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Saturday, November 14, 2009

Home Built Hovercraft

While not being the most efficient means of transportation, hovercrafts impress by the ability to operate both in land and water.The inherent maneuverability is also an interesting characteristic. While the driving is entirely different from a vehicle with wheels, hovercrafts are able to change direction very quickly, given the fast rudder response (usually located in the rear, and close to the propulsion source).

Building a full scale hovercraft would not meet my available budget, time and ambition. Given the fact that I had some extra RC parts from the model helicopter (servos, ESC and battery), I decided to buy an airplane brushless motor plus propeller, and build an hovercraft with household parts (essentially a plastic food container, some cardboard and plastic bag). With the help of my friend hot glue I put all the stuff together, resulting in a high performance RC hovercraft.

Here is a detail of the brushless motor and propeller:

With the two rudder fins, turn response is improved at the cost of a slight increase in drag:

The parts list used for assembling the hovercraft were:
  • One EK5-0086 1000 RPM/Volt 45 g brushless motor;
  • One 7x6E propeller;
  • One 18A brushless ESC (originally belonging to the Art-tech Falcon 3D helicopter);
  • One 1300 mAh 11.1V 3s1p LiPo batery (from the same Art-tech helicopter as well);
  • One 41 MHz PPM receiver (compatible with the Art-tech E-fly 100C transmitter);
  • One 9g micro-servo;
  • One 165x240x65 mm plastic food container;
  • Some cardboard;
  • 0.8mm wire;
  • 5 mm thickness 400 mm height wooden pole;
  • Plastic ties;
  • Bolts, nuts and washers;
  • 0.1 mm thick plastic bag.
  • Soldering iron (mostly for heat-cutting the plastic);
  • Carboard cutter;
  • Scissors;
  • Screwdrivers;
  • Pliers;
  • Ruler;
  • Caliper;
  • Pencil.
The skirt was simply made of plastic bag with help of hot glue to attach it to the bottom of the hovercraft.

Cardboard was used to make the air cushion internal separator and duct:

This part is still to be perfectioned, as the lift performance is not the best (the skirt practically does not raise from the ground, and hardly moves in less regular surfaces).

The following video shows the hovercraft in action, performing several 180 degree turns and spins:

Another video shows the same hovercraft equipped with an onboard camera, providing a first person view of the drive:

Sunday, May 31, 2009

Remote controlled flight

In my adventurous quest at being able to fly with my feet on the ground, I begin to understand the true fun of this hobby and how it stimulates the mind to understand the physics behind flight, and in the particular case of the helicopter how stable hovering and forward flight is possible.

Even though I have a flight stabilization unit onboard, I have diminuished its authority to almost zero so that most of the effort is left for the pilot behind the cyclic.

Flight simulator software greatly helps in ensuring that minimum training be taken before wrecking the real aircraft. It allows basic orientation skills and familiarization with the remote control to be achieved.

However, for full flying skills to be achieved, the simulator does not replace the real helicopter. Even the best simulator only offers a limited imitation of the behaviour of the real scenario. There are many more variables wich are very hard to simulate, and we find these in a real flight. Often a representation of our R/C model is not available, so we are forced to use a machine with closest similarity to our model. Sometimes it is no good enough.

Another aspect is the field of vision: in the simulation we are limited to a field of view of roughly 70º (depending on our distance to the screen, and its size and shape). In the real world, the field of view is of 180º (or more). So much more neck movement is necessary to keep track of the aircraft.

In the simulator there aren't many factors of distraction (unless you keep your IM client running in the background :)). In the real world there may be several elements of distraction, including people and animals (specially dogs).

Once the first steps of simulator training are completed (and we are comfortable with the transmitter, the control gymbals and orientation - make sure you can properly fly nose in and nose out without trouble), it is time for flying the real deal. A calm windless day and a place with a lot of space (more than 20 m x 20 m) and soft ground are essential for having the least ammount of surprises. At this stage training gear is essential if you don't want to break your blades all the time.

Safety cannot be disregarded and you must make sure there are no people walking around in your flying field. If you bring company along with you, make sure they are at least 10 feet behind you and stationary. It is never too much to remind that RC helicopter is the most dangerous model aircraft activity. Imagine these as airbourne lawn mowers. The spinning blades can easily chop off a finger or seriously cut a leg, not to mention the damage it can cause to the face.

While this blog post is not indented to be a exhaustive beginner manual for helicopter flight (you can find a lot of good documentation in the web regarding this), it is worth mentioning that the first training step is to achieve proper hovering skills. Do the best effort to maintain a steady position while hovering at one or two feet off the ground. When you first take off, you will notice the helicopter drifting to the left (or right in case of counter-clockwise rotating rotor head) even with the cyclic properly trimmed. This is the tail rotor propulsion, which is specially noticeable at low altitude. During takeoff you must compensate by applying some right cyclic.

The next move is the transition to forward flight. Here you must make sure you have a lot more available space, as you will easily achieve high speeds. In forward flight the helicopter behaviour is much more similar to that of an airplane. The cyclic behaves much closer to the ailerons and elevators of an airplane, and the anti-torque (or tail pitch) similarly to the rudder. While forward flight is somewhat easier, mistakes can however be much more costly. One aspect to consider is the distance: the further the helicopter is from the pilot, the harder it is to properly identify its attitude. Orientation mistakes become much more likely, so a lot of concentration is necessary to make sure the mistakes do not take place.

Here is a video I have taken with my onboard camera, showing some forward flight and a nice 60 feet climb:

Sunday, April 5, 2009

Getting airborne (on a scale sort of way)

The ability to fly have been a long envied feature of birds. Since the early days of the history of mankind, there have been the ambition to fly like a bird. The same way the conquest for new lands and the seas have been seeked and achieved by people (both for political reasons and for the bare survival), the skies have also been one of the goals of humanity. While many have envisioned and prototyped flying machines (going back to the time of Leonardo Da Vinci), only during the the 20'th century this dream was fully achieved, with the Wright Brothers having created in 1903 the first controlled powered flight. The war effort strongly stimulated the development of this new, efficient way for people to move, control, watch and dominate the territories.

Techonology envolved so fast during the 20th century that besides huge improvements having taken place with the airplanes and other flying vehicles, the industry also found feasibility and a market for flying machines that would not have useful functionality at sight. The combination of aeronautics with radio technology gave birth to products developed to fulfill the sheer fun of flight, but in a small scale, with the feet on the ground, safely behind a remote control.

While these small machines were great for people to better understand the dynamics of flight and how the control surfaces influence the behaviour of aircraft, still there was no solid purpose other than entertaining the model flight enthusiast.

Today, while model aviation is still present as a hobby crossing many different ages, its industry have also boosted the development of new ideas in the field of robotics. With complex computing machines becoming small and light, it becomes possible to put a lot of autonomous behavior on board of a model aircraft.

With the combination of techonologies in the form of packaged miniature devices, it becomes possible to have all the essential flight instruments in a small unmanned airplane or helicopter that can stabilize, fly by itself and follow a plan previously uploaded by the pilot.

A network of sensors allows useful information to be collected and transmitted in realtime, turning a unmanned aircraft into a potentially useful machine.

Conventional unmanned aircraft require the pilot to have visual contact with it, at a distance close enough do discern its attitude, heading, relative speed and altitude. The limited ability of the human vision to recognize these parameters, after some training make navigation from an external point of view possible. Training enables the human brain to acquire the ability to steer the aircraft the same way as if the person would be onboard the aircraft. The pilot have to mentally translate the action to be taken, into appropriate radio control input. The kind of input a person has to provide is conditional to the orientation of the airplane or helicopter relative to the individual. A trained pilot has all this reasoning mechanized in his brain, so that the response is immediate and he doesn't have to "think" about each control input. The timing is critical, and a unexpected delay most likely will cause a crash.

In the particular case of a remote controlled helicopter, input is particularly time critical. It is commonly said that "an helicopter is a machine that doesn't want to fly". In order to keep an helicopter hovering or in forward flight, constant pilot input is usually necessary, as model helicopters are inherently unstable (given its small weight, the lightest air turbulence will impact its stability, and the lightweight nature of the main rotor prevents the inertia from being strong enough to keep the helicopter level during a significant ammount of time).

Today some flight stabilization solutions designed for model aircraft exist and are available on the market for the enthusiasts of r/c airplanes and helicopters. Two main types of solutions exist:

  • Optical system, based on four far infrared sensors (4 thermopile sensors placed at 90º angle from each other) - this system relies on the thermal contrast between the ground and the sky to keep the helicopter level against this reference. It has a very fast response, but it has the disadvantage of being useless in closed spaces.
  • Gyroscope based - most r/c helicopters today have a piezoelectric gyroscope. These are however limited to providing yaw rate or heading hold functionality to help the pilot maintain a steady tail, irrespective of the direction of the wind. A 3-axis gyro provides the additional degrees of freedom enabling not only yaw stabilization, but also swashplate or cyclic stabilization. Besides controlling the tail servo, it also controls the two or three (in the case of a CCPM helicopter) cyclic servos. This enables the helicopter to be precisely level without constant input from the pilot. Some of these devices also feature an optical sensor (similar to that of an optical mouse) that also provides position hold up to 5 meter altitude. This enables the helicopter to be completely steady in the air, without the pilot having to touch any of the control gymbals.
The first solution is usually the cheapest (around 50 euros), being the second usually 6 to 10 times more expensive.

In my experience as a r/c flight enthusiast, I've decided to acquire a small (400 class) electric helicopter. I've chosen a Art-tech Falcon 3D. My budget was limited, and the offered functionality and degree of reliability seemed reasonable for the low price, so I decided to buy it.

At the beginning, given the lack of simulator experience I've failed to perform successful flights and soon made some expenses with new main rotor blades. While the training gear is helpful, it still doesn't prevent all of the crash situations. A bad choice of take-off space also makes things harder.

I've decided to play safe and get the stabilization gear:

After acquiring some flight expertise I went for the acquisition of a flight camera:

Here is a flight session with the onboard camera: