djklein21
Jun 12, 2007, 03:02 PM
I have noticed that one of the most misunderstood and most overlooked set up in both slope and thermal gliders is roll-yaw coupling. Most pilots don't notice or understand how to set up some effective mixes to make your glider roll axially and turn efficiently.
I am hoping this thread can answer some basic questions about glider set up for combating roll-> yaw coupling
For starters let’s define some key terms
Roll- a planes movement about its fuselage (rolling axially about its fuselage, usually controlled through its ailerons)
Yaw- a planes movement about its vertical axis (usually controlled through rudder movement)
Pitch- a planes movement about its horizontal axis (a planes movement about the axis projected through its wings, usually controlled by its elevator)
All deflection measurements are taken at the actual flight surfaces, not the settings in your transmitter. All settings can be a coupling of mechanics and electronic programming, so all meaningful discussion about deflection % is a measurement taken at the contol surface.
As it has been stated in the thread discussing snap flap, lift always comes with a drag penalty. When an aircraft gives an aileron deflection for a turn (lets assume a left turn) the outside aileron moves down to increase the lift on that wing, this increases the drag on that outside wing. The inside wing's aileron moves upward, decreasing the lift on that wing and also decreasing that wings drag. The plane has now banked into a left hand turn. The nose of the plane has however been pulled right, away from the intended left turn. This is called a skid. The increased drag on the outer wing tip and decreased drag on the inner wing tip has adversely coupled the roll and yaw movement of the plane. This is most dramatic in high aspect ratio planes, as the drag created on the wings has a larger moment arm to act on. Full scale pilots have an indicator to tell them if they are skidding, or opposite slipping). On gliders this is usually a piece of yarn taped to the windshield. This lets the pilot know if he or she is flying directly into the moving air. This is what we want to do.
Since most of us begin on small 60" gliders, we usually either don't worry about this or we combat this through aileron differential. Aileron differential refers to an aileron setting that makes the aileron travel further up than it does down. On most small planes we don't have a rudder, or if we do it isn't that effective. This makes us focus on aileron differential as the only cure to roll-yaw coupling issues. To keep our plane from yawing with a rudder command we usually add about 50% aileron differential, but all planes are different. If we combat roll-yaw coupling with aileron diff only we will get a plane that falls as it rolls back and forth. We have simply decreased the lift on the inside wing more than we increased the lift on the outside wing. So to effectively trim a plane to roll axially and efficiently we will need to add some rudder mix to the ailerons.
It is important to note that all of these trims are for a plane flying a single speed and that these mixes change with speed, C.G. and weight. Airspeed changes these trims the most, so we should concentrate on that for now. Low speed affects this mix more than high speed. Or in other words this adverse yaw is greatest at low speed, so your rudder mix and differential should be higher for slower flight modes. Tom Copp has shared with us all that he sets his slope racing planes up for two different conditions completely. Or to restate that for clarity he has separate programs for light air and heavy air with multiple flight modes for each. He has a 50 second air program, and a high 3X.XX second air setting.
I start by tuning my rudder mix first. I put about 40% differential in my ailerons and guess at a rudder mix. I program a switch for higher rudder mix and one for lower. Than I fly the plane and see which is closer. I adjust and try again. Once I get as close as I can, I switch to adjusting my aileron diff.
After my rudder setting is close, I look for a plane that doesn't drop its wings as it rolls. To restate this, the planes cg doesn't fall or rise when given a roll command. This is hard to see if you haven't got your rudder mix close. I fly the plane at the speed I am interested in and tune appropriately. When it is the best I can, I switch back to rudder tuning. It can take a few cycles to get all of this stuff right, but the plane will fly much better when you are done.
Remember these settings are flight speed dependent so use a three position switch to have a plane that flies well at all speeds.
This is an early write up, based on my experiences only. I am by no means an authority on this subject, so take my advice with a grain of salt. Enjoy, and please feel free to add to this discussion.
For more information look to Kevin Newton's site
http://www.knewt.com/planes/setup.htm
Mike Smith has made a great write up for tuning a HKM Sharon for TD flight at the USA F3B team site, under his bio
http://www.teamusaf3b.com/
I am hoping this thread can answer some basic questions about glider set up for combating roll-> yaw coupling
For starters let’s define some key terms
Roll- a planes movement about its fuselage (rolling axially about its fuselage, usually controlled through its ailerons)
Yaw- a planes movement about its vertical axis (usually controlled through rudder movement)
Pitch- a planes movement about its horizontal axis (a planes movement about the axis projected through its wings, usually controlled by its elevator)
All deflection measurements are taken at the actual flight surfaces, not the settings in your transmitter. All settings can be a coupling of mechanics and electronic programming, so all meaningful discussion about deflection % is a measurement taken at the contol surface.
As it has been stated in the thread discussing snap flap, lift always comes with a drag penalty. When an aircraft gives an aileron deflection for a turn (lets assume a left turn) the outside aileron moves down to increase the lift on that wing, this increases the drag on that outside wing. The inside wing's aileron moves upward, decreasing the lift on that wing and also decreasing that wings drag. The plane has now banked into a left hand turn. The nose of the plane has however been pulled right, away from the intended left turn. This is called a skid. The increased drag on the outer wing tip and decreased drag on the inner wing tip has adversely coupled the roll and yaw movement of the plane. This is most dramatic in high aspect ratio planes, as the drag created on the wings has a larger moment arm to act on. Full scale pilots have an indicator to tell them if they are skidding, or opposite slipping). On gliders this is usually a piece of yarn taped to the windshield. This lets the pilot know if he or she is flying directly into the moving air. This is what we want to do.
Since most of us begin on small 60" gliders, we usually either don't worry about this or we combat this through aileron differential. Aileron differential refers to an aileron setting that makes the aileron travel further up than it does down. On most small planes we don't have a rudder, or if we do it isn't that effective. This makes us focus on aileron differential as the only cure to roll-yaw coupling issues. To keep our plane from yawing with a rudder command we usually add about 50% aileron differential, but all planes are different. If we combat roll-yaw coupling with aileron diff only we will get a plane that falls as it rolls back and forth. We have simply decreased the lift on the inside wing more than we increased the lift on the outside wing. So to effectively trim a plane to roll axially and efficiently we will need to add some rudder mix to the ailerons.
It is important to note that all of these trims are for a plane flying a single speed and that these mixes change with speed, C.G. and weight. Airspeed changes these trims the most, so we should concentrate on that for now. Low speed affects this mix more than high speed. Or in other words this adverse yaw is greatest at low speed, so your rudder mix and differential should be higher for slower flight modes. Tom Copp has shared with us all that he sets his slope racing planes up for two different conditions completely. Or to restate that for clarity he has separate programs for light air and heavy air with multiple flight modes for each. He has a 50 second air program, and a high 3X.XX second air setting.
I start by tuning my rudder mix first. I put about 40% differential in my ailerons and guess at a rudder mix. I program a switch for higher rudder mix and one for lower. Than I fly the plane and see which is closer. I adjust and try again. Once I get as close as I can, I switch to adjusting my aileron diff.
After my rudder setting is close, I look for a plane that doesn't drop its wings as it rolls. To restate this, the planes cg doesn't fall or rise when given a roll command. This is hard to see if you haven't got your rudder mix close. I fly the plane at the speed I am interested in and tune appropriately. When it is the best I can, I switch back to rudder tuning. It can take a few cycles to get all of this stuff right, but the plane will fly much better when you are done.
Remember these settings are flight speed dependent so use a three position switch to have a plane that flies well at all speeds.
This is an early write up, based on my experiences only. I am by no means an authority on this subject, so take my advice with a grain of salt. Enjoy, and please feel free to add to this discussion.
For more information look to Kevin Newton's site
http://www.knewt.com/planes/setup.htm
Mike Smith has made a great write up for tuning a HKM Sharon for TD flight at the USA F3B team site, under his bio
http://www.teamusaf3b.com/