Sub-fin damage

[northwest]

Since I don't fly competitively I actually rearly "land" my sailplanes, catching them instead. Occasionally though I target the hi-start chute to within 5 or 10 ft. I fly a Thermal Dancer from polecataero.com (very excellent performer) where the fin is mounted on top of the boom. The plane may slide as much as 10ft after landing on cut grass. On a very low catch (or a botched catch) the tip of the boom may strike the ground. Now when this stuff happens with an Ava/Danny/Topaz what really happens to the fin? These sub-fins must really take a beating if your arrivals are not always precise. Actually I'm considering one of these said RES planes for when I don't feel like flying full-house and to give the spoiler a try, but those sub-fins are a real concern.
Any comments from the Ava/Danny/Topaz flyers would really be appreciated.
Thanks.

[solo6796]

They're plenty strong. No prob.

AJ

[IBWALT]

On my Super AVA I put an extra layer of monocote on the bottom of the vert stab. This is the only thing that I have done to the tail. Never had a problem with the strength of the stab and rudder.

Walt

[solo6796]

Flying tomorrow Walter?

AJ

[rcko]

On my Blaster DLG, sub fin like Ava, the botton lowest rib on the "sub" fin did buckle after a few failed hand catches. The fix was relativelly minor, make a small 1/32" wide by 1/16" to 1/8" deep slot about 1/2" long and insert a cured piece of directional carbon sheet with grain running chordwise. Glue with thin CA and sand carbon strip to contour. This added enough stiffness to keep the LE of the sub fin and lowest rib from buckling.

On my AVA this failure has never happen after 3 dozen flights or so. The fin is made out of pretty thick stock and if this was ever to happen then a similar fix to that performed on my Blaster would likely suffice.

I do recall seeing a thread in this forum under Ava build where one particular builder build a what would attach to the boom and protect the sub fin/tail

[rdwoebke]

I have never had an issue with my Bubble Dancer or Allegro Lite (especially the Allegro, that thing is a tank) sub fin.

I find with my sub fin planes that once they touch town the sub fin sort of acts like a "skeg" and it pretty much stops.

Ryan

[tomcat5109]

I've been flying my Topaz for 3 years and have only broken the sub-fin once. While flying on a new field in central Texas on one particular landing, the sub-fin drug on a half-buried rock and broke a single balsa stick. It was an easy fix and re-cover. Have not had any other problems in the hundreds of other landings it made. The rock landing would ding anything including fiberglass. My opinion is sub-fins are not a problem on low mass sailplanes.

[Andy W]

This was a problem on my AVA. Poor design - the lower block was not braced. Covered in my AVA-E build thread..

The AVA fin has no cross-bracing in this part. The back edge of the thick balsa block was simply glued to the vertical carbon tube that acts as a hinge for the rudder. The damage is minimal, it just shifted 1/4" or so - I just peeled back the covering there (which I had applied after installing the servos, refer back to how I cut thru the rib - visible in that picture - to install the HS5125's) so I can work on the damaged area. Also, if you look at the part that would have touched the ground, it's in perfect shape - there's not even a ding in the balsa, so obviously this "damage" was the result of light contact..

The fix? A drop of thin CA to re-attach the block to the bottom of the CF tube, and then cross-brace that area with a diagonal rib. I may also put in a vertical rib to help spread the load, or better yet, carve a 1/2" balsa square piece to fit snug against that tube, and glue it in place..
..a



827307
.. is what happens when the sub fin contacts the ground repeatadly on spot landings (that's with a 2" skeg behind the CG so the model slides nose-down)..
..a

865853
.. and the fix, just a rounded out balsa block, CA'd to the tube, to help distribute the load. Now to build a taller skeg to prevent contact..
..a

[northwest]

Thanks for all the info guys. Sounds like most have had no worries but I just feel that this is a potential problem area with these planes. I emailed Hobby Club asking if they stock a moulded fin that attatches to the boom-tip of the 3.0m Danny/Topaz but no reply yet (wonder what's taking so long?).

[Garvey]

I guess this is an area where the Sopano scores since it has no sub-fin.

[Roger_n_MtAiry]

I had the same concern when I was building my BD. I cut three strips of 1/64th ply about 2", 2 1/2" and 3" long, laminated them to the bottom of the sub-fin, sanded them to the airfoil shape with tapers at the ends. Covered in the normal fashion. I get a little abrasion of the covering but nothing more. Over 50 flights so far and haven't seen the need to repair the small scuffed area yet. I do fly the BD off of mowed grass. If I flew off of a rougher surface, my results would "vary".

What you mainly have to watch out for is dragging one wing in on landing which results in a sideways impact on the sub-fin. That WILL break something.
Cheers,
Roger

[Wing-span]

Why do these types of gliders need sub-fins anyway? (Danny/Topaz/Ava etc) Why are they not just taller in the first place! They could easily have the same surface area. I think it is just a fashion taken from the free flight and DLG guys, right? Are there any real benefits?

[rdwoebke]

What you mainly have to watch out for is dragging one wing in on landing which results in a sideways impact on the sub-fin. That WILL break something.


Nah... I have done that a ton on my Allegro and some on my BD. This kind of thing happens when you are doing "contest" landings. The sub fin is stronger than people thing.

Of course, I don't live in the desert, I live in the midwest and all our fields are covered in grass (if not grass, it is probably mud).

Ryan

[rdwoebke]

Why do these types of gliders need sub-fins anyway? (Danny/Topaz/Ava etc) Why are they not just taller in the first place!

Drela says he does it because it imparts less twist on the boom. The boom can be made lighter that way. If you look at the Supra plans, it does not have the sub rudder and he specifies a stronger in twist boom.

Ryan

[Wing-span]

This makes me wonder about the intended longevity of the aircraft in the first place if the aerodynamic twisting load imparted by the rudder on these large models is actually distoring the tailboom. The diameter and wall thickness of composite boom's should be easily capable of absorbing the 'load'.

The boom should be ridged enough in the first place to inhibite twisting and bending loads under 'normal' aerodynamic conditions. Launch conditions can and should be regarded as normal. Any twist or bend in the boom will fractionally and temporarally effect the 'trim alignment' of the aircraft creating drag.

[rdwoebke]

This makes me wonder about the intended longevity of the aircraft in the first place if the aerodynamic twisting load imparted by the rudder on these large models is actually distoring the tailboom.

Of course, you realize that when one makes the design, different amounts/kinds of materials could be optimum to increase bending strength, bending stiffness, twist, etc... And the trade off is mass. Look man, I don't design em, I just build em.... If you don't like the sub fins, take them off /leave them off your planes. :)

The sub fin on the models at least in my experience also helps with contest type landings. It sort of makes the plane "stick". But that is just me. I'm going to keep em! :) But that is just me!

Good luck with your building/flying!

Ryan

[mwhitman]

Any comments from the Ava/Danny/Topaz flyers would really be appreciated.
Thanks.

All of the Topaz gliders I have seen in our club (including mine) use a v-skeg from www.superskeg.com. It's very effective in keeping the glider from sliding. It also adds another 1 3/4" of clearance below the wing.

Catching it is still fun though.

Mike

[northwest]

Could the aerodynamic effects of the spoiler be the reason for the added fin area below the boom? I see a Topaz RES with split spoilers, but this particular model does not carry the sub-fin. My Thermal Dancer is a carbon boom plane and flies nicely with all of the fin on top. No spoilers here, just flaps. Is there really a corellation between boom mass and the use of a sub-fin in design?

[rdwoebke]

Well, you could compare the weight of the Supra boom per the plans and the weight of the Bubble Dancer boom.

BD plan calls for the boom to weigh 36 grams:

http://www.charlesriverrc.org/articles/bubbledancer/PDFs/bd_V3.pdf

Supra plan calls for boom to weigh 56 grams:

http://www.charlesriverrc.org/articles/supra/all%20PDFs/supra_fuse.pdf


I suspect Mark would have liked to have had the Supra with a sub rudder. He states he left the sub rudder off the Supra due to a rule in F3J that in theory disallows it (weather it would be enforced or not is unknown).


The vertical tail has no sub-fin. This is an F3J requirement that the only thing protruding from the bottom of the fuselage is the tow hook. The boom has been designed to take the twisting moments generated by such a tail.


Ryan

[John Walter]

Wingspan:

The issue is maximizing performance by reducing the weight of the boom. The torsion loads can be significant especially on an RES ship. Obvioulsy this can be overcome with a bias wrap on the boom, but it adds weight behind the CG thus requiring added nose weight.

Perhaps the extreme example of boom twist is my Grand Esprit. Twist of the FG boom during launch is very noticable. It might not have been a big issue as designed in the '70s for low power winches, but with a BD style spar it can now take a fast launch and a heavy foot on a Real Balls winch. :D

[Fly2High]

It may not be that the boom from torsional loads will fail rather it might be that the twist will not cause a pure yaw from rudder deflection. you will get some pitch in there as well due to the twisting boom

[northwest]

It may not be that the boom from torsional loads will fail rather it might be that the twist will not cause a pure yaw from rudder deflection. you will get some pitch in there as well due to the twisting boom
Good point.
We know the booms will not fail in flight. Didn't know this twisting effect could be serious enough to cause pitching effects from rudder at low thermalling & landing speeds.
So how could we explain a v-tail config ? I see a Topaz V on hobbyclub.com
Anyhow I see that the "Danny XL" has a moulded fin. Hopefully they're available in red/yellow and not only what is advertized. Hoping to save $50. with the regular 3.0m Danny.

[ploof]

I had been flying a this DLG to electric conversion for quite some time a few months ago I had grabbed a tip on landing, and sent the sub-rudder into the ground pretty hard shearing it clean off, after on the feild repairs of maybe 10 times. Monday afternoon I finaly went for a total makeover, I cut out this profile at nearly the same surface area as my DLG vertical stab, with the same area for the control surface, this has worked out to no ill effect of any sort, the rudder responses are very similar, and there has been no noticable change what so ever in the rest of the flight.

Take care, Ploof

[Fly2High]

Ploof,

the reason you may not experience any change is that you boom may be stiff enough to handle the loads. It is all boom dependent what I was describing.

As far as I understand, the sub fin only helps when you have a weaker boom layup. Basically, this is a way to save some weight. from the boom.

[ploof]

Fly

I started this stab about a week before this thread started, as the read went on I ended up getting a little nervous, last week end I snapped off the stab again on landing for the last time, this was getting very old. I figured among booms DLG specific would be on the stronger side. I'm very glad things worked out that old stab was getting to be like a persistent rash, or something.

Take care, Ploof

[John Walter]

I had been flying a this DLG to electric conversion

An important point is that this ship is not used for DLG. The yaw load during a discus launch puts great stress on the fin (and hence the boom). The yaw loads during normal flight are much less and during launch. Therefore the asymetry of the fin creates limited twisting moment on the boom.

[northwest]

Good word from Hobbyclub. Moulded fins are available for the Danny & Danny XL. One is a sub-fin the other is a normal fin, but with a moulded extension underneath which protects the rudder. I'll own one of these early next year.

Thanks for all the views.