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Old Jan 28, 2010, 01:38 AM
It's Prime Time for Slime Time
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OLD Gerasis DG-800 build (have glider, what now?)

Sooooo, I bought a glider from my LHS that they've had on their shelves for they say over ten years. It still has the 499.99 price tag on it but they let it go for $250 to me to get it out of there. It's a PG Gerasis DG 800 360, which from what I've gathered with some initial google research, is a scale model of a DG-800 full scale glider, and has a wingspan of 3.6 m, which, to me, seems REALLY long. Background on me, I'm a glow/electric airplane/heli pilot who's been flying those things since my dad started taking me to the field in 1985. I've never flown RC gliders. I was impressed by the elegance in the planform of the wing, and the delicate design of the empennage, and above all, that it is a scale model. So now I have this thing, and there's no instruction for construction, and I have no one to 'guide my sword' as Inigo Montoya might say. It has ailerons but no flaps, and has spoilers that rise laterally out of the wing, in addition to the rudder and elevator on the T tail. The fuse is glass, and looks like it needs to be sanded down on the seams and perhaps painted. The wings look to be foam core with balsa sheeting and covered in a white film. My primary questions are as follows:

Where do I mount the aileron servo(s)? There are circular cavities that I can see through the covering that look like a place for some sort of control mechanism, but I don't know if a servo would fit out there, and even if I did put it out there, there's no elegant way I can see of embedding the servo wire in the wing. The spoilers have their own embedded pushrods that are preinstalled, and there seems to be a second pushrod guide in front of the this that might be for aileron control, but I'm not sure, and if it is, this would be inconvenient for transport as there's no way that big wing is going to stay installed. Maybe this is common practice in the glider world, and that's why I'm here is to get those types of questions answered.

Is this thing going to need some serious ballast to balance out? Do you guys mount all your servos at the foremost extremity of the nose to try to get as much moment arm as possible? There's plenty of room in the cockpit for gear, but it looks as if I'm going to have to fabricate the trays and mounting positions. Again, if anyone has built this kit, I would appreciate some ideas and experiences on setup, ie, did you use a pull-pull system on your rudder? What type of control horns are needed? The hinges on the rudder were already broken off when I pulled it out of the box, so I'm going to get some Robarts for it, but do the horns need to be small so as to minimize drag?

The wing spar tube is no tube at all. It looks to be a solid steel rod, which seems to be WAY overkill, but more importantly, it also seems too short! It would have been better in my mind to lengthen the tube and either use Carbon fiber tube or rod of the same diameter. Would this be a bad idea to modify it in this way?

I'm guessing that it's ok to belly land this glider, with the smooth rounded lower fuselage surface, but I'm sure the scale glider had an undercarriage of some sort. I might add it later, but I'm fine with belly landing if you all think that's a good idea.

Is this a good glider? Is it hand launchable? A good slope soarer? I have a friend who does paragliding and says there's a good hill to maiden it on, so I guess I'll have to see. Am I getting myself into a quagmire here fellas?

thanks in advance for your expertise.

Austin
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Old Jan 28, 2010, 07:26 AM
MTT
I care about rising air !
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West Chester, Ohio
Joined Feb 2002
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Welcome to the world of scale sailplanes !
Another victim of a serious addiction......

Lots of questions, some I can answer, some not...

First of all, I don't now this particular model so I can't tell you anything about it's flying characteristics.
But Gerasis models generally enjoy a good reputation.
When you ask about whether it is hand launchable, what exactly do you mean ?
At a good slope, my guess is that it would be most certainly hand launchable.
That said, most modern type scale gliders are also good at the slope, but landing can be tricky depending on your slope and landing area.
If you mean hand launchable as in tossing it from your hand to check CG and throws, don't !
Most scale gliders need more flying speed than you can provide with a toss, unless you are an olympic javelin thrower...

Aileron servos : The round cavities in the wings are exactly for that, mounting the aileron servos there. Usually there is also an conduit in the wing to pull the wired through, some models even come with the wires already installed.
Mounting the servos in the wings is the best way to go, makes for a short and slop-free connection between servo and control surface.
Some people like to wrap the servos in shrink tube, and then glue them to the inside of the wing with Goop or something similiar. I don't like that approach very much, and prefer to install a plywood plate in the cavity, to which I then can attach some kind of servo mount.
On a model like this, servos such as the Hitec HS-81MG, or 85MG, are more than adequate for the ailerons.

Ballast : All scale gliders require a healthy amount of nose ballast to achieve the proper CG location. sometimes it seems like a lot, but you have to remember that this is a scale mode of a real sailplane, in which you have arounf 150 - 200 lbs of of ballast in the nose, disguised as the pilot....
I read a lot here on rcgroups about people agonizing about the weight, but you have to remember that modern typpe scale gliders are not meant ot be floaters, they need a certain amount of weight, to be able to perform properly.

Belly landing is OK, as long as you don't have to land on asphalt or concrete....
If you fly mostly at the slope, where landing fields are usually small, and there are no runways as such, it is probably better to not have a retract, or, if you have it, not use it.
For aerotowing, it is better to have a retract, and of course, it is more scale like.

Rudder : Of course you can use a regular pushrod made of wood or carbon fiber, or bowden type pushrods, but I like pull-pull setups best.

Elevator servo : This is a little controversial, on any model in the 4 m wingspan range, I like to install the elevator servo in the tail, close to the elevator. On a model like your DG, the elevator servo would be installe in the fin, at the bottom, with a short pushrod connecting the servo to the elevator.

On bigger ships, say 5m or more, the elevator servo(s) are often mounted in the horizontal stabilizer itself.
Of course, mounting the servo in the tail will require more ballast weight in the nose, but in my experience, the increase in overall weight is negligeble, and the more direct and precise elevator actuation, and ease of assembly at the field are worth it.

Don't quite understand your question about the wing joiner, maybe you can post some pictures.

Keep asking, there's lots of folks aaround on this forum which will be able to help you.
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Old Jan 28, 2010, 08:03 AM
bie
maidenflight-postponer
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Germany
Joined Oct 2004
788 Posts
Hi Austin,

I own a Gerasis DG-800 for almost 15 years now and wrote about it in other threads here in the RC-Groups. May I suggest, that you use ĄSearch this forum" with "Gerasis DG-800"? You might find some comments on that plane.
If there are still questions to be answered after you read everything Iím more than glad to help you!

Regards,

Andy
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Old Jan 28, 2010, 09:47 AM
It's Prime Time for Slime Time
b7gwap's Avatar
Joined May 2004
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Quote:
Originally Posted by bie View Post
Hi Austin,

I own a Gerasis DG-800 for almost 15 years now and wrote about it in other threads here in the RC-Groups. May I suggest, that you use ĄSearch this forum" with "Gerasis DG-800"? You might find some comments on that plane.
If there are still questions to be answered after you read everything Iím more than glad to help you!

Regards,

Andy
Hallo Andy,

I actually did a search for my glider, and got only one thread! but it was somewhat helpful, I just wanted to ask some specific questions about it from you, the experts.
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Old Jan 28, 2010, 10:00 AM
It's Prime Time for Slime Time
b7gwap's Avatar
Joined May 2004
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Quote:
Originally Posted by MTT View Post
Welcome to the world of scale sailplanes !
Another victim of a serious addiction......

Lots of questions, some I can answer, some not...

First of all, I don't now this particular model so I can't tell you anything about it's flying characteristics.
But Gerasis models generally enjoy a good reputation.
When you ask about whether it is hand launchable, what exactly do you mean ?
At a good slope, my guess is that it would be most certainly hand launchable.
That said, most modern type scale gliders are also good at the slope, but landing can be tricky depending on your slope and landing area.
If you mean hand launchable as in tossing it from your hand to check CG and throws, don't !
Most scale gliders need more flying speed than you can provide with a toss, unless you are an olympic javelin thrower...

Aileron servos : The round cavities in the wings are exactly for that, mounting the aileron servos there. Usually there is also an conduit in the wing to pull the wired through, some models even come with the wires already installed.
Mounting the servos in the wings is the best way to go, makes for a short and slop-free connection between servo and control surface.
Some people like to wrap the servos in shrink tube, and then glue them to the inside of the wing with Goop or something similiar. I don't like that approach very much, and prefer to install a plywood plate in the cavity, to which I then can attach some kind of servo mount.
On a model like this, servos such as the Hitec HS-81MG, or 85MG, are more than adequate for the ailerons.

Ballast : All scale gliders require a healthy amount of nose ballast to achieve the proper CG location. sometimes it seems like a lot, but you have to remember that this is a scale mode of a real sailplane, in which you have arounf 150 - 200 lbs of of ballast in the nose, disguised as the pilot....
I read a lot here on rcgroups about people agonizing about the weight, but you have to remember that modern typpe scale gliders are not meant ot be floaters, they need a certain amount of weight, to be able to perform properly.

Belly landing is OK, as long as you don't have to land on asphalt or concrete....
If you fly mostly at the slope, where landing fields are usually small, and there are no runways as such, it is probably better to not have a retract, or, if you have it, not use it.
For aerotowing, it is better to have a retract, and of course, it is more scale like.

Rudder : Of course you can use a regular pushrod made of wood or carbon fiber, or bowden type pushrods, but I like pull-pull setups best.

Elevator servo : This is a little controversial, on any model in the 4 m wingspan range, I like to install the elevator servo in the tail, close to the elevator. On a model like your DG, the elevator servo would be installe in the fin, at the bottom, with a short pushrod connecting the servo to the elevator.

On bigger ships, say 5m or more, the elevator servo(s) are often mounted in the horizontal stabilizer itself.
Of course, mounting the servo in the tail will require more ballast weight in the nose, but in my experience, the increase in overall weight is negligeble, and the more direct and precise elevator actuation, and ease of assembly at the field are worth it.

Don't quite understand your question about the wing joiner, maybe you can post some pictures.

Keep asking, there's lots of folks aaround on this forum which will be able to help you.
Thank you for the wealth of info! I will dig into this further and get some pictures up. I appreciate it!
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Old Jan 28, 2010, 10:51 AM
SoarScale
United States, WI, Wind Lake
Joined Nov 2004
814 Posts
I'll add to MTT's comments where necessary:

Quote:
Where do I mount the aileron servo(s)? There are circular cavities that I can see through the covering that look like a place for some sort of control mechanism, but I don't know if a servo would fit out there, and even if I did put it out there, there's no elegant way I can see of embedding the servo wire in the wing.
The round cavities on European built planes means one of two things - either a bellcrank mounted to a round wooden base with a pushrod through a hole in the core and a servo in the fuse, OR the insertion of a Robbe, Multiplex or other vendors plastic servo housing that fits the smaller servos with wires through the hole in the core back to the fuse.
Quote:
The spoilers have their own embedded pushrods that are preinstalled, and there seems to be a second pushrod guide in front of the this that might be for aileron control, but I'm not sure, and if it is, this would be inconvenient for transport as there's no way that big wing is going to stay installed
.
Typically white electrical or plastic tape is used to hold the wings on this small of a model sailplane IF no other mechanism was installed by the manufacturer. If you choose to use a bellcrank in the round cavities (wouldn't be my choice but maybe yours), the control rod and clevis will stick out at the root end of the wing and insert into holes drilled in the wing root of the fiberglass fuselage. Care is obviously needed when transporting the wings to ensure the clevises and rods are not damaged.
Quote:
Is this thing going to need some serious ballast to balance out? Do you guys mount all your servos at the foremost extremity of the nose to try to get as much moment arm as possible? There's plenty of room in the cockpit for gear, but it looks as if I'm going to have to fabricate the trays and mounting positions
.
On a plane this size, I would recommend servos in the nose with either a single control rod or pull-pull where feasible. This will minimize ballasting and overall weight.
Quote:
What type of control horns are needed? The hinges on the rudder were already broken off when I pulled it out of the box, so I'm going to get some Robarts for it, but do the horns need to be small so as to minimize drag?
Simple fiberglass (printed circuit board) or G10 fiberglass control horns are fine and they do not need to be that small for drag minimization. There is significantly more drag produced by the flying surfaces and fuselage than all the control horns put together!
Quote:
The wing spar tube is no tube at all. It looks to be a solid steel rod, which seems to be WAY overkill, but more importantly, it also seems too short! It would have been better in my mind to lengthen the tube and either use Carbon fiber tube or rod of the same diameter. Would this be a bad idea to modify it in this way?
Remember that the strength of a wing is, or should be, built INTO the wing spar, not in the length of a wing joiner. If Gerasis sold this sailplane with the wing joiner at the length you mention, then the wing system was designed to take flight loads. The joiner therefore, has two functions, to provide the bending strength requirements at the root AND to attach the wings to! You should not need a longer wing rod and you can easily replace it with a solid carbon fiber one (assuming you can find one of the correct, probably metric, diameter). Remember this however, steel wing joiners will bend before they break, carbon wing joiners simply explode when they fail - there is very little warning of a pending failure in a carbon joiner.
Quote:
Am I getting myself into a quagmire here fellas?
Not a quagmire but don't over engineer it. Keep it simple and follow what you already know from your other flying and model experience. There is very little more you need to know to build a sailplane like this.
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Old Jan 28, 2010, 11:49 PM
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I have that plane. Happy to report it flies great.

Don't worry about the weight of the wingrod. I fly mine on the slope in light to medium lift and it thermals beautifully. You'll also see it can land fairly slow with the stock spoilers.

I thought the fuse felt a touch flimsy, so I added an extra layer of 4-oz s-glass from wingrod forward, and then I cut out a plywood floor from some 3/32nd ply. That stiffened the fuse up quite a bit. I initially had three servos in the nose but eventually ended up with only one for the rudder up there, two by the wing root for spoilers, one in the tail for the elevator, and the wing servos.

I got some double-centering on my elevator servo, so I put a JR-341 inside the vertical stab, and made a small metal joiner for the elevator. Bottom line, no more double-centering at all. You'd think the elevator mounted that far behind the CG would be a problem, but it really wasn't. It was really quick to cut out the balsa rudder post and install the elevator servo, but I had to carefully replace the hinges to do it- and I used the Robart hinges to replace the stock ones.

I linked up to two basic full-size servos inside the fuse to the spoilers with plastic ball joints for a quick release. It's a little klugey but works really well, so I'm not ready to change it. I also ran the aileron servo wire in through the same hole, so it's a bit of a squeeze, but it all goes together really easy at the hill. A layer of white electrical tape holds the wings on.

I made a wing bag out of bubblewrap, and it all fits in my Corolla with a bunch of other planes, so no problem there. I usually leave the tail on, but thats the lazy talking. A long snowboard or ski bag will fit everything nicely.

I'd leave the wingrod alone. If anything, this plane it slightly too light. Dont worry about how much noseweight it takes, just balance it on the center of the wingrod.

I used JR-351 servos in the wing, and didn't bother with servo cans- I just spot-glued the servos in with 5-minute epoxy. Servo cans are a nice touch though. You can make your own horns out of PC circuit board material and a #51 drill, IIRC.

Keep an eye on the canopy latch- Mine really didn't hold the canopy on all that well, and a good bounce on landing would make the canopy pop off.

It's really an honest flier. I have a great time doing fast fly-bys with mine on the hill. It's actually reasonably easy to land due to it's weight. I mixed in about 5% up elevator with the spoilers to keep it from pitching down. I wouldn't bother with a retract for slope flying.

PM me for throws and stuff. I'll see if I can find some pics.
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Old Feb 04, 2010, 10:45 AM
It's Prime Time for Slime Time
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Thanks for the first hand experience! Well, I've taken some of your advice to heart. I put a HS-65HB in the rudder for elevator control, and set up a pull-pull for the rudder with a JR ST47 all the way up front controlling it. I've rigged my own tow release mechanism up front as well. Now I've come to the wings. You say you were able to worm all three servo wires through the pushrod guide eh? I'll have to try that. I'm loath to slice open that beautiful wing. I put it all together on my stand on my workbench to get a look at it and think things through-man this bird has a long wing! It's deceiving until you get it all together! Do you think that HS 65 HBs would be good aileron servos, too?
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Old Feb 04, 2010, 10:53 AM
MTT
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West Chester, Ohio
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Even if you have to make a conduit for the wires, there's no need to slice open the wing, thankfully...

If there is no conduit in the wing for wires ( which I doubt, since it has cavities for the servos ), or if the conduit is just to small, you can easily cit a conduit into the foam core using this method :

Get a brass tube with the diameter of the conduit you want to have, and long enough to reach from the wing root to the servo cavity. Sharpen one end, and cut grooves in it so that you have a saw-tooth pattern. Put the unsharpened end of the tube into the chuck of your drill. Draw aline on the wing from the servo cavity to the wing root, , align your "drill bit" with it, and cut a conduit into the foam.

The 65's would be OK I think, but I would use 81's, or 81MG's, they are only slightly larger, have a little more torque, and cost less, to boot..

From www.servocity.com :
65HB = $24.99
65MG = $35.99
81 = $17.49
81MG = $23.49
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Old Feb 04, 2010, 05:50 PM
It's Prime Time for Slime Time
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Originally Posted by MTT View Post
Even if you have to make a conduit for the wires, there's no need to slice open the wing, thankfully...

If there is no conduit in the wing for wires ( which I doubt, since it has cavities for the servos ), or if the conduit is just to small, you can easily cit a conduit into the foam core using this method :

Get a brass tube with the diameter of the conduit you want to have, and long enough to reach from the wing root to the servo cavity. Sharpen one end, and cut grooves in it so that you have a saw-tooth pattern. Put the unsharpened end of the tube into the chuck of your drill. Draw aline on the wing from the servo cavity to the wing root, , align your "drill bit" with it, and cut a conduit into the foam.

The 65's would be OK I think, but I would use 81's, or 81MG's, they are only slightly larger, have a little more torque, and cost less, to boot..

From www.servocity.com :
65HB = $24.99
65MG = $35.99
81 = $17.49
81MG = $23.49
Thanks for the idea! I found that if I curved the servo wires into a triangle and CAed them to a .032" rod already inserted into the pushrod guide, I was able to convince it to come through, although I needed to re solder it once it was through.

I ended up going with some Hyperion metal gear digital wing servos, they're super thin and have almost 100 oz/in torque. Overkill, I know, but they were only ten bucks more than the 65 MGs. Now to do the other side... losing motivation. Need to get it up in the air, get excited about it.
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Old Feb 04, 2010, 10:33 PM
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I'll try to remeber to bring my camera home from work and shoot a pic of my wingroot.
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Old Feb 05, 2010, 02:22 AM
It's Prime Time for Slime Time
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Originally Posted by slopemeno View Post
I'll try to remeber to bring my camera home from work and shoot a pic of my wingroot.
That would be very helpful, thanks!
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Old Feb 05, 2010, 11:53 AM
It's Prime Time for Slime Time
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Soo, I've been doing some searching, and can find very few threads on rocket launching.. Is this because it's a bad idea? I've found some nominal references to the practice, but no actual threads or posts. If my glider weighs 6 pounds, couldn't I get a rocket motor that puts out 4 lbs for 3 + seconds and mount it securely near the CG underneath the fuselage, then disconnect the mount and let it fall back to earth with a parachute? Is three seconds enough time to get up there? I noticed that apogee rockets sells reusable rocket motors. What if I put two of these side by side and ignited them simultaneously or even did it in series as a booster stage? I imagine that the thrust speed is much faster than the flying speed of the glider, but the mass should keep it in check, and I could climb out at a steep angle to use the wing to its best advantage. Do people still do this or is it a forgotten practice?
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Old Feb 05, 2010, 10:38 PM
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Uh, NO.
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Old Feb 05, 2010, 10:46 PM
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Here's the wing root. Note how the spoiler linkage and the aileron servo wired are close, bit not *too* close.
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