Step 12.3, Designing the panel

It turns out that making the instrument panel is not such a hard thing to do, but there are a few questions that must be answered first.  Probably the most important question from a monetary point of view is whether or not the panel will be IFR certified (Instrument Flight Rules).  The cost of an IFR panel is largely dictated by the FAA requirement that the GPS and or VOR  and or localizer receivers be certified to operate within the national air traffic system.  This alone can easily add $10K to the cost of the full featured Visual Flight Rules panel.

With that in mind, it looks like I'll be doing a VFR panel, but what exactly does this mean?  A panel equipped for VFR flight means the aircraft is instrumented for flight under Visual Flight Rules. Basically, the aircraft can be operated only under clear weather conditions. This does not seem to be such a large impediment to me, since I don't have a burning desire to fly in inclement weather nor am I certified to do so.

Next question, how will the panel be in instrumented, that is, what style of instruments to choose?

The most basic choice is between a Glass panel or a panel with steam gauges.  Despite the confusing name, a glass panel is not actually made of glass.  It simply refers generically to an instrument panel that displays its data on some kind of screen, usually an LCD like a computer monitor.  Likewise, steam gauges do not actually measure steam pressure.  The term 'steam gauge' refers, in a somewhat derogatory fashion, to the round dials that traditionally adorn the aircraft instrument panel.

One of the big advantages of a glass panel is that all or most of the instruments are displayed on a single screen.  This can simplify the panel and reduce weight.  It also permits new functionality to be added to the display without having to physically change the panel.  On the other hand, one big advantage to the steam gauge approach is that if the power should go off, the steam gauges won't know it -- they don't need power to operate.

Alas, I chose the glass panel approach because it is the more modern way, and it offers the most flexibility going forward.  Still, I'm not entirely comfortable with the little problem of power or the accidental loss of of it, so I will be adding a separate ASI (AirSpeed Indicator) and altimeter, both steam gauges.  

Deciding the location and number of switches and their function was the hard part for me.  Since I had already decided the number of displays I wanted (2) and the brand that I would be using, Garmin, a lot the functionality has already been decided for me.  What remains mostly reduces to turning on or off lights or avionics.  To get started, I made a list of the things I thought I might like to control:

• Master power / Alternator field enable
• Engine start / mag select -- key switch
• Auxiliary fuel pump
• Avionics master
• Autopilot Enable
• Control Column passenger enable
• Strobe light
• Navigation lights
• Taxi light
• Landing light
• flaps
• pitch trim
• roll trim
• Wig Wag enable
• Pitot heat
• Seat heat
• Dome light

To arrange the switches I followed the idea that the switches should be placed in roughly the order they might be used in a typical flight, left to right.  This order makes about as much sense as any other, with the added advantage that this order will utterly befuddle any Hebrew, or Chinese plane robbers :)

With all that settled, I then laid out the panel using online software from frontpanelexpress.com. Their software allows one to import the panel outline, which I downloaded from Van's in dxf format. This way I could be sure that the panel would fit properly.  I was also able to get some pre-made 'macros' (FrontPanel express terminology) for some of the switches and instruments online.  One RV7 builder in particular, Brian Chesteen, was very helpful.  After that it's just a matter of placing the components, or rather, the cut-outs for them.  There is a lot of measuring involved to get the sizes of the switches and other components properly specified.  A fair amount of guessing is required.  Like how much clearance is needed over here or how much to allow for powder coat over there.  And so it goes...

When the design is finished, the front panel express application totals up the machine time it will take to cut the panel and how many tool changes are required.  It then gives you the bad news, which of course is the price. 

This is the point where I returned to the design to try and reduce the number of tool changes, as well as reducing the sizes and the number of labels as well.  After a few iterations, I got the price down some and simplified the panel a bit.  Both of which are good things.  It is a simple matter to place the order right from the application.  All they really need is your credit card and shipping address. Everything else is specified in the application while you design the panel. The cost of the panel includes infilled engraved labels which are done by the CNC router along with all of the other machining.  Sending this work out probably saved me a month of work.

And the finished panel?  Glad you asked:

My next task is to begin attaching the instruments to the panel.  A bit of filing was necessary here and there. but there were no show-stoppers.  I made adjustments to the design file, so if I ever need to order another it should be perfect.

Bracketry to hold the radio stack.

AML34 switches by Honeywell.  Custom engraved caps by engravers.net

Once all of the switches are in, there is still a lot of wiring to do on the back side.

And here is the nearly completed panel being test fit.

Very briefly, the two LCD screens on the left half of the panel will display the flight data and moving map, respectively.  In the center are the backup ASI and altimeter. To the right is the radio stack containing (top to bottom) the auto pilot panel, the audio panel, and two GTR-200 COM radios.  The big hole to the right of the radio stack is the glove compartment.  Finally, across the bottom are the aforementioned switches.

A cover plate below the second radio allows room for future expansion.  The radio stack was sized to allow a GTN-650 NAV/COM/GPS radio to replace one of the GTR-200 radios + cover plate.  This will convert the panel to a fully certified IFR panel.  The panel is ready -- I'm just waiting to win the lottery now.  And if the lottery doesn't pan out I still have a long lost relative from Nigeria that died and left me a pile of cash.  I just need to pay some expediting fees and...

The one item on the panel that was not previously mentioned is the annunciator panel. It's the row of 6 colored switches located just above the second LCD display.   That will be the subject of an upcoming post.

Step 12.2, Trim and Pitch Servos

At this point in the build I'm installing boxes and getting ready to begin wiring.  The last two servos to install before beginning the wiring are the roll trim and the pitch servo.

The roll trim servo allows one to compensate for lateral imbalance.  This could be caused by uneven fuel use between the two fuel tanks which are located in the wings or even just flying without a passenger.  In the extreme, one tank completely empty and one full, the roll torque applied to the aircraft would be on the order of 21 gallons x 6lbs x 4' = 504lbs!  That's quite a lot of force trying to roll the airplane.  Those interested will note that aviation fuel weighs 6lbs per gallon and the 4' figure is just an estimate on  how far it is from the center line of the fuselage to the center of the fuel tank. Because the passenger sits so close to the center line of the aircraft he makes much less of a impact to the balance:  A 200lb passenger x 1' from the center is only 200lbs roll force applied.

Whatever the cause of an imbalance, the roll trim servo is designed to apply and opposing force by moving the ailerons in the opposite direction.  Fortunately for the servo, it doesn't take that much force to move the ailerons -- very little force at all.  Partly because of mechanical advantage in the linkage, and partly because of aerodynamic forces acting on the ailerons.

Because not much effort is required to trim the airplane, the roll trim servo (pictured above) is not so much larger than a large RC model servo.

The next servo to be installed is the pitch servo which is controlled by the autopilot.  The pitch servo moves the elevator via a connection to the elevator push tube.

The control arm length is adjusted  such that the servo is at its center position when the elevator is in its neutral position.

The pitch servo is installed behind the baggage compartment.  Pictured below is the servo and the push-tube that connects the elevator to the control column via the elevator bell crank.

With the pitch servo installed the next step is to attach it to the bell crank with the control arm.

Now the wiring begins.  Having the fuselage on the rotisserie makes accessing the interior a lot easier.

Pulling all the wires is a pretty big job which is made more difficult by the very small spaces that must be accessed.

At this point in the build I found it necessary to stop pulling wires momentarily to step back and complete the airplane's overall electrical schematic. Although proceeding without a schematic is much faster, it will inevitably lead to me leaving out some necessary circuit.

And so, a week or two elapses without much apparent progress as I pound out the schematic in my office.  Its a good time to think through how the various systems tie together.  Fortunately, I don't have to figure out everything myself as I bought some of the harnesses for the Garmin boxes from Stein Air to save some time.

But a lot of the wiring goes to the panel which has yet to be designed.  I can't finish the schematic until I finish the panel designed.  Next time: It's panel time.

Step 12.1, Fairing the canopy

When the canopy is attached to the frame the canopy's edge is flat against the wind.  A fiberglass fairing is constructed to smooth the transition from the cowling to the canopy providing aerodynamic as well as aesthetic advantages.

The process is simple:  keep adding fiberglass layers until the transition forms a smooth arc.

The loose fabric visible on the top is not fiberglass.  It's called peel ply.  The objective of peel ply is to protect the surface of the curing fiberglass when it is planned to add additional layers.  When peel ply is used it is not necessary to prepare the surface with sanding or cleaning prior to bonding the next layer.  Just peeling the peel ply off leaves the surface clean, and the texture of the fabric gives sufficient bite to adhere the next layer of glass.

The first few layers are made black with dye because the fiberglass overlaps the canopy edge and is visible from the inside.

After five or six layers the transition is beginning to flatten out into a nice curve.

The profile of the curve is made uniform with the aid of  the reverse side of a sanding block. 

The thick black tape protects the canopy from the sandpaper.

And finally, the canopy in its finished state.  Note the tie down strap.  It's keeping the canopy from tipping over and falling off of the roll around cart while I finish off the underside.