I saw this image of a Cessna 172's tail posted on another question and it made me wonder why the beacon light is basically a cylinder, and not a more aerodynamic, teardrop shape?
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$\begingroup$ Biggest reason would be light spread. $\endgroup$– ratchet freakCommented Mar 10, 2015 at 15:23
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$\begingroup$ I thought about that, @ratchetfreak, but I would think that a clear housing would help, or, with the current lighting technology, the lens could be shaped on the outside with the appropriate bits on the inside to ensure proper light distribution. $\endgroup$– FreeManCommented Mar 10, 2015 at 15:47
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1$\begingroup$ Cessna 172's non retractable landing gear probably would have added much more drag than the lights. $\endgroup$– Lie RyanCommented Mar 11, 2015 at 0:11
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1$\begingroup$ Note that the two small antennas you see in the photo, would probably be of similar aerodynamic problem value, than, the shaping of the beacon. $\endgroup$– FattieCommented Mar 11, 2015 at 3:08
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1$\begingroup$ @LieRyan: So? The entire fuselage adds more drag than the lights. Doesn't make it pointless to discuss the lights. $\endgroup$– Lightness Races in OrbitCommented Mar 11, 2015 at 17:55
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3 Answers
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That beacon lens (and most of the strobe lenses shaped like it) are actually Fresnel lenses.
They are optically designed to produce a specific plane & intensity of coverage, and tinted for a specific color. (The color, intensity, and plane are specified in the FARs & applicable Technical Standard Orders if you want to go hunt down the requirements.)
The lens optics are easier to see on the split (Red/White) lenses:
A Fresnel lens is not the only way to meet these requirements, some anti-collision lights do it by sheer intensity (while retaining the "jar" shape, mainly for installation compatibility), and as GdD pointed out there are more aerodynamic options available. The Fresnel lens method is usually adequate for smaller/slower aircraft however, and the drag penalty at relatively low speeds isn't significant enough for designers to worry about.
answered Mar 10, 2015 at 17:15
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Your assumption that all are round is faulty, plenty are teardrop shaped. 
You'll see round ones on many slower aircraft where streamlining isn't as big an issue, however on faster aircraft they are almost always aerodynamic.
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4$\begingroup$ It's interesting that some lights on small aircraft are streamlined (wingtip lights come immediately to mind), but others are disgustingly draggy (like the beacon in the question). $\endgroup$– voretaq7Commented Mar 10, 2015 at 21:20
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1$\begingroup$ @voretaq7 The wing is critical to lift so makes sense that you'd think hard while putting anything there that'd disturb the flow. $\endgroup$ Commented Mar 11, 2015 at 16:49
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$\begingroup$ As this just popped up in my feed again, I was rereading it and your understanding of my assumption was faulty. My OP was "Cessna 172... the beacon light" there was no blanket assumption that all GA aircraft lights are round and not aerodynamic. The question was about this particular light on that particular model. Anyway, back to letting sleeping dogs lie... $\endgroup$– FreeManCommented Apr 3, 2019 at 11:45
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Taking a look to the photograph you are showing not only the lights are not very aerodynamic, you can see as well that the fairing on the tail could have a softer transition, the rivets used are not the ones used in commercial het airplanes and transition between plates is also not smooth. You can see that the aerodynamics is open to improvements.
The answer is overall airplane optimization.
Cessna 172 is an small airplane of 3 passenguers and one pilot. Customers of this airplane will be mainly of 2 kinds:
- Small airlines making short flights between 2 places very close.
- Private use.
Notice that although the airplane has an autonomy of 1200km, usually is not able to compete in routes of that distance as airplanes like A320 will be able to offer a cheaper ticket. Is a typical airplane for travels between islands with low traffic.
In that context...
- For the airline, as there will no be a big traffic, the main cost will be amortization of the airplane, also trips between 2 points will be usually very short, so the most part of the use of the airplane will be in ground taxing the airport.
- For the private use, it will be an airplane not used regularly and the flying time, in the life of the airplane, will be really limited.
In the overall context of the lifetime of the airplane the fuel consumption (driven by the aerodynamic drag) will be smaller than the effect of the amortization of the airplane (capital costs) and maintenance.
In order ot reduce aerodynamic drag and fuel comsuption in general (weight reduction), that will imply extra costs on each manufactured airplane:
- Add-hoc design of components not using standards.
- Higher quality standards and control.
- Higher costs of development (NRC).
- High costs materials.
- More costly manufacturing shapes.
Basically the manufacturer (Cessna) needs to make a decision between:
- Making a relatively cheaper airplane with higher fuel consumption.
- Making a costly airplane efficient in fuel consumption
In that trade-off, for this specific model and my understanding of the customer objective, is more attractive to create a cheaper airplane.
answered Mar 10, 2015 at 22:08