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Old Feb 18, 2009, 04:31 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
425 Posts
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Super twichy wing design. Why?

Can any aerodynamics legends out there give me some pointers as to why this wing is super twichy?
The cord at the root is 450 the width is 620 and the cord at the tips is 115.
I kid you not, the elevons when used as ailerons, only have to move 1 to 2 mm and it really tips over. Using the elevons as an elevator is not a problem.
How can I make it not as tippy?
cheers
rosco

P.S. the block that makes the leading edge is sanded to the airfoil shape
pp.s. The airfoil is a modified MH60
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Old Feb 18, 2009, 07:32 AM
Registered User
Joined Feb 2009
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CG is tail heavy
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Old Feb 18, 2009, 08:58 AM
Herk
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Virginia USA
Joined Jun 2007
1,638 Posts
If the CG was off, the elevator control would be the one that is twitchy. If the elevator is not too sensitive CG is probably about right.

You need to reduce the size of the control surfaces to reduce roll sensitivity. Also moving them closer to the center will make them less sensitive.

I suggest that you cut the outboard half of the elevons loose and fasten them to the wing. Using only the inner half of the elevons will make roll control less sensitive. You may have to increase the elevator part of the elevon movement to compensate for this reduction in area.
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Old Feb 18, 2009, 10:07 AM
ApachePilot
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Joined Oct 2006
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I would agree, split your ailerons in half, usint the inboard portion for ail/elev...ie elevons. and secure the outboard portions to your airfoil...



ApachePilot
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Old Feb 18, 2009, 10:36 AM
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cjbucher's Avatar
Raleigh, NC
Joined Sep 2004
2,551 Posts
Not sure if this could cause a problem or not but your vertical stabilizer (rudder) looks a bit small.
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Old Feb 19, 2009, 05:31 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
425 Posts
Thanks for your inputs. I believe your thoughts.

It has to be a bad design issue on my part, because if you look at a multiplex 'Funjet' the elevons are quite large in comparison to the wing size, but they are no where new as 'tippy'

What things could be done in redesigning it to make it less 'tippy'?

Thanks again.
cheers
rosco
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Old Feb 19, 2009, 09:05 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,695 Posts
Your included angle on the two leading edges looks like it might be a little too acute as the two form a less than 90 degree angle. I don't know that 90 degrees or more is a rule of thumb or anything for designing Deltas but there must be some broad guidance or examples in successful builds that might give you some clues.

If the angle is less acute it has the effect of making changing the length to width ratio of the design and additional width will have the effect of there being more wing area that the ailerons have to act on and that will also slow down the roll rate too.

Jack
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Old Feb 19, 2009, 10:00 AM
ApachePilot
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Joined Oct 2006
1,866 Posts
Rosco,

Looking at your design, I may have come up with a questions that may help you in your issues.

1. you report that there is no issue with elevator.
Perhaps your cOg is close or spot on in this direction. How about your lateral balance? Does it balance at that same spot side to side?

2. you said that if you move aileron control more than a mm side to side it gets real twitchy. Have you tried increasing the size of your winglets, ie extending them past your elevons? Do you have a computer radio that you can set rates and expo? Have you tried it?

I have a few other questions but will await your answer..

Hope we can get this figured out for you...

ApachePilot.
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Old Feb 20, 2009, 02:43 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
425 Posts
Thanks ApachePilot...I appreciate your time.

Well it goes a but like this.
I'm not sure how to answer your first question, although, this may be an answer... the left half of the wing weighs the same as the right half.

With the your question about expo etc, I need to have a normal amount of throw on the elevons to make the elevator have enough power. So is saying that, the ailerons are so influential, that the servo arm only has to mave a mm to make it roll substantially, so there is not even a chance to get expo to work for myself. The servo's that I am running are digital and have pretty good resolution(can't remember the name, but they are nice)

I have a JR PCM9XII and know how to program it very well. And just as a little bit of background, I have had many planes, have made many wings, fly a hotliner, have a pylon racer, designed sportsplanes in cad and had them laser cut, vacuume bagged wings etc,etc I don't say that to big note myself, just to give some background of my knowledge.
I hope that answers you questions well enough.
cheers
rosco
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Old Feb 20, 2009, 02:45 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
425 Posts
P.S. the balsa on the tail fin did make it more flyable, just not near enough
cheers
rosco
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