1 hour ago, jon b said:

It has to do with the inertia on large aircraft, getting them to accelerate needs a large input in the form of engine thrust. You could accelerate them by pitching down of course but the amount of height you would have to trade to get them to accelerate would be disproportionate, especially on the approach and you’d end up being unstable and low.

if you've done some high school physics I think its easier to think of it in terms of energy.

to ascend you need to add potential energy, this can come from bleeding off kinetic energy (i.e. speed), thermals or from engine power

to fly straight and level you only need to replace the kinetic energy lost to drag

when you descent potential energy gets turned into kinetic energy (i.e. speed), to keep the same speed there is only one way - lose it to drag.

Hello Jon,

thanks a lot for your answer. The question arising is based not on light aircraft but on jets, the 737 specifically. The point that I don`t understand and is still open to me:

I am on a landing approach, VFR, no crosswind/winds at all, no failures, 1000 feets above ground level. I realize that I am 10 meters off the centerline to the left of the runway. If I just use the rudders, this is what`s happening in FSX:

- My heading changes from 346  to 356, I continue to fly at this heading for 5 seconds. I realign back to 346° heading. What has happened: I am still 10 meters off the runway. So I didn`t change my position at all.

In X-Plane:

-My heading changes again from 346 to 356, I continue to fly at this heading for 5 seconds. At realigning (back to 346°), I moved geographically in the longitudinal axis 10 meters to the east, so I am again in the centerline.

In both cases I didn`t use the ailerons. The plane is always perfectly aligned to the horizon, so no bank angle which could turn the aircraft. In FSX I didnt change my longitudinal position at all, in X-Plane I did.
Furthermore, if I continuously apply rudder in X-Plane, the plane changes its true longitudinal position continuously, in FSX it does not.

So my question arising is, whether FSX or X-Plane is right.
My thought is that X-Plane is right here, because if I apply rudder, change my heading to 15°, it should fly this direction. Realigning back to 0°, I determine that I changed my longitudinal position. This way, I can reach the aiport. In FSX I cannot.
So if you say you can steer a light airplane by applying rudders only, that does mean that you can`t do that in jet airliners?

I think that it should be possible, no matter what type of plane it is. It should be possible to turn the plane from 0° to 180° just by applying rudder, (let`s say we are in a virtual world where there is no rolling existing, our ground is basically just air in a 2 dimensional word.) If the rudder remains in the applied position, the wind pushes the plane to the oppositie direction, so I can change my heading from 0° to 180°, just by using rudder, while in FSX I can`t and if I interpret it right, it`s also not possible at jet airliners in reality.

I am sorry for the length I have written, concerning it is a very basic topic for professional pilots like you, but unfortunately I cannot explain it better and I am also very curious about it.

Edited January 5, 20215 yr by BigDee

Hi Big Dee,

when you use the rudder, the plane will initially yaw (turn around the vertical axis) away from its movement trajectory. This means the nose is pointed to the left (or right) of were you are going.

Now a couple things happen:

1.) The "relative wind" is not coming straight from the front anymore. The plane is pointed a bit sideways. So now the relative wind on the side of your fuselage exerts a force and pushes the aircraft into the direction the nose is pointed.

2.) The relative wind will hit the "upwind" wing more straight-on than the "downwind" wing - so the upwind wing will get better lift and start rolling the plane towards the direction the nose is pointed. You would actively need to counter this with "opposite" aileron to keep the wings level.

If you DO force your wings to stay level, eventually the trajectory of your plane will change due to the sideforce of the wind on the fuselage and rudder. The new direction it travels to will be towards the side the nose is displaced. Since the rudder input stays the same, this will repeat over and over again - basically the plane is "skidding" through the turn, with the occupants getting pushed sideways, like in a car going through a turn.

Note that this is called "uncoordinated flight" and not the recommended way to turn, but in theory it works like that.

So MSFS is suffering from the same inability to "skid" through a turn that also plagues FSX and all variants before that. Kicking the rudder in those simulators would only temporarily point your nose over - but not really change the flightpath. X-Plane has it correct.

Cheers, Jan

 

As a general rule of thumb I’m inclined to say the physics of x-plane will be more accurate than FSX. 
 

From what your describing you’re basically putting the aircraft into a slip condition. The nose may be pointing 5 degrees to the left of where it was but the aircraft will initially continue on its original track due to its inertia, but keep the yaw in place  and the aircrafts track will eventually change to match where the nose is pointing. So yes, using the simplified model you described you could turn the aircraft around just using rudder. 
 

If you’re talking about a 737 handling technique then no, don’t use the rudder to track the localiser on approach. 
 

The 737 has a swept wing. Imagine the wing as an inverted V or better still look down on it from above in An external view the simulator, now imagine you punch in the left rudder hard and the aircraft yaws 30 degrees left.

If you can imagine the aircraft is still going on its original track through the air with the nose pointing 30 left of its track. You’ll notice that the right hand wing is now effectively lengthened and is presenting its leading edge at about  90 degrees to the airflow , (if the wing sweep was for the sake of argument 30 degrees) So now you have effectively a long straight wing on the right hand side, this will produce a lot of lift.

However the left hand wing will have had its sweep increased from 30 degrees originally plus another 30 degrees of yaw so now it’s effectively 60 degrees, this presents a very swept and narrow wing to the airflow producing a much reduced lift vector. 
 

Imagine the right wing , long and straight like a gliders wing, ideal for producing high lift. The left wing , short and swept like a fighters, producing low lift and low drag. The result is the right wing will rise quite strongly producing a marked rolling action.  Which is why we don’t use the rudder on swept wing aircraft like a 737 on approach.

If I were to continue to example you would notice that although the right wing is producing much more lift than the left wing its new profile also starts producing a lot more drag as well , which will then try and yaw the aircrafts nose back round to the right , which will  then reverse the original situation.

This sets off an oscillation as the two wings battle each other, which is the so called Dutch roll effect . This is why airliners employ yaw dampers to try and smooth this Effect out.

As I mentioned, this discussion needs a flight instructor who can explain this sort of thing much clearer than I can.

So in short, to answer your question, yes x-plane sounds the more authentic representation, however if your in a 737 don’t use the rudder on approach!

Edited to say while I was typing this it appears my learned colleague Jan has just explained exactly the same thing above, in a clearer way !

Edited January 5, 20215 yr by jon b

Thank you very much Janov and Jon for your answers and taking your time to help on it. Appreciate your help as real world jet airliners a lot. I had to read it 2 time pluse once more but I understood it.

I tried to do a little research on that subject in the past and I red the same info, not to use rudders (in real world). So I misinterpreted not using with not possible. The swept wings are the hindering. I tried a Cessna in X-Plane with straight wings and it works much better than a 737. Now I possibly understand the meaning of Dutch roll. If you wouldn`t have told me about the side effect of swept wings I guess I had to think a lot of time to understand why it rolls in the same direction on a 737. And now finally I do know that X-Plane is correct.

Either way, FSX lacks some important feature and I am asking myself how such a crucial element missed in FSX. A real world pilot would notice it instantly, so something is (or better say was) wrong.

I am not talking about MSFS on purpose because I do expect nothing from it. So, X-Plane is a real fascinating tool.

Edited January 5, 20215 yr by BigDee

Well, MFS does skidding turns too 😁

Just as well designed modules did in fs9 and fsx...

Lot's of interesting contributions here. I would only add that I look forward for future developments of both XP and MFS in terms of flight dynamics. Each has their flaws / limitations, although from a strict modelling pov XP offers a worth more of possibilities, at least for now...

 

I’m sure you’ve spotted my deliberate mistake there.

As Jan correctly says the aircraft would be in a skid , not a slip as I’ve written.

An aircraft skids to the outside of a turn and slips to the inside.

8 hours ago, jon b said:

I’m sure you’ve spotted my deliberate mistake there.

Technically Jon did not make a mistake, really - if at all then it is a nomenclature thing.

There are many different names for the same condition of uncoordinated flight (when the nose is not pointing into the direction of travel laterally). Sideslip, forward slip, skid, etc. They are - aerodynamically - all the same thing, the different names stem from different purpose or conditions of applying/suffering uncoordinated flight.

Some other things to consider when this happens:

• Airflow shadowing on aerodynamic surfaces (the air coming in "sideways" will be blocked/disturbed from flowing freely by the fuselage/rudder)
• Asymetric shape of the fuselage/rudder sideview area, leading to roll input (because the rudder "sticks out" the top and will therefore help "roll" the aircraft when hit by sideforce wind)
• Engine inlet oblique airstream (which is problematic in jet engines and therefore intentional slipping is not allowed in jet aircraft)
• Dynamic loads on the rudder assembly during uncoordinated flight (which snapped off the rudder on AA587 https://en.wikipedia.org/wiki/American_Airlines_Flight_587)
• The danger of entering a spin when operating close to stall AOA and then loosing airflow on one wing first due to asymetric airflow properties

So all things considered - asymetric flight is not what aircraft are made for. It can be used in a deliberate way in controlled regimes for certain aircraft - but the pilot should be aware of the possible ill effects it brings. Regular use is increasing drag to steepen an approach (i.e. in gliders), the "sideslip" crosswind approach, shooting off-flighpath in combat aircraft and certain conditions even in airline flying (crab removal in crosswind before touchdown, small slip after engine failure to minimize drag from control deflections).

Cheers, Jan

 

On the slip vs skid states, a very easy to follow source:

Why Skids Are More Dangerous Than Slips | Boldmethod

IIn the whole history of glider flying in Portugal there were only 2 fatal accidents, and a few more non fatal but one of them resulting in serious injuries to the pilot, a seasoned airforce pilot, also glider pilot, who suffered an accident in the 90s of last century during a base turn where a skid resulted in a spin after the outside ( up ) wing suddenly dropped. The glider was a Mucha Standard 

Jon and Jan's contributions are excellent, but here's a nice ( theoretical but not difficult at all to read ) text from one of the links I suggested above:

11  Slips, Skids, and Snap Rolls (av8n.com)

Just as Jan correctly wrote: "The term skid denotes a particular type of slip that occurs when the airplane is in a bank and the uncoordinated airflow is coming from the side with the raised wing"

which is something I sometimes have difficulty explaining to CFIs 😕 who never really understood the concept of slip / skid 🙂

 

Edited January 6, 20215 yr by jcomm

On 1/3/2021 at 7:26 AM, jon b said:

As a general rule of thumb light aircraft are controlled an approach by using pitch for speed control and power for rate of descent.

larger aircraft it’s the reverse.

I've heard this and definitely believe it. Is there a general size aircraft that this become more apparent? I am doing my multi-engine training in a Baron and am surprised how much more apparent the "pitch for airspeed, power for altitude (descent rate)" concept is in a light twin.

Edited January 11, 20215 yr by snglecoil

On 1/11/2021 at 3:12 PM, snglecoil said:

I've heard this and definitely believe it. Is there a general size aircraft that this become more apparent? I am doing my multi-engine training in a Baron and am surprised how much more apparent the "pitch for airspeed, power for altitude (descent rate)" concept is in a light twin.

larger aircraft do a mix of both, and is part of the autopilot, for example on the 744 speed is generally controlled by the auto throttle, but during climb phase throttles are maxed and speed is controlled by pitch, during the main descent throttles are idled and the aircraft glides down to save fuel, then it switches back to throttles to maintain airspeed and pitch to control rate of descent.

Something like the space shuttle is all pitch control to manage the energy of the approach. (show extra aircraft from old versions, then cusomise location, special starts -> Full re-entry)