Dihedral
04-06-2008, 04:06 PM
#1
Dihedral
Is the purpose of dihedral to add 'self-righting characteristics' to the plane?
Does this work by lowering the center of mass away from where the fuse meets the wings (creating a moment arm), so that the force of gravity on the fuse has a greater moment about the axis where the wings meet the fuse?
I have seen both low-wing commercial planes with dihedral, and a high-wing military plane with 'reversed dihedral'. Would this be to nearly eliminate the moment arm and thus the moment about the plane caused by gravity so that the plane does what the pilot tells it to, and not what gravity wants it to do?
Thanks.
Does this work by lowering the center of mass away from where the fuse meets the wings (creating a moment arm), so that the force of gravity on the fuse has a greater moment about the axis where the wings meet the fuse?
I have seen both low-wing commercial planes with dihedral, and a high-wing military plane with 'reversed dihedral'. Would this be to nearly eliminate the moment arm and thus the moment about the plane caused by gravity so that the plane does what the pilot tells it to, and not what gravity wants it to do?
Thanks.
04-06-2008, 05:07 PM
#2
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RE: Dihedral
It does help distribute the center of gravity towards the middle of the plane, it also adjusts the airflow as well, funnelling it appropriatly for stability. I used to think all planes needed it, only to find out, it's absolutely in the way when flying in any sort of wind, so I tend to completely ditch it with my creations. An added benifet for getting rid of it is that you do have more control since you aren't fighting the plane trying to make it's own adjustments without your input. You'll know you are ready to get rid of it when you no longer let the plane adjust itself, where you are correcting it before the plane has a chance to do it on it's own.
04-06-2008, 05:49 PM
#3
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RE: Dihedral
The dihedral works by increasing the lift on the low side. A wing tilted from level has less lift that one that is level. The greater the angle, the less lift. With dihedral, both wings are at an angle to level. As the plane rolls to one side, the high wing looses more lift, and the low wing gains lift as it goes to level. The high wing starts dropping and the low wing comes up. This is how the self righting is accomplished. When the plane is level, both wings are tipped an equal amount so there is equaliburium.(sp?). Of course, this assumes equal wing pannel sizes, balanced tip to tip, no warps and ailerons balanced. All of these can overide the dhideral effect.
Don
Don
04-06-2008, 05:54 PM
#4
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RE: Dihedral
The amount and type of dihedral depends on the wing location and purpose. More advanced skill level models want no coupling between the rudder and roll. So if the airplane is low wing'ed, some dihedral is required. If some degree of stability in roll is desired or you want pro-rolling forces with rudder to turn the airplane, more dihedral is required. Removing all dihedral from a low wing design will cause it to adverse roll when rudder is applied.
If the wing is high mounted on the fuselage, then far less is required to maintain neutral or pro-rolling with rudder. It is in these designs that you may find anhedral used.
If the wing is high mounted on the fuselage, then far less is required to maintain neutral or pro-rolling with rudder. It is in these designs that you may find anhedral used.
04-06-2008, 07:46 PM
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RE: Dihedral
ORIGINAL: Campgems
The dihedral works by increasing the lift on the low side. A wing tilted from level has less lift that one that is level. The greater the angle, the less lift.
Don
The dihedral works by increasing the lift on the low side. A wing tilted from level has less lift that one that is level. The greater the angle, the less lift.
Don
How does the wing know that it is on the low side so it can produce more lift ?
04-06-2008, 09:06 PM
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RE: Dihedral
ORIGINAL: chashint
Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ?
ORIGINAL: Campgems
The dihedral works by increasing the lift on the low side. A wing tilted from level has less lift that one that is level. The greater the angle, the less lift.
Don
The dihedral works by increasing the lift on the low side. A wing tilted from level has less lift that one that is level. The greater the angle, the less lift.
Don
How does the wing know that it is on the low side so it can produce more lift ?
If the vertical stab doesn't have enough area, the side slip yaw will roll the plane back to level but the plane will overshoot level flight and then re-overshoot level flight the other way, and on and on. This is called "dutch roll".
04-07-2008, 11:19 AM
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RE: Dihedral
Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ?
How does the wing know that it is on the low side so it can produce more lift ?
As menionted, that's only part of what is going on though.
Btw, some full-scale aircraft appear to have anhedral on the ground, but in the air, the wings flex a lot under heavy load, and are actually rather flat when the plane is being flown as designed. (see B-52)
04-07-2008, 10:50 PM
#9
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RE: Dihedral
Dihedral both makes the plane flatten out from a rolled position and gives some protection from side slip errors. From any position, a plane with lots of dihedral and a flat bottom wing will return to straight and level flight if you stop giving it control inputs. That's assuming that the plane has enough altitude to make its recovery.
04-08-2008, 08:47 AM
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RE: Dihedral
There's nothing magical about flat bottoms on airfoils. The flat bottom really doesn't give you any extra handling compared to other airfoils with approprate amounts of camber.
04-08-2008, 01:36 PM
#11
Moderator
RE: Dihedral
You're right, but the flat bottom does give more lift as speed increases so in an unintentional dive flat bottom wings will pull the plane out of it.
04-08-2008, 01:42 PM
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RE: Dihedral
My point is that those charastics aren't due to the wing having a flat bottom. All airfoils increase in lift as airspeed increases, even symetrical ones.
What is more often the case with trainers is that the wing is set at a rather high incidence, plus a fairly far forward CG results in the pulling out of a dive you describe. You can cause the same effects with a symetical airfoil, and you can make a plane with a cambered airfoil (such as a "flat bottom" airfoil, which is just another airfoil with camber) NOT do those things.
What is more often the case with trainers is that the wing is set at a rather high incidence, plus a fairly far forward CG results in the pulling out of a dive you describe. You can cause the same effects with a symetical airfoil, and you can make a plane with a cambered airfoil (such as a "flat bottom" airfoil, which is just another airfoil with camber) NOT do those things.
04-08-2008, 04:50 PM
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RE: Dihedral
>> Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
04-08-2008, 04:52 PM
#16
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RE: Dihedral
ORIGINAL: yetti831
Are there any good introductory texts that would cover all of this?
Are there any good introductory texts that would cover all of this?
BASICS OF R/C MODEL AIRCRAFT DESIGN by Andy Lennon
It's the best single book for modelers who want a decent understanding of the basics but aren't looking for an Aeronautical Engineering degree.
It's published by Model Airplane News, and most LHSs can get it for you. The ones around here stock it.
04-08-2008, 05:50 PM
#17
RE: Dihedral
ORIGINAL: mitchell170
>> Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
>> Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
I understand what you're saying here. However, even though the now horizontal wing (right wing in right bank) wants to create more lift vertically, the opposite wing (left wing) wants to create more lift horizontally, in the direction of the bank. Regardless of bank angle, there is still the same amount of air passing under and over the wings; they do not know if they are horizontal or not. (There's a post like this in the aerodynamics forum, and its debated there, too.)
I'm a mechanical engineering student. I haven't taken fluid dynamics yet (next semester
), but I believe that the dihedral issue is more based on simple mechanics. Imagine, for an exaggerated example, that the dihedral was 90 degrees, so that when the plane is flying level, the wings are each at a 45 degree angle. If you bank to the right so that the right wing is now horizontal (45 degree bank right) you will have an unbalanced wing with the weight of the plane completely at the end. Gravity will pull down on the plane, until the force is balanced on the left side by the left wing generating some vertical lift.
Now imagine the same plane with no dihedral in a 45 degree bank. Gravity will have a much harder time 'righting' the plane because there is vertical lift being generated by the left wing, and less vertical lift being generated by the right wing.
Unfortunately I haven't flown a model plane yet! This theory would be proved if the dihedral had less self-righting characteristics at higher speeds, which means that as the lift force of the wings increases (in the direction perpendicular to the surface of the wing), the effect of gravity becomes negligible. If you could fly really, really fast in a plane with dihedral, it should have almost no self-righting characteristics at all (if my postulate was correct).
04-08-2008, 08:08 PM
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RE: Dihedral
>>I understand what you're saying here. However, even though the now horizontal wing (right wing in right bank) wants to create more lift vertically, the opposite wing (left wing) wants to create more lift horizontally, in the direction of the bank. Regardless of bank angle, there is still the same amount of air passing under and over the wings; they do not know if they are horizontal or not. (There's a post like this in the aerodynamics forum, and its debated there, too.) <<
See, I told you it was OLD physics.
I think you are correct. The force vectors have to sum the sam, regardless of the wing orientation. That gives you equal rotattional moments around the axis (fuselage centerline).
I'm getting too old for this, and it's been to long.
bob
See, I told you it was OLD physics.
I think you are correct. The force vectors have to sum the sam, regardless of the wing orientation. That gives you equal rotattional moments around the axis (fuselage centerline).
I'm getting too old for this, and it's been to long.
bob
04-08-2008, 09:20 PM
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RE: Dihedral
ORIGINAL: mitchell170
>> Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
>> Why does the wing being "tilted" away from "level" make any difference in the lift being generated compared to the other wing ?
How does the wing know that it is on the low side so it can produce more lift ? <<
Some of my old physics here, and don't even think about asking "how old".
The lift generated by a wing with no dihedral is purely vertical in straight and level flight. The lift generated by a dihedral wing has both a vertical and a horizontal component in straight and level flight. Think of it like this..........draw a horizontal line representing a wing and then a vertical arrow from the wing. As you tilt the wing (add dihedral) the arrow now tilts slightly toward the fuselage. Because the arrow is now tilted the amount of pure vertical lift is reduced. The plane flies straight because the other wing has just the opposite horizontal component, and they balance each other.
Now, picture the right wing rolled to the right in a shallow turn. When the bank angle is equal to the dihedral angle the right wing is now completely horizontal, and it's "arrow" now points straight up. More lift. At the same time the effective bank angle of the left wing is now the dihedral angle PLUS the bank angle, and it's arrow now has even MORE horizontal component. Less lift.
As a result the natural tendancey of the plane will be to roll back to the left (level) since the right wing is producing more lift than the left. When it gets back to level the lift is now the same for each wing and it tends to want to STAY level.
With a straight wing, no dihedral, the horizontal component of each side is going to be equal at any given bank angle and the plane will tend to remain banked even if you neutralize the controls. The rudder complicates things and makes the above a bit of over simplification, but if you play with the drawings and the arrows you'll see what is happening.
Bob
Even in a vertical dive, the rudder will roll a plane if the wing has dihedral and gravity has no part in it
04-09-2008, 09:22 AM
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RE: Dihedral
Imagine, for an exaggerated example, that the dihedral was 90 degrees, so that when the plane is flying level, the wings are each at a 45 degree angle. If you bank to the right so that the right wing is now horizontal (45 degree bank right) you will have an unbalanced wing with the weight of the plane completely at the end. Gravity will pull down on the plane, until the force is balanced on the left side by the left wing generating some vertical lift.
It also doesn't explain why moving the CG rearwards will make the plane's roll response to rudder generally increase.
There is a LOT going on.
My favorate site for how things fly is here:
http://www.av8n.com/how/
It's aimed at private pilots, but almost all the info is the same for models, and it's very complete.
It's explination of how dihedral works is in in section 9.1. The short version is that if you look at the airflow as it's seen by the individual wing halves, the AOA of each side is slightly different when you have dihedral and an airflow that is not aligned directly with the fuse centerline. This means that no matter what direction the plane is flying, if the airflow is coming slightly from the side (because of rudder deflection or whatever), then lift will be increased on one side and decreased on the other, causing a roll.
04-09-2008, 05:06 PM
#22
RE: Dihedral
Thanks. I made a Free Body Diagram if you will of a plane with dihedral, and one without. The angular momentum balance made so much sense I thought I'd figured it out. What I failed to see (most due from my never have flown a model airplane before!) is that as the plane changes positions, air is not coming at both wings equally, and that the AOA (angle of attack) is constantly changing with respect to the position of the plane.
It still is a rather simple, yet effective tool for trainer aircraft. It's funny how things can make so much sense after shown to you, and you think 'why didn't I think of that before', like the wheel!
It still is a rather simple, yet effective tool for trainer aircraft. It's funny how things can make so much sense after shown to you, and you think 'why didn't I think of that before', like the wheel!
04-09-2008, 05:38 PM
#23
RE: Dihedral
Two important things to remember here:
1. A wing ONLY generates lift PERPENDICULAR to the surface of the wing. If the plane is level both wings generate the same amount of vertical lift PERPENDICULAR to the ground (or gravity). IN level flight a wing with dihedral will also generate LIFT toward the center of the plane. Both wings being equally angled will deliver equal amounts of lift. Bank the plane and the "high" wing will develope LESS vertical lift and more lift toward the center. This also tends to add to the turning effect. The low wing generates slightly more lift vertically and less "inward" lift. The result is that the plane will try to return to level unless externally influenced.
2. Fully symetrical wings do not generate ANY lift at any speed unless they have a positive angle-of-attack relative to the wind. Flat bottom wings will generate lift without a positive angle-of-attack that varies with speed.
1. A wing ONLY generates lift PERPENDICULAR to the surface of the wing. If the plane is level both wings generate the same amount of vertical lift PERPENDICULAR to the ground (or gravity). IN level flight a wing with dihedral will also generate LIFT toward the center of the plane. Both wings being equally angled will deliver equal amounts of lift. Bank the plane and the "high" wing will develope LESS vertical lift and more lift toward the center. This also tends to add to the turning effect. The low wing generates slightly more lift vertically and less "inward" lift. The result is that the plane will try to return to level unless externally influenced.
2. Fully symetrical wings do not generate ANY lift at any speed unless they have a positive angle-of-attack relative to the wind. Flat bottom wings will generate lift without a positive angle-of-attack that varies with speed.
04-10-2008, 09:08 AM
#24
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RE: Dihedral
About your point 2, all airfoils have an angle attack at which they generate no lift at any airspeed. As I said, there's nothing magic about a flat bottom (other than being really easy to build on both the wing and fuse side of things). With most flat bottom airfoils, the AOA for 0 lift is going to be several degrees negative, maybe -3 or so, dependingon the exact airfoil of course.
Also, remember that AOA is measured from the center of the LE to the center of the TE. A flat bottom airfoil set up so that the flat bottom is at 0 degrees is actually sitting about 1-3 degrees possitive AOA. So when you build a plane with a flat bottom airfoil, and put that wing saddle at 0 in relation to the horizontal stab, you're really building in a few degrees of possitive incidence.
What is true is that the more camber you have, the more lift you generate at a given airspeed and AOA (within reason, and with plenty of exceptions). Since a flat bottom airfoil has a fair bit of camber, you get an airfoil with pretty good lift. Combined with a straight wing, lots of wing area, and a fairly thick airfoil, you get some gentle handling properties. But it's not all about the flat bottom, that's all I'm saying.
However, most all of the really good basic trainers DO have a flat bottom airfoils. Some of the reason for this is historical and some is marketing.
Also, remember that AOA is measured from the center of the LE to the center of the TE. A flat bottom airfoil set up so that the flat bottom is at 0 degrees is actually sitting about 1-3 degrees possitive AOA. So when you build a plane with a flat bottom airfoil, and put that wing saddle at 0 in relation to the horizontal stab, you're really building in a few degrees of possitive incidence.
What is true is that the more camber you have, the more lift you generate at a given airspeed and AOA (within reason, and with plenty of exceptions). Since a flat bottom airfoil has a fair bit of camber, you get an airfoil with pretty good lift. Combined with a straight wing, lots of wing area, and a fairly thick airfoil, you get some gentle handling properties. But it's not all about the flat bottom, that's all I'm saying.
However, most all of the really good basic trainers DO have a flat bottom airfoils. Some of the reason for this is historical and some is marketing.
04-10-2008, 11:51 AM
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RE: Dihedral
Any plane, whether or not it has dihedral, will roll with rudder input. If I remember ground school correctly when I was taking flying lessons, think of angle of attack as your speed control and throttle as up/down control. Model planes react somewhat differently than real aircraft because of the huge amount of power in relation to the weight of the aircraft but the basic principles still apply. When you give it rudder the plane starts to turn and as it turns, the outside wing travels faster, lifting that side of the airplane more than the inboard side. Lift is generated relative to the wing, not the ground so given enough altitude to beat gravity, you can do a full roll or more with just rudder. Some of the early RC stuff had only rudder and throttle yet was capable (in the hands of an expert) of some fairly sophisticated aerobatics. The dihedral helped the plane to recover more quickly.
Neal
Neal