Physics at Penn State Harrisburg

Steve Carabello

2004.11.9: Eighth meeting: Little construction, but plenty of planning for further designs and controls.

Participants:
• Doug Hains
• Thomas Haley
• David Reynolds
• Kelly Twomey

The balloon had become far more deflated in one week than prior weeks.
• Possible reasons: there was no tape covering the hole, which could allow more helium to escape; the seams may not be as solid as they once were; the store room is undergoing far more significant temperature changes that previously, causing expansion and contraction of the gas.
• In its deflated state, it was no longer able to support its own weight plus that of the wires (though with just a bit of support of the wires, it was able to float).
• After re-inflating, the balloon plus structure had a net lift of about 2.5 g, with the rotors off.

Other items were tried:
• 3 button-cell batteries (1.5 V), of the type used in small laser pointers: total mass (including tape holding them together): 5.98 g
• Note: these batteries were not able to drive the motors at anywhere near a good speed, nor were they able to last for a long enough time. We will need to investigate how much resistance is in the motors, and how much is in the long connecting wires.
• Additional structure, to support the batteries and allow for controls, will be needed. We may use straws, coffee stirrers, or balsa wood. The density of a certain low-density piece of balsa wood was about 121 kg/m³.
• We considered using permanent magnets and electromagnets to provide the force to push the hanging mass around, for control. Electromagnets would likely use a ferrite core; this material has a density of about 1700 kg/³.
• The mass of one small cylindrical neodymium magnet was about 1.12 g.

• After further consideration, it seems likely that a mechanism using a gear, servo, and rubber band will be simpler to construct and lighter in weight than any electromagnetic device we could build. Details are still uncertain at this point, but that will be our plan.

Items to look for before the next meeting:
• A second large balloon.
• Any remote-control devices that we could scavenge.
• Straws and coffee stirrers to consider for additional structure.

2004.11.2: Seventh meeting: connected everything together, and had a first flight.

Participants:
• Doug Hains
• Thomas Haley
• David Reynolds

First, we made measurements of individual components:
• Lift of the balloon after 3 weeks: 53.8 g
• After refilling the balloon with Helium, its new lift was about 56.2 g
• Lift of the same motors, but with new rotors: about 7 g each. Note: these rotors have been coated with a thin blue shrink-wrap material
• The balsa wood support structure was damaged and repaired since the last time it was weighed. Its new mass is: 13.42 g.
• Mass per length of "invisible tape" (Office Max): 1.4 g/m (calculated from a mass of 0.24 g for a length that was 17.2 cm long)

Next, we used double-sided tape to attach the motors to the support structure, then used invisible tape to attach that assembly to the underside of the balloon itself:
• Mass of the support plus motors: about 55 g
• After everything was connected, the assembly was able to rise, very slowly (notice: lift of about 56 g, needing to support about 55 g). By hanging a small mass from the support structure, it would sink slowly -- even a tiny force was enough to support it.
  

Finally, we connected the motors to two D batteries in series.
• We found a significant amount of rattling of the structure: one of the motors does not operate as smoothly as the other, even though both were of the same type (Radio shack item number 273-0258, 1.5-3 VDC motor).
• When the switch is first closed, the apparatus is able to lift 8 grams (plus the connecting wires). However, the current from the batteries decreases to a steadier, lower level rather quickly, at which time it is able to lift 7 grams in addition to the structure. Note: there is currently more copper wire than we will need; this is to ensure that there will be enough once the final decisions are made regarding the hanging mass.
• When the motors are turned off, the apparatus plus "payload" drops more quickly than we would prefer, though still slowly enough that damage is unlikely.
• Even with uneven lift by the rotors, the device remains horizontal, and seems reasonably stable. Turning on only one motor does cause the blimp to tilt significantly, but it resumes its stable level orientation soon after turning that one motor off, or after turning both on.

2004.10.19: Sixth meeting: repeated the lift measurement after 2 weeks in storage; tested a variety of rotor modifications.

Participants:
• Thomas Haley
• David Reynolds
• Kelly Twomey

Masses, etc.:
• "Mass" of the balloon and 200g weight after 2 weeks: .144 kg
• -- so the lift is now about 56.4 g: we lose about 1 g of lift per week of storage. (Not bad: I'm surprised the loss is this low.)
• Mass of Radio Shack Motor: 17.5g

 
Partially deflated balloon

"Small rotor"
• length of one arm= 5cm
• m= 1.65 g (note: was made with a heavier wire than larger rotor)
• lift: 4g (when motor/rotor close to scale surface)
• lift when raised up above scale: 6-7 g
• Note: raising the motor/rotor causes less blowback onto the scale, making the lift measurement more realistic (so long as our final apparatus does not blow significantly onto the balloon surface).

Trimming the small rotor: making an angled cut from each outer edge
• length of one arm= 5cm
• lift= 8g

"Big rotor"
• length of one arm= 9cm
• m= 1.62 g
• lift= 3g (note: significant rattling of the rotor was observed while it spun; this measurement was while the motor was close to the scale -- all later measurements were with the motor/rotor well above the scale)

Trimming the big rotor: 1 cm cut from each end
• length of one arm= 8cm
• m= 1.45 g
• lift= 8-9g

Trimming the big rotor: 2 cm cut from each end
• length of one arm= 7cm
• m= 1.38 g
• lift= 8g

Trimming the big rotor: 2 cm cut from each end, plus an angled cut near the center hub
• length of one arm= 7cm
• lift= 8g

Trimming the big rotor: 2 cm cut from each end, plus angled cuts near centers and both ends
• length of one arm= 7cm
• lift= 8-9g

2004.10.12: Fifth meeting: measured the lift of a large mylar balloon after it was in storage for one week; planned construction.

Participants:
• Doug Hains
• Thomas Haley
• David Reynolds
• Kelly Twomey
• Daryl Wiest

Balloon-related:
• Balloon net lift today: 57.3 g
• Density of air: 1.20 kg/m³*
• Density of helium: 0.166 kg/m³*
• Based on lift calculations: initial volume of balloon=0.0952 m³
• -- volume of balloon today: 0.0942 m³
• Weight of balloon skin itself: 40.13 g

Motors:
• Motor from airplane (without any extra material): 9.29 g
• CD-ROM motor (including gears and other material): 19.57 g
• Motor that overheated on 9/21: 25.79 g

Miscellaneous:
• Balsa wood support structure: 13.12 g
• Thin enamel-coated wire: 0.42 g/m linear density

2004.10.5: Fourth meeting: predicted, then measured the lift due to a large mylar balloon; disassembled 2 toy airplanes; discussed ideas about methods of mounting the motors to the balloon.

Participants:
• Doug Hains
• Thomas Haley
• David Reynolds
• Kelly Twomey
• Daryl Wiest

Airplane components:
• Motor w/ battery: 23.86 g
• Propeller: 0.81g
• Total: 24.67g

Balloon:
• Diameter: 88 +- 3 cm
• Thickness: 33 +- 3 cm
• Lift calculated: 62g
• Lift actual: 58.something ~ less than 60 est.

Support Structure
• Styrofoam slab: 58g (estimated that we will need only a small fraction of it)

Batteries
• AA battery: 23.0g
• AAA battery: 11.0g
• Laser Pointer battery: 2.0g

2004.09.28: Third meeting: Made a few measurements and calculations of balloons and mylar.

Participants:
• David Reynolds
• Kelly Twomey

2004.09.21: Second meeting: Made assorted measurements and calculations, to get a rough idea of what we're looking for.

Participants:
• Daniel Carfagno
• David Reynolds
• Kelly Twomey
• Thomas Haley
• Doug Hains
• Daryl Wiest

Estimating the lift from a typical party balloon:
• Guess that a balloon has a volume similar to a cube 20 cm on a side. So: V is about 0.008 m³, or (since it's just a rough ballpark estimate) about 0.01 m³
• By looking at buoyant forces, we can find that the amount of mass that a certain volume of gas can lift is equal to: [(density of air) - (density of les dense gas, such as Helium)]*Volume
• For Helium and this volume, that works out to be about 10 g; for Hydrogen, about 11 g.
• If we suppose that the skin of the balloon itself has a mass in the ballpark of 5 g, then the "payload" could be about 5 g per individual party balloon.
• We have the option of using one bigger balloon (purchased or made ourselves), or using several smaller ones. Advantages for one bigger one: there would be less skin material per amount of gas enclosed, giving more "payload" for a given amount of Helium. Advantages for using several smaller balloons: easier to adjust, easier to find materials.

Estimating the spin speed we would need for the rotor:
• Taking the hand-spun rotor toy, we can come up with a rough idea. Suppose, in our hands, we apply something close to a constant angular acceleration. Then, we can use our equations and some measurements to get reasonably close.
• (omega)_i = 0
  (omega)_f = ?
  (delta)t=0.25 s (estimated)
  (delta)(theta) = s/r where s is how far it moved against one surface, measured to be around 10 cm, and r is the radius: we measured the diameter of the shaft to be about 0.32 cm, so r=0.16 cm.
• (delta)(theta)=1/2[(omega)_i + (omega_f)]*(delta)t
• Running through this calculation gives about 500 rad/s, or about 5000 rev/min. Based on the estimates we made, we guess this is likely an overestimate, but we would be surprised if it were 10 times too high or too low. So, we have a rough idea.

Measuring the angular speed of motors, using the rotary motion sensor:
• Label the wheel of the motor as 1, and the wheel of the sensor as 2.
• Since there's no slipping, then at the edges v₁=v₂ so r₁*(omega)₁=r₂*(omega)₂
• This leads to (omega)₁=(omega)₂*(r₂/r₁)
• We found we could easily get above 1000 rpm with motors, but the rotary motion sensor could not allow us to find speeds above about 1500 rpm for the motors. Still, from the measurements we could tell we were in the right ballpark, even with the few motors we had on hand.

Measuring the lift from one rotor/motor combination:
• The simple toy rotor happened to fit reasonably well on one (but only one) of the sample motors we had on hand.
• We hooked this up to the power supply, and placed it (clamped in) on a digital balance, and set it up to push down, rather than lifting up, in order to ensure that the rotor would remain attached to the motor.
• Because the rotor was completely symmetric, we can expect that the push down would be equal to the lift up if we reversed the spin direction by reversing the polarity of the wires going to the motor.
• When the motor was going at maximum speed, we got a reading of 47 g; the mass of the motor plus rotor alone was 30 g, leading to a force available capable of lifting about 17 grams in addition to its own mass.
• Note: at this speed, this particular motor was excessively heated, causing the plastic to begin to melt, and causing an obvious smell of burning plastic. So, it appears that this particular motor would likely not be the best one to use for this purpose. However, we know that, at the very least, it is not hard to find a motor/rotor combination capable of supporting its own weight.

I've ordered a pair of inexpensive remote-controlled airplaines from a catalog; hopefully they will arrive before the next meeting next week. We plan to use their motors and batteries, and perhaps some of their materials can be used to build the support structure for our machine.

2004.09.14: First meeting: discussed the idea in general, and listed some concepts that will be worth credit, as listed on the assignments page.

Participants:
• David Reynolds
• Chris Kramm
• Kelly Twomey
• Thomas Haley
• Doug Hains
• Daryl Wiest

2004.08.13: This page was created.