Conference vmszoo::rc

OK - first Flying Stab area.

I need to build a light tail - don't we all.
Surely if I can reduce the area of the stab I save weight.

So what are the symptoms of a flying stab that is slightly
undersized?

What if it is slight oversized?

What if it is grossly under or over sized.

Don't count weight change as a symptom.

Bye          --+--
Kay R. Fisher  |
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     Kay,
    you run a computer radio.  Why not use a v tail? 
    
    Tom

    The answers would depend somewhat on whether the stab area was reduced
    by reducing the chord or the span or both. Let's assume that it
    was changed by reducing both, and the aspect ratio stays exactly
    the same as the original.
    
    Slightly larger/smaller area: Defining slightly as <10%, I would
    predict little or no visible difference within 95% of the flight
    envelope. This presupposes that there is not some other marginal
    aerodynamic condition existing in the design, which might be *pushed
    over the edge* by stab changes.
    
    Grossly larger area: Pitch sensitivity increases. Less stab travel
    is needed for a given pitch change. Increased drag. Lower speed
    might be noticeable.
    
    Grossly smaller area: Opposite of above. Tail may drop more noticeably
    just off launch. Efforts to regain pitch sensitivity by increasing
    stab travel will result in an earlier stab stall, resulting in even
    greater drag, loss of control. Drag contributed by the stab may
    wind up being greater than with a larger stab, due to operating
    at larger deflection angles more of the time.
    
    I love notes where you con pontificate shamelessly. This could get
    better than SOAPBOX.
    
    Terry

>                   <<< Note 1300.2 by SA1794::TENEROWICZT >>>
...
    you run a computer radio.  Why not use a v tail? 
...    
Boy - it didn't take long for this note do digress.

I hate V-tails and my computer radios don't like them
either.  They think V-Tails are something on a Beech Bonanza
and they expect ailerons and elevator on the right stick
and rudder on the left.  To convince your radio you have
a 2 channel glider V-Tail you have to convince him it is
really a flying wing (Elevron).

Back to the question - what are the symptoms of too much
and too little full flying stab area.

Bye          --+--
Kay R. Fisher  |
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This should be fun. I'd point out though that the answers to the above
questions are applicable to both powered and un-powered planes. Here goes...

Stab too small:

    The biggest problem should be pitch stability. Taken to the extreme
    what you wind up with is a flying wing, which would need some sort
    of reflex in the airfoil to be stable. What happens would depend on 
    whether the CG was placed so that the stab needs to create downforce or
    lift. If the stab needs to create downforce then at some point
    it becomes to small to keep the nose up.  If the stab needs to create
    lift then tail will just drop, and keep dropping, and the whole plane
    will probably tumble end over end.

Stab too big:

    I'm not sure that this would create a fatal problem. Taken to the
    extreme what you get is a canard. As the stab surface area goes up
    the aeordynamic center will also shift rearward so the CG can be moved
    further to the rear. Other than excess drag i'm not sure what the 
    handling implications are. 

Glenn Schrader

        Changing the wing chord, wing area, stab area, or stab moment all
        change the neutral point (center of pressure) of your model. The
        stability of the model is the relationship between the CG and the
        CP. Thus any of the above change the stability of the model.
        
        Changing any one of these variables without adjusting one or more
        of the rest will make your model more or less stable in flight.
        For small changes, it may mean that you need more or less stab
        deflection to control your model. Decreasing stability of a model
        can make it harder to control, but it can also let you do things
        that a more stable model cannot. An extremely stable model is not
        good for aerobatics, but makes a great trainer.
        
        Larger changes can result in a model that is neutrally stable.
        Anything beyond this point results in an unstable model. No matter
        what you do, it will not fly, unless your reaction time is as good
        as on of the guidance computers used on the Space Shuttle or X-29.
        
        Many reference books discuss glider stability. There are all sorts
        of equations that will tell you how stable your model is, and
        allow you to predict what effects all of these changes will have
        on your model. Look in your locak public library, or the AMA
        catalog you just received for a good set of reference books.

    no need to get overly complicated...

    Too small, the effect is obvious, same as too small a veritcal fin.
    Loss of stability and control.

    Too big?  Well, I've seen planes fly that have Stabs the same size as
    the Wing, so clearly, in practice, you can go as big as you want.  As
    stated, a Canard has the Stab larger than the wing (in a way).

    The only thing you really need to avoid is choosing an Aspect ratio for
    the stab that is near to, or lower than that of the wing.  You ALWAYS
    want the wing to stall before the stab.  Higher aspect ratios stall
    later (I may have this backwards, I can't remember).  A stall of the
    Stab can result in a condition called "Deep Stall", where loss of stab
    effectivness do the the stall can't be escaped, and the plane crashes.

    The same condition can occur if the wing turbulence blankets the stab.


    Randy

This is getting good - I think we have a consensus.

Next question (but if you have any comments on Flying Stab Area
please put them in as well).

If I had an adjustable length tail boom and could vary the
length of the tail feathers what would happen as I shorten
or lengthen it?  

Again assume no weight change and that somehow the correct CG is maintained.

Bye          --+--
Kay R. Fisher  |
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    As the tail boom length gets shorter, pitch and yaw sensitivity
    to control inputs increases, and external forces have a greater
    effect on neutral stability. There may be an increased tendency
    to Dutch roll when turning.
    
    Longer tail boom: opposite of above.
    
    Terry

The tail portion of the fuse is essentially a lever arm. The stab exerts a
force on the lever. As the tail lengthens the stab needs to generate less
force to have the same effect so that stab can have a smaller area. A shorter
tail will need a larger stab since it doesn't have as much leverage. Another
effect is pitch damping. A longer tail will have greater damping will be
more stable than a shorter tail. A long tail with a smaller stab will create
less profile drag but the additional structure will increase weight which will
in turn cause an increase in the induced drag created by the wing. Another
problem would be the structural integrity of a long tail. 

Glenn Schrader

    And you still have a potential deep stall problem with the tail in the
    "shadow" of the wing

    On the subject of tail feathers, what effect does a swept stab
    have over a conventional stab.  Also what is the aerodynamic advantage 
    of a T-tail as it relates to sailplane models.  I am modifying my
    Sagitta 600 aand have considered making it a T-tail.  It was very
    pitch sensitive in standard config.
    
                                              Thanks,
    
                                              Jim 
    

    A swept stab moves the center of pressure back a little, giving
    the same effect as increasing the aft fuselage length.
    It can also reduce stab drag if the tips are done just right.
    
    A T-tail puts the stab up out of the wing turbulence, where it will
    be more effective for a given area. Some schools of thought say
    that on a steep climb out, such as just after launch release,
    the wing can blank the airflow over the t-stab resulting in less
    control. These are religious issues, take your pick.
    
    A t-tail won't make much difference in pitch sensitivity, if anything
    it might get worse, if the stab becomes more effective.
    Have you tried moving the CG forward a little on the Sagitta ?
    
    Terry

    I've heard from one glider pilot that a T-tail has less drag than a
    regular tail, but his argument sounded pretty weak. I thought it was up
    there primarily to get it out of the wing wash, and also away from the
    ground on landings.
    
    On tail moment, Helmut has mentioned that he likes the gliders with
    longer tails because "they groove better"... his words. And for a
    treatment of the subject that seemed based a lot more on fact than
    conjecture, see the articles on dihedral and spiral stability in Model
    Aviation. The point that stood out in my mind was the link between tail
    length and stability in a turn. When you are circling, as in a thermal,
    the natural tendency of the plane is to spiral in tighter and tighter.
    This is a function of dihedral and coefficient of lift and some other
    things I don't understand. The circling flow of air over the vertical
    stab creates a force that counters the tendency to turn in. The longer
    the tail moment, the greater the canceling force. So some designs will
    feel more comfortable in a turn than others.
    
    Dave

    Several advantages of a T-tail that I picked up at an excellent
    seminar by Phil Renaud yesterday, on F3B trimming and flying
    techniques:
    
    The stab acts as a tip plate at the top of the fin and reduces
    span-wise, ie vertical, airflow along the fin. This reduces turbulence
    and drag around the fin and stab.
    
    A T-tail design will usually have less of a pitch change as flaps
    are lowered. This is because the stab is up out of the downwash
    created by the lowered flaps. A stab mounted lower would feel this
    downwash on its top surface, pushing it down, raising the nose and
    requiring more down elevator compensation to keep the nose level.
                                                                  
    Phil also mentioned that he and some other F3B pilots are getting
    some new Fisher (German) F3B planes. They are very light(for F3B
    designs) at ~85 oz. and use T-stabs with separate elevators. It
    gives very positive elevator control with smaller control deflections.
    1190 DM delivered to the U.S. We should see them by late spring.
    
    Terry

    This is an interesting note so I think I will try y to keep it going.  
    
    Some 20 years ago, a friend and I worked with Frank Zaic ( you ole
    timers may remember him since he designed many "Thermal" gliders).
    Frank designed a ply fuse which was of the pod and boom type.  We
    began by using a Clark Y (flat bottom) air foil withe a constant cord
    of 9" for the middle part of the wing and a tapered cord for the
    outside part of the wing with polyhedral.
    
    We started with a 7' span and gradually increased only the span to 14'
    without changing any other part of the plane.  From observation (Kay's
    point) we found that a 12' wing span gave us the most desirable
    results in that we maximized performance (duration, pentration, etc).
    Since both of us has the same glider with 8 different wings, we would
    fly "sise-by side" to attain the same flying conditions.
    
    Please note that Frank did all of the al the mathe work in designing
    each wing span and we did the flying.  We had a heck of a log of fun
    doing this project over a two year period of time.
    
    By the way, we did manage to place in the top 1/3 in most of the meets
    back in the good "ole" days.
    
    Regards,
    
    Ollie  

    I am currently awaiting the arrival of plans for a Balsa USA Allure,
    which is a 2-meter design from the mid 80's, it has a 9% airfoil, V-
    tail with ailerons.  From the pictures I have seen it looks like a
    constant chord wing.  Since no modern designs that I have seen have
    a constant chord, what performance drawbacks/enhancements are
    asasociated with consstant chord wings?  I would think vortex induced
    drag would be higher, but maybe tip stalls would be less?  
    
    
                                                   Thanks,
    
                                                   Jim

    A constant chord wing does not have the theoretically desired
    elliptical lift distribution, so you're right, induced drag is higher.
    
    But it's easier to design/build and that's the main reason to use
    it.
    
    Tip stalls may be less, but less than what ? A tapered wing with
    washout will be less tip-stall prone than a constant chord with
    no washout. All else being equal.
    
    Which reminds me, I'm awaiting a Jade Impulse, backordered from
    Hobby Shack, also V-tail with ailerons.
    
    Terry

    Jim, I have had good luck flying constant cord wings by turning the
    wing tip downward (either by using 1/16th plywood or block balsa sanded
    to a very thin thickness).  Some "experts" call this an inverted
    horner tip.
    
    The tip is usually approvimately 1 inch downward and fits from the
    leading edge to 1-2 inches past the trailing edge.  This tip supposedly
    keeps the tips flying and the vortex of each tip does not begin until
    the air has passed the wing area.
    
    If you decide to use this suggestion, you must keep this type of tip
    90% to the fuse.  Otherwise, you may have some interesting flight
    performance from your ship.
    
    Let us inow if you decide to go this way.
    
    Good luck on your project.
    
    Regards,
    
    Ollie         

    Well this is an interesting note.  Right up my street.  Next question:
    
    What is the effect of varying the vertical stab/rudder aspect ratio.  

    I have a HOB 2x4 that I am using to learn slope flying.  It is a real
    solid ship except that the tail keeps comming off/breaking.  Maybe it
    is something to do with my cartwheel landing style.  It can survive a
    single cartwheel on landing, but the tail breaks off on a double
    cartwheel. 

    I am just about to do a third tail.  On the second tail I made the
    horizontal area smaller, pushed it back and gave it a full width
    elevator.  I made the vertical fin a little larger but shorter and the
    rudder wider.  It worked, but did not last long enough to get a lot of
    real experience.  I will build the third tail stronger but the same
    shape as the second unless someone can give me some good pointers. 

    What flight characteristics of your 2 X 4 are you trying to change?
    
    Have the tail mods changed things in the direction you hoped/expected?

    Theoretically, increasing the fin/rudder aspect ratio will
    reduce drag. In the real world you'll never notice the difference
    with that plane and in the conditions you're operating in.
    
    Changing the vertical surface area can change the spiral stability
    which includes things like how responsive is the rudder, how well
    it holds a given turn/bank angle, etc. Again you're not going to
    see massive differences unless you go way beyond the bounds of
    normality in surface area.
    
    Slope flying conditions can mask subtle changes more readily than
    thermal flying. The 2X4 wasn't meant to be world beating design,
    so don't expect radical differences in the way it flies after any
    mods.
    
    Keep practicing, you'll notice that that does more than anything
    else to improve your planes performance. ;^).
    
    Terry
                  
    

Well there is a good point, what are my goal in making the mods.  Reasons are:

1. I had to build a new tail
2. I have just read Dave Thornburg's book 
3. A smaller tail may be able to handle my landing style better. 
4. I do not like the look of the 2x4 tailfeathers

The 2x4 has a pointy tail in plan and this makes it bigger than it need be.  In 
section it has a square trailing edge with no taper.  People had commented that 
the vertical fin looked to be too small and it seemed to have a slight stability 
problem from this.  The goals were to rectify these problems. 

I flattened off the top of the rudder and made it slant up a more at the bottom 
and to make up for this I made it longer.  So it has about the same area as the 
previous rudder but more moment.  However the trailing edge is about 20% 
shorter.  After making it, I notice that the vertical fin/rudder is now longer
than it is high, which is the opposite of most planes.  This set me wondering 
whether I had done the right thing.

I did not get a lot of flight testing before I learned a valuable lesson about 
the need to build a strong tail.  However I did notice that the rudder was less 
effective than the previous one.  

The more general question is what is the best shape for a control surface, 
short and fat or long and thin, and why.  

    It sounds like you did all the right things to correct your problems.
    
    On a slow flying simple design such as the 2X4, a shorter, fatter
    (low aspect ratio) tail will work okay.
    
    In general, a long, thin tail (high aspect ratio) will have less
    drag than a short fat one. It will also probably be more fragile
    and will be less responsive at low speed.
    
    So which one is "best" is based on the flying conditions it will
    be in, most of the time.
    
    Getting a smooth radius to the leading edges of the fin and stab,
    and sanding in a little taper with a thinned and rounded trailing
    edge, will reduce drag quite a bit on those slab balsa tails.
    Never have a square corner on any control surface exposed to the
    airflow.
    
    Increasing the surface area of the rudder or elevator won't
    hurt, but try not to have less than the stock surface area.
    
    Terry

    Like Terry says, a high aspect ratio tail will have less drag & be more
    efficient than a low aspect ratio one, but it will also provide a lot
    more leverage to break it off in the event of a crash. Full size
    performance sailplanes that don't have 'T' tails usually have high aspect 
    tails for the same (low drag) reason.
    
    Terry - why would it be less efficient at low speed though?
    
    Malcolm

    Ummm.. I think I said "less responsive", meaning that if a design
    with a smaller tail area is optimized for a certain (higher) speed
    range, then it will be less responsive at a lower speed than a tail
    with greater area would be at the same low speed. 
    
    The large tail would possibly be more draggy or overly sensitive
    at the higher speed.
    
    Terry