I’ll Pound Y’all

Let’s get wingfoil technical, OK? The photo shows a lift analysis of a#SkyDock airfoil at an attack angle of 4 degrees. At this angle, we’re generating a coefficient of lift of 1.39 with a drag of .0144. This produces a Lift / Drag ratio of 96.73.


In order to generate a lift of 660 pounds (#SkyDock gross weight); the induced drag from the entire wing is 7 pounds.

(Real world values vary substantially… topic for another post…)

Now let’s get practical and estimate in-flight speed to exactly carry the airplane at a 4 degree angle of attack. How fast does the airplane have to go to balance lift & aircraft weight exactly?

Let’s do this in English units, not metric.

It’s easy. Using the lift formula, which looks complicated but isn’t:

Lift = (0.5) ρ v² A Cl

ρ = air density;
v = velocity;
A = Area of wing;
Cl = Coefficient of lift;

We want to solve this equation for velocity. So the equation is rearranged as follows:

v = ((Lift * 2) / (ρ * A * Cl)) ^ 0.5

And now we solve.

Lift = 660 pounds (SkyDock @ gross weight)
ρ = 0.074887 pounds per cubic foot
A = approximately 120 square feet
Cl = 1.39

The answer is … 10.28 feet per second or 7 mph.

Oh My! that’s not correct! NOT EVEN CLOSE!

The problem is the we have solved without taking into account the conversion of force into pounds. Our result should have been intermediately calculated using the relatively obscure unit of ‘poundal’, with a consequent conversion to pounds.

It kind of reminds me of my youth, when I’d be avoiding some bully in the hallway, and he’d ‘pound y’all’ if I got in his way. Different situation… I digress.

A poundal is a unit of force, unlike the bully, who was a unit of terror. At acceleration of one gravity, it is equivalent to a pound using this formula:

1 poundal = 0.031081 pounds.

Fixing the formula from above,

v = ((Lift * 2) / (ρ * A * Cl * 0.031081)) ^ 0.5

And solving one more time using the same variables

Lift = 660 pounds (SkyDock @ gross weight)
ρ = 0.074887 pounds per cubic foot
A = approximately 120 square feet
Cl = 1.39

The corrected answer is … 58.31 feet per second or 40 mph.

Now we know: A SkyDock flying at 40mph with zero flaps and angle of attack of 4 degrees will generate exactly the lift required to balance offsetting gross weight of the airplane.

The author of this post, James Wiebe, has a degree in Math from Tabor College, Hillsboro, KS.

Wing Spar Load Test on SkyDock

Hi K.,

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We ran our negative G load test today on our SkyDock prototype.  As the loading was far in excess of the anticipated flight load, this may be considered an ultimate load test.   It was successful with two glitches.

Gross Weight:  660 pounds
Weight of wing structure 62.5 pounds per wing (outboard of cabin).
Zero Fuel Weight:  570 pounds (90 pounds of fuel; 45 pounds per wing in outboard section).
Testing to 3.8G’s, negative load.
Each sandbag weighed.
1)  Plane was loaded generally in accordance with spreadsheet.  See attached.
2)  Conservative methodology:  stations 0 and 1 weight loads were placed at stations 2 and 3; this increased load on wing.
3)  Glitch 1:  Rivet structure showed signs of failure on both sides on front of “D” cell strap assembly.  Rivets pulling out of carbon fiber “D” cell.  Will be redesigned.
4)  Glitch 2:  center test support structure was placed under carbon fiber in such a way that carbon fiber skin was deflected upwards in tension into internal foam ribs, but just on one side of support structure.  This caused slight permanent compression into foam inside.  Carbon fiber skin is unharmed.  Repair is easy; just inject some expanding foam at affected area.  If / when test is repeated, test support structure will be completely under spar hard points.  This is a testing failure, not a structure failure.
Thanks for your assistance in analyzing the design of this wing.  It’s always fun to see a light thing hold a lot of weight.
Some comments on deflection:  we measured approximate deflection of 4 inches at station 9.5; (this is where we had our safety support stand).  Each section of the wing has a different moment of inertia; the center section is obviously the strongest.  The outboard section has a much lower value; and the third would be on the massive spar strap that connects the outboard and inboard together.  I don’t think we’ve calculated how these play together for predicting deflection; but in any case, the wing is acting like it has an average moment of about 8.  That’s pretty impressive, as I recall the moment of the outboard sections was designed to be around 4.2; the improvement is no doubt because of the carbon fiber bonded to the top and bottom of the spars.  What do you think?
Best Regards

Breakthrough Capacitive Fuel Probe

Product Preview:  Belite Capacitance Fuel Probe

Last Revised March 8, 2016



Cap Probe

Capacitive Fuel Probe controller module.  Functional prototype shown.  Case not shown.


Probes for fuel tanks have often used the principal of “capacitance” to determine the level of fuel within the tank.  These probes are usually constructed of a thin conductive center probe surrounded by a liquid permeable sheath and a grounded jacket.  Commonly available in lengths from 12” to 24” (or longer), these probes suffer from the following problems:

  • Although many claim to be bendable, in our experience we’ve found them to frequently short and fail when bent.  Bendability is a requirement to fit through access holes and at a diagonal positioning from the hole to the bottom of the tank.
  • Presence of minute amounts of water within the fuel may cause these probes to fail. Water is a very good conductor of electricity (specifically when non-pure or contaminated.)
  • Calibration of capacitance probes is affected by changes in the dielectric quality of the fuel in the tank. Specifically, a mix of AvGas with autogas, or a mixture of alcohol within that autogas will cause capacitive probe readings to fail.
  • We believe that almost all capacitive probes on the market are designed with older technology circuits that are less immune to high electrical noise environments, such as what is found in aircraft. While this is a more subjective claim, we have solid foundation in making new claims of noise immunity with our new design.
  • Most capacitive probes require intrusive, large holes in the top of the tank in order to mount the probe puck.


Belite’s new Capacitive Fuel Probe Solves Problems

Our new probe resolves these issues and offers substantial benefits.  In particular, the probe has the following features:

  • The probe is a flexible wire assembly. It may be routed from one corner of the tank to the opposite corner, and held in place at the far corner using a simple mechanical hook.  It does not care what path it takes, as long as that path is surrounded by the fluid to be measured.  It also does not care (within reason) the length of the wire within the tank, and will accommodate any length between 6 inches and 40 inches (one meter).    The wires are jacketed with Fluorinated Ethylene Propylene (FEP).  This material is amazing.  Quoting one of the manufacturer’s websites:

For instance, in the automotive industry, chemical transference is often needed to deliver vital fluids throughout a vehicle’s complex engine. With this in mind, the flexibility and high working temperature characteristics of fluoropolymer tubing can provide custom applications in long lengths on a variety of vehicles.

Since fluoropolymer tubing is a fantastic insulator, it can offer electrical insulation and chemical resistance that makes it a great splicing aid for high continuous service temperatures in corrosive environments.

Source:  http://www.fluorotherm.com/the-different-applications-variations-of-fluoropolymer-tubing/

  • Our probe will not fail in the presence of water. (Water will affect the reading, but the unit will continue to function in an accuracy degraded mode.)
  • Our probe electronic module includes a calibration button which is used to set the Empty and Full position of the fuel tank. This button may be optionally routed to a pushbutton switch in the cockpit, so that the fuel probe may be recalibrated to Full after the tank is filled.  Therefore, full position calibration is automatically recompensated for type of fuel in the tank – 100LL, AutoGas, with or without alcohol.
  • This probe was designed using an integrated circuit with a novel tuned inductive filter on the probe input. This has demonstrated a great deal of noise immunity.
  • The wire assembly may exit the tank via a vent hole or any other hole. In a good tank design, this would be in that portion of the tank reserved for fuel expansion as the fuel warms up on a hot day.

Cap Probe Wire Module

Prototype wire assembly.  Not final shipping version, but a pretty good representation.

How does this new fuel probe work?  What are its features?

  • The design uses a very stable temperature compensated capacitive oscillator in conjunction with a tuned inductive filter. As fuel surrounds the wire probe, the capacitance changes.  The circuit is capable of resolving very small changes in fuel height and converts this to an output voltage, which is linear to the fuel level.  The more fuel, the higher the voltage.
  • The circuit is compatible with any type of gasoline, alcohol, or jet fuel. Kerosene is just fine.
  • The fuel probe unit provides a linear +5 volt output, compatible with almost all modern fuel display gauges and EFIS displays.
  • We specifically recommend our new RADIANT fuel gauge as a companion to the fuel probe.  While the product preview document you are reading is about the Fuel PROBE, we’ll interject for a moment and show you a few photos of our RADIANT fuel GAUGE.  Here is a screen shot of the fuel gauge:


Screen shot of RADIANT fuel gauge.  Left tank is empty and right tank has been drawing down fuel for approximately 10 minutes.  
We recommend this RADIANT fuel gauge for these reasons:  it has a great 2 minute sloshing filter built in; it shows 15 minutes of fuel trend history, and it is a full color, it is nearly weightless, it is sunlight readable display.  $200.

Now, back to the fuel probe:

  • The controller board on the fuel probe has one switch. For calibration: after powering up, press it quickly to set the “low” fuel point.  After filling with gas in the tank with the probe, press it longer (a few seconds) to set the “high” fuel point.
  • A remote switch may be mounted in the cockpit. Pressing it will reset the “high” calibration point.  To avoid accidental high point resets, this requires a five second touch on the switch.  This allows automatic recalibration after every tank filling.
  • For hard core techies: the unit’s connector also provides a +5v regulated output.  This allows a differential analysis of the signal level, as the output DAC is technically ratiometric to the +5V supply.  If you understand that, you’re an electrical engineer.  If you don’t, just ignore it.
  • For hard core OEMS: this product is also available with digital RS-232 serial output, periodically broadcasting the fuel level at 9600 baud.  This allows direct digital attachment to a variety of EFIS systems, assuming support from the EFIS vendor.


What does the Belite Capacitive fuel probe cost?  When is it available?

  • The unit is priced as follows: Unit + 6” probe — $200; Unit + 12” probe — $210; Unit + 40” (1 meter) probe — $250.  Custom lengths are available; contact factory.  Includes electronic unit in small case; wire probe of 6”, 12” or 40” length; power connector and pigtail harness; short coaxial cable to connect together probe to module.
  • First shipments are projected in Q2.
  • Orders received between March 8 and April 30 receive a 25% discount. In order to reserve your delivery position and receive this discount, we must charge your card at time of order.  All such orders are expected to ship by May 30.
  • A similar special is available on our RADIANT fuel gauge.   Orders received between March 8 and April 30 receive a 25% discount. In order to reserve your delivery position and receive this discount, we must charge your card at time of order.  All such orders are expected to ship by May 30.

Want to see a demo video?  Navigate here:


Cap Probe Demo Video Screen Shot

UltraCub Rib Construction — Aluminum ribs with cap straps

If you are building aluminum ribs (or aluminum / carbon fiber strips), the top and the bottom “T Strips” have to be glued on with 2216 or similar epoxy after they are riveted in place.

2216 is a 3M epoxy which is specified for use with aluminum bonding.  Surfaces must be scoured clean (eg,: 180 grit), then cleaned with acetone, before a liberal fillet of 2216 is added on all sides.

Tape was used to stabilize the strips while glue was applied and setting.

The tip and root rib get a square aluminum tube reinforcement.

We also recommend a 5″ vertical piece of angle on each rib, to prevent vertical crushing.  (Not shown, will provide pic soon.)

Rib Construction

Rib Construction

Root and Tip Rib reinforcement Root and Tip Rib reinforcement-2

How to cook trout in the wild, with wine and Tabasco.

In today’s cooking lesson, we’re going to learn how to prepare James’ amazing!!! mildly hot trout recipe.

The very first thing to do is to clean your trout.  It’s easy — cut off the head, gut them, clean out the innards, and leave the skin on.  The skin will come off easily while being cooked, so don’t worry about it.

Then, retreat out of the wild into a cabin with a stove and a frying pan.  This is a very important step — don’t get it wrong.

Start the preparation by melting a half stick of butter or margarine in the frying pan.  I used margarine, but you should use butter.  Really.


Melting a half stick of butter in a frying pan, over low heat.

Once that has melted and is lightly bubbling, insert the trout into the pan.  It is helpful if the tail sticks slightly over the edge — this will help impress your friends.


Trout beginning to cook in bubbly butter. The trout tastes best if the tail sticks slightly over the edge of the pan. No, I’m not explaining why that is so.

The next step provides some extra flavor — throw in 3 or so ounces of your favorite red wine.  The best wine is a cheap red, straight from Santa Clara, CA.  (Inside joke.)


Red wine in the pan.

Now, another critical step.  Locate an abused old pizza pan, such as shown in the following photo.


Abused old pizza pan.

Place the pizza pan over the frying pan.  You should be able to hear the butter / wine mixture lightly bubbling away.  Don’t set the heat too high — a low heat is fine.  (The stove I used in the cabin worked fine with the dial set to 2 (out of 10.))


Abused pizza pan is now covering frying pan and allowing trout to simmer in the wine / butter mix.

Allow 6 minutes to pass.  Then remove the pizza pan.  *Flip the trout using a spatula or fork.*  Your trout will now look like this:


Half cooked trout.

Insert a knife under the skin; peel off the skin.  It’s fairly easy to do.


Removing the skin off the half cooked trout.


Now the skin has been removed from this side. It’s starting to look very tasty — but wait! It’s still only half cooked.

Sprinkle four drops of Tabasco onto the half cooked fish.  If you wish, you may spread the Tabasco over the side of the fish.  Also add salt and pepper to your desired sodium level at this time.


Tobasco drops have landed. Also salt and pepper to taste.

RECOVER the pan with the pizza lid and allow to cook on very low heat for 6 more minutes.  AFTER six minutes have passed, turn the heat up to a medium / high number.


BRIEFLY turn up the heat!

After about 45 – 60 seconds at the higher heat, remove the fish from the pan.  With a little effort, you can leave the bottom layer of skin (which you just slightly seared) on the bottom of the pan.

Then, transfer the fish to fine china.  Consider adding a side of Lay’s Sweet South Heat Barbecue potato chips — a truly excellent choice.

This meal pairs well with a ridiculously cheap red wine in a plastic tumbler.


James’ hot trout with hot potato chips and red wine in a plastic tumbler.


Bon appetit! The trout flakes off the bones — and it was truly delicious.

From swimming in the stream to eating it at the table, this took about 75 minutes or less.  That’s fresh trout.

Let me know how this turns out for you.  Bon appetit!