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Old 01-05-2012, 02:12 AM   #1
pattern14
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Default V-tail formula

Hoping someone can pont me in the right direction here, I have a set of original pz radian wings, bought for next to nothing at a closing down sale. I've already built one fuse for it, and it flew quite well, but I'm planning on making a very light version as a pure sailplane. is there a formula for working out V-tail area and angle? Any ideas appreciated
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Old 01-05-2012, 11:27 AM   #2
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Download 'sailplane calc' (the v tail version) here: http://www.tailwindgliders.com/Files.html

Steve
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Old 01-05-2012, 12:32 PM   #3
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the areas need to be right in the vertical and horizontal projections

included angle seems to "look" best at ~ 100 - 100 degrees, but the rest of it is up to personal taste

so in essence if the top view and side views are known, then the V tail is whatever triangle fits in the height and width in the rear view, but the shape needs to encompass a pretty similar area

hope that helps
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Old 01-05-2012, 12:55 PM   #4
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According to the experts the 'projected area' method produces an under-size v-tail. The rule of thumb I've seen put forward by those who should know (Prof Mark Drela for one) is that total area of surfaces should be the same for a conventional or V tail. Typically the angle of the 'v' is about 110 degrees.

Here's Don Stackhouse's method for sizing a v tail (assuming the conventional tail size is known):

For a crude, but surprisingly accurate "rule of thumb", for essentially the same stability and control authority, a T, V or conventional tail should all have the same TOTAL area.

The V-tail angle determines how much of the tail's effectiveness is allocated to yaw stability vs. pitch stability. If you have a successful conventional tail and you want to convert it to a V-tail, add the (stab+elevator) and (fin+rudder) areas together, then divide by two. This is the area for each of your V-tail panels. Now divide the fin+rudder area by the stab+elevator area. This is the tangent of your V-tail's dihedral angle. Find the arctangent, and that's the dihedral, measured on each side from horizontal. Multiply by 2, then subtract the result from 180, and you get the angle between the panels of your V-tail.
Lots of V tail Q's and A's, including de-bunking of the projected area method toward the bottom of the page here: http://www.djaerotech.com/dj_askjd/askdesign.html
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Old 01-05-2012, 01:07 PM   #5
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according to martin simons "the effective areas are those found by projection horizontally and vertically" although he concedes it may need to be slightly more in practice

also, he notes that spin recovery can be poor because of stalling a tail unit with the coincident movements required (full down and opposite rudder)
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Old 01-05-2012, 02:11 PM   #6
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Projection method is wrong. Go to the DJ Aerotech site and look through the questions and answers. I believe there is also something on the Charles River club site.
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Old 01-05-2012, 02:20 PM   #7
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Originally Posted by scruffy1 View Post
according to martin simons "the effective areas are those found by projection horizontally and vertically" although he concedes it may need to be slightly more in practice

also, he notes that spin recovery can be poor because of stalling a tail unit with the coincident movements required (full down and opposite rudder)
What addition is that from? The latest (fourth edition) in section 11.6 'tail unit drag' (page 136) on discussion of v-tails says:
..the required total area for such an arrangement is no less (than a conventional tail), and may be slightly more in practice...
This could be a correction from the projected area method that may have appeared in earlier editions?
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Old 01-05-2012, 03:09 PM   #8
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My own experience is that a "same projected area" V tail works well particularly with a plane that only has a fixed fin (no rudder) anyway.
Even if there are no real aerodynamic advantages a V tail is much less likely to get damaged on landing and is more rigid than a T tail.
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Old 01-05-2012, 10:31 PM   #9
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Thanks for the input everyone; my choice of a V-tail is part practical/part experimental. Tried inverted V's on twin boom planes, with moderate success. My flying field is so rough now, that I'm doing all I can to protect things on landing......maybe next spring they will plant grass, cheers
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Old 01-06-2012, 12:35 AM   #10
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Originally Posted by JetPlaneFlyer View Post
What addition is that from? The latest (fourth edition) in section 11.6 'tail unit drag' (page 136) on discussion of v-tails says:


This could be a correction from the projected area method that may have appeared in earlier editions?
yep i'm old enough to be using the 2nd edition; but it says basically the same thing

the direct initial quote was from his lighter tome "model flight" which is probably just as old, and i qualified it having looked in the bigger book

all good info; thanks to all for educating me and giving me more resources to plunder
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Old 09-23-2012, 08:36 PM   #11
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The "projected area method" gives you a tail that can make the same elevator force, OR the same rudder force as the supposedly "equivalent" conventional tail, but not both at the same time! That could be a problem in things like spin recovery. To make the same combined forces, you need the same total area.

That said, if you take the same total area and divide it among two panels instead of three, the resulting bigger panels will have better spans, or Reynolds numbers, or both. OTOH, there's the debate about "destructive interference", which in my experience seems to be overblown. Most likely it's counteracted by the improved spans and Reynolds numbers.

I have NOT seen justification for needing more than the same total area.
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Old 09-24-2012, 01:18 AM   #12
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Originally Posted by Don Stackhouse View Post
The "projected area method" gives you a tail that can make the same elevator force, OR the same rudder force as the supposedly "equivalent" conventional tail, but not both at the same time! That could be a problem in things like spin recovery. To make the same combined forces, you need the same total area.

That said, if you take the same total area and divide it among two panels instead of three, the resulting bigger panels will have better spans, or Reynolds numbers, or both. OTOH, there's the debate about "destructive interference", which in my experience seems to be overblown. Most likely it's counteracted by the improved spans and Reynolds numbers.

I have NOT seen justification for needing more than the same total area.
hi don

long time no see on the forums since you helped me with my catalyst design

i revisited your directions in creating a v tail for my cuckoo, after the supplied wood wasn't really up to the task

you can check it out over here

thanks again for your help
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Old 09-26-2012, 10:04 PM   #13
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On the matter of spin recovery I have found a V tail seems to resist the plane entering a 'flat' spin as it maintains a more nose down attitude resulting in an easy recovery, albeit in a steep dive.
Could it be that in a spin the 'leading' tailplane part blankets the 'trailing' one, resulting in a net upward force?

This not surprisingly this spins beautifully (either way) but only with the stick full back.
Click image for larger version

Name:	Fin15sep12.JPG
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Centre the stick and it recovers instantly after any number of rotations (max 10 so far!)


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Old 09-27-2012, 01:01 AM   #14
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That has been my experience on a wide variety of V-tail models over a wide range of model sizes.
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Old 03-09-2013, 06:19 AM   #15
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My experiments with v-tail conversions incorprated the tail assembly mounted on a pivot shaft, with an incidence adjuster screw.After test flying and adjusting incidence I viewed tail incidence angle from the nose, if there was any angle the tail is was too small.V angles were copies from previous kits,areas were copies of previous stabilizer sizes lengthened when angled up equivlent to rudder hight. Pretty crude but good enough for my purposes.
Turned out the tail area was a little small.Interesting expirement.
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Old 03-10-2013, 12:33 AM   #16
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Originally Posted by v2flier View Post
My experiments with v-tail conversions incorprated the tail assembly mounted on a pivot shaft, with an incidence adjuster screw.After test flying and adjusting incidence I viewed tail incidence angle from the nose, if there was any angle the tail is was too small.V angles were copies from previous kits,areas were copies of previous stabilizer sizes lengthened when angled up equivlent to rudder hight. Pretty crude but good enough for my purposes.
Turned out the tail area was a little small.Interesting expirement.
Thanks again for the input. I'm still looking at building a V tail for the remains of the radian. I ended getting an original pz fuse, but after fixing the nose x amount of times, repairing the breaks and cracks every rough landing in only moderate winds, I went back to the EPP fuse. I now have an ST330 Easyfly ( for a fraction of the price; ie $80.00 opposed to $400.00), which has better quality foam than pz stuff, and have started using that with the radian wings fitted. I'll experiment with wing incidence before adding the V tail to it. So far it looks pretty promising. Cheers
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