trp1956

My Feedback: (1)

I've been looking at DLG and other slope gliders and was wondering why most of them -and pretty much every DLG - have rudders that extend *below* the elevator. It would seem more likely to cause damage to the rudder when landing, since there would be no way to prevent the bottom of the rudder from hitting the ground. Just curious.

trp1956

My Feedback: (1)

Anybody?

SushiHunter

Not sure the actual size this is but looks relatively small where landing them on grass probably wouldn't hurt it. Hard surfaces such as cement, rocks, hard dirt, etc. would probably do some damage though.

W7APD Alan

Just a guess, but I suspect the reason is to eliminate twisting the tail boom. As light at those things are built, if the rudder were only above the boom, it would probably apply enough torque to twist the tail boom a bit. That would in turn make the whole thing squirrelly and a bit unpredictable. Balancing the rudder above and below the tail boom means that only sideways bending would occur, which would have less interaction with the rest of the airplane.

But that's just a guess.

trp1956

My Feedback: (1)

thanks for the replies, I appreciate it!

longsoar

I would imagine twist is not an issue here, nor damage on landing. Many people catch their GLGs on landing and carbon boom would not budge by amount of twist rudder produces.
My guess, the main part is: if rudder is all under the line of a mass center - it produces proper banking moment which requires less aileron input (if equipped). If no ailerons used - that way rudder produces even better banking moment. Here we see a compromise.
It is more about proper balance and control surfaces input than damage control when it comes to such "negligible" weights of 5-10oz for the whole glider.
I am not saying I am right, but that's what I would've done.

W7APD Alan

The banking that occurs on rudder inputs is not the result of the rudder, per se, but is because of the dihedral in the wing. When the rudder yaws the aircraft, the wing that is on the outside of the turn is suddenly at a higher angle of attack to the relative wind, and the inside wing's AOA is lowered. Hence, increased lift from the ouside wing and less lift from the inside wing, so the aircraft rolls into the turn.

longsoar

I kinda knew that for over 40 years but thanks for reminding Swept wing produces similar result.
I was kinda thinking outloud on the issue of design. The same principal goes for V-tail put up-side-down. It produces banking momentums with the rudder input. Rudder on the bottom works much better than rudder on the top. It's just not always feasible to implement.

aeajr

My Feedback: (2)

I have been flying DLG for years. As stated above once you get good at it you catch the model most of the time.

However if you are flying on grass and landing on grass the tail typically do not get damaged. Now if you are landing on blacktop or concrete then I might put a little tape or even some glass on the bottom to make it a tougher. But any extra weight is always to be avoided, especially in the tail.

trp1956

My Feedback: (1)

Composite gliders uses what looks like CF reinforcement on the bottom edge of the rudder.

BMatthews

With the massive yaw angles and energy of the side arm launches I'd suggest that it is about the twist of the boom. A fin and rudder that is all above the boom will both twist and bend the boom and that likely induces some angular change in the stabilizer which then affects the launch path. And with how close to neutral stability these would be set to a small change of the stabilizer angle will produce a pretty radical change in the climb path.

At least that's my story and I'm sticking to it....

longsoar

Yeap dude! I am with you on a launch loads. That's one thing I did forget about - and it's pretty violent on whole construction.

speedracerntrixie

My Feedback: (29)

How about the location of the rudder being able to increase yaw power so that turns can be made with less bank thus keeping as much of the wing as possible exposed to lift?

W7APD Alan

Autocorrect strikes again?

longsoar

Yeah man. I hate technology nowdays! Can't do a thing without an idiotic thing trying to correct you!

aeajr

My Feedback: (2)

If you don't like the electronic assists, just turn them off. Easy Peasy!

BMatthews

It doesn't work that way. For starters the most efficient way to turn is to make the wing do the work. And the best way to do that is to bank the wing so there is no side slipping. Side slip just creates drag with little lift.

On top of that a model with rudder and elevator only relies on the wing's dihedral to work with the yaw angle to produce a shift in the angle of attack for the two wings. It's that shift in the angles of attack that the wing uses to roll the model into the turn and back out.

Yes, technically having a high or low placement for the rudder produces or lowers the slight roll force. But the rudder sits so close to the center line compared to the area in the wings that this really is a drop in the bucket. So lowering the fin's area to be more in line with the boom really is all about the launch forces on these models.

Of course once it's done once with success it becomes a style thing. Hence we see gobs of larger winch launch models with similar fuselage and tail designs now that mimic the style even though they'll never see any great amount of whipping forces unless they cartwheel on landing.

speedracerntrixie

My Feedback: (29)

I will agree with the " in style " designs however keeping s much of the wing exposed to the lift does work. I was very active in sailplanes in the mid '90s . Mainly F3B, back then a 3 meter sailplane weighed 75 to 90 oz so was a little more work to thermal then the current crop of light weights. I had great success doing flat turns while working lift. Maintaining the core of the thermal with rudder but holding a slight amount of opposite aileron. Yes it does induce some drag but sailplane fuses have minimal side area so it's not the same as an Extra 300.