Determining Rudder & Fin amounts of the Vert. Stab

[atjurhs]

Hey folks another design question:

I'm wondering if there is some calculation or "rule of thumb" to help me determine the percentage/amount of the Vertical Stabilizer that should be given to Rudder control vs. the fixed Fin? Basically, where should I plan for the Vertical Stab's hingeline?

I've already calculated total Vert. Stab surface areas to achieve a Vertical Tail Volume called "Vv" (shouldn't it be called "Vertical Tail Surface Area", but I suppose that's a differnt discussion) within the correct range of 0.02 - 0.04 for a PolyGlider, now I need to determine how to divide up that total Vertical Tail Volume (surface area) between the Rudder & the Fin.

THANKS in advance!

[Ollie]

"---now I need to determine how to divide up that total Vertical Tail Volume (surface area) between the Rudder & the Fin."

Use the Vv is to make it for stability. The maximum rudder attack angle and rudder area (not fin) is to use yawing control power.

In the case of no fin area, then the vertical tail is a "flying rudder." It depends on the aspect ratio too. If the airfoil's Cl maximum the profile attack angle plus induced attack angle equals total attack angle. For example, if the maximum Cl is 0.8 and aspect ratio is 2.5 then the total angle of attack is about 17 degrees.

In the case of hinge between fin and rudder, then the vertical tail is classed as a airfoil with a flap. It is complex to do engineer design. You must find the polars for the airfoil with flap maximum angle.

[atjurhs]

Ouch Ollie! I was hoping for something much more simplified! I kind of figured that the total EDA should also get accounted for, but I'm hoping for something far less complicated.

So what I am really hoping for is a "close enough rule of thumb"

Using the BD, I measured the Rudder/Fin at the mid-span mark and found about 55% rudder and 45% fin. Then I went through and calculted the surface areas of the two and computed the ratio and found about 65% rudder and 35% fin (hopefully Mark's numbers match these fairly close). So this surface area calc is my "back-up plan" for something "close enough" in case there is no easy to calculate "rule of thumb"

Can anybody say that I would I be really way off the mark if I use the 65% rudder and 35% fin suface area rule?

[BigTilly]

There are plenty of RES planes from yesteryear that had much smaller rudder fractions than that. They flew just fine. There is nothing wrong with a large rudder/total fin ratio, but if you have a large rudder (e.g. a flying fin), you need to make sure that your linkage is rock solid and your servo centers well, other wise you'll chase the rudder trim all over the sky.

[Ollie]

Just copy Dr. Drela's designs. If you make the rudder area too small then you increase the maximum angle and make the drag too much.

I like the flying rudder and found more than enough yaw control power. It is simple to design.

[atjurhs]

Ollie and Tilly, thanks so much for your thoughts and help!

I know how to design/implement a full-flying horizontal stab using either the traditional through the fin method like on BOTs and using a V-mount on a boom as on the BD/Supra. But I can honestly say I haven't ever seen a full-flying rudder design on any plane. BTW, this particular rudder will be going on a RES that will be using a full-flying stab with a V-mount and boom like the BD/Supra. So have you got a pic of how to implement it on a boom? Although I don't know that I will actually utilize this method - just interested to see it.

One more related question:

I understand the mechanical advantage of having more surface area closer to where the pushrod is attached, BUT is there any aerodynamic reason why Rudder's are typically larger/wider at the bottom and narrower at the top? Other than the mechanical disadvantage, it would seem to me that having more surface area higher up would (1) be in less turbulent air not being disrupted by the wing, and (2) also induce a roll component in addition to the yaw effect.

[BigTilly]

In full scale planes, rudders are narrower at the bottom for structural reasons. The control surface hinges drive load into the structure as a function of the area of the control being deflected. If you had to react a rudder that was bigger at the top than the bottom, you couldn't taper the fin vertically. This would put a large mass at the tip and would lead to more dynamic problems than a structure that tapered to the tip.

I always assumed that model planes were similar because it was pleasing to the eye (since they looked more like full scale planes that way). There are models with rudders that do as you suggest, but I don't think that they impart dramatically more rolling moment. The distance from the top of the rudder to the centerline is still very small compared to the wingspan.

[Ollie]

Design a vertical all flying tail. Put the axis so that the vertical 25% area ahead and 75% behind the axis line. The servo load is near zero. Put the axis 90 degrees to the tail boom. The 90 degree 1/8" carbon rod is fixed to the boom. The rod is about 2" above the boom and thru a 1/8" hole in the boom. The boom is wrapped with kevlar thread before and after the rod. The thread is wrapped with CA or epoxy so that the boom is not split in a crash. The boom is extended to the vertical tail trailing edge so the vertical tail isn't dragged a ground loop. An 1/8" ID aluminum tube receives the carbon rod and the tube is part of the vertical tail spar.

[BMatthews]

I generally use about a 40:60 to 50:50 fin:rudder area split for a poly model. It's worked out fine for me all these years.

The rudder and fin should roughly maintain the same chord %'s as it tapers from the base to the tip. That way when you deflect the rudder the overall camber change and resulting angle of attack change is consistent over the whole surface. At least that's my story and I'm sticking too it... :D In practice the aspect ratio of the fin and rudder is so low that you can get away with almost anything that looks reasonable.

I prefer a fixed fin and movable rudder for a few reasons. Less likelyhood of flutter being induced. Using a variable camber with a clean hingeline is supposed to produce a bit less drag at some deflection angles compared to an all flying surface. And finally in a flip over (not all that rare in contest landings) a fixed fin is less likely to suffer damage.

[DACeller]

Some saved links on the subject -
http://library.thinkquest.org/2819/glider.htm
http://www.nesail.com/articles.php?PHPSESSID=206dc55c655dad6b18f96cdb0e1 6538e
http://www.southernsoaringclub.org.za/a-basic-design.html

[ChuckA]

I normally use about equal fin and rudder areas. For RES models, the handling characteristics are determined by the wing dihedral angle and fun/rudder area. Calculations can get you in the right ballpark but fine tuning is often required to get the handling characteristics YOU want.

Back in 2000, I designed a new high performance RES model that used a three piece wing different from what I had been flying for the previous 20 years so I did all the calculations and came up with a model I named LilAn. Performance met expectations but the handling characteristics left something to be desired. After I folded the wing at the 2001 Nats, I rebuilt it enough to do some experimenting with fin/rudder areas and wing dihedral angles.

The first step was to tape a 1 inch strip of balsa to the rudder. Some improvement but not enough. I then increased the dihedral of the out board wing panels from 5 to 7.5 degrees. Much better but not quite enough so I extended the height of the fin and rudder by almost an inch. Problem solved and I used the results in the design of the next LilAn.

The photo shows the final modifications to the tail. The modifications were made with tape and balsa with no attempt to pretty it up. After all, this was a test bed and the repaired wing would never be able to take a full winch launch.

[atjurhs]

...The modifications were made with tape and balsa with no attempt to pretty it up...

"Beauty is in the eye of the beholder".

[atjurhs]

Is there anyway to determine if I have too much rudder and will acquire "Dutch Roll" behavior?

[Ollie]

Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic modes (others include phugoid, short period, and spiral divergence).

If the vertical tail area is too small, it means not enough yaw-roll damping (Dutch roll).

[BMatthews]

Is there anyway to determine if I have too much rudder and will acquire "Dutch Roll" behavior?

It's not the rudder or fin areas on their own. It's the total vertical surface area. The hinged portion doesn't affect the behaviour.


Dutch roll comes from too little fin/rudder area. Go a bit larger and it's all good. Go too large and the model becomes spirally unstable and wants to fall into the turn. The steeper the turn the more it wants to fall into the turn.

The dutchroll>spirally unstable range of response is also linked to the CG positions. Moving a CG rearwards lessens the Dutch roll effect but sharpens the spiral instability. Moving it further ahead does the opposite in each case.

[markdrela]

I Go too large and the model becomes spirally unstable and wants to fall into the turn. The steeper the turn the more it wants to fall into the turn. This is one of the most persistent myths in modeling. If the dihedral and tail-arm/span ratio are sufficient for positive spiral stability, as is the case with all good r/e gliders, then increasing the vertical tail will increase spiral stability, not decrease it. If you don't believe this, take a look at a typical poly DLG. Their VTs are absolutely enormous by free-flight standards, but they clearly have positive spiral stability, since they will fly stably hands-off indefinitely.

A fairly accurate indicator of spiral stability is Blaine Rawdon's parameter "B":

B = EDA * (tail_arm/span) / CL

If B < 5, then the glider is spirally unstable.
If B = 5, hen the glider is spirally neutral.
If B > 5, then the glider is spirally stable.

Note that the VT area doesn't even appear in this criterion. The VT merely determines how stable or unstable the glider is, depending on which side of the B=5 threshold it is. i.e. VT area affects
a) how fast it spirally diverges for the B<5 case, or
b) how fast it returns to level flight for the B>5 case.

The bottom line is that you really can't make the VT too big on a r/e glider. The bigger the VT the better it will handle. The only real drawback is more weight and drag.

[atjurhs]

Ahhhhh ha! I thought EDA had to come into a simple "rule of thumb" calc sooner or later :)

THANKS Mark!

Ok guys, can somebody (not to use up Mark's time on something so simple) help me with the claculations terms:

I'm guessing "tail arm" is the distance from the wing's TE to the vertical fin LE ?

Span of the wings? or the vertical fin?

CL is usually coefficent of lift for the wing. But for what Attack Angle and since I'm using the AG34-38 series of airfoils which do I pick?

THANKS again, I'm really glad not to just be repeating what everybody else does but to actually have some math/physics/aero foundation behind what I want/need to do for my scratch designed Bama Buzzard.

[markdrela]

I'm guessing "tail arm" is the distance from the wing's TE to the vertical fin LE ? Distance from CG to VT's quarter-chord.

Span of the wings? or the vertical fin? Wing span.

CL is usually coefficent of lift for the wing. But for what Attack Angle and since I'm using the AG34-38 series of airfoils which do I pick? Typical thermalling CL is between 0.6 - 0.8 , depending on airfoil camber. The angle of attack is irrelevant here.

[BMatthews]

This is one of the most persistent myths in modeling. If the dihedral and tail-arm/span ratio are sufficient for positive spiral stability, as is the case with all good r/e gliders, then increasing the vertical tail will increase spiral stability, not decrease it. If you don't believe this, take a look at a typical poly DLG. Their VTs are absolutely enormous by free-flight standards, but they clearly have positive spiral stability, since they will fly stably hands-off indefinitely.....

Sorry Mark but I'm going to have to call "shens" on this one. Your thoughts on the size not having a max size goes counter to my own observations and experiences.

Over the years and many models of both free flight and RC I've found that having poly or other higher dihedral amounts definetly reduces the sensitivity of the vertical tail to area sizing with the results being a wider range of usable sizes. But there is still an optimum size to achieve a vertical volume coeffieint that will be in harmony with the projected side area of the wing due to the dihedral. Go too far in making the vertical area larger and eventually the model will become spirally unstable.

Examples;

I had a Top Flite Metric. As sold to me the CG marked on the side was set darn close to the nuetral point. To get away from the harsh "three click trim range" this produced I moved the CG slightly further forward. Suddenly the tendency to fall into the turn during some steeper banks went away. This was my first intro to this whole issue. Another way to have "fixed" this would have been to use a touch more dihedral to bring the tail volume and wing's side area into harmony.

The Goldberg Comet Clipper Mk1 is a popular old time free flight model. However it's well known in the SAM community that this model when trimmed for good free flight performance with a rearward CG is spirally on the edge and it's a death sentance if the model drops into a steeper turn. The fix is to always run the model with a nice big cowling on it for more foward area which slightly reduces the true vertical tail volume coefficient by a hair.... a very critical hair as it turns out. Apparently the issue was noted back in the day since the Mk2 came out with the same center angle and added tip dihedral to deal with the issue. Obviously what it did was bring the wing's projected side area into line with the tail's vertical volume coefficient. I've seen two Mk1's firsthand that suffered from this issue. One was cured when the owner added the cowl back on and the other sadly is no longer with us thanks to this problem.

Yes the DLG's you're talking about do have larger fins than the free flight models. But even with those if you were to double the area on some of the larger ones I'm positive you'll notice that the handling in turns suddenly requires some "top rudder" to hold them from tighening up or if it's extreme enough that the model develops a tendency to fall into a spiral even from level flight.

A few years back someone did a chart of vertical tail volumes for a bunch of the classic gliders of the time. It was interesting to note that the values grouped in at a lower zone for all the aileron models of the time and that the poly models had a higher optimum value but there was still a specific value.

Blaine's equation leaves far too much out of the picture for my tastes. By totally ignoring the tail size let alone how the tail size to tail moment ratio interacts with the wing's and in some larger fuselage designs' projected side area. I know Blaine has produced a LOT of good stuff over the years but I'm afraid that this isn't one of them. As a quickie indicator for one aspect yeah, it's fine but the rest of the recipe needs to be considered as well.

[markdrela]

The Goldberg Comet Clipper Mk1 is a popular old time free flight model. However it's well known in the SAM community that this model when trimmed for good free flight performance with a rearward CG is spirally on the edge and it's a death sentance if the model drops into a steeper turn. Aha. Here's the main source of contention. The "spiral death dive" often referred to by free-flighters is really a combination of spiral instability and pitch instability (or "tuck-in" in RC jargon). Calling is simply "spiral instability" is not correct, at least as defined by any classical book on airplane stability (Etkin, Nelson, etc). If you look up spiral stability in any of these texts, there is no mention of CG position, simply because it's largely immaterial there.

Blaine's equation leaves far too much out of the picture for my tastes. By totally ignoring the tail size let alone how the tail size to tail moment ratio interacts with the wing's and in some larger fuselage designs' projected side area. I have to disagree here. Blaine Rawdon's criterion is a somewhat simplified version of the classical spiral-stability criterion, e.g. equation (5.42) in Nelson. The classical criterion has been around for a long time, and it works. Blaine's simplification assumes that only the VT contributes to the glider's yaw stability and yaw damping. This assumption is quite accurate for most if not all RC gliders. Maybe not for some old-timer gliders with their deep front fuselages.

In any case, the spiral stability of concern to the RC r/e glider pilot is the same as the classical definition, not the FF definition. For example, a spirally unstable r/e glider (in the classical sense) will tend to steepen its turn, and require some opposite rudder, which is clearly undesirable. Whether it is spirally unstable in the FF sense is largely a pitch-stability and CG issue, not a turn handling issue.

So I'll stand by my statement that increasing the VT area on a spirally-stable r/e glider will not cause it to become unstable --- quite the opposite.

This still leaves the question of why VT area affects the "spiral death dive" of a FF model. One reason I've noticed back when I was flying FFHLG gliders is that a large VT prevents the rapid yawing "flip" from climb into level flight, leaving the glider in a steep turn. The turn then initiates the "spiral death dive". The same glider, when lightly tossed into a level glide, showed no tendency to spiral in.

[BMatthews]

...So I'll stand by my statement that increasing the VT area on a spirally-stable r/e glider will not cause it to become unstable --- quite the opposite......

I'm only stating what I found from my own experiences over a number of models that were both my own and that I witnessed the flights at issue belonging to others. I'm satisfied with my thoughts based on those observations and have proven them to myself.

In one case it wasn't a poly model but an aileron model that I wanted to learn about this stuff from. I deliberatley started with too much vertical area and started out playing with the CG range to some extent. It's not a major influence but if the vertical area is to one extreme or the other then a shift in CG will acentuate or reduce the effect. From that first step I found that the spiral stability was related to the CG position. With too much vertical area and a pitch stable far rearward CG the model would not even fly straight in conjunction with the excess vertical area. It would constantly try to fall off to either side. Further back reduces the stability and promotes spiral dive steepening/tightening unless countered by control inputs. From there I started reducing the vertical area from flight to flight until the area became small enough that the tail suffered from poor tracking and 'dragging low" in turns and adverse yaw showed up from use of the ailerons even at higher speeds. Obviously at this point I had too little damping from the small size. At one point where I hit the sweet spot it was a delight to fly and tracked well in turns. So with a change of vertical area I went from spirally unstable where it was like balancing a broomhandle on your palm to the opposite end of the spectrum where it was sloppy with a poor sense of direction.

A lot of your DLG's have sheet surfaces for the tails so they should not be too hard to modify or swap a fin and rudder. I invite you to double the vertical tail's area where the area is well above what we consider normal and see what happens.

[markdrela]

A lot of your DLG's have sheet surfaces for the tails so they should not be too hard to modify or swap a fin and rudder. I invite you to double the vertical tail's area where the area is well above what we consider normal and see what happens. Jeez. A narrow-wing poly DLG isn't all that different from a typical F1A ship in layout, except that the DLG has something like 5x larger vertical tail volume. Surely 5x larger is "well above normal" from a FF'ers viewpoint. How much bigger does it need to get? 10x? 20x?

In any case, I've done the classical stability analyses of even absurdly large VTs (for example, the Bubble Dancer with quadrupled VT area). It remains spirally stable.

[BMatthews]

Jeez. A narrow-wing poly DLG isn't all that different from a typical F1A ship in layout, except that the DLG has something like 5x larger vertical tail volume. Surely 5x larger is "well above normal" from a FF'ers viewpoint. How much bigger does it need to get? 10x? 20x?

In any case, I've done the classical stability analyses of even absurdly large VTs (for example, the Bubble Dancer with quadrupled VT area). It remains spirally stable.


I'm not sure. However I do know that the way my Top Flite Metric flew that it was near to the edge.

Our typical free flight models for modern designs tend to run to verticals as small as possible. It helps the models with reacting to thermal currents to tend to hunt around in the circle and self center in the lift.

[Ollie]

If you love wisdom, forget myth and ego. Be humble. Then, learn about the truth.

[atjurhs]

Mark,

I'm sure I'll be first in line and knocking others down to buy your book! When is it coming out in print?

[ChuckA]

I have never had to reduce fin and rudder area, however I have often had to add fin and rudder area to get the handling I wanted.

When I first began reading MAN in 1947, C.H. Grant had a column about model design and he was a great believer in the Center of Lateral Area school of free flight design. It was a battle between the CLA and pylon schools of design. Anyone remember the Wedgie? Fin area was critical in either case but I don't think the free flight arguments have much applicability to modern RC sailplane design. The high thrust free flight models such as Carl Goldberg's Viking and similar free flight designs of the 60s were really pylon models designed by the CLA theory.

[atjurhs]

A fairly accurate indicator of spiral stability is Blaine Rawdon's parameter
Note that the VT area doesn't even appear in this criterion.


Yep, that does seem a bit counter-intuitive. I suppose it is one of those terms that cancels out in a much more complicated equation :rolleyes:



A fairly accurate indicator of spiral stability is Blaine Rawdon's parameter "B":

B = EDA * (tail_arm/span) / CL

If B < 5, then the glider is spirally unstable.
If B = 5, hen the glider is spirally neutral.
If B > 5, then the glider is spirally stable.


The calcs for my Bama Buzzard (http://www.rcgroups.com/forums/showthread.php?t=720772) came out to 6.5, 5.6, or 4.9 depending upon which value of CL was input into the calc. I would have to I think I'm pretty much good to go :)



The bottom line is that you really can't make the VT too big on a r/e glider. The bigger the VT the better it will handle. The only real drawback is more weight and drag.

Well my rudder/fin is sporting 100 sq.inches! That thing looks huge already! I'd hate to have to add onto it!

THANKS for all the help guys!

[markdrela]

Yep, that does seem a bit counter-intuitive. I suppose it is one of those terms that cancels out in a much more complicated equation :rolleyes: That's exactly right. Spiral stability depends on the ratio yaw_damping/yaw_stability . Ignoring constants of proportionality, these two quantities are given by the following products:

yaw_damping = VT_area * tail_arm^2
yaw_stability = VT_area * tail_arm

So the ratio is proportional to...

yaw_damping/yaw_stability = tail_arm

i.e. the VT area cancels out, and one factor of tail_arm remains. This is precisely why Blaine Rawdon's criterion has the tail_arm factor.

If there are other aerodynamic surfaces which significantly influence yaw damping and yaw stability, the VT area and other areas will not cancel, and this simplified analysis doesn't hold. One example is a small forward fin, like those used on compass-guided FF gliders, or just a deep forward fuselage. Such forward fin area will increase yaw damping but decrease yaw stability. So the ratio
yaw_damping/yaw_stability
will strongly increase, and spiral stability will increase as well. This is why old-timers saw improved spiral stability from deep forward fuselages. The "center of lateral area" theory was an effort to explain this, but didn't address the real mechanism.

In any case, one could add a forward fin to an RC glider to improve its spiral stability and any overbanking tendency. However, if the forward VT doesn't have a moving rudder, it will reduce the rear rudder's authority (and look goofy besides :rolleyes: ). Increasing the dihedral is a better fix.

[Robglover]

In any case, one could add a forward fin to an RC glider to improve its spiral stability and any overbanking tendency. However, if the forward VT doesn't have a moving rudder, it will reduce the rear rudder's authority (and look goofy besides :rolleyes: ). Increasing the dihedral is a better fix.

Uh Oh. Todd is really into "goofy looking." I can see this new design project now.... :eek:

[atjurhs]

Uh Oh. Todd is really into "goofy looking." I can see this new design project now.... :eek:

Oh Rob, you're just jealous of my "pantyhose shirt" :eek: Sure keeps ya cool on those hot thermal fields! I'll get ya one for Christmas :D what color would you like yours to be?