Spring-V full-flying stab mount - RC Groups
Thread Tools
Jun 29, 2009, 11:13 PM
Registered User

Spring-V full-flying stab mount

Hello everyone,

As promised, here is a thread dedicated to the Spring-V mount. The purpose of the mount is to provide a very light, relatively low drag, easy to make and install, tool-less full-flying horizontal tail mount. This is one of those designs that will make you go DOH! when you see it.

In the first installment(s) in this thread, I'll post pictures of the mount showing the various features, and showing how the stab is attached to the fuse.

I'll provide text descriptions of how the tools to make the mount are produced, how the mount stock is made, and how the mounts are formed from the stock. I'll give directions for installation to a stab.

But I'm not going to be able to do the builds along with you all; I simply haven't the time right now as I want to get Zone V2 out to everyone ASAP. So I've asked for volunteers from the community to do the actual work of making these things and showing how it is done. Some volunteers have stepped forwards... I'll provide advice from my testing and experience. The process is simple and does not take all that much time. So I encourage anyone interested to build along and post whatever you come up with!

Enjoy, and thanks to the volunteers!

Gerald Taylor

PS - Thanks to my friend Alan Anderson for taking the pictures for me!
Last edited by G_T; Jun 29, 2009 at 11:42 PM.
Sign up now
to remove ads between posts
Jun 29, 2009, 11:31 PM
Registered User
First round of pics, showing the results.

Jun 29, 2009, 11:41 PM
Registered User
It's getting late and there are other things I have to do tonight, so I'll start posting how this is made tomorrow. But for the really lazy and cheap method of making the form, you'll need a strip of this stuff...

Jun 30, 2009, 01:11 AM
Registered User
soarluck's Avatar
very nice work ,,duh! is rite ,,its to simple ,,thks SL
Jun 30, 2009, 07:28 AM
But I am learning
I_Can't_Fly's Avatar
Thanks Gerald, you were right about the simplicity of the build. I would assume all that is needed to prep the LE stock is some packing tape and wax. I am interested to see how to align the two pivot points. Great work, and even more props for sharing with the community.

Keep it up, Brian P.
Jun 30, 2009, 07:33 AM
Red Merle ALES VI
Curtis Suter's Avatar
That's great Gerald!

I can't wait to see the rest.

Jun 30, 2009, 09:36 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar

I have already made up the tooling. How many layers of what material are you using? It looks like maybe 2 layers of carbon "stretched" so that the fibers are running closer to perpendicular to the boom? +/-25 degree tape maybe? Also, would it be beneficial to bag with peel ply to try to drive lower resin content, or would you consider that obsessive overkill?

Tom Siler
Columbus, OH
Last edited by tom43004; Jun 30, 2009 at 09:47 AM.
Jun 30, 2009, 10:27 AM
But I am learning
I_Can't_Fly's Avatar
In some PM communication between Gerald and myself last week, this might help.

Originally Posted by G_T
The layup is 3 layers maximally sheared 1K carbon, then 2 layers of 1.7oz Kevlar in normal orientation, then 3 more layers of maximally sheared 1K carbon. The area around the pivot holes gets an additional layer of 1.7oz Kevlar in normal orientation, and an additional layer of 1K carbon in normal orientation. On my next layup test I plan to replace the inner two layers of 1.7oz Kevlar with two layers of normal orientation 1K carbon. I suppose I could even use glass but the carbon has a lower density. Also I may add some additional Kevlar reinforcing around the pivot hole, but I haven't decided that yet.

Jun 30, 2009, 11:37 AM
Registered User
Hello everyone, next installment time!

Required and desired tools:

The leading edge stock is used to make the form for the V stock. The location of the holes that are drilled into each V are basically at the flat edge of the stock. That sets the height and makes a gauge for the job. So keep a section of the stock for that. On that piece of stock. take care to make the end perpendicular, so it can be a guide for drilling the holes so they line up nicely. If they are off a tiny bit it won't matter because of how installation lines everything up, but get as close as you can! Otherwise it will look unprofessional.

Another piece of the stock can be used to make a convenient cutting jig. When I cut Vs from the Spring-V stock material, I use either a dremel with a cutoff disk (annoying but works) or a bandsaw (works better but hard on the blade). If one wants to use a bandsaw, then a jig could come in handy. Take a section of the leading edge stock, and glue it to an upside-down L-shaped support block, so it is held parallel to the bandsaw table but V stock can slip over it. The leading edge stock must be raised a touch for the stock to clear on the bottom, and just be suspended out a bit, because the stock goes past the leading edge. Now this jig is easy to use with a mitre guide on the table of the bandsaw. Just set one angle and cut, then set the second angle and cut - out comes a V. It makes it easy to create reproducible parts. I've done it a few times, freehand, once at Phil's, and believe me you want the cutting jig! But it is not a necessity.

The form for making the Spring-V stock is more than just a strip of the leading edge material. The location for the holes to be drilled is at the flat on the LE stock, so it is necessary that the form go past that. An easy way to do it is to take two pieces of the LE stock and glue them back-to-back, then gently dress up the joint so it disappears. That's what I did on my form. One could also use a block of balsa glued to the back of the LE stock and cut/sand flush. That might actually be superior but either will work. You get about one Spring-V mount per inch of stock so make it a length for however many you want to make at a time. But for various reasons I'd recommend no more than 6 or 7 inches of stock the first time you do this! You'll see why when you try it.

Take the Spring-V form and cover it with clear box tape. The stronger, stickier stuff is better. Wide is necessary. One wide strip, centered on the peak of the V, works well. You don't want any seams in the area of the stock that will be kept. Seams are ok in areas that will be trimmed off. Cover any exposed wood otherwise you will likely have to scrap the first molding attempt! Wax well. Or I suppose one could use partall but I haven't done that for this. It might be better if it doesn't cause any reaction with the tape.

One needs a piece of mylar, waxed of course! It should be rectangular, long enough to nearly cover the V form, and wide enough to reach most of the way to the back of the form. But not all the way!

One needs a small vacuum bag and breather. Doing this work is going to wrinkle up your bag so just make up a small one for molding work. Keep your wing and tail bags nice!

Two rubber bands are also needed.

I think that's it for the special tools!

Jun 30, 2009, 12:19 PM
Registered User
Dimensions, dimensions... And other considerations.

Full-flying stabs have characteristics that are slightly different than having a fixed stabilizer and a movable elevator. This needs to be taken into account for best results.

The pitch range for a full flying stab is less than the pitch range for an elevator to generate the same shift in zero lift angle. So on average, the servo horn should be shorter and/or the elevator horn should be longer. Also from my experience expect to put 10% expo on the elevator to make it feel like what you are used to... at least when starting out.

In my case, I use the JR DS285 for elevator control on DLGs. It is the smallest, lightest servo I've found that responds to single trim clicks when the trim step is set to its highest resolution. Most if not all smaller servos respond only to every second or third click of trim. This makes it impossible to do fine trimming of airspeed. And fine trimming of airspeed is worth a lot, particularly for min sink settings where the airspeed range for best results is only a couple tenths of a meter per second wide. That is much finer than most realize... If your servo can't do fine steps, and do them reproducibly, then it is pot luck whether you really get a good trim or not.

On the Zone V1 prototype, the servo horn pivot to hole distance is 3/16". That is about as small as it can be done! For the stabilator horn, I use one of Will Newton's excellent horns, made as long as I can get it. Measured along the surface of the horn (which is tilted) I have the hole 3/4" below the bottom of the stabilator.

Spring-V dimensions: Base of mount is 7/8" wide. Mount at location of holes is 3/8" wide. I have the centerline of the mount swept back slightly as it gets the overall mass of the mount slightly forward but really this is probably too small a difference to detect. I do it anyway.

The pivot holes on the Spring-V end up 3/8" above the boom once installed. So the bottom of the stab ends up ~5/16" or less. This is fairly low for a V mount and helps minimize boom torsion. It is made possible by having the horn outside the V rather than inside.

The pivot pins are .030" or .032" K&S music wire stock. This is bent into small C-shaped pieces. I basically use the end of needle nose pliers as my bending guide. You can see the length from the pictures... It doesn't need to be quite this long! Leave the hidden side of the C a little long for now. It will get cut flush later. Just make ABSOLUTELY SURE that the part which will insert into the Spring-V is rounded out well. Sometimes cutting flares the metal. If this is not removed, then the hole in the Spring-V must be larger than the wire, and this introduces slop and premature wear.

The stabilator mount pieces which receive the C-pins are 1/2" long by 3/8" high by about 1/32" thick laminated pieces of carbon. I just glued some scrap together to make these. Just make sure they are beefy enough to anchor the C-pins. Likely even plywood would work here, but not light ply.

The Zone V1 prototype wing has an AR (Aspect Ratio) of 10.5, and my stabilator is sized accordingly. If you use a higher aspect ratio wing, then you can shorten the span of the stabilator by about a cm, and reduce the root chord by a mm or two. But not necessary...

The cores for my stabilator are Zone-Horizontal, of course Each core is 7" long (call it 18cm), has a root chord of 3 1/4" (call it 8.3 cm) and a tip chord of 1.65" (call it 4.2 cm). The sweep is set so that the 25% MAC is straight across the stab. Final trim span is 13.6" (34.5 cm). You can see the profile in the Zone-Horizontal thread if you are interested. For this stab, the pivot pin location ends up being 7/8" (2.2 cm) back from the leading edge. Phil and I used the Supergee-II layup using 1oz Kevlar, and added an oval reinforcing patch of glass to the bottom skin only. We used Spyder for the cores. They are so thin there is basically no weight penalty. I'm just providing this stab info because this is an efficient, well-proven design. It should remove some guesswork.

Whatever stab design you use, the pivot pin location MUST BE slightly behind the 25% MAC (Mean Aerodynamic Chord). This is counter-intuitive. But if this is not done then the stabilator might flutter at high speeds and this would be BAD!

Jun 30, 2009, 12:28 PM
Registered User
Adjusting the shape of the leading edge stock for a better boom fit...

From what I've seen, generally the stock will result in a radius that is a little small for best fit to the boom. THIS IS WHAT YOU WANT!

Think of the Spring-V as a nutcracker, and the boom is the nut... Get the picture? So to ensure the boom survives, it must be stiffened. The easiest way to do this is to leave the "point" of the V a touch too "pointy". Then when the Spring-V is glued to the boom, the very small gap on the underside is filled with glue. This makes the boom in that area tremendously stiffer, so the upper part of the boom which is unsupported does not see much deformation when the V is squeezed.

Jun 30, 2009, 12:39 PM
Registered User
Layup and bagging:

The Spring-V layup that I've used to date is listed in post 9.

When doing the layup, take care to get good wetout. Don't try to make your first parts dry. A dry layup here would likely result in failure. The weight difference won't be enough to matter. The Spring-V mount is under a gram anyway. You'll be hard pressed to make a lighter mount.

Just wet out the fabric well, and blot out any obvious excess epoxy. You could perhaps put it into the bag for a couple of min with paper towels to dry it a little but don't overdo it. At least not on the first attempt!

Take the mylar with the fabric, and wrap it around the form. Secure with rubber bands on the ends. Pop it into the bag, with a breather draped over it. Hold the bag down around the form so as to push the mylar down onto the form, and apply vacuum. If you don't hold it down as vacuum is building, it might push the mylar away from the form which is not good. Ask me how I know I tend to use a fairly high vacuum when making these, higher than I'd use on foam core wings.

Jun 30, 2009, 02:05 PM
Registered User
BTW, feel free to experiment with different layups! What you want is a moderately stiff spring that can be bent the required distance, plus a little, without breaking. When making the bend test, use a section of boom or dowel to prevent failure in the "point" of the V where stresses would be concentrated. The V will fail at a much lower stress if not supported some in the middle. If you make a heavier layup, then the V can be shorter along the boom and at the top. A lighter layup could be made longer to compensate. So there is certainly some room to experiment! But if you go too heavy it will break rather than bend.


Get the V fabricated and the holes drilled for a precise fit to the C-pins. Go ahead and install the C-pins in their mounting blocks. Do this by drilling a pair of holes for the pins in each block, and inserting a C-pin. Epoxy the pin in place. Then flush cut the protruding extra part of the pin on the side that goes into the stab. Blend a curve on the part that will stick below the stab. Put a slight curve on the other side if necessary to match the curve of the airfoil. In any event remove the excess material there down to the surface of the pin. That part will butt against the upper skin of the stabilator.

With fine sandpaper, remove the gloss on the inside of the Spring-V where it will mate to the boom. Clean well with alchohol. Take the Spring-V and put it on the boom, and stick a piece of .030" or .032" wire through the holes. Align parallel with the wing. Tack in place with a little thin or medium CA of high quality. You may find this easiest to do by placing the plane upside down on its wingtips and checking that the wire is parallel to the table. The weight of the wire will help hold the Spring-V in place for tacking. Remove wire and saturate joint with medium CA. Make sure the small gap below the boom is filled. Suggest not using kicker... Result will be stronger. Just be patient.

Put the rudder pushrod housing between the V and the boom, and glue it there with medium CA.

Use rubber reinforced CA (the black stuff used to mount tires for RC cars) to form a fillet on both sides of the V. This fillet MUST be capable of taking flexing without being brittle, which is why I use the rubber reinforced CA. Don't use kicker. Perhaps other adhesives will work well for the fillet - GOOP perhaps?

When that is all cured, measure the spacing between the outside edges of the Spring-V. This is the spacing to be used on the INSIDE of the mounts that go in the stabilator. The Spring-V is not under any spring tension when the stabilator is installed. Tension will result in stiction due to the friction, causing double centering.

Put some masking tape on the bottom of the stabilator and lay out the cuts for the C-pin mounts. Get the pivot point just behind the 25%MAC. This is very important!

Cut a slot for one of the mounts, including removing a little clearance for the pin itself so the skin is not distorted when the part is pushed into place. Check that it fits well, then use a dental pick or other small sharp instrument to carefully remove a little bit of foam (or balsa) between the skins around the mount location. You only need to remove a little bit. Mix up some epoxy with some milled fiberglass. You want to add milled fiberglass until the mixture creams. Don't use 5-min epoxy as it is not good for most applications. 15-min is fine, or certainly laminating resin is fine! Fill the slot with the splooge, and insert the C-pin mount. Get it set so that the pin is parallel to the bottom of the stab. Put a little fillet of splooge around the mount. Keep it little, or it will interfere with the Spring-V! Let that side cure.

Slip this first pin into the Spring-V, and verify the location marked for the second cutout. Better to check than to be sorry! Make the cut, but make it slightly longer than the C-pin mount so there is room to move it forwards or backwards a touch for alignment purposes.

Stick the C-pin mount onto the Spring-V, and stick the stabilator into place. Verify that the second C-pin mount is not too tall to allow the stab to mount parallel to the wing. Sand it down a touch if necessary.

Install the second C-pin mount. When the epoxy is not yet set, mount the stab on the Spring-V, and align the stab parallel with the wing and perpendicular to the tailboom. Keep the pin parallel to the bottom of the wing skin. Note you can't do that if the V is under compression! Let cure. Don't glue the C-pin mount to the Spring-V! A little wax on that will help prevent this.

Make the stabilator horn, and mark a slot to cut in the bottom skin so that the hole in the horn is aligned front-to-back with the pivot holes in the Spring-V. Cut the slot at an angle, but be careful not to cut the upper skin. Again enlarge the foam or balsa inside a touch to form a hard point. Make and install the stabilator pushrod with L-bend facing outwards away from the Spring-V. Put horn on the L, and insert into the stabilator. Check that everything works ok, and that the pushrod does not quite rub against the Spring-V. Glue the horn in place, filling the slot with epoxy or foam safe CA and making a small fillet.

The purpose of the hardpoint-and-fillet method is that the skin is sandwitched between these two so it has nowhere to go. It also greatly enlarges the area of the glue bonds. This helps prevent failure.

Round the front and back edges of the Spring-V and the stabilator horn to reduce their drag. Seal the edges of the Spring-V with thin CA. Let it wick into the Kevlar. Also do the same thing at the drilled holes, then re-drill.

Make sure everything moves fairly smoothly, and lightly sand as necessary with very fine sandpaper if needed.

I think that about does it! Unless I've forgotten anything, that is the whole process, probably in way more detail than you need. Have at it, and show us what you create!

Jun 30, 2009, 02:42 PM
But I am learning
I_Can't_Fly's Avatar
For thoes looking to determine the MAC of your horizintal, here is a quick link I use. Mac How Too

Good Luck Brian P.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Full flying stab mount j.m. Hand Launch 19 Sep 22, 2008 05:54 PM
Help! Full flying stab mount? LJH Electric Sailplanes 7 Jan 23, 2008 09:57 PM
full flying stab...in a T tail cosmo Thermal 1 Mar 20, 2003 01:49 PM