V-tailed Legend

[ejett]

Several years ago I saw in one of the model mags (probably MAN) a photo of a V-tailed Airtronics Legend. I was intrigued at the time, but now I have 2 T-tailed Legends and a new unstarted kit.

If anyone has successfully done a conversion like this, I would appreciate your advice on how to go about it. I would be interested in trying something different on this kit.

Regards,
EJ

[Ollie]

Add the area of the vertical tail (fin plus rudder) to the area of the horizontal tail. This total should be the area of the new V-tail. The included angle between the v-tail panels should be about 105 degrees. The aspect ratio of the tail should be about four. Pick a planform that is pleasing to the eye and design a structure within that planform. Put the hinge line at 75% of the chord of the V-tail. I would let the hinge line of the V-tail extend about 1/2 inch aft of the aft end of the fuselage molding. I would make the control horns of 1/32 sheet aluminum and angle them so the linkage falls directly aft of the tail boom. This will allow the pushrods to exit the aft end of the tail boom in a straight shot. Deside if you want a fixed or removable tail. Design the tail mount accordingly. Cut away all but about the bottom inch of the fin. The cut line should be parallel to the centerline of the tail boom.

You should be able to save about two ounces in the aft end and about six ounces of lead in the nose.

While I have never owned a Legend, I have studdied its plans and I have designed and built many successful V-tailed models.

[schrederman]

My good friend Bill Maserang of Ft Worth bought a pair of V-tails from Fred Sage and he flys his Legend-V very well. He cut the tail off even with the top of the fuse and mounted a piece of plywood in there to mount the tails on. It realy came out OK. There may be pictures of it on the SLNT web page. (Soaring League of North Texas)

How's the stretched Legend flying?

Jack Womack

[ejett]

Ollie:

Thanks for your input. I am filing your info away for the project.

Jack:

The stretched Legend is a fine flier; better than I am for sure. Thanks for the tip. Bill's Legend looks nice with that V-tail.

Regards,
Ed

[doppler]

To convert a conventional, or T-tail, into a V-tail configuration, you can't just add the vertical and horizontal areas of the stabilizers and devide that by 2, as Ollie suggests. Well, actually you can, but this will result in tail surfaces that are bigger than they need to be.

The whole point of a V-tail is that the tail is doing "double duty", and can therefore be SMALLER in total area than a conventional tail.

Here's how you do that: (Everyone remember their trigonometry?)

The PROJECTED horizontal area of the V-tail must equal the actual horizontal area of your conventional tail. This must also be true for the vertical stab.

First, you have to solve for the angle between the V tails. This usually comes out between 100-110 degrees on most sailplanes.

Let's call the angle between the V-tails THETA.

THETA = 2*(180-ARCTAN(X/2Y))

Where X = 1/2 the area of the Horizontal stab+elevator, and Y = the total area of the vertical stab+rudder.

I could walk you through the derivation of this, but it'd just put you all to sleep. :)

Now that we have the proper V angle determined, we can size the area of each V surface.

V = X/(cos((180-THETA)/2))

Next, multiply this value by 110%. This provides some conservatism in your tail area, while still reducing the overall tail size from a conventional arrangement. Also, there is a little interference drag that musses with the effectivness near the crux of the V. You should see about a 20-30% reduction in total tail area with this method. Very nice for thos competition ships where every last bit counts.

Everything else that Olie suggest I agree with whole heartedly!

Hope this helps.

Eric

[ICTHRMLS]

Originally posted by schrederman
My good friend Bill Maserang of Ft Worth bought a pair of V-tails from Fred Sage and he flys his Legend-V very well.
When he keeps it out of the power lines....;)

[ejett]

Thanks everyone for your input. I guess the issue I still need some suggestions with is how to modify the tail structure to attach the V-tails. Bill's plane looks very nicely done, at least in the photo, but the photo does not really provide any insight into the structure and attachment.

If anyone has any photos or sketches of how to mount the tails to the fuse, I would appreciate seeing how others have done it. The photos do not have to be specifically of a Legend, but I would appreciate some detail on just how to make this important structural connection. This would be my first V-tail construction so I do not have any previous experience to draw from.

Regards,
Ed Jett

[Ollie]

You need to make some basic choices.

Can you accept a fixed tail mount (the lightest and easiest to build option). A one piece removable tail is the next lightest and easiest. A two piece, removable tail is heaviest and hardest to build.

You need to deside the basic structure of the tail panels for mounting design to proceed. Will the tail be open framework? Will it have a flat plate airfoil?

What construction techniques are you comfortable with? Do you only do razor blade carpentry in balsa? What experience do you have with composites? What kinds of epoxy, fiberglass and carbon fiber have you worked with?

What did you have in mind?

Eric's method of sizing a V-tail will work nicely. It will result in about a 2% reduction in overall drag. You get this improvement in efficiency by trading off a little yaw control authority and by reducing the pitch and yaw damping a bit. With the smaller tail area, the CG has to be a bit more forward for the same stability. The choice boils down to a small trade off between improved efficiency and improved handling.

[ejett]

Hi Ollie:
I would be leaning to making a fixed tail installation. As far as construction is concerned, I am pretty good at bashing balsa and have done some sheeted foam, but I do not have a set up for bagging.

What I had thought about was to go with sheeted foam tails with an appropriate airfoil shape. I am somewhat tempted to buy some bagged tails from somebody in the business.

Regards,
Ed Jett

[Ollie]

With a foam core and wood or fiber glass skins, here is the proceedure:
1. Bevel the roots of the panels so that the panels meet at the desire angle.
2. Hold the two panels bottom to botton so the edges match at the tips and along the edges. Check that the beveled edges match too so that the joint will not be skewed to the centerline and the panels will have matching incidence when joined.
3. Tape the bottom of the joint with masking tape. Open the joint and apply epoxy. Close the joint and wipe off any excess epoxy. Run a piece of masking tape from tip to tip to keep the joint closed. Check to see that the roots are perfectly aligned before the epoxy starts to set up.
4. When the joint has cured, run a piece of masking tape completely around each panel about an inch from the joint.
5. Cut three strips of 1.5 ounce, light fiberglass cloth on the bias. All the strips should be long enough to go all the way around the joint. The widths should be about one inch, 1-1/2 inches and two inches.
6. Apply the strips to the joint starting with the narrowest width and soak the fiberglass with laminating epoxy. Blot off the excess epoxy with toilet paper and allow the joint reinforcement to cure.
7. Shape the fiber glass edges where you cut off the fin so that the edges match the bottom contour of the V- tail. The tail incidence is set by this contour so it is important to take into accout the centerline of the tail airfoil ans shape the edges so that the incidence is zero relative to the center line of the tail boom. Let the hinge line fall about a half inch aft of the end of the fuselage.
8. Mount the wing center section on the fuselage. Draw a center line down the length of the work bench. Draw a perpendicular line at one end. The perpendicular line should be longer than the span of the tail assembly.
9. Place the fuselage upside down and align the center line of the fuselage with the centerline on the work bench. Place the V- tail assembly under the fuselage so that it seats in the saddle edges you have prepare in the fuselage. If the wing lifts off the workbench, block it so that the span remains parralel to the bench top. Slide the fuselage along the centerline until the trailing edge tips of the V-tail assembly fall over the perpendicular line.
10. Make sure everything is centered and aligned and block all the parts in place with bricks, lead pigs, paint cans or whatever. Double check that the V-tail is seated to result in the proper incidence.
11. Cut six more strips of light fiberglass cloth the length of the tail's engagement with the fuselage.
12. Epoxy three strips, each side, onto the bottom of the V-tail assembly and the fuselage, with half the width of each strip on each side of the joints. Blot off any excess epoxy.
13. When the joint is cured, remove the masking tape and sand off any rough spots on the fiberglass strips.
14. If there is a gap in the fuselage in front of the V-tail, fill it in with a soft balsa or foam block. Sand it to a pleasing contour and cover it with a couple of layers of light glass in epoxy.

That's all there is to it. Just 14 easy steps.

[markdrela]

Originally posted by doppler
The PROJECTED horizontal area of the V-tail must equal the actual horizontal area of your conventional tail. This must also be true for the vertical stab.


The projected area method is not correct. It will give an undersized V-tail. As you deduced, you get a projection factor from the tilt of the V-tail force vector. But you get another (same!) factor from the fact that the V-tail airfoils see a reduced airplane angle of attack or sideslip because of their tilt. So the V-tail's stabilizing power in each direction is doubly reduced by the projection factor in that direction.

When you do the math, the required total V-tail area turns out to be about the same as the equivalent conventional tail area. The actual conversion equations are here:

http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm

[ejett]

Dr. Drela:

Thanks for your reply. I appreciate everyone's input on this project.

Olllie:

Your 14 steps are quite detailed and easy to understand. Thanks to you as well for taking the time and effort to outline this construction.

To All:

I don't have a firm schedule regarding making this modification, but I will submit photos and notes when I get it done.

Regards,
Ed Jett

[aeajr]

Old Thread but you might be able to provide some help.

I still have the Airtronics Legend but it has never flown as well as I think it can. I am trying to reset the wing incidence but don't know what the recommended spec would be. I am not at 1.5 degrees.

If I set the stab at 0 degrees on my GP incidence meter, the wing measures 1.5 degrees below the zero. Any recommendations?

It used to be more but I added 1/32 in shims under the rear wing hold down to raise the back of the wing about 6 months ago and it seemed to fly better. So I bought the incidence meter and want to set it correctly.

But I have no idea if I am right on the money or way off.

Any advice would be appreciated.

[ejett]

Interesting how things come back around. I took the red Legend that Jack Womack built to Woodcrafters again this year and it flew very well. My son took 2nd place in the 3 min ladder event which was a 4 way tie, but the tie breaker was the longest flight and he had the 2nd longest of the event. And he wasn't trying for longest flight even though I told him while he was up that was what he should do. Oh well, it flew great until a mishap Saturday with a winch line. In a testiment to how strong this plane was, the right wingtip panel was the only part of the plane to sustain any structural damage. The wingtip structure has been repaired and awaits my urge to recover it.

But, my son said something today about he would like to try to build a V-tailed Legend and I told him that I had a set of bagged tails I bought back when this thread was young for just this project. Maybe with him egging me on, we will actually embark on this project in the near future.

When I pulled the tails out and showed them to him, his words were "This is happening."

:D

EJ

[aeajr]

I envy your son's involvement. How old is he?

[ejett]

I envy your son's involvement. How old is he?

Old enough to know better.

EJ

[bobby legue]

Good answer!

[jett_av8r]

Yeah, i'm old enough that its more than a mere pasttime or "hobby"....more like an obsession, lol..... CJ

(BTW, I'm the aforementioned son, lol....)

[ejett]

Well, the program is underway. A few days ago, I came home from work to find the fuse of my first Legend in the workmate. With no tail. It had been run through the bandsaw, cutting the vertical tail off flush with the top of the tailboom.

We have spot glued the tails on the fuse. After the epoxy cures up, we will double check alignment then apply the fiberglass cloth reinforcement, fill in the gap in front of the V-tail and shape it.

BTW, the tails were some bagged CF fabric, kevlar, foam that I had purchased from some one that offered them to me back when this thread was started. So, instead of going through a fancy calculation, I decided to just use them. They did Look About Right.

Will post again when we are done and provide a picture of the finished product.

EJ
Athens, LA