Need another aero-d discussion...

[Mace2004]

But of course what the heck do wing fences have to do with longitudinal stability?

They reduce spanwise flow and delay onset of tip stall which causes a nose-up pitching moment.

[bozon]

Think of the wingtips (the portion of the wings ahead of the elevons) as separate airfoils.  Remember that the elevon only affects the AoA of the wing in front of it (the "local AoA"), the rest of the wing can have a different AoA.  When both elevons deflect upward they do create a lower (or negative) AoA and downward force but only on the wingtips.  Since the wingtips are behind the CG that creates a nose-up pitching moment exactly as does a conventional horizontal tail. This pitching moment will increase the AoA of the rest of the wing and therefore create additional lift.

OK, now I get what you mean. So what makes the elevons works is that most of the wing area they affect is behind the rest of the wing area (easily if they are far out of the triable base) - in other words, the center of lift for the elevon-affect wing is farther aft than the center of lift of the rest of the wing. I imagine that this requires the center of mass of the plane to be quite far back and close to the center of lift to give the elevon area much more lever-length relative to the fixed wing area. Interesting. Horrible for a fighter that needs to be more than a straight-line drag racer, but interesting.

[bozon]

Now back to my original question on planes that have lots of roll with rudder input, I believe the  primary consideration is dihedral.

Interesting, never considered the effects of the dihedral before. But this also means that planes like the Harrier or some large cargo planes that have a negative dihedral will roll the opposite way when flying uncoordinated, or is this designed in such a way as to cancel the advancing/receding wing induced roll?

[Stoney]

Interesting, never considered the effects of the dihedral before. But this also means that planes like the Harrier or some large cargo planes that have a negative dihedral will roll the opposite way when flying uncoordinated, or is this designed in such a way as to cancel the advancing/receding wing induced roll?

Anhedral in these aircraft is typically introduced to lessen their stability in the roll axis.  The top mounted wings introduce a great amount of inherent roll stability, so the designers add anhedral to make them roll better.  The Carolina Lawn Dart, for example, has a very snappy roll rate.

[dtango]

They reduce spanwise flow and delay onset of tip stall which causes a nose-up pitching moment.

You obviously know something about   

[dtango]

OK, now I get what you mean. So what makes the elevons works is that most of the wing area they affect is behind the rest of the wing area (easily if they are far out of the triable base) - in other words, the center of lift for the elevon-affect wing is farther aft than the center of lift of the rest of the wing. I imagine that this requires the center of mass of the plane to be quite far back and close to the center of lift to give the elevon area much more lever-length relative to the fixed wing area. Interesting. Horrible for a fighter that needs to be more than a straight-line drag racer, but interesting.

The elevons must be much bigger because they tend to be closer to CM than conventional tail configurations which are farther away.

[dtango]

Interesting, never considered the effects of the dihedral before. But this also means that planes like the Harrier or some large cargo planes that have a negative dihedral will roll the opposite way when flying uncoordinated, or is this designed in such a way as to cancel the advancing/receding wing induced roll?

Wings located at the top of the fuselage like the harrier and cargo planes are more stable with anhedral while planes with wings located at the bottom of the fuselage are more stable.

[bozon]

Wings located at the top of the fuselage like the harrier and cargo planes are more stable with anhedral while planes with wings located at the bottom of the fuselage are more stable.

Yes, but HT's point was that the an/dihedral affect the rudder induced roll.

[Mace2004]

They reduce spanwise flow and delay onset of tip stall which causes a nose-up pitching moment.

The problem isn't unique to deltas but to all swept wings.  Here's what's probably the most famous (guess I should say infamous) example...the "Sabre Dance":  http://www.alexisparkinn.com/photogallery/Videos/2006-4-19_F100_Crash.mpg

[Mace2004]

Yes, but HT's point was that the an/dihedral affect the rudder induced roll.

You have to differentiate between dihedral/anhedral angle and dihedral effect.  The dihedral angle just refers to the physical angle from the airplane's horizontal plane while dihedral effect is the rolling moment created due to sideslip.  

Dihedral effect is much more than just the contribution of dihedral angle, it's the result (the sum) of all of the stability factors including dihedreal/anhedral angle, wing sweep, wing position, the vertical tail size and position, the shape of the fuselage itself, engine torque and P-factor.  The fact that a wing might have anhedral is simply the designer's attempt to create the net effect he's after when all the other components are taken into account.  The designer will want some specific amount of positive roll stability, high for a transport plane and relatively low for a fighter. The anhedral of the Harrier doesn't cause it to roll away from yaw, it just reduces the positive roll stability of the high wing.  

Most people think of sideslip as being generated by yaw induced by using the rudder.  This allows us to do things like rudder rolls but sideslip also occurs just from rolling an aircraft whether that be with aileron or by an external disturbance such as turbulance.  Say you're cruising in level flight.  You must have positive AoA to sustain flight which means the aircraft's X axis (axis through the centerline of the aircraft) is at some angle above the relative wind.  When you roll the airplane it rolls around this axis but, momentarily as the aircraft rolls, the relative wind is now seen as sideslip and the aircraft will resist the roll due to dihedral effect.  In a general sense this is good otherwise the aircraft would want to continue to roll over onto its back but for an aircraft that requires high maneuverability lower roll stability is a necessity, hence the anhedral of the Harrier.  Roll stability may be lowered by the anhedral but it's still positive.

The F4 Phantom is a great example of balancing competing requirements: http://www.bing.com/images/search?q=f4+phantom&FORM=BIFD#focal=8aab32cc45ab393883de18d887cfe93f&furl=http%3A%2F%2Fimages.surclaro.com%2FScreenshots%2FFreeStyle%2Ffgr2.jpg  

The wingtips have dihedral to increase roll stability while the stabilators have anhedral which would decrease roll stability...why?  (Simple answer here) In the F4's case, the stabilator was given anhedral to provide inproved directional stability similar to the way that most modern fighters have either two tails, ventral fins, or both.  You can see how much they add to the surface of the vertical tail in this picture: [urlhttp://www.bing.com/images/search?q=f4+phantom&FORM=BIFD#focal=3714b1f9b06156a10cdadc6fc8f4e076&furl=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F3%2F3b%2FF-4_Phantom_II_VF-301.jpg][/url]  Unfortunently, this anhedral would have a destabilizing effect in roll so, when they added the anhedral they added dihedral to the outer wing panels to achieve the balance they wanted.

[dtango]

Yes, but HT's point was that the an/dihedral affect the rudder induced roll.

Just to clarify high wings are inherently more laterally stable then low wings.  In low wings the concept of dihedral is applied to increase stability.  In high wings as Mace mentions anhedral is added to decrease stability so that the aircraft will roll.

The aerodynamics behind this is due to cross-flow of air around the wing and fuselage.  For a low wing airplane in a right side-slip the aoa on the right wing is lower while the aoa on the left his higher resulting in lower lift on right and higher lift on left.  Dihedral angle is added to compensate for this destabilizing force.  In a high wing aircraft the effect is opposite thus naturally stabilizing.  For high-wing airplanes anhedral is added to compensate so that the aircraft will roll better.

How all this relates to HiTech's statement is that rudder applied into the direction of roll beyond a coordinated roll results in sideslip in the opposite direction of the roll.  This sideslip then results in more rolling moment in the direction of roll thanks to low wing dihedral.  On a high-wing airplane the effect is opposite.  Because of the anhedral the sideslip in the opposite direction of roll a counter rolling moment is produced against the direction of roll.  This would have the effect of reducing overall roll.  That's what I mean about the high-wing anhedral being more stable in our given situation.  I suppose if anhedral, wingspan, sideslip being extreme enough the aircraft could roll the opposite direction but the counter rolling moment would need to be great enough to overcome aileron created rolling moments.

[Mace2004]

Sorry, I see I messed up that link, here's it's fixed:  http://www.bing.com/images/search?q=f4+phantom&FORM=BIFD#focal=3714b1f9b06156a10cdadc6fc8f4e076&furl=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F3%2F3b%2FF-4_Phantom_II_VF-301.jpg

[RufusLeaking]

You must have positive AoA to sustain flight which means the aircraft's X axis (axis through the centerline of the aircraft) is at some angle above the relative wind.  When you roll the airplane it rolls around this axis but, momentarily as the aircraft rolls, the relative wind is now seen as sideslip and the aircraft will resist the roll due to dihedral effect.

This would explain the increased tendency to rudder roll at low speeds/high AoA in the T-38.

Pardon the slight tangent, how does all this fit into the concept of "coordinated turns?"  I was only taught to use rudder in a turn in Cessa 152s and 172s (T-41s).  The only time I used rudders in jets (T-37, T-38, KC-135) was in simulated engine failure training, cross wind landings (not the -38) and various contact and formation maneuvers (not the -135).

[dtango]

Pardon the slight tangent, how does all this fit into the concept of "coordinated turns?"  I was only taught to use rudder in a turn in Cessa 152s and 172s (T-41s).  The only time I used rudders in jets (T-37, T-38, KC-135) was in simulated engine failure training, cross wind landings (not the -38) and various contact and formation maneuvers (not the -135).

I have no clue about the T-37's & 38's.  The KC-135's however I think have a yaw damper that automatically coordinates the rudder to deal with yaw.

[RufusLeaking]

I have no clue about the T-37's & 38's.  The KC-135's however I think have a yaw damper that automatically coordinates the rudder to deal with yaw.

The yaw damper was the rudder axis of the autopilot on the KC-135A.  I can't remember any discussion of this helping to coordinate turns.  The standard practice was to keep the rudder autopilot engaged when flying manually to prevent the nose from swaying side to side.  There may be some vaildity to your coordination theory.

The KC-135R has an Engine Failure Assist System (EFAS) for take off, which will automatically deflect the rudder in the event of engine faliure on takeoff.  I believe, but can't exactly remember, that the rudder axis of the auto pilot was upgraded at the same time.  And a true yaw damper added?

Man, I'm getting old.  I used to know this stuff, chapter and verse.