High AOA Aerodynamics
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High AOA Aerodynamics
I've noticed that at high AOA the Super Hornet deflects its ailerons and stabilators in opposite directions. The following image shows the effect. The ailerons and rudders are positioned in the orthodox manner, as the F/A-18F executes a roll to the right, however the stabliators are deflected in way that traditionally supports a left hand roll:
http://www.airliners.net/open.file/0852458/L/ I think that yaw/adverse yaw control may be what were seeing, but I don't understand exactly what is happening.
I would have expected stab deflection opposing aileron deflection to cancel both roll and yaw tendencies produced by those surfaces, leaving the rudder as the only effective aerodynamic control. I guess I'm trying to say, 'why aren't the stabs and ailerons left at their neutral, zero deflection angles'?.
Thanks
Obi
http://www.airliners.net/open.file/0852458/L/ I think that yaw/adverse yaw control may be what were seeing, but I don't understand exactly what is happening.
I would have expected stab deflection opposing aileron deflection to cancel both roll and yaw tendencies produced by those surfaces, leaving the rudder as the only effective aerodynamic control. I guess I'm trying to say, 'why aren't the stabs and ailerons left at their neutral, zero deflection angles'?.
Thanks
Obi
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Which way was the plane rolling?
Because I might make a wild guess that it's actually rolling LEFT wing down i.e. in the direction that agrees with the diff-stab deflection. Perhaps, at Hogh AoA, the aileron effectiveness actually reverses (it is the case that at high AoA aileron effectiveness CAN reverse; whether it does on the Hornet I don't know). In which case ALL the surfaces are commanding a consistent left roll; they just aren't deflected normally.
If the roll was to the right, it's possible that the stab position is in some way related to relieving v-tail loads or improving rudder effect; but that seems a thinner explanation.
Because I might make a wild guess that it's actually rolling LEFT wing down i.e. in the direction that agrees with the diff-stab deflection. Perhaps, at Hogh AoA, the aileron effectiveness actually reverses (it is the case that at high AoA aileron effectiveness CAN reverse; whether it does on the Hornet I don't know). In which case ALL the surfaces are commanding a consistent left roll; they just aren't deflected normally.
If the roll was to the right, it's possible that the stab position is in some way related to relieving v-tail loads or improving rudder effect; but that seems a thinner explanation.
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Because they do whatever the computers tell them to do that instant.
It is possible the airplane was NOT in stable flight, but transitioning/turning/rolling. The computers will often have the control surfaces moving wildly in that case to retain stability.
It is possible the airplane was NOT in stable flight, but transitioning/turning/rolling. The computers will often have the control surfaces moving wildly in that case to retain stability.
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The Hornet's not statically unstable in roll or yaw, is it? Maybe neutral in pitch? In which case there shouldn't be too many wild motions, and they should at least be coherent if there are.
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Originally Posted by Intruder
Because they do whatever the computers tell them to do that instant.
It is possible the airplane was NOT in stable flight, but transitioning/turning/rolling. The computers will often have the control surfaces moving wildly in that case to retain stability.
It is possible the airplane was NOT in stable flight, but transitioning/turning/rolling. The computers will often have the control surfaces moving wildly in that case to retain stability.
I've seen it many times in the airplane I fly (B777). Even in cruise, perfectly levelled off, sometimes you see the flaperons up and down to correct a mild turbulence. Just the FBW system at work.
The Hornet flight control system can do some awesome things (fly by wire)!
When flying in formation at slow speed, with no manoeuvring, the flight controls are always doing their "funky chicken" moves. You can see the control surfaces fluttering away keeping the aircraft in smooth balance, but the pilot isn't even moving the stick. All the control surfaces can move independently of eachother as they are all computer controlled.
The pilot in that shot is definitely in a right roll (ailerons & rudders). As mentioned, the stabs look backwards to assist that move, but the "computer box down the back" figures it all out and does what it needs
The Hornet also has what is called an RSRI (Rolling Surface to Rudder Interconnect). You jam in full aileron, and the rudders automatically deflect to assist.
Without the flight control computers helping the pilot out, it will fly like a brick and a very real chance you'll lose control. Fighter jets are normally quite unstable, which allows them to manoeuvre well.
When flying in formation at slow speed, with no manoeuvring, the flight controls are always doing their "funky chicken" moves. You can see the control surfaces fluttering away keeping the aircraft in smooth balance, but the pilot isn't even moving the stick. All the control surfaces can move independently of eachother as they are all computer controlled.
The pilot in that shot is definitely in a right roll (ailerons & rudders). As mentioned, the stabs look backwards to assist that move, but the "computer box down the back" figures it all out and does what it needs
The Hornet also has what is called an RSRI (Rolling Surface to Rudder Interconnect). You jam in full aileron, and the rudders automatically deflect to assist.
Without the flight control computers helping the pilot out, it will fly like a brick and a very real chance you'll lose control. Fighter jets are normally quite unstable, which allows them to manoeuvre well.
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Mad (Flt) Scientist:
Yes it is definately a roll to the right. The Super Hornet is said to be statically neutral to slightly unstable longitudinally. In the picture although the stabs are deflected differentially to the left, they are also at a net trailing edge down position.
This sounds quite plausible 
Intruder & machbias:
The effect is more than a transient, it lasts as long as the high AOA roll does. The flight control system continues to make small adjustments to the stabs while the aircraft performs the manoeuvre. The adjustments made however are usually to both stabs symmetrically, i.e the angular difference between the stabs remains constant. The adjustments are basically in the pitch axis.
This condition always exists when the Super Hornet executes a high AOA roll, also as part of the roll damping logic when the pilot neutralises lateral control, the deflections of the control surfaces are reversed to they appear as a mirror images of the picture posted.
TruBlu351:
Yes you are correct. Many aircraft exhibit the trait of fluttering surfaces to maintain precise attitude. Most of the time its relatively easy to understand what the FCS is trying to achieve, as the control surfaces are deflected traditionally to deal with flightpath and attitude excursions. The control surface repositioning also usually coincides neatly with these excurions. In this case however its not so clear.
Obi
Yes it is definately a roll to the right. The Super Hornet is said to be statically neutral to slightly unstable longitudinally. In the picture although the stabs are deflected differentially to the left, they are also at a net trailing edge down position.
If the roll was to the right, it's possible that the stab position is in some way related to relieving v-tail loads or improving rudder effect; but that seems a thinner explanation.
Intruder & machbias:
The effect is more than a transient, it lasts as long as the high AOA roll does. The flight control system continues to make small adjustments to the stabs while the aircraft performs the manoeuvre. The adjustments made however are usually to both stabs symmetrically, i.e the angular difference between the stabs remains constant. The adjustments are basically in the pitch axis.
This condition always exists when the Super Hornet executes a high AOA roll, also as part of the roll damping logic when the pilot neutralises lateral control, the deflections of the control surfaces are reversed to they appear as a mirror images of the picture posted.
TruBlu351:
Yes you are correct. Many aircraft exhibit the trait of fluttering surfaces to maintain precise attitude. Most of the time its relatively easy to understand what the FCS is trying to achieve, as the control surfaces are deflected traditionally to deal with flightpath and attitude excursions. The control surface repositioning also usually coincides neatly with these excurions. In this case however its not so clear.
Obi
