rkn704

I have just got my first 3 channel glider and computer radio. I now have the ability to mix rudder and ailerons but am not sure quite how to start off.

I assume I have the rudder and ailerons working in the same direction. i.e. For a left turn rudder moves to left when viewed from above and behind, left aileron up and right aileron down.

Approx what percentage of rudder movement should be mixed in for full aileron movement.

I currently have about 10% of rudder movement for full aileron movement but can't really see any difference to flying with the rudder fixed in the neutral position.

I'm flying a Phoenix Model Products Banshe. I'm not yet up to controlling all three surfaces myself.

rscarawa

YOu should not need much rudder on most sailplanes for coordiated turns. I would start out without mixing and see how it flies. Enter a turn in any direction you prefer. Give it about a 45-60 degree roll. While the plane is turning infront of you (you are looking down on the top side of the plane) see if the fuselage is following the flight path of the plane. When you are uncoordinated, you should see the tail hang lower than the nose indicating that you are slipping sideways in the air. Try your turn again and match in some rudder and see if the fuselage is atleast level with the horizon. If it is, you are getting closer. If you coordinate your turn properly, the tail should be slightly higher than the nose when you are in a steep turn. This means that the fuse is pointed into the air stream. When you get the feel of how much rudder is needed to do this, then try to mix in enough to make this happen. It could be that 10% is enough to keep a coordinated turn. How much rudder you need is a function of the aspect ratio, length of the fuse and a couple other odds and ends. When I turn my Spirit 100, it takes just a tap of the rudder to knock the fuse in line for a turn. That may be all you need.

I hope this helps.


Scot

aeajr

My Feedback: (2)

I presume you are using a computer radio.

For aileron/rudder mixing, the airplane mfg will often give recommendations.

How much you add depends on what you want from the plane. The R/A mix gives you a smoother turn, which is important to thermal sailplanes. Doesn't seem to be as important to slope planes. Always important on scale plans when you want to fly with a scale look in the air.

The % will vary by plane. I would keep adding and see what happens. I would start at 25% rudder to full ailerons. Make sure your mix is turned on as some radios allow you to turn the mix on and off from a switch.

Fly that for a while, maybe 10 flights. Then add 10% and see if you notice a change.

On my Legend 3M Thermal sailplane I think I have 40% rudder at full ailerons. I started at 25%. I have been advised to turn the mix off on launch from hi-start or winch, so I have it set on a switch.

I am just starting to set up a Prism 3M. I will start that with the mix off and see how she flies. Then I can turn on the mix and will start at 25% and see how she flies. I will then start increasing on subsequent flights and see where I like it. The maker suggests this mix but does not suggest a %.

I also have a 3.6M Ventus 2C scale sailplane for aerotowing. I have the mix on a switch. No mix on tow. When I release the tow line, it turns on the mix. This one is new, so again I am starting at 25% as I have no guideline on this plane.

I turn the mix off on winch launch and on aerotowing based on advice form experienced pilots.

rogerflies

My Feedback: (3)

The Banshee looks like it could do decent aerobatics, but it won't do an axial roll with the rudder and ailerons coupled. I'd recommend leaving it uncoupled so you can learn to use the rudder. It'll fly well enough, and nobody's going to mistake it for a scale ship anyway.

Getting used to flying with the rudder is easy enough. Start out by putting in too much so you can see the effect, then back off till it looks right. It's a very subjective thing, and you'll know when it looks "right". Full-size planes have a turn coordinator that tells the pilot when he has the right amount of rudder input. You'll just have to watch how the tail hangs down toward the inside of the turns.

It's also fun to learn using opposite rudder to do slips.

Flying full-scale planes taught me how to use the rudder, because you can't land them in a crab. This knowledge helped me fly giant-scale RC power planes in a very realistic manner. The rudder-induced slip is great for one wheel landings when there's no crosswind, or for landing in a crosswind with the nose of the plane pointing down the runway. The ailerons let you FLY down the runway, the rudder lets you POINT down the runway. You use the direction and amount of both inputs to get the desired result: plane flying straight down the runway with the nose pointing straight down the runway.

Roger

aeajr

My Feedback: (2)

rogerflies points out valuable information about rudder use.

I have mine programmed so a switch turns the mix on and off. Sometimes I turn it off just because I want to do what rogerflies suggests, I want to play with it, experiment, learn. Other times I just want to fly so I turn into on.

On gliders, which typically have long wings and no landing gear, the rudder can be very valuable near the ground. If you find yourself coming in off angle ot the desired flight path, or are trying to work into a spot landing and you are near the ground, using the ailerons to bank the plane can be tricky. You can hang a wing tip if you are not careful.

This is where I make most use of the rudder, when I am near the ground and trying to tune my landing. I try to keep the wings level and "yaw" it into the spot. Just another technique to play with to improve your control.

rkn704

Thanks for the tips. I will be trying them out next time. I do need to learn to use the rudder properly but for starters I will be using it mixed in. I have plenty of even more basic things to practice like landing it.

aeajr

My Feedback: (2)

As noted above, landing is one of the places where the rudder is most valuable, but for riding thermals, the mix setting is nice and I would encourage you to use it.

You can always override the mix by simply moving the rudder stick, so if you are into a turn and want to see what more rudder would do, just add some from the rudder stick while you are flying. Most radios will allow you to override the the setting just by providing input from the stick.

Likewise on landing you still have full contorl of the rudder so try it out on landing. You will like it. Get it low and on a nice approach, then just keep the wings level and do your final adjustments with the rudder. It works very well.

rscarawa

I am going to try the aileron/rudder mixing on a switch to see if that clean things up. I was thinking that a good way to determine the amount of needed rudder would be to hook a 8'-10' ribbon right behind the tow hook and see if the ribbon is inline with the fuse when I roll into a turn. Does that seem like a fair test?

aeajr

My Feedback: (2)

Interesting! I have no idea if that is valid, but it sounds cool!

rogerflies

My Feedback: (3)

I think the ribbon will work pretty much like a turn coordinator, but in the opposite direction. The TC is similar to a glass vial level. If the plane is sitting on level ground, the bubble is in the middle of the vial because the forces on the plane are balanced. In a coordinated turn, the forces are balanced, the air flow is in line with the fuse, and the bubble is in the middle of the vial, even though the wings aren't level. You "step on the bubble" to get the turn coordinated.

If the ribbon is in line with the fuse, you're in a coordinated turn. If the ribbon is toward the right wing in a left turn, you need more left rudder.

A clever idea. Sounds like it's worth a try.

Roger

HankTurner

I have seen the ribbon test demonstrated in a magazine and it seemed to work quite well. When coordinating turns there are many factors (or variables) in the equation, such as: airfoil, wingspan, aspect ratio and airspeed, to name a few.
When the aileron on the outside of the turning radius creates excess drag, this causes the plane to yaw in the direction opposite the turn. This is referred to as adverse yaw. Large slow flying planes are affected the most.
One helpful remedy is the use of differential on the ailerons. Some full-scale sailplanes even incorporate spoilers to help coordinate turns. They act by decreasing the lift and increasing the drag on the wing toward which the turn is to be made; thus, unlike ailerons, they have no adverse yaw. I've not tried this with RC planes, but using various amounts of differential and rudder does help counter adverse yaw in turns. The exact amount will depend on the aircraft's flight characteristics and one's individual flying style.

Hank

rscarawa

I was looking at my Spirit Elite manual (have not flown it yet) and they say to start with 1" or rudder movement for full aileron deflection. That is a wide rudder too. Maybe that will give you some insite on where to start. You can go to the great planes website and download the manual if you want to see it. Grab the ARF manual. It is thinner.

rscarawa

I was kind of wondering if you wanted to reduce adverse yaw, start with 100% differential on the ailerons. If that is not enough to give you the roll rate you desire, try 90% diff and so on until you get a roll rate you can live with with the least amount of rudder input. I have not tried this, but if I think my Spirit 100 needs more aileron, I may go from 50% diff to 40% diff and add more rudder. Sounds like a descent way to tune the plane. Any thoughts?

HankTurner

Yeah Scot,
That sounds good. There again, a lot depends on the type of plane and pilot style/skill. If it's a fast high performance glider, then you may not want to use as much differential or mixing, though it does make for some interesting inverted flight.
I once had an old 4m. Craft Sail Air built with and ailerons and little dihedral. If your not familiar with this plane, it's a dinosaur of a thing, a big gas-bagger with a full flying stab... Beautiful!! Anyway, I used full aileron differential, 1 to 1 aileron/rudder mix, and it flew great!

Hank

rscarawa

When you say 1 to 1, do you mean you had equal up down movement on the ailerons and you mixed in rudder, or you only used up aileron movment?


Scot

aeajr

My Feedback: (2)

What is the recommended full rudder throw, forgetting rudder mixing. If it is 1.5" for example, that would be about 65% rudder at 100% aileron for your plane. That provides a % point of view if that is how the radio calibrages the mix. That is how my 9C does it.

rscarawa

The recommended starting point for the rudder is 2". So that is a 50% mix. I did not think it would be that high. On my Spirit 100, I just kind of lean on the rudder a little bit. Maybe 10-15%, but then again, I have a modified tail section.

aeajr

My Feedback: (2)

Thanks for providing that spec. This provides a reference/range for those who are reading and learning from this thread.

Mine does not provide a recommendation, but I started at 25% and worked my way up. I run 40% on my Legend, so that number does not surprise me.

rscarawa

recommended aileron is 3/4 up and 3/8 down.

HankTurner

I used only up aileron, with full rudder mixed in.

I sure wish I still had that plane. After my last post, I searched online and found that the owner of the rights to the plane (and Dreamworks) retired.

Hank

rscarawa

If you were only using up aileron, why did you need rudder in the same direction? I figured if you needed rudder at all, you would need a hair of opposite rudder. Was there something different about your wing design?

Scot

HankTurner

Yeah Scot,

The SailAir has a very thick, flat-bottom, low aspect ratio, airfoil. It was designed for polyhedral, with an option to build the wings flat with just a tiny bit of dihedral. Mine had the flat wing configuration with barn door type ailerons. It flew very slow, and took a lot to initiate a turn. However, this is a very old 4m design, and definitely not the norm.

Hank

F-89 Driver

I’m just gonna jump in here with some observations that might have an effect on this discussion, but then again - - maybe not.

Modern jet airliners have autopilots. But, what’s not generally known is that they’re actually just two-axis autopilots. That is, they only engage the elevator and the ailerons. Those are the only two functions that are activated when the autopilot is turned on. The rudder is actually controlled by a full-time autopilot of its own. It’s called a “yaw damper.”

The aerodynamics involved with modern jet airliners gets to the point that the airplane will become “squirrelly” at high altitudes if it’s left for the pilot to try to keep things aligned about the vertical, or yaw axis. Without a yaw damper, an aerodynamic condition known as “Dutch roll” will soon overtake the plane. (Note: This is NOT the so-called “Dutch roll” maneuver practiced by light airplane pilots. Somehow, through the years CFIs adopted this term without realizing that it already applied to the yawing phenomenon that occurs in swept-wing airplanes. In this situation, the nose and wingtips will oscillate through continuous figure-eight motions in the sky. At high altitudes, the airplane can become almost uncontrollable, and is very susceptible to an erroneous control input at the wrong moment. (It’s a tricky procedure to master.)

To preclude this from happening, the rudder axis of the airplane’s autopilot is functional at all times. In fact, it’s common to have TWO yaw dampers installed just for redundancy’s sake. It’s that important to comfortable, controlled flight.

The first airplane that I flew with a yaw damper was the F-89-D - - almost 50 years ago, now. (Hence my “RCU name.”) On the F-89 this device was called the “sideslip stability augmenter.” We just called it “Little Augie.” The next plane I flew that was thus configured was the Boeing B-727. By this time Boeing had changed the name simply to “yaw damper.” And, believe me, both or these airplanes tended to get VERY squirrelly if the yaw damper became inoperative at high altitude. (At ANY altitude, be it high or low, in the case of the F-89-D.)

With a functioning yaw damper modern airliners are flown with the pilot’s feet flat on the floor. There’s no need to coordinate rudder input for skid-less or slip-less turns ‘cuz the yaw damper will automatically apply the correct amount of rudder to assure that the drinks in the cabin don’t spill out the side of the cups. The only time rudder is used is in engine-out situations, and in the aforementioned crosswind landing situation.

The point I’m getting to is that there are model airplane autopilots available. I’ve never flown model helicopters, but from what I read in the ads, the most-used type of autopilot for model helicopters is one that is used to counteract yaw.

It seems to me, then, that if this “anti-yaw” autopilot were installed in any model (power or sailplane), it would result in perfectly coordinated turns without need to worry about rudder/aileron mixing. The model airplane “yaw damper” would automatically introduce just the right amount of rudder to guarantee a coordinated turn every time.

There, I’ve tossed-in my own 2-cents worth. If the pennies in this forum eventually add up to a dollar, then my input might be worth noting. Srill, if perfectly coordinated turns are critical to the task, a small autopilot might well be the answer.

rscarawa

Are you talking about a gyro from a helicopter in a sailplane? Will that really keep the tail in line? Would you need a heading lock or a regular gyro?

drela

Sorry, but this is wrong. A yaw damper reacts to yaw *rate*, not to yaw angle. It tries to eliminate yaw wiggling, but does nothing to eliminate a steady-state sideslip. In fact, it can easily make the slideslip slightly worse.
Example: Say you're flying with the yaw damper disabled, and you are in a steady right turn with some amount of sideslip into the turn because of a poor rudder position. If you then turn on the yaw damper with no other input, you will find yourself slipping into the turn even more. The yaw damper senses the steady right yaw rate, and tries to stop it by feeding in some left rudder, which worsens the sideslip angle.

To touch on the original question... When a sailplane does a coordinated turn entry roll, rudder and ailerons have to move together into the turn. But when it's in a steady coordinated turn, the rudder and ailerons have to be held opposite -- rudder into the turn, and aileron slightly out of the turn. Therefore, there is no aileron->rudder mix setting which will produce "aileron-stick-only" coordinated flight in both situations. You have to learn to use the ailerons and rudder independently. See here for more on this topic:
http://www.polecataero.com/articles/rudder/index.html

The importance of independent ruddering strongly depends on the type of aircraft. The larger the span/turn_radius ratio, and the larger the tail_length/turn_radius ratio, the more important ruddering becomes. Among all aircraft types, sailplanes typically have the largest ratios, and make the largest demands on effective ruddering (aerobatic aircraft excepted).