Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Rudder angle and roll coupling

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Old 05-22-2018, 02:59 AM
  #26  
UStik
 
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BMatthews, your CAD drawing is intuitive but it didn't let me rest because my intuition was that things are far more complcated. I put my old trainer model in CFD (XFLR5) to see what happens. The model is very simple, its rendering is crude, and I wouldn't trust the computed results too much. The only result I trust, though, is that things are too complicated to draw any firm conclusions. Look at this example:

The 5:1 aspect ratio, 60" span wing has NACA 2415 airfoil. The model weighs 75 oz and flies 40 mph at 3 degrees AoA. On top of a square horizontal stab sits a tapered vertical stab which is swept 25 degrees at its 25% chord line. Rudder is 30% of chord line and deflected 30 degrees to the left. Both stabs have NACA 0009 airfoil.

The total view shows pressure coefficient values as colours (warm is positive, cold is negative) and streamlines from the surfaces' trailing edges. Strong vortices on the tail.
The vortices seen from the rear bottom left side, no general direction noticeable except sideways.
Top view shows streamlines of vertical and horizontal stabs, and even wing, are intertwined.
Side view shows streamlines from wing and horizontal stab as bunches. Strong vortices on top and bottom ends of rudder. Streamlines from rudder trailing edge go both upwards and downwards following the vortices, no identifiable net flow.
Same in rear view, but interesting is asymmetric flow on the horizontal stab. Its left half blows down, its right half up. Even the wing's wash is influenced by the strong tail vortices.
Now what is called "downwash" in XFLR5. The side view shows only vertical vectors, top view only horizontal. That means only the stream components across flight direction are shown. All vectors start at a stab's trailing edge, both vertical and horizontal. Those of the former are much bigger due to deflected rudder.
Rear view shows very long vectors coming from the tips, where the vortex centers are. Can be disregarded here. The vector field from the rudder's trailing edge is what I would call counterintuitive. I couldn't tell any net down force, for me it rather looks like a pretty much horizontal force. (The moment is specified in the first picture in the bottom right corner.)

For what it's worth.









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Old 05-22-2018, 08:09 AM
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Well dang, that's interesting. It's one of the cool things about a forum; you never know when someone will show up with the software to test the hypothesis. I found the impact on the impact on the horizontal surface airflow to be interesting. While I still hold to my thought that it probably doesn't impact my own flying, I can see where it might be useful knowledge for someone setting up a plane for precision aerobatics. Going with the theme of the discussion, it would be interesting to see a comparison between a simulation of a vertical rudder hinge line and a series of rudder hinges with increasing degrees of sweep. Thanks for the graphics UStik.
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Old 05-23-2018, 01:36 AM
  #28  
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Here you go: Vertical rudder hinge line looks only slightly different. One could be tempted to believe that the downwash vectors on the rudder trailing edge make for an overall down component, meaning the airplane's nose would go down as well. And - more downwash from the left elevator half than upwash on the right. Does that mean a down-pitching moment and adverse yaw? Anyway, rudder trailing edge seems to be more important than hinge line.







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Old 05-23-2018, 02:05 AM
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Next experiment, with rudder hinge line swept by 30 degrees (was 15 degrees in the first experiment and zero in the second).
Net downwash seems to go more upwards again, so there seems to be a small nose-up effect of swept rudders.
The rudder moment (see lower right corner of total view pictures) is smaller on swept rudders.
Again adverse roll not only from vertical tail but from horizontal as well. (Was that the original question?)








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Old 05-23-2018, 03:04 AM
  #30  
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Last experiment: Rudder hinge line now at even 45 degrees.
Net sidewash from rudder trailing edge again a bit more upwards (compare how the arrow heads are above the wing).
The effects on the horizontal tail seem to be bigger at more sweep, more downwash left and less right.
Rudder moment again smaller with more sweep. Maybe that's why aerobats have vertical rudder, it's substantially more effective (-0.0118 swept 45, -0.0193 unswept). I didn't find an explanation of this parameter yet so I'm not sure if it's really effect or simply the "shorter" rudder moment arm.







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Old 05-23-2018, 04:27 AM
  #31  
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Comparison: Lift distribution on wing, horizontal and vertical tail for 0/15/30/45 degrees rudder hinge line sweep.
There is a downforce on swept rudders.
Swept rudders are less effective than vertical rudder.
Lift distribution on horizontal tail is most distorted by vertical rudder.
Last picture shows stab's unspoiled lift distribution (rudder not deflected).

The differences are best seen when the pictures are downloaded and scrolled in the picture display program.











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Old 05-23-2018, 09:01 AM
  #32  
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Thanks UStik, This gives me a fresh appreciation for those people who work to squeeze that last 0.5% of performance out of an aircraft. I've seen a lot of discussion through the years about the impact of airflow over and around the wing but have always tended to think of the tail surfaces as functioning in isolation. The impact of the rudder deflection on the horizontal tail is interesting. It is certainly possible that I am misinterpreting your data but my understanding, at this point, is that to some relatively small degree, deflection of the rudder and its impact on the horizontal surface results in a rolling force that would tend to act against the rolling force created by the wing when the rudder yaws the aircraft.
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Old 05-23-2018, 09:37 AM
  #33  
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I'm not quite sure if all these calculations are correct, especially regarding the interactions. But yes, what we see should be a rolling moment of the horizontal stab. It's especially clear in the first two cases where the right stab half has virtually no "lift" but the left half has substantial "lift".

Wing lift is drawn upwards, stab lift downwards. I think that means it's pushing down because it's hit by the wing downwash, which is distorted by rudder deflected to the left. So the left stab half pushing down (the right half not) would make for a small rolling moment to the left (we are looking from behind).

The rudder's "lift" is drawn rightwards, and that is correct since rudder is deflected to the left. The vertical stab pushes the airplane's tail to the right, its nose to the left, so the left roll moment by the stab makes for a bit proverse roll, in roll direction. This view exactly from behind is hard to interpret.
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Old 05-29-2018, 10:15 AM
  #34  
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Thanks for the time you put into those displays. I know the 'puter does the heavy work but it still takes time to set up the scenarios and then capture and post the pictures.

It is quite the eye opener to see how the deflection and lift from the rudder affects the lift from the stabilizer. That is not something I'd have considered.

It is also interesting to see the way the lift from the fin and rudder increases between 0, 15 and 30 then seems to flatten out. This supports what I've seen in real life where once the rudder moves past around 20 I found that I didn't really notice much of any additional turn response speed.
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Old 05-30-2018, 08:47 AM
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Originally Posted by BMatthews View Post
Thanks for the time you put into those displays. I know the 'puter does the heavy work but it still takes time to set up the scenarios and then capture and post the pictures.

It is quite the eye opener to see how the deflection and lift from the rudder affects the lift from the stabilizer. That is not something I'd have considered.

It is also interesting to see the way the lift from the fin and rudder increases between 0, 15 and 30 then seems to flatten out. This supports what I've seen in real life where once the rudder moves past around 20 I found that I didn't really notice much of any additional turn response speed.
This brings to mind what someone at the flying field said regarding one of my planes. They asked if the ailerons weren't a bit small, referring to the strip ailerons. The airplane, a high wing sport plane with a straight, symmetrical wing, has an adequate, average even, roll rate. It might be more accurate to talk about this in terms of Reynolds Numbers but I don't have enough knowledge of fluid dynamics to go there. The point that I am trying to get to is that I would bet there is a point of suddenly diminishing returns on control surface deflection. Control surface deflections also increase drag in ways that may work with or against those forces described above. Airplanes that operate in a relatively slow speed envelope such a 3-D planes and indoor aerobats seem to be able to take advantage of more extreme surface deflection than faster airplanes. It would probably consume more of UStik's time than he has to spare but it would be interesting to see graphic representations control surfaces and deflection across different air speeds. I guess the bottom line is that for any given deflecting surface there is a point where it crosses over into being an air brake.
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Old 11-25-2018, 06:59 AM
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simply put,.....(layman's terms)...with a swept rudder hingeline,....some of the air stream spills off the top of the rudder because the sweep vectors the airstream upward. in a bank, this directs the nose of the plane up that keeps the plane from slipping down as it looses vertical lift from banking. it essentially does the job that giving a little rudder input does when banking.
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