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Old Jan 20, 2013, 02:03 PM
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Knoll53's Avatar
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Fixing the flutter is all about stiffness / rigidity. It's different than strength. It's either the joiner, the spar or the control surfaces. You could stiffen the spar by vacuum bagging on some carbon tow top and bottom at the sub spar location and the main spar. Are your elevons stiff?...or do they wiggle a little bit?

One easy test to fix the spiral dive (which is probably a tip stall) is to set the neutral point for the elevons up a bit and the flaps down a bit, so that they balance each other out. Similar to crow. This way you have increased the wash out of the wing. Fly it and see if it still does the spiral dive when doing those slow turns.

Kent
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Old Feb 23, 2013, 11:52 AM
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Winterthur ZH, Switzerland
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Hey, have you done any further testing yet?
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Old Feb 23, 2013, 01:49 PM
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Australia, QLD, Maryborough
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Hi sfluck, After repairing the damage I have been waiting for a fine day with low wind and finally had an opportunity yesterday to have another fly.
Topsky finally got back to me with the recommended CG, which was about 10mm back from where I was using. I removed some nose weight to put it 6mm in front of the recommended spot rather than make a drastic change.
Yesterday I flew very early in the morning with very light winds. I had 3 flights and was once again impressed by the great glide/sink ratio. I have been flying another high aspect wing that also has a tendency to spiral dive which is sometimes unrecoverable ( a foam wing so no damage so far on impact ), so I was cautious to keep the turns fairly flat.
For trimmed level flight the elevons now have no extra reflex, that is, they are level with the uncut surfaces of the wing. I am still not happy with the flex in the wing join so I did not want to risk a dive test to confirm CG.
It was too early in the day for much thermal activity and the winds started to rise a bit so I put off further testing for now.
I have scoured the nurflugel forum and find it unusual that this wing has no washout at all. It uses a MH45 airfoil and I did read one post that suggests that reflexed sections need no washout?
Miniphase has some wings with the outboard section leading edge cut back to reduce the spiral effect. I am a little reluctant to do that on a $300 wing but it is an option.
Turbulators have been suggested but I'm not sure how to go about that.
Kent also made a suggestion that I may try regarding control surfaces.

I will continue testing as weather permits.

Greg.
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Old Feb 23, 2013, 03:34 PM
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Winterthur ZH, Switzerland
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I would try by putting turbulators (thick tape or something about 0.5-1mm high) at about 30% of the chord on the upper side of the wing. You could find out the optimal position for turbulators by interpreting the mh45 polars, but I personally are not familiar with it enough to say anything useful. I would just try it at 30%. You could try by only applying turbulators on one wing, and leave the other wing original. Then you could try to force the manta into a spiral dive (both ways) and then you could look if it is better the one way or the other. You could try that, it wouldn't be that much effort with maybe a big revenue.
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Old Feb 24, 2013, 12:21 PM
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Winterthur ZH, Switzerland
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Quote:
Originally Posted by Knoll53 View Post
One easy test to fix the spiral dive (which is probably a tip stall) is to set the neutral point for the elevons up a bit and the flaps down a bit, so that they balance each other out. Similar to crow. This way you have increased the wash out of the wing. Fly it and see if it still does the spiral dive when doing those slow turns.
And I would definitely try this first! You say that the wing has no twist built in, with that method you can check if this is the main problem. Very few effort for maybe best possible result.
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Old Feb 24, 2013, 01:52 PM
Whiskey Tango Foxtrot
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Australia, QLD, Maryborough
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Now that the wing is repaired and is back in the air, trying the control surfaces deflection as suggested by Kent will be the next alteration I will make. I only want to try one mod at a time.
Yesterday I programmed the Mid Flaps position to the mild crow setting so I can launch in normal mode then switch on or off as required.

We are currently experiencing flooding rains Down Under so testing will have to wait.

Thanks again to all who have made suggestions.

Are there any other Manta Ray flyers out there?

Greg.
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Old Feb 25, 2013, 01:40 PM
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Winterthur ZH, Switzerland
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I would love to get one, but there are very very very few pictures around. Would you mind doing something against this?

I'm a bit concerned about that instability issue. When I entered the parameters (root 20cm, tip 15cm, sweep 44.5 cm, MH45) into FLZ_vortex, it predicted instable flight. When I "built in" some wing wist the error disappeared. But I am far away from understanding this program, so don't worry .

What might help as well is to write your issue in the Modelling Science section, just brief with plane parameters and the issue per se. They always know an answer for almost everything.
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Old Feb 25, 2013, 04:02 PM
Whiskey Tango Foxtrot
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sfluck, here are some pics as requested.
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Old Feb 25, 2013, 05:18 PM
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Quote:
Originally Posted by guitarcher View Post
Hi sfluck, I removed some nose weight to put it 6mm in front of the recommended spot...... I had 3 flights and was once again impressed by the great glide/sink ratio..
You moved the CG back and noticed better glide.....Yes, I'll believe that! Nice work!!! I had the same experience with my Horten Xc. The loss of control was scary though.
Quote:
I have scoured the nurflugel forum and find it unusual that this wing has no washout at all.
I agree. I wouldn't call it a mistake, but no washout in a swept wing is, at best, a missed opportunity. Swept wings are more likely to tip stall, so a little washout is welcomed. Add in a lot of washout and you can go "Horten" and remove the winglets.

Maybe you could kill two birds with one stone by removing all the covering and vacuum bagging on some carbon skins and carefully adding wing twist while it cures. If the joiner is too flexible, you'd have to make a one piece wing out of it with carbon skins to add stiffness at the root.....That should keep you busy.

Or of course you could just fly it.

Kent
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Old Feb 25, 2013, 07:46 PM
Whiskey Tango Foxtrot
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Quote:
Maybe you could kill two birds with one stone by removing all the covering and vacuum bagging on some carbon skins and carefully adding wing twist while it cures. If the joiner is too flexible, you'd have to make a one piece wing out of it with carbon skins to add stiffness at the root.....That should keep you busy.
Wow Kent, I'm not sure what to say. On first thoughts that seems fairly drastic, but then if it won't fly ,well then maybe drastic action is what is required. I'm surprised a model can get to the production phase which such an inherent fault.

Kent, I haven't done any vacuum bagging yet so I will firstly try your previous suggestion of mild crow on the 2M foam wing I posted in the build log yesterday. Zach from Zupair also suggested some mix settings similar to that on the Taborca posted elsewhere.

The wing flutter is definitely coming from the join between the wings and the pod, so as I can fit in my car in one piece, glassing the join is a great idea.

Flooding rains here have put a dampener on any flying for the present. After having great success with building a 1.2 M Alula ( Redwing ) style wing I'm feeling the urge to build a 2M version. The low aspect ratio will make visibility at altitude much easier.

BTW Kent I'm enjoying your 2.5 plank and can't wait to see it finished.

To be continued.

Greg.
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Old Feb 26, 2013, 02:16 AM
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I can't remember if anyone else has suggested it but how about trying some wing fences? It'll be an easy experiment without having to hack the wing about.
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Old Feb 26, 2013, 09:41 AM
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Originally Posted by guitarcher View Post
Kent, I haven't done any vacuum bagging yet so I will firstly try your previous suggestion of mild crow on the 2M foam wing I posted in the build log yesterday.
Greg.
Right !
Programming in some crow is easy and very well may solve the control issues. Just by adding in some crow, you will absolutely increase the washout of the wing and I predict, good things will follow. That crow experience may motivate you to take more drastic measures.
I know from my own experience, if I have a handful of little complaints about a plane, I'll soon not be flying it at all. That's what happened with the Horten 9-V1. Now that I'm taking drastic measures to fix all of those problems, I'm excited about flying it again.

Kent
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Old Feb 26, 2013, 01:21 PM
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Winterthur ZH, Switzerland
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Looking good that wing! Oh I shouldnt, I shouldn't....


Quote:
Originally Posted by miniphase View Post
I can't remember if anyone else has suggested it but how about trying some wing fences? It'll be an easy experiment without having to hack the wing about.
Could you please explain a bit how this could help? Where would you put them, and how would you shape them? I heard that wing fences are often used as a last measure (Like on the MIG15/21 and others)
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Old Feb 26, 2013, 01:25 PM
Whiskey Tango Foxtrot
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Australia, QLD, Maryborough
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Quote:
I can't remember if anyone else has suggested it but how about trying some wing fences? It'll be an easy experiment without having to hack the wing about.
Thanks for that suggestion Miniphase, I'll add that to the list of things to try.

Quote:
I know from my own experience, if I have a handful of little complaints about a plane, I'll soon not be flying it at all.
You're right about that Kent. Its already spent a few weeks on the shelf gathering dust because I had the shts with it. Now I have a few options that may get it sorted.

We're still flooded in here. Several hundred mils of rain after big floods only a month ago. I have been cleaning out the workshop so the planes can be a bit more organized.

Greg.
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Old Feb 26, 2013, 02:09 PM
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Quote:
Originally Posted by sfluck View Post
Could you please explain a bit how this could help? Where would you put them, and how would you shape them? I heard that wing fences are often used as a last measure (Like on the MIG15/21 and others)
They help prevent spanwise migration of the air flow....ie help they stop the airflow moving along the wing rather than across it

I think the general concensus is to put them in line with the elevon break, they can be made out of card or plastic and taped into position

info here- http://www.b2streamlines.com/WingFences.pdf

and here- http://en.wikipedia.org/wiki/Wing_fence

aim for this kind of arrangement....

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