What can cause a DLG that has a sub-rudder (mine has equal rudder area above and below the boom) to roll over on its back on a discus launch? (Assume the total rudder area is adequate).

Tail moment too short?

CG too far back?

I thought the sub rudder would take care of this issue. I have flown the glider in light slope lift and thermaled that way. It flies fine, although a bit tail heavy (tucking from dives).

Thanks


Details:

Bagged 52" wing with AG foils
ailerons
6" dihedral total
non-functioning flat plate rudder
10.5 oz all up weight

The one time I experienced what you are describing, was when the rudder was too small. I can say that without a doubt based on the following experiment.

One day while flying, a rough landing caused the sub rudder on my DLG to snap off. I wasn't ready to quit just yet so I threw again anyway. The plane did exactly as you described yours. It flipped inverted immediately after launch. The harder I threw, the sooner it would flip. I always managed to roll back upright fairly quickly and then the plane flew just fine at the lower glide speed.

Tail moment is a definitely a player in rudder area requirements. The longer the moment, the smaller the rudder can be because it has more leverage. So empenage area and tail moment are interdependent. Thus if you're absolutely sure that your rudder area is adequate then I would have to assume your tail moment is not adequate.

As for CG, DLG launch speed is similar to dive testing. An aft CG will make the plane want to tuck during the launch and a forward CG will make it want to balloon. I've played with CG in relation to launch and only a CG so rearward that the plane is basically unflyable would make it flip inverted on launch.

The only other thing I can think of is poor launch technique. If the pilot hangs on way too long past the optimum release point and cocks their wrist sharply, it's possible to flip the plane almost inverted.

Jim

Dominic,

Do you have photos of your setup?

Paul.

I will work on getting some photos posted. After the failed discus launches (my throwing technique might be a little off because I'm used to launching a poly DLG, but that one I can launch pretty well) I cut off the sub-rudder portion and flew it with just the portion above the boom on the slope. It displayed no yaw problems to speak of, even with the reduced vertical area, but of course we're not talking about launch forces here.

My Alula (different type of craft I know) started doing this on launch after traveling in the car on a hot afternoon running errands on the way to fly. Once in the air and righted it flew OK. One wing had warped up with the leading edge so it rolled too fast to see at launch speed but behaved at flight speed. Fixed the warp and it stopped rolling on launch....just a thought.

Originally Posted by element2tandem

My Alula (different type of craft I know) started doing this on launch after traveling in the car on a hot afternoon running errands on the way to fly. Once in the air and righted it flew OK. One wing had warped up with the leading edge so it rolled too fast to see at launch speed but behaved at flight speed. Fixed the warp and it stopped rolling on launch....just a thought.

Interesting thought. The plane IS carrying some aileron trim to offset the weight of the launch peg. I haven't counter-balanced the other wing yet...

There is a spreadsheet on my website that will calculate the desired tail sizes for any model sailplane/glider. It's pretty easy to use, all you need is a ruler and MS Excel.

The formulas are from Martin Simons book on Model Aircraft Aerodynamics and Mark Drela.

It's called Sailplane Calc.

http://h1.ripway.com/cloudyifr/files.htm

Curtis
Montana

I for one can attest to the usability and usefulness of the results that Curtis's spreadsheet provides.

When originally designing the QFII, I used Curtis's data to confirm my original choices.

Paul.

the second thing [other than the tail] is the boom

i built two bugs ,one had a wippy boom and the other a nice wrapped boom

the larger dia. wrapped boom was fine but the protruded 1/8 cf tube was bad

the boom stiffness is very important also ,equally as important as the tail


xtc

I can sometimes do the same thing with my scratch built plane. When it got really bad, I discovered that my rudder hinge was about to fail. Fixed that and it still had a tendency to roll left on me (right handed). I started working more on my launch and that has helped.

Do you mean that the plane rolls (left or right?) or that it is on it's back at the top of the launch?

You shouldn't have to worry about counter balancing the launch peg, unless it's made of lead How much aileron are you using?

Nm
:d