Rescue Hexacopter Project

 

Design - Body

The next three design contraints: Lightweight, durable, and repairable all pertain to the actual body of the craft. Aluminum plate was one option, it is extremely light and strong, easily workable and readily available. However, it bends easily and would require realignment upon any hard landing.

The only other choice was carbon fiber. Like aluminum, it is lightweight and strong, but it will maintain its shape on hard landings. It is however, hard to work with, and expensive. These disadvantages aside, we chose to design our hexacopter using commerically available carbon fiber panels to maximize the durability of the craft considering the technical flying skill of the end user.

After discussing the various options with Foster Tech Composites students, we decided to go with a 3.1 mm panel for all structural components and 1.1 mm panels for the prop shield. The 3.1 mm thickness will provide a rigid level structure for plates and supports, while the flexibility of the 1.1 mm panels will allow for maximum protection at the smallest weight.

Now would be a good time to briefly discuss the various components needed to operate a hexacopter as each component affects the choice of the next component. First of all, we need six motors and propellors, each motor should be connected to a electronic speed controller or ESC. The ESC controls the amount of electricity which is provided from the battery to the motor. Connecting the battery to the motor without an ESC, would result in the motor burning out from spinning at maximum power. The ESC receives its commands from the flight controller which is controlled by a signal coming from a 2.4 GHz receiver.

Choosing the Motor and Prop

Most motors used on multi-rotors are different from regular electrical motors. Multi-rotor motors are usually what they call outrunner motors. Outrunner motors are generally slower in revolutions per minute (RPM) compared to inrunner motors and thus better suited to spin a propeller. Outrunners generally have much more torque than inrunner as well. Motors are rated in Kv which translates into revolutions per volt. A 1100 Kv motor turns 1100 times in a minute for every volt of electricity applied to it. Thus if 5V of electricity is applied to the motor it would turn at 5500 times in a minute. Upon research, it was discovered the best range of motors for multi-copters was between the 400-900 Kv range. Generally, the heavier the craft, the slower the props should turn.

Before we can begin the body design we have to determine what motor/propeller combination we will be using to acheive flight. The size of the propeller will determine the minimum length of the boom needed on each arm of the hexacopter. It will also determine the minimum size motor needed to spin it efficiently.

Since we didn't have the time nor the resources to test different motors to determine their suitability for our project, we decided to review the many hexacopters already built and flown in our weight class to determine what was used on successful attempts. A motor that was used by several hexacopters for video camera was the Tiger Motor U7 490Kv. We also looked at the NTM 50-50 580Kv motor from Hobby King. A third choice was the Hacker Motor series, manufactured in Germany.

After reviewing the different choices, we decided the Hacker Motors were a little underpowered for our needs. The Hacker Motor series are superbly designed and extremely durable. And with a price of $175 per motor or $1050 for the project it was the most expensive on our list.

The NTM motor was definately powerful enough, but the last NTM motor we used decided to try an scalp me in front of my students when the e-clip holding the motor together let go during testing. It was by far the cheapest at $40 per motor or a total of $240 for the project.

The Tiger Motor U7 was designed for hexacopters up to 18 KG or about 40 lbs. It has a much better quality record than the NTM 50-50 and definately on par with the quality of the Hacker Motors. As far as price, the Tiger Motor came in at $149 per motor or a total of $894.

While the prospect of saving over $650 on the project is appealing, quality must take precedence over saving money in this case. This hex will be a monster and will need top quality components in order achieve the mission. For this reason, we decided to go with the U7.

U7 Chart

Tiger Motor U7 Kv490 Test Data

The above chart shows the test results for various propellor and voltage ranges for the Tiger Motors U7 Kv490. Again, assuming a projected total craft weight with maximum payload of about 25 lbs, we can calculate the minimum thrust needed to keep the craft in the air at about 11264 grams (25 x 2.2 x 1000). To that, we need to add an additional 35% for manuvability while in the air. This gives a total minimum thrust of about 15200 grams. Since we have 6 motors, we can calculate that each motor should be responsible for about 2532 grams of thrust for minimum manuvability. In reviewing the chart, we looked for a voltage/prop combination which would yield a thrust value higher than 2532 grams at 65% throttle. The smallest voltage/prop combination meeting that criteria was the 16 x 5.4 prop running at 22.2V. This would give us about 20 lbs of thrust at 50% throttle and a whopping 50 lbs at full throttle.

Thus, from the test data provided by Tiger Motors, the prop size of 16x5 will be what we use to model the hexacopter. From the 16 in propellers we can calculate the minimum size of the craft to be over 48 in. (16x3) including propellers. Since we want to include a prop shield on the final model, this extends the total size to about 51 in. at it widest point.

By using the prop as the "measuring stick" we can calculate the prop shields to be 17 inches in diameter, allowing a half-inch between the inside of the shield and the tip of the propeller. If all the shields were connected together, that would place the centers of the motors exactly 17 inches from each other and 17 inches from the center of the craft.

Initially, we decided on a center plate that would be about 12 inches in diameter, with each arm extending radially from the center plate. Because of the protrustion of the arms into the center plate, very little room was left for other components. So we created a layered body with distinct layers for electronics, arms, batteries, and landing gear. This design was deemed too bulky and was scrapped.

Radial Arms

Radial Arm Design

The design we decided to go with was a spiral design, where the arms are connected to the edge of the center plate parallel to the center point. By attaching the arms to the side, we can use all of the interior portion of the center plate for electonics and other components. This allows for much smaller profile and diminished the weight of the hexacopter substantially.

Spiral Arms

Spiral Arm Design

We changed the center plate to be a 17" hexagon and used this for our center structure. Using AutoDesk Inventor 2014, we created the center bottom plate by creating a hexagon and dimensioning it to 17 inches, and extruding it out 3.1 mm. As we added parts and structures, we constantly revised the center bottom plate to accomodate the additions.

 

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