Help with Final Project: Build an Object That Jumps Up Mechanically

[xrotaryguy]

I would guess that a sideways winding would wound along a boom with a tail fin on it. That way, one sructure would be used for two purposes. That'd reduce the mass of the machine. I don't know that the verticle height gained by mass saved would be greater than the height lost by mechanical complexity and frictional losses though.

 

[jonlai9]

Thanks for all those ideas xrotaryguy, unfortunately a launcher is not possible for this project as the entire apparatus must leave the ground.

The bowl-cage structure has most of the bowl sections removed so that you are left with only a frame resembling a bowl. I would think that the idea is all about stability. Placing the centre of thrust and center of gravity close together means that large torques will not be developed and therefore give it vertical stability (that is the structure remains upright - something that rockets are notorious for, which makes them dangerous[you've got a hard projectile with a lot of power endangering objects at ground level]). That plus the way the air flows over the bowl will also increase its vertical stability. The structure do tend to slip sideways if any wind are present, which will decrease the vertical height obtained (it is now spending its time moving along a diagonal).

I do not understand why you are considering winding the elastic sideways, rather than lengthways for this project.

Oh, so it only uses the frame of a bowl. I think I understand what you mean, but by any means do you think you can do a quick image search online and see if a similar "toy" appears? I want to be on the same page in terms of knowing about how the "toy" looks like so we don't misunderstand each other.

About the winding of the elastics, I am not considering winding it sideways, I really want to wind it up lengthwise (ie. when you roll an elastic band around your finger), however it's just that your diagram showed the elastic winding up sideways. I'm just wondering if it was really meant to show to wind up sideways because that's the elastic force that we are used to, instead of the elastic force attained when it is twisted on its side.

 

[xrotaryguy]

]I am not considering winding it sideways, I really want to wind it up lengthwise (ie. when you roll an elastic band around your finger)

Oh, now that's an interesting idea. But, doesn't that mean the part of the rubber band that is on the outside of the coil will have considerably more tension on it than the windings at the center of the coil? It seems to be like you'd get a pretty good initial torsional force, but after that the force would fall off very quickly.

I think that the torque produced by a rubber band that is twisted would have a more uniform rate of derease. Perhaps that's why rubber band powered air planes wind the rubber band that way.

 

[jonlai9]

Oh, now that's an interesting idea. But, doesn't that mean the part of the rubber band that is on the outside of the coil will have considerably more tension on it than the windings at the center of the coil? It seems to be like you'd get a pretty good initial torsional force, but after that the force would fall off very quickly.

I think that the torque produced by a rubber band that is twisted would have a more uniform rate of derease. Perhaps that's why rubber band powered air planes wind the rubber band that way.

I see, thanks for explaining that. I do understand why one would wind it up sideways as if you were twisting it now, even though it would heavily decrease the release elastic force.

But would regular rubber bands be sufficient if I followed suit and wind it up sideways? I just did an experiment with the rubber band hanging from my hand spinning a pencil and the force was not very strong, at least not strong enough so that it could possible lift off the surface and go a feet or so. Are there certain types of rubber bands that would be more effective in this situation or should I try alternatives like a piece of malleable metal to use to wind it up?

 

[xrotaryguy]

Are we sure that a rubber band isn't a spring? I guess the instructor just doesn't want you to load a spring and fling something up in the air with it. Any way I think that twisting the rubber band is the best way to go. You could just buy a bunch of rubber band powered air planes, and use the parts to make your helicopter. You'd definitely have enough rubber bands to get the thing pretty hgh!

Now then, back to rockets, which will go way higher Here's a site that has a pretty sinple setup.

http://www.antigravityresearch.com/

Here's a vid of their rocket car. You can see in one of the clips just how fast it goes. I'm sure you can get one to fly quite high

http://www.antigravityresearch.com/HTMLobj-872/Rocket_Car_384kbps.wmv

I'm sure that you can get a rocket up without a launch pad. I'm sure it'll take a few test fires, and some fine tuning to get the rocket to fly straight without a pad, but it is totally possible. I have launched large estes powered rockets that were only going 2 or 3 miles per hour when they cleared the launch pad, and they flew nice and straight Take a look at the arial photo they took with one of their rockets. It looks like it was taken from a good 1,000 feet. You can totally smoke your class mates with somehting like this.

 

[xrotaryguy]

Here's a bit on how to deploy a parachute. This stuff is pretty cool.

http://ourworld.compuserve.com/homepages/pagrosse/h2orrecsys1.htm

 

[jonlai9]

Are we sure that a rubber band isn't a spring? I guess the instructor just doesn't want you to load a spring and fling something up in the air with it. Any way I think that twisting the rubber band is the best way to go. You could just buy a bunch of rubber band powered air planes, and use the parts to make your helicopter. You'd definitely have enough rubber bands to get the thing pretty hgh!

Now then, back to rockets, which will go way higher Here's a site that has a pretty sinple setup.

http://www.antigravityresearch.com/

Here's a vid of their rocket car. You can see in one of the clips just how fast it goes. I'm sure you can get one to fly quite high

http://www.antigravityresearch.com/HTMLobj-872/Rocket_Car_384kbps.wmv

I'm sure that you can get a rocket up without a launch pad. I'm sure it'll take a few test fires, and some fine tuning to get the rocket to fly straight without a pad, but it is totally possible. I have launched large estes powered rockets that were only going 2 or 3 miles per hour when they cleared the launch pad, and they flew nice and straight Take a look at the arial photo they took with one of their rockets. It looks like it was taken from a good 1,000 feet. You can totally smoke your class mates with somehting like this.

So how exactly does this rocket car work?

For me to be able to incorporate something like this, I would need to be able to pump the thing up with pressure, and have it release the pressure only once I have told it to (press a button). Except for the fact that it uses water pressure, I'm unsure of how it exactly works, can you explain to me? Thanks.

 

[andrevdh]

Mechanics of the grasshopper's jump:

http://www.st-andrews.ac.uk/~wjh/jumping/legsprng.htm"

 

[xrotaryguy]

Well, it's really just a 2 litre bottle that is half full of compressed air, and half full of water. The open end of the bottle faced down, and it's cap has a hole drilled in it. The hole acts like a nozzle on a rocket motor. There have to be fins at the bottom of the rocket too.

Once the rocket has been partially filled with water, the cap is screwed on, and a hose from a bicycle pump is pressed into the hole in the cap. The bicycle pump has to have a special hose on it to pressurize the bottle through the hole. Evidently it is a good idea to have a good pump too since you're looking for about 90psi (like Lance Armstrong would use)instead of 40psi (like Pedro and Napoleon Dynamite would use).

With the bottle pressurized, a cork must be left in place. One of the links that i posted has a system for this. However, since you are not allowed to leave anything behind, you would need a different cork. I was thinking that a floating cork would be good. Then, once the rocket is sitting on the ground, you could reach under it, and push the cork into the rocket, and momentarily plug the hole by holding your finger over it. You'd want to let the cork float all the way to the top before pulling your finger away from the hole. You also want the rocket to be sitting still before pullingyour finger away. If the water is sloshing around or swirling, your rocket will not fly straight.

One of those links has a great write up on how to deploy the parachute. You've GOT to read it. It's pretty cool what these guys can do with a 2 litre bottle.

 

[jonlai9]

Mechanics of the grasshopper's jump:

http://www.st-andrews.ac.uk/~wjh/jumping/legsprng.htm"

Thanks, it was an interesting read, although it talks about motors using the idea of grasshopper's legs. I can see how that works, but if I were to not use any electronic devices, would that be possible? I imagine I would need some kind of spring that will give that action into the leg.

Would this type of implementation actually work and have it jump a good distance UPward?

With the bottle pressurized, a cork must be left in place. One of the links that i posted has a system for this. However, since you are not allowed to leave anything behind, you would need a different cork. I was thinking that a floating cork would be good. Then, once the rocket is sitting on the ground, you could reach under it, and push the cork into the rocket, and momentarily plug the hole by holding your finger over it. You'd want to let the cork float all the way to the top before pulling your finger away from the hole. You also want the rocket to be sitting still before pulling your finger away. If the water is sloshing around or swirling, your rocket will not fly straight.

One of those links has a great write up on how to deploy the parachute. You've GOT to read it. It's pretty cool what these guys can do with a 2 litre bottle.

Let me get this straight. It's a 2L bottle tipped upside down.

But what I don't really understand is how exactly will the water NOT spill out if there is a hole in the cap? From what I'm understanding and the limited knowledge I have of analyzing it, what's pushing the rocket sideways or forward is water pressure that came from the pressurized air. Wouldn't that mean that the water would be pressurized outward for it to move? If the water is not what's pushing it, what is supposed to be making it move?

 

[andrevdh]

When I originally read your thread this was my first line of thought - how do insects do it? But I think it will present one with quite a challenge to implement it. I do not think it is suitable for your particular project (too complicated), but it might give one ideas - maybe it can be adapted? There are basically only one motion necessary to load the mechanism (the red extensor muscle contracting) which could be an elastic that is wound up. This causes the spring to bend (a piece of flexible wood or fibreglass similar to that used for a bow). At some stage of the contraction of the spring the connecting point (insertion point) of the extensor muscle on the tibia is in such a position that it actually helps the tibia to swing back, which initiates the jump. Apparently one do need some tension on the flexor muscle to keep the tibia folded in during the contraction which one can implement with another elastic held under constant tention.

What worries me is also the question that you raised - how should the body be orientated for the jump to be succesfull? I would guess that a grasshopper raise on his front legs just before he jumps.

 

[jonlai9]

When I originally read your thread this was my first line of thought - how do insects do it? But I think it will present one with quite a challenge to implement it. I do not think it is suitable for your particular project (too complicated), but it might give one ideas - maybe it can be adapted? There are basically only one motion necessary to load the mechanism (the red extensor muscle contracting) which could be an elastic that is wound up. This causes the spring to bend (a piece of flexible wood or fibreglass similar to that used for a bow). At some stage of the contraction of the spring the connecting point (insertion point) of the extensor muscle on the tibia is in such a position that it actually helps the tibia to swing back, which initiates the jump. Apparently one do need some tension on the flexor muscle to keep the tibia folded in during the contraction which one can implement with another elastic held under constant tention.

What worries me is also the question that you raised - how should the body be orientated for the jump to be succesfull? I would guess that a grasshopper raise on his front legs just before he jumps.

Thanks, it does give me some ideas, but with my limited knowledge I'm unsure of how I can apply it here. It was a very interesting read, however, I never would have attempted to know how grasshoppers jump before :P

If I were to do a grasshopper for my project, I can imagine that it would be extremely mechanical, and I don't know how I can somehow press a switch or pull a string to make it jump, lol...

 

[jonlai9]

^
Anyone have any good ideas?

 

[tim_lou]

balloons? are they allowed?

 

[jonlai9]

balloons? are they allowed?

Nope. It would've been so easy otherwise.

 

[xrotaryguy]

Thanks, it was an interesting read, although it talks about motors using the idea of grasshopper's legs. I can see how that works, but if I were to not use any electronic devices, would that be possible? I imagine I would need some kind of spring that will give that action into the leg.

Would this type of implementation actually work and have it jump a good distance UPward?



Let me get this straight. It's a 2L bottle tipped upside down.

But what I don't really understand is how exactly will the water NOT spill out if there is a hole in the cap? From what I'm understanding and the limited knowledge I have of analyzing it, what's pushing the rocket sideways or forward is water pressure that came from the pressurized air. Wouldn't that mean that the water would be pressurized outward for it to move? If the water is not what's pushing it, what is supposed to be making it move?

I would imagine that you have started your project at this point since we are so close to the end of the semester. However, I might as well reply any how. The water would have to be kept in with a cork of sorts. I think that it would be best to have a cork that is less dense than water. That way, you could push the cork into the rocket with your finger to launch it. The cork would just float to the top of the water so as to keep from blocking the hole in the cap. No worries about your finger, you're only dealing with maybe 60 psi ish. It won't hurt :) It might tickle a little... and you might get kinda wet. Again, you'll need to have the rocket quite stable when it takes off. You don't want it to fly sideways.

 

[jonlai9]

I would imagine that you have started your project at this point since we are so close to the end of the semester. However, I might as well reply any how. The water would have to be kept in with a cork of sorts. I think that it would be best to have a cork that is less dense than water. That way, you could push the cork into the rocket with your finger to launch it. The cork would just float to the top of the water so as to keep from blocking the hole in the cap. No worries about your finger, you're only dealing with maybe 60 psi ish. It won't hurt :) It might tickle a little... and you might get kinda wet. Again, you'll need to have the rocket quite stable when it takes off. You don't want it to fly sideways.

I think I"m having trouble understanding still - the cork floats to the top (or rather, bottom of the bottle since it's tipped over), but that means the water comes out due to gravity, no?

 

[andrevdh]

The water rocket works by the same principle as a real rocket. Visualize someone standing in a boat and throwing balls to the rear. As the preson throws the balls he need to apply a push to them. According to Newton's third law the balls push back on the person in the opposite direction, that is the balls push the person (and thereby the boat) to the front of the boat.

In this case the pressurized air in the bottle provides the push and the water acts like the balls (with the inverted bottle the water is below the pressurized air and is pushed out of the opening of the bottle towards the bottom). It all happens very fast though and the water is expelled within the first two meters of its launch. After that the push is over and it becomes a projectile being launched vertically.

 

[jonlai9]

The water rocket works by the same principle as a real rocket. Visualize someone standing in a boat and throwing balls to the rear. As the preson throws the balls he need to apply a push to them. According to Newton's third law the balls push back on the person in the opposite direction, that is the balls push the person (and thereby the boat) to the front of the boat.

In this case the pressurized air in the bottle provides the push and the water acts like the balls (with the inverted bottle the water is below the pressurized air and is pushed out of the opening of the bottle towards the bottom). It all happens very fast though and the water is expelled within the first two meters of its launch. After that the push is over and it becomes a projectile being launched vertically.

Thanks for the explanation - it's not that I don't understand how it works, it's that I don't understand how NO WATER will spill out of it in the process of the air pressure pushing the water out, which is supposedly why the rocket is moving in the first place?

 

[andrevdh]

You could insert a tight fitting cork stopper with a bicycle valve into the opening. Then you need to experiment to find the maximum pressure you can pump it up to without letting the cork come out (wear a raincoat).

Tie the cork to the bottle with a piece of thread (nothing must be left behind ...). I would think that the thread need to be a bit longish so as not to disturb the launch too severely (75cm?).

For a launching pad you could model a three legged structure out of a piece of stiff wire to keep it upright and glue it to the bottle.

Now comes the messy part. Lubricate the bottle with vaseline (and your hands) and clap its sides quickly between your hands to over pressurize the bottle - lift-off!

 

[jonlai9]

Ok guys, I finally started building.

I used the propellor design modelling a helicopter. First, I tried it without the bottom long support that supposedly balanced the apparatus and this made BOTH the body and the propellor turn, in directions opposing each other. When I added it, there was still minor turns but it was not significant. I'm guessing the balance also adds rotational friction between the ground and the object?

But in either cases, I was unable to get the unit to fly up. It would simply rotate at fast speeds, but would not move up. My predictions are that the mass is too heavy - right now, I'm using a plastic cup as a body, with the propellor's span at around 8 inches. I was speculating that if I used paper or cardboard, the elastic force might be too great for it and break. Another friend suggested I use surgical tubing to replace elastics. Recommended?

Am I doing anything wrong here?

 

[andrevdh]

you need to increase the rotational inertia of the body so that it does not rotate while the propeller spins. What I suggested originally is several long arms (stiff wire) with vertical fins (dart fins) at the ends. If this do not work try replacing the fins with little washers or like to increase the rotational inertia. The fins will have a lot of drag but little mass thereby preventing the cup from rotating. You also would need the centre of mass to be beneath the propeller to stabilize the structure.

Is the cup inverted with the propeller resting on top of the bottom of the cup. How did you fix the elastic to the cup?

 

[jonlai9]

you need to increase the rotational inertia of the body so that it does not rotate while the propeller spins. What I suggested originally is several long arms (stiff wire) with vertical fins (dart fins) at the ends. If this do not work try replacing the fins with little washers or like to increase the rotational inertia. The fins will have a lot of drag but little mass thereby preventing the cup from rotating. You also would need the centre of mass to be beneath the propeller to stabilize the structure.

Is the cup inverted with the propeller resting on top of the bottom of the cup. How did you fix the elastic to the cup?

I used paperclips that connect the propellor and another paper clip to connect to the bottom of the cup - or top, since it's inverted.

I added two "blades" to the button to cause some friction, and now it does not spin, however, the propellor doesn't seem to be strong enough to bring the object up. I tried it with a plastic cup, then I went to McD's and grabbed a paper cup, no go either. Both would spin fast, but not enough to lift it into the air. Why?

 

[andrevdh]

I am not completely clear on your design, but I foresee two major problems. A propeller works by pushing the air back. This causes the air to push it (and whatever is connected to it) forward (or upwards in this case). I get the impression that the air that is pushed back by the propeller just runs into the cup thereby cancelling the lift obtained by the propeller. One also need to keep the friction between the propeller and the body to a minimum. Try and insert some types of slippery washers (teflon) in between the propeller and the (long thin tube) body.

What is this button you are talking about?

 

[jonlai9]

I am not completely clear on your design, but I foresee two major problems. A propeller works by pushing the air back. This causes the air to push it (and whatever is connected to it) forward (or upwards in this case). I get the impression that the air that is pushed back by the propeller just runs into the cup thereby cancelling the lift obtained by the propeller. One also need to keep the friction between the propeller and the body to a minimum. Try and insert some types of slippery washers (teflon) in between the propeller and the (long thin tube) body.

What is this button you are talking about?

Sorry, I meant bottom.

I totally understand what you are saying, but not knowing why it does not work. Could it be that the propellors are not pushing enough air?

I forgot to mention that I have already added (metal) washers between the propellor shaft and the cup.

One of my classmates mentioned that he was using a similar design but he used a box as a body, where the box is almost the width of the propellors (I'd say around 80%). Because this is competition-based, he won't show me, but he told me about it a little. However, if the "box", or body, was almost as big as the propellor's span, woudln't that not make sense since the air pushed down would not be able to push off the ground? However, he's succeeded in lifting it off the ground - a good 2-3 feet, which is not much, but I'm glad to hear that it actually works.

Now I just don't understand why mine doesn't fly UP.

 

[andrevdh]

Imagine yourself standing in a boat on a lake. Your feet are planted wide along the long axis of the boat. Tired of fishing all night you raise your arms and twist to the one side. In order to do that you need to push with your feet on the boat in the opposite direction. This means that the boat will rotate a bit in the opposite direction of the twist. Your rotation is therefore canceled by the opposite rotation of the boat. As far as your current construction is concerned what it means is that the propeller is not pushing nearly as hard on the air as is would have if the body did not rotate. The airflow is therefore weak. How to remedy this? You need to prevent the body from rotating. This is what I meant by increasing the rotational inertia of the body.

Rotational inertia is the property of the body to resist a change in the rotational motion of a body (just like Newton's first law for linear motion).

Rotational inertia depends on two factors - mass and the distance that the mass is from the axis of rotation.

Since we do not want to increase the mass the trick is to put a little bit of mass far from the axis. This is why I suggested to stick long thin pieces of wire in the bottom of the structure. These wires then need to extend to the sides. Little pieces of mass (clay?) on the ends will then increase the rotational inertia of the structure. As a bonus these wires can then be bent to support the structure upright. If the wires are long one might not even need mass pieces on its ends.

I get the impression that you are winding it up really tight (or a strong elastic). Maybe a weaker longer elastic will do - you will then need a longer structure. This will then give a more gracefull liftoff but more sustained (longer) lift as the weaker but longer elastic unwinds.

As far as the box goes. He might have mounted several propellers in series inside of the box, thereby creating a sort of turbo effect. That is the subsequent propellers can speed the air up even more. Why a box and not a pipe which is the more natural choice? I would guess that (lighter - paper and reed) constructing and mounting (flat surfaces) inside of of a box is just easier (think of a box kite). The propellers might be unleased by pulling a piece of wire running along the axis (and off centre) out. This will then release the propellers in sequence.

Another possibility is that he has succeeded in creating the required aerodynamic lift over the box with propellers mounted on its outside (box kites are known to be strong flyers).

Vectron Ultralite Flying Saucer:
http://myweb.ecomplanet.com/SKIP6840/mycustompage0116.htm"