What V-tail incidence to use?

[irish_lord99]

So once again I come to bathe in the everlasting fountain of knowledge (mixed with much opinion :p ) that is RCGroups!

I'm in the process of building a v-tail trainer type plane from my own plans, and I'm not sure what to set the tail incidence at. I've scoured the internet and found many different articles on how to set the incidence, or what it is for a specific kit... but what I really want to know is: "how does the incidence of the V-tail affect the flight characteristics of the model?

I was going to just set it at 0 degrees, but I thought I'd ask around and see what folks on the forum thought.

Thank you very much,
~irish

[vintage1]

Set it the same as you would a normal tail. I.e about 2-4 degrees less than the main wing.

Use a CG calculator on the PROJECTED area of the tail to get the CG in the right place.

[irish_lord99]

So by "projected" do you mean, how the tail appears in a side and/or top view? Not the actual area of the tail, right? Or do I have that backwards?

[Brandano]

The plan view to place the CG for pitch stability. Twice the side view to place the CG for yaw stability. Also remember that the incidence you are looking at is the side view projected incidence

[eflightray]

Here's quite a good CG calculator that include the tail area - All area CG calculator (http://www.geistware.com/rcmodeling/cg_super_calc.htm)

and here's a more simple one using just the wing area - Wing area CG calculator (http://www.geistware.com/rcmodeling/cg_calc.htm)

[vintage1]

It is not posible to determine the CG without taking the tail into account.

[MCarlton]

how does the incidence of the V-tail affect the flight characteristics of the model?

In the same way that the incidence of a conventional tail does, but with the proviso that I would initially set the tailplane incidence at 0 degrees in relation to the fuselage datum and then work the wing incidence to be 3-4 degrees greater than that.

A V-Tail isn't much different to any other kind in the way it works, it just takes a little more care to arrange.

Be aware that in your trainer application, a V-Tail model can sometimes suffer a touch from "tail wagging" and especially if you are planning a high wing "cabin" configuration, the yaw stability and response isn't always as effective as a conventional "cross" tail configuration.

[irish_lord99]

Wow, thank you all very much for the help and input! I think I have enough info to get where I want to go. :D Thanks also for the COG calculator, that'll come in real handy.

~jake

[macboffin]

This is a bit late, only just came across the previous posts ; the angles of the two elements are important ; too "flat" and you get the "Bonanza syndrome" ie a bit of tail wagging sideways at higher speeds . I have flown many Vee tailed aircraft,(some being big UAVs) and have found that an included angle of from 90 to 100 degrees works best.

[MCarlton]

I'm not disagreeing, but in photos 2, 3 and 5, the V-Tail isn't providing all the horizontal stabiliser area and is acting more as a canted twin vertical stabiliser. I would go for something more than 90 degrees personally speaking, if you look at a lot of V-Tail models, especially sailplanes, the angle is between 100 and 110 degrees. Otherwise, the V-tail has to be oversize to provide the right amount of horzontal area.

You don't want too much vertical fin area, otherwise there will be an excessive tendancy to "weathercock" into whatever wind is blowing.

That will depend in part on the shape of the fuselage, different proportions of fuselage side area ahead of the CP and CG will need different amounts of fin area.

Matt

[Mike the Snake]

I've found that at 0/0 my planes tended to hunt, acting like double centering.

I'd start with 1 or 2 degrees positive decalage (up).

V-tails can be very difficult to get Just right.

I added a sub-vert (effectively making it a "Y" tail) to my present glider and WOW, waggle gone, and it tracks unbelievably well now.

[macboffin]

I'm not disagreeing, but in photos 2, 3 and 5, the V-Tail isn't providing all the horizontal stabiliser area and is acting more as a canted twin vertical stabiliser. I would go for something more than 90 degrees personally speaking, if you look at a lot of V-Tail models, especially sailplanes, the angle is between 100 and 110 degrees. Otherwise, the V-tail has to be oversize to provide the right amount of horzontal area.

You don't want too much vertical fin area, otherwise there will be an excessive tendancy to "weathercock" into whatever wind is blowing.

That will depend in part on the shape of the fuselage, different proportions of fuselage side area ahead of the CP and CG will need different amounts of fin area.

Matt If in pics2, 3, and 5 the vee tail is not providing all the horizantal area, where is the rest? As to "Weathercocking" this can only apply to an aircraft on the ground ; once airborne ,the only wind an aircraft knows is the one due to forward motion!

[irish_lord99]

If in pics2, 3, and 5 the vee tail is not providing all the horizantal area, where is the rest? As to "Weathercocking" this can only apply to an aircraft on the ground ; once airborne ,the only wind an aircraft knows is the one due to forward motion!


I would like to respectfully disagree with that. I have been in some real live planes where the cross winds have made landing pretty difficult, and I'm sure you've experienced the same with RC models if you've flown them in the wind. My piloting experience (with RC) has thus far been limited to non-windy days. :)

[Brandano]

But side wind doesn't cause trouble due to weathercocking, it just adds a sideways drift, and forces the plane to do a "crabbing" landing. There's plenty of videos of these on youtube, and yes, they make the landing and takeoff harder, but it isn't the wind turning the plane. Wind gusts now, those are another matter entirely.

[irish_lord99]

Ah... gotcha, thanks for clearing that up. :)

[Sail 'n Soar]

Set it the same as you would a normal tail. I.e about 2-4 degrees less than the main wing.

Use a CG calculator on the PROJECTED area of the tail to get the CG in the right place.

Using the projected area when calculating the correct CG location is a most common error. The change in CL as a function of a change in aircraft angle of attack of the stab is greatly reduced from that of a conventional stab configuration because of the large dihedral angle of the V-tail surfaces. When designing a V-tail group, the total V-tail surface area should be equal to the sum of the vertical and horizontal stab areas of the equivalent conventional tail configuration.

Rather than using the projected area, which is equivalent to using the total V-tail area times the cosine of the V-tail half angle, you should use the total V-tail area times the cosine squared of the V-tail half angle to calculate the equivalent horizontal stab area and the total V-tail area times the sine squared of the V-tail half angle to calculate the equivalent vertical stab area. For example, Ah equivalent = Astab*cos(110/2)^2 for a 110 degree included angle V-tail) and Av equivalent = Astab*(110/2)^2.

Consequently, V-tail designs with ~90 degree included angles have either high aspect ratio wings, excessive equivalent vertical area, or insufficient horizontal area - often compensated for by moving the CG forward.

Go through the math yourself.

[Sail 'n Soar]

Using the projected area when calculating the correct CG location is a most common error. The change in CL as a function of a change in aircraft angle of attack of the stab is greatly reduced from that of a conventional stab configuration because of the large dihedral angle of the V-tail surfaces. When designing a V-tail group, the total V-tail surface area should be equal to the sum of the vertical and horizontal stab areas of the equivalent conventional tail configuration.

Rather than using the projected area, which is equivalent to using the total V-tail area times the cosine of the V-tail half angle, you should use the total V-tail area times the cosine squared of the V-tail half angle to calculate the equivalent horizontal stab area and the total V-tail area times the sine squared of the V-tail half angle to calculate the equivalent vertical stab area. For example, Ah equivalent = Astab*cos(110/2)^2 for a 110 degree included angle V-tail) and Av equivalent = Astab*(110/2)^2.

Consequently, V-tail designs with ~90 degree included angles have either high aspect ratio wings, excessive equivalent vertical area, or insufficient horizontal area - often compensated for by moving the CG forward.

Go through the math yourself.

Corrected equation (left out the sin().
Av equivalent = Astab*sin(110/2)^2

[Sail 'n Soar]

Got ahead of myself and reversed the trig functions. The correct relationships are:

Av = Astab*cos(theta/2)^2
Ah = Astab*sin(theta/2)^2

Where Astab = total area of the two V-tail surfaces
Av = vertical stab area for the equivalent conventional tail
Ah = horizontal stab area for the equivalent conventional tail
theta = the included V-tail angle.

Use Ah in your CG location calculation.

[Mike the Snake]

The Opus is popular for DS. It has a V-tail (there's a ew T-tail version as well).

The MC3 V-tail version has a definitite "waggle" that the T-tail version has much less of (I don't think one can ever actually have NO waggle).

I added a 3rd fin, a sub-vert made from an old crashed Opus tail.

This mod has completely transformed how my MCV flies! It tracks better, and waggles less than the T-tail version now.

The sub-vert also helps keep the flaps from being damaged in "flaps down" landings.