angled vs. vertical rudder hinge line

[capncrunch]

attached is a picture of the fin/rudder of my Direct Connection R/C Tsunami.

I noticed the other day (correcting for ~10mph breeze) that the rudder seems to have both roll and pitch coupling. while roll coupling is pretty much normal (it has to do with the inertial moment of the force exerted on the rudder being offset from the roll axis, right?) the pitch coupling was a little unexpected.

im trying to visualize why the pitch coupling is happening (unless im imagining it) and I can only think that its a result of the rudder hinge line's angle to the vertical. It would follow that the force acting on the rudder has a vertical component as well as the desired component in the yaw axis.


first question - am I right?

second question - how does one design a fin/rudder that will induce pure yaw?

-Barrett

[Bill Mixon]

First Question..

Yes you are right. With a slanted hinge line axis there is some vertical component being created.
However, there are lots of other things that need to be considered in the design of the plane such as wing possition, dihedral, thrust line, fuselage aerodynamics, and on and on.

Second Question..

It is very difficult.
Below is a picture of one of my planes that is totaly symmetrical and therefore rudder almost creates 100% yaw.
This is a freestyle or "3D" plane so it's not the best example.
This is about as good as it gets as far as the best tail section arrangement in my opinion.

Bill

[BMatthews]

capn, if it helps you visualize why it has some up elevator coupling imagine what it would look like if it was deflected a full 90 degrees. The side view of the rudder would look like a very short span elevator with full up in it. That help?

Now as for YOU Bill, I must know more about your fun fly. That is totally intruiging. I take it that it'll knife edge forever. Flat turns too? No gear so you just hand launch it I take it.... or VTO from your hand?

That must be quite the giggle when folks see it for the first time....

PS: If this was me doing this I think I'd put 2 landing gears on the ends of those center fins. Then you could land and take off upright or inverted.... if you can keep track of which is which that is.... :D

[Bill Mixon]

Originally posted by BMatthews
Now as for YOU Bill, I must know more about your fun fly. That is totally intruiging. I take it that it'll knife edge forever. Flat turns too? No gear so you just hand launch it I take it.... or VTO from your hand?

It is something I designed for 3d flying.
Yes, it will KE excellent. Very little stick input to fly KE with 0 coupling. Flat turns, knife edge wall's, square KE loops, etc. Lots of stuff that I haven't seen anything else do.

I originally built this one without any form of landing gear, and yes hand launched it and then pulled it out of the air from a hover at the end of the flight.:)
That was before this years AMA safety code (9) that doesn't permit this type of flying.:rolleyes:
I now have a small wheel on the bottom fin for landing on.

Bill

[hardlock]

"This is about as good as it gets as far as the best tail section arrangement in my opinion"

How does the tail type that has the rudder post completely behind the elevator compare?

I've noticed some 3D planes with these have no inboard angles cut in the elevator roots for the rudder swing since it's not needed.

Any aerodynamic benefit to this style (besides getting the rudder further back) or is it all for just looks/construction reasons? Looks like less elevator leakage at the root maybe or is control surface area the only consideration?

[Bill Mixon]

Any aerodynamic benefit to this style (besides getting the rudder further back) or is it all for just looks/construction reasons? Looks like less elevator leakage at the root maybe or is control surface area the only consideration?

I think it's more of a style thing or "fad".
Obviously the bit more surface on each elevator is of some benifit, but it's not worth it IMO.

I designed the above to get the horizontal stab and elevators as far back as possible. Pitch stability is a problem with the large elevators and large deflections.
If I built it with the rudder and vertical stab further back "longer fuselage" the rudder becomes less effective which isn't good either. I Have one version of this plane on cad file that the elevator is one piece on with the rudder up in front of the elevators in a split configuration.
Exactly the opposite from the norm.

Thinking outside the box.;)
Bill

[hardlock]

"Thinking outside the box"
Boy, I guess so! That may be the best yet as the elevator then has no break allowing root lose and for the rudder it doesn't matter (except knife edge and we start all over again. :D)

I'd just heard that running your ailerons as gapless as possible at their root next to the fuse is better so thought the same logic might apply to the elevator when designing the "perfect" plane.

[Dereck]

Barrett
So, you want good KE with no coupling?

That hingeline in your model looks a tad too much swept - bit of styling there perhaps?

I've got three designs I fly now. The best two at KE are my own design "Puts-E" ten cell and a highly chopped and modified Four Star 40 - the Great Planes CAP 232 actually rolls noticeably with rudder, which means taking out the coupling either with fancy stickwork or computer programming (add opposite rudder to aileron input).

I reckon the CAP's snag is down to excess wing dihedral allied to a lot of rudder fairly high up above the thrustline. The Four Star's rudder was purposely altered to put a lot of area down low.

My OD started with a flat wing - it's a very low AR, low winged monoplane. With top rudder in KE, the model would slowly roll towards inverted - rudder caused a roll away from its direction. Not a lot of fun to fly at first. I added around 2 deg of dihedral to the version I fly now.

It's all a juggling act balancing dihedral, thrustline, vert stab area above and below datum and probably the fuselage shape as well. While you can take some effects out by looking at the design before you start, it is more often down to flying it and figuring out what best to address next to fix the problem.

Ask any pattern competitor how much they fly on trimming flights!

Elevators ahead of the rudder line, with no gap between them and the fuselage? Has to be a styling thing. I liked the look of it so much on the CAP that I embodied the whole lot into my new sports aerobatic design. Snag is, you do end up with either a shorter tailplane moment, or a longer fuselage. Suspect I will pay for the longer fuselage when I load it into my car, though the model is fairly compact.

First question - yes, I think you're right

Second Q - at least keep the fin / rudder area. Make a new one with a vertical hingeline, or sweep it back a little - maybe a fourth of what it is now. I'd up the rudder area at least 50% keeping the fin the same, but that's my thing.

D

[capncrunch]

Thanks, dereck - good answers there.

basically, this is a fast + big plane for me, and the rudder is definitely more for style than for aerodynamics.

i'm not intent on hacking apart the tail of my "pride and joy" model at least until it gets replaced, but it was interesting to compare performance to design and think about how to design for performance in the future.

how does dihedral affect knife edge?

barrett.