Using Alternative Sources for Nav and Strobe Lights

By Scott Gettings, EAA 724

Background
Many experimental builders are concerned with the high prices and lack of flexibility when using products from Whelen or Aeroflash to light their aircraft. In some configurations, a complete nav/strobe system can run the better part of $1,000.00. Ouch. I'd prefer to spend that money on an engine.

Another consideration if you are building in a "lensed" wingtip light for reasons of appearance and/or aerodynamics is that the fancy streamlined light housings are just not necessary. They only add unnecessary cost and weight.

Strobe and nav lights have geometric visibility and intensity requirements for certified and experimental aircraft. These specifications are available from multiple sources, including Aircraft Spruce & Specialty. Basically, you need 360 degree strobe coverage horizontally, plus 30 degrees of vertical visibility. The side position lights need 110 degrees of horizontal coverage, and a white tail position light must be seen from 70 degrees aft. The intensity requirements are a little vague for experimental a/c, but 400 candlepower (cp) for strobes and 40 cp for nav lights appear to be acceptable values. These requirements can be met in many ways, most often by two "external" wingtip combination lights, or two internal wingtip and one tail strobe/light.

Light Intensity and Luminescence
In searching for alternative light sources, I found that bulbs may be rated in lux, lumens, candles, candela, foot-candles, millicandles (mcd) and candlepower (cp). This is confusing, since there are not good conversions among these measures. In the most simple terms, it appears that candles and candela are the same, and a candlepower is one candle of brightness measured at one foot. A lumen is the light produced by one candle shining in one square foot one foot away. A lux is 1 cd at 1 meter. Overall, it sounds like these terms are pretty similar for our purposes. You just need real bright lights on airplanes. For more information, see:

http://www.intl-light.com/handbook/registered.html

Strobes
The prices for aircraft strobe systems are truly shocking. For each location, you need the bulb, its lens, and the power supply. I began searching for alternatives to high aircraft prices since strobes are made for many other vehicles at a fraction of the cost (no surprise here). However, it is quite hard to get light intensity specs for most non-aviation strobes. If this were possible, you could probably find acceptable strobe bulbs in the $10-20 range and similarly inexpensive power supplies.More practical strobe alternatives may include non-standard vendors of bulb-power supply systems or using LEDs. If you plan to use a conventional system, you can get inexpensive bulbs that will put out >400 cp for around $30 at:

Rollison Airplane Company, Inc.
Bloomfield, Indiana
http://www.airplanegear.com/skybright.htm
or visit us at: www.RLSA.us

Call Rob Rollison to get just the strobe bulb if you want to mount it separately. Their power supply is around $150, and will push 3 strobes in flashing patterns. This is far more reasonable than Whelen's unit at around $400. Rollison also carries the 16-guage, shielded 3-wire strobe cable for about $0.50 per foot, which is far less expensive than anywhere else. You need this shielded cable to avoid RF interference.

LEDs
Light Emitting Diodes have come a long way in the last few years. Although LEDs were initially used only for low-light displays, higher powered lights have now come on the market. Unfortunately, the cheap "high intensity" LEDs sold by Radio Shack and most other vendors really don't help us much. Very recently, a company called Luxeon has produced inexpensive, truly high-intensity LEDs you can buy over the internet.

LEDs draw amazingly small amounts of current, such as from 350 milliamps up to 1 amp. Using this little current not only helps in the total power requirements of a plane, but also allows much lighter wiring to the nav lights. LEDs also come on in nanoseconds, last 100,000 hours and have no filaments to break.

LEDs can be used for strobes when both the intensity and visibility requirements can be met. Depending on your installation, this may require multiple LEDs and/or a reflector. For example, replacing a tail strobe/white nav light could be done with a small cluster of Luxeon LEDs. A strobe using LEDs will require a device to flash its power supply, which can be done using a variety of inexpensive options.

The newer, truly high-intensity LEDs can also easily supply the intensity and visibility requirements for nav lights. These can be wired directly since no flashing is required.

Wiring LEDs
Hooking up these lights is very simple and inexpensive. There are two main critical factors: you can't hook them up with the polarity reversed, and you can't over-drive them past their maximum current. You can push them a little, however.

You will need to know the voltage drop of the LED(s), which is called Vf (for forward voltage) and their normal current in milliamps (ma). This is on their spec sheets. Typical values might be 3.2 volts and 350 ma. You can hook as many LEDs in series as you want until the sum of their voltage drops equals your system voltage. For example, if you used four, 3.2 Vf LEDs in series, you will use up 12.8 volts. You probably have 13.4 or so volts in your "12" volt system, so this will work. The basic formula is:

V= V(system)-V(sum of LEDs in series)

If you only want to use 2 LEDs at 3.2 Vf, and you have a 13.4 volt system, you will have 13.4-2(3.2)= 7 volts. You have to dissipate the remaining 7 volts to avoid exceeding the maximum current for each LED. You can do this with a $0.10 resistor.

Ohm's law says V=IR, so
7 volts / 0.35 amps = 20 ohms

Get the nearest 1/2 watt resistor to your value (22 ohm in this case) and put it in series with your two LEDs. Be sure to hook up each LED from positive-negative-positive-negative so you keep their polarity correct. The resistor can be on either side of the LEDs. The LEDs will get a little warm, but the resistor will get hot so keep it from touching anything important.

Since the LEDs are so cheap, you may wish to simply put as many as you wish to "eat up" your system voltage. If you want to get fancy, you can get small, constant current drivers from a number of sources (below) to deliver an exact current. These drivers deliver a constant current to the LEDs, but seem unnecessary for our applications. If your system voltage drops momentarily, the LEDs will harmlessly dim for a second or two. In my experiments, they don't change that much with minor fluctuations. You can also wire LEDs in combination parallel and series if needed, as long as the individual Vf and current requirements are met.

An easy way to build your system is to place a milliammeter in series with your test circuit of LEDs. With a direct current readout, you can play with different size resistors and number of LEDs to optimize your system. You can also use the simple calculator at: http://www.ledsupply.com/techinfo.html . This company also offers LED drivers for less than $20. Another, less expensive supplier is American Backplane, 355 Bantam Lake road, Morris, CT 06763 (860) 567-1568. Neither source of drivers is as inexpensive as a resistor - or even another LED!

Sizing LEDs
The only company I found that offers high-intensity, affordable LEDs is Luxeon. See http://www.lumileds.com/luxeon/products/products_index.html and download the "Star" datasheet. (Log in as a "guest").

The most usable form appears to be the Star in the Lambertian (140 degree) light pattern. It is less than the size of a quarter, has a built-in heat sink, and has multiple solder terminals to choose from. Luxeon stars come in 1-watt, 3-watt and 5-watt sizes. You can only get red (at this time) in the 1-watt size. They put out 44 lumens, where the green 1-watt size only puts out 30 lumens. However, you can get a 3-watt green with around 80 lumens and a green 5-watt with 120 lumens! The green 5-watt have a voltage drop Vf=6.8 volts, so two of these could be hooked directly to a 12 V system. In my experiments, two of the red, 1-watt stars are so bright you can't readily look at them - bright enough! Other experimenters have measured the output from these LEDs with satisfactory results: http://www.jbwilco.com/Cozyweb/navstrobe.htm

For strobes, you could use the white Star III, which produces 80 lumens @1 amp and 3.9 Vf. Five of these would give you 400 lumens. They cost $15 each, so you could make a combination tail strobe / position light for $75 that lasted forever. This is one heck of a lot cheaper than the $180 Whelen tail combination light I bought - and no power supply or heavy cable. You just need a simple switching circuit to flash them.

The best place to buy these is at www.luxeonstar.com. They have the best prices right over the internet.

A combination conventional strobe and LED nav light wingtip with reflector might look like:

The Luxeon Stars and a sample driver alone are quite small:

Hopefully this short article will give builders some inexpensive alternatives to their nav and strobe lights.