Elevator SNAPROLL
10-26-2009 | 02:15 PM
#51
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From: USA
RE: Elevator SNAPROLL
ORIGINAL: RCHUGHES
you can get back up all you want but I know other people on this forum that can Back up my comments, A plane can fly with a CG past the Neutral point I have done it, I know other people that have done it, with the right Airframe it actually does not fly bad at all, does it really matter to you that much??
ORIGINAL: onewasp
So long as we have Bruce's attention.........
Would you please comment on ''flight'' (I use the word loosely) on any plane with the CG located behind (that doesn't mean .001'') the neutral point !
My remarks on this are included in post #15.............and I'm still impressed !!!!!!!!
So long as we have Bruce's attention.........
Would you please comment on ''flight'' (I use the word loosely) on any plane with the CG located behind (that doesn't mean .001'') the neutral point !
My remarks on this are included in post #15.............and I'm still impressed !!!!!!!!
Relationship to aircraft and missile stability and control
* If the center of gravity (CG) of an aircraft is forward of the neutral point, or the CG of a missile is forward of the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that returns the angle of attack of the vehicle towards the angle that existed prior to the disturbance.
* If the CG of an aircraft is behind the neutral point, or the CG of a missile is behind the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that continues to drive the angle of attack of the vehicle further away from the starting position.
The first condition above is positive static stability. In missile analysis this is described as positive static margin. (In aircraft analysis it may be described as negative static margin.)
The second condition above is negative static stability. In missile analysis this is defined as negative static margin. (In aircraft analysis it may be described as positive static margin.)
Depending on the static margin, humans may not be able to use control inputs to the elevators to control the pitch of the vehicle. Typically, computer based autopilots are required to control the vehicle when it has negative static stability - usually described as negative static margin.
10-26-2009 | 02:37 PM
#52
RE: Elevator SNAPROLL
At lower speeds and with the right design flying with the CG a moderate amount behind the NP is possible and practical. The flat foamie guys do it quite regularly. The models you see doing those outragious shows indoors will all typically be flying with likely a - 3 to -5% or so static margin. It makes hovering super easy.
I'm a bit of a coward and my own first flat foamie is flying at pretty much 0% to perhaps a hair negative. It shows me this when I do a vertical dive by not pulling out at all until commanded. And I know of others where they are definetly flying with a bit of negative stability.
Obviously this can't be taken to greater extremes or we would need a computer to fly the models. And as noted flying at reduced airspeeds and higher angles of attack seems to calm the models to where they require little or no correction to maintain a consistent line unless disturbed.
I'm a bit of a coward and my own first flat foamie is flying at pretty much 0% to perhaps a hair negative. It shows me this when I do a vertical dive by not pulling out at all until commanded. And I know of others where they are definetly flying with a bit of negative stability.
Obviously this can't be taken to greater extremes or we would need a computer to fly the models. And as noted flying at reduced airspeeds and higher angles of attack seems to calm the models to where they require little or no correction to maintain a consistent line unless disturbed.
10-27-2009 | 03:49 PM
#54
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From: League City, TX
RE: Elevator SNAPROLL
Guys:
I had this problem with my AW Extra 260 .60-.90. I asked a similar question to the original poster, and received suggestions about reducing elevator throws, being more gentle with up elevator input, etc, all of which were to a degree helpful. The issue I, and I think the original poster, had was that the snappiness of the airframe was / is way beyond the plane being aerobatic. An answer for that airframe was that, simply, the plane was too heavy for its wing area - that is, wing loading was much too high. This situation results in the amount of lift generated for a given angle of attack is too small to support the lift required for positive G maneuvers, and hence the AOA increases to the point of stalling the main wing. Is the AUW of the aircraft OK? An 85cc gasser in a 27% Extra seems like a lot of engine and wt. Might be useful to calculate the cubic wing loading.
http://www.flyrc.com/calculator.shtml
I had this problem with my AW Extra 260 .60-.90. I asked a similar question to the original poster, and received suggestions about reducing elevator throws, being more gentle with up elevator input, etc, all of which were to a degree helpful. The issue I, and I think the original poster, had was that the snappiness of the airframe was / is way beyond the plane being aerobatic. An answer for that airframe was that, simply, the plane was too heavy for its wing area - that is, wing loading was much too high. This situation results in the amount of lift generated for a given angle of attack is too small to support the lift required for positive G maneuvers, and hence the AOA increases to the point of stalling the main wing. Is the AUW of the aircraft OK? An 85cc gasser in a 27% Extra seems like a lot of engine and wt. Might be useful to calculate the cubic wing loading.
http://www.flyrc.com/calculator.shtml
10-27-2009 | 04:20 PM
#55
RE: Elevator SNAPROLL
ORIGINAL: onewasp
Relationship to aircraft and missile stability and control
* If the center of gravity (CG) of an aircraft is forward of the neutral point, or the CG of a missile is forward of the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that returns the angle of attack of the vehicle towards the angle that existed prior to the disturbance.
* If the CG of an aircraft is behind the neutral point, or the CG of a missile is behind the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that continues to drive the angle of attack of the vehicle further away from the starting position.
The first condition above is positive static stability. In missile analysis this is described as positive static margin. (In aircraft analysis it may be described as negative static margin.)
The second condition above is negative static stability. In missile analysis this is defined as negative static margin. (In aircraft analysis it may be described as positive static margin.)
Depending on the static margin, humans may not be able to use control inputs to the elevators to control the pitch of the vehicle. Typically, computer based autopilots are required to control the vehicle when it has negative static stability - usually described as negative static margin.
ORIGINAL: RCHUGHES
you can get back up all you want but I know other people on this forum that can Back up my comments, A plane can fly with a CG past the Neutral point I have done it, I know other people that have done it, with the right Airframe it actually does not fly bad at all, does it really matter to you that much??
ORIGINAL: onewasp
So long as we have Bruce's attention.........
Would you please comment on ''flight'' (I use the word loosely) on any plane with the CG located behind (that doesn't mean .001'') the neutral point !
My remarks on this are included in post #15.............and I'm still impressed !!!!!!!!
So long as we have Bruce's attention.........
Would you please comment on ''flight'' (I use the word loosely) on any plane with the CG located behind (that doesn't mean .001'') the neutral point !
My remarks on this are included in post #15.............and I'm still impressed !!!!!!!!
Relationship to aircraft and missile stability and control
* If the center of gravity (CG) of an aircraft is forward of the neutral point, or the CG of a missile is forward of the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that returns the angle of attack of the vehicle towards the angle that existed prior to the disturbance.
* If the CG of an aircraft is behind the neutral point, or the CG of a missile is behind the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that continues to drive the angle of attack of the vehicle further away from the starting position.
The first condition above is positive static stability. In missile analysis this is described as positive static margin. (In aircraft analysis it may be described as negative static margin.)
The second condition above is negative static stability. In missile analysis this is defined as negative static margin. (In aircraft analysis it may be described as positive static margin.)
Depending on the static margin, humans may not be able to use control inputs to the elevators to control the pitch of the vehicle. Typically, computer based autopilots are required to control the vehicle when it has negative static stability - usually described as negative static margin.
"If you go thru the fence nose first, the car has understeer- etc.."
My old Chev powered Healey, had definate oversteer (CG was 45/55, front to rear).
My TR3 had -well understeer plus under power
10-29-2009 | 02:53 PM
#57
RE: Elevator SNAPROLL
An idea occured to me while replying to another thread. It could be worth a try and might produce a simple fix to this issue. I've guoted my thought from the other thread below.
Actually there MAY be something you can do to help this. On some of my gliders I've had good luck with adding turbulators to slightly delay the high drag mush that comes near the stall. They are also supposed to slightly increase the angle where the stall occurs. You MAY find you have some luck with adding turbulator strips on the outer third of the wing on the side that drops away consistently. On the other hand if it drops off either way about equally then try adding turbulators to both sides. Locating them to do the most good is a guessing game but I'd suggest a double thickness of 1/8 automotive trim striping tape at about the 8 to 10% chord point. If it doesn't help move this strip forward a little towards the 5% point.
Just be aware that a key element of this type of model is the ability to stall and snap violently on demand. To avoid it when NOT wanted it's up to the pilot to adjust to the model's needs. The pilots of the full sized airplanes have to do the same thing. Anyhow the big outcome of this is that the model may not snap the same upright as inverted or the same from one side to the other if you end up with the tubulator on the one side only. Adding more strips of different lengths may or may not fine tune the stalling charactarisitics but I just don't really know. At least it's something that's easy to play with and try different things.
Just be aware that a key element of this type of model is the ability to stall and snap violently on demand. To avoid it when NOT wanted it's up to the pilot to adjust to the model's needs. The pilots of the full sized airplanes have to do the same thing. Anyhow the big outcome of this is that the model may not snap the same upright as inverted or the same from one side to the other if you end up with the tubulator on the one side only. Adding more strips of different lengths may or may not fine tune the stalling charactarisitics but I just don't really know. At least it's something that's easy to play with and try different things.
