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Old Feb 07, 2013, 10:40 PM
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Originally Posted by jyothibasu1k View Post
check this
Jyo,

Again... the cg must be under the center of the motors, not the pivot point.
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Old Feb 07, 2013, 10:40 PM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
126 Posts
can you tell me wat matewelli is try to tell... what is that vertical distance...

where it should be.....


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Originally Posted by leadfeather View Post
Not true... if they use the ailerons (flaps) for pitch control as advised, they will have good pitch authority regardless of vertical cg. However, no control scheme is going to work to control pitch in hover until the craft is no longer tail heavy in hover.
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Old Feb 07, 2013, 10:42 PM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
126 Posts
i got a doubt here, why should be the cg under motor center...

in real planes, the motor is under the wing but not middle of the horizontal central axis of wing ?



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Originally Posted by leadfeather View Post
Jyo,

Again... the cg must be under the center of the motors, not the pivot point.
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Old Feb 07, 2013, 10:51 PM
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Originally Posted by jyothibasu1k View Post
can you tell me wat matewelli is try to tell... what is that vertical distance...

where it should be.....
If you are using fan tilt to control cg (not the best for your design) the torque created to control pitch depends on the thrust line and the cg. If the thrust line goes through the cg you don't create a torque. The best arrangement for this control arrangement is either cg above the pivot point to create a torque in the required direction, or a very low cg so that it acts like weight shift. With your current design you don't have much latitude to doe either of the above.
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Old Feb 07, 2013, 10:56 PM
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Quote:
Originally Posted by jyothibasu1k View Post
i got a doubt here, why should be the cg under motor center...

in real planes, the motor is under the wing but not middle of the horizontal central axis of wing ?
Think of the free body diagram you should have covered in first year physics of your engineering studies.

Check the diagram below. As shown, thrust lines up with the cg... the plane is balanced in hover. If you move the cg back you will create a torque that will pitch the nose up.
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Old Feb 08, 2013, 12:07 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
126 Posts
wat u said is okey..

now can i move my cg location in this fuselage design ..

i am thinking to move my motor

have a look at the pic


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Originally Posted by leadfeather View Post
Think of the free body diagram you should have covered in first year physics of your engineering studies.

Check the diagram below. As shown, thrust lines up with the cg... the plane is balanced in hover. If you move the cg back you will create a torque that will pitch the nose up.
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Old Feb 08, 2013, 12:15 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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@matwelli

pic of present working model
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Old Feb 08, 2013, 12:24 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
126 Posts
hopes the 2nd option helps me...



Quote:
Originally Posted by nadine View Post
Hey,if you want only 2 motors to have like osprey,you need kk2 board with osprey twincopter firmware,from Lazzyzero repository!
But remember for tilting you need some spare channel from rx and not more than 45 tilt!!! this is the board:
http://hobbyking.com/hobbyking/store...rol_Board.html
and one video with osprey twincopter firmware:
http://www.youtube.com/watch?v=koNSNQ5tUIg
Or if you want,you could try to go only with gyros and mechanical mix,like this,but remaines only the problemm to translate from hovering to normal flight!:
http://www.rcgroups.com/forums/showthread.php?t=1577718
enjoy....
ps:the simple way is like Leadfeather sayed to you,to use system like Foam&tape does used in his plane with Flybar!!!!
http://www.rcgroups.com/forums/showthread.php?t=1319946
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Old Feb 08, 2013, 04:45 AM
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Originally Posted by jyothibasu1k View Post
wat u said is okey..

now can i move my cg location in this fuselage design ..

i am thinking to move my motor

have a look at the pic
Yes, moving the motor is good. It lines up the fan thrust with the cg.

There is another alternative. The goal is to have the the cg correct for fixed wing flight and for hover. Another way to accomplish this (with your original motor configuration) is to move spar back from the leading edge of the current wing design about 15mm. Continue to use the spar as the pivot point. This way the pivot point is 15mm behind the cg for fixed wing flight and also 15mm behind the cg for hover flight, the craft is always in balance.
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Last edited by leadfeather; Feb 08, 2013 at 05:53 AM.
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Old Feb 08, 2013, 06:55 AM
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Quote:
Originally Posted by leadfeather View Post
Yes, moving the motor is good. It lines up the fan thrust with the cg.

There is another alternative. The goal is to have the the cg correct for fixed wing flight and for hover. Another way to accomplish this (with your original motor configuration) is to move spar back from the leading edge of the current wing design about 15mm. Continue to use the spar as the pivot point. This way the pivot point is 15mm behind the cg for fixed wing flight and also 15mm behind the cg for hover flight, the craft is always in balance.
Good point lead.
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Old Feb 08, 2013, 07:00 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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moving the spar apart means.. like the F&T did in his having the rod at middle of the wing location




Quote:
Originally Posted by leadfeather View Post
Yes, moving the motor is good. It lines up the fan thrust with the cg.

There is another alternative. The goal is to have the the cg correct for fixed wing flight and for hover. Another way to accomplish this (with your original motor configuration) is to move spar back from the leading edge of the current wing design about 15mm. Continue to use the spar as the pivot point. This way the pivot point is 15mm behind the cg for fixed wing flight and also 15mm behind the cg for hover flight, the craft is always in balance.
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Old Feb 08, 2013, 07:48 AM
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Originally Posted by jyothibasu1k View Post
moving the spar apart means.. like the F&T did in his having the rod at middle of the wing location
Yes, move the spar towards the middle of the wing...about 15mm a distance equal to the motor offset.
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Old Feb 08, 2013, 08:35 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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first i want to try by moving motor center to the pivot and cg line..... next i will do what you said regarding spar

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Yes, move the spar towards the middle of the wing...about 15mm a distance equal to the motor offset.
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Old Feb 08, 2013, 10:03 AM
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United States, CA, Sunnyvale
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The simplest solution by far would be to just move the battery forward until the CG is directly under the motor shafts when the wing is vertical. Yes, this would also move the CG forward for forward flight, but this thing is so short coupled and the tail is so small, it is closer to a flying wing than a conventional airplane. Being a little nose heavy in forward flight might just help keep it under control. Besides, I think you need to focus all of your energy on getting a stable hover. You can evolve the design to transition to forward flight later. For the moment you need to learn to crawl before you learn to run.
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Old Feb 09, 2013, 12:38 PM
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United States, CA, Sunnyvale
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I don't think you will have much pitch authority with the existing design. I am talking about using the control surfaces on the wings, not the issue raised by Matewilli. He was talking about titling the motors for vectored thrust. When in hover mode, if you move both wing flaps towards the tail, the resulting force vector will come from about the hing line and point mostly to the rear. I would estimate that the hinge line is just an inch or two from the aricraft CG, giving you very little moment arm to work with. There isn't much you can do about it, except to mount the battery as high as possible. It's an inherent limitation of this particular design concept.

Regarding your wing and CG issues discussed. Ideally the CG for the wing and motor assembly alone (not the entire aircraft) will be at the wing pivot point. That way when the wing tilts the CG for the aircraft will not change. Also the thrust line for wing mounted motors should be in line with the CG of the wing and motor assembly, otherwise it will generate a torque that will make life hard for your wing tilt servo. This CG location will also reduce gravity loads on the wing tilt servo and minimize shock loads due to a bad landing.

I see your wing spar is sagging when the wing is in forward flight position. that leads me to believe it is not one continuous spar but two spars with a center joiner. That is a poor structural design. You want a single spar. Check the strength and stiffness of your design by setting the wing to forward flight mode and lifting the aircraft by the wing tips at the CG. If the spar bends excessively even when bouncing the plane up and down a little it is a bad design and will not survive forward flight.

I see your wing spar is set on the top of your wing. The is very bad aerodynamically. A flat plate wing isn't very good to begin with, but to put the spar on top is the same as a "spoiler" and will cause separation of the airflow, and great reduction of lift, and a large increas in drag. Better to put the spar along the bottom side of the wing where the positive pressure will help prevent airflow separation.

Your horizontal tail needs to be positioned as high a possible, usually on top of the vertical stabalizers. Otherwise when you transition to forward flight the prop wash will hit the tail and push it down causing a sever nose up pitching moment.

Back to your pitch authority problem in hover. The best solution is to turn it into a tri-copter. Even a small motor in the tail will give you much better pitch authority in a hover. You should have acceptable yaw authority using the wing ailerons in hover mode, but this has the disadvantage of making right yaw in hover mode also be left roll in forward flight mode, which is very confusing. Better to use a tilting rotor in the back as is normal for a tri-coptor as it will have little or no effect on yaw in forward flight mode.

In order to make a tri-coptor configuration work you need to have some weight carried by the rear rotor in a hover, but you still need to work it out so the CG is correct for forward flight as well. Mounting the motors out well ahead of the wing will help do that. I know this contradicts what I said earlier, but I was speaking of your existing design then. Now I am speaking of a tri-copter variant.

All design is a compromise of requirements and a balance of forces. You need to balance them in two flight modes and everywhere in between. That is what makes VTOL so difficult.
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