DLG Rudder design

[James B]

Simple question really. Why is it DLG's have rudders that hang down way below the fuselage. It seems (to me) unique to these types of craft. It has always puzzled me.

[Plummet]

I'm guessing...

(DLG == Discus Launched Glider I assume.)

Think of the fin and rudder as a wing. When the rudder is used this "wing" generates sideways lift. The idea is to push the tail sideways so that the model points in a new direction.

If the fin and rudder are all above the line of the fuselage then the sideways lift will also tend to try to twist the fuselage, so as to make the model bank.

Viewed from behind the model, if the rudder is applied to move the tail to the right, (that is a left turn) then the twist will also make the right hand wing drop, which will tend to cause the model to turn as well, but to the right. The banking is opposing the rudder.

Putting equal areas of the fin and rudder above and below the fuselage means that the twisting effect is removed. The rudder will not cause banking and so will be more effective.

Plummet

Edited By Plummet on 31/03/2013 13:54:15

Edited By Plummet on 31/03/2013 13:54:49

[chris basson]

For a Guess Plummet that Sounds VERY Likely & I for one chose to Believe You!

CB

[James B]

A good explanation, but why would this design be unique to DLG's?

[Plummet]

Posted by James B on 01/04/2013 10:14:09:
A good explanation, but why would this design be unique to DLG's?

It's not. Many aircraft have rudders that extend to the bottom of the fuselage, and often the rudder is much wider at the bottom than the top.

A glider only has a limited store of energy, be it in the form of height or speed. When it runs out the flight is over. Waggling control surfaces increase the drag, and so wast energy. I would imagine that balancing the rudder will mean that less aileron waggle is needed - and so less drag is caused.

Plummet

[James B]

I think we maybe talking at cross-purposes.

I meant rudders that extend past the bottom of the fuselage, like Hyperflights Elf>

**LINK**

or probably a better example, the Slopeside E385 DLG. If you look at the manual for this model, it comes in two versions, the DLG with the aforementioned Rudder design or the HLG version, the difference being it has a tow-hook and a rudder that stops at the fuselage. The only difference between the models is the launch method.

**LINK**

[James B]

I think we maybe talking at cross-purposes.

I meant rudders that extend past the bottom of the fuselage, like Hyperflights Elf>

**LINK**

or probably a better example, the Slopeside E385 DLG. If you look at the manual for this model, it comes in two versions, the DLG with the aforementioned Rudder design or the HLG version, the difference being it has a tow-hook and a rudder that stops at the fuselage. The only difference between the models is the launch method.

**LINK**

[James B]

I think we maybe talking at cross-purposes.

I meant rudders that extend past the bottom of the fuselage, like Hyperflights Elf or probably a better example, the Slopeside E385 DLG. If you look at the manual for this model, it comes in two versions, the DLG with the aforementioned Rudder design or the HLG version, the difference being it has a tow-hook and a rudder that stops at the fuselage. The only difference between the models is the launch method.

[birdy]

I am under the impression that it is to minimise the torsional loads on the boom of DLG; during launch the forces on the rudder are rather large to say the least, and if this were to cause the boom to twist, it'd probably damage to boom in the long term and also mess up the control linkages to both the eleveator and rudder, possibly with unfortunate results.
I'd imagine that this is compounded by the want of the designers to have a less draggy rudder - a large amount of energy is usually lost during launch due to the rudder's drag. This can be achieved in part through a higher aspect ratio of the rudder, but this - for an above the boom fin - means moving the centre of force further from the boom, increasing the torque. By extending the fin and rudder below the boom the aspect ratio can increased and the centre of force moved closer to the boom, reducing the torque.

[James B]

Ahh, that makes sense. I'd never thought of the differing forces on the rudder with a DLG lauch as compared to a HLG launch. The spinning motion of a DLG launch could have a twisting force on the rudder and i suppose if its extened below the fuse this would balance it out somewhat.

Sorry about the triple post by the way, don't know how that happed