Aviation technical quiz

[Nick - A2A]

Fog, FG?

Yep, fog it is! As I say, it was a cryptic question, but then those METARs can be pretty cryptic too.

The French "mince" which apparently translates to "thin" or "thin slice" is used to denote shallow fog, or MIFG in METAR terms. As Esa says above, in the general context of foggyness, MIFG would more logically seem to represent mist and fog but that would presumably be too straightforward! At any rate, MI as a description can only be applied to FG.

Anyway, as MI is now taken, we need to rely on another bit of French to describe mist: "brume". Therefore we end up with another esoteric METAR code: BR.

There's a bit more info on some of the French terms which are responsible for the less obvious METAR codes here: http://www.cfidarren.com/r-metarmystery.htm.

Cheers,
Nick

[Levkovvvv]

It was probably one of the most difficult ones up to now. Especially since most of us use apps to decode METAR-s nowadays. A beer each for Nick and AKar (this forum has no age restriction so we'll settle for beer as a reward and not something more NSFW )

So here is one (and it is a lot easier than the last one )

Lets say you are cruising in your second generation fighter jet at mach 1.5 ish at FL 400-500 and you retard your throttle, and trim the airplane about 5 degrees nose down and start your descent. You start looking through your flight suit pockets to see if you lost that napkin on which that hot waitress wrote her phone number when your plane pitches nose up very sharply. The air is calm, plane is in perfect condition (or about as perfect as any of them get once they leave the factory). What happened?

[AKar]

That was an interesting vocabulary question. I went totally on wrong tracks for long, trying to figure out some device or system; at least I recalled and checked out these French metar codes, of which mince was one. Asked the other way, I wouldn't have remembered the words behind BR, MI, and the very common around here, GR and GS (and so on) - they just come from some French word!

Lets say you are cruising in your second generation fighter jet at mach 1.5 ish at FL 400-500 and you retard your throttle, and trim the airplane about 5 degrees nose down and start your descent. You start looking through your flight suit pockets to see if you lost that napkin on which that hot waitress wrote her phone number when your plane pitches nose up very sharply. The air is calm, plane is in perfect condition (or about as perfect as any of them get once they leave the factory). What happened?

Do we assume we slow down into low transonic regime, out of primarily supersonic flight?

-Esa

[Levkovvvv]

Yes, throttle is closed, we are descending, our airspeed goes down and speed of sound goes up, we will be hitting transsonics and below.

[AKar]

Then we'd run into 'inverse Mach tuck', so to speak, as its effect on the trim diminishes.

-Esa

[Levkovvvv]

I actually had to google Mach tuck, because we don't use that name in our language, but yes, that would be it. Airfoils center of pressure is at roughly at 25% MAC at subsonic speeds and roughly at 50% at supersonic speeds, and as your center of gravity remains unchanged, your aircraft's longitudinal stability changes dramatically as you pass Mach 1 either direction. It was an easy one, and I actually had this exact situation happen to me in the sim, although I can't remember exactly what I was flying... I was quite surprised it was modeled.

[AKar]

It was an easy one, and I actually had this exact situation happen to me in the sim, although I can't remember exactly what I was flying... I was quite surprised it was modeled.

In DCS? I've run into this kind of surprises with them. I really am overdue to do some review of flight mechanics and applied aerodynamics...my voluntary study time has been taken up by mostly electrical stuff recently, actually, as it turns out I finally end up beginning the MSc in electrical engineering by Autumn this year. Something I should have started way earlier. I'd figure it will keep my sim time properly limited.

Of course, a related question would be to ask why all-moving stabilators are generally used in supersonic airplanes of more recent design, instead of a separate elevator surface, but that would be too easy to google for.

Instead, let us throw in another easy one! After the MH 370 and AF 447, the discussion about flight recorders that would deploy in-flight, or at impact, to break free from the lost airplane to help to find the box itself, and that way, the remains of the flight eventually, and also to increase the survivability of the recorder itself, has surfaced again. Now, one flight recorder of this kind has been in widespread aviation use in line service for decades.

Name the type(s) that carries this.

-Esa

[Levkovvvv]

Actually, that was in FSX I am not really sure if it was the aerosoft EE lightning or something milviz. DCS Su-27 also behaves realistically in transsonic region but it has way too much artificial stability to have "mach tuck". You can however notice a very real increase in stability once you go supersonic. I shudder to think how little control you would have if it didn't have an all moving tail

As for your question, I know Navy airplanes use them, as far as military goes, and for civilian use, maybe some helicopters that fly over water, oil rig resupply etc, but I really don't know the types.

[AKar]

As this is rather googleable as well, I spit the answer I was after right away. The DFIRS (Deployable Flight Incident Recorder Set) has been standard issue on F/A-18 series since at least later C-models. It is installed on the upper rear fuselage, on the LH side of the centerline, in between the vertical tails. It consists of a rather survivable and floating body with slight gliding ability to soften the land, under a carbon plastic panel (one of those please-do-not-step-on locations), and is launched off the airplane by a small explosive. Either ejection sequence activation or triggering of an impact switch pops the box off, if I recall.

Not sure if a similar concept is used on the other types as well.

-Esa

[AKar]

Okay, let us have a one of good-to-know stuff!

In the video below, and in many others filmed in GA airplanes, one often hears an annoying whine on the audio taken from the airplane's system, that is clearly dependent on the engine RPM. Sometimes this is audible by other means as well.

[youtube]http://youtube.com/watch?v=1uFlReovvnI[/youtube]

Explain what this is all about.

-Esa

[Levkovvvv]

I knew you couldn't resist posting electrical stuff And it would have been a difficult one had I not had that happen to my car a couple of years back The noise is caused by the alternator, to be exact the ripple voltage that hasn't been properly rectified by the Gretz or other rectifying circuit or filtered out by the capacitors. It can be caused by a lot of things, including but not limited to faulty diodes, faulty capacitors, corroded or in any other way damaged connectors between the alternator and the battery, or any other stuff that increases system impedance, damaged antenna cables...

It gave me quite the scare when I first heard it while tinkering with my car radio, antenna wire insulation was damaged, and the battery was old with corroded terminals, while testing something, I let some other wire touch the spot with the bad insulation on the antenna and I was greeted by that rpm dependent whine. My first idea was that I was somehow hearing the spark plugs firing or impulses from the tachometer but I eventually replaced the antenna cable and it went away. Swapped the battery too, because winters over here can be a real bitch if your batt is weak.

[AKar]

Yes, the ripple is the answer. To me, this sounds strong enough over a wide range of frequencies that is not just ghosting over, jumping the wires capacitively. Obviously, if the audio amp likes it, it may just as well amplify a slightest signal. (Just to try this, take an active speakers' audio plug off and touch one or both of the signal collars with your finger. The AC hum should immediately sound.)

If we speak of generators such as found on the (T)182T, which are a subset of the group called alternators, we find them typically being three-phase, full-wave rectified. Their natural output after the rectification has a fundamental frequency of six times the alternator's running speed, and they in general show relatively little harmonics and low amounts of ripple voltage.

Now, in this case, the main buzz or whine appears to be at or about 4600 Hz at takeoff RPM. This is almost precisely six times the fundamental frequency of the alternator output, if we use the drive speed of 3.2:1 for the alternator in comparison to the crankshaft. Is it possible to trace a failure to a single component, or a group of components, based on this finding?

-Esa

[Levkovvvv]

I learned that thing about audio amp picking up anything the first time I foolishly disconnected my guitar cable from the guitar first instead of from the amp first while the amp was on. Sound of hot lead touching the metal jack boat (it is a Strat) is just torture, even worst than you touching the hot lead with your fingers.

Hm, well, if the alternator's fundamental frequency is 6 times it's running speed, you get 6 positive waves with each turn which would indicate that the rectifier is working as it should. And since the biggest "capacitor" in that system that should filter that out is the battery, we would say it is a corroded battery connection. And I say we because my buddy who is more of an electrical wizz than me is sitting here and he concurs I know it is kind of cheating, but this ain't a competition

[AKar]

In T182T, Lycoming TIO-540-AK1A is used. Its rated speed is 2400 rpm, or 40 Hz. Two alternator options are provided, we pick the one with 3.2 drive ratio. So, it would turn at speed of 128 1/s. Further assuming a three-phase alternator with six-pulse rectification, the waveform of the electrical generator system to the DC output would be a rised cosine pulsetrain with six pulses per rotation of the alternator. The fundamental frequency is thus 768 Hz.

In the audio sample, we have no significant spectral content at that frequency, albeit there is a nearby, moderate peak at about 670 Hz; without further analysis I'd consider it unrelated. The main 'whine' you hear on the audio track is at approximately 4600 Hz at takeoff RPM - too close to the expected sixth harmonic of the alternator's fundamental to ignore, and it makes the highest peak noise in the audio. Presence of even harmonics implies half-wave asymmetry - this is of course expected as we have a rectified circuit! If there was some significant issue in the electrical system, I'd expect much more stuff going on at lower frequencies. I can't quickly figure any process that would create strong sixth harmonics in particular in this electrical system, as much of the rest of the spectrum is clean, lacking telltale signs of certain fault conditions. It is certain that this noise is something that is not completely filtered, and the audio amplifier rejects it poorly; it is noteworthy that this close to the audio low pass filter's cutoff frequency in a typical aeronautical VHF radio, used to stick the audio into the transmission bandwidth; it may or may not be related. Getting deeper into the system, I'm sure the mechanism behind this noise could be traced with good certainty, but for now, I leave it in the air!



-Esa

[Nick - A2A]

Okay - here's a picture question. What do these three each have in a ratio of 2:1 instead of the more usual 1:1?







As a clue, I've left the identity of each in the image link.

For extra points, can you tell me company that's attempting to bring this principal into 21st century light sports aviation?

Cheers,
Nick