Challenger Ultralight Aircraft Construction

Panel Instrumentation

One of the decisions left to the Challenger builder is the design of the instrument panel. The instructions on page 22 of Section 2 of the assembly manual could be summarized with “Now go build an instrument panel”. It leaves considerable leeway for the builder. It does not give very much direction.The instruments included in the kit are quite Spartan and not really adequate especially with regard to engine monitoring.

Many modern instrument panels are quite complex. The modern ultralight pilot wants much more than was considered normal in the early 1980’s when the Challenger was first designed and manufactured. In fact, many Challenger instrument panels today rival those installed in much larger GA aircraft.

Above: A complex Challenger instrument panel installed by Lake Aviation.

The following are some suggestions and considerations for building an instrument panel on a Challenger.

Instrument Panel Material

The standard panel material supplied with the kit is high quality plywood. It is seldom used. Most builders use a piece of aluminum sheet. Aluminum sheet is heavier, but also stronger and easier to cut. It is better suited to use with a panel that is populated with a large number of gauges and switches. It is also better suited to mounting heavier and deeper instruments such as an EIS, VSI or a transponder.

The panel should be primed and painted whether it is aluminum or plywood. This will preserve the material. The paint should be flat not glossy so that the pilot does not encounter glare off of the panel surface. Bare aluminum can be primed using galvanized metal primer available at any hardware store.

Above: A blank aluminum instrument panel for a Challenger. This needs to be primed and painted. Instruments must be checked for clearance with the framework behind it.

The panel should be mounted slightly tilted upwards. If the panel in a Challenger is mounted vertically, it will be difficult for the pilot to read the gauges mounted lower on the panel. Test fit the panel first to ensure that you can read all the instruments when you are sitting in the seat.

The panel should also be trial fitted to ensure that the gauges do not interfere with any of the metal tubing forward of the panel. Especially critical is the curved aluminum tube (part number FWF) that passes horizontally forward of the panel location. This tube may interfere with the larger and deeper gauges such as the altimeter or a VSI (vertical speed indicator) if you chose to install one.

Finally, ensure that the panel does not interfere with your legs when you are seated.

The panel should be mounted on vibration isolating fasteners to protect those precision instruments from the bumps and vibrations that travel through the airframe. See the Accessories section of this web site. They are threaded on both sides with a rubber body in between. Four of these with 1/4-20 size threads have adequate side load capacity to hold the panel. They are threaded male on both sides and easily mate with mounting brackets.

What Instruments to Include

The gauges included with the Challenger kit are an ASI, Altimeter, Tiny Tach, 1 CHT, a fuel gauge, a compass, and the ignition switch. That would make a bare bones panel to say the least.

It is recommended that the aircraft have a few more gauges to fully monitor the engine parameters including both cylinder head temperatures (CHT’s) and exhaust gas temperatures (EGT’s) for both cylinders. Also recommended is a voltmeter and perhaps an ammeter to monitor the health of the battery and alternator.

A fuel gauge is optional as it should be remembered that fuel level gauges should never be trusted in an aircraft. The pilot must physically check the fuel tank volume with a dip stick or a visual level gauge before each flight. The best fuel gauge in flight is your clock. You should have well in mind how much time you have in your fuel tank.

Other items to include in your instrument panel planning are the radio, GPS, and electrical switches. Many builders opt to place the GPS and Radio on RAM mounts clamped to the two 1″ aluminum tubes in front of the cockpit (part number DT-1). This frees up panel space and brings these 2 instruments closer to the pilot for better viewing. It also allows easy upgrades to these two instruments without disturbing the panel. Radios and GPS’s are constantly having upgrades and enhancements. You are much more likely to upgrade your GPS than your CHT. Having a RAM mount will preclude a complicated panel installation change, especially if you opt for a larger model.

Electrical switches and any circuit breakers or fuses installed in the cockpit panel should include spare stations for future add ons. It is a very messy task to drill another 1/2″ hole in a panel that is mounted to, and fully wired, in an aircraft.

Analogue or Digital

Finally, the instrumentation decision comes down to whether the builder will use individual conventional analogue “steam” gauges or a new all in one electronic engine instrumentation system (EIS) or even an electronic flight information system (EFIS) with built in engine monitoring parameters.

At one time, cost was a major deciding factor for this choice as the EIS units were much more expensive than the combined cost of a comprehensive suite of analogue gauges. That is no longer the case as EIS systems have come down considerably in price to the point where the two options are about even in cost. EFIS systems are more expensive but that is because they also include additional flight information not generally used in an ultralight like the Challenger.

So how does one decide between the two options: individual analogue needle pointing gauges or one flat panel display of digital readouts. The decision is not as simple as one might think especially in an ultralight environment. Let’s examine the pro’s and con’s of the two options.

Analogue Instrumentation

Advantages

Analogue gauges have the advantage of being quick and easy to read to give the pilot an instant indication of what is happening to the engine or the airplanes position. For example, the analogue CHT gauge will show in a glance that the engine is in the “green” and there is nothing to worry about. A quick glance at the ASI on final approach will confirm airspeed. The ASI is usually the big gauge beside the altimeter at the top of the panel. It is easy to find and read. The pilot’s eyes are kept looking up and out of the cockpit, which is where they should be looking in ultralight flying.

If an analogue gauge fails in service, it is only one gauge in a suite of many gauges. It is easily reordered and replaced. The cost and time lost are very minimal. The complexity of replacing an analogue gauge is very small. Three or four screws and a couple of wires that pull off and push onto posts at the rear of the gauge.

Analogue gauges are easy to install and wire up. Most have only 2 or 4 wire connections. The connections are often posts that the installer pushes electrical connectors onto from nice wire harnesses provided with the gauge. The installation wiring diagram is simple and easy to read.

I think
that a nicely laid out, well equipped panel of gauges looks really cool. My non-pilot friends that I take up flying always wonder in amazement at my panel and ask if I know what all those gauges do. I reply with a swaggering air of confidence that yes I do know their exact purpose and function.

Disadvantages

Analogue needle pointing gauges are not as accurate as their modern electronic cousins. They do not remember their readings so that they can be downloaded later to a laptop for a graphic analysis. A full suite of analogue gauges will take up a considerable amount of room on a panel in a small ultralight like the Challenger.

Above: An analogue panel from the rear. Note the need for the Altimeter and ASI to clear the curved frame tube at the top of the picture. The Compass is located at the top, well away from the electrical gauges to minimize magnetic deviation from their magnetic fields.

Digital Electronic Engine Instrumentation Systems (EIS)

Advantages

An EIS is a very small package compared to the suite of analogue gauges. It is a marvel of electronic miniaturization. It fits in a very small panel and leaves lots of panel room for other items such as the radio, GPS, a cup holder, cell phone, and so on. It requires only one hole cut into the panel and will do the work of 8-12 other gauges. That certainly simplifies the process of cutting panel holes and lining them all up.

Modern electronic gauges are generally more accurate than their analogue predecessors. The read out of the digital gauges is more accurate leaving no room for error. The engine RPM is not about 5000 rpm, it is exactly 5035 rpm at that very moment in time.

Many EIS units have a remarkable memory that can, upon request, spit out a years worth of engine parameters so the aircraft owner can see a pattern of engine performance at a glance. This is a very helpful tool in determining the health of an engine, detecting a trend in changing performance, and trouble shooting any problems. There is little room left for doubt as to what exactly is happening inside the fire breathing monster.

Electronic instruments can be programmed to sound alarms at preset levels. For example, EGT and CHT’s can be input to alert the pilot when these levels enter the yellow or red zones. This may help prevent an engine failure or engine damage.

Disadvantages

Many pilots report that the digital read out from an EIS is distracting. The digital readouts are so accurate that they are constantly changing. The change in digits is noticeable in the peripheral vision and keeps drawing the pilots eye to the instrument. This often results in heads down flying that can be dangerous in the VFR world that ultralights inhabit.

Having an EIS puts all your instrumentation eggs in one basket. A problem with an EIS often means removing the unit from the panel and returning it to the manufacturer for service. The aircraft is often left unserviceable. One could fly a Challenger with a malfunctioning EGT gauge, but it would not be wise to fly without any engine monitoring in place. A new analogue EGT gauge will be in the mail and on your door step in a few days. An EIS return for repair will be weeks.

Many of us in the world of ultralight flying enjoy the pure simplicity of it. An EIS or EFIS is a modern complicated gadget that will require reading and understanding yet another user manual for yet another electronic gadget. Set points for alarms will need to be input. Outputs will have to be zeroed and calibrated. Multi-function knobs and buttons will have to be figured out. Some of us revel in that sort of thing, some of us loath it. To each his own in his own airplane.

Instrument Panel Layout

Although the Challenger Assembly instruction manual includes a reference photo of a Challenger instrument panel, RP-3F-2, it comments that this panel is not standard and is for reference only. No tips are given for instrument location. That is unfortunate because many panels are poorly laid out. The location of aircraft instruments should follow a logical pattern that reflects priority and importance in flight.

The primary, most critical gauges also happen to be the largest. They are air speed and altitude. The 3 primary rules of flyinr are Air Speed, Air Speed, and Air Speed. These rules become more important as your altitude gets lower. Therefore, these two gauges should be located at the top of the panel in the center where they are easily seen and read quickly with a glance. No head motion should be required, just the slight movement of the eyes.

Above: A well laid out analogue instrument panel with a full suite of engine monitoring gauges. The switches are labelled for easy identification. Nothing will interfere with the control stick.

Secondary gauges are the engine monitoring gauges such as the tachometer, CHT and EGT. These are important in monitoring the health of the engine but not in flying the plane. These can be located in a neat row below the primary flight instruments. This would include an EIS.

Other items such as switches, fuses, breakers, and the ignition, can be located below the engine monitoring instruments. Other instruments not critical to flight such as the hour meter or clock can likewise be in this tertiary location.

None of the switches should interfere with the movement of the control stick. Keep in mind that the control stick will have the brake lever on it and the ignition will have a key sticking out of it. Therefore, do not put the ignition switch in the center of the panel, in front of the control stick. This may result in interference and a control issue.

The location of the switches and ignition should be logical and not haphazardly placed in left over spaces. The logical place for the ignition and start button is on the left side near the throttle. This allows the pilot to hold the control stick with the brakes on using the right hand, while pushing the start button and adjusting the throttle on start up with the left hand.

Switches should be grouped according to similar functions. Nav, strobe and landing light switches are one group. Radio, GPS, intercom, and transponder are another group of like function. The master switch should be set apart. It should also be a switch with a capacity to carry the entire electrical load. 30 Amps is typical for
a Challenger.

The switches and breakers should be labelled. Some builders use label maker tape. Others use engraved plastic stick on labels. 3/16″ is usually an adequate letter size. The aircraft registration should be located on the panel in 3/8″ letters, front and center.

What To Do With Avionics and Electrical Equipment

The Challenger kit does not have any provision for placing the electrical equipment such as the voltage regulator and electrical wiring buses or additional avionics such as an altitude encoder. The builder must come up with an idea for locating such equipment.

A solution to this is to build a platform in the nose to mount electrical equipment. This platform can be made using 1/2″ aluminum angle and .025″ aluminum sheet metal. Be sure to leave enough clearance for the nose cone, the pilots feet (with boots on) and any long instruments mounted on the panel (including room for wiring harnesses). Also, if you have the flip up nose cone option, you must leave clearance for the hinge.

The platform can be about 14″ wide and can be 6 to 10″ in depth. Do not make it too high above the pedals or you will not leave enough room for the electrical components. Small sheet aluminum squares on the sides will give the platform framework rigidity and stability.

Above: The Avionics Platform Mod in the Nose of a Challenger

Conclusion

A well designed panel will make flying easier, more pleasurable and safer. A well equipped panel gives the pilot peace of mind. It will enhance the value of the aircraft. It will also be a source of personal pleasure when your friends say “Wow! Do you actually know what all those things do?”