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Old Mar 03, 2013, 12:01 AM
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Pasco Tri Cities, Washington, United States
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Switching to a V tail

I am about to build a Pegasus tow plane. It has a 109" wingspan and a large rectangular/box fuse. Normally it is built with a standard horizontal stab and elevator as well as a fin and rudder. I'd like to replace the standard tail with a set of V tails. How do I determine how large the two ruddervators need to be to provide the same rudder and elevator authority? Any help will be appreciated.

Larry
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Old Mar 03, 2013, 08:23 AM
An itch?. Scratch build.
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South Wales U.K.
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It's not so much how big it is, as what you do with it.

The ruddervators could be the same size as the original aircraft's elevators, as you use both of them for rudder. (The 'how big' part).

Then select the amount of throw for each control, elevator or rudder, to suit how she flies. (The 'what you do with it' part).

Anyone who thought the first statement rude, cut 100 matchstick size pieces from a length of 1/4" square balsa as a penalty.

A man with a 'V' tailed Eagle.
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Old Mar 03, 2013, 10:13 AM
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Florida
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Many sources recommend that you increase the area of the V tail to be 33% greater than the area of the elevator+stabalizer area of a conventional set up. I've found that works pretty well.
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Old Mar 03, 2013, 10:17 AM
Red Merle ALES
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United States, Mt, Helena
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Larry,

If you have a spreadsheet program I have a conversion that uses formulas from Dr. mark Drela.
http://tailwindgliders.com/Files.html#Files

One caveat, he and Don Stackhouse have had lengthy discussions in the past with different views. I think in the end they agreed to disagree, at least that is my take from two highly educated folks. I have gone from a V-Tail to Cruciform tail and I liked the Drela results better.

Curtis
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Old Mar 03, 2013, 11:17 AM
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This works quite well. I use it most of the time.
http://www.charlesriverrc.org/articl...tailsizing.htm
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Old Mar 03, 2013, 11:49 AM
Red Merle ALES
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Quote:
Originally Posted by Sparky Paul View Post
This works quite well. I use it most of the time.
http://www.charlesriverrc.org/articl...tailsizing.htm
Those are the formulas in my spreadsheet.

Curtis
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Old Mar 03, 2013, 06:37 PM
Ascended Master
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All-flying vee tail...
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Old Mar 05, 2013, 02:40 PM
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Quote:
Originally Posted by CloudyIFR View Post
... One caveat, he and Don Stackhouse have had lengthy discussions in the past with different views. I think in the end they agreed to disagree, at least that is my take from two highly educated folks...
The only difference of opinion we have is in the calculation of the tail dihedral angle. Dr. Drela prefers the constant stability method, which comes up with a little more dihedral than the constant control authority method. It's a fairly minor point in most cases, and there are some other mitigating factors that apply to models in particular the blur the differences even more.

What we both agree on, and so does the mathematics, is that a V-tail should have the same TOTAL area as the vertical tail+horizontal tail of the "equivalent" conventional or T-tail.

NOT the same "projected area" in the top view as the horizontal, and same projected area in the side view as the vertical tail. That's the "projected area method", and gives you a seriously undersized V-tail. It can make the same rudder force, OR the same elevator force, but not both at the same time!

Likewise, rules of thumb about some flat percentage increase from the horizontal tail's area, regardless of the vertical tail area. That will lead you astray as well.

Just simply make it the same TOTAL area.
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Old Mar 09, 2013, 11:03 AM
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What are the thoughts re adverse yaw for an upright V-tail versus an inverted V-tail and proverse yaw?

Jim R.
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Old Mar 09, 2013, 11:19 AM
Ascended Master
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The inverted vee tail is better!
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Old Mar 10, 2013, 02:37 PM
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Thank you, Sparky Paul! I'll save your detailed illustrations and expose some people I know to the benefits of the inverted-V tail.

Jim R.
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Old Mar 10, 2013, 03:02 PM
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Quote:
Originally Posted by JRuggiero View Post
What are the thoughts re adverse yaw for an upright V-tail versus an inverted V-tail and proverse yaw?

Jim R.
I believe you're referring to adverse roll in this case, not adverse yaw. Adverse yaw is something ailerons do.

Yes, an upright V-tail does try to cause adverse roll, i.e.: roll the airlane in the opposite direction from the yaw command it is making. However, because in most cases the tail's span is far less than the wing span, this effect is usually negligible.

Yes, an inverted V-tail does try to roll the plane in the same direction as the yaw command, which is usually good when trying to roll in and out of a turn, although it might not be for rudder inputs during the turn.

However, the inverted V-tail typically pays a high price for this in the form of increased fuselage weight and drag. One of the principle advantages of the upright V-tail over a conventional tail is that it does not drag its tips through the weeds and rocks during landing, which for models is a critical determinant of the required aft fuselage strength. OTOH, an inverted V-tail insists on being part of the "landing gear", and so is far worse in this regard than a conventional tail or an upright V-tail. You need to either beef up the tail section and both tail panels to handle this, or use a twin-tail boom layout, in which case you now have two tail booms instead of one, plus extra weight in the inboard wing structure to carry those tail boom loads to the fuselage.

Overall, the upright V-tail's adverse roll is normally less of an issue than these other problems.

Twin tail boom arrangements, sometimes with inverted V-tails, are moderately popular on some UAVs. Typically this is for planes with pusher props (frequently to keep the prop clear of sensors mounted in the nose, despite the efficiency losses that are virtually inherent in pusher props). Using a twin-boom tail helps keep people away from the prop when working around the plane on the ground.
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Old Mar 10, 2013, 06:02 PM
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The wing rocking of an upright vee-tail just looks wrong! And makes for shaky videos.
The inverted vee with twin booms is a smoother flier.
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Old Mar 11, 2013, 06:15 AM
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depending on the size of the moving surfaces you also have to consider how much an inverted V tail will add an anhedral effect to the plane as a whole. Usually it's not a big deal, since the tail span is very small compared to the wing, but it can be a major factor on a "split boom" configuration. Also, still in the split boom setup, unless the wing spar is very rigid the wing twist induced by the tail surfaces can have the plane roll opposite the yaw input.
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