Stab and rudder position

[dusty IV]

I just trashed together a small electric glider of about 48"WS from scratch. I'm ready to glue on the tail features. Why are there different layouts for the tail feathers?

I can place the rudder at the extreme end of the tail with the stab well head of it. For appearance I like this layout best.

or I can place the rudder on top of the stab

or I can place the stab at the extreme end of the tail with the rudder well ahead of it.

There must be reasons designers choose one of these methods but I haven't got a clue. Any comments?

[MarkusN]

It's mostly about control authority in the respective direction.

E.G. manned gliders used to have the stab in front of the rudder. They had enough pitch authority, and the shorter tail made them more nimble. Another advantage was simple assembly and some ground clerance when landing in high grass without having to go for all-out T-Tail (which has a few structural problems.) If maximum agility is not a concern you can gain a little performance by using a smaller stab at a longer lever.

The long wings OTOH have a lot of inertia around the vertical axis that has to be overcome by the rudder. Which hast an aerodynamically ineffective design to boot. So they gave the rudder the maximum lever at the extremity of the tail.

I have seen "rudder before the stab" designs mostly on gliders which need to high-start well. That way the rudder is not in the wake of a high-incidence stab in the high pitch up attitudes that can happen during high start.

[JetPlaneFlyer]

I believe the biggest driver for tail layout, at least as far as 'sports' models are concerned, are structural/construction issues and aesthetics. I'm sure any of your suggested solutions will work fine aerodynamically.

Generally the longer the moment arm between wing and tail/rudder the more effective these surfaces will be so if you do mount one of the surfaces further forward on the fuselage it's area may have to increase slightly.

Steve

[dusty IV]

Good answers, thanks. Pic of glider attached. I can use a rudder as shown or the rudder shown laying down. This one will extend below the center line. This will land on the grass so I'm not worried about rash. So what's best and why?

This was an attempt at a fast build. I use a carbon fiber shaft front to back on the fus. White foam was wire cut and speared for the pod. Fastest and easiest fus I've every built . About 2 hours time or less. Very light at 2 oz and strong. I have to add the wing pylon yet. Wish I had thought of this years ago. Molds suck.

I have the new small 123 VPX cells for power. Looks like I will be under 18oz.

[eflightray]

Back in the very early days of model gliders, there were a few designs where the vertical stab and rudder were underneath the fuselage.
I remember being given an A2 size glider called the Revenge with and under slung fin.

I'm not sure of the exact reasoning, other than it kept the horizontal stab up out of the way, and also gave a negative AoA when the model of on the ground and so less chance of it being blown over by the wind.

[MarkusN]

...rudder shown laying down. This one will extend below the center line. This will land on the grass so I'm not worried about rash. So what's best and why?...
Experience has shown this to be necessary on discus lauch gliders with their high rudder moments. I don't know if it's about inertia or aerodynamic forces, but with the strong centrifugal forces and large yaw angles at high speeds this design proved superior.

Back in the very early days of model gliders, there were a few designs where the vertical stab and rudder were underneath the fuselage.
I remember being given an A2 size glider called the Revenge with and under slung fin.
Ah yeah. My very first glider had this design as well. (A1, went by the moniker of Piccolo (Italian for "small one").) I believe it was about good high start characteristics.

[Brandano]

The T tail on full size glider designs is to reduce the number of intersections between flying surfaces (compared to a cross section) while still keeping the horizontal stabilizer out of the tall grass on a landing. There might also be considerations about the wake coming from the wing-fuselage intersection, since gliders usually have a fairly short wing chord compared to a very long tail boom and the wake from the wing might well have time to move back up by the time it hits the stab.

[JetPlaneFlyer]

Back in the very early days of model gliders, there were a few designs where the vertical stab and rudder were underneath the fuselage.
........I'm not sure of the exact reasoning,
I think the usual reason is to allow the pop up stab dethermaliser to operate without the fin getting in the way ;)


Brandano... the biggest driver for the 'T' tail is to get the stab out of the turbulent wake of the wing and fuselage which allows the stab to operate more efficiently and therefore it can be smaller which produces less drag.

Down side it the added structural weight needed in the vertical stab and aft fuselage. Models with 'T' tails have a bad habit of snapping the fuselage just in front of the vertical stab :( .