Voltage regulator (aka Pitot power) - part 5

Heated Pitot power supply

Pitot power supply housing

Technically it is no longer a voltage regulator, but I decided to keep the title series going to make it easier to follow my quest to intelligently power the heated Pitot tube, so please excuse the title.

As you might remember, the last time I took a stab at this I had reached unsatisfactory results after testing my own voltage regulator based power supply design.

The biggest issue was that the air blast simulating the flight environment sapped way too much heat from the tube, rendering it useless. On top of that, time to reach full heat was also substandard, taking up to 12 minutes to reach 200ºF.

In other words, while I successfully tamed the heat to the Pitot tube, I could not control it as I needed to.

Looking back to a comment from a reader at the bottom of “Voltage regulator - part 1”, I started thinking that perhaps I could program a micro-controller (basically a tiny computer) to purposefully turn on and off the full battery voltage to the Pitot tube in order to reach my desired temperature more quickly. 

Not only that, but I might be able to use a thermocouple to report back the actual temperature of the probe to the computer, and cleverly adjust the power output based on that information. I could then automatically add more power ON time for decreases in temperature due to higher airspeed, rain, or icing conditions, or more power OFF time for the opposite set of circumstances, like a raise in ambient temperature, or a slower speed (slower air blast) during the approach and landing phase of the flight.

It was worth a try, so I started freshening up my C++ skill set, this time as it related to micro-controllers.

Pretty good book, however prior C++ knowledge is definitely advantageous.

Believe me when I say the title of this book was misleading, it definitely took more than 24 hours to get it all figured out, but from what I learned I was hopeful I had a better path to reach my goal.

What I would need to test this idea were an Arduino-style micro-controller, a thermocouple, a 10 amp relay to handle the full 7 amps coming from the battery all at once, and a breadboard.

Everything is held together by a custom 3D printed base

The system worked, and I could control the temperature at will from ambient temperature to the full 700ºF that the full ON battery power is capable of. Of course the software took the longest to get right, and went through many revisions, but I am happy with it now. 

In a later version of this system, I added a second thermocouple to be buried into the foam of the Long EZ’s nose, right outside of the G10 Pitot tube insulator. This second sensor makes sure that if there were any high temperatures reaching the foam, the probe’s heat would get switched to a lower power mode and not burn up the airplane’s nose.

Note the 2 thermocouples

The nicest feature of this new system is its ability to react dynamically to changes in the environment without input from the pilot, and maintain any desired temperature within approximately ±15ºF, after a very short adjustment period.

Let’s take a look at some test results…

These test results are an order of magnitude better than those of the voltage regulator

The three series represent different sampling rates of the thermocouple, with the highest one generating the most precise tracking of the set temperature of 200ºF as expected.

Completely satisfied with the results of these tests, I started prepping for the first test unit, using an even smaller microprocessor.

CAD drawing of the interior of the planned test unit

Eventually, even this layout will have to be shrunk to better fit the small confines of the EZ’s nose.

Zortrax M200 delivering the goods in ABS plastic

Testing the layout in real life

All wired up and ready to go

Vented enclosure cover

For this test unit, I am also planning on adding a cockpit display of Pitot and Foam temperatures using a serial OLED 1.5" (3.8 cm) color display.

Stay tuned…

Longeron repair

Left side (22.5 hrs)

Today I'll show you how I repaired the busted longerons. If you don't know or don't remember why they have to be repaired, check out my previous post about it. 

I will start with the left one. The right one will be made whole in the same manner.

Here's how it all went down...

The first order of business was to develop the easiest, most reliable, and reproducible way to precisely cut the longerons with a 7:1 ratio. You see, while this cut is pretty easily accomplished over the bench, it is no longer so once you are working suspended in mid air over the fuselage, nevertheless the parts still have to match precisely for the fix to be strong.

After a few failed wooden jig models, I decided to leverage the power of the recently acquired 3D printer to design a precise  7:1 custom wedge that would accurately mate to the longeron, minimize the chances of mistakes and misalignments, and allow me to concentrate on operating the hand saw correctly, instead of keeping track of multiple variables at once.

Ok… I did have to make three of them before I got one that I was happy with, but that seems to be the perpetual theme of this build anyway. 

Warning: Some pristine spruce longeron stock was harmed in the testing of these ABS jigs.

Yep, the nail holes were 3D printed right into the sides.

Pretty wild, isn’t it? Let see it in action on the fuselage!

The lower tab was mede for pushing against the sidewall (as depicted).

Using a drill to transfer the nail holes to the longeron

The small ABS "bridges" ensure perfect placement of the jig on either sides of the longeron

The "bridges" get cut away in the process

Almost there

ABS is pretty tough, and resisted the saw well with only minimal damage.

I decided to remove the busted foam and fill the space with flox.

Outer skin left intact to form the new old-shape

Tongue depressor wrapped in duct tape forms the back end wall 

Adding flox

Another tongue depressor forming the new inner sidewall (less sanding that way)

A day later, with all the masking removed.

Same thing after a little sanding

Outside view of the floxed part. The marked delaminated triangular piece will be replaced soon.

Using the same ABS jig on the table this time, I cut the longeron stock that would replace the broken one.

Last action this jig will ever see

On the inside of the fuselage I added two UNI plies that replaced the missing inner skin. This part of the repair was a bit of an academic exercise, since a big hole will be cut right over the top of the repaired section to fit the Center-Section spar.

Red is the UNI patch I just added, purple is the hole I will have to cut for the Center-Section spar.

With the new inner skin in place, I test fitted the replacement longeron on the fuselage, and with the tiniest amount of sanding I was ready to mix some flox.

Test fitting the longeron splice

Flox added to the joint

Longeron splice curing overnight

Same thing viewed from below

The next morning every distant doubt about lack of strength vanished after I tried picking up the fuselage by the new longeron splice uneventfully.

"Yep, that will do!"

Now, according to my engineer guide, the most important part of this repair was to take place. The replacement of the UNI outer skin (layer by layer, oriented accordingly to the original specifications) had to be carried out correctly.  A lot of this replacement fiberglass would eventually be swallowed by a gaping hole for the Center-Section spar, but some would not, and it would have to tie in to the lower layers as specified on page 3-22 of the plans, section 4.2 (Large Defects). 

How to fix large defects

A visual representation of how the fix is to be accomplished

The first area of concern was a slight delamination of the outer skin. This would have to be cut off and the surrounding area included in the overall skin replacement scheme.

The top right corner has to go

Foam and flox is revealed underneath

UNI plies exposed. Note the different orientations of the plies.

Corner absorbed into the overall repair scheme. The purple area will be removed for the CS spar

First off though was replacing some missing foam with micro.

Damming the corner

Building up the foam with micro

Whole area prepped and ready

Then came two UNI plies in the original orientations down to the 3” line, followed by a bigger BID piece extending 1” further (4” line on the fuselage). Plenty of peel-ply went over it all.

First UNI layer cut to size to the 3" line

Peel-ply going over the 2 UNI and 1 BID plies

A view from inside the fuselage

A day later the peel-ply came off, and the repair to the outer fuselage skin was completed.

Peel-ply removed. Note the large area to be removed for the CS spar.

Close up of the repair

Repair seen from the front looking back

The inside of the longeron still laid bare, but would receive one ply of BID, plus one more over it all, next.

Inner part of the longeron will get 2 plies of BID

Area sanded and prepped

Pure epoxy mixed with a little Cab-O-Sil to prevent runs

Cab-O-Sil is a fumed lightweight silica thickener used to reduce the flow of epoxies on vertical surfaces.

Flox applied from a plastic bag

Flox fillet under the longeron

2 ply BID pre-preg

Pre-preg folded to follow the contour of longeron and sidewall

Pre-preg plastic backing removed

Peel-ply over the repair for the overnight curing cycle

After it cured, I removed peel-ply, duct tape, and newspaper. Then I sanded the rough edges smooth, and this is what I was left with...

I'm very happy with the results of this repair.