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Old Jul 23, 2011, 01:38 PM
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zetland76's Avatar
Ilkley, UK
Joined Apr 2007
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What size fin and rudder do I need?

I have rebuilt the body on an old rudder and elevator only, slope soarer. (I couldn't bear to scrap my first ever built up wing). This trainer wing has quite alot of dihedral.
The problem. It is very sluggish to turn this model and I have to hold over full rudder for quite some time. What is the recommend for fin and rudder shape and size. My immediate thought is to make the rudder wider. Or should I make the fin and rudder taller? Or is it the dihedral on the main wing to blame?

Thanks

Mike
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Old Jul 23, 2011, 02:17 PM
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domll's Avatar
Norwich.uk.
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Sounds like the dihedral: She's trying to stabilize the flight and fighting any rudder input.
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Old Jul 23, 2011, 05:09 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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Post a picture of the beast and we will be able to make an informed recommendation.
Just now my crystal ball doesn't seem to be working
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Old Jul 23, 2011, 05:21 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
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Quote:
Originally Posted by domll View Post
Sounds like the dihedral: She's trying to stabilize the flight and fighting any rudder input.
Actually the opposite is true.. More dihedral makes a plane turn better on rudder, that's why rudder only planes almost always have plenty of dihedral and aileron planes don't have much, if any. Dihedral is what makes a plane roll when you add rudder (see fig 7 on attached pdf). As a rule of thumb for rudder only you want about 7 deg of dihedral per panel.

I'd suspect either not enough dihedral or too small a rudder or too short a tail moment, or a combination of all of these factors.. as Whiskers say's a photo would help.
Getting all technical, you can calculate the rudder size/moment arm and dihedral angle required for good rudder control response by using the design parameters in the attached .pdf.. page 6, formula 10 (basically bigger rudder on a moment arm tail and more dihedral = better rudder control authority)

Steve
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Old Jul 24, 2011, 10:33 AM
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Ilkley, UK
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This is a picture of the plane.Name: DSCF0001.jpg
Views: 56
Size: 330.6 KB
Description: A picture of the plane. Thanks for all the comments so far.
Thanks for all the comments so far.
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Old Jul 24, 2011, 11:16 AM
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Aberdeen
Joined Mar 2006
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Looks to have plenty of dihedral. More rudder area seems to be the way to go. I'd keep the shape pretty much as you have it but add a little more height and more width and make the fixed part smaller and the moving part larger. You also need generous deflection.. at least 30 deg each way if not more.
Even with ample rudder area you cant expect a R/E glider to have 'snappy' response to rudder especially when flying slow.. It's normal for there to be a little delay in response.
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Old Jul 24, 2011, 04:06 PM
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Cape Town, South Africa
Joined May 2008
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I am inclined to agree, you need more rudder area, possibly start by butt joining with cocktail sticks and tape a piece of 1/8" balsa the same size as your rudder onto the back of the rudder, making sure to leave space for the elevator movement. If that improves the turning you are going in the right direction, easy to change the size of the add on balsa pieces until she behaves correctly. Then you can make yourself a neater vertical and rudder if need be.
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Old Jul 24, 2011, 05:47 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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For a RE slope glider you need a heap of rudder authority.
You get a bit cross-wind on the slope and the dihedral banks the plane to turn down wind, and the rudder really needs to exert massive force to fight the wing and get the whole thing back on track.
I'd put on a rudder of twice the area, with a forward aerodynamic balance, and give it a 40 degree deflection each way.
On a sloper with a similar problem I added ailerons... That fixed it
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Old Jul 25, 2011, 07:47 AM
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slebetman's Avatar
Malaysia, Selangor, Kajang
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For me, it looks like too little dihedral. RE gliders should have 12 degrees of dihedral per wing: 24 degrees total. That looks closer to 9 degrees per wing. That's the rule of thumb for thermal gliders. I don't know much about slope soarers.
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Old Jul 25, 2011, 08:16 AM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
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Give something like this a go, with plenty of throw:
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Old Jul 25, 2011, 12:12 PM
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Ilkley, UK
Joined Apr 2007
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Thanks Whiskers, I'll give it a try.
Just fixing my PSS Spitfire (lost the lead-lined spinner) and Middle Phase (battery got loose) then I'll get back to the RE
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Old Jul 25, 2011, 01:37 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
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Quote:
Originally Posted by slebetman View Post
For me, it looks like too little dihedral. RE gliders should have 12 degrees of dihedral per wing: 24 degrees total. That looks closer to 9 degrees per wing. That's the rule of thumb for thermal gliders. I don't know much about slope soarers.
It's hard to say how much dihedral it has from the photo but i agree with you that something in the 9-12 deg range is where ideally it should be at.. I said minumum 7 deg in my last post but i ment to type 9.

Steve
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Old Jul 25, 2011, 05:07 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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Quote:
Originally Posted by zetland76 View Post
Thanks Whiskers, I'll give it a try.
Just fixing my PSS Spitfire (lost the lead-lined spinner) and Middle Phase (battery got loose) then I'll get back to the RE
Cool!
Let us know how it goes, and if I need to eat a big slice of Humble Pie
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Old Aug 07, 2011, 03:11 PM
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zetland76's Avatar
Ilkley, UK
Joined Apr 2007
51 Posts
Made the rudder taller with the counter-balance as recommended. Was better but not very much however, I put the quicklink in a hole closer to the rudder giving the rudder much more throw. That did the trick. Not as fast as my aileron models but much faster for the RE model.
Taped a spycam to it and got some reasonable shots on my local slope.
Slope soaring --- Baildon Moor (2 min 37 sec)


Thanks all
MIke
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Old Aug 07, 2011, 04:27 PM
M16 CIII/AR15/FN 2000 and PS90
Denile's Avatar
United States, AR, Waldron
Joined Jan 2005
521 Posts
Very nice flying spot. Are those bomb craters that can be seen on the base of the hill.
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