UFO Introduction with Mission Objective and Desired Look

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By Tuan Vo, Project Manager

A vertical takeoff and landing (VTOL) aircraft is one that can ascend vertically into the air and land the same way.  There are many quadcopters and hexacopters that have achieved this feat. The use of electric ducted fans (EDF) to accomplish VTOL and hover flight is rare and have not been widely established.  This project will attempt to use EDF’s to achieve VTOL and hover flight in a fashionable manner.

Mission Objective:
Build a VTOL aircraft modeled after the UFO from movie The Day the Earth Stood Still.  The VTOL will utilize some of the major components provided by the customer and be controlled wirelessly to fly safely around a classroom.

The desired look: (taken from the movie)
1


Rough 3D model:
2


Exploded View:
3

Determining the Number of Fans

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By Juan Montano

Hello, this study will try to determine the number of fans that would be required for the UFO. The number of fans used in the UFO must meet the criteria of achieving lift off at 50% +/- ∆.

First, we determined the weight of each component currently provided to us by the customer:

Fan, ECS, Battery, Structure (Old), XBee, Tinyduino, Gyro

Component Weight
Fan 115g +/- 5
ECS 30g +/- 5
Battery 380g +/- 5
Structure

(top, mid,bottom)

 400g +/-5
Xbee 7g +/-5
Tinyduino 8g +/-5
Gyro 2g +/- 1

 

Now, we try to determine how the mass of the structure will change as we decrease the number of fans. First, we will assume a linear regression of the diameter size of the UFO as we decrease the number of fans. We will then assume that the correlation between percentage of volume and mass is in a 1:1 ratio.

 From a different study, we determined the difference in diameter between 4 & 6 fans was between .7-1.5 inches. Using the max difference, we get ∆d=0.75”. Assume the UFO is a cylindrical shape, we will use the equation of a cylinder to determine the total volume of the UFO. We note that the old structure had d=11” and h=3”.

  

Fans Volume of UFO % Ratio to 6 fans Estimated UFO Weight
3   180.3/284.96*100=63% 253g
4   212.5/284.96*100=75% 298g
5   247.4/284.96*100= 87% 347g
6   284.96/284.96*100= 100% 400g

 

Now we determine the mass (g), and thrust for x number of fans.

Part/#fans 3 4 5 6
Fan 345g 460g 575g 690g
ECS 90g 120g 150g 180g
Battery* 380g 380g 380g 380g
Structure** 253g 298g 347g 400g
Xbee 7g 7g 7g 7g
Tinyduino 8g 8g 8g 8g
Gyro 2g 2g 2g 2g
Components 832g 977g 1122g 1267g
Total: 1085g 1275g 1469g 1667g

* We will assume a single battery as the power supply for the UFO, given to us from the customer. We may modify this, based on the battery trade-off study.

** Determined above using the old structure

Using the average thrust provided from last semester’s post burn studies, we have 502g of thrust provided by each fan.

Fans/Thrust Total Provided Structure+Components=UFO Lift UFO Control % Control
3 1506g 253+832=1085 421 28.0% +/-5
4 2008g 298+977=1275 733 36.5% +/-5
5 2510g 347+1122=1469 1041 41.5% +/-5
6 3012g 400+1267=1667 1345 44.7% +/-5

 

 

For the case of Multiple Batteries:

Here, we will assume that the mass of the battery will change with the amount of fans present. We will use the battery provided by the customer and determine the mass to be 380*x/6, where x is the current number of fans. Therefore, we will have:

Fans/Thrust Total Provided For Components For Structure UFO Lift UFO Control % Control
3 1506g 642 253 895 611 40.6% +/-5
4 2008g 850 298 1148 852 42.4% +/-5
5 2510g 1059 347 1406 1104 44.0% +/-5
6 3012g 1267 400 1667 1345 44.7% +/-5

 

 

In conclusion, we can see that 6 fans would give us approximately 50% thrust control. However, we would like to point out that there can be modifications made to the battery and/or the structure to provide ourselves with 50% thrust control using a smaller number of fans. Below, we will see how much weight the structure would have to be to provide that requirement.

Appendix A:

Here, we are going backwards to determine the maximum amount of weight our UFO (structure and/or battery combination) should be to provide a 50%+/- ∆ thrust control per number of fans. Using similar assumptions as above, we will note our fixed values.

Number of fans Fixed Thrust 50% Thrust Fixed Weight Remaining weight for battery/structure
3 1506 753 452 301
4 2008 1004 597 407
5 2510 1255 742 513
6 3012 1506 887 619

 This table above shows the fixed amount of total thrust we have and the set in weight determined by the components that are required in the UFO (Fan, ECS, Gyro, XBee, Tinyduino) and whose weight we can’t change. The remaining weight column shows how much weight the battery and structure should be to be able to maintain a 50% thrust control.

Since we can allow some leeway in the amount of thrust we should provide for control, we also show a 45% thrust control table below:

Number of fans Fixed Thrust 55% Thrust Fixed Weight Remaining weight for battery/structure
3 1506 828 452 376
4 2008 1104 597 507
5 2510 1380.5 742 638.5
6 3012 1656.6 887 769.6

 

 

 

Motor Battery Selection

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By Jake Rice

UFO Abducted must be able to fly around a classroom using ducted fans driven by brushless DC motors. To achieve untethered flight, a battery must be used to power the motors. In selecting a battery for the UFO, the three main considerations are weight, maximum discharge current, and energy capacity. A lithium-polymer battery will be chosen due to the low weight and high discharge current of lithium-polymer batteries relative to other common types. Because the speed controllers for the motors are rated for 14.8V, the choices are limited to lithium-polymer batteries with 4 3.7V cells in series.

The main factor limiting battery choice is the maximum current draw of the motors. To maintain safe operation, the battery must be rated to supply more than the maximum current potentially drawn by the motors. Each motor has a maximum current draw of 27A, and the 4 motors have a combined maximum current draw of 108A.

In addition, the energy capacity of the battery must be large enough to ensure that the UFO completes its flight. Based on previous research, the UFO will be able to maintain level flight at under 70% of the maximum motor throttle, and at 70% throttle the motors draw 44.68A. To meet the 2 minute minimum flight time requirement, the capacity of the battery must be higher than 1489mAh.

The following chart compares several acceptable batteries. The battery purchased last semester is at the bottom of the chart.

 

Name

Cost

Weight (g)

Capacity (mAh)

Maximum Discharge Rate (C)

Maximum Current Draw (A)

Maximum Flight Time (min @ 70%)

Capacity/Weight Ratio (mAh/g)

Turnigy nano-tech A-SPEC G2

$49.77

296

2600

65

169

3.49

8.78

Turnigy nano-tech

$34.47

268

2250

65

146.25

3.02

8.40

ZIPPY Flightmax

$31.27

368

3000

40

120

4.03

8.15

ZIPPY Compact

$35.57

350

3300

35

115.5

4.43

9.43

Sky Lipo

$64.72

349

3000

40

120

4.03

8.60

ZIPPY Flightmax

$25.78

329

2650

45

119.25

3.56

8.05

Thunder Power G8 Performance Pro

$94.99

284

2700

45

121.5

3.63

9.51

MaxAmps LiPo

$109.99

327

3250

150

487.5

4.36

9.94

 

Of the batteries compared, the MaxAmps LiPo has the best capacity/weight ratio, the second-highest capacity and a far higher maximum current draw than any of the others. Using the Thunder Power or one of the Turnigy batteries would reduce the weight of the UFO. However, due to the higher capacity and the cost advantage due to the fact that we already have it, we should use the MaxAmps battery to power the UFO’s motors.