V-Tail Math

[Munin]

Hello everyone.

I am working on two scratch built planes running glow and a gilder.
This time I have decided on going vtail as my air field is not as furry as a golf course.

After having read several articles by respected people like Mark Drela and DJAeroTech. I am still confused by their hardened math workout.

My question is as follows.

How to calculate Vtail total area with preset 100 degree angle based on the following variables: Main wing area, Fuselage length from TE ?

Sub question: any percentage rules on how large the control surfaces should be on a vtail?

Thank you for any input on this matter.

/Munin

[CloudyIFR]

Munin,

I really like the flying wing in your Avatar! Nice.

If you have a spreadsheet program download "Sailplane Calc" and "Convert a Cruciform Tail to a V-Tail and Vice-Versa", the formulas are all in there in a simple and easy to use manner. The formulas are from Martin Simon's, Mark Drela and DJ AeroTech.

Curtis
Montana
www.TailwindGliders.com

[HerkS]

Munin - Odin's Raven

On a plane with a fairly conventional layout, I've used 14% of the wing area as a starting point for the total area of the tail surfaces.

My experience indicates that V-tail control surfaces work well when their width is about 20% to 25% of the local width of the whole tail.

A V-tail is good for rough landing areas -- keeps it out of the weeds and rocks.

[Munin]

Herks, Cloudy

Thanks for your reply.

Here is a sample shot from the book I am using to find e.g. conventional tail size. The author all so states that vtails should be 10% greater then conventionals. Any comments ?

Cloudy.
I dont seem to figure out how to read the auto generated tail size from your spreadsheet based on the given input on the wing and fuse.

Thanks again.

/Munin

[HerkS]

Munin

CloudyIFR is off flying an out of town charter - so may not be able to respond for a couple of days.

Put your complete design into his VTail spreadsheet. After you've entered all of the data and selected your balance point etc. Then go to the results page and see how the numbers for your plane compares to the data ranges that are listed as typical for different types of aircraft. In the case of tail sizing your numbers should fall in a range that is suitable for the type of aircraft you are designing.

His spreadsheet is for sailplanes, but will work for your power model too.

There is more information on the tail sizing page that puts calculations based on your numbers in the left hand column - then gives quite a bit of info about the ranges that are appropriate for different types of models.

If you have any specific questions about your design, you could send a copy of your completed spreadsheet to me and I'd be happy to give you feedback on any issues that you are unclear about.

email to soartec@aol.com

[CloudyIFR]

Munin,

Well Herk has answered it pretty well.

Like he said if you email the saved spreadsheet it'll help look at what you're doing, also if you haven't read the tutorial on how to use the spreadsheet I recommend that as well, you can find it at RCSoaring Digest here:

http://www.rcsoaringdigest.com/pdfs/RCSD-2008/RCSD-2008-04.pdf

Curtis
www.TailwindGliders.com

[Don Stackhouse]

A V-tail should have the same total area as an equivalent conventional tail. I have not found it necessary to make it significantly larger, and I would find a book that recommends that to be suspect.

OTOH, any book that recommends making the total V-tail area any smaller than the supposedly equivalent conventional tail (such as the old "projected area method", where the V-tail has the same area in the top view as the equivalent horizontal tail, and the same area in the side view as the supposedly equivalent vertical tail) is just plain WRONG. Dead wrong. As in: it could kill your airplane.

As far as making that area some specific percentage of the wing area, that method is fundamentally flawed, due to gross oversimplification. There are a whole list of things that enter into the calculation, the main ones being the length of the tail moment arm in comparison to the wing's mean aerodynamic chord ("MAC", which for a tapered wing is typically a little bit larger than the average chord), and the tail moment arm in comparison to the semispan (half the wing span).

As far as tail dihedral angle of a V-tail, that's analogous to the ratio of vertical tail to horizontal tail on a conventional or T-tail. Just as those don't all have the same ratio of fin to stab area, the tail dihedral of a V-tail has to be matched to the individual aircraft. In general, more span needs more fin, and more wing chord needs more stab. Therefore, a higher aspect ratio for the wing will need more tail dihedral, while a lower aspect ratio wing needs a flatter tail dihedral.

As you apparently have already found, there is more detail on this in the "Ask Joe and Don" section of our website: www.djaerotech.com

as far as initial sizing goes, read the articles on "tail volume coefficients". A Vht of around 0.5, and a Vvt around .05 is usually fairly safe. I also look at what I call the "dynamic tail volume coefficients" (my term), which use the square of the factors containing the tail moment arm, as a measure of dynamic stability, which tends to be even more important to handling qualities than static stability.

As far as relative percentages for the control surfaces, books for full scale aircraft such as Abbott & Von Doenhoff's "Theory of Wing Sections" recommend about 20-25% of the flying surface chord. However, that's for high Reynolds numbers ("Re") and fully turbulent flow. At our Re's, the flow is generally laminar, and would rather be laminar and separated than turbulent and attached. The closer to the trailing edge you get, the more this is true. For this reason, it's usually best to put the hinge lines further forward, maybe 30-40% of the chord, or in some cases even more than that (such as about the same spot where you would normally put a turbulator). Often the determining factor becomes structural considerations rather than aerodynamics.

[Airboatflyingshp]

Don and others what about the hybred type V -T topped tail how do they work I can see structural advantages but what about the way they work?

[Don Stackhouse]

Structurally looks very good. I don't see that as a variation of a V-tail, more like a twin-fin version of a T-tail. The verticals are not going to contribute anything significant to pitch stability, which takes it out of the "V-tail" category.

Aerodynamically looks like a great way to make all sorts of interference drag. However, it does keep the height relatively low, and probably does very well when taxiing on the water, with the end plate effects of the water below and the stab above, and the benefits to the rudder's induced drag of what amounts to a biplane rudder. However, a simple water rudder would be better for that situation.

It would be interesting to hear from the plane's designer. They must have some reason they thought it was the best way to go. At first glance, whatever that reason was is not readily apparent.

[Airboatflyingshp]

Hi Don here's the site http://aviakb.ru/avk/eng.htm google page translate and the Beriev link http://www.beriev.com/eng/core_e.html one has the same form of tail the other has motors mounted on the fins/rudder elements........ there is a large flying model by the works team in the water-plane links stick I set up. I don't think either of the manufacturers omitted water rudders they are too powerful once immersed to omit, there is clearly a set of reasons why both manufacturers have gone down this route, I'm intrigued.

Something new with a V tail - NZ the Shearwater....pretty see picture below.
http://www.seaflight.co.nz/
More pics plus a plan view of this here half way down page
http://www.rcgroups.com/forums/showthread.php?t=442033
Fantastic http://www.rcuniverse.com/forum/m_2..._11/key_/tm.htm laddies gone and built one, sure is pretty and now here http://www.rcgroups.com/forums/showthread.php?t=844247 looks good and seems to fly like the real thing.

[Airboatflyingshp]

Ive had a thought - thinking on K Kusomas swept (on the T10 T15 and T16 Lake Hawk) T tails and the HP Victor then other MORE CONVENTIONAL craft eg Beafighter and Saro princess with conventional tails but that required stabilizer dihearal to balance their flying caracteristics............might this not also achive simillar but without a loss of lift as well as achieving the twin rudder effect?

[Don Stackhouse]

... there is clearly a set of reasons why both manufacturers have gone down this route, I'm intrigued...

In the case of Beriev, it's definitely not a variation of V-tail, the fins are nearly vertical in both cases. It appears to be to facilitate their rear loading ramp. The ramp makes the aft fuselage wide enough to support a nice space between the twin fins, and using the T-tail arrangement for the stab gets it up above the turbulence from the fat fuselage, and lets it act as a brace between the tops of the two fins. No aerodynamic benefits, but lots of structural ones.

[Airboatflyingshp]

Thanks Don - Adding the Beriev model link http://www.rcgroups.com/forums/showthread.php?t=571807&pp=15 for those interested, by the way Laddies model of the Shearwater is an exemplary flier in even in far less than perfect conditions. Morph the proportions a little and it could become an amphibious soarer like K Kusomas glorious T16 Lake Falk/Hawk.

I have a glass glider fuse which I would like to equip as a V tail so this is going to be a usefull link, when I get to working out the correct proportions, if I put up pics dimensions and wts would you care to advise :o :) at least so I stand a chance of getting it right ;) :)

[pmackenzie]

Don and others what about the hybred type V -T topped tail how do they work I can see structural advantages but what about the way they work?

FWIW, my guess for the float plane in the picture is that they wanted to have a rudder in the prop blast for improved single engine performance.

Pat MacKenzie