Aero ‘lectrics

Lights, generator, compressor, action.


When we parted company last month, I started with some general comments about off-grid hangar power. As an electronics engineer, I have learned more about electrical engineering this month than Ive learned about anything in a long time.

Lets get started with the 120-volt dinopower source, a gas generator. A bunch of tradeoffs need to be considered when choosing a generator, such as average power output, peak power output, noise, run time on a tank of gasoline, size, weight and a couple of dozen minor items that enter the picture. All things considered, I came to the conclusion that a generator that could put out 120 volts at 15 amps average, 20 amps peak would be adequate for my operation. Multiplying amps (15) by volts (120) gives us a minimum of 1800 watts average and 2400 watts peak. There are probably a hundred makes and models of generator that fit that bill, but the $279 Harbor Freight reconditioned #55362 caught my eye for a couple of reasons. First, it is 2200 watts average (18 amps) and 2800 watts peak (23 amps). Second, it comes with a Subaru Robin engine, which has garnered good reviews for economy, quiet operation and dependability. The claimed run time on a full fuel tank at 50% load is 13 hours, or about 3 hours past my capability of wrenching on an airplane in the heat of summer and the cold of winter. (The HF #55362 is available new as HF #92863 for $399.)

If this is the generator of choice for you, Id suggest a quick trip to the Robin Subaru website to download the EX13 owners, parts and service manuals that do not come with the generator. Yes, I know it uses the EX17 engine, but the EX13 series engines include the 13, 17, 21 and 27.

The rascal is quiet, almost unbelievably so. With the generator genning out behind the hangar and the exhaust stack pointed away, I have to really listen to make sure the thing is still running. It is as thrifty on fuel as the manual says, and it is light enough for me to rassle around by myself. It stows neatly between the hangar rails, and the engine cowling is a cinch to move around with the $25 furniture dolly (HF #93614) fastened to the generator with four worm clamps.

The Air Compressor

This research took me nearly a month to complete, and Im not yet fully up to speed on compressors. To see what I mean about not completely understanding, you may want to visit for Richard Kinchs compressorratings.

Also, I admit to an informed mess-up. So as not to be one who doesn’t do his homework, I went to the HF web site and downloaded the owners manual for the 90385 air compressor. It says that it has a 2.5-horsepower motor, and that neatly corresponds (at 750 watts per horsepower plus an efficiency fudge factor) to the generator I chose. It also says that you should use it on a 15-amp circuit. That should have been my first clue that we were playing a numbers game. If you assume an 80% efficiency rate for the motor, you find that you need a 2300-watt generator. That will stretch the little generator right to the limits, but because it is not running continuously, we ought to be OK. But the owners manual says it can run from a 15-amp house breaker, which is (15 x 120) 1800 watts. Wait a minute: One calculation says 2300 watts, and the other one says 1800 watts. Whats going on? Go back to the truetex site and see what Kinch has to say about “rated” versus “real” horsepower.

At any rate, hooking this compressor directly to the generator caused the voltage to drop to 85 volts during run time. As if that wasn’t bad enough, when the pressure dropped and the compressor kicked back online, it killed the generator engine. Flat stopped it cold. That dog aint gonna hunt.

So I boxed it up and sent it back to Harbor Freight. To the company’s credit, as the running/starting amperage wasn’t listed in the data sheet, it picked up the FedEx charges for a returned item that weighed about 70 pounds with crating.

We are now without a compressor. What to do? Sears has a nice line of one-horse compressors that might work, and that generator certainly should hold it, no matter the starting surge current. And so it did. The little $99 Sears (#15310) 3-gallon compressor is the perfect ticket for pumping tires, doing compression checks, and all the light stuff that goes on in the hangar. Remember I said last month that if you are building a metal airplane and have to drive fifty bazillion rivets, then this series of articles isn’t for you. However, for average maintenance loads, this is the perfect combination. The generator output drops less than 2 volts under load and less than 5 volts when plugged into the shop electrical distribution system with a 50-foot #12 extension cord running from the generator to the distribution box.

Were I to do it over again, Id probably pay the extra $50 for the 7-gallon Harbor Freight compressor (#16639) and have more than double the amount of air available with the same exact motor the smaller 3-gallon unit has.

While we are on power tools, all I really want is a small drill press and a small grinder/buffing wheel. Harbor Freight comes through again with the keyless chuck bench drill (#44505, $60) and the 6-inch grinding wheel (#37822, $46) with a brass wire wheel (#47936, $2). Caution: Stock up on half-inch washers before you try fitting the wire wheel to the grinder. You will need about six of them to position the wheel so it doesn’t hit the housing when it turns.

Good News, Bad News

Neither the compressor nor the grinder or the drill press drop the voltage more than a couple of volts when they are on and operating, and thats not enough to even make the lights flicker. Oh, the lights? I thought youd never ask. Again, the Internet becomes our friend. An engineer named Don Klipstein (author of “The Contributions of Edsel Murphy to the Understanding of the Behavior of Inanimate Objects” [EEE (Electronic Equipment Engineering magazine) Vol. 15, No. 8, August 1967] and other lesser known papers) has written a bible, or at least the Pentateuch, on lighting of all sorts, from incandescent to fluorescent, to LED, to strobe and beyond. His web site ( and especially his (undocumented) lamp efficiency page at is a gold mine of lighting information.

It turns out that the old favorite 48-inch, fat 40-watt fluorescent tube (T12, or 1.5-inch diameter) has a light output after ballast loss of about 65 lumens/watt. (A lumen is defined by Klipstein elsewhere in his literature, but for me to quote it here would take forever.) Suffice it to say that the two tubes, at 80 watts together, give a total light output of 5200 lumens.

However, some clever engineer did his math and found that the most efficient diameter for a fluorescent tube is about 1 inch, which gives the most light for the least power. The 32-watt T8 bulb gives almost 95 lumens/watt, or 6080 lumens for both bulbs…an increase of 17% more light on 20% less power, for a total increase in efficiency of 37%.

The bad news is that if you go to the HandyManStore to buy a T8 fixture, it will cost you about $60 for a single two-bulb fixture plus the lamps at $3 apiece. Thats $400 worth of fixtures and lamps for a typical six-fixture hangar installation.

The good news is that if you buy a cheapo $10 shop light and throw away the bulbs and ballast, you can replace them with a $14 T8 electronic ballast (ballast #E-758-F-232, $13.45) and $1.20 T8 bulbs from the online store at The site also gives you a choice of color temperatures from warm 3000K bulbs to cool 5000K blue-white bulbs . The bulbs are a little over a buck apiece, but you have to buy a full case of 25, and the shipping cost adds about a buck a bulb to the price.

Even Better

The genius who invented the small-diameter T8 bulb made the pin dimensions at the ends of the bulb identical to the old T12 bulb, so the shop-light sockets will fit the small, efficient T8 bulb. Net at the bottom? Splice in the new electronic ballast, put the whole thing together just like the directions say, put your new bulbs in, and away you go; $28 for the whole fixture including bulb cost, or $168 for the whole hangar. The electronic ballast is warranted for 7 years, and the bulbs have a 20,000-hour life rating. Thats nearly six years at 10 hours a day, 365 days a year. No hassles for many, many years.

Wow. Generators, air compressors, drill press, grinder and shop lighting, all for less than $500. How can I top that? See ya next month, and Ill do my best.

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Jim Weir
Jim Weir is the chief avioniker at RST Engineering. He answers avionics questions in the Internet newsgroup–Maintenance. His technical advisor, Cyndi Weir, got her Masters degree in English and Journalism and keeps Jim on the straight and narrow. Check out their web site at for previous articles and supplements.


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