Blackbird From United States, joined Oct 1999, 3436 posts, RR: 5 Posted (2 months 4 weeks 1 day 8 hours ago) and read 1996 times:
I know this sounds remarkably silly... why does the rudder when deflected (being above the planes center of gravity) not roll the plane the opposite way it's deflected? In other words, right rudder, left roll?
Is it just because the yawing motion (say right-yaw) produced also produces a rolling movement (say right-roll, due to asymmetrical airflow over the wings) to go with it, that neutralizes this tendency?
Or is there another reason the rudder doesn't roll the plane the opposite way it yaws it because the fin is above the center of gravity?
SlamClick From United States, joined Nov 2003, 9938 posts, RR: 72 Reply 1, posted (2 months 4 weeks 1 day 8 hours ago) and read 1996 times:
It does.
It is possible to rudder-roll many airplanes.
If you are thinking airliners, you should know that the rudder pedals are nothing more than footrests, and not even our favorite footrest any time all the engines are running. We roll in and out of turns with our feet flat on the floor or resting against the pedals and let the yaw damper operate the rudder. I flew a DC-3 a while back and humiliated myself (I am typed in it) because I'd utterly forgotten what those pedals were for.
The [airliner] rudder has a large area because we may have to lift off with one engine inoperative and the other at takeoff thrust. By the time we have the flaps/slats cleaned up the rudder travel (throw) is restricted. By the time we get to cruise the thing hardly moves at all. So it has its center of pressure maybe twenty feet above the CG. The wings have theirs maybe forty or fifty feet outboard from CG and have a lot more area than the rudder. Then we have ailerons out at the very tips, maybe 70 to 100 feet from CG - lots of arm! And we have roll spoilers. The rudder just doesn't have much opportunity to roll the airplane once we are underway.
Now I suspect that, say, a T-38 would be rather different.
One more thing to think about regarding rudder. Let's make it a Cessna 150. To make a turn you will apply aileron and rudder into the turn - your early lessons being focused on teaching you how to keep the ball centered. When the bank is steep enough you will center the ailerons to stop the roll-rate and use the rudder to center the ball, which means you will end up centering the rudder. You will continue the turn with backpressure on the elevators alone. In a sustained, coordinated turn the elevator will be the only surface with any noticeable throw. We turn with elevator.
Starlionblue From Greenland, joined Feb 2004, 12881 posts, RR: 57 Reply 2, posted (2 months 4 weeks 1 day 7 hours ago) and read 1963 times:
SlamClick explains it well (how else?) I would add that rudder roll is an interesting design consideration for trijets. Once of the disadvantages of the DC-10/MD-11 style rudder is that it is quite far from centerline compared to the L-1011's rudder. So you get more more roll when it deflects than if it were closer. I believe this is why the DC-10 wing engines had to be closer to the fuse than the L-1011 wing engines, leading to a stronger (=heavier) wing.
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474218 From United States, joined Oct 2005, 2829 posts, RR: 3 Reply 3, posted (2 months 4 weeks 1 day 6 hours ago) and read 1945 times:
Quoting Starlionblue (Reply 2): I believe this is why the DC-10 wing engines had to be closer to the fuse than the L-1011 wing engines, leading to a stronger (=heavier) wing.
The L-1011 has a large single rudder, while the DC-10/MD-11 has a much smaller duel segment, double articulated rudder. Because the L-1011's larger rudder provides more rudder authority, the wing engines could be mounted further away from the fuselage centerline.
Starlionblue From Greenland, joined Feb 2004, 12881 posts, RR: 57 Reply 4, posted (2 months 4 weeks 1 day 6 hours ago) and read 1937 times:
Quoting 474218 (Reply 3):
The L-1011 has a large single rudder, while the DC-10/MD-11 has a much smaller duel segment, double articulated rudder. Because the L-1011's larger rudder provides more rudder authority, the wing engines could be mounted further away from the fuselage centerline.
Ah. Quite.
The DC-10/MD-11 is also smaller due to less usable fin height. I guess it's all part of the same puzzle.
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BE77 From Canada, joined Nov 2007, 72 posts, RR: 0 Reply 5, posted (2 months 4 weeks 1 day 6 hours ago) and read 1927 times:
OK Blackbird - I never thought about it like that before, so now I am confused too.
You would think you would get a rolling moment about the CG when you deflect the rudder - actually, you must. Something else bigger is happening though, since it doesn't work that way?
I tend to actually use those 'footrests' all the time, as a BE77 does not have aileron trim. If I need both hands for things like map reading (I'm old school - my gps does not have a moving map ) then I just use the rudder to pick up a wing when it drops. (Note - yes, the BE77 ailerons are spring connected to the rudder, but the same thing works in a buck and a half too).
Back to the conjecture...maybe the diehedral of the wing push forward by the induced yaw acts to generate a lot more lift, therefor overcoming any roll moment about the cg??
FutureUALpilot From United States, joined May 2000, 2172 posts, RR: 6 Reply 6, posted (2 months 4 weeks 1 day 6 hours ago) and read 1928 times:
I'm guessing its because it makes the outside wing travel faster, generating more lift and creating a tendency to roll towards the same side as the depressed rudder pedal. (Thinking as though one were to apply rudder in level flight)
Moose135 From United States, joined Oct 2004, 630 posts, RR: 7 Reply 9, posted (2 months 4 weeks 1 day 5 hours ago) and read 1921 times:
Quoting SlamClick (Reply 1): Now I suspect that, say, a T-38 would be rather different.
Yes, but I don't remember using the rudder for turns in a T-38, mostly just roll and pull. The T-37 you used rudder in turns, and when I went to the KC-135, I needed to re-learn using rudder around the pattern.
BE77 From Canada, joined Nov 2007, 72 posts, RR: 0 Reply 10, posted (2 months 4 weeks 1 day 5 hours ago) and read 1898 times:
Quoting SlamClick (Reply 8): Get rid of that habit before you ever fly swept-wing aircraft
Sure hope I have to!!! Not likely without a lot of help from the lottery though.
Quoting SlamClick (Reply 8): guess there was something wrong with my explanaton.
Nah, nothing wrong, with your explanation. I was just trying to sort out the dynamics wrt the Newtownian physics of the issue in my own head. Like Blackbird posted, I know that a rudder acts differently than an aileron, but it is the why that is interesting - and I at least never thought about it before - for the last 29 years, I 'knew' the rudder behaved as it did, without ever thinking about the counter-intuitivensss that Blackbird highlighted.
Starlionblue From Greenland, joined Feb 2004, 12881 posts, RR: 57 Reply 11, posted (2 months 4 weeks 1 day 4 hours ago) and read 1880 times:
On the opposite end, you have fighter aircraft that use their elevators (and I believe even rudder(s)) as extra ailerons. They have elevons and I guess the rudder(s) are "rudderons".
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Jetlagged From United Kingdom, joined Jan 2005, 1862 posts, RR: 7 Reply 12, posted (2 months 4 weeks 1 day 3 hours ago) and read 1864 times:
Quoting BE77 (Reply 10): Nah, nothing wrong, with your explanation. I was just trying to sort out the dynamics wrt the Newtownian physics of the issue in my own head. Like Blackbird posted, I know that a rudder acts differently than an aileron, but it is the why that is interesting - and I at least never thought about it before - for the last 29 years, I 'knew' the rudder behaved as it did, without ever thinking about the counter-intuitivensss that Blackbird highlighted.
Hardly counter-intiutive. All pilots are taught "effects of controls", and that includes roll induced by rudder. Only a non pilot or someone who has not studied aerodynamic stability and control would find it surprising.
Ailerons induce yaw as well as roll (which is part of the reason why you need a rudder!). Lateral and directional control and stability are closely interlinked. It is not like a ships rudder, as the only means of making a turn. You could make a rudder turn in an aircraft but it would not be very efficient. Better to use the tilted lift vector instead.
Actually a rudder works just like an aileron. It changes the local aerofoil camber and thus changes the lift the surface produces. The only difference is the axis the surface acts in.
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474218 From United States, joined Oct 2005, 2829 posts, RR: 3 Reply 13, posted (2 months 4 weeks 1 day 2 hours ago) and read 1839 times:
Quoting Jetlagged (Reply 12): Actually a rudder works just like an aileron. It changes the local aerofoil camber and thus changes the lift the surface produces. The only difference is the axis the surface acts in.
Jetlagged:
Congratulations, you are the first person (beside myself) that understands that the rudder doesn't push the nose of the aircraft. But that the the tail end is sucked in the the low pressure created by deflecting the rudder.
Hah! I worked with a guy once at a company who, although not trained as an engineer, was very technically astute. We used vacuum pumps and such at that job, and he'd often explain something by referring to the "negative pressure" created on something-or-other. That annoyed the hell out of me
Quoting Jetlagged (Reply 12): Actually a rudder works just like an aileron. It changes the local aerofoil camber and thus changes the lift the surface produces. The only difference is the axis the surface acts in.
As pointed out, however, the rudder has far less moment-arm to work with. I'd imagine that the increased lift on the downward-moving wing (due to the roll caused by rudder deflection) would cancel out a fair amount of the rolling force provided by the rudder.
Quoting SlamClick (Reply 8): Get rid of that habit before you ever fly swept-wing aircraft.
For what reason, out of curiousity?
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PhilSquares From Ireland, joined Mar 2004, 3717 posts, RR: 53 Reply 17, posted (2 months 4 weeks 22 hours ago) and read 1795 times:
Quoting SlamClick (Reply 1): Now I suspect that, say, a T-38 would be rather different.
The reason has to do with the effectiveness of the actual control surface. On conventional straight wing aircraft, the rudder works very different than it does on swept wing aircraft. I only have about 80 hours in light aircraft, so I am far from being an expert, but I do have about 22000 hours in swept wing aircraft.
I was a T-38 instructor, many years ago and one of the things we would do as a demonstration of the swept wing characteristics of the T-38. We would do a full aft stick stall, which was just what it says. The aircraft would enter a conventional stall, where on a swept wing aircraft, the wing stalls from the wing tip inward. Thus, the conventional controls, ailerons, become ineffective. As the stall develops, the wing is completely stalled, and there is no longer any buffett and the IVSI is pegged at a 6000/FPM descent. However at higher angles of attack, the rudder is extremely effective as a control surface and you can still do a rudder roll by a smooth application of rudder.
In a swept wing aircraft, at higher angles of attack, the ailerons which are normally located on the outboard sections of the wing, are less effective due to the stalled condition that exists. The rudder since it's not effected by the ever increasing angle of attack remains effective.
Starlionblue From Greenland, joined Feb 2004, 12881 posts, RR: 57 Reply 18, posted (2 months 4 weeks 21 hours ago) and read 1777 times:
Quoting Vikkyvik (Reply 16): Quoting SlamClick (Reply 8):
Get rid of that habit before you ever fly swept-wing aircraft.
For what reason, out of curiousity?
My guess is that you risk dutch roll and all sorts of other nasty behaviors. Or could end up parking your aircraft in a residential neighborhood on Long Island.
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) then I just use the rudder to pick up a wing when it drops. (Note - yes, the BE77 ailerons are spring connected to the rudder, but the same thing works in a buck and a half too).
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