerable to damage during the crash landing, and so needed to be tough and fracture resistant.

For the competition this was a length of bamboo knitting needle.

 

Bearings

In order to minimize bending forces on the bearings, a pair of rubber bands were used one either side of the frame to balance out the load trying to twist the bearings, and the bands ends were looped through an aluminium carrier to hold them square and true.

 

 

The wooden framework was made from 'P' section wooden beading, which has a fairly large flat area to act as a bearing surface, but also has the head of the 'P' to give strength.

 

Low friction was achieved by using nylon bolts, and ptfe washers which were cut from a 'non-stick' oven lining sheet.

 

 

Each bolt was tightened until the joint locked, and then backed off ¼ turn to give a small working clearance and hence give free movement, but minimum slop.

Launch Pad

As the frog launches, the legs move together, scraping across the launch pad. Potentially this can loose a lot of energy, so the same ptfe oven liner was used as the launch surface, together with some spherical plastic beads on the foot ends to give minimum contact area.

 

Drag

The fact that the frog changes from 'short and fat' to 'tall and wide' during the launch obviously helps the aerodynamics. The eyes and webbed feet were needed for to get the frog looking 'froglike' so they were made functional as well by mounting them at an angle to impart spin to the frog during flight to increase stability.

 

Triggering

Getting a good clean release is key, and I used 2 loops of fine nylon fishing line wrapped round the tensioned frog to hold it 'short and fat' once the rubber bands had been tensioned.

 

To launch it I melted the fishing line, in the early days with a small butane heater triggered remotely by a pair of bicycle calipers, and then in the interests of reliability, I changed to using a model rocket fuse…. basically a short length of wire which gets red hot for a few seconds when you put a fresh 9v battery on it.

 

Many contestants suffered from unreliable launch mechanisms… there is little more embarrassing than having the cameras running, music and drum rolls, 'Prepare to compete' over the speakers, a deathly hush, a confident button push followed by absolutely nothing… apart from a slow handclap!

 

Trajectory

Originally the competition brief was to jump over a bar, which would have required substantial accuracy, so a variable angle launch pad was made, and the angled eyes and webbed feet were fitted to produce a stabilizing 'spin'.

 

'Frogsbury' Flop

In order to clear the bar most efficiently the frog should go over the bar laid on its side, and the addition of enlarged webbed feet vanes, ensured that the centre of drag was behind the centre of gravity so that it achieved this 'frogsbury flop' over the bar.

 

As it turned out, the competition was done with no bar, so accuracy of trajectory didn't matter.

 

However, causing the wide mouthed frog to be horizontal at the top of its flight gained about ¼ m extra jump, as it was the lowest point on the leaper was measured.