Nearly wrecked my Gerasis 3.6m DG-800! - RC Groups
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Oct 10, 2010, 01:21 AM
It's Prime Time for Slime Time
b7gwap's Avatar

Nearly wrecked my Gerasis 3.6m DG-800!

Had my buddy launch it off the slope, it was a nose high launch, so I fed in a little down and it nearly lawn darted down into the slope it pitched downward violently. I pulled back on the stick, noting that it took almost full back pressure to pull it out of the dive. Much to my dismay, as the aircraft returned to nearly zero pitch attitude, it stalled. Or at least that's what it looked like, an abrupt dropping of the right wing and rapid loss of altitude and downward pitching movement. I was able to recover again, but I had to hold about 85% up elevator just to keep the nose level, and any more than that and it would stall as described. I was shaking, grateful for the substantial ridge lift off the slope that was giving me a much needed buffer zone as I struggled to keep the bird airborne and under control. I brought it around to the landing area, threw up my airbrakes, and the thing just started to float. I no longer needed the up elevator, and it sailed right past me, right on down the slope! So I pulled the brakes back in and fought the thing for another climb and go around in the pattern. I was able to get it down on the second attempt, albeit with a light wingtip drag and sideward slide. The canopy popped off, but I was just glad to have it down safe. I decided not to try to fly it again, and just enjoyed the simplicity of my non-powered Cularis for the rest of the day. When I got it home, I put it up on the balance machine, and it showed a little nose heaviness. It's the only possible cause that I can determine. My question to you folks is- do gliders like the DG-800 have a very narrow CG range? Would having the CG range 1/4" ahead of the wing spar cause what I experienced today on the slope? I've got it balanced out now, but I'm worried about another flight!
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Oct 10, 2010, 11:20 AM
Will fly for food
davidjensen's Avatar
I have a Gerrisis 3M fox and it is heavy. I assume your DG is also which will make your DG is a fast flying ship. The CG range (front to back) will be not much more than 1/4" wide and may not be centered on the wing rod. If your CG was more than 1/4" in front of the range it may cause this probem but usually spoilers require up elevator comp. I think you have a stab incidence problem. Moving the CG 1/8" will not make much of a change in the situation. I don't know how much throw you have in the elevator maybe too much or possible too little. Also check your elevator linkage for any binding or restricted movement.
Oct 10, 2010, 12:24 PM
It's Prime Time for Slime Time
b7gwap's Avatar
The DG is 6 and a half pounds AUW. I'm no sailplane expert so I don't know whether that's too heavy or not. I got the kit (it's an old one) from a hobby shop going out of business who could never sell it at full price for the full twenty years it sat on the shelf. Unfortunately the instructions were MIA, so I don't have a documented CG range. A member here suggested I use the spar as the ideal CG location, which is what I did.

I should also point out that it has flown one time before, this was its second flight. The maiden was conducted using a piggyback launching system using a 60 size trainer as the launch vehicle. This worked well and released the sailplane at about 700 feet up. I had my throws way out of whack, I discovered. I had way too much in both aileron throw and elevator throw, so much so that the adverse yaw had to be manually overridden with every input. The sailplane was twitchy, but controllable and made a safe glide back to earth. My flight battery "might" have been slightly fore of where it was on the first flight, and I suppose if the range is as small as 1/4", that could have been the difference. I also went in and dumbed down my throws, added expos, mixed in some up elevator to the air brakes, added some aileron to rudder coupling and increased rudder throw.

The horizontal stabilizer is exactly where it was on the maiden, and does not appear damaged or tweaked, and the linkage is intact. (I used a TREX 500 swashplate link for the pushrod, very beefy and slop-free )

I'm going to take out a little of the elevator expo, leave the throw where it's at, and move it back to the spar on CG (it really wasn't that far off) and try it again I suppose.

Does anyone have any original manual reference to the PG Gerasis 3.6m DG-800's CG range?
Oct 10, 2010, 08:56 PM
Registered User
I don't think this is a cg issue, or at least I don't think even a severe forward cg would do this. I seriously doubt an incidence problem would do this either. Possibilities you might check into (and it could be a combination of these):
-radio problem
-incorrect elevator compensation mix
-sick servo with busted gear or pot problem
-loose servo mount or control linkage
-pilot error (flying too slow??)
-weird gusts or something, and/or rotor behind an obstruction
Oct 10, 2010, 11:53 PM
auto-tune remix
slopemeno's Avatar
I have one that I've flown extensively, and it's a big pussycat.

I have really small elevator throws, and some rudder mixed into the aileron throws. It thermals really well, and lands pretty easily with 5% up trim mixed into the spoilers. I also balanced mine on the wingrod, if that helps.

One thing that I found was mine had elevator double-centering, due to the "snake" pushrod. I removed my rudder and rudder post, and glued a JR-341 elevator servo inside the stab. THAT made a big difference- now it's very easy to make accurate, fast passes on the slope, and to carve big loops.

I didn't get a manual with mine either, but if you search my older posts I gave out my throws to some guy in 2005-6.
Oct 11, 2010, 10:21 AM
dare to be dull
bigjohn's Avatar
Originally Posted by slopemeno

One thing that I found was mine had elevator double-centering, due to the "snake" pushrod. I removed my rudder and rudder post, and glued a JR-341 elevator servo inside the stab. THAT made a big difference- now it's very easy to make accurate, fast passes on the slope, and to carve big loops.
I'm not familiar at all with this plane, though I do fly an EMS DG-800. I was figuring it was a sloppy servo connection as well as I read down the thread. Get rid of the "snake"
Last edited by bigjohn; Oct 11, 2010 at 10:23 AM. Reason: imadumbass
Oct 11, 2010, 01:49 PM
High Lift Coefficient
Sailhigh's Avatar
I don't think it's a CG problem either. In fact being nose heavy, it should be more stable and less sensitive to large control movements. Long tail moment scale sailplanes need surprisingly little elevator movement to obtain the desired pitch attitude. Adverse yaw is common in high aspect ratio wings if you don't use aileron differential which should also be coupled to the rudder. I think if you make these adjustments and have your helper throw the plane out straight or slightly down with the wings level and you letting the plane gain some speed before applying any elevator control my yield you quite different results. I've seen 2 other Gerasis DG-800's (one was slopemeno) and they both flew rather well.

Oct 11, 2010, 02:18 PM
auto-tune remix
slopemeno's Avatar
If I can assemble it tonight I'll give you CG, throws, and expo settings.
Oct 11, 2010, 02:53 PM
It's Prime Time for Slime Time
b7gwap's Avatar
Thanks for the info and ideas, gentlemen. I'll get her up again with slopemeno's info and report what I find.

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