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Old Jul 18, 2013, 02:54 PM
"Flying is my life!"
FlyVA's Avatar
United States, VA, Salem
Joined Oct 2002
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Above and below DLG tails for F3X?

Usually the bottom of a vertical stabilizer starts in the fuselage and extends upward. Mostly seen on DLGs, vertical stabilizers are equally 50% above and 50% below the fuselage so as to minimize the boom twist leverage generated when discus launched

Are there any aerodynamic disadvantages to an “above and below” vertical stabilizer on a larger design?
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Old Jul 18, 2013, 03:38 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
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I don't know to be honest, but I suspect there is a problem in that you might not get far in to the comp with a under over tail in some f3x classes. E.g f3f and f3b with ballast.
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Old Jul 18, 2013, 03:55 PM
In F3J size does matter!
roydor's Avatar
Israel
Joined Nov 2006
836 Posts
In DLG's it's important to balance the torsion loads on the tail boom for structural reasons.
In F3X the loads are lower considerably so it isn’t needed.
Also, F3X models may get the rudder damaged if it’s lower than the fuselage on landings and it can be a little dangerous when launching.
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Old Jul 18, 2013, 03:57 PM
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I also believe it violates some of the model rules for arrestment on F3B and F3J models. i.e. no skegs
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Old Jul 18, 2013, 04:05 PM
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Tuomo's Avatar
Jyvaskyla, Finland
Joined Aug 2003
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Quote:
Originally Posted by djklein21 View Post
I also believe it violates some of the model rules for arrestment on F3B and F3J models. i.e. no skegs
Ava is legal for F3J.
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Old Jul 18, 2013, 06:10 PM
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FlyVA's Avatar
United States, VA, Salem
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All good points fellows but back to the original question.

Are there any aerodynamic disadvantages to an “above and below” vertical stabilizer on a larger design?
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Old Jul 23, 2013, 09:53 AM
Marc PUJOL
Joined Feb 2010
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Quote:
Originally Posted by FlyVA View Post
All good points fellows but back to the original question.

Are there any aerodynamic disadvantages to an “above and below” vertical stabilizer on a larger design?
Yes ther is alttle advantage for a below fin. But it is a it marginal.
The main advantage is that it allows to stop the plane (it's an allowed skeg)
The main drawback is the resistance in torsion during landing. But with a good boom, you can manage.

I have experienced for 2 years with my Genoma series (4M F5J planes) without any issues

Marc
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Old Jul 23, 2013, 03:33 PM
F3B and F3K
RetoF3X's Avatar
United States, TX, Dallas
Joined Mar 2009
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The german Bizarre team (with Reinhard Vallant) had an F3J model called Free Willy for the win (FW FTW), which had a symetric vertical.

http://bizarreteam.forumieren.de/t4-...rste-einblicke

I however only saw them flying with the Free Willy (which had the normal tail).

I did not understand the rationale for the symmetrical tail on a J plane.

Best,
Reto
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Old Jul 24, 2013, 01:51 AM
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United States, CA, San Luis Obispo
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The justification for DLG's is to add yaw damping (with a larger tail) without creating a bunch of torque on the tail boom. You would have to make heavier booms to take the twisting of a top only vertical stab.

The tail booms on F3J models have much larger diameters and thus larger torsional stiffness. So building a large enough vertical stab doesn't impact the overall weight budget of the model like a DLG.

You might make an argument about proverse roll moments with the application or rudder, but that is a REALLY small effect with a properly sized tail.
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Old Jul 24, 2013, 06:20 AM
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Jyvaskyla, Finland
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Originally Posted by Avaldes View Post
The tail booms on F3J models have much larger diameters and thus larger torsional stiffness. So building a large enough vertical stab doesn't impact the overall weight budget of the model like a DLG.
Some time ago there was a launch video over here. Pike Perfect launching and tail twisting torsionally in a quite alarming way. I would not mind having a torsionally stiffer tail booms in my F3J planes.

DLG:s have proportinally much larger vertical stabs. The design trend in F3J is towards the very same direction - with tailbooms getting longer and structural weight lower. Only time will tell what the dominant tail configuration is after 5 or 10 years. Waiting for the 2022 WC...
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Old Aug 01, 2013, 04:23 PM
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OK...this thread has run its course so I'd like to share what my beliefs were when the question was first asked. The symmetrical (or close to it) should:
(1) not hurt performance
(2) reduced torque load (twist) on boom
(3) provide "center" rudder drive point
(4) raise the horizontal stab higher away from ground damage
(5) legally lower the model skidding distance on the ground
(6) be a hell of a lot better looking than a big butt cruciform tail...IMHO of course.

Mission accomplished. Thanks to those who posted.
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