Does radar altimeter work over the ocean? I'm wondering if radio waves are reflected by the sea. If so then the usual radar altimeter would work. If not, then is there any other fallback besides barometric altimeter?

According to the wiki article, for some reason radar altimeter uses radio waves (instead of microwaves like usual radar). This is why I put in the radio tag.

Yes.  In the Navy, our primary source of altitude indication under 5,000ft is the radar altimeter.  The AGL over the ocean conveniently also happens to be the MSL as well.


What course of action to take when having to go missed during IMC and lost comms? Under FAR Part 91 operations what course of action should a pilot take if radio communication is lost after being cleared for an instrument approach in IMC and then a missed approach becomes necessary due to inadequate visibility?

91.185 doesn't say anything about missed approaches but there is some information in the AIM 5-4-21:

  In the event a balked (rejected) landing occurs at a position other
  than the published missed approach point, the pilot should contact ATC
  as soon as possible to obtain an amended clearance.
  If unable to contact ATC for any reason, the pilot should attempt to re−intercept a published segment of the missed approach and comply with route and altitude instructions.
  If unable to contact ATC, and in the pilot’s  judgment it is no longer appropriate to fly the published missed approach procedure, then consider either maintaining visual conditions if practicable and reattempt a landing, or a circle−climb over the airport.

That indicates that ATC expects you to fly the published (or instructed) missed approach procedure if possible, or climb over the airport if not.

As for what happens after going missed and climbing to a safe altitude, that's up to you. Even if you were in contact with ATC, they would ask for your intentions because they don't know why you went missed and whether you want to try the same approach again or divert. ATC also doesn't know your filed alternate airport (see the AIM p. 5-1-24) and there's no requirement to go there anyway. It's up to you as PIC to determine the best course of action; ATC will help by keeping other aircraft away from you as far as they can.


Clarification on autogyro thrust formula I'm slightly confused regarding the formula quoted here:
How to calculate the amount of thrust of a autogyro during auto rotation?
When they say thrust is equal to the weight.  Do they mean use earth weight directly i.e. 50Kg or am I supposed to calculate thrust as weight force, i.e. F = m * g?
If its the latter can I just use summary of component forces to determine vertical thrust, resultant from a horizontal axis of travel?

In vertical autorotation, with a constant sink velocity, the lifting force produced by the rotor is exactly equal to the weight of the gyro, since any difference between weight and lift would cause an acceleration, and we're considering a constant sink velocity, with zero acceleration.
In a dive under autorotation, with a stable dive path, the force produced by the rotor is higher than the weight of the gyro, since the tip-path plane of the rotor is tilted back, so you have a rotor drag component that has to be added (vectorially) to the vertical component (equal to the weight) in order to calculate the total rotor force.


Could there ever be a fully solar powered airliner capable of transatlantic flights? Could a new or a modified existing airliner, e.g., a Boeing 737, be fully powered by solar energy and make the normal length trips aviation-fuel powered aircraft make?

Short answer: No

Long answer: No. Look at the solar impulse project, this is some of the best solar-electric technology that's out there, and it's barely able to carry one person, very very slowly. If you took an airliner, covered it in the best solar cells available and connected it to the best electric engines available it probably wouldn't even be able to taxi.

Some science here: The maximum energy density of the sun on the Earth is about 1300 watts/meters squared. About 30% is lost in the atmosphere so the best you'll get on the surface of the earth is about 900 W/m2. The 737, for example, has 102 square meters of wing area, that would equate to about 90000 watts (90KW) of sun energy hitting the wing. The best power conversion we have of solar to electricity we have is about 10%, so you would get at most 9KW of power from a 737 solar array. As we see from this answer it takes 90 MW (megawatts) of power to get a 747 into the air, assuming it takes about 1/3 or that (30MW or 30000KW) to get a 737 into the air, then that array will only generate 0.3% of the electricity needed, and that's the best possible case in the strongest sun. If it's cloudy or at night you are completely out of luck.

Any solution based on current battery technology is addressed here, and it's very much a no at this point.

Theoretically it is not possible that a solar powered passenger airplane could be as good as a jet-a1 powered passenger jet, this is because the energy from the sun is not sufficient even if you had 100% conversion to electricity. If battery technology were to become orders of magnitude better to the point its energy density compared to liquid fuels then it very well could, this is a long way off, the best batteries that are currently lab concepts are about 1/5 of the density of liquid fuel. Once batteries get there then solar power could charge them for the flight. It's actually just as possible that solar power could be used to make liquid fuel in a renewable way from CO2 and other elements in the environment. Bio-fuels are solar powered, but we can't make much of a dent in fuel use with them without taking food off of people's plates, and personally I'd rather eat!


How does the performance of an aircrafts propeller change with respect to distance from obstructions? I am interested to know if there are any changes in propeller performance (thrust, efficiency) with respect to distance from obstructions such as the fuselage or wings?

I am particularly looking for studies or experiments. Mostly interested in tractor/puller configurations but info for pushers would be nice to know too.

A propeller accelerates air, and the stream tube going through the propeller disc can be idealized as something which is initially wider than the propeller disc and contracts while being accelerated (Froude's propeller hypothesis). Now two types of obstruction are possible:

Obstructions to the flow near the propeller axis, and
Obstructions to the flow near the stream tube.

It is quite obvious that something which blocks the flow will reduce the propeller's efficiency, and since flow speed is higher behind the propeller, the same obstruction will reduce performance more if placed behind the propeller disc. This is why pusher designs show better performance with the same engine power.

But also objects near the outer edge of the propeller can reduce efficiency. When a wall on one side ahead of the propeller prevents air from flowing towards the disc, the flow though the propeller disc is disturbed and efficiency will suffer. Similarly, a wall past the propeller will prevent a symmetric outflow, and the stream tube will attach to the wall (Coanda effect). This can be used to the advantage of high lift devices on wings, but will generally reduce efficiency.

For experimental data, I recommend to read NACA reports like the one for engine cowling development or this one on fuselage-propeller interference.


What are Water Vapour Resistant (WVR) engine bags made from? What specific material are Water Vapour Resistant (WVR) engine bags made from?

The specifics will depend on the product. Bags by Aerotest are made from supported PVC textile and coated with acrylic lacquer.


Are the N1 and N2 tachometer generators installed on 727 / JT8D engines line replaceable units? Each JT8D engine on a 727 has two tachometer generators as part of the engine tachometer system to measure N1 and N2 speeds. Are these generators line replaceable units? How long would replacement of just one generator take?

Usually, they are. In case of the JT8D engine, the N1 tach generator (inside the nose bullet), atleast is a LRU. The AMT Society even has a competition which involves the removal of N1 tacho generator.


Weapon vs normal aircraft track I've read that some radars can detect stealth aircraft or aircraft with low radar cross section but are not powerful enough to allow a "weapons" track or allow a missile or another aircraft to lock on to it.
What is the difference between the 2 radar tracks? Is one less accurate in terms of pinpointing the aircraft range by a few meters or kms?

Virtually any radar in the world can detect and track a stealth aircraft as well as to provide a “kill chain” to direct and guide weapons at it.  The key is at exactly what range they can do this and that’s a function of the radar cross section of the target.  Stealth aircraft typically, depending on the bearing and azimuth a radar illuminates them from have an RCS on the order of a BB pellet; a conventional aircraft eg a SU-27 has an RCS around the size of a two story house.  Certain kinds of radar sets like low frequency radars can detect stealth aircraft much better than X-band radars can but are inferior for miniaturization or for providing weapons guidance, limiting their effective ranges.


What is the best way to reapply for a denied Class 3 medical certificate? To Become a helicopter pilot is a major life goal of mine. My aunt was a naval pilot, when she took me up in a helicopter it was one of the most memorable experiences of my life.

Background: Today I have a job as a software developer at a successful company, I have graduated college in engineering. I work out regularly and eat healthy, I am able to complete 15 reps of pullups. I have excellent vision. My temperament is mild. I am reliable and prompt at work and I treat others as I would like to be treated.

Last year I was denied my class 3 medical license primarily due to being prescribed an antidepressant, which I was prescribed in February of 2015 as an off label treatment for ADD, I never took a single pill as the side effects scared me, also I am not depressed so it was a little off putting. There were complications due to my having received some counseling for alcoholism in college. In reality I made some mistakes in college and did not drink responsibly on a few different occasions, I have never been a regular drinker. It was possibly the new environment and the fact that I had no experience drinking. I have never been arrested, I have no DUIs or any of that nonsense. Receiving this news was very difficult considering the cost, time, effort and the fact that I did not even take any of the medication.

Currently I am not taking any medications or being treated for any issues. Job is going well, I experience a reasonable level of fulfillment and even volunteer on occasion. Getting my pilots license is a major goal of mine. Recently I decided to cut down on drinking, I now almost never drink unless it's with family or old friends. Realistically that ends up being one or two drinks less than once a week, I would not mind completely abstaining.

My primary question is: would it be worth it to again pursue the class 3 medical? Or will I be denied once more at this point? If this is the case, are there additional measures I can take to ensure that I get this license?

Take steps and do your homework BEFORE applying for your medical through FAA MedXPress!
I suggest discussing your concerns with a qualified Aviation Medical Examiner or beforehand, assuming you are a member, with AOPA Legal and Medical Services. AOPA has an excellent list of resources on health conditions that may affect certification.
Because a possible history of counseling may qualify as treatment, consider the alcohol and drug abuse guidelines. Note the phrase complete abstinence.

1. To be considered for medical certification, you should write to the FAA, stating that you have completed at least one full year of recovery with evidence of complete abstinence after being treated for drug or alcohol abuse. Although the Part 67 regulations require that there be a sustained period of abstinence of no less than the preceding two years, pilots may be considered for recertification as early as one year after successful treatment with appropriate documentation.
2. The FAA will require the following:
a. Your complete treatment records and discharge summaries.
b. An evaluation conducted by a professional who has had training in diagnosis and/or treatment of addictions. These professionals include substance abuse counselors, psychologists or psychiatrists, other physicians with training in addictive disorders, or members of the American Society of Addiction Medicine (ASAM). (A psychiatric and psychological evaluation may be required if the initial evaluation is inconclusive).
c. Evidence of successful completion of an inpatient or intensive outpatient program with a documented commitment to abstinence.
d. Participation in an acceptable aftercare program consisting of individual and group counseling sessions for at least 12 months.
e. Establishment of a monitoring system that includes a physician with expertise in substance abuse disorders, and
f. Additional monitoring reports from employers, family physicians, or others, as well as alcohol testing when indicated.

The mental health section has a page on ADD/ADHD. Summary excerpt is below.

Because of the potential for significant underlying psychiatric or psychological problems and the side effects of medications, the FAA does not currently allow medical certification for a diagnosis of ADD/ADHD requiring the use of medication.
For medical certification consideration, you will need to have
discontinued all psychoactive medications for at least the preceding
90 days.  After being off the medications for 90 days, a
neuropsychological evaluation should be conducted.  The
neuropsychological evaluation must also include the following three
tests:

Trail Making Test
Wisconsin Card Sorting Test
Paced Auditory Serial Addition Test (PASAT)

If the airman is an adult who has been prescribed one of the
stimulant-type drugs for ADD, but has not been diagnosed with the
disorder, the medications will have to be discontinued for three days
before undergoing the neuropsychological evaluation noted above.

Because you may have a history of treatment with antidepressants due to the past prescription, the mental health section has a page on special issuance for depression with antidepressant usage. Summary excerpt is below with emphasis in the original.

Effective April 5, 2010, the FAA began considering individuals for
special issuance medical certification who are being treated for
depression with one of four specific antidepressant medications.  All
classes of medical certification will be considered, but individuals
who are granted a special-issuance medical certificate under this
policy may take only one type of antidepressant medication limited to
the following four medications: Fluoxetine (Prozac), Sertraline
(Zoloft), Citalopram (Celexa), or Escitalopram (Lexapro). All these
medications are SSRIs, antidepressants that help restore the balance
of serotonin, a naturally occurring chemical substance found in the
brain.
Increasingly accepted and prevalently used, these four antidepressants
may be used safely in appropriate cases with proper oversight and have
fewer side effects than previous generations of antidepressants.
While the focus of this policy statement is on individuals being
treated for depression, the FAA realizes that these four medications
may be used to treat conditions other than depression.  It should be
noted, therefore, that, in all instances, the FAA will continue to
consider applicants and make determinations on a case-by-case basis
under the special-issuance process just as it always has.
…
No regulatory changes are being made under this policy.  Further, the
FAA continues to believe that applicants requiring use of multiple
antidepressant medications or use of any other psychotropic medication
in conjunction with any one of the four specified in this policy will
not meet the criteria set forth under this policy.  The use of
psychotropic medication continues to be disqualifying under the
medical standards and special-issuance certification will be granted
only after thorough analysis of each individual case presented and
only when appropriate conditions and limitations are in place so that
the applicant may safely be permitted to operate an aircraft.  It
should be noted that as new information becomes available and
recommendations from the medical community change it may be necessary
for the FAA to again revise its policy.


Are emergency actions an appropriate part of a captain's briefing in a light aircraft? It is generally considered that giving a captain's briefing when flying passengers in a light aircraft for recreational purposes is a good thing to do. I'm asking about flying people who may not be used to light aircraft, such as friends and family (as opposed to other pilots, who we often fly with).

In general, I give a few pieces of information:

What will happen on the ground
When to avoid talking (i.e., when it looks like I'm listening intently and noting something down, when we're approaching to land)
Where we're going, what will happen during the flight
What to do if they feel unwell

Then there is the bit I'm going to ask about; people are used to hearing about "emergency exits", and evacuation procedures etc. when flying on an airliner, so I usually consider it prudent to supply the same sort of information about a light aircraft emergency:

How to open the doors if jammed by damage
To get out as soon as we come to stop
To move as far away from the aircraft as possible, even if I am unable

However, I'm starting to wonder if this sort of information would be scaring the bejesus out of my inexperienced passengers. In the next couple of years I'd like to fly my kids who are still fairly young. I definitely do not want to make it a scary experience for them when the time comes.

How do other light aircraft pilots approach this conundrum? Be safe, and supply the important information while possibly scaring passengers, or avoid the "nitty gritty" of an emergency knowing that it is a rare occurrence indeed?

It strikes me that I could provide this information if the time comes to make an emergency landing, however knowing my own limitations I know that if that time does come, I will probably be too overwhelmed with aviating to be thinking clearly enough to provide the important information.

What is the "right" amount of emergency briefing?

It sounds like you are covering the safety material you need to from a regulatory and good sense approach. Trying to convey an emergency exit briefing during an emergency is not going to work, you need to give it all at the beginning. If you get an engine fire on startup you need your passengers to be ready.

I find that people appreciate that you give a briefing as it shows a bit of "professionalism" that re-assures them, rather than scaring them it shows you know what you are doing. I do cover the same material, and I usually do it slightly tongue and cheek in the similar manner as a commercial airline. For example when I talk about emergency exits I point to the doors to the left and right, then say something like "if there is an exit behind you then things really have gone wrong". That's just my personal style, there's no hard fast rules on it. If people decide not to fly with you after your briefing maybe you are going about it wrong though.

The approach I take with small children is different than that which I take with adults. The first flight in a light airplane is a very novel experience for anyone, with kids I explain what the experience will be like a lot more than adults. An adult will understand what you mean by climb usually, but saying "the airplane will lean back a bit and we will go up" makes more sense to a little kid.


What is the loud noise of a helicopter when the rotors are not yet moving? What is the loud noise when a helicopter is ready to start flying, but the rotor is not yet rotating?

For example at Trump's "chopper talk", when he is intercepted by reporters on the way to Marine One? The helicopter nearby makes really loud noise already, which makes it hard to communicate between reporters an Trump - so it seems to be something important. It's not the rotor itself, as it's not rotating.

The sound you're hearing is the APU (Auxiliary Power Unit) running.

Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to be bigger and heavier in order to produce enough torque to get the engine moving fast enough. The bigger the engine, the bigger the starter (and the batteries to drive it). At some point, it becomes impractical to have an electric starter on a big turbine engine.

That's where an APU comes in. It's essentially a small jet engine. Small enough, in fact, to be started electrically. Once it's running, it feeds high-pressure air to spin the main engine fast enough for it to start. It can also provide electricity, run the A/C, etc.


Why do planes always land on rear wheels instead of the nose wheels? Various times, I have seen and felt myself that the plane always lands on the rear wheels, but not the front wheels. Why do the pilots land it on the rear wheels instead of the front? Does it have some advantage to it or is it a rule?

Is this required for all the planes (irrespective of size, type) to land only on the back wheels?

Aircraft land on the main wheels. For aircraft with nose wheel it is the back ones, but for aircraft with tail wheel (also called “tail-draggers”) it is the front ones. In either case the main wheels are very close to centre of gravity and carry most of the aircraft's weight, the nose or tail wheel only carries a small fraction of it.

The aircraft must land on wheels that are close to the centre of gravity (longitudinally). If they were not, the force on the wheels would create a moment that would violently pitch the aircraft. Actually, tail-draggers tend to bounce on landing a bit because there still is some moment left and in case of tail-draggers it pitches the aircraft up.

Early planes were almost all tail-draggers, because that layout is more robust, handles unpaved runways better and has less drag when the gear is fixed. It however provides poor directional control especially once the tail lifts off the ground during take-off roll. Later most designs started to use nose-wheel, because it provides better directional control, is directionally stable, makes the floor level when the aircraft is standing so loading is easier and visibility during taxi is much better and with invention of retractable landing gear the drag didn't matter any more.


Why are there so few aircraft that had inhabited wings? According to an answer by John Frazer

  The only successful plane with inhabited wing might have been the
  Ju-38

Is this true?

In the whole history of aviation there was only one aeroplane with inhabited wings (so that people were able to stay in the wings, not just for repair purposes)?

What are the reasons that there were so few of them?

There are a few more aircraft with seats in the wings, but you are right as long as passenger seats are concerned: they appeared only in the G-38. Now I could also mention the Soviet Kalinin K-7, which was supposed to seat some of its planned 120 passengers in its wing, but that design never went beyond early flight test of the first prototype.

Professor Hugo Junkers was convinced from the start that the best aircraft would only consist of the wing and engines, but the size of real-world aircraft did require a fuselage to place pilot and passengers. Only when wing thickness grows to 2 meters would it be feasible to place people inside (the root thickness of the Junkers G-38 was 1.7 meters, that of the K-7 was 2.33 meters). That was only possible with the largest aircraft of their time, like the Messerschmitt 323 which had engineer's positions in the leading edge of the wing.

The Illustrated London News article about the Me-323 from 1943 contained the picture above (source) which indicates the engineer's cabin between the two inner engines of each wing.

The simple reason for the scarcity of inhabited wings is the limited chord and thickness of practical wings. Back in the days of propeller airplanes, the relative wing thickness could be 20% or more, but with modern jet transports, even 14% are on the high side (this text contains an extensive collection of data). Next, the wing of airliners is full of other stuff:

Leading and trailing edge devices
fuel tanks
pumps, pipes and control linkages

which leave no space for passenger seats. Only in the days of slow aircraft with low wing loadings was the inclusion of passenger seats even conceivable, and only in the largest wings. The G-38 had a wing loading of only 80 kg/m² while the A380 has one of 680 kg/m², so its wing area is only three times larger than that of the G-38 and its maximum root thickness about the same. If you now consider that just 6 out of the 34 passengers on the G-38 could sit in the wings, it should become clear how infeasible it becomes to place passengers in the wing of even the largest aircraft of today.

Junkers went even farther in turning the blended wing into reality when they (together with Messerschmitt) had to submit proposals for a large cargo glider in 1940. While Messerschmitt scaled up their then-largest aircraft, the M-18d, by a factor of three to arrive at the conventional Me-321, Junkers opted for an all-new design which was only loosely based on the G-38 and blended the cargo volume into the center wing. Their Ju-322, however, turned out to be almost uncontrollable. In the haste of the development, nobody cared to check how effective the far too small and too closely coupled tail section was.

Theoretical concepts of aircraft which place their payload in the wings have focused mostly on freighters, like in this NASA study (PDF!).


In this MD-11 stall test video, are we seeing the tail stalling first? Watching this short stall test video for the MD-11, one can notice obvious buffeting (starting at about 0:04) of the tail section, while at the same time – at least apparently – there doesn't seem to be any major buffeting of the wings.
I've also read Why are aircraft designed so that the wing stalls before the tail?
Considering that vision (especially in a degraded old video) isn't a great gauge for what's happening, I thought to ask this question with two related components:

Are we really watching the horizontal stabilizer stalling first?
Either due to a bad design, CG issues, or trim errors, is it possible to have the tail start buffeting before the wing, but not stall before it?

The entire tail is shaking (not just the horizontal stabilizer) not because it's stalling but because it is in the turbulent wake of the main wing.  The tail is generating downforce as you can see from the up elevator.  The main wing's nose down pitching moment increases when the center of pressure shifts aft at stall, as well as the change in the overall center of lift outboard and aft due to wing sweep as the root stalls with the outer end still flying.
The increase in pitching moment exceeds the downforce that the tail is able to produce, and the wing wins the force balance tug-of-war with the tail, and the airplane pitches over.
If the tail itself stalled, the nose would pitch over radically from the near complete removal of downforce, as if the pilot suddenly shoved the stick forward. That's not what is happening in the video.  When this happens the tail's local AOA can increase further due to the pitch over and and a partially stalled tail can stall completely as if you sawed it off, and the airplane ends up pointed at the ground, as demonstrated in this testing in a Twin Otter.
I recall another incident in in a Twin Otter in the Canadian Arctic where tail stall was induced by the pilot pitching over hard after a zoom climb right after takeoff.  Extreme pitch rotation at low speed means very high local AOA at the tail's leading edge. The pitch over rotation cause the horizontal tail's stall AOA to be exceeded when the pilot re-applied up elevator, and once exceeded it just got worse, and the Twin Otter just pitched over straight into the ground.


What aircraft is this twin prop? Sorry it's the only picture I have.

The aircraft is almost certainly a Beechcraft B200 or B200C King Air, with a combined Cargo-and-Airstair Door, as can be seen in these photos:

(source)

(source)

The airstair door is essentially a door-within-a-door and opens downwards as usual:

(source)

The larger cargo door opens upwards:

(source)


Why are there so many ADS-B ground towers in western Colorado? I was reading up on Appaero's new ADS-B OUT transponder and clicked over to the Stratus coverage page for some reason. Then I glanced at the map, and noticed something interesting:

Western Colorado is, well, ADS-B tower central. What gives? Chicago and the northeast are close, but not nearly as dense.

As GdD commented, the cluster you noticed is not near Denver, but Montrose, CO. The reason for this cluster is Wide Area Multilateration (WAM) with ADS-B. This technique uses many sensors distributed around an area to provide better coverage in areas with mountainous terrain that blocks a single radar or ADS-B station. Western Colorado is one such area that experiences delays when ATC does not have sufficient radar coverage to support operations in inclement weather. Montrose is one of the areas that the FAA is using to develop this technology. This "constellation" serves the following airports:

Gunnison-Crested Butte Regional (GUC)
Montrose Regional (MTJ)
Telluride Regional (TEX)
Durango-La Plata County (DRO)

See also this technical presentation about the WAM project, and the lease for the tower sites.

Also, somewhat related, see this NTSB safety recommendation regarding ADS-B coverage serving SAR needs after a crash near Montrose.


How is a conflict between aircraft with TCAS I and II resolved? Let's say there are two aircraft in a risk of collision, one of them is equipped with Traffic Collision Avoidance System (TCAS) I so it will get only a Traffic Advisory (TA), however the second one is equipped  with TCAS II, and it will get both TA and Resolution Advisory (RA). What is the procedure to be followed if there is a risk of collision, the TCAS II aircraft must follow the RA, and the TCAS I must follow the ATC instructions or what is the procedure in this case?

TCAS 1 will only give a Traffic Advisory (TA). The crew will lookout for the other aircraft and take evasive action if necessary. They also may contact ATC for instructions. They will follow ATC instructions unless they see that the instructions will bring them into the path of the other aircraft.

TCAS II will give a traffic advisory first, then a Resolution Advisory (RA). The crew has to follow the resolution advisory and inform ATC of their maneuvering (e.g. TCAS climb, Big Jet 123). They are trained not to follow ATC instructions that oppose the TCAS RA.

Note that TCAS II RA's are always in the vertical plane; TCAS II can not give instructions to turn.


What are my take off minimums? Requesting for some clarification.
Part 91 operator. 0-0 visibility and ceiling minimums
Departing Rwy 15 at ADS
Can I depart with a standard 200 ft/nm cross DER 35 ft. since I don't need to comply with the 400-2 1/2 wx? (0-0 ...right?)
or
Do I fall under the std.(which from what I understand, is ...0-0?) w/ min. climb of 325' per NM to 1100?

What this tells me is that I can take off with any legal visibility/ceiling for my type of operation if I can maintain 325’ per nm climb. If I can only accomplish the standard 200’ per nm climb, I need 400’ ceilings and 2.5 miles of visibility, regardless of operation type.

Essentially it’s saying that there’s something out there that you’ll hit if you can’t maintain the 325’ per nm climb gradient. If you can, great, go. If not, you need the increased visibility to see and avoid it.


Altimeter disagreement. What's the crew procedure to deal with that? The crew is facing a situation, in which, the altimeters indicate different altitude (the port and starboard ones - cpt´s and FO´s). Is there any procedure to perform an IFR landing?
If we consider a light aircraft, what the crew will do?

Since you describe a captain and FO, I'm going to assume that you are asking about a larger aircraft, like say an airliner or business jet.

Part of the certification of these types of aircraft require them to have a third (standby) altimeter in addition to the two primary altimeters.

When there is a discrepancy, the typical procedure (keep in mind that a pilot should always follow the manufacturers checklist in these situations) is to cross-check the three altimeters and find the two that agree so that the bad one is then excluded.


In India, what is the difference between "local flying area" and "airspace"? Can anyone define what is a "local flying area" and an "airspace" and what is the difference between the two? It would be good if the definition is as per Indian rules.

A local flying area is airspace that is designated by local air traffic control for general flying not involving circuits and landing. In the US it is something that is loosely referred to as "the practice area." In this area aircraft can perform practice and other maneuvers, and air traffic control will be aware of it and can warn other aircraft to be on the lookout for this. In India it is common for these practice areas to be segregated for flying school aircraft, and medium to heavy aircraft.

To put it another way, all local flying areas are part of a country's airspace, but only a fraction of the airspace is designated as local flying areas. I hope this helps.


Is it legal to make a non-military stealth plane? Just as military camouflage can be legally worn by civilians on most jurisdictions (e.g., in the U.S.), can I create a stealth aircraft for non-military use? More precisely, can I register and fly the thing?

Yes you may, but you need to carry a mode C transponder while above 10,000 ft or in some types of controlled airspace and have it switched on. Which pretty much negates the stealth capabilities of your aircraft.

To quote the linked AOPA page:

  According to 14 CFR 99.13, no person may operate an aircraft into or
  out of the United States, or into, within, or across an ADIZ
  designated in subpart B unless operating a transponder with Mode C.
  Certain exemptions might apply to aircraft that were not originally
  certified with an engine-driven electrical system; see 99.13(d).


What is the correct equation for rotor solidity? I'm working in a project that uses"blade solidity ratio", or "rotor solidity", the problem is, there are several articles which defines the equation in a different way changing in the section of the Radius analysis. I found 3 equations that I will describe below:

$$1. \frac{Nc}{2πR}$$ $$2. \frac{Nc}{2R}$$ $$3. \frac{Nc}{R}$$
N=Number of Blades, c= Chord Length, R= Radius of the rotor

Which one is correct or there is some case where each of this equation is applied?
Thanks.

Rotor solidity is the area of the rotor disk that is actually occupied by blade area.

Area of rotor blades = $N \cdot c \cdot R$

Area of rotor disk = $\pi \cdot R^2$

$$Rotor \ solidity = \frac{N \cdot c}{\pi \cdot R} \tag{4}$$

So all three of the equations OP found would not be usual. Good reference books are:

Helicopter Performance, Stability and Control by Raymond R. Prouty
Principles Of Helicopter Aerodynamics by J. Gordon Leishman.

Both these books use equation (4) for rotor solidity. It is possible for a factor 2 to pop up in the coefficients which are usually considered regarding disk area and solidity: $C_T, C_P$ and $C_Q$. From Leishman:


Why do buses at some airports differ from conventional ones? Why do buses at airports differ from conventional ones?
Why do they have such wheels or cover wheels at all?

Source.

Apron buses are dedicated to transportation in the airport area, the one your show is located near the air museum at Salzburg airport / LOWS) in Austria. Similar buses are very commonly seen at airports:

Apron bus. Source.

The bus is traveling on short distances, it's maximum speed is 40 km/h, the price is lowered in proportion of the simplified design (more technical data).

Regarding the hidden wheels, considering the speed and the apron flat surface, this is possible for aesthetic reasons (many additional advantages come for free, e.g. fuel savings, protection against debris and stowaway). More importantly, the bus is a kneeling type type designed to accelerate passenger transfer. The floor can be lowered to:

To board and leave the bus more rapidly (no or small step).
To be accessible to everybody with no help, including when seated in a wheelchair (no lift required).

Protecting the wheels also keeps clear the space needed to lower the bus, and prevent accidents. This may be of some particular interest in a region subject to important snow falls, like in Austria.

Wheel access is provided by a panel:

Aero ABus

But this type of buses also exist with some or all apparent wheels:

Source.

Airport buses may be wider than regular road buses, they are not subject to public road regulation:

Source.

The model shown in the question can carry 100+ passengers in standing position (5 seats).

Source.

Apron buses can also have a bridge included (see also mobile lounges):

Source.

Unrelated comment: Salzburg, in addition of being the birthplace of Mozart and a famous ski resort, got an airport well known for its difficult approach during winter snowstorms, with the aircraft flying close to the mountain sides.


How to take into account wind in aerodynamic forces balance? I'm trying to make a basic simulation for horizontal drift of a falling object due to wind. I need the force balance so that I can find out the acceleration of the object from the wind.

The object is initially dropped with zero horizontal velocity with the only force acting on the object being forces applied by the wind which is a constant velocity. I would like to use an observer on the ground as a reference frame. The projectile will be dropped directly above an observer and will accelerate in the direction of the wind until reaching steady state (velocity of the wind).

I want measure how far the projectile moves in the horizontal direction while it falls to the ground. I am getting confused on how forces are applied to the object.

The way I originally thought the only force was wind pressure applied over the surface area of the object.

$F_{wind} = \frac{1}{2}\rho v^2 A$

I was unsure however, if there was any help from the drag force. Does the drag force add to the force from wind pressure? Is the wind force just the drag force with a different name?

EDIT: Adding some clarification to the problem setup.

Additional response to edited question: object is dropped with an initial groundspeed of zero: the only forces present as object falls are gravity and aerodynamic drag.  Drag is a function of airspeed, which has a vertical component as well as a horizontal component.  If you use the airmass reference frame, the ONLY significance of wind is that 1) the initial condition is that airspeed equals wind speed, and 2) during the time the object is falling there will be some horizontal translation between the airmass reference frame and the ground reference frame, which will affect the final answer when converted back to the ground reference frame.  See notes below, and links at end, about not being able to solve for vertical and horizontal accelerations independently of each other.

Read on for more (some content was originally aimed at a broader version of the question)  :

Cutting to the chase, is the object being dropped from a freely drifting balloon, or from a tower, or from an airplane?  I.e. is the horizontal airspeed initially zero, or not?  Dropping from an airplane is more like dropping from a tower than dropping from a freely drifting balloon, because the initial horizontal component of airspeed is not zero.

Once an object has accelerated to freely move with the wind, the horizontal component of airspeed is zero, so the wind exerts no force on the object. This makes calculations easier.  In theory, if the initial horizontal component of airspeed is non-zero, it will take a infinite amount of time for the horizontal component of airspeed to fall all the way to zero, just as it takes an infinite amount of time to fully achieve the vertical terminal velocity.  In practice, after a few tens of seconds the horizontal airspeed will probably be very close to zero.

If the object is initially released with zero groundspeed, then the horizontal component of the airspeed vector is initially not zero, but rather is equal to the wind speed.   It turns out that this has an effect on the object's vertical acceleration as well as the object's horizontal acceleration, even if the body is a perfect sphere!  (See links below for more.)

If the object is dropped from an airplane, wind or no wind, the situation is essentially the same as if the object is dropped from a tower in wind.  The key point is that the horizontal component of airspeed is not zero at the instant of release.

If horizontal as well as vertical airspeed, force, and acceleration components are present at the instant of release, then you need to take care to properly consider the true magnitude and direction of the net aerodynamic force acting on the object.  You need to write your equations in terms of the airspeed vector, which also is equal to the ground speed vector (including the vertical component) plus the wind speed vector.  You can't treat drag due to wind speed, drag due to horizontal ground speed, and drag due to vertical falling speed as independent entities, because drag is dependent on airspeed squared, and all these components of motion contribute to airspeed.

Ultimately it's up to you whether you want to work in terms of the airmass reference frame (which moves with the wind) or the ground reference frame, so long as your equations appropriately deal with the x, y, and z components of the airspeed vector.  It will generally be much easier to work in terms of the airmass reference frame, and view the ground as moving carpet down below, like one of those airport people-mover beltways.  Just bear in mind that IF the object is released with zero groundspeed, then the initial condition is that the horizontal airspeed equals the wind speed.  IF the object is dropped from a freely drifting balloon, then the initial condition is that airspeed is zero.  IF the object is dropped from an airplane, then the initial condition is that the dropped object's airspeed equals the airplane's airspeed.  Regardless of which is true, once the appropriate initial condition is set, forget about the wind, except for checking at the end to see how much the ground moved relative to the airmass reference frame, during the time of fall.

In other words, if the object is released from an aircraft with a known airspeed vector, and the airmass (wind field) is uniform, then you can ignore the wind while you do your calculations in terms of airspeed, and then you can add on the "drift" relative to the ground due to the translation of the airmass reference frame relative to the ground during the time of fall.

It appears from your question that you hoped to be able to compute horizontal acceleration components independent of vertical acceleration components.  In any case where the object's initial horizontal airspeed is not zero, that's not going to work.  Just as the horizontal airspeed at any instant affects the rate of change of vertical airspeed, so too does the vertical airspeed at any instant affect the rate of change of horizontal airspeed.  See links below for more.

On the hand, if you are just dropping objects from a freely drifting balloon in a uniform wind field, then you can ignore most of this answer.  In that case, horizontal airspeed, force and acceleration are zero.

These links should be of some use to you: see all associated links as well:

https://physics.stackexchange.com/questions/176513/calculating-wind-force-and-drag-force-on-a-falling-object/176515#176515

https://physics.stackexchange.com/questions/491600/flight-time-of-spherical-bullet-fired-horizontally-versus-dropped-vertically-e/491771#491771


Why don't aeroplanes use propellers with more blades like the intake on a fanjet? The air intake on a fanjet typically has many small angled blades. A wind pump in an American film might have many broad slatted blades. My desk fan has 3 very large scooping blades and seems quite effective.

It seems to me that if each blade provides thrust, the more blades you have, the more thrust you might expect. It also seems to me that a propeller with more blades could be smaller and turn more slowly, potentially avoiding going supersonic while providing the same level of thrust.

I ask because I'm building a drone and considering using six small multibladed props.

What are the downsides of adding more blades to propellers?

Every blade will create its own boundary layer and its own vortex sheet. It is more efficient to use fewer blades with deeper chord, because the forward part of a boundary layer contributes most to friction drag.

To keep the lift coefficient on the propeller blade sections in a reasonable range (0.6 to 1.0) for efficiency means that blade chord will be reduced, which will make them less stiff. Again, it will be better to reduce blade count to arrive at a more viable design. All the thrust the propeller creates is pulling on those skinny blades, and they must be strong enough to withstand this force.

Only when the propeller disc loading increases do more blades begin to make sense:
 When engine power increases, the propeller disc area should also grow, but this growth is limited by the resulting speed of the blade tips. Once the flow speed there becomes supersonic, the drag at this section of the blade increases without a corresponding increase in thrust. To avoid that the next best option is to increase the solidity ratio of the propeller, called also the activity ratio.

Make no mistake, this is bad for efficiency. But if there is enough power available, adding more blades is the best way out.

You are right, a lower prop speed allows to increase its diameter, but while tip speed will drop by less than the reduction in prop speed (after all, flight speed should not change), the available thrust from this propeller will drop by the square of the speed reduction, since thrust is proportional to the dynamic pressure on the blades. And thrust you get only from the circumferential fraction of the local speed at the blade; flight speed does not count here and does not help to mitigate the reduction.


How do door seals work? The pressurization in the cabin requires the doors have their seals working properly. How do they prevent pressurized air from leaking from the doors?

How do they prevent pressurized air to leak from the doors?

They don't. The seal does not have to be perfectly airtight.

The requirement is that it will create enough impediment to the air that wants to gush out, where "enough" is defined by the capability of the conditioning system to input new air into the cabin.

In other words, everything is fine as long as the air mass that gets out of the doors is less than the maximum air flow that the air conditioning system can input into the cabin.

Note also that, as stated on Wikipedia, aircraft are equipped with one (or more) "out-flow valve(s)" and "pressure relief valves": these valves intentionally let air out of the cabin to avoid over pressure in the cabin and thus they prevent possibly dangerous scenarios.


Has any accident been prevented by bending or breaking the rules? I wonder if there is an incident that pilots saved the plane or minimized the damage by bending or breaking the rules like Sully? (As it's known Sullenberger started the APU at first which was the 15th thing in QRH.)

Thanks in advance.

First I have to admit that I have no idea what FAA regulations said about passengers taking part in controlling an airplane in 1989, but I guess it was at least frowned upon, even if the passenger happened to be a pilot.

The United Airlines flight 232 made a stunningly successful "landing" at Sioux City airport, much due to the fact that they took an extra crew member from the cabin to control the throttles of their severely disabled aircraft.

Unfortunately 112 people lost their lives as the plane cartwheeled after a very rough touchdown, and one might ask what is so successful about that... well: 184 people survived a situation that was in subsequent simulations deemed to be impossible to manage.

The throttle operator was a UA training check airman Dennis Edward Fitch, riding as a passenger on this ill-fated flight. It has been stated in many sources again and again, that without his presence in the cockpit, the outcome would have been much worse, even the worst possible.

It just so happened, that Fitch had been practicing a similar scenario in a simulator after a fatal crash of Japan Airlines flight 123 in 1985. The reason of that crash was total loss of hydraulics, Fitch wanted to find out if an airliner could be controlled with throttles only. Luckily, it was somewhat possible.


What will the impact to GA be if DTC DUAT goes away? Data Transformation Corporation issued the press release below.

  FOR IMMEDIATE RELEASE

  DTC DUAT Contract Extension

  May 21, 2015 — Data Transformation Corporation, DTC DUAT which
  supplies the DUAT Service for the FAA for the past 25 years, has
  announced that it has been granted a 60 day extension by the FAA to
  continue to provide the FAA DUAT service. DTC DUAT has always been
  committed to making the DUAT Service better for General Aviation
  pilots by adding new features and services making it easier to get
  weather briefings and file flight plans. DTC would like to inform all
  of its users and third party vendors that we are fully operational and
  plan to continue operations during and after the 60 day period. DTC
  DUAT is pursuing alternative avenues to provide the DUAT Service
  capabilities to its users following the 60 day extension period.

  For more information on this topic or anything about DUAT, please
  contact our marketing manager, Doug Priestley, at (800) FAA-DUAT
  (322-3828) or by email at
  marketing@dtcduat.com.

If the alternative avenues do not produce the desired results and DTC DUAT operations cease, what will be the real-world, day-to-day effects on general aviation?

I think that there would be little practical impact, except for those people who are heavily invested in DTC DUATS specifically and have to switch to another system.

First, there are two DUATS providers: DTC and CSC. Even if DTC goes away, CSC is still available, at least for now, and it provides exactly the same service as DTC (see this question).

Second, Lockheed Martin Flight Service (LMFS) provides all of the information and flight plan filings that DUATS does, and also provides briefings with a human briefer. So there is a completely separate alternative to DUATS available, although admittedly switching tools always has some cost in terms of time and learning at least.

I assume the main impact would be on people who use DTC DUATS as their primary flight planning and filing tool and use it directly, rather than via another tool. For example, I use Garmin Pilot with CSC DUATS but I could switch to DTC simply by changing the data provider option. Some flight planning tools also support LMFS flight plan filing and I guess that more and more will in future, if only for competitive reasons. But if you use the DTC DUATS website directly and are very experienced and comfortable with it then switching to LMFS or anything else would have a learning curve.

But in the end, people won't quit flying or even cancel flights just because DTC goes away. They'll just complain a bit, choose a new platform (probably LMFS) and get on with it.


How do I find the floor of controlled airspace on sectional charts? So, I understand finding floors and such when seeing something a fraction like 45/10 or 39/SFC, but I've been reviewing and gotten a couple of questions that look like this:

What is he floor of the controlled airspace overlying the Coeur D'Alene Airport?

The answer is "At the surface" but I can't figure out where it says that?
The second question said this:

What is the floor of the controlled airspace overlying the Sandpoint Airport?

The answer is "700 feet MSL". Cant find that either.

It does not say it anywhere because the answer is not variable like class B airspace but lets dig in.

The question here is really trying to get you to identify the airspace that surround Coeur D'Alene Airport and we can see the dashed magenta line encircling it. If you take a look at the legend you will see that denotes

  Class E (sfc) Airspace.

The same goes for Sandpoint Airport which is surrounded by a faded magenta line. The legend again denotes

  Class E Airspace with a floor 700ft. above the surface


How does downwash affect angle of attack? Many aerodynamics textbooks, as well as many answers on here and similar websites claim that the downwash downstream of a wing induces a net angle of attack that is lower than just looking at flow direction and chord orientation would have you believe. They then often go on to say that lift is perpendicular to this "induced flow direction", explaining that the component parallel to the originally perceived flow direction is induced drag. I struggle with this idea, because my current understanding of lift dictates that downwash itself is a product of lift generation and therefore a product of angle of attack.
Also, when looking at visualisations of flow fields around a wing, we can see both upwash ahead of the wing and downwash behind it. Intuitively, I'm inclined to think that the downwash, being downstream of the wing, can't really affect the flow dynamics around the wing anymore. Conversely, the upwash, being upstream of the wing, should affect the flow around it, increasing angle of attack and therefore lift produced.

The only explanation I've come up with is this, though I'm not sure if it's correct:
Since any wing producing lift must introduce a net downwash on the surrounding air, the average air movement over the whole wing must also be negative. I guess my problem with this explanation is that I've always thought of angle of attack as a function of just chord orientation and flow direction. Is it correct to assume that, looking at the flow field closely around a wing, "traditional angle of attack" doesn't matter as much since the large induced flow velocities in front of the wing will make actual "aerodynamical" angle of attack, i.e. the angle at which the oncoming air actually hits the wing (as opposed to the angle between freestream flow direction and chordline) differ significantly?
I apologise if my question isn't very comprehensible, I had quite a hard time formulating it. In any case, I'd be glad for an answer, and I'll try my best to clarify what's unclear.

Is it correct to assume that, looking at the flow field closely around a wing, "traditional angle of attack" doesn't matter as much since the large induced flow velocities in front of the wing will make actual "aerodynamical" angle of attack, i.e. the angle at which the oncoming air actually hits the wing (as opposed to the angle between freestream flow direction and chordline) differ significantly?

Yes. Just witness the angle at which slats are pointing down: They are oriented to the local direction of flow which is strongly up at the leading edge when the coefficient of lift is high.

Typical landing configuration of an airliner wing, from an article by A. M. O. Smith, McDonnell-Douglas, in Journal of Aircraft, Vol 12 No 6, 1975. As always: Converging streamlines indicate accelerating flow and falling pressure while diverging streamlines show decelerating flow and rising pressure.
Note that the double slotted flap here is instrumental in inducing this steep local flow angle: Without it, the wing would not produce nearly as much lift and the suction on the upper side would be much weaker, causing less local bending of the direction of flow.
Note as well that the angle of attack of the airfoil is 0° while the streamlines entering the drawing on the left already have a marked upwash angle. The same happens in reverse at the right side where the flow shows a distinct downwash. This is a 2D simulation and at infinite distance to the airfoil the direction of flow is strictly horizontal. On both sides, because this airfoil does not produce induced drag in 2D flow (an effect also known as d'Alembert's paradox).
On a real wing, however, the tip effects reduce the lift curve slope so the local wing section will show a lower lift coefficient at the same geometrical angle of attack. Now suction and upwash are reduced (but still exist) and the air flowing off the wing leaves it with an added downward speed component. The far-field flow pattern now does not any more have the symmetry of equal up- and downwash. Instead, the downwash angle is increased to twice the magnitude of the reduced upwash angle because the influence of the free vortices in the wake must be added. The result is a backward tilt of the sum of all pressure forces acting on the wing which we call induced drag.


Mushing glide vs forward slip sink rate Source: Wolfgang Langewiesche - Stick and Rudder; An Explanation of the Art of Flying

I ran across a mushing glide in Wolfgang Langewiesche's - Stick and Rudder; An Explanation of the Art of Flying book and across this topic.

The author says: This gait is sometimes used by a skilled pilot, during an approach to a landing, to steepen the descent and at the same time make sure that the airplane will not pick up excess speed, as it would if the descent were steepened by diving.

I think that the same (or similar) effect can be achieved with forward slip, which, unlike mushing glide, points the nose down, thus giving a pilot a better view during approach while having a higher airspeed as a hedge from the stall. Besides better visibility and higher stall safety margin I assume (but with no scientific proof) that the descent angle with FS might actually be steeper, especially if a pilot slips at Vfe thus creating a large amount of drag.

That being said I wonder which flight condition/maneuver allows steeper approach and if there's any particular benefit of mushing glide to FS? What are your thoughts of this kind of approach?

It really depends on the aircraft. Some have plenty of rudder authority and sideslip very well, others are better at stall onset with still plenty of elevator and aileron authority.

Now first to the term "mushing glide": This happens at very low speed when the airplane flies far at the backside of its envelope. Flow on the inner wing is already partly separated which causes the high drag. The nose is up because of the extreme angle of attack and the stick is pulled back close to its rear stop.

Not every airplane will be pleasant to fly: Some will show abrupt stalling and cannot be held in this mushing glide. Others have less stall margin between inner and outer wing and the pilot risks an inadvertent roll when the outer wing has an airfoil with poor stall characteristics (think 5-digit NACA series) and one side stalls first.

But there are also many airplanes which can be held easily in this attitude and with responsive ailerons and elevator, too. Those are the ones which lend themselves to a mushing glide. I would only fly this after extensive stall testing at altitude, calm winds and when I know exactly how the airplane reacts to control inputs.

On the other hand, if there is plenty of rudder authority to achieve a substantial sideslip angle in a sideslip, I would't think twice and pick this approach. Ending it is possible with little to none altitude loss whereas a mushing glide needs to be ended at enough altitude to pick up some speed for the flare. Visibility is better and the risk of stalling a lot smaller.


What is the average angle planes wings they need to fly? The planes I am talking about are everyday passenger plane's, not fighters. Do you know what the angle needs to be for it to create enough lift so the plane can fly?

The angle of attack airplanes fly at while cruising is called the optimum angle of attack (around 4 degrees) which meets the max.(L/D).


Why weren't the rotor tips of the Djinn directly fed from the turbine's exhaust? The Sud-Ouest Djinn seems to be pretty much the only tip-jet helicopter to have any sort of commercial success. It used a turbine to drive a compressor, which itself fed slightly warm air to pipes in the propeller blades, which itself exited at the tips, making the propeller turn.

This patent from the 90's says that warm air would be more efficient, as exhaust efficiency is limited by the speed of sound, and hot air has a higher speed of sound. As such, the patent is about putting a heat exchanger between the turbine exhaust and the air compressor exhaust.

But then, why didn't they simply plug both exhausts together to go drive the tips, similarly to a high-bypass turbofan? Why simply evacuate the turbine exhaust instead?

The soot in the burned exhaust will eventually clog the ducting. The higher temperature will deform the rotor, or tougher heavier materials will be needed.

The stream of a  high BPR turbofan (ignoring the size) when channeled into narrow ducts will raise the stream velocity, but lower its pressure, which will be an overkill in terms of thrust needed.

Therefore "slightly warm" clean air is best.


What is the highest indicated airspeed ever achieved in a traditional aircraft? It is fairly well known that the SR-71 holds the record for highest speed for a traditional aircraft (not a rocket/able to land and takeoff under its own power). However since the SR-71 flies much higher than other aircraft its indicated airspeed is comparatively low.

I was wondering what the fastest indicated airspeed achieved is and by which aircraft?

The world low-altitude speed record was set by Darryl Greenamyer in a rebuilt F-104 on 24 October 1977 at 988.26 mph (869.67 kts; 1'590.45 km/hr) and it wasn't really at sea level (mud lake Nevada on a hot day density altitude can be quite high so we don't know KIAS exactly). That airplane is by most considered to be the fastest due to small wind and frontal area. And it might also become one of the fastest land vehicle ever as some clever guy thought that getting rid of the wings was a good idea...

The SR-71 had a designed airspeed limit of 500KEAS. It was design for high altitude flight and they did not bother about structural strength to withstand low altitude fast flight or high g loads flight as you can see in the full flight manual.
Other jet fighter were known to fly fast at low altitude during mission like the F-4 with operating procedures of 1.34 Mach (863 mph, 759.5kts, 1,389 km/h).
Finally more recently the F-16, not known as a fast airplane was nonetheless designed the limit of 800KIAS in mind. The canopy was meant to melt at 850KIAS and the engine supposedly would blow up at 854KIAS. And numerous reports show that the pilot had to pull back the throttle in order not to exceed the 800kts limit at sea level.
The reason why we don't have higher speed recorded is just that there are no need for it, the aircraft where therefore not designed to exceed "reasonable" speed and no-one was willing to push the limit further. The risk versus benefits to bet on some engineers over-designing their aircraft so that they would fly above their limit speed is just not worth it. But I'm pretty sure if you where to "floor" a MiG-29/31 or a f15 up to failure you would certainly exceed those speed. It's like the longest flight, some records won't probably ever be tried again.


What are the structures on the sides of and aft of the canopy on Israeli F-16s? I know Israel has highly modified F-16s, but just what are the structures on the sides and top, aft of the canopy? See this image of an Israeli F-16 compared to a normal one:

Vs

They're conformal fuel tanks, a type of external fuel tank designed for a specific aircraft type to reduce drag and other effects.

In general, they increase fuel load and free up stations for weapons or even more fuel.

In particular, the F-16 is a small platform with a modest fuel load and thus has a moderate radius even when clean. But when it's combat loaded with sensors and weapons, the additional weight and drag quickly eat into its combat radius, so one or two drop tanks are semi-permanently hung from its wings and/or belly. Don't quote me, but the CFT's almost double the F-16's internal fuel capacity.

The F-16 wasn't originally conceived for ground attack, carrying bombs for many of its missions, but it quickly grew into the versatile multirole platform it is today.

The USAF can better accommodate the F-16's short range because it flies a huge fleet of ~450 aerial tankers to service 2000 fighters, whereas the IAF operates just 11 tankers for 400 fighters. (Obviously other USAF a/c also require tankers, not just fighters.) Also, the IAF doesn't have the luxury of other nearby friendly air bases it can fly out of. Thus, they use CFT's.

With the benefit of hindsight, the original F-16 design team would probably have enlarged the internal fuel tanks from the start had they known how the F-16 would be used.


Does huge rotating AWACS radar affect flight characteristics? I would think that having a huge mass spinning around should produce massive gyroscopic forces when turning or pitching.
There must be some clever way of minimising or eliminating the effects... How is it done?

(An example of the sort of thing I am talking about)

On the E2 Hawkeye the dish does not rotate very fast (ie, very slowly), and so the torque is probably negligible.  Usually the radomes are designed to help generate lift, and while certainly not as effective as a wing, do offset their own weight quite a bit.


What is the theoretical maximum velocity of electric aircraft? I'm wondering about theoretical maximum velocities of supersonic electric aircraft. I think the fastest demonstration of any type of aircraft has been close to mach 10. This was done using a rocket engine, so propulsion was not limited by air characteristics as much as an electric motor + compressor's propulsion would be (I think).

Under best conditions; Assuming an incredibly high power to weight ratio of motors, extremely aerodynamic design, strongest materials, etc.

Theoretically, does anyone know what the limiting factors in the velocity of an electric compressor propelled supersonic aircraft would be (barring energy use)? Does it depend on size, etc?

Could it reach mach 2+?

Thanks a lot in advance for any input.

Since you seem not to be concerned about the usability of this hypothetical electric aircraft, let's make some simplifications:

The aerodynamics are so good that it does not create any drag. This is completely unrealistic but helps a lot to simplify things.
Structure, controls and propulsion are so advanced that they leave one third of the take-off mass for batteries. Who needs payload anyway?
Efficiency is 80%. Not even electric propulsion can be lossless.
Energy storage is the best current technology has to offer. This would be about 1.8 MJ per kg of battery for non-rechargeable lithium cells.

To accelerate a battery of the mass m to a speed v needs an amount of energy $$E_\text{kin} = \frac{m}{2}\cdot v^2$$
Flying Mach 2 at sea level is impractical; it helps to climb to at least 6.000 m (20.000 ft) to do so. This adds potential energy: $$E_\text{pot} = m\cdot g\cdot h$$
Conveniently, the result is independent of mass, so the potential maximum speed is $$v_\text{max} = \sqrt{2\cdot\frac{1}{3}\cdot0.8\cdot1{,}800{,}000\ \mathrm{m^2\ s^{-2}} - g\cdot h}$$
If we solve this for 6000 m, the speed at the end of the acceleration is 949.3 m/s, which is already Mach 2.966. If you prefer to use rechargeable batteries (1 MJ/kg), the maximum speed drops to Mach 2.15.

If you start to add a reasonable figure for drag, you won't even make it to the speed of sound before the batteries are empty. Currently, propellers are the most sensible way to convert electric energy into thrust, and large, low-speed propellers are the most efficient type. If you think to drive a conventional compressor with an electric motor, its subsonic efficiency is already far worse than the assumed 80%. Even its mass is not yet competitive with conventional gas turbines.


Is this "bearing" sentence utterly incorrect? There is often tremendous confusion between bearing, track and similar terms.
On Wikipedia, I saw the following sentence:
NOTE CAREFULLY, THE FOLLOWING SENTENCE IS WRONG...

In aerial navigation, bearing refers to the actual compass direction
of the forward course of the aircraft.

...NOTE CAREFULLY, THAT SENTENCE IS WRONG.
PLEASE NOTE I believe that quote is totally incorrect, do not quote this page!
I refer to the "where the aircraft is going/pointing" as either "track" (the vector of motion over the ground) or "heading" (where the nose is pointing). Whereas, "bearing" means bearing regarding some distant object, (that could be either relative or absolute), and does not relate in any way to "where the aircraft is going/pointing".
{Indeed, you could just be standing somewhere - not moving and not in a vehicle - and very much talk about "bearings" to various landmarks.}
In other words, I have always thought this diagram is correct, and the only usage.
My question for actual pilots, controllers, etc, is this! ...
In fact, am I correct that the quoted sentence above is totally wrong?
Further: am I being "too dogmatic"? Hence specifically: is it the case that, in aviation, perhaps you sometimes do use "bearing" loosely to mean track or heading?
Let me repeat myself: I'm pretty sure I understand what bearing means and what track means.  But the wiki quote in question, says the opposite: in fact, is it just plain utterly 100% incorrect, a typo? Or, am I naive... ie, you dudes sometimes do use "bearing" to mean what I call track?
{Just TBTC ... for me "bearing" cannot be used about "us". The sentence "our bearing.." is meaningless.  You use bearing about another object. "What's the bearing to blah..." [Of course, that could be implicit, ie, "is our bearing still 123?" meaning, I'm still talking about bearing to Sydney as in the previous sentence...].}
As a curiosity: physics programmers (for games, whatever) constantly use these terms, and of course, we have to use them really precisely like any programming. You can imagine a character or whatever (perhaps a car in GTA!) has all these qualities, they are discussed constantly in relation to physics of the virtual objects in all senses, AI paths of tanks, etc etc.  Next time I scream at someone "that's not bearing you idiot, that's track" I want to be on a sound footing!

Short answers

Here's my answers to your questions, in order:

Yes, you are correct that the quote is wrong.
Yes, you are being too dogmatic.

2(a). By the way, you are also wrong.
The quote is incorrect.  Bearing is properly used the way you describe.
I misuse words all the time, but it's not common for me (or anyone I know) to swap bearing and track.

Longer explanation

The quote isn't correct because magnetic track properly refers to the actual direction that the aircraft is moving, as read off the magnetic compass. However, you say that you refer to:

  the "forward course of the aircraft" as either track or heading. (Track, vector over ground, heading, z-axis of vehicle airframe.)

Track and heading are not the same thing, and neither one is necessarily the forward course of the aircraft.  Track is the actual vector over the ground.  Heading is the direction in which the longitudinal axis of the aircraft is aligned.  Course is the correct word for the forward course of the aircraft; the intended (not actual) vector over the ground.

So you've got the definition of bearing right, but you've got track, heading, and course all mixed together.

I've been known to say "bearing" to mean "track", but it's usually only just after I've said "What be our", and just before I'm about to say "ye scurvy dog?"


For a modern airliner, wings are tested to withstand 150% of the limit load. What kind of weather would produce turbulence at 100% limit load? I often see people citing the wing flex tests, but with very little reference as to what kind of forces they're designing the plane to take.

I am specifically referring to modern day jetliners such as the Airbus A330.

The wings are tested to withstand 150 of the ''most extreme'' forces the plane will encounter. But what does this mean exactly? Not too long ago, a 777 experienced extreme turbulence, and the aircraft experienced vertical accelerations of +3 G. Would that have still been under limit load for the 777?

The 1.5 relates to the margin between permanent bending (limit or yield load) and breaking (ultimate or breaking load).  Transport Airplanes are good to 2.5 limit load at gross weight.  Pulling more than that may or may not bend the wings permanently.  For ultimate load (pulling the wings off, it's 3.75Gs minimum using the normal 1.5 safety factor)

Those are minimums.  There is a lot of safety margin in the stress calculations so I would not be surprised at all to see an airliner take 3Gs without permanent damage even at max gross.  If the airplane is below its maximum zero fuel weight (a function of the pax and baggage load, which is where the wing bending is coming from) there is more margin, depending on how light you are.

Then there are the two speed limits to protect the structure to consider.  Maneuvering speed is the speed below which a sudden maximum elevator input will reach stalling AOA (thereby unloading the structure) before limit load is reached.  Turbulent Air Penetration Speed (TAPS) is the speed below which a theoretical worst case vertical gust will cause stalling AOA to be reached before limit load is reached.

As far as weather conditions, the biggest risk is flying into a thunderstorm cell and hitting extreme vertical gusts (mainly the up going air shaft in the center of the cell), and next to that would be Clear Air Turbulence which is usually related to crossing jet streams and passing through the shearing turbulence sometimes caused at the boundary between the jet stream and the adjacent air mass (jet streams reside in the little corner at the very top of a frontal boundary, in a little triangle of space at the top edge of the warm air mass, next to the adjacent cold air mass and the stratoshere above).  Another one is crossing rotors (spinning horizontal shafts of air) that reside below the crests of mountain waves that form downwind of mountain ranges at high altitudes.

For that eventuality you have to slow to TAPS any time you expect to hit severe turbulence like picking your way around thunderstorm cells or flying into areas where Clear Air Turbulence is reported.  In the absence of a CAT report, a cautious capt may slow to TAPS anyway if the crew notices that they are passing across a jet stream, just in case (you know you are crossing a jet when your ground speed suddenly changes and the autopilot starts making a sudden heading change to stay on track - it all goes back to normal when you leave the jet).

In theory, staying below TAPS provides protection against permanently bending the wings same as staying below MS protects from bending the wings from a full on elevator input.  If the 777 in the incident hit 3 Gs, it suggests that they were above TAPS when it happened but even if it was fully loaded, with the fudge factor in the structural design it's not surprising it wasn't bent from an encounter only half a G above limit load.


Turbo-electric engine vs high by-pass-ratio Turbo fan, which config. would seem more efficient? (Liquid Hydrogen Fuel) Would be a turbo-electric config using 2 turbofans as generators for a 150 pax. aircraft more efficient than a very high bypass rationed Turbofan that burns hydrogen?
If not, then why are all companies like Airbus, Rolls Royce and Safran etc. working towards this?
Also, why would they just use their Highby pass Turbo-fans, that ways they don't have to invest billions and also have very high efficiency and less pollution (CO2, Nox, etc.)?
If we can argue about 15 years down the line, what would be a better solution? And why?

Turbo-electric configurations do not offer any meaningful advantages for commercial airliners.

Aircraft engines have extremely high power-to-weight ratios. These would be greatly degraded by adding motors and generators.
For instance, the RR T406 offers power in excess of 10 kW/kg. Large turbofans are even better. Most electric generators and motors are worse. The best have the same power-to-weight, but since they only transfer power, this increases the weight of the engine system by a factor of 2.5-3.

Wings are already the optimal location for both the generator and the propulsor.
You want your heavy engine where the lift is created, and that's the wings. You want your propulsors away from the fuselage, and that's either the wings or the tail.
Thus, there is no need to move the power any distance.

Hydrogen is a suitable fuel for turbofan engines and requires no other intermediate devices to convert it to thrust.
There is no significant need for hybrid airliners, as there's no stops to recover energy in flight.
Weight means fuel burn. Engines are heavy: in an A320neo, for example, they take up 14% of the weight. Increasing it 2.5x will make the aircraft 20% heavier. After adding payload and unusable fuel, this results in 15% more fuel burn, if the motors and generators were perfectly efficient.
Since they are not perfectly efficient, a naive replacement would result in even worse fuel consumption. This could be offset by perfecting propulsor size, speed, and placement, but the end result is still that a turbo-electric aircraft will be less efficient than a similar turbofan aircraft.
As to hydrogen itself, it is not the best aviation fuel. However, it is usable. The overall energy efficiency will be worse, due to the loss of cabin space to tanks. Whether that can be offset by clean production is a question outside of aviation.
Regardless, a 15-year horizon is so short that it can be safely said that hydrogen or electric or hybrid airliners will not be anything more than concepts and experiments in this timespan. 75 years, that could be a topic for debate, on appropriate sites.


Can the four-engined Lockheed JetStar II fly on one engine? As we know, there are no four-engined business/private jets currently in production. Some Dassault Falcon models are actually trijets, but this is not the case. The only four-engined business jet in serial production was Lockheed JetStar line.

I'm wondering, in case of extremely rare emergency event, when 3 out of 4 16.5kN TFE7313 engines shutdown, could JetStar constantly fly on some flight level without loss of altitude using only one engine?

I have 3000 hours in jetstar.... The answer is no... It will not fly on one... Two yes and you really don't need that much rudder.. In the simulator with two out I used to put the inboard operating engine at 98% and modulate the outboard engine to control speed/sink doing that it really only needed 1/4 rudder trim


Do services still apply foam carpet on belly landings? From decades I have been reading that, fire services would spray foam on the runway, in order to avoid fire, on a belly landing. Is this procedure still being used? If not, what would be the safest procedure?

Firefighting services still apply foam path, atleast outside US, though the practice is no longer encouraged.

FAA, in its 2002 CertAlert 02-04 actively discourages it:

  The FAA does not recommend the foaming of runways for emergency landings and warns against the practice with any foam other than “Protein” foam....  It is recommended that ARFF personnel decline to foam a runway when requested by a pilot because they do not have the specialized equipment and protein foam

FAA lists a number of reasons for this decision (It had actually recommended foam paths for emergency landings in 1966, before withdrawing it in 1987), including,

Time may not be always available for foaming
The use of foam in foam path should not affect the subsequent firefighting abilities.
Foaming the runway should not affect the movement and safety of other aircraft.

ICAO too, does not recommend for foaming the runway in its Airport Services Manual, stating,

  15.1.2 The effectiveness of runway foaming is not fully substantiated by the real evidence of operational incident studies.

ICAO also considers the various reasons for foaming the runway and analyses them:

  a) Reduction in aircraft damage. ... The data available from a study of emergency landings made with, and without, the application of foam show
  that no significant reduction is achieved in the risk of fire or in the extent of damage by the foaming of runways.

  Reduction in deceleration forces. ... It is to be noted that from what is
  known to date, the braking action of an aircraft on a foamed runway will only be slightly worse than that on a wet runway, assuming non-freezing weather.

  Reduction in friction spark hazard. ... tests have shown that aluminium alloy metals produce no friction sparks capable of igniting aircraft fuel vapours ... on either dry or foam-covered concrete or asphalt runway surfaces. ... Titanium friction sparks, capable of igniting aircraft fuel vapours, could not be effectively suppressed by runway foaming in any of the scale research tests ...

  Reduction in fuel spill fire hazard. From all that is known of the fire suppression qualities of foam and the scale research tests, it is clear that a foamed runway would have no appreciable effect on the fire hazard of fuel vapours in the atmosphere over the foam.

ICAO also points out that the primary firefighting vehicles should not be used for foam laying operations; Also, the time taken for the operations and replenishment is a concern.

It is to be noted that, though the practice is no longer recommended, fire services are not barred from using it if the pilot asks for it and facilities are available.

In a related note, US armed forces seem to use it.


To what extent is the clerical work (i.e. amending flight plans, logging, etc.) of a center air traffic controller now automated? Specifically, do center controllers have to manually amend every flight plan now that En-Route Automation Modernization (ERAM) is fully implemented in the USA? To what extent is the process automated?

I know that, once the flight is en route, if the computer predicts a conflict the controller will need to okay a flight amendment. When flight plan requests are sent in and the center computer checks their viability, do controllers still have to give the okay to amend them and send them back?

Essentially, my question is this: what do FAA center air traffic controllers currently (I say currently because of when ERAM was implemented) do besides talking to the pilot?

... Do center controllers have to manually amend every flight plan now that ERAM is fully implemented? To what extent is the process automated?

Yup. ERAM does not take away tasks from controllers, it just makes them easier.

  For controllers, ERAM provides a user-friendly interface with customizable displays. Trajectory modeling is more accurate, allowing maximum airspace use, better conflict detection and improved decision making. — FAA

Controllers still fill out and pass on flight strips. Physical flight strips are being phased out by the Advanced Electronic Flight Strips (AEFS), for center controllers and all the other facilities.

  AEFS replaces traditional paper flight strips, and manual tracking of incoming and outgoing flights, with an electronic flight data display. The system can be updated through a touch screen or mouse. Controllers no longer need to physically carry a flight strip across the control room. A simple swipe of the finger sends the data to another station. This allows the controller to stay engaged with the traffic at all times. — FAA

AEFS will eventually be replaced by the Terminal Flight Data Manager (TFDM).

It wouldn't surprise me if they print out those flight strips, just in case.

Source: FAA


What effect do slats have on descent rate? During descent, assuming the same speed and idle thrust, is the rate of descent greater in a clean configuration or with slats?

If the slats would increase L/D, they would not be retracted in cruise.

TomMcW is right, any high lift device increases drag when compared to a polar point that is within the regular operating envelope of the clean wing. How much so can be answered when looking at old wind tunnel data published by Sighardt Hörner in his classic "Fluid Dynamic Drag":

Figure 6-20 from Fluid Dynamic Drag comparing different versions of the Fieseler Fi-156 "Storch" slat. Although it is not explicitly stated (and the original report in the Jahrbuch der deutschen Luftfahrt-Forschung of 1940 is not available online), it must be assumed that the lift coefficients of (a), (b) and (c) are similar. After all, Mr Hörner worked at Fieseler in Kassel and contributed to the aerodynamic design of the Fi-156, so he should have been intimately familiar with this airfoil.

Version (a) at the bottom is with a gapless slat and produces a drag coefficient of 0.015. Version (b) is with a gapped slat but blocked flow, and the drag coefficient shows a small increase to 0.018. Once the gap is opened (c), the drag coefficient jumps to 0.03. Note that at a higher angle of attack and lift coefficient (d), the drag coefficient is slightly reduced (0.028). But it is safe to assume that a deployed slat roughly doubles the airfoil drag.

If thrust and speed are not changed, deploying slats will increase sink speed. If I further assume that this descent is done at optimum L/D (zero-lift drag equals induced drag), then a doubling of the zero-lift drag of the wing will increase total drag by maybe 17%, details depending on the share the wing has in overall drag (I assume for the figure above that this share is ⅓).


What happened to the Fokker 100 revival project? What happened to the Fokker 100 revival project by NG Aircraft?

They wanted to build a better Fokker 100 called the Fokker 120NG/Fokker 130 in 2010-2014 which originally should be completed this year.

NG Aircraft still exists, or at least, they still have a website. The big roadblock is financing the development of the aircraft. I did a search in Dutch government records: the most recent mention is in 2016, indicating that at that time sufficient funding has not been found yet to start development in earnest.

LinkedIn shows 11 employees.


Why do we have to perform a full ADIRS alignment between latitudes of 2 degrees North and South on the A320 family? In the latest FCOM revision under the preliminary cockpit preperation normal procedures chapter ADIRS alignment has been added. However most of the conditions remain the same except for a new line which has been added wherein a full ADIRS alignment has to be performed at airports where the latitude is within 2 degrees North and South.
Could anyone explain with a reference why Airbus has decided to add this ?
As per my company FCOM this is only applicable to certain MSNs (mainly our newest A320-251N's)

SOP is modified to introduce a new condition to perform a complete IRS alignment during cockpit preparation. When departure airport is located between latitudes 2° north and 2° south, a full alignment must be done. This is to avoid excessive drift of Honeywell ADIRU Block III during the subsequent flight.


How prevalent is traditional radar tracking of commercial aircraft? In many media reports regarding air incidents such as AirAsia Flight QZ8501: What We Know About the Missing Plane from NBC News I often see a phrase such as the following:

  By the time it had been given clearance, just four minutes after its last correspondence, the jet had disappeared from radar, Tjahjono told the news agency.

From various maps I've seen it appears the aircraft went missing over 100 km from any major land masses / airports and it made me wonder how far commercial aircraft would normally be tracked by traditional radar that sends a ping and waits for an echo from a typical airport?

I'm guessing in many cases media tend to use the term radar interchangeably with transponder technologies such as ACARS but I'd still be interested to know the limits of typical radar installations and how prevalent it remains in modern commercial aviation.

There are many surveillance technologies available to track aircraft.

Radar is the most common technology used for ATC (Air Traffic Control) surveillance. Basically there are two varieties:

Primary radar

This type of radar sends a high energy radio pulse which reflects of the aircraft. The time delay of the received echo is a measure of the distance.
The characteristics of the transmitted radio pulse. Since aircraft don't have to participate for this radar to work this radar principle is mostly used for Air Defence purposes.

ATC primary radar usually has a maximum range of about 50-100NM, but can exceed 200NM. Primary radar is mostly used for airspace protection (monitoring unauthorized access) and airport surface control. These radars have a limited maximum range but a very low minimum range and provide reasonable accuracy.

Long range primary radar is expensive to operate. Due to the free space signal loss, long range radar needs to transmit a very powerful pulse and receive an very weak echo. The associated infrastructure (large antenna, active cooling) and power requirements make long range primary radar very expensive.

Military use of primary radar includes over the horizion (OTH) radar which has ranges well over 1000NM. These radars are geared towards detection of incoming missiles and aircraft and not suitable for air traffic control due to their lower accuracy.
Secondary radar

Secondary radar is the most common surveillance technology used by ATC.

This type sends a radio signal to the aircraft's transponder to which the transponder replies with its own radio signal. Again the time delay is a measure of the distance. The transponder can encode additional information about the aircraft (such as altitude or identity) in the reply. This radar technology relies on the correct functioning of the radar transponder.

Because the radar relies on transmission of energy from the aircraft, the transmitter needs less power than a primary radar. The maximum range of secondary radars can exceed 250 NM for aircraft flying high enough (above the horizon). Not all secondary radars are operated at that range though, airport radars are often limited to 50 - 100 NM.

Radar signal horizon

Then there are a number of surveillance technologies which are a bit different than radar:

(Wide Area) Multilateration

This technology uses multiple receivers spread over an area. By accurately comparing the times that an aircraft's transponder signal arrives at the different receivers the system can calculate where the signal must have been transmitted from. The transponder signal must be received by at least four receiver to get an accurate position. The accuracy of multilateration is best in the area surrounded by receivers; outside that area the accuracy of the system degrades quickly. A provide with enough receivers, a Wide Area Multilateration system can cover vast airspace volumes.
ADS-B

Automatic Dependent Surveillance - Broadcast is a surveillance technology that relies on the aircraft transmitting their position twice per second. A simple antenna and receiver is all that is required to receive these positions. Since the signal comes from the radar transponder, the above signal horizon figure applies. A very sensitive receiver can detect high flying aircraft far away. Not all aircraft are yet equipped with ADS-B but most passenger transport aircraft are.

The company Aireon is equipping satellites with ADS-B, so in 2018 there will be oceanic ADS-B coverage.
ADS-C

Automatic Dependent Surveillance - Contract is again a surveillance technology that relies on the aircraft transmitting their position. Contrary to the broadcast counterpart which transmits to all interesting parties, ADS-C addresses its transmissions to the contracted ATC facility. The data goes via a satellite link or over VHF when the aircraft is flying over land. Because of the satellite link, coverage is global (except the poles).
ADS-C reports are less frequent that radar or ADS-B reports, typically the system only reports at specific reporting points.

ADS-C is not installed on every aircraft, mostly on intercontinental transport aircraft. It is used mainly over the Pacific and the Atlantic.
Verbal position reporting

Not really a surveillance technology, but used in areas where there is no other means of surveillance available. The pilot reports his position over the radio to air traffic control. The maximum range of the radio transmission is the limiting factor here. Using HF radio, global coverage is achievable. The transmissions suffer a lot from static noise and sometimes other aircraft have to relay messages if no proper direct communication between ATC and the aircraft can be achieved.


Do airlines ever clean their planes? Looking at some photos, I saw that a lot of planes are very dirty (on the fuselage, not inside or in the cabin), and it is said that the dirt on the fuselage affect performance. Also insects affect the aerodynamics of the plane, so, do airlines ever clean their planes?

In addition to repaints, aircraft are also periodically washed and treated to maintain the paint quality and reduce the build up of dust/grime/salt and other particulates.

There are automated systems installed at some airports to clean airplanes as they taxi over these areas. Think of it as a drive through car wash, but for airplanes.

The very first applications of these were actually for the US Navy because they have a requirement to wash aircraft due to their proximity to seawater. A company called tammermatic developed a system for the navy to clean airplanes directly on the field using a set of powerful jets. Images from the tammermatic website

Then there is the Skywash system (PDF brochure), developed by PMW (a company that makes concrete pumps) to wash aircraft automatically. It was first used by Lufthansa in 1997 (image cc: ARNE DEDERT / AFP):