Rudder Size?

Hello Rick,
Many thanks for the information.
I`ll do some calculations over the weekend regarding wing area and percentages etc. to see where i`m at with the current set up and take it from there.

I did add some ballast yesterday as it got quite windy (25mph ish), which is a lot for a Rudder/ele. and she did penetrate well all things considered, I just felt that the rudder command could have been better, hence my post.

Like I say, I`ll see what calculations come out at.
Thanks again for your help
Kev

Hi Kev.

I was interested to see what others would suggest, before adding my bit.

My involvement with developing rudderless thermal soarers (DLGs and rocket gliders) since 2004 has taught me about vertical stabiliser areas.

The ideal vertical stab size is (IMHO) dependant on much more than wing area alone. 

Too much fin area will give rise to spiral instability.

In layman's terms, this means once in a turn, the model will tighten itself into a corkscrew dive, unless correction is applied.

So, too large a fin area and the nose drops into the turn, and likewise, if the model has too little fin area, the reverse is true, ie. it will tend to slow down or may even stall.

The ideal fin area, is a product of CG location, Wing Dihedral and sweep angles and fuselage lateral area. 
As you might expect, the ratio between tailplane and fin areas has a bearing on model handling... if the tailplane is small, the CG has to move forward, the further forward the CG, the greater the tendency towards spiral instability - the cure for which is a smaller fin or greater dihedral. So you can probably now see why I say there is a relationship in the handling and the ratio between vertical and horizontal stabiliser areas.

When trimming fin area, I do this in flat calm, evening air. I set the model to fly just off the stall (ie as slow as possible) while turning, without any tendency to change it's rate of turn. If I then set the model back to flying in a straight line, with the same elevator position it will slow slightly and stall. 

This state of trim gives the advantage of being able to quench a stall before it happens, with either down elevator trim or a turn-initiating rudder input. 

When trimming, I start with a fin that is obviously too large, test fly it, trim off 1sq" from the TE of the fin, then test fly and trim off again, until the turns are exactly as I want them, then verifying that by taking off even more area, and noting a deterioration in the handling. Once the optimum area is known, a new vertical stabiliser is made to that size. Simples!

As you mentioned drag, it has to be said, that a smaller fin will have less drag than a larger one, providing they have similar profiles, and if corrections need to be made because the vertical stabiliser is not the optimum size, then that will create more drag still.

It would be interesting to see what, if any, handling effects, your enlarged rudder will give.

There is a lot of Good Fun to be had in altering fin areas, enjoy it and let others know! 

Best regards,

Jef

Hello Jef,
Hope you are well.
Many thanks for your input. It makes very interesting reading and plenty of food for thought.
One point I did pick up on was when you mentioned about the model slowing in a turn, possibly indicating a small fin area, as my model has been doing just that, even when inputting a little down elevator.
The other point I would make, is the fact that this model is made up from bits and bobs.  ie: Fibreglass fuz from one swapmeet, wings from a different meet etc. The root wing chord matched the fuz so I went for it. I then built in some dihedral, and made the tail feathers, followed by working out the c/g.
So all in all i`m surprised it fly`s at all given my limited knowledge of workings out. 

Although i`ve been in the hobby for donkeys years, when you build from plans or kits, all the mathematical stuff is done for you and therefore I guess that has now been my downfall so to speak.

Anyway, I`m learning now so I will see what happens when I try the bigger rudder and take it from there.
 
All the best
Kev.

Kev,

Excellent stuff - please keep us posted with any other mods and their results.

Just out of interest, at the point that you are measuring the rudder deflections, what is the chord of the rudder?

Jef

Hi Kev.



Yes it tells me you aren't using anything like the 30 degrees of deflection that most Rudders have.
If you have the same deflection ANGLEs available on both rudder setups, then the reason your first rudder was a bit slow to respond would be too little deflection. You should have been getting about 45mm of movement from centre, on the 75mm chord rudder.

Sounds like your set up is ok as it is, a smaller total fin area has less drag all of the time (ie including when not turning)... a correctly trimmed (or very slightly smaller) vertical stab area has less drag in the turns, as the minimum of input is required. No point me going into details, as I've already done so in this thread, but if you are holding rudder input on, all the way around the turn (when there isn't a thermal there pushing you out), then the total fin area is already on the small side (it isn't lifting enough).

In case you are wondering, am basing all this ("mathematical and technical" stuff) on the fact that your 105mm chord rudder is only offset by 30mm at full deflection.

Pretty sure (without actually using trigonometry) that 30degrees deflection would give about 60mm of movement on a 105mm rudder chord.

Without knowing the fin and rudder profile section data, I would not be able to comment about whether a large rudder moving less is more efficient than a small rudder moving more. Or in other words I don't know!

When fighting to stay in strong thermal lift, a rudder with greater authority is infinitely better than one that has insufficient control, regardless of which has greater drag!  

Hope this actually all makes sense, if not please ask for clarification on any, or all, of it.


Jef

Hi Kev,

   Have a look at Curtis Sutters' site, tailwindgliders.com;  he has a truely excellent spead sheet that is available for open use that will be of great help to you in this matter.  I have used it, and have learned a great deal.

   You'll like it too!


  soarguy