Rudder mixing

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Rudder mixing

Postby rogueqd » Thu Sep 01, 2011 4:16 pm

I've seen quite a few people mention mixing rudder in with their ailerons. I thought I'd have a play with that on my simulator, it works great, but I'm confused as to why.

As far as I know, rudder is used to control yaw. Landing in a cross wind for example. But when mixing it with ailerons everyone seemed to say that the plane rolls faster in to (and out of) a turn. With that in mind I mixed the rudder so that it deflected in the same direction as the ailerons, to make the plane roll faster, but this ended up causing the plane to roll slower. So I put it back to "normal", left rudder with left ailerons, and the plane rolled fast as expected, but when you look at the plane from the rear, the rudder seems to be conflicting with the ailerons, making the plane roll in the opposite direction.

How does that all work?
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Re: Rudder mixing

Postby RCModelReviews » Thu Sep 01, 2011 11:28 pm

I don't bother mixing rudder with anything -- that's what your eyes, brain and thumbs are for -- creating coordinated turns :-)

Some people mix rudder with ailerons on the AXN Floater because in standard form, the ailerons aren't overly powerful. On this model, a little rudder/aileron mixing significantly improves the responsiveness of the model but being a 3D flier, I naturally tend to use the rudder in turns when/where necessary.

On a model with lots of dihedral (like the AXN), there is a lot of coupling between yaw and roll. If you put in left rudder, the model will roll to the left. In fact, it's very easy to fly this kind of model without touching the ailerons.

However, in a flat-wing model, the rudder will produce virtually no roll so operates independently.

You'll notice that as you fly larger models (especially scale ones like the Piper Cub), you will need to start adding rudder in the turns or the tail will hang down and the model will kind of side-slip as it turns.

The HobbyKing EPP FPV model does this a lot so I find myself using quite a bit of rudder in the turns to make sure it turns properly.
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Re: Rudder mixing

Postby disco stu » Fri Sep 02, 2011 12:48 am

Reason for rudder in the first place: As you would know, when you roll with ailerons one wing goes down and one up, and lift = drag and down generally means faster. This basically means that when you roll to the right, without rudder input, the plane naturally yaws to the left as a response the effects of the ailerons and banking on the aircraft, and then right rudder is used to over come this. If you've ever flown in a full size aircraft, glider especially, you can see this at work with the bit of wool on the canopy. Once turning some rudder is needed to overcome the fact that one wing is moving faster than the other wing, and is therefore creating more drag (bernoulli eqn from memory will show this, if your maths/physics orientated), but that is a lot less than the rudder needed in the first part of the turn

I just thought I better mention why the rudder is needed
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Re: Rudder mixing

Postby Sid Sideslip » Fri Sep 02, 2011 2:57 am

In power planes I was taught to "step on the ball" of the turn and bank indicator, i.e. whatever side of the plane that ball went that was the side you pressed the rudder pedal down to centre it again and so make a coordinated turn.

Gliders, being all primitive, didn't usually have powered gyro instruments, like a turn and bank or the more modern turn indicator (shown here courtesy of Wikipedia). Glider pilots can also work out sideslip and skid by which side of their mouth they are dribbling out of. :lol:

Image

Btw, the mark the little aeroplane wing is aligned with indicates a "standard turn" of 2 minutes to turn 360 degrees.
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Re: Rudder mixing

Postby rogueqd » Fri Sep 02, 2011 3:08 am

Sid Sideslip wrote:Btw, the mark the little aeroplane wing is aligned with indicates a "standard turn" of 2 minutes to turn 360 degrees.
Well I never knew that.

But nobody has actually answered my question, which was... If you apply left stick, so the left aileron goes up and the right goes down, you would think that right rudder (the rudder deflects to the right) would increase the roll (acting like a third vertical aileron), but it seems to have the opposite effect and left rudder actually seems to increase the roll, why?
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Re: Rudder mixing

Postby RCModelReviews » Fri Sep 02, 2011 3:34 am

It doesn't increase the roll-rate because the rolling effect of the rudder is insignificant when compared to the yawing effect it creates.

If you look, you'll notice that the ailerons are a significantly greater distance away from the centerline of the model (drawn from the nose to the tail). This gives the ailerons a huge amount of leverage to roll the model.

The rudder however, is relatively close to the centerline so it doesn't have as much leverage to *roll* the plane but it is a long way from the center of gravity so it has a huge amount of leverage to yaw the plane (around that CG).

So, when you apply rudder, it tries to yaw the opposite way to the roll and thus, slows down the roll because in most aircraft, when you yaw, the wing that moves forwards generates more lift so tends to roll the model towards the wing that yaws towards the rear. There are a number of reasons for this roll effect including the fact that the advancing wing will be traveling faster and any dihedral will act in a way that increases the angle of attack of the advancing wing.
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Re: Rudder mixing

Postby rogueqd » Fri Sep 02, 2011 3:38 am

RCModelReviews wrote:the wing that moves forwards generates more lift so tends to roll the model towards the wing that yaws towards the rear.
Ahh, now I get it, thanks. :)
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Re: Rudder mixing

Postby rogueqd » Fri Sep 02, 2011 3:48 am

Also, since you mentioned it before...
RCModelReviews wrote:I don't bother mixing rudder with anything -- that's what your eyes, brain and thumbs are for.
I was thinking of when I finally get an FPV setup. I could mix rudder with ailerons and leave the rudder chan free for camera panning, since I won't have any head tracking. Is that clever, or trying to be too clever?

RCModelReviews wrote:However, in a flat-wing model, the rudder will produce virtually no roll so operates independently.
Hmm, all (both) my planes have dihedral. You've got me thinking a flat wing one would be good practice. Maybe my first 3d plane? I'd also like a slow flyer as some of the parks around are a bit small to fly the AXN in. Could you recommend a good slow flying trainer that might be able to do a bit of 3D? Preferably cheap and easy to repair too, lol.
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Re: Rudder mixing

Postby RCModelReviews » Fri Sep 02, 2011 4:41 am

You can slave the rudder and aileron together -- as to how good it will work depends on the airframe so it's "suck it and see" to find out for sure.

If you want to learn 3D then get one of those depron or EPP foamies. You can fly them indoors or outdoors on a calm day and they're really tough so when you dumb-thumb them, there's probably not going to be any damage - and what damage you do get can be fixed with a hot-glue gun.

Don't make the mistake of trying to learn 3D with a scale model. You want something that's light and tough so that it will fly slow and take the knocks.

Your simulator is also a real help.
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