How to See in the Dark This section gives a bit of insight into how to make a model aircraft visible in the dark. It provides some very basic level information about how the human eye responds to light as well as presents some considerations for configuring a lighting system for maximum visibility and efficiency.

Understanding Visibility

Consider Figure 1. When a lamp is emitting light, the light travels away from the bulb in every direction within three-dimensional space. The only light that we can see is the light that happens to shine directly into our eyes. This means that from a distance, we can really only see one beam of light from the source at any given moment in time. The rest of the light is lost into space and is useless to the pilot. If there were four lamps mounted on the exterior of a model aircraft - one on each wing tip, one on the nose and one on the tail - a pattern much like the one depicted in Figure 1 will be visible. This works reasonably well, but is very inefficient and wasteful, and all that is really visible is four points of bright light.

Figure 1

Bright points of light rarely, if ever, appear as single points. Usually, the light striking the eye will be affected by (and scattered within) the eye, itself, causing the point of light to appear as a star, as if short strings of light are stretched outward around the perimeter of the point. This phenomenon can become extremely pronounced as a source of bright light moves closer to the observer. If a light source is too bright, it can create a situation where the light scattering in the eye can actually wash out other surrounding details. This glare is often evident in sunlight during daylight hours, especially while driving in the late afternoon, and can be rectified to a large degree by wearing sunglasses to filter the light. However, wearing sunglasses at night is not advisable.

Now, think of a light bulb up close. If the glass of the bulb is clear, then the filament inside will be clearly visible. If that is the case, then when the lamp is emitting light, you will notice that the light is coming entirely from the filament. It will appear as a glowing white squiggly wire. This is because the only light that you can see are the rays that are entering your eyes directly from the filament. Most of the light being emitted from the filament, however, escapes your detection and is scattered about in all directions, flooding the space around it. If the light reflects off an object's surface and back into your eye, then that object's surface will become visible. Reflected light is what enables us to see things around us.

With that in mind, imagine a light bulb that is white and opaque. Bulbs that are white have a thin powdery coating on the inside of the glass, which serves to diffuse the light from the filament. This allows a wider area of light to become visible from the bulb when the lamp is turned on. When the lamp is turned on, it appears to emit light from the entire surface of the bulb, and the filament is no longer prominently visible. What this means is that more of the light that is generated by the filament is intercepted over a wide surface area, redirecting more of the available light toward your eyes than you would ordinarily see from a naked filament.

This leads to Figure 2. Figure 2 illustrates an extension of the white bulb concept by placing a thin, opaque surface (such as white Econo-kote) between the bulb and the observer. The light striking the surface will become diffused, much as it would along the surface of a white bulb, reflecting in scattered directions. This makes better use of the light emitted from the lamp, because it captures a wider area of light rays, resulting in a larger visible area as opposed to a single ray of light from a distant, externally mounted bulb. By placing lamps inside an airframe, and using a thin film covering, the aircraft skin can be made to glow. The light emitted from such an arrangement is less intense than an externally mounted bulb, but there is more of it to see at any given moment in time, and the pattern begins to look more like an airplane. Figure 2 shows how such a lighting pattern may appear on an aircraft in the dark.

Figure 2

"Less is more" is the motto from my school of thought. Using the diffusion concept, it is now possible to illuminate more of the aircraft - making use of much more of the available light. But it is electrically expensive to power an incandescent lamp. It can take upwards of 165mA to power a single bulb, and I have heard of lighting configurations that draw as much as 1.8 Amps!

There are a number of disadvantages to using incandescent lamps in addition to their power requirements. For one, they can be vulnerable to vibration. When a piece of tungsten wire (i.e. the filament) is heated with an electrical current to the point where it is emitting a bright light, it becomes considerably weaker than when it is cold and dark. If a bulb mounted on a model aircraft were suddenly subjected to a strong adverse vibration from the engine or from a hard landing, the hot tungsten filament could be broken rather easily. Because of this risk, it is advisable to incorporate some redundancy into the system by adding more lamps, thus effectively doubling the already outrageous power requirements. Another disadvantage is that bulbs are rather bulky and can come loose from their sockets if wires are not directly soldered to them. It is also difficult to replace bulbs that are mounted inside the airframe. Regardless of any external influences, an incandescent bulb will eventually burn out over time.

So, how does one get more out of less? A little creative thinking is all it takes. Once you stop thinking that you need intensely bright lights to illuminate your aircraft, you can begin thinking about alternative light sources. The diffused lighting idea can be extended yet another step further. Usually, within a wing, it is only possible to light up a single section between ribs with a single incandescent lamp because of the opaque bordering ribs. However, a single lamp should produce enough light to realistically illuminate at least one third to one half of an entire wing panel (if not more). So, what does one do - use transparent ribs? You could. But try to think a little further outside the box. What is the real objective here? The objective is to spread light across the surface of a wing (or fuselage) to make the whole thing glow. So how can this be accomplished?

Consider the incandescent bulb once again. Imagine it mounted inside a wing, between two ribs, and that the wing is covered in white Econo-kote. How much light from the bulb is actually visible? There is still a lot of lost light, absorbed by the ribs, the leading edge, and the trailing edge, or being reflected internally by the wing covering only to be absorbed by one of those surfaces. Plus, the surface of the Econo-kote between the ribs will be quite bright - most likely much brighter than you really need! It's not a very efficient process. [I prefer Econo-kote over Mono-kote, because it is thinner.] The question becomes, then, what is the minimum amount of light needed to effectively light up a specified area? Well, imagine if you could slice the bulb up into tiny pieces and distribute just the light you needed to just the areas you needed it in. How much of the aircraft could you light up by doing that? I would venture to guess that there is enough light available within a single incandescent bulb to effectively illuminate one half to two thirds of an entire .40-size model aircraft! If only the light actually could be managed that way...

Fortunately, there is a solution. Instead of considering a single, massive light source, why not utilize a number of very low-power lamps? Imagine if you could take one narrow beam of light from an incandescent bulb and place it in a particular spot on the aircraft - even use it in a diffused manner to light up a small area. How many of those beams do you think you could gather from a single bulb and distribute within your model airplane? When you begin to think of it in this way, you begin to realize just how much light is wasted when you use a bulb. I keep revisiting this concept, because I want to convey a solid understanding about just how wasteful and inefficient incandescent bulbs are and to get you to start thinking more creatively.

So what is the alternative to using incandescent lamps? Light Emitting Diodes! LEDs come in many shapes and styles, colors and power requirements. I found a number of LEDs that I like to use from a surplus electronics mail-order company in Arizona. A single LED can produce ample usable light with as little as 10 mA (more or less, depending on placement). There is still some wasted light from an LED, but it is minimal compared to an incandescent lamp. The majority of the light emitted from an LED is focused in one direction if the LED is housed in a T1 or T1-3/4 package. This makes them directional, which means they can be aimed at a particular area to light it up (such as the wing film between ribs). There is still some light leakage around the sides of an LED, but not nearly as much (percentage-wise) as a lamp.

There are many advantages to using LEDs. They are very small and lightweight, they consume very little power, the light they produce can be directed where you need it, and they are not subject to vibration failure. They last forever (for all practical purposes), therefore, they can be mounted inside a model aircraft without much worry of ever having to replace them in the future. I like to use the super-bright red variety of LEDs, because they produce a brilliant, highly visible red light. A single LED will light up the space between two ribs quite nicely. I usually mount them in the wing in webbing between the upper and lower spar, aimed toward the rear. The light strikes the upper wing surface, illuminating the area, and ample reflected light illuminates the bottom area. One method that I found to be extremely effective is to cover the upper portion of the wing with white Econo-kote, and the bottom portion of the wing with transparent red Mono-kote (I have not tried green). This allows the reflected light from the underside of the white Econo-kote to be visible, as well as the light reflected off the ribs and spars. I also like to put one LED on each wing tip, facing forward, so I can see a reference of where the wingtips are when I make a landing approach.

To me, the biggest advantage to using LEDs in place of incandescent lamps is their power requirements. I use a five-cell battery pack in my lighting system to provide a 6-Volt power source. I can run three LEDs in series (with a resistor) on a 6V battery. If I run the LEDs hot at 20mA, that's 20mA x 6V = 120mW for a single strand of three. Compare that to a single incandescent lamp consuming 165mA @ 2.4V, or 396mW of power! Let's do a little math here just for fun. Imagine using an 8-cell battery, producing 9.6V, to drive four (4) incandescent lamps in series, drawing 165mA through the circuit. Total power for the circuit is 9.6V x 165mA, or 1.584 Watts! Now, consider the series circuit consisting of three LEDs that I mentioned above, consuming 120mW of power. If you divide the power used by the incandescent lamps (1.584W) by the power required for a strand of three (3) LEDs (120mW), you get 13.2 strands - let's call it 13 strands, even. 13 strands of 3 LEDs gives a total of 39 LEDs, which means that for the same power required to drive four incandescent lamps, you could drive 39 LEDs! Let's work the numbers backward for a 13-strand LED lighting system. Assuming each strand of three consumes 20mA apiece, that's 20mA x 13 strands, or 260mA. 260mA @ 6V dissipates a total of 1.56W. That's actually 24mW less than the 1.584W required for the four bulbs. And instead of a simple four-point lighting configuration, you now have an entire airplane glowing in the dark!

At least I do, anyway.

43 LEDs! 6V @ 155mA! 930mW!

If you really want to have some fun, drop the LED current down to 15mA per strand. 15mA x 6V = 90mW per strand. 1.584W/90mW = 17.6 (truncate to 17) strands of LEDs! 17x3 = 51 LEDs! 17x90mW = 1.53W! THAT'S 30mW LESS AND 12 LEDS MORE THAN THE 39 LEDS RUNNING AT 20mA APIECE! And at 10mA per strand, you could power 26 strands (78 LEDs!) on 1.56W! That's 260mA for 78 LEDs! Are you starting to get the picture?

It's fun to work the numbers and to experiment to see just how dim you can effectively run an LED and still be able to fly, but the important thing is to keep your head on straight. You need to maintain a margin of safety in your design, so try not to fly on the fringes. Your lighting system should be bright enough to see under any reasonable flying conditions.

As mentioned previously, contrast is key to visibility. As you gain experience in night flying, you will discover that nature provides a wide range of conditions that affect visibility. A friend of mine was once flying at night and was suddenly engulfed in a fast-moving, dense fog! Fortunately, he managed to bring his plane down in one piece - don't ask me how. Even clouds can present a problem. Clouds hanging over a city will tend to reflect the city lights. Therefore, if the sky is mostly cloudy or overcast and you are flying near a city, the contrast between your aircraft lights and the sky will be reduced, making it just a little more difficult to see your plane. Also, if you have any ideas about flying through the sunset (one of my most favorite activities), keep in mind that we are able to see objects because of light that reflects off their surface. Therefore, since the sun sets to the west, it is best to keep your plane to the east so that it is illuminated as much as possible by the diminishing sunlight. If the lights on your plane are bright enough, they will slowly become visible as the ambient light level drops and your eyes begin to adjust to the increasing darkness.

If you fly toward the west at this time, you face one of three problems. First, if the sun has not gone below the horizon, yet, you risk accidentally looking into the sun, causing temporary spot blindness (usually just long enough to bring a plane down rather abruptly). Also, with the aircraft between you and the natural light source, all you will see of your aircraft will be its silhouette, eliminating much detail, making it difficult to determine the orientation of the aircraft. Finally, as you look in the direction of the setting sun, the iris of your eyes will respond, shrinking your pupils, limiting the amount of light entering your eyes, making it even more difficult to descern details. During this time of dusk, you can actually sense the iris in your eyes opening and closing if you face north and fly back and forth between the east and west.

Flying through the sunset is not recommended until you have gained considerable experience flying at night and have a suitable lighting system. I have not flown through the sunset using the cyalume chemical light sticks, so I do not know if they are bright enough to become visible when you need them to. Dusk presents the worst possible lighting conditions since it is a transitional period between light and dark - and depending on your latitude, dusk can be relatively short lived (near the equator), or last a good, long time (closer to the poles). And depending on your climate and the time of year, you may also be faced with other problems, such as moths and other bugs that like to come out at dusk and get in your face (the only thing I can figure is that out here in the desert, they must be attracted to the moisture in my eyes, since that's where they always seem to be headed).

I fly with a relatively dim set of LEDs - about 11mA apiece - and my airplane (a Goldberg Eagle II) is still quite visible, even through dusk. If you keep in mind the principles conveyed in this text, you will be able to construct a lighting system that works well for you.

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Copyright 1999, William Hubbard

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07/28/2000