pitch coupling
12-04-2002 | 04:04 AM
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pitch coupling
Rudder inputs on some planes tend to cause the plane to pitch either to the canopy or the landing gear.
You can either fly the coupling out manually or set up a Mix if you have a computer radio to automatically input elevator when rudder is applied.
You can either fly the coupling out manually or set up a Mix if you have a computer radio to automatically input elevator when rudder is applied.
12-04-2002 | 07:36 PM
#3
Senior Member
Pitch Coupling
As wgeffon stated coupling of controls is pretty common especially on Scale Aerobatic models. Over the years we have tuned the pattern models of eliminate this and it has become a minor issue.
Another common coupling effect is roll coupling.
Basically what happens is the rudder is of such a shape and the wing and tail are in such a position that the plane will not respond with pure yaw when the rudder is applied. Sport models and Fun fly types live with this all the time....But Aerobatic models fight this...the Reason is You are trying to fly in straight lines...you want rudder to only give Yaw and nothing else..no rolling or pitching component.
There are ways to eliminate these coupling effects...
Pitch coupling is tied to CG the more aft the CG the more the plane will pitch to the belly. The more forward the CG the plane will pitch to the Canopy. Now Wing and stab placement have a huge thing to say about this. That's why the pattern models for the most part have this effect designed out.....You can raise or lower the stab in relation to the wing as you make design changes....This will tend to effect the way the Horizontal stab works and where the forces are applied....thus solving the problem....In Scale Aerobatic models this become tougher as the plane still has to look like the Extra or the CAP or Edge....
Roll Coupling is a function of Dihedral in the wing...or wing placement. If the plane is rolling in the direction of the rudder applied this is called Proverse Roll Coupling....This means either the wing has too much Dihedral or the with the dihedral in the wing the wing is too high in the fuse....Adverse Roll Coupling is when you give right rudder and the plane rolls left. This means not enough Dihedral or the wing is too low in the fuse for the given Dihedral....
See Dihedral acts to move the effective location of the wing up. So More dihedral makes the plane fly as though the wing was higher in the fuse...While less Dihedral makes it fly like the wing is lower in the fuse....A mid-wing Plane like an Edge would have less Dihedral on principle than a Low wing plane like an Extra 300L...
Dihedral is also a created with the taper of the wing....A prime example is the thickness of the airfoil at th root compared to the tip...Many of the pattern designs when you lay the wing upside down...top....to a flat table the wing lays flat. Some would say there is no dihedral but in fact there is because of the airfoil section and how it tapers...this acts as Dihedral.
So Now that you know what it is....How do you solve it?
Well back in the days before mixing you would cut the wing and increase the dihedral....or you would move the stab lower in the fuse....But in todays modern world there is no need to scrap the plane just mix some aileron or some elevator into the rudder...Meaning that when rudder is applied you get the proper direction Elevator or aileron. In small amounts this will not affect the way the plane flies but when you start getting large amounts or mix say over 10% the plane will not perform as well. A design change should accompany the next version you build. But like I say the Scale Aerobatics planes are limited to look like the real thing so only a minor amount of change can be made....and sometimes this is not enough.....
I hope this helps you understand it.....and this is why the top end radios like the JR 10X have built in mixes to dial out these unwanted coupling effects....And you can create your own Program mixes to do the same things....Look at the plane you are flying is it an Advance 40 or a Tiger2 or is it a precision machine like a Smaragd, Hydeout, or a Carden 40% Extra.....the amount of coupling and mixing of controls are design Dependant and a good designer will work out many of these bugs for you....Like I said most of the pattern models are pretty dialed in and most of the top end Scale planes have minor coupling issues but many of the 40-120 sized arfs of Scale Aerobatic models are not as refined...You could expect more issues with these as they are designed around looks and fun performance not necessarily precision work.
A good resource is
www.nsrca.org
There is a trimming chart posted there. This trim chart will tell you what the tests are to determine the problem with trim and what the solution is to solve it....Its very valuable info.
Trimming a model is much much more that just moving the trim tabs on the TX and making it fly straight and level.
Troy Newman
Another common coupling effect is roll coupling.
Basically what happens is the rudder is of such a shape and the wing and tail are in such a position that the plane will not respond with pure yaw when the rudder is applied. Sport models and Fun fly types live with this all the time....But Aerobatic models fight this...the Reason is You are trying to fly in straight lines...you want rudder to only give Yaw and nothing else..no rolling or pitching component.
There are ways to eliminate these coupling effects...
Pitch coupling is tied to CG the more aft the CG the more the plane will pitch to the belly. The more forward the CG the plane will pitch to the Canopy. Now Wing and stab placement have a huge thing to say about this. That's why the pattern models for the most part have this effect designed out.....You can raise or lower the stab in relation to the wing as you make design changes....This will tend to effect the way the Horizontal stab works and where the forces are applied....thus solving the problem....In Scale Aerobatic models this become tougher as the plane still has to look like the Extra or the CAP or Edge....
Roll Coupling is a function of Dihedral in the wing...or wing placement. If the plane is rolling in the direction of the rudder applied this is called Proverse Roll Coupling....This means either the wing has too much Dihedral or the with the dihedral in the wing the wing is too high in the fuse....Adverse Roll Coupling is when you give right rudder and the plane rolls left. This means not enough Dihedral or the wing is too low in the fuse for the given Dihedral....
See Dihedral acts to move the effective location of the wing up. So More dihedral makes the plane fly as though the wing was higher in the fuse...While less Dihedral makes it fly like the wing is lower in the fuse....A mid-wing Plane like an Edge would have less Dihedral on principle than a Low wing plane like an Extra 300L...
Dihedral is also a created with the taper of the wing....A prime example is the thickness of the airfoil at th root compared to the tip...Many of the pattern designs when you lay the wing upside down...top....to a flat table the wing lays flat. Some would say there is no dihedral but in fact there is because of the airfoil section and how it tapers...this acts as Dihedral.
So Now that you know what it is....How do you solve it?
Well back in the days before mixing you would cut the wing and increase the dihedral....or you would move the stab lower in the fuse....But in todays modern world there is no need to scrap the plane just mix some aileron or some elevator into the rudder...Meaning that when rudder is applied you get the proper direction Elevator or aileron. In small amounts this will not affect the way the plane flies but when you start getting large amounts or mix say over 10% the plane will not perform as well. A design change should accompany the next version you build. But like I say the Scale Aerobatics planes are limited to look like the real thing so only a minor amount of change can be made....and sometimes this is not enough.....
I hope this helps you understand it.....and this is why the top end radios like the JR 10X have built in mixes to dial out these unwanted coupling effects....And you can create your own Program mixes to do the same things....Look at the plane you are flying is it an Advance 40 or a Tiger2 or is it a precision machine like a Smaragd, Hydeout, or a Carden 40% Extra.....the amount of coupling and mixing of controls are design Dependant and a good designer will work out many of these bugs for you....Like I said most of the pattern models are pretty dialed in and most of the top end Scale planes have minor coupling issues but many of the 40-120 sized arfs of Scale Aerobatic models are not as refined...You could expect more issues with these as they are designed around looks and fun performance not necessarily precision work.
A good resource is
www.nsrca.org
There is a trimming chart posted there. This trim chart will tell you what the tests are to determine the problem with trim and what the solution is to solve it....Its very valuable info.
Trimming a model is much much more that just moving the trim tabs on the TX and making it fly straight and level.
Troy Newman
12-04-2002 | 09:13 PM
#6
My Feedback: (41)
Thanks
Hey Troy thats what I call a good explanation. I know that it took time and a boatload of knowledge to explain that. I printed it out for my files. Still while most of the pattern planes fly true I have found that many of the widebody designs need refinment. Thanks Mike Mueller
