Sunday, April 4, 2010

Performance Indicator Panel

Cockpit instrumentation is normally divided into two broad categories: control panels house switches, levers and other actuators used to control the aircraft, whereas performance indicators tell the pilot what the aircraft is doing (not a bad thing to remember if you ever have to land a plane in an emergency: in the dizzying dozens of dials, what are you looking at? Control or performance indicators? Start there and you may actually make it!).

In balloons, three performance indicators are most important: vertical speed, indicating how quickly one is ascending or descending; altitude, indicating the distance from the balloon to the ground; and temperature, usually that of the gas inside the balloon but also sometimes of the ambient atmosphere outside the balloon.

I've just built a prototype of my main performance indicators panel. Why this is a long, vertical panel I'll go into later; for the moment I'm just showing the construction process (after months of researching and buying the various instruments).

First, opening holes in the wooden panel to admit the main indicators. A simple wooden panel is strong and light:

Second, after the main panel was painted black (the less visual clutter, the better), applying a largely matte-black foam slab to insulate instrumentation, a bit, from vibration:

Third, using machine bolts to attach the various instruments:

Fourth, most indicators locked in:

Fifth, all instruments locked in:

From the top:

1. OAT Indicator (Outside Air Temperature): later, this will be paired with a BGT (Balloon Gas Temperature). The relationship between ambient (atmospheric) and balloon gas temperature is important for a number of reasons.

2. VSI (Vertical Speed Indicator): indicates the rate of climb or descent in hundreds and thousands of feet per minute.

3. ALT (Altimeter): indicates the distance (in feet, here) between balloon and ground; this is entirely based on atmospheric pressure, and the altimeter has to be properly adjusted before flight (and, preferably, before landing, with information radioed up from the ground, before landing). This altimeter is good to 25,000 feet.

3a. PRESS 1 (pressure gauge), the white dial mounted on the left margin of the panel: this is a simple vacuum pressure gauge, indicating ambient atmospheric pressure in mmHG, a useful check on altimeters (3) and (4).

4. ALT (Altimeter 2): also indicates the distance, in feet, from balloon to ground, but this altimeter is rated to 50,000 feet and 'takes over' after ALT (3) (see above).

5. CLOCK. This is simply a timepiece; it is rated for military aircraft and designed to withstand al variety of temperatures, pressures, and shocks.

To the right, a snake-neck lamp is attached to illuminate the indicators in the dark. This is a back-up for a general illumination system I will build in later.

In the final photo I'm wearing a pressure glove with a pressure-restraint coverall; a third, fireproof overglove will cover these; here I'm just getting a feel for how the glove interacts with the indicator panel. No detail may be overlooked.

Vertical space between instruments is left open for a number of indicator lights I'll be installing later.

Note that while I have an electronic VSI/Altimeter (which weighs 1/10th of the panel I've built above, and I carry strapped to my thigh during paraglider flights) the panel I've built above is entirely composed of instruments that are (a) independently operated (failure of one does not mean failure of another) and (b) require no power. I'll carry the electronic system as well, but I don't want to be knocked out by a simple power failure; also, the LED displays of electronic devices, in my experience, fail (go black, or blank) at -30F to -40F, temperatures I can expect at high altitudes; so I'm sticking with simple, independent, robust, non-powered devices as my main flight performance indicators.

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