Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Stoney on June 27, 2010, 10:18:22 PM
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After that recent spat of over-the-top aerodynamics discussions, I find myself jonesing for another. Anyone got anything cool to talk about?
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LOL! I'm too busy photoshopping :P. How about "what generates lift?" :t :t :t Very evil topic for those that have an inkling for how evil it is.
Tango
412th FS Braunco Mustangs
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LOL! I'm too busy photoshopping :P. How about "what generates lift?" :t :t :t Very evil topic for those that have an inkling for how evil it is.
Tango
412th FS Braunco Mustangs
I think we tried that one last year. I remember because I was chastised for offering up "angle of attack" as an answer.
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How about "what generates lift?" :t :t :t Very evil topic for those that have an inkling for how evil it is.
Magic?
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Magic?
More precisely ... witchcraft. :noid
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No no no. Not angle of attack or magic or witchcraft, that's foolish and it's really quite simple. As the wing moves through the air it attracts a bunch of little critters in the air called Bernoullis (also known as "Lifties"). They run around on the top of the wing to hold it up. When the Bernoullies get scared or upset because of things like having the wing tilt too much causing the air to get rough and tumble they jump off and the wing stalls. If you get the airflow smooth again they'll come back and hold your wings up again. Of course you can't see Bernoullis....they're invisible....obviously.
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me! me! I got a good one for ya :)
there was a recent post about the Stipa-Caproni Flying Barrel, which I've never seen before so I had a quick look at http://en.wikipedia.org/wiki/Stipa-Caproni (http://en.wikipedia.org/wiki/Stipa-Caproni). I noticed this:
The duct itself had a profile similar to that of the airfoils, allowing the fuselage to provide the airplane with additional lift
and the additional lift provided by the airfoil shape of the interior of the duct itself allowed a very low landing speed of only 68 km/h (42 mph) and assisted the Stipa-Caproni in achieving a higher rate of climb than other aircraft with similar power and wing loading.
would the aerofoil profile of the duct provide any lift? and if it would, surely it would cancel out as the duct is circular and the lift component would therefore resolve to 0? or is it just wikiBS?
:)
edit: :lol mace
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"would the aerofoil profile of the duct provide any lift? and if it would, surely it would cancel out as the duct is circular and the lift component would therefore resolve to 0?"
Any tube with positive AoA will generate some lift and especially if the tube has a profile it will provide more lift which is not cancelled out, no matter which way the profile's camber is set or if it is symmetrical. If the tube is not symmetrical but the profile's camber is always "up" both on lower and upper side of the tube it will provide even more lift.
That's my take on it.
-C+
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Umm yes. I need to know what the coefficient would be if the left rear elevator is shot off of a C47 at 65'. And how does the effect the vertical trajectory during troop drops. :headscratch:
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No no no. Not angle of attack or magic or witchcraft, that's foolish and it's really quite simple. As the wing moves through the air it attracts a bunch of little critters in the air called Bernoullis (also known as "Lifties"). They run around on the top of the wing to hold it up. When the Bernoullies get scared or upset because of things like having the wing tilt too much causing the air to get rough and tumble they jump off and the wing stalls. If you get the airflow smooth again they'll come back and hold your wings up again. Of course you can't see Bernoullis....they're invisible....obviously.
They really do exist. I have proof! Look, the mother of all Bernoullis'!
(http://thetongsweb.net/images/gremlin.jpg)
Tango
412th FS Braunco Mustangs
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More precisely ... witchcraft. :noid
Hogwarts?
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After that recent spat of over-the-top aerodynamics discussions, I find myself jonesing for another. Anyone got anything cool to talk about?
Hmmm...
How to model the airplane directional stability in simulation?
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Hmmm...
How to model the airplane directional stability in simulation?
I know very little about coding, but mathematically couldn't/shouldn't it be treated as a symmetrical airfoil? IE. the fuselage and the vert stab.
I know, all I did was state obvious. :( Won't even try commenting on the coading part. :)
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After that recent spat of over-the-top aerodynamics discussions, I find myself jonesing for another. Anyone got anything cool to talk about?
Why some planes roll more then others when applying rudder?
HiTech
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Why some planes roll more then others when applying rudder?
HiTech
I shoot: Different distance between the wings' and rudder's aerodynamic centers?
EDIT: (on the Z axis)
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I shoot: Different distance between the wings' and rudder's aerodynamic centers?
EDIT: (on the Z axis)
Cigar for the winner :aok
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My guess would be the size of the rudder (aerodynamic force that is exerted) and how far the force is exerted from whatever axis the aircraft is rotating about. Torque, moment, or couple (depending on if you are talking to a physicist or engineer) is basically the rotational equivalent to force and is defined as T = R X F (read: torque equals R Cross F) or the torque equals the vector cross product of the displacement vector and the force vector.
I guess in one sentence it would be that a larger or taller rudder would cause the aircraft to roll more than a smaller or shorter rudder. Bigger meaning larger force, and taller meaning a larger displacement vector.
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I know very little about coding, but mathematically couldn't/shouldn't it be treated as a symmetrical airfoil? IE. the fuselage and the vert stab.
I know, all I did was state obvious. :( Won't even try commenting on the coading part. :)
I'm currently using a symmetrical airfoil aoa data to generate the Z-axis torque, but it's oscillating pretty easily now.
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Cigar for the winner :aok
Hopefully not a cigar from Zetanine's thread :uhoh
http://bbs.hitechcreations.com/smf/index.php/topic,291543.0.html (http://bbs.hitechcreations.com/smf/index.php/topic,291543.0.html)
:rofl
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I'm currently using a symmetrical airfoil aoa data to generate the Z-axis torque, but it's oscillating pretty easily now.
Rgr, I might reply with a PM in a bit. Allthough I'm pretty sure you have everything in place that I'm thinking about and more.
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I believe this is a very minor force and rolls right with left rudder if the rudder is above cg.
HiTech
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Why some planes roll more then others when applying rudder?
I'll bite.
Another part of the effect is the length of the wings. When yawing one wing travels faster and the other slower than the center of the plane - the longer the wings, the larger is the airspeed difference and the more lift is gained/lost - the lift difference leads to a roll. This gives a pronounced roll when kicking the rudder and then a more mild sustained effect.
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As the wing moves through the air it attracts a bunch of little critters in the air called Bernoullis (also known as "Lifties"). They run around on the top of the wing to hold it up. When the Bernoullies get scared or upset because of things like having the wing tilt too much causing the air to get rough and tumble they jump off and the wing stalls. If you get the airflow smooth again they'll come back and hold your wings up again. Of course you can't see Bernoullis....they're invisible....obviously.
In Flight Screening, we were taught about "Lifties."
You forgot the part about them stamping on the wings and tail prior to jumping off (wing buffet).
Mace obviously had military flight training.
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Wow no one want's to discuss one of the word I can not spell?
And no, that's not every word, just close.
HiTech
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Haha, well I had thought about posting a nice nasty set of equations on roll & yaw/sideslip coupling to discuss your topic and elaborate a bit more on what bozon already mentioned but I'm having too much fun elsewhere!
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quick question - for a properly trimmed aircraft is the thrust line the same as the motion vector? ie. in level flight should the thrust line be parallel to the ground?
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OK maybe some aero-engineer can enlighten me on how delta wing, elevator-less planes work. I mean planes like the Mirage or F-106.
Normally the torque to keep the wing in an angle against the airflow is provided by a separate elevator and a leverage due to the elevator being far aft of the main wings. In a delta the wing needs to both produce the lift and the counter torque to keep it in a constant angle of attack. How does that work?
(http://idfaf.110mb.com/Pictures/MirageIII/MG-32.jpg)
(http://www.militaryfactory.com/aircraft/imgs/convair-f106-deltadart.jpg)
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Does the wing configuration really make a difference? If the control surface resides aft of CoL the angle change of the main foil will always be the same as the movement of the control surface i.e up - up. Then think of canard. It resides in front of CoL so it acts more like a flap or spoiler i.e. the angle change of the main foil is always the opposite to the control surface i.e. up - down. Even if that sounds logical one would think that employing a trailing edge flap would cause a significant nose down tendency and it must do so but it also needs a counter force from elevator surfaces or e.g. a steep landing angle where some part of the airframe acts as a lifting force as in Draken J35 which had a tail wheel to protect the exhaust nozzle on extreme angles. The counter force still needs to be more than the force of the flaps or the nose will come down. Thus it seems logical that nearly all modern deltas also have a canard to assist in vertical control.
http://www.vectorsite.net/avj35.html
This is also related to discussion we had years ago about the pitch tendency when deploying flaps in WW2 era planes. Some pitch down, as seems logical, but some are documented to pitch up? Is that actually more related to position and trim of the tail plane?
-C+
Ed. The Draken does not use flaps at all, AFAIK, but its large wing area to create lift or to slow down, thus the extreme angles and thus the tailwheel because the it uses high AoA to shorten take-off run..
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Normally the torque to keep the wing in an angle against the airflow is provided by a separate elevator and a leverage due to the elevator being far aft of the main wings. In a delta the wing needs to both produce the lift and the counter torque to keep it in a constant angle of attack. How does that work?
What torque are you talking about?
Elevators are for controlling pitch. If the engine thrust is in line with the center of gravity, there should be no adverse affect on pitch. Jets are affected much less by engine torque than older prop planes.
In a conventional airplane, the center of gravity is forward of the center of lift. The horizontal stabilizer, to which the elevators are attached, actually pull down.
I am not sure about how a delta wing handles this. On a delta wing plane, the trailing edge control surfaces act as both ailerons and elevators (elevons).
Thus it seems logical that nearly all modern deltas also have a canard to assist in vertical control.
Canards are forward of the center of gravity, so they pull up. There is an efficiency in that the canard is helping to lift the weight as opposed to a conventional tail, which pulls down against the main wing.
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What torque are you talking about?
Longitudinal stability net torque / moment, I assume.
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What torque are you talking about?
torque = radius x force (vector product)
Elevators push down against the lever that is the body of the plane from the center of lift to the tail. This is HOW elevators control pitch and has little to do with engines.
Does the wing configuration really make a difference? If the control surface resides aft of CoL the angle change of the main foil will always be the same as the movement of the control surface i.e up - up. ...
Not quite. The contro9lo surface is not a separate part from the whole wing. It only changes the profile. It all boils down to how the pressure is distributed over the wing to produce the net torque to tilt the wing into a new angle of attack. What I don't know is how it magically gets distributed so it easily balances itself in a new steady angle of attack. This is the case of paper planes that can glide quite well and are easily self-balanced. Why do they glide and no just fly like an arrow in an arc into the ground.
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torque = radius x force (vector product)
Elevators push down against the lever that is the body of the plane from the center of lift to the tail. This is HOW elevators control pitch and has little to do with engines.
This should read.
Elevators push down against the lever that is the body of the plane from the center of GRAVITY to the tail. This is HOW elevators control pitch and has little to do with engines.
What torque are you talking about?
In the case you are question the term "torque" it is not being used in the engine torque since, but rather in the generic form of a force causing a rotation.
Now back to my original question on planes that have lots of roll with rudder input, I believe the primary consideration is dihedral.
HiTech
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quick question - for a properly trimmed aircraft is the thrust line the same as the motion vector? ie. in level flight should the thrust line be parallel to the ground?
It can be but doesn't have to be. When you say properly trimmed for what speed are your referring? Properly trimmed for 120kts or 500kts? You can be properly trimmed for any speed yet your AoA will be different and therefore the relationship between the thrust line and motion vector will change. Of course, in a very general sense yes the trust line is parallel and you can assume that it's designed to closely match the motion vector at the aircraft's best cruise speed but it would match for only one specific condition. Also, there are plenty of aircraft in which the thrust line is actually angled down to provide additional lift (generally for takeoff and landing) or in other direction to counteract torque effects and P-factor.
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In the case you are question the term "torque" it is not being used in the engine torque since, but rather in the generic form of a force causing a rotation.
Roger that. Even though the physics term, "torque," is correct in the discussion of forces acting on moment arms, I am not used to hearing it with regards to the pitch of an aircraft. When discussing the x-y-z axis, it is common to use roll (about the x), pitch (about the y) and yaw (about the z).
Torque, in my experience, is used to discuss the engine/prop effect on roll. For example, I could never get a Cessna 152 to spin opposite the engine's torque. A T-37, a jet, would enter a spin in either direction.
Now back to my original question on planes that have lots of roll with rudder input, I believe the primary consideration is dihedral.
Not sure. In a swept wing aircraft (KC-135), the yaw induced would change the angle of the leading edge of the wing, with one side reducing the sweep angle to the relative wind and the opposite wing increasing the sweep angle. The decreased sweep increased the lift on that side, and the plane would roll in the direction of the rudder deflection. And, I would not characterize it as "lots of roll." It was easily cross controlled with opposite ailerons.
T-38s, with no dihedral, were prone to "rudder roll"' at low speeds, high AOA. So much so, that cross wind landings were made in a crab, with no rudder input.
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OK maybe some aero-engineer can enlighten me on how delta wing, elevator-less planes work. I mean planes like the Mirage or F-106.
Normally the torque to keep the wing in an angle against the airflow is provided by a separate elevator and a leverage due to the elevator being far aft of the main wings. In a delta the wing needs to both produce the lift and the counter torque to keep it in a constant angle of attack. How does that work?
(http://idfaf.110mb.com/Pictures/MirageIII/MG-32.jpg)
(http://www.militaryfactory.com/aircraft/imgs/convair-f106-deltadart.jpg)
Pretty much the same as a conventional wing and tail aircraft does. I have a crude picture but can't seem to upload it right now so you'll have to picture it yourself.
The delta has a Center of Pressure and CoG just as a tailed aircraft but obviously the wing shape is different. The control surfaces are at the trailing edge of the outer portion of the delta wing. Control surfaces essentially only affect the portion of the wing directly ahead of them by changing the local AoA. The affected surfaces (the portion of the delta wing directly in front of the control surfaces) are behind the CoG and CoP just as in a conventional tailed aircraft except they're displaced outboard.
Use the control surfaces differentially (opposite directions) and they act as ailerons and the airplane rolls but use them together (i.e., deflect them the same direction) and they create a pitching moment to control AoA just like an elevator....hence the term Elevon (as RufusLeaking mentions). Make sense? This is also the way a flying wing works except the Elevons can split to create drag for yaw control taking the place of the vertical tail.
The pure delta is an interesting but limited design for a wing. It's very easy to make them thin but strong and the high sweep generates vortices over the wing's surface that improve lift. The sweep also keeps the wingtips behind the shock wave created by supersonic flight so they're outstanding for high speed flight but they kind of suck for turning. Their instantaneous turn rate is very high but the design bleeds like a stuck pig so sustained turning is generally bad and a delta has stability issues at high AoA. They also have very high takeoff and landing speeds with a very nose-high attitude while landing and cannot use trailing edge flaps.
There have been many attempts to eliminate the delta's limits but you don't see many pure deltas around any more. Even so, almost all modern fighters include some aspect of the delta. Tailed deltas like the MiG021, A-4, and F16 became very common in the 60's. Modern tailess deltas like the Typhoon, Gripon, and Rapale all use canards in front of the wing. Modern non-delta winged fighters like the F18 still use aspects of a delta design by inducing the delta's vortices with leading edge extensions.
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The delta has a Center of Pressure and CoG just as a tailed aircraft but obviously the wing shape is different. The control surfaces are at the trailing edge of the outer portion of the delta wing. Control surfaces essentially only affect the portion of the wing directly ahead of them by changing the local AoA. The affected surfaces (the portion of the delta wing directly in front of the control surfaces) are behind the CoG and CoP just as in a conventional tailed aircraft except they're displaced outboard.
Use the control surfaces differentially (opposite directions) and they act as ailerons and the airplane rolls but use them together (i.e., deflect them the same direction) and they create a pitching moment to control AoA just like an elevator....hence the term Elevon (as RufusLeaking mentions). Make sense? This is also the way a flying wing works except the Elevons can split to create drag for yaw control taking the place of the vertical tail.
And here is my problem with this description: to increase AoA, the elevons deflect UP. On a normal wing, deflecting the ailerons up lowers the AoA, the lift and this is the wing that drops in the roll. So why does deflecting the elevons up, which according the this description should lower the angle on attack of that portion of the wing ends up increasing the pitch of the plane and increasing the lift.
This should read.
Elevators push down against the lever that is the body of the plane from the center of GRAVITY to the tail. This is HOW elevators control pitch and has little to do with engines.
Of course. Misplaced terms.
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And here is my problem with this description: to increase AoA, the elevons deflect UP. On a normal wing, deflecting the ailerons up lowers the AoA, the lift and this is the wing that drops in the roll. So why does deflecting the elevons up, which according the this description should lower the angle on attack of that portion of the wing ends up increasing the pitch of the plane and increasing the lift.
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.
For rolling, the elevons act exactly as do conventional ailerons. You want to go left, the left elevon comes up creating a downward force, the right elevon goes down creating an upward force and the airplane rolls left. Again, this is the same as a conventional aircraft.
The fact that the elevons operate differently for pitch and roll means that controlling them is more complex that conventional controls. In mechanical systems, the pilot's input (pitch or roll) goes into a box called a mixer that sorts them out to move the elevons in the right direction, either together or in opposite directions. With fly-by-wire systems, it's easy as it's all controlled by software.
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LOL! I'm too busy photoshopping :P. How about "what generates lift?"
this is a no brianer...differences in air pressure generate lift!
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this is a no brianer...differences in air pressure generate lift!
Oh boy...
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this is a no brianer...differences in air pressure generate lift!
Absolutely, but what creates the differences in pressure? :t :t :t
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http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
:confused:
-C+
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I have a very brief moment to post and wanted to add to the discussion on delta wings. Rufusleaking and Mace have discussed in general how delta wing aircraft deal with longitudinal stability when there isn't a horizontal tail. Essentially the elevons are trimmed to counteract pitchup or down moments as needed. Bozon pointed out the F-102 as the example aircraft in question to discuss the topic. That's an instructive aircraft actually because delta wing's have other longitudinal stability issues that can affect them that require more than just elevon control surfaces to address. Here is an image of the F-102:
(http://thetongsweb.net/images/f102a.jpg)
Notice the 4 "mystery thingies" I've annotated on the wings. Those little "mystery thingies" I've pointed out are really important in dealing with uncontrolled pitchup of the F-102 in certain flight regimes.
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http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
:confused:
-C+
:t :t :t
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And here I was thinking this would deteriorate into a discussion about hip to waist ratio, i.e. the "Coke Bottle" needed to get the 102 supersonic.
As they say good fences make good neighbors. :salute
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As they say good fences make good neighbors. :salute
:aok
But of course what the heck do wing fences have to do with longitudinal stability? ;)
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:aok
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.
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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.
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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?
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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.
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They reduce spanwise flow and delay onset of tip stall which causes a nose-up pitching moment.
You obviously know something about :airplane: :D
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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.
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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.
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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.
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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 (http://www.alexisparkinn.com/photogallery/Videos/2006-4-19_F100_Crash.mpg)
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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 (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.
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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.
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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 (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)
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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).
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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.
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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.
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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.
A yaw damper was probably the first electronic augmentation to flight controls ever designed. It's a fairly simply analog rate detector that will input a few degrees of rudder opposite sensed yaw. While it's designed specifically to counter dutch roll (the "swaying side to side" you mention) it also provides some (limited) automatic correction for coordinated flight. While jets don't have to worry about all those nasty effects from that spinny thing up front the use of ailerons will still have a tendancy to produce adverse yaw (yaw away from the turn) because the upgoing wing is producing additional lift and therefore increased induced drag while the downgoing wing produces less lift and less induced drag. It's this differential that will yaw the plane away from the roll and turn. A yaw damper will counteract the yaw within its limited authorities and provide a coordinated turn, again this is within limits.
Yaw dampers evolved into Stability Augmentation Systems (SAS) with damping in pitch, roll, and yaw but their authority is still fairly limited in the amount of control deflection they can command. Over time these analog systems have been replaced by digital computers and became CAS and TCAS type systems with lots more control authority and new features like aileron/rudder interconnects and then, eventually, pure fly-by-wire. The newer systems are smart enough that they can provide the right control inputs regardless of what the pilot does.
For instance, the F14 had only a 1960's era analog SAS and when pulling greater than 17 units AoA the pilot had to know not to use lateral stick to roll but instead use the rudder. At high AoA a pure aileron roll produced so much sideslip that the airplane would actually roll opposite of the stick input...it would start in the correct direction but the induced sideslip would blank and stall the upgoing wing while the sweep of the downgoing wing dramatically increased its lift and the airplane would snap roll the other direction, violently if you were fast enough. This was the negative side to dihedral effect and swept wings and the SAS didn't have the ability to counteract the problem while it did help some. However, using rudder instead of stick yawed the airplane into the desired roll direction producing sideslip from the opposite side and dihedral effect would then roll the plane nicely. (This is the same cause/effect as the rudder roll you saw in the T38). The F18's fly-by-wire system is smarter. It has two flight control computers that receive the pilot's input from the stick/rudders. The computers look at the flight conditions and decide what commands to send to the flight controls. At high AoA it will washout aileron and replace it with rudder even if the pilot just jams the stick to the side. The result was the same as in the F14...the F18 pilot just doesn't have to be as good since he only has one vote in three. :lol
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Thanks mace. I also wonder if SAS's now include spoilers as well to provide "proverse yaw" . Seems possible. I suppose if I do some digging around and pile through some of my aero texts etc. the answer is there somewhere!
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Thanks mace. I also wonder if SAS's now include spoilers as well to provide "proverse yaw" . Seems possible. I suppose if I do some digging around and pile through some of my aero texts etc. the answer is there somewhere!
I don't see why that wouldn't be possible, of course spoilers seem to be mostly limited to big wing planes and gliders now.
The F14 had them but the SAS only worked through the rudders and stabs. It's the last fighter I can think of that used them and that was due to the swing wing. They worked in conjunction with the stabs with the wings out and they were disabled with the wings back and the stabs did all of the roll control. Being purely electrically controlled (but hydraulically powered), it was easy to switch them on and off based on wing sweep angle. They also allowed full-span trailing edge flaps for CV suitability, direct lift control on approach, ground-roll braking, and produced no twist in the high-aspect wing so there was no possibility of control reversal at high speeds. Of course, they also reduced the need for the yaw SAS to get involved in simple turns.