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

[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).

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