FIn/Rudder

Hi

Just putting some thoughts together from What I have read so far
Your thoughts and comments please to help clarify things for me.

When I was a kid into C/L we all put on heaps of rudder but that was probably not the best way. Seems most favour controlling line tension with LO position. Enough rake to balance line drag and centrafugal force and another 1 - 1.5 deg of yaw to get a little lift overhead off the side of the fuselage.

So were not using fin to counter line drag at all then?

If not then fin is just for damping out yaw wobbles in gusts and manouvers etc. Then fin offset ought to be set to whatever yaw we are aiming for, say 1 - 1.5 deg, is that right?

OK so now fin and rudder, I am assuming a fixed fin/rudder for now. The whole fin is a very low AR and so will produce loads of induced drag if it is loaded to even moderate Cl. If we use a rudder, that introduces camber and makes the fin capable of heigher Cl and very heigh induced drag. I ran some figures on fin drag budget and found it way more efficient to use a larger symetrical fin carring the same load at small values of Cl than to use a smaller fin with rudder. OK some see the drag as benificial in straightening up the plane but at small angles, side loads are more effective than drag so I'll go that way for now.

So how large should the fin be as a % of wing area?

Yes there is a delema that while a high AR fin is more effective, it raises the CP on the fin above the centre line of the plane and introduces a roll effect. While perhaps not very practical, a subfin can counter this rolling problem.

Anyone tried subfins with any success?

You see in theory at least if you get a left yaw, the faster right wing lifts, rolling the plane left. The fin reaction is also left to counter the initial yaw but that exacebates the left roll. A subfin would counter the roll at least in upright flight. Maybe the best that can be done is to try to keep the tail of the fuselege and therefore the base of the fin as low as possable.

OK desiginers, your thoughts?

You got some of it right, but not all. Al Rabe reported on another thread that his high fin on the Bearcat wasn't a problem at all. I don't know. Never worried about that.

As to per centage of wing area, well, it all depends on moments, fuselage side area, prop size--there is a lot to it.

On the yaw thing, we don't design yaw into the plane. In fact, the outward yaw can be detrimental in overhad tension. But there is a built-in yaw just from the fact that the plane is always flying in a left turn. The leadout angle is set for the angle needed to equal the sweep of the lines. We do not induce yaw with the leadout setting, nor do we try to counter it that way. The plane must be trimmed in its own right and then the leadouts set to follow the plane.

LAST EDITED ON Jun-15-04 AT 00:27 AM (CDT)
 
Ken,

The basic purpose of any rudder on a stunt ship has nothing to do with line rake or line drag. It is there to add stability in the Yaw axis. Making it movable adds the possibility of using it as a trim device and an active control to compensate for gyroscopic precession of the propeller.

Your thoughts are highly technical and assume, in my opinion, greater deflections and higher CL of the deflected surfaces than actually occur with a properly adjusted movable rudder. Yes, there is a theoretical left rolling moment from a deflected right rudder. You can speculate all you want about the theoretical possibilities for adverse reactions from using movable rudders, but the fact remains that my airplanes with movable rudders won the NATs three times, the Walker twice, second in the World championships, three NATs third places and a fourth. They never failed to qualify. Not bad for operating under the rolling, twisting, dragging handicap of a system so dogged with adverse reactions. Since you apparently haven't actually experimented with various rudder configurations, I assume that you are throwing out air balls. Theorize all you like, and ignore the obvious, that properly trimmed, movable rudders can be an asset. In my airplanes, they certainly haven't proven to be a detriment.

I'll say it again. In my experience, there is NO OBSERVABLE rolling moment with a proper rudder deflection. Even with the 11" tall rudder on the BBQB Bearcat, all of which is above the thrust line, there is NO OBSERVABLE rolling tendencies as a result of rudder movement. For most people the facts in evidence should be sufficient to quell this endless speculation as to possible dire consequences of having moving rudders without sub rudder area to balance the rolling moment.

Paul Walker is using movable rudders on his Mustangs. Maybe Paul doesn't understand how much he is handicapping his airplanes, but then, what would he know about building and trimming competition airplanes.

Hi Al

Yes I know there is a need for adjustable trim in this regard but I am not up to that yet. I did not mean it to be taken that way.
My intention was to get some thoughts sorted out given a simple fixed fin/rudder was used and not tackle the moveing rudder at this point.
As kids, we used to use a fixed fin and then a fixed offset rudder on the premis this would help yaw out and maintain line tension. I was refering to this practice when indicating that a larger symetrical fin is more efficient. Given that practice is probably an inappropriate use of fin/rudder anyway, I am also asking for some guidelines on fixed fin size, AR and angular setting able to damp down yaw variations. Also to straighten out where my understanding may be off or some concepts not as important as my reading suggests.

The concept of calculating fin area based on side area, similar to working out NP and static margin just does not work on R/C I assume it would not work on C/L either so is there a way that will give a good initial fin area. Then how do you tell if the fin is effective enough? Is that just subjective or can we make a more difinitive judgment from observation like can be made with R/C?

I am very into design and coming from a RC sailplanes background, C/L is somewhat different so I'm just trying to sort out the basic trim concepts before adding complexeties like moveing rudder. Not my intention to rubbish your famed rudder at all.

LAST EDITED ON Jun-15-04 AT 01:01 PM (CDT)
 
Ken, Welcome to the workd of control line stunt!

It is my opinion that most builders of stunt ships use vertical fin/rudders too small and often, located at less than the extreme end of the fuselage for adequate stability in the yaw axis. This is the primary cause of loss of line tension on outside maneuvers. I think the subject is too dynamic to respond to calculations of size, CL or AR. For example, there is just too much difference in the dynamic forces involved between an airplane doing a hard first corner in the hourglass and the second and third corners. In the first corner the airplane has plenty of speed and gravity is acting toward the bottom of the airplane. In the second corner the airplane is much slower after a very draggy first corner and the long climb to the top of the circle. Control forces here are dramatically different and gravity is now acting toward the inboard wing tip. Gyroscopic effects are still at full force, and perhaps greater if the engine has a substantial 4-2 break. While gyroscopic forces are possibly greater, yaw stability is severely lost from the speed reduction of the corners and climb. The airplane, a that point is poorly able to resist the effects of gyroscopic precession, so, typically, there isn't much tension to work with in those corners without a movable rudder. My best suggestions for a successful competition stunt ship is to use a fairly large fin/rudder, located well aft, for stability, and to make it, at least trimable, if not operable. The ability to screw in or out a little more offset is a valuable trimming device. Moving rudders naturally incorporate this ability to trim the (neutral elevator) rudder offset for optimum line tension in maneuvers. If the fuselage/fin/rudder side area is large enough for solid stability in the yaw axis, then rudder movement may be omitted but, in my opinion, the requirement for rudder adjustability for trimming remains.

Al,

I think you make a good point that often, we assume something is true based on logic or math or some observable phenomenon. But no one ever actually tries it. Much like the logic behind "X" planes; some things you just have to build and fly and see what happens. May turn out that everyone was right and the concept doesn't really work, but sometimes, it turns out that a concept is quite valid.

I can echo what Al rabe says regarding the fin "It is there to add stability in the Yaw axis. Making it movable adds the
possibility of using it as a trim device and an active control to compensate for gyroscopic precession of the propeller"

a few months back I did an experiment with a model that had an inverted landing and lsot the fin, which needed to be replaced.
I made up about 4 differnt thickness starting from symetrical airfoil only 1 cm wide max, to a Fully 1/2 airfoiled section (simialr to the nobler with a falt back.) that was a good 1" thick at the highpoint, and I personally found largest airfoiled Fin, to offer better results in terms of level flight tracking, which resulted in better line tenison in all manouvers. With my new ship, I retained this configuration.

I dont use an adjustable rudder. I dont feel the need. ( not to say others DONT benifit. )

Hi

For the moment, I want to ignore gyroscopic presession and just concentrate on aerodynamic stability. I'll consider other complexities later on.

On unteathered models, lateral stability is a ballance between diheral and fin effectivness. Small fin and large dihedral result in yaw instability (duch roll) while large fin and small dihedral result in spiral instability (spiral dive). So there are 2 distinctly different behaviours that can be observed and acted upon get a ballance between them.

On C/L models, the kind of side slip that leads to spiral instability would seem to be absent. There would therefore seem to be no upper limit to fin effectivness.

Is that supported in practice, no adverse behaviour from oversized fins?

What behaviour can be attributed to CL models with too large a fin?

What if the model was rolled in on a gust, would a large fin result in a spiral dive into the circle as the above principles suggest?

Is there a way to avoid this?

>Al,
>
>I think you make a good point that often, we assume
>something is true based on logic or math or some observable
>phenomenon. But no one ever actually tries it. Much like the
>logic behind "X" planes; some things you just have to build
>and fly and see what happens. May turn out that everyone was
>right and the concept doesn't really work, but sometimes, it
>turns out that a concept is quite valid.

Ron B.
F4Fguy

The concept does work,and the concept is valid.

While it's true you can trim almost any airplane to fly well,it's also true that to get the most out of any airplane,you can't beat a properly adjusted working rudder.

Ron,

Well, we're going to find out anyway. I just finished the installation of said working rudder on my miraculously saved PA plane. Hopefully, this will overcome the rather pronounced yaw experienced during outside turns at 45 degrees and during the top of the hourglass. There's a lot of adjustment available in the setup so we'll find out how in works this weekend.

why you guys are talking fin rudder off set,i thought i might ask,i have just started a brodak me 109 profile kit to which i will fit a os 35fp.instuctions state to use no rudder offset and this is shown on the plan.every model i have built has had some offset.what should i do offset it a bit or biuld as per instuctions.thanks guys Damien;)

To take the question a little further, as stated, all the older designs showed some rudder offset, some quite a lot. I have been for some time leaving the offset out and it works out OK. My question is, what about the designs, the Nobler comes to mind, that have an airfoiled rudder or some that actually airfoil the fuselage (Magician). In keeping with modern thinking, should the rudders/fuselages be built straight and symetrical or airfoiled?

Randy,

I am very curious for the results of your experiments. As I described in thread #9248 the installation of the RabeRudder worked very well for my particular airplane with a too short fuselage for the rudder to be effective in stabilising sudden inside yaw tendencies (such as when flying outside corners at low speed). B4 installation of the RabeRudder I tried to improve things by using more rudder deflection but that proved to be detrimental for other parts of the pattern. It will be important to also adjust line rake so that the mechanical forces and aerodynamical forces aren't working against each other. But I am convinced that you can substantially improve by optimising your trim in this respect, And hey, its so nice when the A/C keeps pulling during these parts of the flight.

LAST EDITED ON Jun-16-04 AT 11:44 AM (CDT)
 
Bram,

I never had any line tension problems at all. The plane pulls only slightly less overhead than it does in level flight. From the handle, I could see what looked like a slight yaw at the top of the outside square loops and at the top of the hourglass. The plane would nose in a bit then sort of rock back and forth on the yaw axis for 10 or 12 feet then straighten out. Folks seeing it from outside the circle said the yaw was pretty pronounced. Adding an adjustable rudder helped a bit, but didn't get it to go away. I've had the leadouts from very far forward to very far back with some improvement, but again, no real help.

So, we'll see what a wiggly rudder does. Should help at least.

If used intelligently and not overdone. Start with very little movement which will give you about what you are getting now and add just a bit at a time. The (elevator neutral) rudder offset is still another trim situation completely and the trimming process should address them separately.

Al,

As a start, the rudder moves almost not at all from neutral to full up. From neutral to full down give me about 3 degrees of outboard offset. The rudder is set for no offset at neutral.

I hope to fly it this weekend and we'll see what we get.

Ken, well trimmed plane has typically well ballanced leadouts to CG and thus there is only very little lift on rudder (does not matter what is the offset - the AoA is iprtant here). So it means the roll and induced drag is not a problem at all (or at least not comparable to other forces from centrifugal force and wing lift).

Regarding downfin - well I have it as well as all typical designs. Didn't you recognize than C/L stunt ships has very tall fuselages, especially under the tail? That fuselage side is also part of rudder area and thus area over leadouts and under leadouts is very similar.

All very interesting. Combat models with no rudder fly just fine. On the other hand I saw a Boxcar Chief (a fine flying Old Time Stunt airplane with a high wing and leadouts below the wing) lose its rudder in flight and instantly go into a tight spiral into the ground.

>Ken, well trimmed plane has typically well ballanced leadouts to CG >and thus there is only very little lift on rudder (does not matter >what is the offset - the AoA is iprtant here). So it means the roll >and induced drag is not a problem at all (or at least not comparable >to other forces from centrifugal force and wing lift).
>Regarding downfin - well I have it as well as all typical designs. >Didn't you recognize than C/L stunt ships has very tall fuselages, >especially under the tail? That fuselage side is also part of rudder >area and thus area over leadouts and under leadouts is very similar.

igor

Hi Igor

I had noticed. I Thought the deep fuselage was to aid line tension overhead and was curious about the very low AR fins. Not as effective in yaw damping as higher AR fins due to induced incidence. So I was trying to gauge how important people think the roll effect form taller fins is.

Still I would like an answer to if it possable for a fin to be too large on a CL plane and how large is too large?

:)

OK, I will ask a question instead of answer. If you have any axial torque from rudder, how you are going to distinguish torque from that large prop we use. And how the rudder interacts with axially rotating incoming air from prop?

I would like to know where I can get the information on setting up the Rabe Rudder. Are there any diagrams available that show the set up?

LAST EDITED ON Jun-20-04 AT 02:20 PM (CDT)
 
Actually, it was explained in great detail in the November/December 2001 issue of Stunt News. It is very easy to add to existing stunt ships but is typically Set for too much movement which can cause as many problems as it solves. IT IS A TRIM DEVICE, not a primary control. Apply gently for best effect.

The device is simple. It was first shown in the Bearcat article in the march 1970 AAM. That article also showed, for the first time MY invention, the sliding block adjustable leadouts. The movable rudder linkage was also shown on the Mustunt drawings and the photograph in the Stunt News article. It usually consists of parts of two nylon horns and a piece of threaded pushrod. No more-no less.

>To take the question a little further, as stated, all the older >designs showed some rudder offset, some quite a lot. I have been for >some time leaving the offset out and it works out OK. My question >is, what about the designs, the Nobler comes to mind, that have an >airfoiled rudder or some that actually airfoil the fuselage
>(Magician). In keeping with modern thinking, should the >rudders/fuselages be built straight and symetrical or airfoiled?
>Herman Green

Hi Herman.

One way to think of an airfoil is as a symetrical teardrop body imposed over a cambered centre line. If the camber is zero ( a straight reference line), the airfoil will be symetrical. It will produce no lift at zero incidence and the same lift (in opposite directions) at the same incidence angles in either direction form zero incidence.

If we graph the incidence on the horizontal and lift on the vertical axis, we get a fairly straight line until near stall. The slope of this line is very similar for all airfoils, sizes and speeds.
There are some notable exceptions at very low speeds.

When we camber (curve the centre line) above the reference line, the whole incidence/lift curve moves up. The whole lift range is shifted in the positive difection, the airfoil stalls at smaller negative incidence, producing less negative lift but at positive incidence, the stall occures at higher incidence and maximum lift is increased.
It also means that at zero incidence, the airfoil is producinq some lift. Zero lift occurs at a somewhat negative incidence. This angle is commonly denoted as Beta. This is the angle between the reference line and zero lift.

There are many kinds of camber lines and a flat botom airfoil like any other asymetric section can simply be considered to have a mean camber line that it is symetrical about.

What all this means is that if you compare a symetrical fin with a flat bottom airfoil fin of the same size and thickness where both reference lines are aligned the same, then the flat botom one is working at a higher aerodynamic angle of attack AAOA. To have the two fins working to the same AAOA and producing the same load, you would have to align the Beta lines of the 2 fins. The symetrical fin will appear to be set at a higher incidence geometrically (GAOA) but both are at the same incidence aerodynamically AAOA. Now as we increase the incidence of each fin by the same amount, both fins will continue to operate at the same AAOA and produce the same load until stall but the symetrical fin will stall first.

It is far easier to deal with symetrical airfoils as Beta is zero so you know what AAOA it is trimed at but a cambered section requires some performance data like polars to determine this. Another advantage of cambered airfoils is that minimun drag occures at some positive lift AAOA so drag can be optimised for the required lift. Another feature is that the centre of lift varies with AAOA so there is a varying pitching moment as well.

An airfoiled fin is working at a higher AOA and producing more yaw load than it appears however, depending on the camber, this may only be a degree or two.