jcstalls

My Feedback: (7)

Greatings, I am new to this board, and if there is anything that I can do to assist anyone, please let me know.

I am finnaly getting my hands on a Sagitta XC kit. I am fully aware that this plane is no longer competitive in its class.
I plan on taking about a year, or so, to complete it.

I have read some post on mods and agree with them.
I am also open to all the wisdom and experience of those who have built and flown this ship.

So far I plan on adding 6" to the tail. Increasing the stab size and beefing up the bearings.
I also plan on installing different spoilers, ya know the ones that only work when you want them to.

Thanks guys, Jared (Tip) Stalls

Ollie

It is imperative that your stab redesign have the pivot line pass through the aerodynamic center of the stab to avoid flutter or blow back problems. The aerodynamic center is located at 25% of the mean areodynamic chord for a single taper planform. The cut out at the root to allow rudder deflection complicates matters by making it a double taper planform with the 25% chord line swept at two different angles. The aerodynamic center of any planform is at 25% of the chord that passes through the centroid of the area. You can find the centroid by making a scale model of the planform out of cardboard or any stock of uniform density and finding the point where the cardboard balances.

When flying such a large plane near the limits of vision, it is very hard to judge the airspeed which can build up to dangerous levels. By the time you realize it may be going too fast, a gust or abrupt control input could blow the wings or stab off. For this reason it might be a good idea to go to the next larger size joiner rods for the wing and tail. Half inch diameter carbon joiner rods for the wing and 3/16 carbon joiner rods for the stab are not unreasonable. Then the sparcaps on the wing root panels become the weakest link and can be reinforced with .056 thick carbon.

With provisions for several pounds of ballast, for days with very strong lift, you will have a plane that is more competitive. The E205 airfoil has good high speed performance.

jcstalls

My Feedback: (7)

Thanks Ollie, good ideas.
For the spar caps I will do a progressive CF tow lamination, and a wrap of kevlar thread.
Jared

Ollie

The wet layup of carbon tow is good for a compression strength of 50,000 to 75,000 PSI depending on how straight you can get the fibers and how dense you can pack the fibers ( 10 to 15 times stronger than spruce). The prepreg based, precured strips from CST or ACP are 3.6 to 5.5 times as strong as wet, laidup tow.

While you are at it, you may want to fully sheet the root wing panels for additional torsional rigidity.

jcstalls

My Feedback: (7)

I will look into the precured strips. I will make a test run of both methods and see which one fails first.
When I used the CF tow wet years back on my Pulsar, I made a jig for compresing the tow. I believe I started with one full lenght piece and continued inward subtracting 6". This was about 12 years or so back, the wings flexed on zoom launch, but never failed.
Fulling sheeting the inboard panels is a great idea.
Thanks, Jared

Ollie

You will save material and have the strength of the carbon more closely track the bending load if you start with a full length and make each suceeding piece half the length of its former piece. You can stop halving the remaining piece when its length is shorter than the root joiner tube. The total length of carbon used will be twice the overall length of the spar for each spar cap.

Since the carbon is anywhere from 10 to 55 times as strong as spruce, you can leave out the spruce and save weight. The long thin caps of carbon will fail in buckling before they fail in compression. To prevent buckling, use thick end grain balsa between the caps and wrap the whole spar assembly in kevlar tow and laminating epoxy.

If this modification process goes on unchecked, there will be little of the original design and material used in the final model.

PeterH

I helped an acquaintance test-fly his gorgeous brand-new XC one day, short flight wings-level shallow approach. When we walked over to recover the aircraft after landing, we saw that the fuselage sides split away from the bottom forward of the towhook.

My impression was that the materials did not keep up with the growth in mass of the airplane - - the fuselage was 1/8 ply and there was no gluing area to the joints in front, and the triangle stock's grain was not used to triangulate the joints, just run fore-aft. All that wood split in the middle.

I was asked to 'repair' the fuselage, which I did by obtaining a Bob Sealy fiberglass fuselage and making the swap. Much longer tailboom, very nice handling, no more explosions on ground contact. Not much work if it's done first, as compared to building up a wood fuselage only to see it fail on its first landing.

I also swapped out the dial-cord spoiler pulls with a servo in each wing for spoiler, a much easier and far less fiddly thing to do when assembling the airplane for flight.

Ollie

All great suggestions, Peter.

I can't believe the builder of the wood fuselage didn't glass the inside and outside of the fuselage forward of the wing trailing edge.

If the fuselage hadn't failed on the first landing, the tow hook would have been ripped out on the first winch launch.

This just underscores the proposition that the Sagitta XC isn't a good project for the inexperienced.

jcstalls

My Feedback: (7)

Thank for the input.
Sorry, I have been just too busy.

Ollie, with the 1/2" CF joiner, natualy I will have to replace the spars and trim the ribs a little.

I also do plan on glassing the fuse with some added cf at the boom. I will also be glassing the inner fuse.

I will keep an eye on the weight but I think a wing loading of around 14oz. will be just fine.

Jared

jlkirst

I had an XC for several years before it was ripped off out of storage. It was built be a friend, Tony Palethorpe, back in the 80's. I flew it a reasonable amount. Fun airplane.

Re. bigger stabs: I have seen other Sag Xc's blow off half a stab and fly fine, so bigger may not be neccessary. Definately recommend increasing the stab thickness. It is way too thin and you can actually stall the tail if you are out screwing around (remember, they fly well, but they can't be flown like a two meter beater). I was planning on a stab with a thickness in the 9-10 % range, not quite fully sysmetrical (better response at zero angle of attack).

Also reinforce the fuse at the leading edge of the wing (prevents dorking damage of a ten pound airframe).

Also, get yourself a thermal snifffer or the like, they really make the thing a kick to fly

Hope this helps, Have fun

Jim Kirstine