roll coupling on knife edge - problem
03-21-2009 | 05:19 PM
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roll coupling on knife edge - problem
I have a 64" Edge (a no name arf kit I got on RCU a year + ago). It has an unusual knife edge behavior. I roll it onto knife edge, give it some rudder to keep it flying and the rudder input rolls the plane back to level. I can compensate by giving aileron but what the?
I checked the angle of incidence - wing to horizontal stab and they are dead parallel - angle is 0. The wing alignment pins are adjustable if need be so the angle of incidence is adjustable.
Wing is fully symmetric, tail surfaces are flat - no airfoil.
I no very little about aerodynamics, could someone tell me what I can do to eliminate the adverse roll coupling - I don't want to do channel mixing.
I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen - could that be causing it where there is more surface area above the thrust line than below it?
Thanks in advance.
I checked the angle of incidence - wing to horizontal stab and they are dead parallel - angle is 0. The wing alignment pins are adjustable if need be so the angle of incidence is adjustable.
Wing is fully symmetric, tail surfaces are flat - no airfoil.
I no very little about aerodynamics, could someone tell me what I can do to eliminate the adverse roll coupling - I don't want to do channel mixing.
I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen - could that be causing it where there is more surface area above the thrust line than below it?
Thanks in advance.
03-21-2009 | 05:23 PM
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RE: roll coupling on knife edge - problem
Quite normal.
You can either put the opposite aileron in yourself or do a rudder to aileron mix.
Moving the CG will also have some effect but I'm assuming that you have that where you want it.
You can either put the opposite aileron in yourself or do a rudder to aileron mix.
Moving the CG will also have some effect but I'm assuming that you have that where you want it.
03-21-2009 | 09:23 PM
#4
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RE: roll coupling on knife edge - problem
ORIGINAL: Robert_Ellis
I have a 64" Edge (a no name arf kit I got on RCU a year + ago). It has an unusual knife edge behavior. I roll it onto knife edge, give it some rudder to keep it flying and the rudder input rolls the plane back to level. I can compensate by giving aileron but what the?
I checked the angle of incidence - wing to horizontal stab and they are dead parallel - angle is 0. The wing alignment pins are adjustable if need be so the angle of incidence is adjustable.
Wing is fully symmetric, tail surfaces are flat - no airfoil.
I no very little about aerodynamics, could someone tell me what I can do to eliminate the adverse roll coupling - I don't want to do channel mixing.
I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen - could that be causing it where there is more surface area above the thrust line than below it?
Thanks in advance.
I have a 64" Edge (a no name arf kit I got on RCU a year + ago). It has an unusual knife edge behavior. I roll it onto knife edge, give it some rudder to keep it flying and the rudder input rolls the plane back to level. I can compensate by giving aileron but what the?
I checked the angle of incidence - wing to horizontal stab and they are dead parallel - angle is 0. The wing alignment pins are adjustable if need be so the angle of incidence is adjustable.
Wing is fully symmetric, tail surfaces are flat - no airfoil.
I no very little about aerodynamics, could someone tell me what I can do to eliminate the adverse roll coupling - I don't want to do channel mixing.
I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen - could that be causing it where there is more surface area above the thrust line than below it?
Thanks in advance.
If the model returns to S&L in the same direction as applied rudder, right rudder for example results in right roll, this is remedied by decreasing dihedral in the wing. The amount of decrease is proportional to the severity of the return to S&L. 1 to 2 degrees usually is enough.
If the model rolls in the direction opposite applied rudder, then the model wants to roll inverted when you apply rudder. For this condition, you would need to add a degree or so of dihedral
If the wing is plug-in and the tube aluminum, bend the ally in the middle slightly and tack glue it in the fuse socket with one drop of ca, and fly it. If the wing is one piece , carefully cut the top sheeting or glass at center and go about 70% down. Don't cut the bottom. Wedge a small wedge and drip epoxy in the center section, and reglass the top center section. This will effectively reduce the dihedral
Thrust line, cg, fuse area distribution have nearly nothing to do with the fix you are after
Good luck
MattK
03-22-2009 | 12:46 AM
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RE: roll coupling on knife edge - problem
MattK,
A right rudder makes it roll right. And it is an aerobatic plane - no dihedral. So you're saying I should give it negative dihedral (wings dip down)? It has an aluminum wing tube.
Would increasing the area of the rudder such that it has more of it's area lower (below the horizontal stab) remedy the situation?
A right rudder makes it roll right. And it is an aerobatic plane - no dihedral. So you're saying I should give it negative dihedral (wings dip down)? It has an aluminum wing tube.
Would increasing the area of the rudder such that it has more of it's area lower (below the horizontal stab) remedy the situation?
03-22-2009 | 01:05 AM
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RE: roll coupling on knife edge - problem
Tall fin and rudder is scale on the Edge, the full scale has a very tall fin and rudder also. The competition pilots are using a mix to fix that problem. We will have a different value for right top rudder than left top rudder. Putting negative dihedral will make that plane look odd, and create other bad flight habbits in other flight attitudes.
Dan,
Dan,
03-22-2009 | 01:25 AM
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RE: roll coupling on knife edge - problem
Dan,
I didn't think giving it negative dihedral was a good solution for me. I have a Four-star 60 that exhibits the same roll coupling but I assumed it was because it was a low wing and had a fair amount of dihedral.
I am surprised the edge does it tho. I am just training myself to compensate with the sticks to get it to behave better.
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
Do the competition guys have it set by some switch so they only get the mix when they want it?
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
I didn't think giving it negative dihedral was a good solution for me. I have a Four-star 60 that exhibits the same roll coupling but I assumed it was because it was a low wing and had a fair amount of dihedral.
I am surprised the edge does it tho. I am just training myself to compensate with the sticks to get it to behave better.
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
Do the competition guys have it set by some switch so they only get the mix when they want it?
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
03-22-2009 | 07:54 AM
#8
Senior Member
RE: roll coupling on knife edge - problem
ORIGINAL: Robert_Ellis
Dan,
I didn't think giving it negative dihedral was a good solution for me. I have a Four-star 60 that exhibits the same roll coupling but I assumed it was because it was a low wing and had a fair amount of dihedral.
I am surprised the edge does it tho. I am just training myself to compensate with the sticks to get it to behave better.
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
Do the competition guys have it set by some switch so they only get the mix when they want it?
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
Dan,
I didn't think giving it negative dihedral was a good solution for me. I have a Four-star 60 that exhibits the same roll coupling but I assumed it was because it was a low wing and had a fair amount of dihedral.
I am surprised the edge does it tho. I am just training myself to compensate with the sticks to get it to behave better.
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
Do the competition guys have it set by some switch so they only get the mix when they want it?
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
Competition guys often will use flight modes which are switched on or off at specific times in their schedule (spins or snaps for example), where the models have specific
set-ups entered which allow ease during those maneuvers (much more elevator or throtthe to rudder mix for example). We don't want to fly this way all the time, only for specific maneuvers.
Are you absolutely certain the model has no dihedral??? Zero dihedral for a tapered wing means that the wing's center is flat....that is, if you cut the wing in half across its area, the projected surface is flat. In order to have that, the top of the wing droops a bit and the bottom has slight dihedral. I suspect that the top of your wing is flat but the bottom has dihedral which results is a couple degrees dihedral overall. Remove some of that and try again.
Or not....I am giving you THE answer. It's your choice whether to use it or not
MattK
03-22-2009 | 11:32 AM
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RE: roll coupling on knife edge - problem
ORIGINAL: Robert_Ellis
1)
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
2)
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
1)
If I do a channel mix, then it will have adverse side effects when I do stall turns, and even giving it rudder on landing (we sometimes have a nasty crosswind).
2)
I have a 25e Hyperion Yak 55 that flies awesome. You roll it on edge, give it rudder and it just has a very slight pitch coupling.
- Rob
When landing crosswind you are 'on' all controls all the time.
2) Remember that ship fondly as there aren't too many that will fit that description.
Simply true.
Some use a switch for mixes some do not.
"IF" the rudder is causing a proverse roll coupling (or adverse) then it will do it all the time not simply in knife edge. Knife edge may be the most pronounced visually but the condition is occurring every time you use rudder. Speed will make it more noticeable as will time 'on' the rudder.
It simply isn't possible to trim an airplane to be dead neutral in all conditions all the time.
Wind direction and velocity will change that.
I usually choose to fly it through all but the most severe couplings. That way you are never too surprised by wind changes.
The full scale guys trim it "about right" then compensate.
Simply a learning process and after awhile you aren't even aware that you are compensating at all.
03-22-2009 | 12:46 PM
#10
RE: roll coupling on knife edge - problem
Awe- you are goingto kill the sales of some radios which ar loaded with switches
lets see - high idle switch cut off switch
roll couple switch - on n on
On helis I can se the hold switch necessity but on fixed wing stuf - th switches are simply presets for rates - in my book. On flippy flop stuf (3D) a low elevator setup is good for non flop flying
other than that and retracts (?) I find that just learning to fly the thing is easier and better - for me.
lets see - high idle switch cut off switch
roll couple switch - on n on
On helis I can se the hold switch necessity but on fixed wing stuf - th switches are simply presets for rates - in my book. On flippy flop stuf (3D) a low elevator setup is good for non flop flying
other than that and retracts (?) I find that just learning to fly the thing is easier and better - for me.
03-22-2009 | 02:36 PM
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RE: roll coupling on knife edge - problem
Robert: May I suggest taking the plane to the field and putting it through the following flight test to see if you need the rudder to airleron mix in other flight manuevers/lines. A good pilot will not put a mix "in" or assign it to a swtch until he/she has flight tested for it. There is no way to "look" at the plane, or take instruction from this forum and get it correct. Your answers are waiting for you in the air. Please do the follow to obtain your answers.
1. Fly staight and level and then give rudder only, see if the wing dips in the same direction that you are giving rudder. Most planes do this. This is the same direction the wing is moving in KE flight that you describe. If so, the rudder to ail. mix described above is needed on that line too.
2. Do a hammer head stall turn (you'll need to do several). If you are hammering with left rudder, see if the left wing dips to the gear, most airplanes do this, they do this because the wings are NOT stalled in a hammerhead. The outer panel (the one that is arcing in the largest circle) has more air over it than the inner panel (smallest arcing one), and that wing lifts while the plane is "in" the hammer. Usually causing the hammer to torgue off, and cause a correction of the downline to exit on the same line as entry. The KE mix decribed in the above posted will help this to stay on plane (geometric plane), or counter this, help this not to happen, etc. So now that mix is needed on that line also....
3. Put the plane into a vertical upline with some speed, get a good running start at the upline. Right after you establish vertical apply a little rudder (as in to do the top half of a KE loop), see if the plane rolls with the yaw command. If so, you need the rudder to airl. mix on that line too.....
Most competition guys who run that mix have it on all the time. They have found that it is needed on most lines whenever the rudder is applied. Firsts find out if you too need it all the time. If so, just make sure all switches are inactive on the switch selection screen, and it will be on all the time.
Hope this helps,
Dan Baker
1. Fly staight and level and then give rudder only, see if the wing dips in the same direction that you are giving rudder. Most planes do this. This is the same direction the wing is moving in KE flight that you describe. If so, the rudder to ail. mix described above is needed on that line too.
2. Do a hammer head stall turn (you'll need to do several). If you are hammering with left rudder, see if the left wing dips to the gear, most airplanes do this, they do this because the wings are NOT stalled in a hammerhead. The outer panel (the one that is arcing in the largest circle) has more air over it than the inner panel (smallest arcing one), and that wing lifts while the plane is "in" the hammer. Usually causing the hammer to torgue off, and cause a correction of the downline to exit on the same line as entry. The KE mix decribed in the above posted will help this to stay on plane (geometric plane), or counter this, help this not to happen, etc. So now that mix is needed on that line also....
3. Put the plane into a vertical upline with some speed, get a good running start at the upline. Right after you establish vertical apply a little rudder (as in to do the top half of a KE loop), see if the plane rolls with the yaw command. If so, you need the rudder to airl. mix on that line too.....
Most competition guys who run that mix have it on all the time. They have found that it is needed on most lines whenever the rudder is applied. Firsts find out if you too need it all the time. If so, just make sure all switches are inactive on the switch selection screen, and it will be on all the time.
Hope this helps,
Dan Baker
03-23-2009 | 04:58 AM
#13
Senior Member
RE: roll coupling on knife edge - problem
Most competition guys who run that mix have it on all the time. They have found that it is needed on most lines whenever the rudder is applied. Firsts find out if you too need it all the time. If so, just make sure all switches are inactive on the switch selection screen, and it will be on all the time.
If you look at that mixing as many do, it takes on a more sensible perspective. It's making the rudder function just a yaw control function. And what's bad about that? Nothing.
03-24-2009 | 09:34 AM
#14
Senior Member
RE: roll coupling on knife edge - problem
BTW, it's flying day for the retired guys at one of my club fields. So I'm about to head out. I'm taking an alltime favorite, a WorldModels Ultimate46.
That little sucker is QUICK. And from day one, it had a very pronounced rudder couple. Lot's of pitch with any amount of rudder and some roll. Takeoffs and landings were a pain because adjusting your flight path with the rudder gave pitch changes you really didn't need. It's kewl advice most give about learning to fly the knife edge without mixes and you'll become a better flyer. You will. They are right about that. But ask any of them how they practice dealing with the unwanted pitch and roll they get landing. You'll probably get a blank stare in response. And then watch 'em land.
That little Ultimate got mixes worked into it's programming on the TX about the 2nd day it flew..... back in 2006, I think it was.
There is no shame in adjusting a model to respond to rudder with just yaw. And no glory from cutting the model up and rebuilding to adjust that rudder response instead of fixin' wid' mixin'.
That little sucker is QUICK. And from day one, it had a very pronounced rudder couple. Lot's of pitch with any amount of rudder and some roll. Takeoffs and landings were a pain because adjusting your flight path with the rudder gave pitch changes you really didn't need. It's kewl advice most give about learning to fly the knife edge without mixes and you'll become a better flyer. You will. They are right about that. But ask any of them how they practice dealing with the unwanted pitch and roll they get landing. You'll probably get a blank stare in response. And then watch 'em land.
That little Ultimate got mixes worked into it's programming on the TX about the 2nd day it flew..... back in 2006, I think it was.
There is no shame in adjusting a model to respond to rudder with just yaw. And no glory from cutting the model up and rebuilding to adjust that rudder response instead of fixin' wid' mixin'.
03-24-2009 | 10:45 AM
#15
RE: roll coupling on knife edge - problem
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
03-24-2009 | 12:54 PM
#16
Senior Member
RE: roll coupling on knife edge - problem
ORIGINAL: littlecrankshaf
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
03-24-2009 | 01:21 PM
#17
Senior Member
RE: roll coupling on knife edge - problem
ORIGINAL: Danny Baker
Robert: May I suggest taking the plane to the field and putting it through the following flight test to see if you need the rudder to airleron mix in other flight manuevers/lines. A good pilot will not put a mix "in" or assign it to a swtch until he/she has flight tested for it. There is no way to "look" at the plane, or take instruction from this forum and get it correct. Your answers are waiting for you in the air. Please do the follow to obtain your answers.
1. Fly staight and level and then give rudder only, see if the wing dips in the same direction that you are giving rudder. Most planes do this. This is the same direction the wing is moving in KE flight that you describe. If so, the rudder to ail. mix described above is needed on that line too.
2. Do a hammer head stall turn (you'll need to do several). If you are hammering with left rudder, see if the left wing dips to the gear, most airplanes do this, they do this because the wings are NOT stalled in a hammerhead. The outer panel (the one that is arcing in the largest circle) has more air over it than the inner panel (smallest arcing one), and that wing lifts while the plane is "in" the hammer. Usually causing the hammer to torgue off, and cause a correction of the downline to exit on the same line as entry. The KE mix decribed in the above posted will help this to stay on plane (geometric plane), or counter this, help this not to happen, etc. So now that mix is needed on that line also....
3. Put the plane into a vertical upline with some speed, get a good running start at the upline. Right after you establish vertical apply a little rudder (as in to do the top half of a KE loop), see if the plane rolls with the yaw command. If so, you need the rudder to airl. mix on that line too.....
Most competition guys who run that mix have it on all the time. They have found that it is needed on most lines whenever the rudder is applied. Firsts find out if you too need it all the time. If so, just make sure all switches are inactive on the switch selection screen, and it will be on all the time.
Hope this helps,
Dan Baker
Robert: May I suggest taking the plane to the field and putting it through the following flight test to see if you need the rudder to airleron mix in other flight manuevers/lines. A good pilot will not put a mix "in" or assign it to a swtch until he/she has flight tested for it. There is no way to "look" at the plane, or take instruction from this forum and get it correct. Your answers are waiting for you in the air. Please do the follow to obtain your answers.
1. Fly staight and level and then give rudder only, see if the wing dips in the same direction that you are giving rudder. Most planes do this. This is the same direction the wing is moving in KE flight that you describe. If so, the rudder to ail. mix described above is needed on that line too.
2. Do a hammer head stall turn (you'll need to do several). If you are hammering with left rudder, see if the left wing dips to the gear, most airplanes do this, they do this because the wings are NOT stalled in a hammerhead. The outer panel (the one that is arcing in the largest circle) has more air over it than the inner panel (smallest arcing one), and that wing lifts while the plane is "in" the hammer. Usually causing the hammer to torgue off, and cause a correction of the downline to exit on the same line as entry. The KE mix decribed in the above posted will help this to stay on plane (geometric plane), or counter this, help this not to happen, etc. So now that mix is needed on that line also....
3. Put the plane into a vertical upline with some speed, get a good running start at the upline. Right after you establish vertical apply a little rudder (as in to do the top half of a KE loop), see if the plane rolls with the yaw command. If so, you need the rudder to airl. mix on that line too.....
Most competition guys who run that mix have it on all the time. They have found that it is needed on most lines whenever the rudder is applied. Firsts find out if you too need it all the time. If so, just make sure all switches are inactive on the switch selection screen, and it will be on all the time.
Hope this helps,
Dan Baker
Like I said before, it's up to you to use the free advice and it's also up to you to chase your tail til the cows come home. I'm dunn with this thread
MattK
03-24-2009 | 02:35 PM
#18
RE: roll coupling on knife edge - problem
ORIGINAL: mjfrederick
I think a better solution would be to move the CG forward and add some positive incidence to the wing. Moving the CG aft will just make the aircraft more unstable.
ORIGINAL: littlecrankshaf
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
Now, if your plane is pitching hard to the gear as well on knife edge this technique will be of little help and your best recourse will be a dihedral adjustment.
03-24-2009 | 04:43 PM
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RE: roll coupling on knife edge - problem
I snuck out to fly this morning and tested for roll coupling in all the attitudes and indeed right rudder made it roll right in every scenario.
I tried mixing a bit of opposite aileron - 10% initially which didn't look like much at all. It ended up being really close - actually a little too much - eventually tuned it to 7% and it flies much better - even landed easier in the crosswind I had this morning.
MattK may be right that changing the dihedral would be the better solution, but bending the wing tube seemed a bit difficult - especially to get it right and fine tune - plus if you go too far, you can't really undo it.
Anyway the plane flies SO MUCH better that I am happy with it as it is now. I appreciate all the help.
- Rob
I tried mixing a bit of opposite aileron - 10% initially which didn't look like much at all. It ended up being really close - actually a little too much - eventually tuned it to 7% and it flies much better - even landed easier in the crosswind I had this morning.
MattK may be right that changing the dihedral would be the better solution, but bending the wing tube seemed a bit difficult - especially to get it right and fine tune - plus if you go too far, you can't really undo it.
Anyway the plane flies SO MUCH better that I am happy with it as it is now. I appreciate all the help.
- Rob
03-24-2009 | 05:53 PM
#20
Senior Member
RE: roll coupling on knife edge - problem
It's good that we have transmitters with mixes available.
Because not all of us are adjusting our latest design and care to rebuild the model to get the sucker to fly cleanly. And a lot of us are flying ARFs for a reason. Same reason applies to rebuilding the ARF as it did when we chose to assemble the ARF.
I flew my Ultimate biplane today. Rebending it's wing tube wouldn't have been an option. It flew like gangbusters like always. If I remember back a couple of years ago when it got trimmed, it took about 3 flights to work out the mixes. It's been an excellent ARF.
Because not all of us are adjusting our latest design and care to rebuild the model to get the sucker to fly cleanly. And a lot of us are flying ARFs for a reason. Same reason applies to rebuilding the ARF as it did when we chose to assemble the ARF.
I flew my Ultimate biplane today. Rebending it's wing tube wouldn't have been an option. It flew like gangbusters like always. If I remember back a couple of years ago when it got trimmed, it took about 3 flights to work out the mixes. It's been an excellent ARF.
03-24-2009 | 07:54 PM
#21
RE: roll coupling on knife edge - problem
I listened to one of the top pilots in the US say he uses mix and leaves it on all the time also explaining why. (It made a lot of sense to me) Truth or Fiction, only he knows for sure but his explanation sure sounded a lot like Dan's above. I don't really know any of you guys here on the forum; maybe someone is Andrew Jesky or some other big name but when I tried this gentleman's suggestions I was very pleasantly surprised.
03-25-2009 | 08:54 AM
#22
RE: roll coupling on knife edge - problem
Before mixes were available in TX the pattern guys figgured out how to setup a model to fly with little or NO roll/pitch couple.
Really-
I made -back in 1977 a mechanical linkage which auto combined rudder deflection and elelevato rinput to compensate
It worked but was a pain to setup- I just set the CG right and all was cool.
As radios progresed it became much easier to correct various layouts and force arrangements with TX mix
As flying changed to include maneuvers which made the planes LESS stable - the input of mix to stop belly tuck -bacame commonplace
Som of the theories about this are really goofy but basically the problem results form loosing or lessening the forces of the tailplane (stab)in other than stable level flight This plus bad lateral area setups
A Hard yaw can cause pitch easily
same as knife edge.
By shifting CG forward -the force needed to hold the plane level -in level flight (up trim on the elevator)
acts to stop the down pitch when the model rolls on to it's side. (gravity then acts only on the side area of the model- ) It won't solve pitch due to yaw in some cases but it helps.
All this fiddle fu-ing around can make a plane into a POS as other flight attitudes are ALL affected.
Really-
I made -back in 1977 a mechanical linkage which auto combined rudder deflection and elelevato rinput to compensate
It worked but was a pain to setup- I just set the CG right and all was cool.
As radios progresed it became much easier to correct various layouts and force arrangements with TX mix
As flying changed to include maneuvers which made the planes LESS stable - the input of mix to stop belly tuck -bacame commonplace
Som of the theories about this are really goofy but basically the problem results form loosing or lessening the forces of the tailplane (stab)in other than stable level flight This plus bad lateral area setups
A Hard yaw can cause pitch easily
same as knife edge.
By shifting CG forward -the force needed to hold the plane level -in level flight (up trim on the elevator)
acts to stop the down pitch when the model rolls on to it's side. (gravity then acts only on the side area of the model- ) It won't solve pitch due to yaw in some cases but it helps.
All this fiddle fu-ing around can make a plane into a POS as other flight attitudes are ALL affected.
03-26-2009 | 06:58 PM
#23
RE: roll coupling on knife edge - problem
I can't disagree with that Dick but I have a plane that when the CG is setup to do really good waterfalls, harriers, parachutes, etc. tucks to the belly. As I see it I basically have two choices; move the CG and live with an airplane that doesn't fly the way I want it to under most conditions or leave the CG alone and mix out the belly tuck in the radio which, by the way, also shows up in every attitude when rudder is applied, not just in knife edge flight. Mixing it out is so incredibly easy with no ill effects that I can see I believe I'll stick with it. I know others will see it different but that's my two cents.
03-26-2009 | 10:16 PM
#24
RE: roll coupling on knife edge - problem
I do the same thing for some of my models. The BIG gassers are the easiest to combine aft cg and still get smooth performance.
The precise flying some hope for, does not include the nuetral or unstable setup which makes for really good high alpha flying.
My foamies -I just fly - the cg is aft but I just fly around the "tuck".
Different flying styles-
My point was that the tuck n roll can be trimmed out of most aerobatic designs -if they are basically setup for stable flight.
-
The precise flying some hope for, does not include the nuetral or unstable setup which makes for really good high alpha flying.
My foamies -I just fly - the cg is aft but I just fly around the "tuck".
Different flying styles-
My point was that the tuck n roll can be trimmed out of most aerobatic designs -if they are basically setup for stable flight.
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04-03-2009 | 07:10 AM
#25
RE: roll coupling on knife edge - problem
Interesting discussion.
Quote: "I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen"
Likely there is significant unbalance in rudder area above and below wing or thrust center line. If mixes don't make it happy, a redesigned rudder might. Can easily be tested by taping on a hunk of cardboard adding area probably to the lower section.
Quote: "I notice that the rudder planform is not all that scale - more tall/narrow than others I have seen"
Likely there is significant unbalance in rudder area above and below wing or thrust center line. If mixes don't make it happy, a redesigned rudder might. Can easily be tested by taping on a hunk of cardboard adding area probably to the lower section.
