Are aircraft with squirrel cage fans as wings viable? I'm having trouble finding it, but a few years back I saw a YouTube video where a gentleman had designed and built an aircraft that used what I can only describe as squirrel cage fans for the upper surface of the wings. As far as I recall he made some bold claims about how efficient and capable this design was.

I was skeptical at the time and haven't seen this design since my original viewing of the video.

Does anyone know anything about this design? If so, is it viable at the scale of your average drone? Is it viable at the scale of a transport category aircraft?

Unfortunately, I can only speak for MAV design, but the lab I work in at the University of Maryland (UMD) has done some work on a similar design called a cyclorotor.  Here's a paper that might have some information for you (https://www.researchgate.net/publication/301680815_Development_of_a_quad_cycloidal-rotor_unmanned_aerial_vehicle?ev=prf_pub), but I'll try to sum up here.

The neat thing about these aircraft is that you have, in theory, a very efficient and simple rotor system.  Instead of a rotor blade where only the outer portion of the blade is moving at a desirable speed, the entire blade is moving as fast as you want it.  Similarly, instead of the complexity of a swashplate and needing to actually tilt a rotor in the direction you wish to travel, you can actuate the pitch of each blade in a cyclorotor arrangement with respect to its location in its rotation (analogous to cyclic blade control) and generate a thrust vector in virtually whatever direction you want...all without tilting the aircraft's body.  I've copied a figure from the paper I linked to illustrate this:

In this image, you have 4 blades rotating around on a drum.  It's not quite the "squirrel cage", but similar.  Each blade is allowed to pitch, as on a Ferris wheel, as it spins around.  To govern this pitching, each blade is attached to an offset bar near the center of the drum's rotation.  Hence, as the blades spin around the drum, their pitch is passively modified, forcing them to generate thrust in a particular direction.  Change the orientation of the offset bar, and the cyclic pitch of the blades changes accordingly, thus modifying the direction of the thrust vector...all without changes the vehicle's flight attitude.  For reference, a helicopter has to tilt over in whatever direction it wants to go (generally speaking), increasing drag area and, based on rotor dynamics as well, limiting its response time.  Plus, it could be handy to keep a stable, level platform that can still move around (say for filming or something like that).  It is a really neat feature to have.

The paper is likewise optimistic about the efficiency of the design.  However, measuring efficiency is a VERY tricky thing to do in a way that you can truly compare two designs.  For example, disc loading is typically measured on a helicopter as the gross weight of the aircraft divided by the rotor disc area.  On a cyclorotor, if you want to compare on that metric...what is the disc area?  The paper utilizes the projected area of the cycloidal rotor (the "drum", if you will, the forms the fan wing concept).  Some argue that utilizing that doesn't give you a true apples-to-apples metric to compare with because well...how IS that projected area and the weight it lifts necessarily comparable to a single-rotor helicopter's disc?  Even for coaxial helicopters, I've heard substantial arguments about how to best reduce a coaxial rotor system to a single rotor system for a true apples-to-apples comparison...and I'm not sure you could find one that involves disc loading for this present case of the cyclorotor.

Power loading (gross weight divided by installed power, so the higher the quotient, the better hover efficiency you have -- more weight for less power) seems like the go-to metric then, since it has less "wiggle room", per se.  However, now that you have that, how do you still say that two helicopters at a given power loading are better or worse than each other?  Power loading doesn't capture everything (only hover efficiency), so an additional parameter is necessary.  What is being held constant between the two designs that gives a basis to say whether or not a given design is actually more efficient than another?  In the UMD paper I've linked to here, it posits disc loading...which, as said, may be a questionable way to do that.  These are the two of the "go-to" parameters of helicopter performance, so we might need to make something new.

Hence, I think that the best answer that I have is that the design is still being felt out, and I'd be wary of people who posit massive efficiency increases.  There is no free lunch, and efficiency is a tricky thing to nail down.  I've seen more CAD of fan-wing designs than I have vehicles (at least, for those larger than the MAV scale), so I'm inclined that think it's not living up to expectations for the larger vehicles, but that's pure opinion.  Also, this said, I know of at least 5 of these vehicles that have been made and flown at UMD, though none over 10 pounds gross weight.


What to do as an international student without a driving license that is needed for PPL Training in Germany? I am required by the Luftamt Südbayern to provide information from the driving suitability register from the Federal Motor Transport Authority in Germany but I do not have a German driving license as I am an international student.

Anyone with any ideas as to how I should approach this?

You do not need a driving license to request information from the German Register of Driver Fitness.

The Kraftfahrt-Bundesamt has a website in English with information on how to request your information free of charge: https://www.kba.de/EN/ZentraleRegister_en/FAER_en/Auskunft_en/faer_auskunft_inhalt_en.html?nn=642934


How do ETSOAs/TSOAs expire? EASA can issue an ETSOA (Euro-Technical Standard Order Authorization/Approval) for certain equipment design, to simplify installation authorization on an aircraft type design.

I found no notice of such an ETSOA expiring at a certain date or for a certain certification basis. Still, at some point regulations evolve and ETSOA'd equipment may stop meeting those regulations. How is this managed?

As an example: suppose a few years ago, an equipment was designed  and manufactured as per some ETSO, and approved by EASA for that ETSO under a certification basis including the now obsolete DO-178B.

Now or in the near future, an aircraft manufacturer wants to use that equipment on a new aircraft type that is required to use the updated DO-178C. What would the aircraft and the equipment manufacturers have to deal with as extra certification work for that equipment? And in the case there is no extra work, how will the EASA deal with new regulations in the years to come?

(this question is not specifically targeted at EASA's ETSOAs: consider the same question for FAA's TSOAs and similar mechanisms in other countries)

From the Code of Federal Regulations CFR 14

§21.613   Duration.
(a) A TSO authorization or letter of TSO design approval is effective until surrendered, withdrawn, or otherwise terminated by the FAA.
(b) If a TSO is revised or canceled, the holder of an affected FAA letter of acceptance of a statement of conformance, TSO authorization, or letter of TSO design approval may continue to manufacture articles that meet the original TSO without obtaining a new acceptance, authorization, or approval but must comply with the requirements of this chapter.

From the FAA's page on TSO Cancellation/Withdrawal:

What is the difference between a "Canceled TSO" and "Withdrawal of a TSO authorization"?
A "Canceled TSO" means that the TSO has been rendered inactive and the FAA will no longer issue new TSO authorizations against that particular TSO. However, any existing TSO Authorizations obtained prior to the cancellation of the TSO are still valid and may continue to be manufactured and marked in accordance with that authorization.
The "Withdrawal of a TSO authorization" means that a specific authorization letter is withdrawn by the FAA and the approval to manufacture that particular article is terminated.
In summary, if the TSO is canceled, you can still make the article but you can't apply for a new authorization to that particular TSO revision level. If the TSO is withdrawn, you can't even manufacture the article under the TSO system (i.e. the FAA won't let you put a TSO label on it).

An example of the FAA suspending a TSO Authorization NavWorx ADS-B


Why is the F-35 engine nozzle exposed? I was wondering why the F-35 engine is exposed. The F-22, F-16 and the YF-23 Black Widow have the exhaust nozzles of their engines blended flush with the fuselage.

(YF-23 Black Widow)

(F-35)

The YF-23 Black Widow clearly has two engines which are indented into the plane. However the F-35 has one engine which is exposed at the back. So I have two questions:

Why does the F-35 have only one engine unlike the older models?
Why is the F-35's engine exhaust nozzle clearly more exposed to bullets and rockets than older models?

The F-22 is intended (and the competing YF-23 was intended) to be an air superiority fighter to replace the F-15.  The F-15 was a twin-engine model, as was the F-4 Phantom that it replaced.  Twin engine planes have excellent "survivability," meaning they can take a lot of hits and still stand a chance of making it home (you can lose an entire engine and still stay airborne).  They also are much higher-performance.  The F-15 can takeoff, stand on its tail and climb vertically to over 40k feet.  I've seen F-16s do something similar but they couldn't climb as high and they couldn't carry as much fuel and/or armaments while doing it.

The F-35 is intended to be a replacement for the single-engine F-16.  The F-16 is a capable workhorse but it's more of a budget, multi-role aircraft, meaning it was designed, from the get-go, to do some air superiority AND some ground attack, operating on a tighter budget.  We have considerably more F-16s in the Air Force inventory than we've ever had (or will have) F-15s and/or F-22s.

The F-15 used the same engine as the F-16 (two to choose from: either the P&W F100 or the GE F110), but two of them.  When I was in the Air Force

F-15's cost about USD 50 million
F-16's cost about USD 35 million

Two engines and a larger, heavier airframe (albeit less sophisticated) added up to a significant price premium.  In air combat, above 25k feet, my money would be on the F-15.  At altitudes below 25k feet, my money is on the F-16.  With an inherently unstable airframe and a computerized fly-by-wire system to tame it, the F-16 was wickedly maneuverable.  The increased power of two engines on the F-15 working against the greater weight really loses the advantage at lower altitudes.

The "buried" engines in the F-22/YF-23 are trying to improve the stealth characteristics.  The F-22's thrust vectoring, coupled with twin engines, make it more stealthy than the F-117 and faster and more maneuverable that the F-15 or the F-16.  The YF-23 was arguably faster, but less maneuverable.

The F-35 is intended to cost less than the F-22 (debatable as to whether that will be the final case).  And certain models of the F-35 are capable of a trick the F-22 simply can't match: STOVL.  It can do a Short TakeOff and Vertical Landing.  For the latter, the rear nozzle pivots downward 90% and a "lift fan" opens up in the forward fuselage.  The F-22 was never intended to be able to do this.  "Burying" the rear nozzle AND trying to do that would add a LOT of weight (and additional cost) to the aircraft.


What is an FAA Notice? Does it have legal force? The FAA issues Notices and other non-legislative rules (e.g. Advisory Circulars). These documents are distinct from the laws in Title 14 of the Code of Federal Regulations. What are the differences between them? Do they have legal force?

As SSumner pointed out, the FAA produces all kinds of paper - some of which is "regulatory" and/or have legal force, and some of which isn't.  I won't go into all of the specific document categories, but I'll talk about a few that most of us will find interesting.

Handbooks and Manuals
The FAA actually publishes full-length books, if you know where to look!
The FAA has written books on flying an airplane,  flying balloons, risk management, buying & owning an airplane, and a whole bunch of other topics which provide broad information that you can use to familiarize yourself with a topic.
Advisory Circulars
Advisory Circulars (ACs) are the FAA's way of explaining their regulations.
They're broken down into sections which generally match up with the FARs they're interpreting.  Some ACs may rival the lengths of handbooks, but they are much more narrowly focused, generally dealing with a single topic and often much easier to understand than the legislation they're interpreting.
Airworthiness Directives
When an unsafe condition is identified in an aircraft or component which requires action the FAA can issue an Airworthiness Directive (AD), requiring that specific actions be taken.
Often the required action is compliance with a manufacturer's Service Bulletin (or compliance with a service bulletin is also an acceptable method of compliance with an AD).
There are also Special Airworthiness Information Bulletins (SAIBs), which are like "Mini-ADs": The FAA is calling operators' and mechanics' attention to something which may not merit an AD. SAIBs are informational - you don't have to comply with them - but it's certainly a good idea to read them and pay attention to the things they're highlighting.
Orders
An "Order" describes procedures used by the FAA. For example, JO 7110.65U describes various aspects of how ATC works.
Procedures described in Orders may be modified by Notices.
Notices
A "Notice" is just an agency's way of saying "Yo! Pay attention! Something's happening!"
Your question seems generally concerned with internal notices which I linked to in the title. These generally modify Orders, like this one.
The FAA can also issue "Public Notices" about changes that affect airmen and the   general public, or pretty much any other topic they'd like.  Public Notices are published on faa.gov as well, but the easiest place to find those is through the FAA Safety Team's SPANS system which is searchable and has a much nicer interface than digging through the FAA site by region.

As for whether the non-legislative documents have legal force the answer is a very well-qualified "maybe".  Some certainly have the force of regulation – Airworthiness Directives being the most obvious – and for our purposes I think that's what you're really concerned with, because the law says you have to comply with the regulations.
Orders and Notices are similarly "regulatory" for the agency: The FAA has to comply with its own orders, so ATC has to be operated the way JO 7110 says it's to be operated.

For stuff like ACs, Handbooks, etc. we discussed this in chat a while back talking about the answer to FAA Order 8900 and the FSMS - the gist of it is that the non-legislative documents the FAA produces are generally interpretations and guidance, but they are "effectively regulatory" in that if you go against them you need to have an excellent reason for doing so, and be prepared to back it up with appropriate justification and evidence if asked.

As a classic example, alterations to aircraft are expected to be performed in accordance with manufacturer's maintenance procedures, or "other methods, techniques, or practices acceptable to the Administrator" which are described in AC 43.13 (43.13-2B deals with light, non-pressurized aircraft and is an interesting read).

A mechanic doesn't have to comply with the guidance in AC 43.13, but if a mechanic were to modify an aircraft without conforming to that guidance they would need to supply supporting documentation as to why their work is acceptable, such as a statement from a Designated Engineering Representative, otherwise the FAA could deem the work "unacceptable" (and from a liability standpoint, the mechanic could wind up in serious trouble should something go wrong with the aircraft as a result of that work).


Does object mass have a significant bearing on wind interference? I'm learning to fly RC quadcopters, and before I go outside with an expensive, possibly dangerous (if it were out of control and hit something) large unit, I've decided to practice flying for a while on a smaller entry level "palm" drone (if I lose/break it, I won't be out more than $30).

While trying out the palm drone outside, I've noticed that a very tiny gust of wind will knock this thing out of the sky, or at least into a very different direction then intended. Intuitively I want to assume the larger drone will do better against fair-weather Kansas winds; however I've also learned not to assume that my ground based experience translates well into aviation experience. So

TL;DR

Will a larger, heavier drone be less impacted by wind gusts than a smaller palm drone?

Scaling laws are your friend here.

While the area which can be attacked by a gust grows with the square of the length increase, the mass of the drone will grow with the cube of length. This translates into higher disc loading of the props, which in turn requires them to operate at a higher dynamic pressure. The same wind gust will change forces relatively less on a bigger drone. It is the same with aircraft: Light indoor models have the same problems outside that you experienced with the small drone.

It gets even better when you look at the capability to resist rotation: Here the moment of inertia is the deciding factor which grows with the product of mass and length squared. This means the bigger drone will better resist attitude changes by the length increase to the fifth power. To be fair, gust moments will increase with the cube of length increase (area times lever arm), leaving you still with an advantage of length squared. This also means that all changes will happen more slowly, which improves the chances that the control system (the MEMS sensors onboard as well as your control inputs) will keep the situation under control.


Is it possible to get a medical certificate from Canada and a pilot license in America? I posted a question prior to this but it was too long and confusing. I have done some research and so I will ask:

If I obtain a medical certificate from Transport Canada (TC), and get my commercial pilot license through the FAA, can I work in the US as a commercial airline pilot and take school there?

I am a diabetic, so I can't obtain a class one or two medical certificate in America, but I can in Canada.

You can still fly in US airspace if you get a medical certificate from Canada, but I don't know if that's like from Canada into America.

Feel free to edit this question so it's more clear, I will include requirements from TC and such below.

On the second link, can someone explain it a little more to me? I am not great with understanding such things, I am sorry!

TC medical certificate regulations

Pilot medical certificate international use

(This is similar to this question - and see this answer in particular - but there may be some points here that deserve a separate answer.)

First, can you use a Canadian medical instead of a US medical, with a US pilot's certificate? The answer is no, with one (?) exception. If it were possible, then people could shop around other countries to find the easiest medical for them, which would be a huge loophole and would allow them to simply ignore the FAA's medical requirements. As a general statement, the country of your pilot's certificate and medical certificate must be the same.

The only exception I'm aware of is when you hold a foreign license and medical, and then ask the FAA to issue a foreign-based private certificate under 14 CFR 61.75. See this question for more information but basically, you would have to hold a Canadian pilot's license and medical first, and even then you would be limited by the privileges of the Canadian license.

Second, could you get a US commercial certificate using a Canadian medical? Yes, but only in a very roundabout way, and even then you couldn't use it. You'd first have to get an FAA foreign-based private certificate as I described above - which means getting a Canadian license first - and that certificate would be fine to use for a US commercial checkride. However, after passing the checkride you'd still need an FAA second-class medical to actually exercise your new commercial privileges.

Finally, another option would be to get a Canadian commercial license and medical, and then fly only Canadian aircraft in the US. I have no idea how practical that is because most countries don't allow cabotage, but maybe there's some way to make it work.

AFAIK diabetes is only disqualifying if you're insulin-dependent; if you're on oral meds you may still be able to get a Special Issuance for it. If you have any doubts about your personal situation then I'd suggest consulting an AME and/or calling AOPA, if you're a member.


Is having a history of seeing a therapist not allowed for pilots? In a recent question someone said:

  I thought about seeing a therapist, but as I want to work at a major company, I don't know if you're allowed to have a history of seeing one.

Is it true that having seen a therapist a bad thing? If yes, why? It doesn't make sense to me.

This can be difficult to answer, but I'll try from a US/FAA perspective.
Military Service
If you want to become a military pilot you cannot have any history of mental or psychological disorder This is from the perspective of starting out to become a military pilot. If you are already a military pilot and have some kind of mental/psychological issue that you need addressed, there is a process for doing that and returning to service. From this standpoint, self-help and "keeping it to yourself" is better than seeking treatment since you will be disqualified outright.
Personal Anecdote
How do I know this? This is exactly what happened to me. I had seen a psychologist when I was 8 or 9 years old (because he was a quack and convinced my mother (who was being treated for depression) that I needed help too). This made it into my permanent medical record. I applied for military service with the US Marines during college with the goal of becoming a pilot. I took the Air-Force Officers Qualifying Test (AFOQT) and passed with a 98% (the highest my recruiter had ever encountered). When it came time for my MEPS review they found the history (I disclosed it at the beginning, they found it via my disclosure and I provided additional documentation when asked) and disqualified me. I appealed to the review board and was denied. I appealed again, denied, and you only get two appeals (and the only recourse after that would be a Congressional/Presidental appointment). They told me I was eligible to enlist, but not eligible to be an officer.
FAA Medical Perspective
The FAA takes a different stance in that you are required to disclose to the AME any mental health visits during the previous three years. The AME will then decide if it warrants additional testing or review by the Office of Aeronautical Medicine (OAM). The FAA can also get this information via other methods (for example if your employer reported you or you are jailed for some kind of mental health disorder outburst).
Certain conditions are automatically disqualifying, such as psychosis, bipolar disorder, and severe personality disorder.
Employer Perspective
There are laws in the US protecting employees from being discriminated against for mental health issues. Many companies have reporting and monitoring programs that assist a pilot with a psychological issue to seek help and get back to flying. The FAA participates in these programs and provides a "rigorous evaluation" before allowing the pilot to return to flying.
So from a commercial service perspective it is important to be open about the issues upfront and early, especially with your AME. Pilots have been arrested for denying mental health history.


When/how to set the inbound or outbound course when intercepting a VOR for a DME hold? I have question for holding pattern.

As I know, when I do DME holding I have to intercept VOR radial and decide entry.

However, when I do this I am so confused and do not know how I can select inbound or outbound course.

For example, One instruction is [Hold SE 325R ABC VOR 13 DME Left]

In this case, my instructor says "try to intercept outbound course 145 and make a holding."

However, another instruction is [Hold NW 330R ABC VOR 12 DME]

In this case, my instructor says "set inbound course 150 and make a holding."

In both cases, the position of aircraft was NW from the ABC VOR, and my heading was 150 and distance was almost 18 DME from the ABC VOR.

This is what I confuse that first instruction was setting outbound, but second instruction was setting inbound course for intercepting the radial.

Could you let me know how I can figure out when I set inbound course or outbound course for intercept one for DME holding?

*I use G1000 which has HSI.

Thank you for your work and time!

Below is a picture possibly showing what your instructor may be telling you. (based on the information you have given in your question)


Is the pod on the right wing of the PC-12 and Cessna 208 a gyrocompass? Hello avgeeks,

First time posting here looking for an answer.
Could someone tell me what's located inside that pod on the right wing of some planes like this Pilatus PC12 or this Caravan 208?
I guess it can be a gyrocompass but i am not 100% sure.

Thank you in advance!!
Safe landings.

Its not the gyrocompass. Its where the weather radar unit is mounted since the prop prevents it from being mounted in the nose like on many larger aircraft. You can find a nice instructional video on how to use it here. The magnetometer used on many aircraft to provide magnetic heading information to the systems is much smaller than those domes.

(source)

If you are referencing the heading indicator when talking about a gyrocompass those are panel mounted. If you are referencing the wet compass (which is balanced in fluid) those are mounted on the column in the center of the windshield typically. A lost of the more modern aircraft in this class have glass panels which use AHARS units and may not have traditional gimbals anymore.


When were the last subsonic turbojet-powered airliners retired from commercial passenger service? The very first jetliners (Comet, Tu-104, Caravelle I-VI, 707-120/-220/-320, DC-8 series 10-30, original 720, CV-880) all used turbojet engines, turbofans not being available until later; when turbofans became a viable option, jetliner manufacturers switched over to turbofans en masse for their new designs (including the later versions of the Caravelle, 707/720, and DC-8), and some of the older, turbojet-powered airliners were converted to also use turbofans, but many already-built jetliners continued to fly under turbojet power for decades.

I have been unable to ascertain precisely how long subsonic1 turbojet-powered airliners continued to carry paying passengers in regular service.  Wikipedia says that the Tu-104 was withdrawn from passenger service in March 1979, and the Comet last carried paying passengers in 1981, but does not provide a citation for either statement, and I have been unable to find any information as to when the other turbojetliners were retired from passenger service.

Can someone help with this?

Yes, I am aware that the Concorde used turbojets; it does not count for the purposes of this question, as it was a supersonic aircraft, cruising at Mach 2, where turbojets are actually more efficient than turbofans, whereas this question concerns those early subsonic aircraft that were only built with turbojets because turbofans were not yet an option.

EDIT: The original intent of this question was to ask when the last subsonic turbojetliners in scheduled passenger service anywhere in the world were retired, but John K's comment raises the good point that these aircraft would have hung on in the dusty corners of the globe for a good long while after they were retired in places like the U.S. or western Europe.  Therefore:

When were paying passengers last flown in scheduled service on subsonic turbojet-powered airliners...

...in First and Second World countries (U.S., Canada, Europe, East Asia, Australasia)?
...anywhere in the world?

EDIT 2: The Trident 3B also does not count, as it was mostly turbofan-powered, using the turbojet only as an additional booster on takeoff for hot-and-high operations.

As far as the T-104, Aeroflot retired the Tu-104 from civil service in March 1979 however it wasn't until after the Pushkin crash in February 7th 1981 that killed 52 people that the type was permanently removed from service.

Follow-up / safety actions
The remaining military Tu-104 aircraft were grounded following this accident.

Source: https://aviation-safety.net/database/record.php?id=19810207-0
The last commercial passenger flight of the Comet 4 was by Dan Air on November 9th 1980.

Comet 4s flew with BOAC until 1965 and with British airline Dan Air until 1980.

Source: http://www.bbc.com/future/story/20170404-the-british-airliner-that-changed-the-world
To answer the actual question of the last in service, that appears to be the B707 for all countries.
Trans World Airlines flew the last scheduled 707 flight for passengers by a US carrier on October 30, 1983

An era in U.S. aviation ended at 3:25 p.m. on Oct. 30, when TWA Flight 4, the last Boeing 707 to fly for a scheduled airline in the continental U.S., touched down at New York City's Kennedy International Airport.

Source: http://content.time.com/time/magazine/article/0,9171,952267,00.html
Saha Airlines of Iran was the last airline to keep 707s in scheduled passenger service until April 2013.

All flights of Iranian airline SAHA have been stopped due to its outdated aircrafts. The airline, which has been established by the Air Force of Iran, has operated for 23 years, but the Civil Aviation Organization of Iran stopped its operation due to outdated aircrafts. SAHA owns four planes, including three Boeing 707s with an average age of 36.8 years.

Source: http://www.payvand.com/news/13/apr/1165.html


What is a "Pod-Pak"? The following image, from the Qantas Heritage Collection (via aussieairliners.org), shows the flight crew inspecting the 'Pod-Pak' prior to departure:

Based on the angle of their heads, I'd assume it's the engine-looking device to the right of frame.

Also, what are the rocket engine-like tubes at the left of frame?

A Pod-Pak is a aerodynamically shaped enclosure to transport an extra engine under the wing.

From 'Flying Magazine', Nov 1959:

  Spare engine in “Pod Pak” is the airlines' modern method of
  transporting extra engines to maintenance points. Nine major airlines
  flying Boeing 707s have ordered Pod-Paks. This spare suspended below
  the wing of a Qantas Airline's 707 shows the Pod Pak suspended from
  the airplane's front wing spar by means of two fittings and to the
  landing geard support rib by four bolts. Fairings streamline ends of
  the engine and normal engine cowls enclose the sides. No structural
  changes are required in the airplane's wing and, when not needed,
  Pod-Pak can be removed completely.

The exhaust pipes of the Pratt & Whitney JT3C turbojet engine on the left of the picture are an attempt to reduce the noise of the engine. They should aid in the mixing of the core exhaust stream and the freestream air which is quite a noisy process.

The successor to the JT3C, the JT3D, was a turbofan. In a turbofan there is a layer of bypass air between the core exhaust stream and the freestream which reduces the noise and increases the efficiency. Later B707s fitted with the JT3D engines did have not have the 20 pipes exhaust. Most of the aircraft which were originally fitted with the JT3C where later retrofitted with the JT3D.


How to glide in a circular pattern? I was just curious as to how you would make a glider glide in a circular pattern ( or helical ).

Will there be a large difference when considering a conventional glider vs a delta wing glider?

Assuming there is no wind, a circular pattern is most easily achieved by flying a constant speed and a constant bank angle. Maintain coordinated flight (no  skidding, no slipping). There is no difference in turn dynamics between a conventional glider vs a delta wing glider.


What's the difference between feathering, and flapping in a helicopter? Aren't both methods used to increase angle-of-attack (AoA)? Blade flapping will change the blades level, while feathering will change the pitch, but if both increase AoA why not just use one method?

Feathering

The collective pitch control, or collective lever, is normally located on the left side of the pilot's seat with an adjustable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e., all at the same time) and independent of their position.

That's what you referred to as feathering, notice it says "independent of their position". It's for collectively increasing or decreasing of the lift on all blades—the engine will keep the RPM constant.

Using the collective in level flight would cause a climb or descent, while with the helicopter pitched forward an increase in total lift would produce an acceleration together with a given amount of ascent.
— Wikipedia

Flapping
Flapping is position dependent as explained here. Flapping is not pilot controlled. It's to counter the dissymmetry of lift.


Where can I find Air Traffic Control information material suitable for beginners and students? I would like to learn more about how the Air Traffic Control system works - are there any resources available which describe the system that would be suitable for a beginner or student?

Another good reference would be the training materials for VATUSA. This is part of an online system called VATSIM where people both fly and provide ATC virtually.

Although some of it does not apply to real operations (and of course none of it should be actual operations), it still covers real procedures based on the actual rules.

The basic guides do a good job of providing an introduction to the basic concepts, but the more advanced stuff is less helpful.

Other regions have their own VATSIM organizations, if you prefer a region other than the US.

In addition to the materials available, participation is also great for learning. Through VATSIM you can get training on the materials and then get hands-on experience working with (virtual) traffic.


If I'm on a IFR flight plan with appropriate IFR reserves, and I'm given a lower altitude which will consume more fuel, what should I do? Let's say I'm on a long-range flight at my requested altitude of FL250, and I am handed off into a very busy sector where the controller tells me "Descend and maintain FL210, for traffic."

At FL250, my fuel burn and capacity is sufficient to make the flight with better than IFR reserves.

But if I have to descend to a lower altitude, my fuel burn will increase and my true airspeed decrease to the point where I would have to stop en route for fuel.

How can I describe my situation to ATC in such as way as they will allow me to stay at 250?  Controllers do have some obligation to get you to your intended point of landing, and if giving you an altitude, airspeed, or vector that will adversely affect your flight, shouldn't there be some way to describe that?

What about saying, "if you give us 210, we're going to have to declare minimum fuel"

As I said in my comment, I don't think there's any single answer to this question because there are just too many variables and the scenario itself seems a little artificial.

But, the first thing you can do is negotiate with ATC. Accept the descent instruction but request a return to FL250 as soon as possible. Offer to fly at another altitude (including above FL250 if possible), or on an alternative route, or with a 10-degree course deviation or anything else that gives ATC options for keeping you at the altitude you want. Of course, there's still no guarantee that the controller will be able to allow it.

The next thing would be fuel management: depending on the aircraft and your current power settings, you may be able to sacrifice speed in order to gain range. But that might not help either if the end result is still that you won't have the reserves you need.

If none of those those things are possible then there's no good option except to plan a fuel stop. Declaring an emergency would be an abuse of the system and could lead to an investigation: you have plenty of fuel, just not enough to get where you want to go, and there's no immediate danger to the aircraft. (I read an NTSB ruling where a pilot had repeatedly said "unable" and eventually declared an emergency simply in order to keep on his preferred route. The FAA grounded him, he appealed to the NTSB and lost.)

In fact, you hopefully already planned for a possible fuel stop because if the winds are so different at FL210 and FL250 that it makes the difference between making it to your destination or not, that's something you should have found out during flight planning. But even if you didn't, or if the wind just changed suddenly, a diversion for fuel shouldn't be a big deal.


How can I identify the FAF on this Jeppesen chart? According to this Jeppesen ILS Approach Chart:

How can the FAF be identified?

Would it be the point at which the G/S is intercepted?

What is the difference between FAF and FAP?

The terminology is different between ICAO and the US.
The following definitions are being used by Jeppesen (Introduction to Jeppesen Navigation Charts):

  FINAL APPROACH FIX (FAF) — The fix from
  which the final approach (IFR) to an airport is executed and which identifies the beginning of the final
  approach segment. It is designated in the profile
  view of Jeppesen Terminal charts by the Maltese
  Cross symbol for non-precision approaches and
  by the glide slope/path intercept point on precision
  approaches. The glide slope/path symbol starts at
  the FAF. When ATC directs a lower-than-published
  Glide Slope/Path Intercept Altitude, it is the resultant
  actual point of the glide slope/path intercept.

  FINAL APPROACH FIX (FAF) OR POINT (FAP)
  (ICAO) — That fix or point of an instrument approach
  procedure where the final approach segment commences.

  FINAL APPROACH POINT (FAP) (USA) — The
  point, applicable only to a non-precision approach
  with no depicted FAF (such as an on-airport VOR),
  where the aircraft is established inbound on the final
  approach course from the procedure turn and where
  the final approach descent may be commenced. The
  FAP serves as the FAF and identifies the beginning
  of the final approach segment.

So the FAF looks like this for non-precision approaches:

And for an ILS approach the FAP is number 8 in the following image:


How does the battery master switch activate the battery contactor in Cessna 152? I have trouble understanding how the battery master switch activates the battery contactor in this schematic from the Cessna 152 manual. This is the model from 1979 or later with an Alternator Control Unit.

Image source: Flight Literacy

I believe that the battery contactor is a relay that connects the top and right contacts (as in the image), when the left contact is powered.

Initially, all switches are off, the alternator is not turning, and there is no oil pressure:

The left contact of the battery contactor is connected to ground via the battery master switch in off position.
The battery contactor is not powered. It disconnects the battery from the connection through the Ammeter to the primary bus.
The battery voltage is distributed only to the clock and the oil pressure switch.
The two diodes close to the battery contactor are in reverse bias and therefore not conducting.

When I change the battery master switch to the on position, I expect that the battery contactor is closing the contact, and connects the battery via the Ammeter to the primary bus.

That requires power on the left contact of the battery contactor.
The battery master switch connects this contact (via "Alternator field circuit" breaker and "Pull off ALT" breaker) to the primary bus.
The primary bus is not powered yet.
In the way I read this schematic, the battery contactor cannot be activated. Nothing happens.

What am I missing?

I think two of my premises were wrong.

The battery contactor is activated by connecting the left contact to ground (instead of 24 V as I thought). It works different from the starter contactor (which has a permanent ground connection).
The battery master switch toggles between ground [on] and floating [off] (instead of primary bus [on] and ground [off] as I thought).

Like this, the plane will be able to start!


Is alpha induced angle real on 3D wing or just math concept? Does alpha induced angle/"effective airflow" really exist in real life on 3D wing or this is just mathematical concept for calculate induced drag?

(same as lifting line theory-circulation theory is just math concept for calcualte lift ,but everyone knows that airflow dont move around wing in circle...)

Question is ,if I put AoA-sensor infront of leading edge at same position at 2D and 3D wing,will be values different or equal?

(theory tell us that angle (A) at my picture below must be less on 3D wing than 2D wing...)

(in my home made wind tunnel ,I cant see difference when I put visual AoA indicator infront of leading edge at  2D compare to  3D wing..so this is reason why I ask is this real thing(maybe angle is too small to see) or just math concept...all this mess with questions about induced drag on this forum ,starts when I do this experiments..)

here is exampels of visual AoA indicators:video 1     video 2

1. Summary

Induced angle of attack is technically a mathematical concept. You can't measure it directly via flow sensors [from an isolated wing]. However, it is extremely useful in understanding and predicting the production of induced drag.

Just like Newton's theory of gravity is a mathematical abstraction; but, if it helps you in understanding and predicting outcomes, you might as well accept it as real. Read the following subsections if you want to understand the potential flow aerodynamic modeling a bit better.

EDIT: provided that you have an extremely long wing (and tunnel width) to call 2D section, and provided you have a corresponding high aspect ratio wing to call 3D section, the difference in AOA measured at the same chordwise location should converge to the induced AOA.

2. Lifting-Line

Induced angle of attack comes from an aerodynamic modeling method, called Lifting Line Theory, which approximates the whole wing as one single bound vortex line that changes in vorticity strength across the span. The change in vorticity is shed via trailing vortices across the span.

Since vorticity induces cross-flow, this model predicts net downwash (downward airflow) at every point along this vortex line. And since the wing is just a line, we can neatly separate out the free-stream velocity, which may have a free-stream angle of attack, or geometric angle of attack, against the line, and a downwash velocity that modifies the flow incidence, which we can call induced angle of attack.

Image ref: https://www.researchgate.net/figure/Lifting-Line-Theory-Scheme-Epps-2010_fig1_273450664

This simplified model predicts surprisingly accurate lift for straight, mid to high aspect ratio wing, in low subsonic flight. It also predicts a new kind of drag, called induced drag, which is totally absent in 2D subsonic [potential] flow.

Now, my question is, is any wing a single vortex line? Of course not. It's a model that gives very useful engineering results. The immediate shortcoming of the model is that it doesn't predict the flow field around the wing well, at all. How can it? It's a line!

3. Lifting-Surface

We can do better by approximating the whole wing as a lifting surface (or its cousin vortex-lattice), with the whole surface producing and shedding vorticity. This model is able to predict the flow field near the wing much better, by producing a different value of downwash at every spanwise and chordwise point. In front of the wing, there will be upwash. So if you measure flow incidence in front of the wing, you will measure higher incidence than the free-stream one.

Image ref: Drela, Flight Vehicle Aerodynamics

At this point, you should realize that the induced angle of attack is no more (because there is different downwash at every chordwise point). But for the same high aspect ratio, straight tapered wing, both lifting-line and lifting surface will agree on prediction of lift and induced drag.

So once again, is a wing a flat surface of vorticity? Of course not. Wing has thickness that lifting surface doesn't handle, and nor does it predict boundary layer.


Are these ballast bags for a hot air balloon? While driving around on the back side of Aspen Mountain (so up around 10,000 feet altitude) we came upon these bags of ash.  They look like they could be opened up on the bottom to let the contents out, our best guess is that they are ballast bags for a hot air balloon.  You can see my brother's legs in the first picture for some kind of scale.

Were we right?

Almost definitely not.

Hot air balloons control their height by changing their temperature, they don't typically use ballast bags. It's possible that these were ballast bags on a gas balloon, but ash seems like a strange choice for weight considering how light it is.


Are there other practical means of 'slowing' down an aircraft, other than parachutes? Whenever an airliner crashes, inevitably questions arise as to how can the planes be made safe and ultimately, how can we avoid loss of lives.

Though statistically, modern airliners are safest ever, but due to the nature and outcome of even a single crash, it affects people psychologically worldwide.

Questions like parachutes for planes or even passengers are numerous on this site. For example, I posted this questions after reading this question.

The way I understand, if an aircraft can be slowed down, by means of a parachute, the resultant damgage to human lives would be a lot less. This led me to wonder, what can be other means of slowing down an aircraft, which would crash otherwise.

For an aircraft to fly, it needs forward speed. If you slow down an aircraft too much, it will stall and fall out of the sky. Airplanes are designed to not slow down. It's not "if a Boeing 737 is slowed down to 80 knots, more people will survive". A Boeing 737 has to fly at 130 knots minimum. If you want a machine that can, it's a helicopter.

Most crashes happen suddenly, e.g. approaching a runway while not paying attention to airspeed. It's not like "Hmmm, I think it's going to crash in 3 minutes, let's arm this system to minimize injury". It's like, "we're stable, runway ahead, everything's good......(terrain! terrain! PULL UP!)......oh sh!t ! Max power!!!". If the pilot can foresee the accident, most of the time he can avoid it altogether.

If you know well in advance you ARE going to crash, chances are the aircraft is completely out of control, spinning around and falling. In this case I think a large enough parachute will be effective.


Are there locations/altitudes with minimum speed requirements? For example, a slower aircraft operating on the North Atlantic Track System would have an operational impact on surrounding/overtaking aircraft as well as potentially increasing ATC workload to ensure separation.

So, are there any formally encoded minimum aircraft speed/performance requirements based on operational environment?

(Maximum speeds are covered in this question and the topic of why aircraft aren't designed to operate at slower airspeeds is covered in this question.)

The way it works is you file your flight plan for the NAT with a proposed Mach #, but when you get your Oceanic Clearance they will assign Mach #s for each segment of the NAT and that's what you have to fly unless cleared for a different speed.  The adherence to assigned Mach #s are the only assurance of separation once out of radar coverage.

Perusing the NAT Operations Manual I couldn't find anything that gives a minimum Mach #, but if you were unable or didn't want to maintain the Mach # assigned, you would probably be given the option of flying down below RVSM airspace, or taking an off-NAT routing like the northerly "Blue Spruce" route (which airplanes use when they have to stop in Iceland for fuel because they don't have the range for the NATs).


Which procedures or devices do ensure the aircraft stop flying after the pilot has ejected? Pilots eject from aircraft when it cannot be controlled any more. Ejection seems to happen without delay and few time to secure the aircraft and its weapons.

Pilot ejection, source.

Thanks to @RonBeyer for pointing out the Cornfield Bomber which also links to a notable accident in 1989 with a MiG-23 flying 900 km and several countries, uncontrolled:

  During takeoff [from Poland], the afterburner failed and the engine
  began losing power. At an altitude of 150 meters and descending, the
  pilot assumed he had a complete engine failure and ejected. [...]
  After flying over 900 km (560 mi) the MiG crashed into a house,
  killing a Belgian teenager.

What are the procedures or features in place to prevent the aircraft to keep flying and explosives to explode?

As you pointed out, the only reason for abandoning an aircraft is because the pilot can no longer control it or successfully land it.  With or without flight crew onboard, it's going down in one form or another.  As such it's pointless to design any additional system to disable or destroy the aircraft as it accomplishes nothing.

Pilots however have been known to direct a stricken or otherwise doomed aircraft away from populated areas, as this Thunderbirds pilot did at a 2003 air show at Mountain Home AFB, in Idaho.

So too was this crash of a MiG-29 at the Paris Airshow in 1989.  Mikoyan test pilot Victor Kobachev ingested a bird in his starboard engine and the jet departed controlled flight;  Kobachev fought to direct it away from the crowd line before he punched out.


Can an Indian operator get approval for ETOPS-120 without first getting approval for a lower ETOPS rating? If an A320 aircraft equipped with CFM 56-5B engines is certified capable of 120 minutes ETOPS by the manufacturer as per its AFM, can an Indian operator directly request operational approval for ETOPS-120 from the DGCA? Is this possible without getting approval for a lower rating first, i.e. ETOPS-75 or ETOPS-90?

Note: the request for approval would be based on (a) AFM certification, (b) ETOPS training  to flt. crew and maintenance, (c) implementation of ETOPS maintenance and operational policies and procedures (d) ETOPS CMP compliance, (e) satisfactory in-service reliabilty and operational experience of approx 10 years with a fleet of 15 aircraft on non ETOPS sectors

The short answer is yes (unless the DGCA has some unusual process with no public documentation). But be careful: the manufacturer does not certify an aircraft for ETOPS; only a regulatory agency can do that. The manufacturer can put in its books what it thinks is possible/acceptable, but without approval from the local regulatory authority, an operator cannot legally use those books for continued airworthiness. Generally, though, the big manufacturers work with the big local authorities worldwide to make sure that the books will pass muster internationally. But that does not mean that individual operators are exempt from needing to obtain local certification for their specific operations and airplanes. Given the basis you've listed, an operator would have a very strong case, in my opinion.


Looking for the correct propeller shape to create a small copter I am trying to create a mini flying quadrocopter using a Raspberry Pi (mini programmable computer). So far I have hooked up the RPi to 4 motors with controls to each. I'm pretty happy with that side of it.

The part I need help with is the design of the propeller; my motors are not massive and I want to ensure I get the best design for maximum thrust to weight as well as manoeuvrability.

What do I need to look for in propeller design considering my limited motors?

Use Mark Dela's XROTOR - it will give you the chord and twist distribution over span for a given speed, torque and thrust (well, two out of three).

It is based on Larabee's theory (PFD!) for a minimum induced loss rotor.

For a practical solution try to design an optimum propeller for the speed and torque of your motors and then try to find something similar in a  hobby shop. Building the propeller yourself will be a major effort - just balancing them well will not be easy.

If you are into doing everything yourself, try to carve a master and make a mold, maybe from silicone rubber. Then you can pour as many propellers as you need. Put glass- or carbon fibers into the mold and fill up with thickened epoxy. For thickening use a mixture of cotton short fibers and Aerosil 200.

The links to the materials are only for background information - I cannot endorse the sources because I have never ordered from those specific suppliers. The method in general will work, though.


What's this stick-like thing on the glass of C-130J? Just was watching this video:
C-130J Shadow Formation Landing
You can see this stick-like thing on the glass of the aircraft.

What's it? a sensor?

It's part of the windshield anti-icing system, and is the same as on the Navy's C-130T. From the NATOPS flight manual for the T model, page 2-150:

The three windshields, the two windows on each side of the windshields, and the two lower windows in front of the pilot are NESA-type. These panels are heated by applying unregulated ac power from the left-hand ac bus to a resistance material between the layers of glass. The ac power is applied by automatic dc control systems that cycle to maintain window temperature within specific limits. A center windshield system controls heating of the three center windshields, and a side and lower system controls heating of the side and lower windows. The two systems are identical except for the amount of total ac power provided.


Is it legal to use handheld radios to listen to airport frequencies in Australia? I am a student pilot at ABX (YMAY) in Australia. It is a single engine aerodrome with three frequencies:

123.25;
125.80; and
the ATIS frequency

I live very close to the airport, directly under the base/final turn. Would I be able to purchase a handheld radio that I could use to listen to the ATC and practise my radio calls? Is it legal?

An Aeronautical License issued by the Australian Communications and Media Authority (ACMA) is required in order to operate a handheld aviation-band VHF transceiver in Australia.

  An aeronautical licence is issued to authorise a station that:

  is not fixed to an aircraft;
  is operated on aeronautical frequencies;
  is operated for purposes relating to the operation of an aircraft, or
  airport or aerodrome operations; and
  in relation to an aircraft
  includes a mobile station operated on board the aircraft or on the
  ground in communication with aircraft.

  An aeronautical licence is
  necessary to authorise the operation of stations providing:

  air traffic control services;
  aerodrome radio information services;
  private company radiocommunications; and
  other airport or aerodrome
  services.

Reference: ACMA

The Australian-based recreationalflying.com tutorial website may also provide more useful information.

(Note: This applies for a transceiver -- i.e. transmit & receive -- rules may be different for receive-only).


Why does only class C airspace have an "outer area"? There is an area called "outer area" near class C airspace. As described by this AIM:

  Though not requiring regulatory action, Class C airspace areas have a procedural Outer Area. Normally this area is 20 NM from the primary Class C airspace airport. Its vertical limit extends from the lower limits of radio/radar coverage up to the ceiling of the approach control's delegated airspace, excluding the Class C airspace itself, and other airspace as appropriate. (This outer area is not charted.)

As it is not charted and there is no FAA regulations about outer area, I would think it could also be set near class B or D airspaces. Why don't any other airspaces but only class C airspace has the outer area?

The Class C outer area is the airspace surrounding Class C airspace out to 20nm, and from the lower limit of radar coverage to the top of the controller's airspace.  The outer area is not charted, although you can see in the white boxes (in the chart below) adjacent to Class C airports that you can initiate communication within 20nm.

ATC offers Class C services (e.g., flight following) on a workload-permitting basis to participating aircraft operating in the outer area, and aircraft in 2-way radio communication will receive IFR/VFR aircraft separation services.
So you can think of Class C airspace as 3 concentric rings: the 20nm outer area, the 10nm shelf area, and the 5nm surface area.
Source AIM 3-2-4


How "powerful" is a wing tip vortex? I know that aircraft generate wake turbulence and wingtip vortices, and the strength of those phenomena are related to the size of the aircraft, but how powerful are wingtip vortices in terms a non-pilot would understand?

For example, if a human were standing inside of a wingtip vortex would it have the strength to knock them off their feet?  If the wingtip vortex were turned vertical like a tornado would it be an EF1, or an EF5?

If we're talking about large aircraft the answer is Powerful enough that you want to stay out of them.

I refer you to this delightful FAA video in which a Cessna (looks like a 172) was flown into the wake of a C-5 Galaxy. The Cessna was rolled through 360 degrees by the vortex.

NASA research has shown that the rotational speed of wake vortices can exceed 150 miles per hour (130 knots).
While the Enhanced Fujita Scale rates tornadoes based on damage a 150 mile per hour wind speed is in the range estimated for an EF3 tornado ("Severe Damage", defined as Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations are badly damaged.) - This should reduce your desire to be anywhere near the wake vortex of a large aircraft.

If we're talking about smaller aircraft (say two Cessnas flying in trail, and the trailing aircraft somehow gets below the lead) it's strong enough to be unpleasant - the wake from the leading aircraft will shake and rock the wings of the trailing aircraft, but it probably won't flip the trailing aircraft over.

Many pilots are familiar with encountering their own wake at some point in flight training: It can happen when practicing steep turns in calm air.
If the weather and timing of the turn are "just right" and you produced a circle rather than an oval yo will hit the descending wake from your outside wing as you roll level at the 360 degree mark. This produces a slight "bump" at the end of the maneuver (visible in this video).


What is the cheapest plane one can buy to fly above Mach 1 and be able to pull 8g plus? Which plane would you suggest for someone who would like to be able to fly fast enough to break the sound barrier, yet also be able to pull lots of Gs?

What would be the price of such a plane, and how much would it cost per hour to run and be maintained?

Ignoring the various practical limitations that 99.9999991% of people would encounter....

The answer more has to do with what is available than what would be optimal. There are not a whole lot of air frames that fit such a criteria and there are even fewer that are for sale to/in the public hands, when talking about things that exist in the single digit realm for-sale quantity is a real factor.

This McDonnell Douglass F-4 is currently listed for 3,950,000 USD and perhaps gives a good basis for what such a plane would cost. The F-4 carries 3335 gallons of Jet fuel, at my local field where fuel is a reasonable 4.50 a gallon its going to cost you 15000 USD to top off the tanks. Fuel burn depends on how you run the aircraft, if you are slamming it at super sonic speeds all the time your hourly burn rate is going to be high.

Maintenance is harder to pin down. These jets were generally operated by the military so you are going to need to find a mechanic out in the world that is willing to deal with it. You may get lucky and find someone who has some military experience to do the work but if you cant you are going to need to hunt down all the maintenance manuals and related documentation for the aircraft which may be harder to come by than you think. On top of that you need to keep two J79 in working order that may already have a lot of time on them.

If you want to hop to the other side of the pond design wise this MIG-29 is listed for 4.60 Million USD and will get you into the super sonic spectrum and allow you to pull some G's.

So for ~4.5 Million and some non determinant maintenance costs, lets say conservatively 500,000 a year for fuel, oil, incidentals, maintenance, some new tires, avionics and other odds and ends you can have all the fun in the sky you want.


Why are modern commercial airplanes still so noisy? After years of scientific research on the performance of commercial aircraft like 777's or A320'S, the cabin noise is still very loud. Modern aircraft have new features like serrated engine nacelles, like on the 747-8 or 787 engines; improved aerodynamics; vortex generators; cabin insulation, etc. So why do aircraft still have so much cabin noise after all these improvements?

Like everything in aviation its a trade off of a few factors.

Efficiency: A non-running engine is pretty quiet but that's of little use to a pilot who needs thrust to actually go anywhere. Now you can have lots of sound suppression in an engine and alter the airflow and what not but in the end of the day jets work on Newton's third law so you still need to throw a hot mass out the back. Any attempt to baffle this will take a toll on efficiency. (Side note, car exhausts have a similar effect on car engines these days).

Weight: The generally accepted way to reduce noise is to insulate against it. What you are really doing is adding material to absorb the mechanical energy that is the sound wave and dissipate it. Generally this is done with insulation of some kind (there is a variety of styles). But like anything this takes a toll on weight and will reduce what you can carry.

Cost: Insulation does wear out and break down over time (its usually some kind of foam), so the more you have the more you need to replace. Also of importance is the fact that most of it gets removed during check to inspect the airframe so having a lot of insulation will add to your down time during checks (this costs the airline money in terms of hours).

If you take a look at some custom-built private planes you will see some of the things they can do in terms of added noise reduction and what those implementations look like. On the other hand active noise control is becoming a cheaper and more efficient technology in headphones which makes a case for not needing to improve insulation (it will eventually be cheaper to have a pair of ANR headphones for everyone than maintain a lot of insulation). A lot of the GA community has moved to ANR headsets which are much nicer than their traditional counterparts.


Are fighter jets designed to be so inherently unstable that a human can't fly one unassisted? I'm told that in order to be more maneuverable fighter jets are designed in a way that makes them impossible for a human to control without the help of a flight computer.  Is this actually true?  Would a modern fighter (like an F-22 or Su-35) crash if the stability computers died?

Further, if this is true, what design techniques are making them unstable and how do they help with maneuverability?

My short answer:

Stability is reduced by shifting the center of gravity aft.
Shifting it past the neutral point makes the airplane unstable, so movements away from the trimmed state are accelerated. This increases maneuverability.
Flight computers are multiple redundant, if one dies the others take over.
Slow unstable airplanes can be flown by a human pilot, but not fast unstable airplanes.

For the long answer, let me first clarify terms:

Static stability is the tendency of a system to return to it's old state after being disturbed. Take a pendulum: If you pull it to one side, it will return to the middle. Eventually.

Dynamic stability is the tendency of an oscillating system for the oscillations to die down over time. Take the same pendulum: It will swing from side to side, and  friction will ensure this happens with ever smaller amplitude.

Now we need to add dimensions, all three of them: Pitch, roll and yaw. An airplane can be stable in one dimension and unstable in a different one. I understand your question such that you ask about the static pitch stability (or longitudinal stability) of fighter aircraft.

The Wright Flyer was longitudinally unstable (see here for more). Once aircraft designers learned that aircraft can be made to fly stable, and learned that this is of immense benefit in pilot training, static stability became a requirement for new aircraft. When war in Europe broke out, the British forces were equipped with a superb training aircraft, but it was so stable that it took effort and time to convince it to change course. They were shot down in droves.

From now on, low stability was a prime requirement for fighters and aerobatic aircraft. Static stability is proportional to the control forces (more precisely: To the hinge moment of the respective control surface), so reducing stability gave pilots more response for the same effort. Longitudinal static stability is measured as the relative distance between neutral point (NP) and the center of gravity (CG). See here for more. Longitudinal static stability is achieved by placing the CG ahead of the NP. Shifting the CG back gives you a more responsive airplane, but also one which is more easily disturbed by gusts.

This is the design technique you asked about. Pretty simple, right?

Once you shift the CG aft of the NP, stability is lost and the airplane will increase deviations from the trimmed state. This can be helpful if you want large angle changes, and quickly. An unstable aircraft only needs a small kick and will do the rest of the maneuver all by itself.

This is how it helps in maneuverability. But it is even more helpful to reduce the inertias, especially around the roll axis, for a faster response. That is why all combat aircraft have their engines close to the center.

Of course, negative stability is not acceptable when you need to take your hands off the stick to get a map out or to pee on a long flight. So without computer control, the limit was a CG position near, but not aft of the NP.

With supersonic aircraft, things got more complicated. Now the aircraft operates in two flight regimes, one where lift acts at the quarter chord of the wing and one where it acts at mid-chord. Aircraft with low static stability become very stable in supersonic flight, and the tail surface has to create a high down-force so that the sum of all lift stays where the CG is. Creating lift always incurs a drag penalty, and in supersonic flight it has to be payed twice: One for the excess lift on the wing (which is needed to compensate for the tail's down-force) and one for the down-force on the tail.

Using a flight control computer offers the possibility to allow the pilot to let go of the stick without the aircraft going off course. Now the stick does not command elevator deflection, but pitch rate, and the CG can be moved back from maybe 12% of MAC (mean aerodynamic chord) to -2%. If you compare the wing areas of stable and unstable jets (Jaguar and Mirage F-1 are prime examples), you will see how much is achieved just by going back with the CG by a few percent of wing chord.

Conventional SEPECAT Jaguar and CCV version (picture from Ray Whitford's Fundamentals of Fighter Design). Both configurations have the same airfield and combat performance!

Can a human still fly such an aircraft? In glider competitions, the more daring pilots fly with relaxed static stability and have no problem to keep the aircraft under control. Even the Wright brothers could handle their unstable airplane, and handling improved when they moved the CG further back (If you want to know why, please post a new question. This answer is getting too long already!). However, the speed of an aircraft's pitch response is proportional to flight speed (and inverse to the pitch moment of inertia), so faster aircraft are harder to control. You can compare dynamic pressure with the stiffness of a spring: A stiffer spring moves the eigenfrequency of a spring-mass system up, and the same is true for the eigenvalues of the equations of motion of an aircraft. Given that the reaction time of a good pilot is at least 0.1 s (and more if he/she is tired), it is impossible to counteract motions with frequencies of more than a few Hertz. The lag means that the reaction comes too late and will support the motion. See this YouTube clip how that works out in practice. This crash was due to wrong signal gains, not a classic instability (after all, the flight computer was still working, but produced too strong elevator deflections).

I would venture to say that a human can still barely fly an unstable jet at low speed (after all, Tom Morgenfeld almost got the YF-22 under control), but once he firewalls the throttle, he will be always behind the plane, and will crash it soon.

Size helps: Bigger planes have lower eigenfrequencies, and light, big vehicles are easy to control, regardless of stability. All Zeppelins were completely unstable in yaw and above the critical airspeed of an airship (again, please ask for a more detailed answer on this aspect) also in pitch, but with one person each for the vertical and the horizontal control surfaces, and enough people on board to rotate them after 2 - 4 hours, nobody felt the need to make Zeppelins naturally stable.

If one computer dies, the others take over. Most unstable configurations have four parallel computers which cross-check their result to catch any malfunction. The Dassault Rafale uses only three, but adds safety by clever algorithms for checking the results.


Calculating lift with lifting-line theory in MATLAB I am attempting to calculate the lift generated by a model wing (~0.8$m^2$ wing area) using Prandtl's lifting-line theory in MATLAB. I am starting with the equation below from Anderson's 2005 book Fundamentals of Aerodynamics:

I have defined the $\frac{2b}{\pi c(\theta)}$ as '$\mu$' for simplicity, moved this back into the first summation and rearranged the above to:

$\Sigma A_n\sin(n\theta)(\mu + \frac{n}{sin(\theta)}) = \alpha(\theta) - \alpha_0(\theta)$

Presumably if I split the half-wing section (i.e from $\theta = \pi/2$ to $\pi$) into 10 segments I can generate a set of simultaneous equations from $n = 1$ to $10$ and $\theta_1$ to $\theta_{10}$. Solving that set of equations should then give me values of $A_1$ through to $A_{10}$. However, the values I'm generating for my coefficient matrix, $B$, (i.e. the $\sin(n\theta)(\mu...)$ bit) seem to be off, reaching 65 for $\theta_1$ and $n =10$. This is turn gives me incorrect values of the $A$ coefficients. Both are shown below:

This leads me to believe something must be off in either my re-arrangement above or my MATLAB code, which is shown below for completeness. It might be inefficient but please ignore that for now! Anyone able to assist with where I might be going wrong? Would be greatly appreciated!

MATLAB Code:

clear all
clc

% Wing segments:
N_seg = 10;
N_seg2 = N_seg;

% Flow:
U_0 = 15.433 % [m/s]
sweep = deg2rad(30); % Sweep angle
U_n = U_0 * cos(sweep); % Flow component travelling normal to wing sweep

% AoA and zero-lift AoA:
AoA = 0;
AoA_0 = -6.4;

% Wing twist, to be confirmed following further design:
twist = 0;

% Initiliase segment chord, theta and AoA matrix:
c = zeros(1,N_seg);
theta = zeros(1,N_seg);
AoA_seg = zeros(1,N_seg);

% Lambda range:
for Lambda = 0.25:0.25:0.5

% Wing area, span, chord etc.
S = 0.8; % [m^2], wing area
b = 1.56827; % Wing span
s = b/2; % Semi-wing span
c_root = (2*S) / ((1 + Lambda) * b); % root chord [m]

c(1) = c_root; % setting root chord as inner segment wing chord
theta(1) = pi/2;
AoA_seg(1) = AoA;

% tip chord [m]
c_tip = ((2*S) / ((1 + Lambda) * b)) * (1 - ((2* (1 - Lambda)) / b) * (b/2) );
% mean aerodynamic chord [m]
MAC = (2/3) * (c_root + c_tip - (c_root * c_tip) / (c_root + c_tip))

    % Segment chord lengths and theta to wing tip:
    for N_seg = 2:N_seg
        % Chord length of each segment, linear interpolation
        c(N_seg) = c_root + (c_tip - c_root)/N_seg2 .* N_seg
        % Theta of each segment;- first segment must not be 0,
        % otherwise singular matrix obtained
        theta = pi/(2*N_seg):pi/(2*N_seg):pi/2;

        % AoA of each segment , linear interpolation
        AoA_seg(N_seg) = AoA + (twist)/N_seg2 .* N_seg
    end
    mu = 2*b./(pi*c);

    % Setting up RHS (B) and LHS matrix:
    for i = 1:N_seg
        for j = 1:N_seg
            B(i,j) = (sin(j .* theta(i))) .* (mu(i) + j/(sin(theta(i))))
        end
    LHS(i) = (AoA_seg(i) - AoA_0) * (pi/180);
    end

    % Finding A coefficients:

    A = B\transpose(LHS)

end

Your general formulation seems fine. Two things to point out:

If you choose to sample only half the wing, then you cannot utilize the relation: $C_L=A_1\pi A$. You would have to do the integration: $C_L=\frac{2}{V_\infty S}\int^{b/2}_{-b/2}{\Gamma(y)dy}$. That being said, maybe you can simplify the expression and get a closed-form result? Haven't tried myself.
Your code doesn't seem to handle for the case where $\sin(\theta)=0$.

You can take a look at the code below for reference:

% Wing area, span, chord etc.
S = 0.8; % [m^2], wing area
b = 1.56827; % Wing span
s = b/2; % Semi-wing span
taper = 0.25;
Cr = S/s/(1+taper);
Ct = taper*Cr;
MAC = 2/3*Cr*(1+taper+taper^2)/(1+taper);
AR = b^2/S;

% AoA and zero-lift AoA:
AoA = deg2rad(0);
AoA_0 = deg2rad(-6.4);

% Discretization
num = 10;
[N,Ts] = meshgrid(1:num, linspace(0,pi,num));
ys = -s*cos(Ts);
Cs = (Ct-Cr)/s*abs(ys)+Cr;

% Construct linear system
B = 2*b/pi./Cs.*sin(N.*Ts)+N.*sin(N.*Ts)./sin(Ts);
if Ts(1)==0
    % Singular case, use l'Hopital's rule
    B(1,:) = N(1,:).*N(1,:);
end
RHS = AoA - ones(num,1)*AoA_0;

% Solve
A = B\RHS;
CL = A(1)*pi*AR;


From which ATC station is push-back clearance requested? When parked at the gate that requires a push-back onto the taxiway, from which ATC station is the push-back clearance requested? Is this the ground or apron station, or is that still departure clearance delivery station (as they better know the slot times, etc.)?

At controlled aerodromes, the aerodrome control tower is responsible for separating aircraft on the maneuvering area (from other aircraft, and from obstacles, vehicles etc.). If a pushback is done onto a taxiway which is a part of the maneuvering area, the aerodrome control tower must approve this first. At large aerodromes, there are different tower working positions, operating on different frequencies and with different callsigns. So the "which exact ATC station" question really varies from airport to airport. You might talk to "xx tower", "xx ground" or something else entirely.

Note that at many airports, the gate area (apron) is NOT a part of the maneuvering area. Technically speaking, ATC is not responsible for separating aircraft outside of the maneuvering area. Instead, pilots, tug drivers, vehicles and other persons on the apron follow different priority rules when a pushback is required (traffic laws normally state that vehicles shall yield to aircraft; the rules of the air have rules regarding aircraft right-of-way).  However, some major airports have designated positions for coordinating this effort, which - from the pilot perspective - looks just like ATC. They operate on a VHF frequency, and typical callsigns are "xx apron" or "xx ramp". Note, however, that such stations do NOT provide air traffic control - technically, they provide advisory service, and they are not responsible if a collision occurs, because ATC separation is not provided outside the maneuvering area. Persons staffing such positions generally do not have to be fully qualified air traffic controllers (this will vary from country to country), and are sometimes employed directly by the airport, instead of an air navigation service provider.

In summary: if the pushback occurs on the maneuvering area, the aerodrome control tower has to approve it. If not, no ATC approval is required, but there might still be other stations coordinating pushback.


If a pilot takes a MEL checkride and later a SES checkride, is the pilot now also qualified for MES/SEL? *Assuming all checkrides are passed.

If a pilot gets a multi-engine land rating on a primary checkride, and later takes a checkride as single-engine sea, is the pilot now qualified for multi-engine sea and single-engine land?

I suppose an unspoken question would be, does a multi-engine rating automatically allow for single-engine PIC privileges, or is the restriction between single and multi-engine a 2-way street?

No. You must take the training and pass the practical test for each add-on rating. See 14 CFR 61.63(c).


Why does the A330 sit nose-down on the ground? The passenger versions of the A330 have a small but noticeable nose-down angle when sitting on the ground:

(Image by contri at Flickr, via russavia at Wikimedia Commons.)

In contrast, A330s built as freighters have a noticeable bump under the nose,1 which allows the nose gear to be mounted low enough down that it reaches the ground with the aircraft sitting level:

(Image originally by Ceecookie at Wikipedia, via Leptictidium at Wikimedia Commons; cropped and modified by me.)

I can think of some disadvantages of having a nose-down body angle on the ground:

Boarding (and, to a lesser extent, deboarding) would be slowed down slightly by the passengers having to negotiate a sloped cabin floor (especially those having to drag very heavy pieces of luggage uphill in the process).
Ground servicing of cabin equipment would be expected to be made somewhat trickier by the sloped floor (a galley cart or other rollable object inadvertently left unattended for a moment would roll away towards the cockpit, potentially causing damage or injury if it has a sufficient length of unobstructed aisle to get up to a good clip).
The nose-down body angle tilts the aircraft forwards compared to the situation that would arise with a completely-level fuselage; this moves the aircraft's center of mass forwards, shifting more weight to the nose gear that would otherwise be carried by the main gear, and, thus:

Requiring a heavier, more substantial nose gear than would otherwise be the case.
Reducing the maximum braking force available from the main gear (and thereby increasing the aircraft's stopping distance).
Slightly decreasing the aircraft's directional stability on the ground (ye olde wheelbarrowe effect).

Indeed, the vast majority of large airliners sit level on the ground, rather than nose-down, so why is the A330 different in this regard?

1: Freighter A330s converted from passenger aircraft have no such bump, as the aircraft's cargo-loading system is fully capable of dealing with the slight slope.

For one reason or another, the main gear was made longer that originally planned and it was decided to avoid the cost and weight also extending the nose gear to match. The nose down angle while rolling along the ground is no big deal one way or the other, except for looking bad and disconcerting passengers who wonder why the plane is tilted forward.

Lots of airplanes are like that, sometimes because of changes made during development, but more typically after fuselage stretches where the main gear had to be made longer to keep the extended tail off the ground during rotation (the flying pencil CRJ1000, which I used to be type rated on, feels like a backwards taildragger in that regard).  Obviously, the freighter needs a level cabin floor for loading, hence the nose gear band-aid; for the passenger versions, it's not really necessary and the blister and extra structure to do all that is just ballast and drag, serving no useful purpose.


How to justify that M_AC=0 for a symmetric airfoil? I learnt that for a symmetric airfoil, the pitch moment around the aerodynamic center should be null:

$$M_{AC}=0$$
$\hspace{11pt}$ How could I intuitively explain this statement?

I also learnt that for an airfoil with positive curvature, $M_{AC}<0$ and  for an airfoil with negative curvature, $M_{AC}>0$. I'm new at my Flight Stability course, and I don't know how to achieve these conclusions. I do know the definition of aerodynamic center, but it isn't enough for me to make such observations.
How could I deduce these statements?

Per definition the moment coefficient in the Aerodynamic Centre is independent of the angle of attack.
Since the airfoil is symmetrical, the moment around the a.c., $M_{a.c.}$ for an angle of attack $\alpha $ will be of equal magnitude and opposite sign at angle of attack $-\alpha $:
$M_{a.c.}(\alpha)=-M_{a.c.}(-\alpha)$

The two statements can only hold if $M_{a.c.} = 0$


How to enter an inverted spin? Spins cannot occur without stalls. To enter a spin, a pilot would hold the elevator up and let speed bleeds off. This is how you'd approach a stall. Just as the stall starts to develop, apply full rudder; now the world is spinning around you.

Now, how does one enter an inverted spin? How do you stall an aircraft while it is inverted?

When inverted, cut power. As speed bleeds off, gradually move stick forward keep your flight level, the nose will come up quite a bit over the horizon. It is important not to start sinking here as that will prevent the speed coming down low enough. When the stick hits the forward stop, kick in full rudder either way.

Enjoy!


What can be approach of balancing canard glider model? There are a lot of instructions out there about balancing classic wing configuration glider models. The center of gravity should be approximately 25% to 30% from the front edge of the wing, which is easy to control. The next step is flight tests and adjustment by adding or removing weight or moving the wing forward or backward. Using this approach I constructed several gliders with stable horizontal flight.

But what can be approach for balancing canard configuration glider model? Is it the only recommendation for the canard wing to produce more pitch applied to the center of gravity then the main wing produces, is it possible to verify this somehow before testing flights?

Static longitudinal stability is defined as the tendency of the airplane to pitch down when it slows down and vice versa. In other words, when the lift coefficient goes up, the pitch moment coefficient must go down. This is expressed in equations like $$-\frac{c_{M\alpha}}{c_{L\alpha}} = \frac{x_N - x_{CG}}{l_\mu} = X\%$$ with X a positive number between 0.1 and 0.2. $x_N$ is the lengthwise coordinate of the neutral point, counted positive backwards, and $x_{CG}$ is the lengthwise coordinate of the center of gravity. The neutral point of both surfaces is at their respective quarter point and the combined neutral point sits between both, proportionally closer to the larger surface. Place the center of gravity X% of the mean aerodynamic chord $l_\mu$ ahead of this common neutral point and your stability is X. 0.2 is a rather high value and 0.1 makes for better performance but also less inherent stability.

Now all that is missing is that elusive neutral point of a canard. To explain what follows, it is the combination of the neutral point of both surfaces scaled by their area and corrected for the downwash of the forward wing which reduces angle of attack changes on the rear wing. Since only the inner wing will be hit by this downwash, we need to get a grip on the fraction of the wing which is exposed. This involves the span ratio of both wings $\frac{b_c}{b}$ and the taper ratio $\lambda = \frac{c_{outer}}{c_{inner}}$ of the main wing.
$$x_{N_{canard}} = \frac{x_{N_{canard}}\cdot c_{l\alpha_{c}}\cdot S_{c}+x_{N_{wing+fuselage}}\cdot c_{l\alpha_{w+f}}\cdot S_{w+f}\cdot\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)\cdot\frac{2+(\lambda-1)\cdot\frac{b_c}{b}}{(1+\lambda)}}{c_{l\alpha_{c}}\cdot S_{c}+c_{l\alpha_{w+f}}\cdot S_{w+f}\cdot\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)\cdot\frac{2+(\lambda-1)\cdot\frac{b_c}{b}}{(1+\lambda)}}$$
In absence of a precise value, use 0.7 for the downwash factor $\left(1-\frac{\delta\alpha}{\delta\alpha_W}\right)$.


What are these Lights on the Center Column between Cockpit Windows on the A320? I have always noticed there is what appears to be three lights on the center column, which separates the pilot and copilot window of the Airbus A320 family. From the looks, the center light is red and two surrounding lights are white. They also seem to be present in other aircraft, like the A350. Though there they are far less visible and integrate more smoothly into the background colors.
My question is: What are those for? I have checked the FCOM and the internet but I could not find any information on them. See the image below, for the lights I am referring to.

These are not lights, they are the Eye Reference Indicators. They are used to find the correct seating position. You adjust your seat such that the white ball is exactly hidden behind the red ball on your side:

Since the A300 Airbus has provided an eye reference indicator on the centre structure of the windshield in all Airbus aircraft (fig.3). It enables flight crew to adjust their seat position so that their eyes position matches the eye reference point. The indicator is a device that is fitted with 3 balls painted red or white. To achieve a correct seating position, pilots must align the red and white ball meaning that the white ball is hidden when in the correct position.

(Airbus - Are You Properly Seated?)
This YouTube video shows how to properly adjust your seat on the A320 (you can see the eye reference indicators at around 1:30): CORRECT COCKPIT SEAT POSITION! Eye-Position-Indicator explained by "CAPTAIN" Joe
They are not unique to Airbus though, some other aircraft have them too, like e.g. the ATR-72 (left) or the Dash-8 Q400 (right):

(image source: theflyingengineer.com)
Boeing uses a different method for seat adjustment, so you won't find such indicators there.


Why does a subsonic nacelle inlet have a throat before the diverging section? Aircraft propulsion 2nd edition by SAEED FAROKHI" p.340

Aircraft propulsion 2nd edition by SAEED FAROKHI" p.123

As we can see from the image, the nacelle slightly converges towards a throat, before diverging. Why?

Is it to produce the lip thrust force? If so, is that force independent of the diffuser thrust (i.e., the  internal pressure rise in the diverging section) ?

And if the cowl lip  represents an airfoil operating at a negative angle of attack, shouldnt the  "lip thrust" be in the opposite direction?

The throat is a byproduct of trying to prevent flow separation at slow airspeeds or high angles of attack.

Below you can see a rudimentary sketch of the intake streamlines for different airflow conditions. As you can see in the first case, the air needs to curve around the tip of the lip at low airspeeds. Flow will separate, if the curvature is too strong. Flow separation can lead to all kinds of problems for the engine. In order to prevent flow separation, a thick and rounded lip is used. Superimposing such a rounded lip on the divergent geometry of the inlet results in a throat as you described.

Image by K. Aainsqatsi - self-made, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=11098086


Can a pilot or owner repair a seatbelt? I have a Cessna 210 with inertia reel seatbelts.  The spring coil on the retention wheel broke.

FAR 43, Appendix A (C).14 allows for owner replacement of seat belts
FAR 43.13.(b) requires that it be restored at least to its original
or properly altered condition.
and FAR 21.9.(a).5  allows a part produced by an owner / operator to be installed

so... can I as owner / operator:

repair the spring and re-install the seatbelt
or
purchase a used seatbelt with a working coil spring (but no good webbing) and replace the coil spring?

You cannot repair a seat belt. You can only replace it.

An owner-operator can perform preventative maintenance on their aircraft. That includes replacement of the seats and/or seat belts.


Do you need 40 hours in a helicopter to get a PPL add-on if you already have an ASEL PPL? FAR 61.109(c) says:

  For a helicopter rating. Except as provided in paragraph (k) of this section, a person who applies for a private pilot certificate with rotorcraft category and helicopter class rating must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in the areas of operation listed in §61.107(b)(3) of this part, and the training must include at least...

The question is, does the 40 hours have to be in a helicopter, or can hours in ASEL meet this requirement? All the training centers seem to be saying it has to be 40 hours in helicopter. Can you theoretically fulfill the other requirements inside the 20 hours training and 10 hours solo in heli and be compliant with 61.109(c) with only 30 hours in heli?

Actually, it's not clear to me if the 20 hours of dual has to be in helicopter.

Considering the difference between legal and smart, you could nonetheless potentially satisfy the training requirements with just a few hours in a helicopter. FAR 61.109 specifically notes that certain requirements must be satisfied in a helicopter:

  61.109(c)(1): "3 hours of cross-country flight training in a helicopter;"
  61.109(c)(2): "[...] 3 hours of night flight training in a helicopter that includes" a 50nm flight and 10 takeoffs and
  landings
  61.109(c)(3): "3 hours of flight training with an authorized instructor in a helicopter in preparation for the practical test..."
  61.109(c)(4): "10 hours of solo flight time in a helicopter..."

Theoretically, the flight training could be all combined, resulting in 3 dual and 10 solo hours in a helicopter, the rest of the requirements being fulfilled in a different category or class of aircraft.

The FAA issued an interpretation (O'Mara, 2012) about these rules in 2012. The discussion was about weight-shift-control aircraft (61.109(j)), but the reasoning is applicable to other sections of 61.109:

  In your letter you indicate that an examiner you spoke with stated that all of the aeronautical experience requirements specified in § 61.109(j), to include the 40 hours of flight time, 20 hours of flight training, and 10 hours of solo flight training must be completed in a weight-shift-control aircraft. That statement was incorrect.

  [...]

  ...[T]he requirement for an applicant for a weight-shift-control aircraft to have 40 hours of flight time may be met by obtaining flight time in any category and class of aircraft.

  Paragraphs (j)(1) through (j)(5) of § 61.109 specify the flight training that must be received by an applicant to receive a weight-shift-control aircraft rating. Flight training is defined in § 61.1 as "training, other than ground training, received from an authorized instructor in flight in an aircraft." Similar to the definition of flight time, it also does not include a limitation that the training be obtained in any particular category and class of aircraft.

That said, this is somewhat academic: in order to get a solo signoff from a CFI, a student would at least require nontrivial training time in a helicopter. I'll also cite another interpretation (Zomnir, 2010) I came across in my research, which has perhaps the most obvious statement in an FAA legal interpretation:

  First, you ask whether your student is required to be proficient to the practical test standards to pass a practical exam. The answer is yes.


What requirements are there for radio systems in aircraft? What are the requirements for radio systems in aircraft? For example, are there requirements about:

Reception ability
Transmit ability
Transmit power

This is primarily about voice communication systems, but information about other radio systems is welcome as well.

What are the requirements for radio systems in aircraft?

Generally there aren't any, at least as far as "being required to have radios".

It is perfectly legal to fly around in most of the United States in a Piper Cub with no radios of any kind.
Indeed, unless it's been retrofitted with one a Piper Cub would have no electrical system at all - so you would only be able to fly under "daytime VFR conditions" in airspace not requiring communication with air traffic control (absent special arrangements).

If you elect to fly an aircraft equipped with radios (as most of us do) there are specific requirements those radios must meet, which are described in Technical Standard Orders (TSOs).

This is not a comprehensive list, but:

For Communication radios these are outlined in AC 90-50D and TSO C-169a.
Transponders must meet specific TSO performance criteria per FAR 91.215
Navigation Radios (VOR/ILS/ADF) must meet specific TSO performance criteria if used under instrument flight rules.
The TSO number for these escape me at the moment but I'm sure you can look them up.
GPS systems must meet TSO C145c / TSO C146c requirements (if used for IFR navigation).
The Emergency Locator Transmitter (ELT) must meet specific TSO criteria.

The requirements obviously vary based on the type of radio, and typically reference other industry standards (for example TSO C-169a for VHF communication radios sends you over to RTCA/DO-186B for the actual performance criteria).


Why do some airplanes have a vertical rudder hinge while some others are canted? I was thinking that canted rudder hinges are used for modern aircraft while the vertical ones are used for old slow airplanes. But it seems that I was wrong since the SST Tupelov Tu-144 and the Concorde also have that vertical (or almost vertical) rotational rudder axis. Then my question is, what is the advantage and this advantage of such a rudder? Which one is better?

The vertical tail of the A380 in the last photo is swept back. Surface sweep is used for high subsonic wings and tail surfaces, to postpone the effects pf compressibility when approaching the speed of sound. And when the vertical tail is swept back, the rudder hinge will be at an angle as well: the rudder chord will be a relatively constant percentage of the vertical tail chord, for aerodynamic and structural reasons.

The two photos of supersonic aeroplanes show vertical tails that are not swept back - they cruise faster than the speed of sound and do not have to postpone any compressibility effect, they fly right through it. No reason to sweep anything back here.

And the photo of the little Cessna with swept back vertical tail: it only looks as if it is trying to postpone any compressibility effects. Of course, it does so by flying way slower than the speed of sound, and the vertical tail is only swept back because it looks super fast.