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Maeko

2011-07-15T06:33:04Z

Maeko a sparless single skin SLE sled using tailored bumps on the leading edge to maintain canopy shape. It is lighter, uses less material, and is easier to make than a standard parafoil.

Flat aspect ratio: 5.1
Area: 1 m²
Tip to Tip: 225 cm
Center chord: 57.4 cm

Maeko is a minimalistic soft kite with nice flying characteristics. It is a result of the experiments done with Foil Nose 3.

Design Highlights

The combination of four key features come together in the Maeko design:

Tailored leading edge bumps.
All flat sewing.
SLE bridle.
Load and sheet tension lines.

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Wireless Configuration

2011-04-09T16:33:03Z

This article describes reconfiguring the wireless access point portion of a home network in order to provide a more reliable internet connection.

Background

My employer provides home internet and telephony. The choice of provider is predetermined and there is no option for selecting alternate hardware.

Connectivity is fine when using a wired connection. However, the wireless connection is quite unstable. The wireless network often experiences problems with temporary and permanent disconnections. I am often greeted with "Dad, the internet is down again!" when coming home from work.

At the place we used to live, the internet was "part of the apartment" and each owner bought their own hardware to make a connection. Back then I bought a small wireless router and installed Tomato firmware. Tomato was chosen because I needed QOS to protect internet voice calls against streaming video bandwith usage. Tomato was so trouble free that I did not think about it until the current wireless problems.

Below I describe how I replace the wireless section of my "all in one" box with a Tomato powered access point ...

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Site Feed

2011-02-20T16:30:03Z

This article describes the bare metal mechanics of a site or blog feed. Layers of abstraction have been piled upon what is actually a simple method for getting information out to the world. Here we uncover the original basics in creating a feed.

At the lowest level, a feed has two parts:

Autodiscovery code in a HTML page.
Feed information in a XML file.

Details follow.

Autodiscovery

Autodiscovery is code in a HTML page which tells browsers (or other software) that the site contains a feed. When visiting a site with autodiscovery, a browser indicates the presence of a feed on the site. For example, Firefox indicates a feed with an icon in the address bar:

Clicking on the feed icon will begin the subscription process for the site feed.

This behavior is triggered by an entry ...

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V Plotter Design

2011-01-27T16:30:02Z

A V plotter is a minimalistic design which uses a pair of steppers, some string, and a pen head to create a plotter. These are sometimes made by students or technology sector employees as a way to avoid "real work".

In this article, I dig into the math behind these machines, and also write a program to calculate the configuration of a V setup needed to produce a working device.

Requirements

What is the optimal configuration of control lines for an area to be plotted? Obviously, we can't have a drawing area above the control lines — our friend gravity sees to that. But, can we do better than hand waving and "somewhere below the control lines" for the plot area? Yes: we think up some constraints and model them with math and code (two more friends!):

Tension: We can imagine that the control lines have to be under some tension in order to be effective. For the purposes of this article we say that both string tensions must be in the range [ m/2, m*1.5 ], where m is the mass of the plotter head. Lines can neither be too slack nor too heavily loaded. The effect of this constraint is to prevent any line from being too close to horizontal or too close to vertical.
Resolution: There is a change in resolution when we map a change of length in one or both of the strings into X and Y coordinates. I.e. Coordinate system conversion causes a non-uniform step resolution. We say that, for each control line, a one unit change causes at most a 1.4 unit change in the X,Y coordinate system. We limit plotting to the area of reasonable resolution. Here our definition of reasonable is a 40% change.

To implement these requirements, we need to know how to calculate them. The next few sections talk math and have pretty pictures.

Line Tension Calculation

The below diagram shows a mass m suspended by two lines. Each line can (and usually will) have a different angle to the X axis. We wish to calculate the tension of each line in the diagram.

To describe the horizontal forces along the X axis (in balance), we write ...

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ssh Configuration

2011-01-16T16:30:02Z

On a Linux or Unix system, ssh is used to make a secure connection to a remote machine. This connection can be used as a communications transport for other programs. In this article I describe how to setup an encrypted ssh connection between two machines. This connection can be used to log in to the remote machine, synchronize files between the machines, or display locally the GUI of a program running on a remote machine.

For the purposes of this article, alfa and beta are the peer machines we wish to configure. Both have a user account named bill.

Install openssh

The first step is to install openssh on machine alfa:

bill@alfa: sudo apt-get install openssh-server openssh-client
bill@alfa: sudo apt-get install rssh molly-guard openssh-blacklist openssh-blacklist-extra

We install both the ssh client and server portion as we will both be ssh'ing from and to these machines. The packages on the second line are optional, but a good idea. Refer to the man pages for more information.

At this point, we could use ssh to access a remote machine (running a ssh server) and be prompted for a password to validate access. Password login validation for ssh is a bad idea because it opens a security hole. Instead we follow the steps below to exchange security keys with the target machine and eliminate the ability to ssh using a password.

Create the encryption key on alfa:

bill@alfa: ssh-keygen -t rsa -C "user bill on alfa"

Enter a filename, for example: bill_on_alfa. If the generated files are not created in ~/.ssh, move them there. The -C comment at the end of the command is optional, but a good idea as it help human readers identify the key. The public portion of the key and the comment ends up as part of the identity string in alfa's authentication agent, and in beta's authorized_keys file.

Now add the RSA identity to alfa's authentication agent. On alfa, type:

bill@alfa: ssh-add bill_on_alfa

Listing the RSA identities in alfa's authentication agent is done by:

bill@alfa: ssh-add -L

You should see the public key parameter of the just generated key as well as the "user bill on alfa" comment.

Since the identity is registered in alfa's authentication agent, we can use the 'short form' of ssh-copy-id to propagate the public key to beta's authorized_keys file.

bill@alfa:ssh-copy-id beta.local

This command behaves nicely, creating the authorized_keys file if none exists, or appending the key if the file already exists.

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Volkite

2011-01-11T16:30:02Z

Volkite is a kite with two skin foil construction on the leading edge and single skin for the rest of the canopy. It is lighter and uses less material than a standard parafoil.

Flat aspect ratio: 3
Area: 1.5 m²
Tip to Tip: 212 cm
Chord: 70.5 cm

The LE has 14 cells comprised of 7 mini-ribs and 6 full ribs. The mini-ribs stop short of the point where the lower and upper skin meet, allowing for air passage between adjoining cells. This also helps to avoid the pucker distortion which can occur when a rib narrows to join two skins.

Working with a (partially) single skinned kite is illuminating because it clearly shows when a section of canopy is not not contributing to lift.

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