Propulsion

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Weights:

  • Motor + Fan + Shroud = 0.932 lbs
  • Motor = 0.804 lbs
  • Speed Controller = 0.322 lbs
  • 2 cell battery = 0.552 lbs
  • 3 cell battery = 0.814 lbs
  • Shroud/Fan mount = 0.060 lbs
  • Fan = 0.044 lbs
  • Motor Cover/Shroud = 0.008 lbs

Notices:

Propulsion group members should email David by 10:00PM each sunday with a record of man hours worked as well as what those hours were spent doing since the previous sunday


Highest Priority:

  1. THURS (Mar 6): More Bigbird shielding static testing ……. meet in the senior design room around 12ish to start charging batteries. Even if you can't make it at 12 you should show up at some point because we need as many people as we can get in order to be efficient.
  2. MON (Mar 10): PDR slides must be completed ……. tasks to be given out thurs/fri to be completed and sent to David Leopold by Sunday night.

Projected Budget:

Unit Cost Quantity Subtotal
Wemotech Midi fan with Aluminum fan $67.95 2 $135
Kontronik Fan 600-15 Brushless $200 2 $400
A high capacity battery $300 1 $300
ESC $200 1 $200
Noise Generator $200
Misc./Const. $500
TOTAL $1735

*TA Note: You should also consider the cost of cell balancers for LiPo batteries (ensure safe charging, although last year's balancer may work fine) and possibly a new battery charger. The one used the past two years is somewhat hard to use and since dealing with LiPo's is very dangerous, it might pay to get a new one with new (safer) technology and a better user interface.


Current Projects:

  • David Terry: acoustics research. what spectrum to look at in static tests (what

frequencies, dbs). how is "noise" defined by FAA regulations?

  • McCracken: engine availability/research/COST (project management wants a cost

projection by the end of the week, FEB 15)

  • Jolene: make sure LabView sound spectrum acquisition is ready. Also start

looking into nacelles

  • Zack: design/build materials & process for static acoustics test. help plan

acoustics test

  • David: plan static acoustics test for Bigbird
  • Stephen: push/pressure Eli to figure out if we can test turbo jet statically
  • Mike Dinh: engine availability/research/COST (project management wants a cost

projection by the end of the week, FEB 15)

  • Ryan: design/build materials for static acoustics test. help plan acoustics

test


LiPo Charging Procedure

1) Connect charger to car battery.
2) Once it starts, push down both + and - and release the - first and then the +
3) Press + button to scroll to the battery type (LiPo), and then press - button to select it
4) In cell select screen, press + until 10 cells are selected, and then press - button
5) In Max Charge Quantity screen, select (in mAh) the value listed on the battery
6) Select the current for charge rate. The max current should be the the capacity of the battery (in mAh) divided by 10 (i.e. a 7500mAh should be charged at a max 7.5A). In this screen, both + and - buttons can be used to select the charge rate. Once it is selected, the "#GO#" will appear on screen.
7) Connect batteries with the balancers.
8) Connect negative to negative, then positive to positive. Tape off any connecters not being used.


Lessons Learned/Hints from 2006/2007

An electric propulsion system consists of a Speed Controller, Motor and battery. Motors can either be "brushless" or "brushed" Brushed motors are cheaper, along with their speed controllers. However, Brushless motors are more efficient and capable of putting out MUCH more power. Honestly, don't even bother looking at brushed motor systems. Motors are rated at a certain max sustainable amp draw. Without getting into too much detail, motors are usually defined by how many "turns" they have. This is nothing more than how many complete revolutions the internal wiring makes around the motor itself. Lower number of turns equals higher RPM and less torque (good for EDF), higher turns equals more torque and less RPM (good for swinging large props).

Speed controllers are also divided into brushless and brushed. In order to power a brushless motor a 3 phase current must be applied, with variable timing to change RPM. All this boils down to an expensive microchip based speed controller. Speed Controllers are rated in Amps at up to a certain voltage. A certain motor/prop combo will draw a max sustained amperage at a particular voltage. Make SURE that your speed controller is rated for more than this value by a healthy safety factor. The last thing you want is a fried speed controller because you cut corners on how much the speed controller was rated for. The 2005 system draws up to 120 amps at 42 volts, and the speed controller was rated for 160 amps at 60 volts I believe… As mentioned before, some speed controllers come with a BEC. Most high power models do not however, as it is assumed that the modeler will go with a separate battery for radio operation for safety and reliability reasons.

As a general rule, going up in voltage will lower your amp draw (you know, that whole Power equation…) High voltage systems are also as a general rule more efficient.

Finally, there are the batteries. These come in three main flavors: NiCd (Nickel Cadmium), NiMh (Nickel Metal Hydride) and LiPo (lithium polymer). Suffice it to say that NiCd are outdated as a propulsion battery source. Their energy density (power over weight) is terrible.

NiMh is much better as far as energy density goes when compared to NiCd batteries. They are rated at 1.2V per cell. Different cells will have different capacity as well as max sustainable current ratings. NiMh batteries will lose their charge over time. This is important if they are used as your propulsion battery, as your max thrust will be much less initially with batteries that have been left untouched for days. As a general rule, charge NiMh batteries right before you fly, and use them hot off the charger.

LiPo batteries are by far the best… period. Their energy density is simply astounding. However, so is their danger potential. Despite all the scary sounding warnings to follow, I can tell you right now that with the type of aircraft you are building, LiPos WILL be required if you want to get off the ground with an electric aircraft and stay under your weight projections while maintaining anything approaching useable flight times. A mishandled LiPo (incorrectly charged or in a very bad crash) will vent toxic fumes and ignite in a self oxidizing chemical fire. Standard A/B/C extinguishers will not put this fire out. As long as they are properly charged, and their outer aluminum casing is not punctured (they are under vacuum) you will be fine. Proper protection within the aircraft to avoid crash damage would be prudent. Each LiPo cell is rated at 3.7 volts nominal. LiPo cells are as a rule not capable of as high of current rates as NiCd or NiMh. This means that you must put multiple cells in parallel. However, this technology is rapidly advancing, and cells with higher "C" ratings come out regularly. C ratings are nothing more than how much current the cell can put out in relationship to its capacity: For example, a LiPo cell with 2,000 mAh capacity with a 5C rating can put out 10 amps continuous. Cells with ratings of 20C are now common I believe. Do NOT over-current a LiPo, this can cause the aforementioned venting/fire. This means you must be VERY careful not to short a LiPo battery pack (a short = super high current). Damaged LiPo cells will swell up and inflate like a ballon, they will not always vent and/or catch fire. If you do swell a cell just remove it to an area where no flammable objects are nearby… leave the cell alone for at least an hour and if it hasn't vented by then it shouldn't at all. Do not attempt to use or charge a puffed LiPo.

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