CG check
06-18-2014, 10:25 PM
#51
Regards
06-19-2014, 12:06 AM
#52
With respect this is the opposite of what I'm saying. Modern designs have plenty of potential pitch stability. The issue is with directional stability. Watch most of the current large fuselaged current designs going round an outside corner and you will see them slip off to one side. An increase in vertical area is what is required to deal with this.
Malcolm
06-19-2014, 01:20 AM
#53
Dag,
With respect this is the opposite of what I'm saying. Modern designs have plenty of potential pitch stability. The issue is with directional stability. Watch most of the current large fuselaged current designs going round an outside corner and you will see them slip off to one side. An increase in vertical area is what is required to deal with this.
Malcolm
With respect this is the opposite of what I'm saying. Modern designs have plenty of potential pitch stability. The issue is with directional stability. Watch most of the current large fuselaged current designs going round an outside corner and you will see them slip off to one side. An increase in vertical area is what is required to deal with this.
Malcolm
If you move Your CG back for Your likings you must be prepared to ; a: Increase stab area. B: Fly With considerable negative trim.
This is "fisiks" to mantain balanse in Space (read air). The Pitch stability increases With larger stabs , thats correct!!!
Best Regards
06-19-2014, 03:59 AM
#54
Dag,
Once again with respect you are stating the obvious and it does not relate directly to my point.
Let me try again: Almost all modern designs have a surfeit of potential longitudinal stability. That is you can balance them over a wide range and still have some. What they don't have is a surfeit of directional or yaw stability (which is also CG dependent) and so when choosing a CG position it is often compromised by the need to retain some yaw stability. What is needed is a redistribution of vertical area i.e. bigger fins. This is something that designers seem to have cottoned on to if you look at Chip Hyde's new bipe and I'm sure Bryan Hebert will have it sussed with his new Allure.
Here's a case in point. My new Hybird bipe when balanced at the designer's recommendation goes to the canopy in knifedge. Up lines are perfectly straight in yaw and pitch and down lines are straight with a smidgen of low throttle/down elevator mix. All my trimming experience tells me it is nose heavy so I start to move the CG back. When I get to 30mm behind the designer's position it starts to straighten the knifedge and it needs to go back further. BUT it has started to wander directionally on pushes and doesn't feel as nice to fly generally. So as a temporary fix I have moved the CG forward to the design point and put in a very small rudder/elevator mix. The real cure is an increase in yaw stability which will require some strakes or other devices on the tail. Unless somebody knows different............
Malcolm
Once again with respect you are stating the obvious and it does not relate directly to my point.
Let me try again: Almost all modern designs have a surfeit of potential longitudinal stability. That is you can balance them over a wide range and still have some. What they don't have is a surfeit of directional or yaw stability (which is also CG dependent) and so when choosing a CG position it is often compromised by the need to retain some yaw stability. What is needed is a redistribution of vertical area i.e. bigger fins. This is something that designers seem to have cottoned on to if you look at Chip Hyde's new bipe and I'm sure Bryan Hebert will have it sussed with his new Allure.
Here's a case in point. My new Hybird bipe when balanced at the designer's recommendation goes to the canopy in knifedge. Up lines are perfectly straight in yaw and pitch and down lines are straight with a smidgen of low throttle/down elevator mix. All my trimming experience tells me it is nose heavy so I start to move the CG back. When I get to 30mm behind the designer's position it starts to straighten the knifedge and it needs to go back further. BUT it has started to wander directionally on pushes and doesn't feel as nice to fly generally. So as a temporary fix I have moved the CG forward to the design point and put in a very small rudder/elevator mix. The real cure is an increase in yaw stability which will require some strakes or other devices on the tail. Unless somebody knows different............
Malcolm
06-19-2014, 04:37 AM
#55
THis is what you wrote. maybe you did't mean it this way??????
Regards
06-19-2014, 08:17 AM
#56
Senior Member
Strakes and extended surfaces work well enough. To me (and I have used them too) they compromise a model and add weight. A better more optimal solution is to take weight out of the tail. Enough weight reduction will require a fore relocation of the wing, meaning longer tail moment.
Current designs with huge forward fuse volumes destabilize the models in Yaw (and in pitch). Add the larger props and contra rotators of today, and yaw destabilization becomes greater still.
I have found that 1" movement of the wing makes a considerable difference in the model's directional stability and general handling in yaw..... It takes lots of effort to reduce aft fuse weight and vert and hor stab weights enough to allow wing movement forward. Whether the effort is worthwhile is up to the individual.
Current designs with huge forward fuse volumes destabilize the models in Yaw (and in pitch). Add the larger props and contra rotators of today, and yaw destabilization becomes greater still.
I have found that 1" movement of the wing makes a considerable difference in the model's directional stability and general handling in yaw..... It takes lots of effort to reduce aft fuse weight and vert and hor stab weights enough to allow wing movement forward. Whether the effort is worthwhile is up to the individual.
06-19-2014, 10:41 AM
#57
Strakes and extended surfaces work well enough. To me (and I have used them too) they compromise a model and add weight. A better more optimal solution is to take weight out of the tail. Enough weight reduction will require a fore relocation of the wing, meaning longer tail moment.
Current designs with huge forward fuse volumes destabilize the models in Yaw (and in pitch). Add the larger props and contra rotators of today, and yaw destabilization becomes greater still.
I have found that 1" movement of the wing makes a considerable difference in the model's directional stability and general handling in yaw..... It takes lots of effort to reduce aft fuse weight and vert and hor stab weights enough to allow wing movement forward. Whether the effort is worthwhile is up to the individual.
Current designs with huge forward fuse volumes destabilize the models in Yaw (and in pitch). Add the larger props and contra rotators of today, and yaw destabilization becomes greater still.
I have found that 1" movement of the wing makes a considerable difference in the model's directional stability and general handling in yaw..... It takes lots of effort to reduce aft fuse weight and vert and hor stab weights enough to allow wing movement forward. Whether the effort is worthwhile is up to the individual.
Jim O
06-19-2014, 12:51 PM
#58
Senior Member
Why do we have long nose moments? We have all been duped by the current trend to composite fuses making things lighter. Lightness is great for our sport but the underlying little secret is lack of lightness distribution. Fuses do not have to be so heavy in their tails. But for simplicity, composite makers use one size fits all in their lay-ups. Couple that with composite rudders, hor stabs and elevators, and you are talking 12 ounces at least concentrated at about 36"-40" in back of the cg. Here's where the E guys have a huge advantage with the battery brick, but don't use it to as good an effect as they could because the tails are so darned fat
Jim, I've done exactly what I say in my gasoline powered Delta which uses a Xigris fuse. BTW, 1" sounds really small but it represents about 2 1/2 % of the tail moment. Other than improved groove with the longer tail, I didn't see adverse effects. Maybe I didn't fly it long enough.....
Jim, I've done exactly what I say in my gasoline powered Delta which uses a Xigris fuse. BTW, 1" sounds really small but it represents about 2 1/2 % of the tail moment. Other than improved groove with the longer tail, I didn't see adverse effects. Maybe I didn't fly it long enough.....
