Sign in to follow this  
jcomm

AirfoilLabs C172...

Recommended Posts

At another thread someone mentioned some open problems with this add-on.

 

I own the C172, and updated it to the latest version.

 

Overall I find it amusing, but there are some notorious glitches in some aspects of flight behaviour, such as the oerbanking tendency when even a mild bank is set, returning the yoke to neutral, at any power settings, turning by the left or by the right.

 

The real C172s are, just as most GA aircraft, roll stable up to a significant amount of bank. For bank angles above 30º ( 45º) opposite aileron is required to counter overbanking, specially if turning through the left, but not at a mild 10º or even 20º bank angle, where the aircraft will tend to return to wings level, specially if turning through the right and with mid to high power settings ( due to prop effects ).

 

In the AirfoilLabs C172, no matter what bank angle and or power / picth settings you start from, the aircraft will always overbank, towards a spiral dive if left at it's own will.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Not to bash, but weird conclusions. What are you basing your observations on? Do you know which engine is moddeled in airfoillabs? Do you know the difference in actual fligh behaviour between 160 and 180 horsepower engines on 172s? between different power settings? Do you take trimming settings into account? Speed?

 

I am all for analyzing things, as long as there is some substance to it. Here you make a general statement on something and say it's wrong without actually saying anything. 

 

I know it sound stupid, but give me few weeks and I will get you a video where I get a 172 into spiral from 15 degree left bank.

Share this post


Link to post
Share on other sites

Lena, I just hope you don't get hurt!  Spiral dives can be dangerous!  Take care!

 

Joke aside - it's basic flight dynamics, and, actually, some knowledge of how various models of the C172 actually fly ...

Share this post


Link to post
Share on other sites

Lena, I just hope you don't get hurt!  Spiral dives can be dangerous!  Take care!

 

Well, entering and recovering from them is a part of every basic training.

I mean, if you gave me a specific situation that you think is wrong and why, then we can discuss it at least. Saying your whole point relies on having "some knowledge about it" is a bit non-substantial.  

I guess, or hope, that you made this thread to actually spark such discussion. 

Share this post


Link to post
Share on other sites

I guess you guessed right... But, I also though what I wrote was clear - namely, that, at any speed, load ( leaving out asymmetric fuel and payload situations ), power settings, trim settings, if, you're flying level, descending, or climbing in this C172 and bank either right, or left, not more than 10º ( first mark ).

 

Return the yoke to neutral...

 

Observe what happens to your bank angle. You can either pull the yoke to avoid diving and gaining speed or simply leave the controls, and you can coordinate or not with rudder ( * ). Whatever you do, and that's the problem, overbanking will be the end result.

 

That's what is wrong.

 

Why? Well I am not going to start here a "Principles of Flight" or "Roll Stability" thread, because my point was exactly to point out that something is obviously wrong with the roll stability in this C172 FDM.

 

( * ) Although my point refers to sideslip situations. In my glider most of the time I use a lot of outside/ opposite stick at 45+º bank angles in coordinated tursn, but that's another story - coordinated turn in a glider, at high bank angles...

  • Upvote 1

Share this post


Link to post
Share on other sites
The real C172s are, just as most GA aircraft, roll stable up to a significant amount of bank.

 

Apparently, the C172 and many GA singles have a slightly unstable spiral mode.

 

From "Airplane Flight Dynamics and Automatic Flight Controls - Jan Roskam":

 

ByHCwfX.jpg

 

1o4ugX9.jpg

sLwqmHd.jpg

Share this post


Link to post
Share on other sites

Going back to what I remember about 172s.

 

Mild roll, and the plane returns to neutral. My Vans RV6, just stayed where you let go of the aileron. It was quite different from the Cessnas. More along the lines of aerobatic low & mid wings.  As to how long the RV stayed there, I don't remember. Never went to the point of testing it.

Another item I tried in the Cessna 172 for flight sim purposes years ago, was to quickly roll the yoke & then let go. The plane would quickly roll back to neutral & past it. Then it would level out. I don't remember any sim models doing the same.  For the Piper low wings; after a certain point of roll, it could tighten up to a steeper bank.

 

Now...............as to how I feel about real life, and the exact modeling, I could care less. Too many planes were different to make assumptions that apply in the majority of cases.  I never entertained the idea of using a sim model to predict what the real plane would actually do. I also never compared the actual  movement of the yoke or stick, to the real thing (arc wise dimensions). My objective with a desktop yoke or stick, is just to get the plane to do what I want.  I don't think in terms of rotating the yoke a quarter circle, or whatever. 

  • Upvote 1

Share this post


Link to post
Share on other sites

Yes, modern Cessnas and especially the "S" model are used in many flight schools and known for good stability and even pretty much feet on floor, just like many GA, even if this characteristics actually tends to be associated with higher overbanking tendency ( use of ailerons and differential deflection and aileron frise hinges ).

Share this post


Link to post
Share on other sites

Sorry to ask this silly question here , but how does the default 172 compare ? I don't own the airfoillabs 172.

Share this post


Link to post
Share on other sites

Apparently, the C172 and many GA singles have a slightly unstable spiral mode.

 

From "Airplane Flight Dynamics and Automatic Flight Controls - Jan Roskam":

 

ByHCwfX.jpg

 

1o4ugX9.jpg

sLwqmHd.jpg

 

 

Missed this one Murmur... Will have a look at it. Thx! Reading through it will help - I also have this one :-) 

 

Larry, thx also for the personal experience notes on the real C172.

Share this post


Link to post
Share on other sites

When I read this thread I feel like -

 

"Gee wow... I like the planes that go ZOOOOOMMMM.  They are NEAT."

 

Just like my four year old boy does. :smile: 

 

Kidding, but only sort of. I understand the above, but my passion in regards to aviation has always been more about the history, design, and operation of the equipment itself. I appreciate the fact that mathematics and physics, or rather, our quest to understand and overcome the limits imposed by mathematics and physics, has been a primary factor in the evolution of aeronautical design, but I've never really had the desire to actually interpret anything beyond the type of pilot report that Larry gave above.

 

For example, I enjoy reading about how undesirable flight characteristics in a given aircraft caused engineers to make changes to try and remove those characteristics, which in turn may of had other effects and forced other design changes in turn, but that's as far down the rabbit hole as I ever care to go. :smile:

 

I always find it fascinating however, that there are so many different facets to this 'hobby' that people can enjoy.

Share this post


Link to post
Share on other sites

I don't like the formulas either... they're a rather formal way of expressing sometimes very intuitive / common-sense interpretations of complex systems...

 

Of course, it's good to have a formal language to express it, and in the field I work in IRL it is actually very important, but I guess most of the 

excellent ideas that ended up in solutions for many engineering problems in aviation, mostly during war time, but also in many other fields, started from rather intuitive / common sense interpretations of the causes ... 

 

There are things I've long learned when I started to fly many years ago, and could only understand in terms of aerodynamics decades after... 

Share this post


Link to post
Share on other sites

I have about 200 hours in the C172S. For reasons that I will never understand, the real a/c has the issue that it is more stable in nose up/down pitch and less stable in the bank attitude than any of the sims that I have flown since 1994, including all of the FS versions, XP, and even the ASA instrument sims. And the same applies to the Fresca sim that the flying club owns. Must be a feature of a software model rather than the real aerodynamic thing.

 

With that said, I agree with the OP that any bank in the real thing, less than 25 degrees, will return to approximately level. That is, a sudden bank change (less then 25 degrees) will result in a rather quick return to level flight. Its's the gradual change in bank over time that makes it susceptible to graveyard spirals.

 

The real thing is extremely stable in nose up/down pitch, unlike almost any simulation. I often think that sims must put people off ever wanting to learn to fly when managing nose up/down pitch.....

 

Can't comment on the AirFoilLabs model- I have it but haven't had any time to even open it let alone fly it....

 

Thanks, Bruce.

Share this post


Link to post
Share on other sites

The real thing is extremely stable in nose up/down pitch, unlike almost any simulation. I often think that sims must put people off ever wanting to learn to fly when managing nose up/down pitch.....

 

 

 

This is due cheap desktop grade controls in my opinion.

 

THE REAL THING:

1. The real aircraft has a 14 or 16" pitch travel on the yoke, you have TONS of granularity to control/maintain the pitch.

2. The real aircraft also restricts your inputs due to the airflow forces over the surfaces pushing agains you. This makes easier and smoother to apply pressure.

 

THE SIMULATOR:

1. Desktop hardware tend to have a very short 3-5" pitch travel at best, every minuscule movement translates into a lot of input for the aircraft due to conversion/mapping.

2. Desktop hardware yokes does not provide you any feedback, you have not the real airflow surfaces pushing against you leading into too much input too easily unnoticed.

3. Also, cheap potentiometers keeps sending spikes, noise while you use them.

 

I moved to a certified force feedback yoke, real pitch travel length, force feedback pressure based on the aircraft IAS to mimic the real thing. All bumpiness and instabilities for nice pitch control went away.

 

It feels now quite similar to the real thing! It is simply amazing when you are flaring and the stall horn kicks in how the yoke elevator force washes out as the real thing.

Share this post


Link to post
Share on other sites

I would agree, other than my experience with the Fresca sim (no "PC sim" and able to be used for instrument currency".

Share this post


Link to post
Share on other sites

Thx Bruce for your additional comments from RL experience flying the 172S, just like Larry.

 

That's my experience too in many flights in th R models, not as a pilot, because I only own a GPL ( now EASA :-( ) but as someone many times allowed to actually fly the aircraft by my airclub fellows.

 

I am also starting to invest on better hardware, but it's very expensive. I just ordered a GNS and Throttle Quadrant from ELITE ( Swiss ) and am witing for delivery and taxes I'll have to pay on top of the already expensive hardware, and I am planning to invest on their yoke too, because I'll be able to use it with ELITE and PSX :-)  The radio stack is still a dream.

 

ELITE provides me with that stability you mentioned Bruce, like no other FSX or X-Plane model I have tried indeed...

 

This being said, I guess that Lena probably considered I was referring to coordinated turns, and in that case she is right in what she points out because indeed if rudder is used during a tighter turn to coordinate it, and we do not use only the yoke, creating a sideslip, overbanking will occur and require outside yoke deflection.

 

Of course overbanking is my "daily" experience in gliders. At 45º+ bank I sometimes have to use almost full outside aileron in some of them!

Share this post


Link to post
Share on other sites

 

 


I moved to a certified force feedback yoke, real pitch travel length, force feedback pressure based on the aircraft IAS to mimic the real thing. All bumpiness and instabilities for nice pitch control went away.

 

Me too, and it's a world of difference. Also trimming feels real now.

 

Have given up with XPlane though. Even with such a yoke I rarely need more than 10% of travel, no matter if default or Addon plane. That's all way too twitchy.

 

Mike

Share this post


Link to post
Share on other sites

Just as a complement, and with a very well written set of "Course Notes" I strongly recommend visiting:

 

http://www.flightlab.net/Flightlab.net/Download_Course_Notes.html

 

and in line with this particular thread what is discussed regarding directional stability in this particular Note:

 

http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/4_LateralDirectional%232BA14D.pdf

 

I think that flight simulators either use the "blade" approach but really do not take into consideration all of the moments created by the various lift and drag generation surfaces, or do not model them correctly, or when using the so called "table-based" approach, use certain coefficients that are innapropriate, or the authors tune those to get some effects better modeled, causing weird behaviour on others...

Share this post


Link to post
Share on other sites

Just as a complement, and with a very well written set of "Course Notes" I strongly recommend visiting:

 

http://www.flightlab.net/Flightlab.net/Download_Course_Notes.html

 

and in line with this particular thread what is discussed regarding directional stability in this particular Note:

 

http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/4_LateralDirectional%232BA14D.pdf

 

I think that flight simulators either use the "blade" approach but really do not take into consideration all of the moments created by the various lift and drag generation surfaces, or do not model them correctly, or when using the so called "table-based" approach, use certain coefficients that are innapropriate, or the authors tune those to get some effects better modeled, causing weird behaviour on others...

 

I think it's important not to confuse static lateral stability with spiral dynamics.

 

Static lateral stability means that if the aircraft e.g. sideslips yawing the nose to the right, then it tends to roll right, and vice-versa.

 

This is the common behaviour of FSX and X-Plane aircrafts. The effect seems to be more pronounced in X-Plane aircrafts.

 

However, a positive static lateral stability is not a sufficient condition to produce also a stable spiral mode. I was surprised to learn that, apparently, even the C172 has a slightly unstable spiral mode, and if left hands off, its bank angle will (slowly) progressively increase until it's in a spiral.

Share this post


Link to post
Share on other sites

Just mentioned the site for the interesting notes available there, and not because of the dynamic nature of the spiral dive phenomenon Murmur.

 

And, in the cited document you can ( and I guess I once sent you this very same link ) find also part of the justification on how an aircraft reacts to a sideslip situation, which was exactly the one at the start of the thread - I give some yoke input, left or right, and return it to neutral, without touching the rudder.  The aircraft enters a sideslip....

Share this post


Link to post
Share on other sites

Just mentioned the site for the interesting notes available there, and not because of the dynamic nature of the spiral dive phenomenon Murmur.

 

And, in the cited document you can ( and I guess I once sent you this very same link ) find also part of the justification on how an aircraft reacts to a sideslip situation, which was exactly the one at the start of the thread - I give some yoke input, left or right, and return it to neutral, without touching the rudder.  The aircraft enters a sideslip....

 

Can you quote or give the precise point of the document you're referring to?

Share this post


Link to post
Share on other sites

There are various instances, more or less directly related to it:

 

pg. 4.1, 3rd paragraph: [...]A stable aircraft yaws toward the velocity vector, but rolls away[...]

pg. 4.5 Clp section: "An aircraft with dihedral effect rolls away from a sideslip (away from the velocity vector)"  and further down

 

"An aircraft with positive lateral stability rolls away from the sideslip (velocity vector) that results when a wing drops, and that usually means back toward level flight (although an aircraft with dihedral effect can go into a spiral dive if the bank angle is high and other moments prevail)."

 

In my real life experience aboard C172s and just as both Larry and Bruce referred, the aircraft tended to return to wings level, even faster if the roll was to the right ( positive ) and the power was in the cruise range or above ( due to prop effects ).

 

If you own the AirfoilLabs C172 you can easily check what I meant, by setting various bank angles and returning the controls to neutral, not using rudder at all, and entering sideslips ( left or right), You can then test with full power, idle power, level, climbing or descending, and see that no matter how small your initial bank is, the aircraft will continue to bank, unless opposite roll controls are input.

Share this post


Link to post
Share on other sites

There are various instances, more or less directly related to it:

 

pg. 4.1, 3rd paragraph: [...]A stable aircraft yaws toward the velocity vector, but rolls away[...]

pg. 4.5 Clp section: "An aircraft with dihedral effect rolls away from a sideslip (away from the velocity vector)"  and further down

 

"An aircraft with positive lateral stability rolls away from the sideslip (velocity vector) that results when a wing drops, and that usually means back toward level flight (although an aircraft with dihedral effect can go into a spiral dive if the bank angle is high and other moments prevail)."

 

In my real life experience aboard C172s and just as both Larry and Bruce referred, the aircraft tended to return to wings level, even faster if the roll was to the right ( positive ) and the power was in the cruise range or above ( due to prop effects ).

 

If you own the AirfoilLabs C172 you can easily check what I meant, by setting various bank angles and returning the controls to neutral, not using rudder at all, and entering sideslips ( left or right), You can then test with full power, idle power, level, climbing or descending, and see that no matter how small your initial bank is, the aircraft will continue to bank, unless opposite roll controls are input.

 

The roll moment due to sideslip is not the only factor at play. The directional (weathervane) stability, the roll moment due to yaw rate, and the yaw moment due to yaw rate, all play opposing roles in the dynamics of the long period spiral mode.

 

Now, as the academic textbook I quoted shows, apparently the C172 has a slightly unstable spiral mode.

 

This means that, on the long period, even when trimmed, if left to itself the bank angle will progressively increase. As a sidenote, this is also the behaviour of the AeroflyFS 1 C172: if you put it in a slight bank angle, it will increase its bank angle, although with a much slower rate compared to e.g. the default X-Plane C172. From the data in the academic textbook I quoted, the behaviour of the AeroflyFS 1 C172 seems quite close to the real aircraft.

Share this post


Link to post
Share on other sites

The roll moment due to sideslip is not the only factor at play. The directional (weathervane) stability, the roll moment due to yaw rate, and the yaw moment due to yaw rate, all play opposing roles in the dynamics of the long period spiral mode.

 

 

They're all mentioned in that same PDF, although in other paragraphs.

 

Two characteristics of the modern C172 that could indeed contribute to overbanking are the use of differential ailerons, which deflect up the aileron on the wing towards which the bank will occur than the one in the outer wing deflects down, causing additional drag and countering the adverse yaw, as well as the use of fries ailerons, both increasing the yaw towards the turn and hence the yaw-induced roll, but apparently and while contributing to make the 172 rather feet on floor during turns, they certainly do not contribute that much towards overbanking...

 

The dynamic instability referred to in Roskam's text probably refers to the first models of the C172, because the aircraft is actually known to be "stable" in this respect, as others in this thread confirmed too from their real life experiences.

 

BTW, the rate of roll when left at it's own will is actually superior in the AirfoilLabs C172 than in the default C172, at least in my X-plane.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this