Forces in a slipping turn I have trouble understanding why the ball (on the turn & slip indicator) falls into the turn e.g. drop to the left when aircraft is rolled left, when in an uncoordinated turn (slipping turn). What are the forces that goes into this?

From reading up online, many sources quote that in a slipping turn, the aircraft is banked too much for the rate of turn. So, the horizontal lift component is greater than centrifugal force. As a result, the ball drops into the turn since there is less g force to "centralise" the ball.

Isn't centrifugal force suppose to be equal and opposite of the horizontal component of lift. How can being in a slipping turn make the centrifugal force lesser? Essentially, what is so special about slipping turn can allow the centrifugal force to change?

Image from: http://avstop.com/ac/flighttrainghandbook/forcesinturns.html

I have trouble understanding why the ball (on the turn & slip indicator) falls into the turn ... in an uncoordinated turn (slipping
  turn). What are the forces that goes into this?

The ball is displaced to one side because the aircraft is being allowed to fly sideways through the air.  The nose is not aligned with the instantaneous direction of the flight path, but rather is yawed to point to the left or right of the instantaneous direction of the flight path.

As a result, the airflow is striking the side of the aircraft and generating an aerodynamic sideforce, which is what causes the ball to displace to the side.

There are many reasons why an aircraft might tend to fly yawed slightly toward the left or the right in a turn.  Most of these reasons tend to promote a slip, rather than a skid.  Other than the most obvious reason of a significant, intentional deflection of the rudder by the pilot, these reasons include the curving nature of the relative wind during turning flight and its effects on tail surfaces (also described as "aerodynamic damping"), and the fact that the outboard wingtip must move faster, and thus tends to generate more drag, than the inboard wingtip.  In an aircraft with a propeller, P-factor can also play a role, tending to promote either a slip or a skid depending on the direction of turn.  The setting of the rudder trim may also be factor.  When the bank angle is changing, adverse yaw from the deflected ailerons and from the rolling motion itself plays a major role, strongly tending to promote a slip when the bank angle is increasing and a skid when the bank angle is decreasing.  In all cases, the cure is for the pilot to use the rudder as needed to compensate for these effects and align the nose of the aircraft with the instantanous direction of the flight path, so the nose is pointing straight into the "relative wind" and the airflow is not hitting the side of the fuselage.

Note that "having too little or too much lift in relation to the bank angle" or "having too little or too much centrifugal force in relation to the bank angle" are not causes of slips or skids.  Rather, they are results of slips or skids.  The causes are the aerodynamic effects listed above, creating a yaw torque that makes the aircraft fly slightly sideways through the air, despite the generally stabilizing effect of the vertical fin.  More on this later.

So why does the slip-skid ball get displaced off center when the aircraft flies sideways through the air?  When the airflow hits the side of the fuselage, this generates an aerodynamic sideforce, acting orthogonal (perpendicular) to the flight path in the direction that is roughly parallel to the wingspan,  This sideforce points toward the high wingtip in a slip, and toward the low wingtip in a skid.  This real aerodynamic force has been completely omitted from the diagram of the slipping turn attached to your question.  This is a major flaw in the diagram.

This aerodynamic sideforce contributes to the net aerodynamic force generated by the aircraft, so that the net aerodynamic force vector is no longer pointing
straight up" in the aircraft's reference frame, i.e. parallel to the vertical fin.

The slip-skid ball always tends to position itself in the curved glass tube at the point in the tube that is exactly perpendicular to the net aerodynamic force vector.  If the ball is positioned at any other point in the tube, it will "feel" an apparent force to the left or right, and will shift position.  If the net aerodynamic force is not pointing "straight up" in the aircraft's reference frame, then the ball will not position itself at the "bottom", i.e. at the center, of the curved glass tube.

Another way to look at the situation is to consider the apparent "load" acting on the ball, due to the combined effect of weight and "centrifugal force".  The ball always tends to position itself in the curved glass tube at the point in the tube that is exactly perpendicular to the "load" vector.  If the "load" vector is not pointing "straight down" in the aircraft's reference frame, then the ball will not position itself at the "bottom", i.e. at the center, of the curved glass tube.

The key thing to understand is that the apparent "load" felt by the aircraft structure and contents, including the pilot and the slip-skid ball, is nothing more than the mirror image of the real aerodynamic force vector generated by the aircraft.  This is explored in more detail in a several answers to the related ASE question "Forces "felt" by pilot, G-meter, inclinometer--are they the aerodynamic forces generated by the aircraft, or the sum of weight+centrifugal force?".

The diagram is misleading because it suggests that some mysterious effect, presumably somehow related to turn rate, yet seemingly unrelated to any real aerodynamic force, is affecting the amount of "centrifugal force" generated by the aircraft or acting on the aircraft.  This is false.  The difference in the magnitude of the "centrifugal force" vector between a "coordinated" turn and a "slipping" turn is due to the way that a sideslip generates a real aerodynamic sideforce vector, as described above.

While it is in fact true that the "load" acting on the ball can be said to be equal to the vector sum of weight and "centrifugal force", this is not a concept that has a lot of explanatory power, unless we understand that the "centrifugal force" is intimately related to the real aerodynamic forces generated by the aircraft.  The "load" vector, which can be said to be the vector sum of weight and "centrifugal force", is nothing more than the mirror image of the real aerodynamic force generated by the aircraft.  The "load" vector is always equal and opposite to the vector representing the real aerodynamic force generated by the aircraft.1

An extreme case of a slipping turn is a forward slip or sideslip (these are actually exactly the same thing, just aimed differently in relation to the target), where the aerodynamic sideforce from the slip is sufficient to bring the turn rate all the way to zero, creating a linear flight path.  Whenever the flight path is linear, the slip-skid ball doubles as bank angle gauge.  The most extreme case of a non-turning (linear) sideslip is vertically-banked sustained knife-edge flight, like we often see at an airshow.  In this case the aerodynamic sideforce vector from the airflow striking the side of the fuselage (plus an additional component due to the engine thrust line being aimed above the horizon) is supporting the entire weight of the aircraft, and the wing's lift vector is zero.  In this case, the "load" vector is simply equal and opposite the aerodynamic sideforce vector from the slip (including the component due the engine thrust), because aerodynamic sideforce vector and the net aerodynamic force vector are one and the same.  The net aerodynamic force vector is equal to the weight of the aircraft but acting in the upward direction, while the "load" vector is equal to weight of the aircraft, acting in the downward direction.

While established in a steady-state turn at a constant turn rate and airspeed, trimmed for hands-off flight, if we then pull aft on the stick to "load up" the wing with "extra" lift to boost the G-load to a higher-than-normal value for the bank angle, or if we push the stick forward to "unload" the wing to decrease the lift vector and drop the G-load to a lower-than-normal value for the bank angle, the turn rate will immediately change, and so will the horizontal component of the "centrifugal force" vector.  The diagram above might tend to lead us to think that the ball will immediately shift off-center.  This is not the case.

In a situation like this, the flight path will curve up or down, introducing a vertical component of "centrifugal force".  The vector sum of weight and centrifugal force will still be "straight down" in the aircraft's reference frame, and the ball will stay centered.

Or to look at it from a much simpler perspective-- when we pull the stick aft or push the stick forward to change the wing's angle-of-attack, we are increasing or decreasing the magnitude of the lift vector, but we aren't introducing any aerodynamic sideforces-- we aren't yawing the fuselage to fly sideways through the air.  Because the net aerodynamic force vector still acts "straight up" in the aircraft's own reference frame-- parallel to the direction that the vertical fin is pointing-- the ball still stays centered.

(For simplicity, we're ignoring the forward or aft components in the net aerodynamic force vector-- the components that wouldn't show up in a head-on view of the aircraft.  The basic explanation here remains the same regardless of whether thrust is exactly in balance with drag, or not.)

Of course, "loading" or "unloading" the wing in this manner during a turn will also drive a change in airspeed, as the flight path curves up or down.  For an aircraft of a given weight at a given bank angle, there is only one value of the lift vector (G-loading) that will yield a steady-state turn at constant airspeed.2

  From reading up online, many sources quote that in a slipping turn, the aircraft is banked too much for the rate of turn.

From the content above, you'll see that this isn't always true.  It can be true given certain constraints, but it is not a concept with a lot of explanatory power.

As noted above, if we are established in a stabilized turn, and then we pull the stick back, or push the stick forward, to boost or decrease the lift vector and G-loading, the turn rate immediately changes, but the ball does not shift off-center.3

The idea that (for a turn at a given airspeed) "in a slipping turn, the aircraft is banked too much for the rate of turn" is only true given the constraint that there is zero upward or downward (skyward or earthward) curvature in the flight path.  And the only way4 that (for a given airspeed) we can have a mismatch between the bank angle and turn rate, while not allowing the flight path to curve up and down, is that we are applying the rudder to expose the side of the fuselage to the airflow and generate an aerodynamic sideforce, or we are failing to apply the rudder as needed to cancel out other aerodynamic torques that are tending to expose the side of the fuselage to the airflow.  If we do this, while adding or subtracting power as needed, we can indeed create the following situations.  Starting from a stabilized coordinated turn at a constant airspeed and altitude, we can--

a) vary the turn rate and turn radius while holding altitude, airspeed, and bank angle constant

b) vary the bank angle and while holding altitude, airspeed, and turn rate and radius constant

The ball will deflect off center in both of these sitations.  It is equally true to say that the ball goes off-center because the centrifugal force vector is no longer correctly matched to the bank angle, or because the load vector (the vector sum of centrifugal force and weight) no longer points straight "down" in the aircraft's reference frame (i.e. parallel to the vertical fin), or because the net aerodynamic force vector no longer points straight "up" in the aircraft's reference frame (parallel to the vertical fin), or because the pilot is using the rudder in a way that exposes the side of the fuselage to the airflow, which generates an aerodynamic sideforce.

The pilot's rudder usage is the key to centering the slip-skid ball, or to intentionally driving it off center.  The sideways force we "feel" in a slip is the real aerodynamic force generated by the air striking the side of the fuselage, which changes the direction of the net aerodynamic force vector, and its mirror image, the "load" vector.  Explanations centered around some sort of "balance" or "imbalance" between bank angle and turn rate, or bank angle and lift force, or bank angle and G-loading, are misleading, and have little explanatory power, and certainly do not apply to aerobatic flight.  These explanations tend to obscure, rather than illuminate, what is really going on.

Some of these sorts of explanations are well suited to a car driving on banked track, or a bobsled going down a banked track, but not to flight, where the trajectory is not constrained to have a specific turn radius, and also is not constrained to have no upward or downward (skyward or earthward) curvature.5

Note that in flight, generally speaking, changing airspeed while holding the bank angle constant creates no tendency toward a slip or skid, though the turn rate and radius will both vary.  As long as the net aerodynamic force vector continues to point "straight up" in the aircraft's reference frame, there will be no slip or skid.  Though again, we can come up with specific contrived situations where varying the airspeed while holding the bank angle constant does cause a slip or skid-- for example if the turn rate and radius are constrained to remain constant as the airspeed is changed.  Again, this can only happen if the pilot applies the rudder to expose the side of the fuselage to the airflow, generating an aerodynamic sideforce.

  I have trouble understanding why

This is not surprising, because you have been provided with faulty explanations, as well as faulty diagrams.  The diagrams attached to your question have omitted the aerodynamic sideforce vector caused by the airflow striking the side of the fuselage.  The diagrams attached to your question give the impression that the net aerodynamic force vector generated by the aircraft is the same in all 3 cases (coordinated flight, slipping flight, and skidding flight), when it actually is not.  The diagrams attached to your question give the impression that the "load" vector can somehow be something other than the mirror image of the net aerodynamic force vector, when it actually cannot be anything other than the mirror image of the net aerodynamic force vector.

To read more about the specific errors in the vector diagrams attached to your question, see the related question What is missing from these diagrams of the forces in slips and skids? and answer What is missing from these diagrams of the forces in slips and skids? .

Other related ASE questions or answers:

(Q) Forces "felt" by pilot, G-meter, inclinometer--are they the aerodynamic forces generated by the aircraft, or the sum of weight+centrifugal force?

(A) Forces "felt" by pilot, G-meter, inclinometer--are they the aerodynamic forces generated by the aircraft, or the sum of weight+centrifugal force?

(Q) What does the balance ball actually indicate?

(A) What does the balance ball actually indicate?

The topic of accuracy of the diagrams illustrated in your question has also been discussed on Physics Stack Exchange.  See--

(Q) Is this vector diagram of the forces at play in turning flight correct?

(A) Is this vector diagram of the forces at play in turning flight correct?

Footnotes

1 -- For simplicity, we're assuming that the slip-skid ball is located near the aircraft's CG, or aircraft's yaw rotational rate is constant, or both.  A change in the yaw rotational rate will create an apparent centrifugal or inertial force that displaces the slip-skid ball to one side if it is located far forward or aft of the CG, and this displacement does not reflect any real aerodynamic force component.  For example, if the yaw rotational rate is increasing toward the left, a slip-skid ball located far in front of the CG would tend to be displaced toward the right, and a slip-skid ball located far aft of the CG would tend to be displaced toward the left.  These second-order effects are beyond the intended scope of this answer, and are also beyond the scope of what pilots are generally able to notice in actual flight.

2 -- For simplicity, we're overlooking the fact that magnitude of the lift vector associated with a steady-state coordinated turn at some particular bank angle and some particular airspeed and angle-of-attack is very slightly different when we are descending or climbing, than when we are maintaining altitude (with respect to the airmass in all cases).  For modest dives and climbs, this is a very minor effect and definitely not something that someone still learning about the basics of slips and skids should concern themselves with, but in the interest of accuracy, this point should not be completely omitted.  To learn more about the "unloading" of the lift vector in a descent or climb, see What produces thrust along the line of flight in a glider? and Does lift equal weight in a climb?

3 -- Surprisingly, Wolfgang Langewiesche's classic flight-mechanics-for-pilots book "Stick and Rudder" (1944), which contains a lot of good material on the physics of turning flight, does make the claim (see pages 205, 219-220, and 223-226) that while excess back pressure on the stick does not cause a slip or skid, too little back pressure on the stick can cause a slip, with the ball deflected to the low side of the turn, which should be corrected by increasing back pressure rather than by applying inside rudder.  This is probably the only faulty idea in the entire book.  The 3rd edition of the book "Modern Airmanship" (1966, edited by Neil Van Sickle, Major General USAF) has similar content on pages 308-309.  By the 8th (1999) edition of "Van Sickle’s Modern Airmanship", edited by John F. Welch, Lewis Bjork, and Linda Bjork, this content had been deleted, while the remaining content on the physics of turning flight had been retained (page 441).  For more, including specific quotes from these books, see https://web.archive.org/web/20180905112047/http://aeroexperiments.org/critiques.shtml .

4 -- Strictly speaking, it's not exactly true that the only way that (for a given airspeed) we can have a mismatch between the bank angle and turn rate, while not allowing the flight path to curve up and down, is to deflect the rudder to the side in a way that exposes the side of the fuselage to the airflow.  But it's a good first approximation.  For a more nuanced view, we have to consider the effect of the sideforce generated by the rudder itself.  For example, we could shut down one engine on a conventional piston twin and apply the rudder just enough to keep the fuselage exactly aligned with the airflow.  Since the sideforce from the rudder itself is not zero, the ball will be slightly deflected (toward the working engine) when the nose of the aircraft is pointing straight into the relative wind.  If we bank into the good engine as needed to cancel all turning tendency, we'll end up flying along in a straight line (zero turn rate) while banked slightly toward the good engine, as discussed in detail elsewhere on ASE, such as here, here, and here (link to be added).

5 -- Speaking of bobslebs on curved tracks, "Hang Gliding Training Manual" and "Performance Flying" by Dennis Pagen both include extensive content-- including an illustration of a bobsled on a track-- purporting to show how, as a pilot enters a turn while flying a hang glider, if he or she doesn't immediately "load up" the wing with an adequate pitch input, the glider will slip sideways through the air, toward the low wingtip.  Also the idea that an intentional reduction of angle-of-attack and lift and G-loading while turning will make the glider slip sideways through the air toward the low wingtip.  See for example pp. 128-129 in "Hang Gliding Training Manual", and page 45 in "Performance Flying".  A similar idea appears on page 70 of Peter Cheney's "Hang Gliding for Beginner Pilots" (3rd edition, 1997).  I've checked these ideas in flight with a yaw string, in hang gliders as well as conventional aircraft, and found no basis for them.  For more on the content in these books, see https://web.archive.org/web/20180905112047/http://aeroexperiments.org/critiques.shtml .


How is human waste taken care of on commerical aircraft? Trains, as far as I'm aware, just dump human waste on to their own tracks. There are often warnings like: DO NOT USE the toilet while the train is stopped at the station voiced through the PAs or signed up on the walls. The Daily Mail, in 2014 had an article mentioning: Train companies are to be ordered to adapt their trains to stop dumping sewage on the tracks.

I always kind of assumed that commercial airliners did the same, but I've never heard any warnings about using the restrooms aside from when the seat-belt sign is on.

How do commercial airliners take care of human waste?

Human waste from the lavatories is collected on-board and later disposed of at the airport after landing.

(Image Source: WikiMedia Commons - Author: Mnts)

Check out this related question here on Aviation.SE:
Where does my pee go when I flush at 35,000 ft?


Could you encode data in a Black Box ping? Black boxes put out a sonar ping at about 32kHz. From what I've read, depending on sea conditions the ping will be audible at the surface out to about 16 km, leaving a search area of around 1024 square km.
By adding a little complexity to the ping circuit, you could encode within it the last received GPS position of the box, before it hits the water. With average sea depth and average sink angles of less than 45° that information would cut the search area down to about 10 square km.
Is there some reason this tech is not used, or is it still in the rigorous testing pipeline these devices must go through?

Current underwater locator beacons (ULBs) have a detection range of only a few km. 16 km would be under exceptionally favourable conditions. So in most cases the GPS data would not be of any additional help. It might tell you where the aircraft was before it hit the water, not exactly where the debris ended up after falling through several Km of water.

An advantage of current ULBs is their extreme simplicity and small size. There's no need for external data-feed and power connections. There are no issues of openings in the casing for waterproof connectors that also need to withstand the g-forces and pressures expected to be endured by a ULB. They are easy to test regularly and relatively easy to maintain.

From what I have read, current proposals for improvements to ULBs mostly focus on

increase duration from 30 days to 90 days
increase range by changing frequency from 37.5 kHz to ~10KHz

There are trade-offs here - longer duration means bigger batteries. changing the frequency may make it harder to distinguish from other sources of sound in the ocean.

An interesting 2011 dissertation describes the difficulties involved  and discusses modulating the beacon signal to include data.


Can I apply for ASEL and Glider class ratings at the same time? I would like to get both FAA Private ASEL and Private Glider class ratings.  I was told I could take lessons for both ASEL and Glider at the same time under a Student certificate.

Can I apply for a Private certificate with ASEL and Glider ratings at the same time? And is it more beneficial to do this than to get ASEL first then Glider?

You "apply" for a rating or certificate by showing up at the examiner with proof of your written test and taking a flight test -- so of course you can only do one at a time! But I think you want to know if you can train for both airplanes and gliders at the same time, under the authority of the same student pilot certificate. Yes you can do that.

However I do not recommend it. They are very similar of course, but there are enough differences that while you're learning to fly, you should just pick one and get your certificate. Then learn the other.

If you want to fly both airplanes and gliders, there is no particular order that is better than the other. I fly both airplanes and gliders; I happened to do airplanes first because there was no convenient glider field close to where I lived at the time. Now I fly both and I feel they complement each other and it's great!

But while you're learning, pick one and get it done; then do the other one.

(This may only be correct with the FAA in the US. And to be fully FAA-pedantic, Airplane and Glider are "categories"; "single-engine land" is a "class" that is available in the Airplane category. Thus ASEL is category-and-class, but Glider is just category.)


What happens when lightning strikes an airplane? What happens to an airplane when hit by lightning?

Since the aircraft is not grounded, the physics are the same as with a car that gets struck by lightning. The aircraft's skin will be charged, but very little current will be induced in internal parts in the process. The outer skin will act as a Faraday cage and protect the inner parts from the electric field.

This is greatly helped by a metal structure. Composite aircraft parts need a copper mesh over all external surfaces to give them sufficient electrical conductivity. The terminals of that mesh must be connected to the metal parts of the structure to make sure that all parts will have the same electrical potential in case of lightning strike.

However, the ionized air caused by lightning will leave burn marks on the surface, and an inspection of the affected part and the electrical equipment is advisable.


Why are planes using rivets & not welded construction? I was motivated to ask by this question: Why are airplanes riveted and not screwed?

Why not welded construction? Is welding these alloys too difficult? I come from a refining background & in a lot of hazardous duties welded construction has become the norm because integrity is easier to guarantee than screws, bolts or rivets.

Just wondering what's different in aviation? Isn't repair, safety, weight & aerodynamics all better with a welded construction?

Short answer: High-strength aluminium alloys are tricky to weld correctly. Aluminium is such a fine material for aircraft structures that the need to rivet it is gladly accepted.
Two things are important:

While steel has a temperature range in which it gets more and more runny, aluminium alloys change from solid to liquid within a few degrees. Also, heat conductivity in iron-based alloys is lower than in aluminium, so heating steel locally will keep the surrounding material cooler and more solid compared to aluminium. While welding thin sheets of steel is trivial, it needs lots of experience in aluminium. For very thin sheets, special equipment like a water-cooled copper backing on which the aluminium sheets rest, so their back is cooled, are needed. Also, the melting temperature of steel and titanium is high enough for it to glow long before it melts, while aluminium will melt without giving you any optical hint of its temperature.

High-strength aluminium is produced by progressively aging and precipitation hardening the material. The usual alloys use copper atoms dispersed through the aluminium matrix which locally distort the atomic lattice and strengthen it. If they are heated and rapidly cooled by welding, the copper distribution would be changed and the material would be weakened around the welding area. To harden the finished structure again is rather impractical in most cases, so riveting is the better alternative.

A third speciality is the oxide layer on aluminium, which has a higher melting temperature than the base material. You need an AC TIG welder to disrupt the aluminium oxide layer, so your choice of welding techniques is rather limited.
Also, riveted structures are easier to inspect and to repair. Most repairs need to remove aircraft structure for access, and a riveted structure is easier to disassemble and to put together again after the repair using slightly thicker rivets.
My experience with aluminium welding stopped at 4mm thick sheets; while thicker ones were easy to weld, I never managed to weld thinner ones. You sit in front of your structure and heat the spot where you want to start welding. Watching it through the darkened head screen you wait until the spot under the arc becomes glossy, which signals that the surface has started to melt. Now you need to add your welding wire like crazy to keep the spot from heating more, and get the spot moving. If you fail to do so, a second later you will have a hole under your arc, because the aluminium has molten completely and has fallen away. Doing this with 2mm sheets was a pure exercise in futility for me - the moment the surface became glossy it fell away already.

Edit:
Thanks to @voretaq7 for sharing the link about friction stir welding in the comments! This is made possible by precise positioning of the parts and a computer-controlled welding head and will see wider application in the future. Eclipse Aerospace claims it helps them to avoid 60% of rivets in their jet aircraft.


What are common downsides to leasing an aircraft to a flying club? I plan on owning my own plane in the coming years and using it to get my PPL.  I've recently been turned on to leaseback with flying clubs.  The pros seem to heavily outweigh the cons, but I always wonder.  What are the common (and uncommon) downsides to employing a leaseback with your own airplane?

A leaseback can be advantageous, especially when you use the aircraft for your own flight training and travel.  But there are some things you need to watch out for:

Other people will be using your aircraft - whether people admit it or not, nobody will take care of the plane as well as you do. If you think you might take issue with trash in the cockpit, a scratch on the panel, or a tear in the seat, you might want to give this a second thought. In my experience, flying clubs seem to have pilots that take better care of the aircraft, since membership is controlled.
You'll have to schedule your flights - just like everyone else (though some concessions are usually made for the owner). The bottom line is, you won't be able to just get up and go fly somewhere at a moment's notice anymore.
You have to play by someone else's maintenance rules - While you have to comply with FARs  in any case, the lessee typically will have stipulations that they (or their designee) will perform any required maintenance on your behalf and your say in the matter will be limited. Of course, you still have the honor of paying for any maintenance performed (typically at little or no discount).
Your capital is tied up - This can be a good or bad thing. It's good in the sense that aircraft tend to be a good hedge against inflation risk. It's bad because you're precluded from using the money on something else. This is one of the main reasons that flight schools/clubs LOVE leasebacks.
If you're looking to make money... it won't happen - most leasebacks only help offset the expense of operating the aircraft. If you're looking for a return on your investment, better to look somewhere else. Do it only to reduce the cost of ownership on something you want anyway.
Finally, YOU take on the financial risk of the aircraft - in most leaseback situations, you are reimbursed for any use, but are still responsible for any finance and indirect (and sometimes direct) maintenance costs. These have to be paid, whether the aircraft is flying or not. And if business starts to slow down... well, I'll give you a hint: it's not the flying club that has to pay to keep your plane airworthy.  This is the main other reason flight schools/clubs LOVE leasebacks.


What is wrong with my understanding about total pressure for a rocket nozzle? I’m using the ideal rocket equations and isentropic flow but cannot determine what is wrong in my understanding about total pressure. Can anyone explain where I am wrong using the following example?

Example:

$\gamma = 1.4 $

Converging-diverging nozzle Area ratio $A/A^\star = 12.0$

Inside the rocket chamber $P_0 = P_t = 12 MPa$

Exit area $A_e = 0.002 m^2$

Mass flow rate $\dot m = 2.5kg/s$

$Thrust=5000N$

Supersonic flow

Atmospheric pressure $P_{atm} = 101.3 kPa$

Solution:

Based on isentropic equations - (e.g. https://www.grc.nasa.gov/www/k-12/airplane/isentrop.html)

Mach No. $M = 4.127$

$P/P_t = 0.0056$

$T/T_t = 0.227$

Substituting in $P_t$ gives:
$P = P/P_t * P_t = 0.0056*12 MPa = 67200 Pa $

Then based on the ideal rocket equation (i.e. https://www.grc.nasa.gov/WWW/K-12/rocket/thrsteq.html)

$Thrust = v_e * \dot m + A_e (P-P_{atm}) $

Rearranging for exit velocity:

$v_e = \frac {Thrust - A_e (P-P_{atm})}{\dot m} $

$v_e = \frac {5000 - 0.002 (67200 - 101300)}{2.5} = 2027.28 m/s$

Then the exit density can be found by rearranging this (also from the isentropic equations website):

$a = \sqrt{\gamma \frac{P}{\rho}}$

Rearranges into:

$\rho = \frac{\gamma P}{a^2}$

Speed of sound
$a = v_e / M = 2027.28 / 4.127 = 491.22 m /s $

Substituting into the density equation:

$\rho = \frac{1.4 * 67200}{491.22 ^ 2} = 0.390 kg/m^3$

And here is the problem/confusion:

From the question the total pressure is $P_0 = P_t = 12 MPa$
This is also supposed to be the total pressure at the nozzle exit because it is reversible/isentropic flow.

But $Total Pressure = Static Pressure + Dynamic Pressure$
$Total Pressure = P + 0.5 \rho v_e^2 $
$Total Pressure = 67200 + 0.5 * 0.390 * 2027.28^2 = 868624 Pa$

And this is nowhere near 12 MPa... so what part am I not understanding correctly?

I'll just answer my own question thanks to Koyovis' comment  :)

Koyovis was right - I didn't realise the difference between how dynamic pressure is calculated for compressible vs incompressible flows.
There are some useful notes on it on pages 4-8 of this link (http://mae-nas.eng.usu.edu/MAE_5420_Web/section5/section.5.5.pdf)

In summary:
Bernoilli’s equation is

$Total Pressure = P_{static} + P_{dynamic}$

For an incompressible flow,
$P_{dynamic} = 0.5\rho v^2$

For a compressible flow, use the isentropic equation
$P_t / P_{static} = (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}$

To get this in a way you can compare to Bernoilli's equation, make total pressure the subject of the equation, add & subtract $P_{static}$ from the right hand side, and rearrange the equation slightly (shown in the link above) gives:

$P_t = P_{static}  +P_{static} ( (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}-1)$

This is the same form as Bernoilli’s equation where the dynamic pressure is:
$P_{dynamic}=P_{static} ( (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}-1)$

Putting in the values from the example gives the correct result.


Why not just drop an engine on fire? I am interested in the case of Qantas Flight 32 and similar incidents, where there was a fuel leak after a number two engine explosion due to the disintegration of the turbine disc. It was a very dangerous situation since the fuel leaks were very close to the failed, then burning, engine. And to make the matter worse, fuel dumping system was down due to hydraulic failure.

Although the crew managed to shut it down, the engine took considerable amount of time for fire to be extinguished completely.

I wonder in those kind of situations, could adding eventually a "drop-engine-on-fire" feature make the situation less dangerous?

You may consider the following:

Modern airplanes could fly with one engine out.
Dropping the engine must be in water to not cause any harm to the people below.
Usually, the engines are mounted on the wing only at three points.
Even if the main evidence of the investigation (that is, the burning engine) will be lost and maybe never found, I think that saving lives is a bit more important.

EDIT :
In response to comments, I think that every feature (even if its seems silly) is subject to pro-con studies. For example, Boing 747 had fuse pins  that are designed to break cleanly in case of excessive loads/vibrations.

The current position of regulators like the FAA is that dropping things off of airplanes in flight is a bad thing. Dropping extremely large and heavy things like engines would be extremely bad. This poses an unacceptable risk to people and property on the ground (or even in the water). Most aircraft with engine fires do not kill anyone on board, so why risk killing people on the ground?

The preferred method of dealing with engine fires is similar to that of fire protection for buildings; take steps to prevent them, and provide methods for putting them out should they start. We don't go blowing up buildings that are on fire just because it might be quicker than putting it out.

So instead manufacturers are required to provide many features to prevent fires. Areas around the engines are separated into zones by firewalls and seals, required to prevent spread of fire. Any flammable fluid is required to be contained and drained, and separated from ignition sources.

If a fire does start, the engines have fire suppression systems installed. The system has to be tested to show that it provides the required suppression to a fire, and the aircraft generally carries enough suppressing agent for at least 2 uses. Even if this doesn't work, the isolation features built in should prevent the fire from spreading, and possibly allow it to burn itself out of fuel or oxygen. In most cases an aircraft is close enough to an airport to land before this becomes an issue.

These measures have been shown to be adequate. When was the last time someone died because of an engine fire on an airplane? Fires are extremely rare, and fires that can't be dealt with by the existing measures are even more rare.

There are many more reasons why dropping engines is a bad idea.

Some designs allow an engine to shear away from the wing in the event of a crash, but this is not designed to work in flight. Aircraft that drop items like weapons in flight require extensive design and testing to make sure that the object will drop away from the aircraft properly. An aircraft in an emergency like an engine fire could be making extreme maneuvers that would make this even more difficult.

Objects like weapons that are designed to be dropped regularly still malfunction. How can you make an engine that's been installed for decades separate reliably? If you are relying on this feature for safety, there will be added cost for designing, installing, and maintaining the system. Those costs could be reduced if you don't rely on the feature as much. But then if the current measures are enough, why accept that extra expense?

Investigators generally take interest in engine fires. If you drop the engine, there goes the best evidence for figuring out how the fire started, and what could have prevented or mitigated it. Something large like an engine would be easier to find than smaller parts, but there is still the risk of loss or damage hindering the investigation. If the problem is bad enough that airplanes are dropping engines, it would be better to fix the problem than address the symptoms.

If you constrain pilots to only dropping an engine over water, this complicates things further. There are already rules about where airliners can dump fuel, but dumping fuel is generally a less stressful situation. The pilots chose to stay in the air and dump fuel, so any emergencies are generally less urgent and they have time and attention for where they could dump fuel. If there's an engine fire that requires jettison of an engine, things are going to be more hectic in the cockpit, and it may not be the best time to be considering where this would be allowed, taking things like trajectory into account. And going back to the cost issue, if this would be limited to only areas over water, you couldn't rely on this option to always be available, so is it worth the cost?

Airliners can fly with an engine shut down, but missing an engine is a different matter altogether. The aircraft is required to be tested and certified in many different conditions with an engine shut down. How can we test this with a missing engine? Do we drop an extremely valuable engine every time we test? Do we just make the flight with one engine, which is still in testing itself? Do we not test, and accept that after going to all the trouble of allowing jettison of an engine, the aircraft may not be controllable in some situations?


Flight planning tools - Data foundation I would be interested to learn how GA Flight planning tools work regarding the calculation of the necessary information for flight plans where you usually would use the manual charts (performance, endurance, range...) How do these platforms provide this info? Most of the info for old planes is from the 50,60s where no digital data has been provided and manufacturers don't give away these data usually. And data like performance, endurance etc is aircraft specific and necessary to do the right calculations when I do it manually.

My goal is to provide a platform to do it digitially. What do you think is the right approach to provide data like this to the platform.

When I start planning for a flight, charts in the manual are needed to plan properly (Landing and takeoff lengths, enroute information, endurance, range) This is mostly provided just as a chart and would be great to work digitally with.

Many of the modern flying apps like Foreflight do most if not all of these calculations for you once you have programmed your aircraft parameters in. As mentioned in my answer to your other question a good deal of this data is specific to the plane. W&B is specific to every aircraft and will vary depending on how the plane is setup. Things like takeoff performance and fuel burn are generally similar for an airframe but since some aircraft have multiple propellor or engine configurations available there is a variance in the numbers. Foreflight allows you to program in climb, cruise and decent fuel burn and will calculate fuel usage for you. If you have target airspeed (climb, cruise, decent) in it will calculate time en-route for you, and you can select your altitude for wind/heading calculations. The only thing I'm not sure foreflight does is takeoff distance (yet).

I have not spent any time behind a G1000 yet but I am under the impression that the various newer glass panel options can do some of these calculations as well.

Larger aircraft have FMS systems to aid in a great deal of this once you get out of the GA world.

You could try and digitize some of the functions within some of the more common POH's (C172, the various Cherokees, C150 etc) but that would not technically be legal for flight since its not the POH for the specific airframe.


How do I calculate this question? find the position of the centre of gravity in % Find the position of the centre of gravity in % of the MAC (mean aerodynamic chord) if the distance from the datum to the LEMC (leading edge of mean chord) is 13 meters, the distance to the TEMC (terminal edge of mean chord) is 19 meters and the distance from the datum to the centre of gravity is 15 meters?

I believe that the Length of the Aerodynamic Chord is $19\ m - 13\ m = 6\ m$, and that the CG is $2 m$ from the Leading Edge of the Mean Chord (LEMC).

Therefore, I believe that the CG in percent is:
$$
\mathrm{\frac{CG}{Length\ of\ Chord}} = \frac{CG - LEMC}{TEMC - LEMC} = \frac{15-13}{19-13} = \frac{2}{6} = 33\%
$$


What weapons do A-10 Warthogs typically carry? What is the A-10 Warthog's typical operational load out? Seeing as it is built a tank buster, does it actually carry anti-tank weapons, and what situations would they be used e.g would they (A-10's) ever use an AGM or ASM on a tank?

The 30 mm GAU-8/A Avenger  with over 1000 rounds is in itself a potent anti-tank weapon, which can fire depleted uranium rounds, among others. It carries a number of anti-armor weapons like the AGM-65 Maverick, Hydra 70 rockets and cluster bombs in 11 external hardpoints with a maximum load of 7200 kgs.

The 'typical' operational layout of aircraft depends on the mission. For example, the image below shows one configuration.

By U.S. Air Force photo by Master Sgt. Blake R. Borsic - http://www.af.mil/shared/media/photodb/photos/000217-F-0656B-004.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=505075

In the above picture, the aircraft can be seen carrying a ALQ-131 ECM pod under one wing and two AIM-9 Sidewinder air-to-air missiles under the other wing for self-defense, along with AGM-65 and bombs. If these are not required (for example, where there is little danger from anti-aircraft fire), they can be replaced with others or the aircraft is flown without one so as to increase range/time on target.

By U.S. Air Forces Central Public Affairs Photo by Master Sgt. William Greer - http://www.flickr.com/photos/dvids/5510306058, Public Domain, https://commons.wikimedia.org/w/index.php?curid=37832505

In this photo, we can see the aircraft carrying only precision munitions and targeting pods.


Why do drones near airports necessitate cancelling flights? From Wikipedia's page on the Gatwick drone incident:

  Between 19 and 21 December 2018, hundreds of flights were cancelled at Gatwick Airport near London, England, following reports of drone sightings close to the runway. The reports caused major disruption, affecting approximately 140,000 passengers and 1,000 flights.

I assume that drones can crash into aircraft, negatively impacting the safety of flights.

Suppose airport staff report sightings of a drone. What should the airport do?
Is this considered an emergency? Why or why not?
Can other planes request clearance to avoid the area?
Have any documented instances of drones resulting in injury to or loss of crew, passengers, or plane?

Re the title question, a plane hitting a crow is enough to take out an engine, as shown in the following video.

Crows weigh about 500–600g. A drone such as a DJI Phantom weighs two to three times as much and isn't nearly so squishy.


Does load factor affect roll rate? To be clear I am referring to a given aircraft at a given weight, speed, attitude, etc everything else the same except one is at say 0.5g and the other at 2.5g will there be a difference in roll rate if both have the same aileron deflection etc.

From my reading I think the answer is no but I am looking for something definitive. The question arises because the typical nose low UA recovery is UNLOAD then roll and recover to the horizon. For what reason is the UNLOAD? Some have suggested that it is because you will get a better roll rate to get the wings level. This I am not sure about.

I believe the main reasons for unloading are that (1) you want to avoid asymmetrical 'g' loads and (2) when holding a large amount of elevator force the pilots ability to input up to maximum aileron deflection is limited.

So I would appreciate someone confirming or denying if there is an aerodynamic reason that roll rate is affected by load factor?

One of the best reasons to unload before using your ailerons is to make sure you are not
stalled.  If you are, using ailerons could reverse and make you roll the opposite way.

Ailerons can be evil when used uncoordinated with rudder, yet very helpful rolling out of an inverted dive.  The danger increases if the aircraft is yawing and near stall.  So it is much safer to recover in 2 steps, unloading first.


Does dihedral improve roll stability in gusty conditions? Against roll instability, should I add more dihedral/a high wing?
I'm trying to build an RC plane that can handle windy conditions. For that I want extraordinary aerodynamic stability.
For longitudinal static stability, I make my planes quite nose-heavy, and compensate for that with a greater tail downforce. That works.
What doesn't work is lateral stability/roll stability. In theory, if a gust rolls my airplane, I will have a sideslip. Due to a high-wing configuration, and a dihedral angle, I will have a rolling moment that will counteract the original roll angle.
Image from avstop.com, modified

So I made my plane attempts have great dihedral/polihedral angles, and a really high wing with a solid surface underneath to further help counteract any rolling.

Any time I went crazy on dihedral and high wings, my planes kept crashing agressively due to very sudden and very powerful side/back flips. At first I didn't understand, as these features should've made my plane almost un-rollable.
However, what I suspect is that these features had the opposite effect. Correct me if I'm wrong, but looking a the first diagram, what I see is that when a plane flying at level flight encounters a sideways gust, it will also make the plane agressively roll over, just as if the sideways gust was from a roll induced sideslip.
Is this a correct assessment? Did these roll stability features make my planes roll around uncontrollably?
Is there a way to have my plane react normally to a sideways gust while having static roll stability? Do I need way bigger vertical stabilizers for these dihedral values?

Dihedral does not help roll stability to a side force gust.  Quite contrary, a gust will roll the plane into a slip, which then will turn the plane downwind (courtesy of your elevator turned rudder).
Firstly, I would not recommend setting a model plane with excessive "static stability" in windy conditions.  You want to be staticly stable, but not so much that a gust will pitch you up into a stall.  Higher wing loading also helps.
But roll stability from dihedral is good for cruising flight, but no match for a side gust, especially in a model.
What you must do to keep dihedral is to put side area below the CG.  This is known as "anhedralling".  Sea gulls lower their wingtips.  Cessna put the wing (and fuel) on top.  Consolidated built their famous PBY 3 Catalina with the engines on top too!  All 3 want to roll into the wind, not away.  This is the key.
A fourth design, the common paper airplane, does this with gusto.  The side "keel" balancing with the upraised wing around the center of gravity is best for cross wind roll stability.
So you may experiment by raising of lowering your CG, adding side area, and glide testing in a cross wind.  Your second model seems to have more of a chance.  You may try a shorter and more square vertical stabilizer, and a bit of weight (a coin or two) on top of the wing at the expected Center of Pressure.


Jettison and Dump fuel - Do they have separate meanings? According to the definitions, is there any difference between Jettison and Dump fuel?  Does one of them use a different procedure, or they are simply synonyms?

In the context of fuel, jettisoning fuel would imply that the fuel is in a container that can be detached while in flight. In the linked image, the large tank below the main body is an external fuel pod that can be jettisoned. Jet with external fuel tank I believe in WWII that P51s would have drop tanks for escorts that would be jettisoned once an enemy encounter was expected.

Dumping fuel would simply mean that there is a type of valve located on the fuel tanks that can be actuated in some way such that the fuel can freely exit. Jetliner dumping fuel

To say differently, if one was holding water in a bucket, pouring the water on the ground would be 'dumping fuel' and simply dropping the bucket would be 'jettisoning fuel'

Reviewing the wikipedia article on Fuel Dumping, the author(s) make no such distinction between dumping vs jettisoning and seem to use jettison solely.

In the context of ATP, after reading the wikipedia article on fuel dumping, I believe that they are considered synonyms. I believe a non-pilot would go more along with my definition however.

I answered in this manner as I believe that the OP is not a native English speaker and that clarification was needed as to distinguishing the two as they seem very similar. I am a native English speaker, but lay no claim to being a master of it.


How much is the Boeing 787 Dreamliner affected by teething troubles now, one year after commencing commercial operations? Boeing's Dreamliners have been hit several times by small bugs, like fires in the batteries or other engineering issues. Are such incidents frequent when new types are introduced, and how much has the situation improved one year later?

You can get a rough idea by searching B788¹ on AvHerald.

Airlines do report technical problems² to their overseeing aviation safety board (in India it is, somewhat unusually, part of the civil aviation authority, DGCA; in most countries it is a separate agency, e.g. NTSB in USA). And those publish them.

Also for technical problems, aviation safety board of country of manufacture would be consulted, so NTSB should know about most technical problems of B787 and it should be possible to find them in their bulletins.

The Aviation Herald collects such reports from various sources. Only incidents that occur in flight get reported and only “interesting” subset of them, but if there were any serious problems with a type, they would come up.

Further if some problem requires mitigation steps to be taken, the civil aviation authorities publish airworthiness directives, that describe steps airlines must take (and under which conditions and until when) to be permitted to operate the aircraft. Since Boeing is based in USA, any airworthiness directive regarding it would most likely be first published by FAA and usually taken over by aviation authorities in other countries.

As far as I know, the batteries were the only serious issue of B787 that required immediate modification throughout the fleet. There are many small ones, like any aircraft, but there does not seem to be anything else that would be significantly more common or serious than on other types.

¹ AvHerald lists aircraft by their ICAO identifiers in the article titles (and full name in the text). The ICAO identifiers distinguish the series, so Boeing 787-8 gets B788.

² Those that affect safety. So non-working in-flight entertainment would not have to be reported, but a short-circuit in it should be.


How do you handle frost on a plane? If you plan to fly on a cold morning and there is frost on the plane, what needs to be done?

Do you have to remove the frost from the windows? wings? tail? fuselage?
How should you remove a thick layer of frost?
Can you rub the frost until it's smooth and take off?

You should definitely not attempt flight if there is frost on the windows that would affect visibility, or on any of the wings or flight control surfaces.  Frost accumulation on such surfaces can reduce the lift generated by your wings, possibly causing a crash on takeoff.

According to the NTSB, frost the size of a grain of salt, distributed as sparsely as one per square centimeter over a wing's upper surface, can destroy enough lift to prevent a plane from taking off.

The cheapest solution is to try to re-position the plane on the ramp so that the sunlight has a chance to melt it away.

You can also temporarily move the plane into a heated hangar until it melts, or ask the FBO to do so for you.  If the forecast is predicting a chance of such conditions the night before, you can usually call the FBO on the night before and ask them to move your plane into their hangar just for the night, in advance of your morning flight.

If you're really anxious to go, you can ask the FBO to apply a heated deicing solution to your plane, but be prepared to pay for it (it's priced by the gallon and the amount needed will depend on the size and amount of frost of your plane).  Not all FBOs will have deicing capabilities, particularly if the region does not commonly experience such conditions.


How is mass flow rate computed? I have been wondering how the mass flow rate is determined by the area of the engine air intake surface and the incoming speed of the air.

In still air, it is understandable that a stationary engine takes in air at the same rate as it is exhausting based on the preservation of the momentum of the air. Then, the mass flow rate through the engine intake surface can be easily calculated.

However, in still atmosphere with the same engine solely mounted to an aircraft cruising straight, level, and at constant speed $v$, it is not obvious to me how to determine the mass flow rate.

In particular, keeping the air intake surface area unity, can we determine the mass flow rate based on $v$ alone?

The short answer is: No. One cannot calculate the inlet mass-flow based on the velocity alone. However in order to "know" the velocity ( $v$ ) you most likely would have measured everything you need (see below).

Your confusion with regards to the in- and outlet mass-flow might be based on the fact that the intake does not "collect" all air upstream depending of the flight-velocity.

The key is not to think of the geometric area of the intake but the area of stream tube around the air entering the intake.

The velocity upstream of the intake is not equal to the air-speed (subsonic flight) because the potential field of the intake decelerates (or accelerates) the flow upstream of the intake and expands (or contracts) the stream tube (from Wikipedia):

In order to calculate the mass-flow we need to determine the speed and the properties of the air.(please observe that the air-speed ( $u$ ) at the intake is not equal the free-stream-spped ( $v$ ) )

Usually the measured properties are:

Total Pressure, $p_\mathrm{t}$ (Pitot Tube)
Static Pressure, $p_\mathrm{s}$ (Static Pressure Port)
Total Temperauter, $T_\mathrm{t}$ (info, examples)
Relative Humidity, $\varphi$ (for simplicity not used below. Humidity changes the heat capacity)

The following equations are used to calculate the mass-flow ( $\dot{m}$ ). Simplifications are commonly made for slow speeds unnecessarily excepting a 5% error. The following approach does not use these simplifications.

The equation which needs to be solved in the end is:

$\dot{m} = A \cdot \rho \cdot u$

Neglecting blockage effects and non-uniformities, the density ( $\rho$ ) and the velocity at the inlet ( $u$ ) are unknown and need to be derived from the measured values.

First the Mach-Number is calculated by solving the following equation (see Nasa) for the Mach-Number, $M$:

$\frac{p_\mathrm{s}}{p_\mathrm{t}} = \left( 1 + \frac{\gamma - 1}{2} M^2\right)^{\frac{\gamma}{\gamma-1}}$

Here $\gamma$ is the isentropic coefficient which is around $1.4$ for air depending on the humidity.

Second, with the Mach-Number, $M$, we can use another isentropic relation to calculate the static temperature, $T_\mathrm{s}$:

$\frac{T_\mathrm{s}}{T_\mathrm{t}} = \left( 1 + \frac{\gamma - 1}{2} M^2\right)^{-1}$

Third, using the ideal gas law the density ( $\rho$ ) can be calculated:

$\rho = \frac{p_\mathrm{s}}{RT_\mathrm{s}}$

Here $R$ is the Specific Gas Constant.

Forth, using the equation for speed of sound, $a$

$a = \sqrt{\gamma R T_\mathrm{s}}$

Finally Fifth, utilising the definition of the Mach-Number, $M$, the air-speed ( $u$ ) can be calculated:

$M = \frac{u}{a}$

Now all missing values are available to compute the mass-flow.
This approach can easily expanded to cover humid air by adjusting $R$ and $\gamma$ with respect to the relative humidity.

As mentioned in other answers: The inlfuence of non-uniformity of the flow caused by boundary layers or inlet distortions (e.g. inlet-ducting) need to be corrected using a calibration.


Are there any unmanned variants of Solar Impulse? Solar Impulse proved to be capable of flying day and night without a single drop of fuel while carrying a pilot. Wouldn't it be easier to design an unmanned solar powered aircraft? Are there any? What about helicopters? Are they more difficult to make? What about automated landing at night time? I suppose there should be a lot of people interested in fuel-free aircraft, but why the only one I've heard of is Solar Impulse?

The raison d'être of Solar Impulse is to carry a single human being around for a long time. Of course an unmanned version would have better performance (I doubt it would be easier to design), but what would be the purpose of such a craft?

Solar Impulse is so fragile that it can only fly in unusually clement weather, so any scheduled service is out of the question. This would need a conventional backup, and if one is around anyway, why not use the conventional backup also on the few days of the year which would allow Solar Impulse to fly? It would also save the staff of meteorologists and ground support that Solar Impulse needs.

For high-altitude observation solar-powered drones have been tested (and found lacking) in the past. Helicopters have such high power demands that solar propulsion is not a viable option.


Why was the boat mounted this way on the underside of the wing on the "Landseaire" flying yacht? After seeing the below image of the "Landseaire" flying yacht here I was curious to know why the small boat mounted to the underside of the plane was mounted the way it was.  At least to me (and @Dave who mentioned it in his answer containing this picture), I found it interesting that the boat would be mounted "backwards" and also so far from the body of the aircraft.  Can someone explain why this position and orientation was necessary or why it was chosen?

The reason is the "flat tail" of the boat. The drag this "flat tail" produces when mounted "reverse" is much less then the dynamic drag it would induce the other way around.

https://en.wikipedia.org/wiki/Drag_(physics)

  I don't see how the link helps explain why the flat side produces less drag than the pointed side.– Ron Beyer

The published cw coefficients (see answer of @jwzumwalt ) show the best value for the drop shape facing towards the air stream.

There is no entry for a reversed drop. But it is evident that is would have a higher cw coefficient.

A closer look at the boat reveals it also has a drop shape. But this is facing to its tail. Therefore the boat creates less drag when mounted "reverse".

  If this was the case, wouldn't the boat cause less drag moving backwards through the water also? – andy-m

It might but the usually draft of the boat is no so deep that a relevant part of the "flat tail" is actually in the water.

source: https://upload.wikimedia.org/wikipedia/commons/2/2d/Bundesarchiv_B_145_Bild-F056332-0004%2C_Bonn%2C_Bundesgartenschau%2C_Seen.jpg

On the other hand a little turbulences at the boats tail could help to keep direction in the absence of  a steering paddel...

  This explanation is wrong - an anemometer spins with the forward facing blunt half of the hemisphere causing the most drag - en.wikipedia.org/wiki/Anemometer – jwzumwalt

The most important differences between the anemometer an the drag experiment is:
The half bowls in the anemometer are hollow which "catches" the wind way better than the flat surface of the filled half bowls in the experiment.


What advantages are there to operate a corporate flight department under Part 135 as a separate entity instead of in-house as a Part 91? This question specifically involves FAA regulations in the U.S.
Some U.S. corporate flight departments will set up a separate business entity for their flight operations to limit the liability of the parent company. Because this business operates with on-demand passenger transport for compensation as its primary business, it must be operated as a part 135 (or 125 if they are running big ships) instead of part 91. In some cases, this new subsidiary will provide additional revenue stream by servicing outside clientele in addition to meeting the travel needs of its parent company, and sometimes they remain exclusive.
If a corporate flight department doesn’t want to use their aircraft to supplement income with charter flights, is liability the only reason to set up and operate as a Part 135 instead of keeping it Part 91?
Are there any other reasons, aside from liability and supplemental income, to bother with a 135?

There is no reason you can't operate as a part 91 department but simply adopt the standards of 135.  Government contracts work this way.  It's all pt91 work but adheres to pt135 rules.
Actually holding a 135 is a lot of work and is expensive.  The training and checking requirements are onerous for your average flight department.  How do you feel about potential career limiting checkrides every six months?  You also need to keep the planes up to a higher standard.  This means observing TBO hours and calendar times.
You don't mention aircraft types, but Continental requires piston engine overhauls at 12 year intervals or upon reaching the time limit, whichever is first.  Even a piston single becomes expensive to operate.


What does mean PPM unit and why it is the unit of flow rate for the ASU(Air start unit)? I was seeing the specifications of several ASU(Air start unit)s and found out that makers of such machines provide the flow rate of the ASU in PPM. Why? why not Volumetric flow rate?

Example:

TS200 Air Start Unit

PPM stands for Pounds Per Minute (not to be confused with Parts Per Million, which also uses PPM), which is a unit of mass flow rate.

The specification gives the mass flow rate rather than the volumetric flow rate because the mass of air going in and out of the ASU is the same, where the volume going in and out is different because of the compression.


What is the security policy for getting into a Control Tower in the US? Are there any regulations the FAA has ruled to get into a Control Tower? Do people need any special kind of security clearance? Are there forbidden items or other special rules?

I'd also want to know if there's a way for non-pilots (your average civilian Joe) to get into a Control Tower.

It is still possible (and in my experience not that difficult) to get FAA tower tours (and sometimes even TRACON or Center tours), though you should be reasonable in your expectations here -- You are probably NOT going to get a tour of the tower at JFK or LAX.

The FAA used to actively encourage pilots with an instrument rating (or pursuing instrument training) to sign up for ATC facility tours under a program called "Operation Raincheck" -- This program is still running, though it's been substantially scaled back (Poke around on the FAA Safety Team's SPANS system, or sign up for notices in your area and keep a sharp eye out. Tours usually have a very limited number of slots available, and they fill up fast!).
(As an example of how quickly the slots fill up, when N90 (New York TRACON) last had a Raincheck event it was full within a few hours of being announced. No, I didn't get a slot.)

At towered GA airports - particularly ones with a lot of flight training - you can also contact the tower directly. The printed AOPA Airport Directory has facility numbers in it but that's a little out of date. You can also get the phone number from Flight Service, the local FSDO, or ask the tower/ground controller for the number if they're not too busy.

When you call the facility explain that you're a pilot/student pilot and would like to arrange to tour the facility (and if possible shadow a controller). They will usually ask for your name, address, airman certificate number, and a few other things to run a check and get approval to let you in to the facility. You will also be expected to have government issued photo ID with you when you show up.

My experience arranging a tour of our local tower was extremely pleasant - they are happy to do the tours and let you shadow a controller, and I even got to pick their brains about some peculiarities in local procedures.
It's a great educational experience, and I would recommend it for any pilot/student pilot.


What is the procedure for a rejected Jet/Rocket Assisted Takeoff (J/RATO)? Seeing the image of a Lockhead LC-130 doing a jet assisted takeoff in snow mentioned in this answer, I wonder:

What is the procedure for a rejected takeoff when using JATO?
Can the JATO rockets be turned off once ignited?

(Image source:  Wikimedia Commons)

JATO (more correctly: RATO) typically uses simple solid-fuel rockets for their ease of maintenance, simple construction, long-term storability, robustness, and reliability. However, a simple solid-fuel rocket has to burn out once ignited, there is no way to shut it down. Techniques for extinguishing and even throttling solid-fuel rockets do exist, but they were developed rather recently, have not yet been commercialized, and clash with the goal of simplicity.
Currently, the "usual" way to shut down a solid-fuel rocket is to rapidly decrease the pressure in the combustion chamber, aka an explosive Flight Termination System, aka blowing up the rocket – not something you want to happen with a rocket strapped to your wings (IOW directly below the fuel tanks). Alternatively, you could get rid of that extra thrust by jettisoning the rockets. Having a dozen or so flaming projectiles uncontrollably flying around the airplane is also not a good idea.
So, you either reject before igniting the rockets or you don't reject at all. V₁ is when you hit the button, which may be as low as 0 kt, depending on the specific application.


Where can I get the FAA data that defines airspace, including MOAs etc.? I was looking at http://www.gelib.com/aeronautical-charts-united-states.htm, where you can download shape files for Google Earth that show US airspaces.  I'm writing some software that has a similar need and need to find a source for this data.  I'm looking for data that defines the extents of airspaces including MOAs, restricted areas, etc.

I have been pouring through the FAA's website with no luck.  The link I referenced above says its source was the National Aeronautical Charting Office (NACO), which I'm having very little luck finding as well.  I think it may have been renamed, thus the poor results.  I also called the FAA and can't seem to find anyone there that knows where to transfer me.

So, does anyone here have any helpful pointers on finding said information?  I want to pull it directly from the source to make sure it is always up-to-date and accurate.

Take a look at the National Flight Data Center.  The data appears to be contained in NASR. I mentioned in my comment that there may be no official source for spatial data, but I think this may include what you are looking for.
These are derived products. The cannonical definitions are the legal descriptions found in FAA Orders with the prefix 7400. At the time of this writing, JO 7400.11 (current version E) covered airspace classes A,B,C,D and E as well as reporting points. The JO 7400.10 (current version C) covered special use airspaces. Both publications can be found at the FAA's Website.


Where can you join a STAR & STAR Altitudes? Do you have to join a STAR at a transition or can you join it at any point?

For example, coming from the south, can I join the DODJE5 arrival at FZT rather than AEX?

DODJE5: https://aeronav.faa.gov/d-tpp/1908/06918DODJE.PDF

Also, if I were to fly this in a standard GA prop aircraft, would the MEA looking numbers (ex. 5,000 for FZT -> CQY) be required?

You can only file, and ATC can only assign, one of the listed transitions. However, that doesn't mean you will actually fly it as charted (unless you lose comms). If you filed lower than the charted altitudes, you are not expected to climb. In practice, ATC mau give you direct a later fix (e.g. FZT) where you'll join aircraft descending to your level. Or, if you're slow, they might put you on vectors off the STAR to keep you out of the way of faster traffic. It all depends on what else is going on around you, so it's difficult to predict what will actually happen.


Does full nose up trim provide approximately the elevator pressure for Vg? I heard during an aviation presentation that full nose-up trim provides approximately the right amount of elevator pressure for Vg with the power at flight idle. The presenter also followed up with that this is a certification requirement. Is this the case? Is there anything in 14 CFR 23 or 25 about it?

This technique is commonly called the "minimum trim glide" - basically you smoothly trim the aircraft to full nose-up after the engine quits (at a rate which won't cause it to "balloon"), or alternatively you establish best glide speed manually and then trim to full nose-up to relive the pressure on the yoke so you can concentrate on other things.

I'm not sure if this is a "certification requirement" (I can't find anything about it in Part 23's discussion of trim requirements, the old CAR 3 regulations, or in AC 23-8C.

Empirically I can tell you that it's true for the loading configurations I've tried it in, on the Piper PA-28 and Cessna 172 aircraft I've flown. I have been lead to believe it is generally true for single-engine piston light aircraft: With no engine thrust full nose-up trim will produce something close to best glide speed. Before relying on it in any particular type of aircraft however I would perform a test at altitude by bringing the power to idle and verifying that you get something reasonably close to best glide speed.

Note that there is a potential pitfall of the minimum trim glide when practicing engine out maneuvers: Upon recovery it is possible to cause a trim stall. As power and airspeed increases in the recovery elevator/trim authority is also increased, and full nose-up trim is generally adequate to cause the aircraft to stall in this situation, therefore special care must be taken to prevent the aircraft from pitching up to an attitude which would cause a stall.


What impact did the Concorde have on business of other sub-sonic airliners? The Concorde was anyways a technical marvel of its time, and a status symbol for society. Its fares were well above the rest, costing USD2999 + taxes for a JFK-LHR journey. But, at the same time there are reports that British Airways indeed made $750 million from it, over 27 years:

  It was not that Concorde operations made losses. In fact, on an
  average Concorde made an operating profit of £30-50 million a year
  for BA. BA reportedly earned £1.75 Billion in revenue for Concorde
  services against an operating cost of around £1 billion during its 27
  years of commercial operation (1976-2003).

So, what impact did it have on the business of other sub-sonic airliners? Probably, economy class would not have been so much stricken, since its fares were more than 3 times higher. But what was the impact on business class travel of other airlines and airliners, between the destinations served by the Concorde?

A few thoughts:

The British Airways Concordes only operated scheduled on the London Heathrow- New York JFK route. This implies this is the route under the most competition. There has been enough pressure on this route even after the demise of the Concorde for several companies (as well as BA) to try business-only services on this route.
By experience, changing flights is never quick in the US since you need to go through customs and add time margin to ensure you make the connection (Think three hours connection time on the ground). I think even before the current security state you'd rather have taken a direct flight than change from a Concorde to a domestic flight if possible.
I don't think the charter destinations would have caused much impact on other airline operations.


How are aircraft identified as unregistered? When someone operates an unregistered vehicle on public roads we know how they are caught: Police routinely pull people over, and they always check registration of vehicles.  In fact, police will frequently "run tags" without even stopping cars.  They have a realtime connection to a nationwide registry that can report the details of the tagged car and whether a tag is expired or stolen.

Apparently there are a good number of fines and prosecutions for operation of unregistered aircraft.  And at least some are reported by ATC.  I'm going to assume that nobody is foolish enough to file a flight plan listing an unregistered aircraft ID.

So how does ATC check aircraft registration?  Do they have live access to the registry?  When someone calls an ID over the radio are there controllers that routinely check the ID's registration?  Is it illegal to make up an ID for radio interaction with ATC?  Are there people that visually watch for tail numbers and discrepancies with radio calls?

Aircraft don't have to be actually flying for people to notice something fishy going on. In particular, there are always people hanging about at airports keeping an eye on everything going on. Furthermore (at least in the US), aircraft registration details are a matter of public record and can be easily looked up online.
For example, here is the registration detail page for John Travolta's private Boeing 707: https://registry.faa.gov/AircraftInquiry/Search/NNumberResult?nNumberTxt=N707JT
As you can see it contains complete registration details including type, validity status, expiration dates, owner name and address, and airworthiness status.


Why did the Beechcraft Starship use a delta wing? From what i've seen delta wings are more appropriate to supersonic designs, and have poor lift/drag ratios at subsonic speeds. The Starship had a cruise speed of 300 kn, so what was the reasoning behind the wing design?

I know that the engine on the back/pusher configuration puts the CoG further back, so you would want to put the wings behind, but the Piaggio Avanti does the same with conventional wings.

Because it was something of a Burt Rutan trademark since his early days. His first designs, the VariViggen and VariEze, had such a wing already, and apparently he saw no reason to optimize the wing of the Starship.

Rutan VariEze in flight (picture source)

The only true innovation was a variable geometry canard, which he even patented. Being only 85% scale, the performance of the Starship prototype was formidable, but once the real production aircraft had been designed with full compliance to airworthiness regulations, the Beech Starship turned out to be less impressive. The additional wetted surface produced more friction drag. Piaggio's design is clearly superior.

By the way, the idea behind the variable geometry canard was sound: In the landing configuration it would balance the additional lift from the fowler flaps on the wing. But the whole mechanism cost more in weight than the reduction in drag during cruise could save.

Drawing from US Patent 4641800.


Why does an aircraft not spin out of control right after touch-down? When a plane is landing, how does it not immediately start spinning out of control?

Given the massive amount of forward momentum and the fact that all the various braking forces (wheel brakes, spoilers, thrust reversers) are applied separately on each side of the plane, if there was even a small imbalance between the two sides wouldn't the plane want to start spinning (yawing) in that direction?

Is this something a pilot has to account for while landing or is there some physical effect that makes it Just Work?

how does it not immediately start spinning?

Physics, and engineering. But I guess that you would like a bit more detail, so let's try dive in.

  wheel brakes

They tend to have a stabilizing effect by shifting some of the weight on the front wheel and allowing the pilots to control the aircraft with more effectiveness.

  spoilers

These are automatically controlled by the Fly-by-Wire system, so if required they would be retracted.

  thrust reversers

If they are functioning properly, the resulting force will go through the vertical plane containing the C.o.G. (but a bit below it), so they do not pose much of a problem.

Another thing you have not mentioned, but I am going to add, is crabbed landings (such as this one), since in these situations the aircraft DO tend to spin a little.
In these cases, in the timeframe between touchdown and braking, 2 things help the aircraft remain on the runway: the vertical tail (with the rudder), and the position of the main landing gear w.r.t. the center of gravity of the aircraft. As it can be seen in this image taken from the lecture notes of the course I followed (the title translates to "Braking run in crabbed landing"), normal tricycle gear aircraft tend to be stabilized by their own landing gear in this situation, while the same cannot be said for old school taildraggers.

So, what really could cause a problem? The answer is wind gusts (especially if the wind is perpendicular to the runway), uneven conditions of the runway (say, a puddle or a thin layer of ice on one side of the runway will cause one set of main gear to have less grip than its counterpart on the other side, causing a yawing towards the side with more grip), asymmetrical failures in the thrust reversers, excessive braking while also turning the aircraft to realign to the centerline (particularly a problem if the distance between the two main gears is not sufficient).

How does the aircraft remain on the runway despite these possible problems? At high speeds, thanks to the vertical tail; and at low speeds thanks to the front nose gear. At in-between speeds, the situation is slightly more complicated: the tail is not much useful anymore, while the front gear would risk breaking if used to correct too much. So, for example, a FbW system can help through the asymmetrical use of spoilers.
An example of what could happen, but seen during a take-off, is illustrated in this video (video kindly provided by MichaelK in chat).


Flying wing vs box wing aircraft design I've been reading some articles online about flying wings aircraft design. But articles about box wing design (of similar depth to flying wing articles) seem harder to find.

For illustration, here are two example aircraft, one of each design, i.e. flying wing and box wing respectively.

Northrop YB-35

Sunny box wing aircraft

From the two images above, I can easily figure out a similarity and a difference between the two aircraft: The swept wing and the vertical stabilizer

What are the major similarities and differences between the two aircraft configurations? For example, if the sunny box wing aircraft doesn't have a swept upper wing, will it still be stable in flight, and is wing sweep necessary for flying wings and box wing aircraft?

A good place to start is the biplane flying wing with endplates that box it in, so that it is both a flying wing and a box wing. The Dunne D.5 of 1910 was the first certified stable airplane ever to fly. It was a swept design, with pronounced wash-out towards the tips, effectively putting the tail at the end of the wing. But its ancestor the D.1 glider had proved stable without any endplates. Although they did improve the stability of the D.5 and D.8, their main function was to reduce tip losses. They even lacked a rudder; the planes turned OK using the ailerons alone but could not land in a crosswind.

By comparison the box wing just flips the lower wing backwards, so it acts as the horizontal stabilizer and there is no need for washout. But it also loses directional stability so a tail fin or similar is necessary. For this reason many box designs, such as the Sunny, sweep it less than the front, so that the swept fore wing and endplates are enough to stabilize it.

The monoplane swept flying wing typically has no endplates. Dunne achieved the same effect by sharply drooping the wing tips. But in most designs, the washout reduces lift at the tips enough to limit tip losses.

Sweep is not strictly necessary. If you use an airfoil with reflex camber, tilted up at the trailing edge such that it has a stationary centre of pressure as AoA changes, it will be inherently stable and no other tricks are needed. But it will be very sensitive in pitch and the controls will not be well balanced. Also, it will be unstable or minimally stable in yaw unless you play tricks similar to Dunne's, such as adding endplates and toeing them in to create differential drag.


How many emergency/abnormal checklists are there for a typical airliner? (Related to, but hopefully not a dupe of, this question)

The latest NASA ASRS newsletter includes the case of an A320 crew that forgot to retract the gear and started detailed troubleshooting without noticing that the gear was still down. The captain said in his report that:

  I instructed the First Officer to coordinate landing at the divert
  airport while I briefed the Lead Flight Attendant and then performed
  the Airframe Vibration Checklist

The mention of an airframe vibration checklist caught my attention because it's such a specific scenario. So, excluding normal operations, how many checklists does a typical airliner (like the A320) have for unusual/abnormal/emergency scenarios?

How many checklists does a typical airliner have for
  unusual/abnormal/emergency scenarios?

Many. A book. The abnormal checklists usually compile to a book and it is located in the cockpit at all times.

As an example, there are 452 pages on the QRH of the Boeing 737. I counted 212 abnormal checklists using the index pages.


Will we ever see a single-engined airliner? Given the trend in the industry to move towards aircraft with two engines, could it ever occur that we see an aircraft manufacturer develop an airliner with just one?

I'm suspecting the answer is no, given the redundancy offered by a twin engine configuration (e.g. if the independent probability of engine failure is one chance in a thousand, the probability of dual engine failure is one in a million.). I know the Virgin Global Flyer used one engine (https://en.wikipedia.org/wiki/Virgin_Atlantic_GlobalFlyer) presumably because the risk of engine failure was acceptable given the task in question.

However, if someone were to manufacture an engine with a probability of failure below, say one in 100 million, could this configuration make sense from an economic perspective?

There are two major problems I see.

Even if you have an incredibly reliable engine how do you prove it is incredibly reliable. I can't seem to find figures for the target IFSD rate for the current higher ETOPs levels, but I find figures for early ETOPs of one per one hundred thousand hours. Lets say you improve that to one per billion hours, how exactly do you demonstrate it's that low? fly for a billion hours? Thats a hundred planes flying for a thousand years. This is why redundancy is the route taken for many safety critical systems, you can argue that the individual components are merely very reliable and then argue that the chance of two of them failing at once is extremely low
Where do you put the engine? If you have a single engine you want it on the centerline. That is not such a problem for props or low bypass turbofans, but it's difficult for high bypass turbofans.

Because of these two factors I doubt we will see airliners move to a single engine configuration unless we see fundamental changes in airliner design and regulation.


Could an airliner exceed Mach 1 in a zero-G power dive and safely recover? (I looked for duplicates. I really did.)
Being as it is that "safety" and this are mutually exclusive:
I am stupid. I take a cruising A320, apply TOGA power, and push zero Gs until I exceed Mach 1. How do I get out of this in one piece? And how much faster can I go before I need to start this process of getting out in one piece?

There is actually some data (albeit limited) on this scenario:
On August 21st 1961 this test was performed in a DC-8. When this test was performed they were supersonic for about 16 seconds which took a lot of planning to pull off. You first need to climb higher than the plane typically does to have enough altitude to pull this off, then make sure you understand how the control surfaces are loaded in the dive. You get out in once piece by planning for it and understanding exactly what's going on:

We took it up to 10 miles up, 52,000 feet—that’s a record—and put it
in a half-a-G pushover. Bill maintained about 50 pounds of push. He
didn’t trim it for the dive so that it would want to pull out by
itself. In the dive, at about 45,000 feet, it went to Mach 1.01 for
maybe 16 seconds, then he recovered. But the recovery was a little
scary. When he pulled back, the elevator was ineffective; it didn’t do
anything, so he said, “Well, I’ll use the stabilizer,” and the
stabilizer wouldn’t run. It stalled, because of the load. What he did,
because he was smart, is something that no other pilot would do: He
pushed over into the dive more, which relieved the load on the
stabilizer. He was able to run the [stabilizer] motor, with the
relieved load, and he recovered at about 35,000 feet.

Remember you're diving fast, really fast, so you likely can't go much faster as you will ultimately run out of time to accelerate:

The Mach number itself isn’t used in a dive as a target because it’s
much more accurate to use airspeed. So every thousand feet I would
read off to Bill the airspeed [he needed] at the next altitude. As we
were coming down, I was talking almost all the time because at a
descent rate of 500 feet per second, every two seconds we were 1,000
feet lower. Looking out the window—which I stopped doing—it looked
like it was straight down.

The trick is to not over stress the airframe.


What the weather condition can affect for flight? What is the most disturbing weather conditions for flight?
On another word: what more dangerous for takeoff/landing: wind, precipitation or something else?

It's actually fairly simple when talking about weather related dispatch delays for the airlines.

Number one items:

In summer, thunderstorms, especially those associated with fronts and squall lines.

In winter, snowstorms with heavy accumulations, or severe freezing rain.

And beyond that, in both summer and winter, extreme low visibility, as in fog.


What is the function of this small hole on the B737's hydraulic reservoir line? This is picture of a B737-800. When you open the bleed system a small amount of air is discharged from this hole (marked with an arrow, lower right). What is it for? And what happens if this hole gets clogged?

AMM Part I Reference 29-09-00, page 3.

Click images for higher-res.

Answering based on ATA 49 system, B757/767 APU Surge Control Valve muscle air plumbing.

Some pneumatic sensing and pressure systems have a moisture bleed hole to prevent icing in the system.  They are typically located at a low point or bend in the line to allow water to escape the system while maintaining pressure.  If the hole gets plugged it can prevent moisture from escaping and ice to form in the line.


Which US airport historically has the most days in which VMC prevail? I live in a part of the country where VMC are starting to become increasingly uncommon, which has led me to wonder - which US airport historically has the most days in which VMC prevail per year?  What sources of information can be used to investigate VMC trends at a given airport?

According to NOAA here are the cities with >80% annual sunshine which I've used as a proxy for VMC.  You'll find these cities are all in the desert southwest region of the US where the climate is arid.

SOURCE:  http://www.ncdc.noaa.gov/oa/climate/online/ccd/pctpos.txt
DATA THROUGH 2004

CITY, STATE            ANNUAL % AVG POSSIBLE SUNSHINE        NO. OF YEARS OF DATA
YUMA, AZ               90%                                   42
REDDING, CA            88%                                   10
LAS VEGAS, NV          85%                                   47
PHOENIX, AZ            85%                                   101
TUCSON, AZ             85%                                   53
EL PASO, TX            84%                                   54

To investigate long-term trends you can access historical data at the NCDC website and analyze the data for specific weather reporting stations.


What is the difference between a ground controller and an air traffic controller? In terms of their roles at an airport. I'm designing a system that would inform its users if there is space on the runway that a pilot could use to land or take off, when there is an obstacle on the runway. From my understanding, a ground controller would use this system however the roles of a ground controller and air traffic controller seem to be similar to me so I'm unsure if both would use the system or just the ground controller.

Ground controller is a "position" (role) carried out by an air traffic controller.  The role of an air traffic controller who is working the ground position is to coordinate traffic moving on taxiways and to and from runways.  The ground controller is in charge of the taxiways.  An air traffic controller may be certified to work multiple positions.

You can think of a ground controller as a type of air traffic controller.

Positions (roles)

There are many air traffic controller roles which an air traffic controller may perform.  Other air traffic controller positions are:

Clearance delivery: A clearance delivery controller gives the pilots their clearances, which details their route.

Ground controller: A ground controller is responsible for traffic moving on the ground along the taxiways.

Tower controller.  A tower controller is responsible for traffic landing and taking off or in the air in the immediate vicinity of the airport.  The tower controller is in charge of the runways.

Departure controller.  A departure controller controls traffic which is leaving the airport.

Enroute or "center" controller.  An enroute controller handles traffic which is flying between airports.

Approach controller.  An approach controller controls traffic which is approaching to land at the airport.

Some air traffic controllers work just one position.  Others work multiple positions.  For example, at a small airport, the same controller may be the ground controller and tower controller.  Or an airport that is busy enough in the daytime to need separate controllers for ground and tower may be need only one controller for both roles at night when there is less traffic.

Example flight

Let's take for our example a pilot who is departing from a large airport, flying under instrument rules, and land at a small airport.

The pilot might first contact clearance delivery.  Clearance delivery will give the pilot her route, which she will write down.  The pilot will then radio ground and ask for permission to taxi.  The ground controller will give the pilot permission to taxi, including the specific route along the taxiways to use to get to the departure runway.  On reaching the runway, the ground controller instructs the pilot to contact the tower controller.  The tower controller will then clear the pilot to take off.  The pilot enters the runway and takes off.  The tower controller instructs the pilot to contact the departure controller.  The departure controller gives the pilot instructions to control the route and altitude during departure.  The departure controller then instructs the pilot to contact an enroute controller.  As the plane moves along, the enroute controller instructs the pilot to contact a different enroute controller.  That controller gives the pilot instructions to descend and approach the destination.  The enroute controller instructs the pilot to contact the tower.  The tower gives the pilot instructions to approach the runway and land.  Since this is a very small airport, the tower and ground positions are handled by the same controller, and on the same frequency, so after the plane touches down, the same controller gives the pilot instructions to taxi to her destination.

Every person the pilot talked to was an air traffic controller.  Each of these air traffic controllers was working different positions (roles).

See also: Air Traffic Controller (wikipedia)