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Old Mar 08, 2014, 02:41 AM
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question: Conventional tail to V tail conversion

Hi to all

If I like to change conventional-tail to V-tail which formula is right:

new V area = same as horizontal stab. surface ..........on conventional tail or
new V area = horizontal stab. surface + vertical surface ........on conventional tail

Thanks a lot, Boris

PS
I already read from djaerotech web page that projected area of horizontal tail is a wrong way
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Old Mar 08, 2014, 04:08 AM
Marc PUJOL
Joined Feb 2010
140 Posts
neither one nore the other.
It is a matter of efficiency.
And the efficiency of a V tail is 0.67 (as an average) less than the X tail one.
this is due to the fact that a V tail see the air with an angle less than a X tail.

If you think V tail is lighter, then you are wrong. At equal efficiency, it is as heavy as X tail...

On top of that there is some interractions between axes with V tail

That's why I do not like V tail.
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Old Mar 08, 2014, 07:27 AM
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Well, that's not exactly true Marc.

Boris, the correct formula (with a few simplifying assumptions) is that the TOTAL area of the two tails should be the same.

A V-tail made via the "projected area method" can make the same pitch force, or the same yaw force, but not both at the same time. To make the same pitch and yaw forces simultaneously, you need the same TOTAL area.

However, Marc, that does not mean that in actual practice the weight is the same. In the case of a V-tail there are only two intersections to reinforce, while a conventional tail requires three. As a result, there tends to be some structural weight savings. This is not just theory, we have made a number of kits (such as our Chrysalis series) that can be built with the builder's choice of V-tail or conventional tail. Stability and handling of the two versions is virtually identical, but the V-tail version does tend to come out a little bit lighter.

Regarding my comment above about simplifying assumptions:
There are some fine points that do complicate the whole question of "equivalency" between the tail types. The one that gets brought up frequently (such as Marc's comment) is the supposed "destructive interference" between the two halves of a V-tail during rudder inputs in particular. In my experience, this partly involves fuselage effects on non pod-and-boom layouts (such as the aft fuselage of the Beech Bonanza), and also is partly due to the amount of tail dihedral. As long as your tail dihedral is less than about 45 degrees from horizontal per side (i.e.: included angle between the tail panels of at least 90 degrees) it does not seem to be a significant problem.

On the plus side for a V-tail, and perhaps explaining why destructive interference does not seem to be as significant as some folks want to expect, is the fact that a V-tail distributes its total area between two panels instead of three. This means that either the panel spans, or the panel chords (and therefore their Reynolds numbers, especially important for models), or both, are greater than the panels of an equivalent conventional tail or T-tail, increasing their effectiveness and efficiency.

In addition, a V-tail has fewer intersections than a conventional or T-tail, and therefore tends to have less interference drag. The worst in this regard would be a cruciform ("X") tail.

Adding all this together, aerodynamically the different tail types tend to come out surprisingly equal, but structurally the V-tail often comes out slightly ahead. Also, from an operational standpoint a V-tail does better than a conventional tail with regards to staying out of the weeds and rocks on landing, but without all the high-mounted mass of a T-tail, so it doesn't create the T-tail's problems of bending and torsion loads on the tailboom during hard landings, which for models is frequently the deciding factor in the required tail structure.

Marc, as far as your 0.67 number, that's going to vary with the details of the tail design, and in any case your point ignores the effects on total drag of the conventional tail's fin and rudder. Not really a valid comparison.
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Old Mar 08, 2014, 08:16 AM
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Thanks a lot for info.
About re-sizing new V-tail ................

Is it be better to enlarge just in lenght V tail stabilser or to enlarge it in lenght and wide too.
thx, boris
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Old Mar 08, 2014, 10:29 AM
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Some of both is probably best. More panel span improves the tail's aspect ratio, but also increases the torsional loads on the tail boom. More chord is good too, but without a span increase some of that is not as effective as it could be. Keeping the overall aspect ratio about the same or just a little higher is probably about the best compromise.
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Old Mar 08, 2014, 11:17 AM
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thanks Don a lot.
i have Spirit 2m. I fly mousty dlg.
I made Spirit before long time ago. I will start to fly it again.
Goal is slow flying and trimming it to fly almoust hands off..
Maybe, some day I could made V tail coversion on it but not sure how it will look esteticly because Spirit has very short tail.
Thanks tor tips,
best, boris
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Old Mar 08, 2014, 02:19 PM
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Properly done, you should not see a significant difference in your Spirit's handling or stability. A properly sized V-tail is equivalent (root word = "equal") to the tail it replaces. There are some subtle nuances, but for all practical purposes, there should be no significant difference. If your Spirit's tail area and or tail moment arm were too small before (and I've heard from other sources that in the case of the Spirit the moment arm in particular is), then they will still be too small with an equivalent V-tail.

The hands-off behavior depends on a lot of things, tail moment arm being a major one. A plane in a thermal turn needs more lift coefficient at the wingtip on the inside of the turn, because of its lower airspeed. In a typical RES (no ailerons) airplane, it does this by yawing to the outside of the turn a little, which interacts with the wing dihedral to increase the angle of attack at the inside wingtip.

Because the plane is flying in a banked circle, the airflow past the fuselage is curved in both the pitch and yaw sense. If the airflow is aligned with the fuselage at the wing, then the "relative wind" at the tail is blowing upwards and inwards towards the inside of the turn. This tends to pitch the nose down (requiring some up elevator to compensate, in addition to the "up" needed to provide the extra overall lift coefficient needed in the turn), and yaw the plane towards the outside of the circle. Increasing the tail moment arm increases this effect.

If the wing dihedral and the tail moment arm are just right, the yaw due to the curvature of the airflow in the turn is just enough to provide the extra angle of attack needed at the inside wingtip, and the plane will hold a perfect thermal turn all by itself, without needing rudder trim.
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Old Mar 09, 2014, 04:41 AM
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Thanks a lot Don for extra info!!!
Best, Boris
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Old Mar 10, 2014, 02:52 PM
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Something like the Essence shown here:
(scroll down to find the V pictuer on the box).
http://www.rcgroups.com/forums/showt...ghlight=spirit

I have a V-tail modified Spirit that took the original H-stab, bent it to 90* included angle (so 45* per side), and extended each surface by 3 or 4 inches (7.5-10cm).

That made for high-aspect ratio ruddervators, and I liked it a lot. One thing to watch-- which ought to be obvious, but I messed it up becasue I wasn't paying attention-- You can't just glue the V on to the slab sides of the fuse like you do with the regular horizontal because the fuse taper wil end up giving the V a bunch of negative incidence you don't wan't. Finish the top of the fuse flat and to the proper incidence angle first. I'll post a photo if I can dig it up this evening.
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Old Mar 10, 2014, 05:08 PM
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Going with increased panel span like that increases the dCl/d-alpha (the slope of the lift coefficient vs angle of attack curve, i.e.: how sensitive the panel's lift is to changes in angle of attack). This makes it "stiffer" in terms of static stability, but reduces the angle of attack at which the panel stalls (however, it does not change the max lift coefficient, that stays the same). Therefore, max control authority stays the same.

The down side is that the torsion loads on the tail boom increase with increased span. However, on models the deciding factor on tail boom loads is usually the loads on landing, which in the case of a conventional tail is usually the loads form the stab snagging on ground obstructions. A V-tail tends to have significantly less trouble with that, even f the panel span is longer.
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Old Mar 10, 2014, 06:04 PM
Marc PUJOL
Joined Feb 2010
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Dear all
When I talk about efficiency, I'm talking dynamic. Not static.
The V tail will act as a tail that see the air with less angle. this means that if it has the capability to have equal force as a vertical tail with same projected area, this hapen at higher angle. Then it is less efficient for a define angle of slippage.

An airplane never fly straight. It always yaw, roll, pitch...

So if you vlook at the dynamic behaviour of your plane, slippage frequency and amortisation factor will be worse for V tail than for X tail for the same projected area.

I made several computations and measures on that topic to say that AVL or XFLR5 predicts good things.

The consequence of this dynamic better behaviour at equal projected surface of the X tail is in Circling ability.

I will not also talk about Cl when trying to circle. There is a flap that is quickly at high angle of flap deflection, and the other one nearly neutral. This conduct to bigger drag.

We must change our way of thinking. All our standard calculations are made without actions on sticks. But we act on them every 2 to 4 seconds. Anf this conducts the plane to fly "surprisingly"... And this has lots of consequences on the performances.

I recongnise very limited potential improvement in V tail on speed condition when the plane is going straight (so very limited time). But for thermaling, this is not realy the case and drawback still exist.

If you think, V is better, we should have Boing or Airbus planes with V. But they have X tails... And they fly straight, want to save weight, etc.

Once again, at the same efficiency, V and X tails have the same weight. And as soon as you have to act on the sticks or when the plane try to recover the straight flight (this means every time), V introduce more drag.

I do believe that there is more "fassion" behind V than real technical advantages.

Sorry, but even if you made some flying V machine, this do not means that other solutions would also had worked.
After several studies, trials and measurements, V is beatiful, and that's it (all?).
Marc
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Old Mar 10, 2014, 10:01 PM
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Marc, again, you are looking at only one parameter instead of the complete equation. Yes, the angle of attack due to a pure pitch or yaw disturbance is less, and the resulting lift coefficient is less (note also that the coefficient of induced drag is therefore also less), but it's acting on the entire area of the tail, not just the stab, or not just the fin+rudder. The total forces generated are the same.
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Old Mar 10, 2014, 10:36 PM
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The original Open Cirrus had an all flying V tail and won the 1967 German Nationals. The Austria and SHK also had all flying V tails. The standard Austria was one of the last wooden ships that was competitive against the then new fiberglass sailplanes. Schempp-Hirth changed the Cirrus for one reason, sales. The people writing checks for sailplanes wanted to see a standard tail.

Global Hawk flies just fine with a V tail. It has an advantage over airliners. No passengers waiting to board at the gate.

I like my V tail 2 meter ship well enough that I'm building a 100" version. Believe me, both have been through the ringer in XFLR5. For small control inputs, the drag difference is negligible and pretty much negated by the higher tail surface Reynolds Number, which reduces drag in all flight conditions.
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Old Mar 10, 2014, 10:43 PM
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I just found that v-tails are harder to transport, whereas a x-tail usually comes apart and packs in a transport box much more easily, that's the only reason I got rid of my v-tail models
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Old Mar 10, 2014, 11:07 PM
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I find them easier to transport, something I became quite grateful for when I was transporting a car full of display models to our booth at the Toledo show every year. A stack of V-tailed fuselages will "nest" together quite nicely, and well as snuggle down into the bottom corner of a car seat, or into the corner of a box. Unless you go with the extra weight of making them removable, a conventional tail "does not play nearly as well with others."
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