Conference vmszoo::rc

1575.0. "Flying Wings" by KAY::FISHER (A watched pack never peaks.) Mon Nov 15 1993 13:13

Usenet info on flying wings.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
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Since there have been a number requests for more information about
my experiences with flying wings via email I thought I'd post some
of my observations to the newsgroup.
 
SPEED RANGE:
There are many variables with respect to the range of speed your wing
will achieve and there is no single factor responsible but the aspect
ratio and wing-loading seem to have the most effect. Reducing the 
aspect ratio will greatly widen the speed range but at the cost of 
efficiency at low speed where the drag produced by tip vortexes will 
become significant.  As the wing-loading increases a greater AOA is
required at any given airspeed so the drag goes up and so does the
rate of descent.
 
Here's a comparison of my own models:
 
                span  chord  A/R   weight loading  airfoil  speed
Power           38"    14"   2.7:1  3lbs   13oz    sym 12%   5-90+mph
Slope           48"    12"   4:1    2.5lbs 10oz    sym  8%   5-40 mph
Thermal         96"    12"   8:1    4lbs    8oz    rfx 10%   5-30 mph
 
Power:
Flying at 5mph requires a very high angle of attack and a bit of
throttle.  Control response is still good because of the prop-wash
over the elevons.  Since there is no yaw control (rudder) on my
wing there are problems in maintaining a low-speed AOA greater than 
about 45 degrees (it will slowly yaw to one side requiring a change of
heading - bank- to correct).  Top speed is limited only by the
ratio of thrust to drag and since I've got a fairly think airfoil
(for strength) it seems to be as fast as most pattern ships but
certainly not in the same class as your average pylon racer.  The low
weight means that speed at full-throttle is pretty-much constant
regardless of the model's attitude.
 
Slope-soarer:
In the case of the slope-soarer a thin section ensures a good top
speed due to minimal drag.  At the bottom end of the speed range
the low-aspect ratio causes quite a rapid rate of descent but this
is a desirable trait on the slope, making landings much easier.  
Control response is decidedly "mushy" at very low speeds.  I've never
had to ballast this bird to fly.  If it's too windy to fly it then
the odds are that nobody else is flying either!
 
Thermal-soarer:
The thermal ship will fly as slowly as my slope-soarer but has a
much lower rate of descent at a given air-speed thanks to the lower
wing loading and lesser drag of the higher aspect ratio.  Top-end
for this ship seems to be limited by the amount of drag created
by the reflexed airfoil.  As with the slope-soarer, control is very
slow at low air-speeds and there's a bit of a trade-off in spot
landings between maintaining good control (speed) without overshooting
the mark (there is an incredible amount of ground effect).  As with
the slope-soarer, I don't bother ballasting this ship as there is very
little improvement in top speed but a noticeable deteriation in the
low-speed rate of sink.
 
CG:
With regards to CG positions, on a wing with a symetrical airfoil
my experiences show that 1/3 chord is about right which means
that if you build your wings like I build mine it's right on the
mainspar.  On a reflexed wing the CG seems to be better if it's
a little closer to the 30% mark, any further back and recovery from
unexpected changes in attitude are too slow.
 
AIRFOILS:
Personally I'm not a fan of reflexed airfoils, to me they look
like an inverted undercambered one so I figure that you're 
sacrificing an awful lot of lift for stability.  I'm continuing
to experiment with sections for thermal-soarers  but I'm most
happy with fully-symetrical for power and slope use.  These ships
are trimmed with the elevons almost flat (ie: no reflex) and the
neutrally stability of the airfoil results in a really "groovy"
airplane (ie: you "fly" the aircraft rather than disturbing it
>from an inherently stable flight-path as is the case with a 
typical "trainer" setup).
 
Basically everything is a trade-off with airfoils and I'm
almost at the point of considering the use of a conventional
high-lift airfoil (ie: negative stability) in conjunction with
a helicopter gyro to create a "fly-by-wire" bird.  I wonder if
any contest rules forbid such a smoke and mirrors approach?  The
idea is to avoid all the extra drag and loss of lift that a
high-lift airfoil creates whilst still producing an aircraft which
can be controlled by mere mortals.
 
VERTICAL STAB(S)
A single fin mounted in the center of the wing on the top-surface
should be the most efficient approach but twin fins look much better.
Problems with this are that the fins are blanketed by the wing at
high angles of attack which can reduce stability at slow speed.
Tip-fins sound like a good idea because they also offer the prospect
of reducing tip vortexes (like a 747, vari-eze etc).  The problem
I've encountered with these is bad dutch-rolling at low speeds without
any noticeable improvement in any other part of the flight envelope.
So.. my advice is.. use twin fins on the top-surface and a smaller
underfin (or two) on the bottom of the wing.  These bottom fins can
also double as landing-gear/runners for gliders.  Use tip fins as well
if you must but keep them small and don't expect anything other than
an asthetic effect.  Sweeping the vertical stab can increase the tail
moment slightly but most people do it because it looks nice.
 
CONTROL SYSTEMS:
Elevons are the best option for most ships.  In the case of the
slope-soarer or power ship with a fully symetrical airfoil and low
aspect ratio you can perform almost any manouver except stall-turns
and knife-edge flight (as well as many others such as an ascending
spin under full-power...this has to be seen to be believed!).  There
is no adverse yaw with these ships.  If a higher-aspect ratio soarer
is your choice then the reflexed wing section that these birds
normally use also tends to nullify the effect of adverse yaw since
both elevons are already "up" at neutral so the upward travelling
surface creates about as much drag as the downward travelling one.
I've built my own RC gear so find that electronic mixing is the best
way to handle elevons.  Regardless of how well they're made, any
form of mechanical mixer is going to introduce slop and as I found
to my detriment many years ago, even a little slop in a fast-moving
powered wing can result in flutter.  My earlier .20 powered
wings (albeit with a highly worked motor) were reaching well over
70mph in flat-level flight and since I had no throttle on the engine
I used to cut it by over-reving in a vertical dive.  Unfortunately
I lost the first bird (using a mechanical mixer) to control-flutter 
when one of the elevons splintered into a cloud of balsa during one 
of these dives (very spectacular!)  Needless to say that now I use
laminated ply/balsa elevons now and always seal the hinge gap.
Rudder control is almost a complete waste of time (and weight) since
as I mentioned, there is an almost zero tail moment unless you extend
the fuselage rearwards (in which case you might as well throw a
tailplane on it as well and go back to conventional designs).
With suitable electronic mixing and full-span elevons control
linkages are an exercise in simplicity and add nothing to the weight.
 
DIHEDRAL:
Leave it out!  It only complicates wing building and in the case of
power or slope-soaring models it just detracts from the wonderfully
neutral handling characteristics.  In the case of a thermal soarer
I've found that angled tip-plates like an inverted hornier(?) tip
do the same job.  Don't be tempted to turn them into full-blown tip
fins though our you'll get the dreaded dutch-rolling.
 
SWEEPBACK:
Same as for dihedral, not necessary.  Lots of earlier flying wings
used sweepback to improve stability (it has a dihedral effect). In
my opinion it only serves to complicate construction, weaken the
structure (ribs aren't 90 deg to the spars/le/te etc). On a highly
tapered wing sweepback will reduce tip-stalling but at the cost of
overall aerodynamic efficiency.  If you *must* use a tapered wing
then sweep the leading edge only and use a slightly thicker section
at the tip.
 
That's enough of a rave for now.  If anyone has any other specific
questions I'd be more than happy to field them.
 


++++++++++++++++++++++ The full NEWS header follows +++++++++++++++++++++++++++
News Article 19750
Path: nntpd.lkg.dec.com!pa.dec.com!decwrl!waikato!aukuni.ac.nz!nacjack!bruce
Newsgroups: rec.models.rc
Subject: Flying Wings
Message-ID: <[email protected]>
From: [email protected] (bruce simpson)
Date: 14 Nov 93 13:26:24 GMT
Keywords: FLYING-WINGS
Distribution: world
Organization: Malleus Maleficarum - Nil Mortifi Since Lucre
Lines: 153

    I must mention that I have concerns about two points in the previous 
    note from Usenet.
    
    1. CG @ 30%. No way will this work.
    
    	Wanna watch a gremlin fly at 30% - not for long!. 
    
    	12-16% of average wing chord is the only chance of a glider or
    	power wing working..
    
    
    2. 	It is mentioned that flutter from a mechanical mixer caused the
       	demise of a powered wing. It isn't the mixer its the position of the
       	central horns. BTW the only flutter I have ever experience on the
       	Gremlins is with the ones that are driven by my 347's and 388 and
       	they use only electronic mixing.
    
    	The long strip elevons are the problem. They must be rigid and
    	preferably tapered to the tips.        
                                         
    
    Regards,
    
    E.
      

    
    Tapered elevons:  I see many Gremlin pictures where they are square at
    the wingtip.  If they should be tapered, how much?  I have thought
    about flutter on mine, especially with a hot 40 on it.  The gaps are
    sealed top and bottom.
    
    Ron

    On the YS45-driven-Grem-rocket I was forced to trim the outer 12" down
    to 1/2" at the tip. This cured the flutter that I got in high-speed-
    -full-power-on-from-great-height-dives....
    
    Stiff ailerons stock really is important if you go to bigger power
    plants.
    
    
    E.

    I'm just starting to look into these wing things for thermal soaring.
    
    Searching through BARCS SOARER back issues I came across the following
    references..
    
    Does anyone have any info or the Coordinates (add to note 1113 please)
    
        FOIL INFO		  REFERENCED MODEL INFO
    Name	 Origin	  	 Name 	          Designer
    ----------------------------------------------------
    CJ-5WD	 Thinned CJ5	 Time Bandit      Kai Erdmann
    CJ-5
    CJ-3309			 MANX CAT         Joe Hanna
    CJ-3309 lab  Martin Lichte
    E182	 Eppler		 MOIN	          Beetz
    				 CLOCK KESTREL    Denis Oglesby	
    				 ICAROSAUR (tip)  Gene Dees
    E178	 Eppler          ICAROSAUR (route)Gene Dees
    S5020-084	 Selig		 PETES PLANK      Pete Cole
    AR-2610-S80  Alfons Rieger
    ----------------------------------------------------------
    
Kai Erdmann, was the editor of Aufwind, Is that still true? 
    
    Thanks Trev

    Trev,
    
    yes, Kai Erdmann still is the editor of Aufwind. They once featured the
    TIME BANDIT, and I'm pretty sure they published the airfoil data. I
    will look it up in the back issues and hack it in, but can't promise
    I'll manage to do this this week or next. Bear with me.
    
    Regards,
              Hartmut

    Hartmut,
    	Thanks, I have done the Time Bandit Co-ords, I will enter them in
    1113. Apparently they used a modified CJ-5 called CJ-5WD. I have used
    it through the FOIL program but it doesn't seem to have enough data to
    smooth very well at the leading edge.
    
    Continued in 1113
    
    Trev

    The folowing is a brief summary of Silent Flight Editor Dave Jones
    impressions of the Kaltenkirchen Flying Wing Meet held in in Osnabruck
    Germany.
    
                           Competition Rules
                           *****************
    
    The talk a couple of years ago was to match the performance of the F3B
    models.  The competition rules were staight out of the F3B book, speed
    and distance on the 150 meter course and 7 min. duration, all launched
    by F3B standard power winches.
    
    
                               Performance
                               ***********
    
    The current designs are matching the duration performance and
    thermalling convincingly.  On the speed course they were about 70%
    of the way there, but this should be gauged against the wing loadings
    used for speed.  Tailless models do not seem to like being heavily
    ballasted, about 13.5 ozs/sq ft being the typical wing loading for
    speed.  In the distance task the models looked pretty reasonable but
    it was difficult to assess the potential of the air without putting an
    F3B model up.  It looked as if they were up to about 70% of what one
    would expect.   It must be said thouogh that this was a big improvement
    over the performance a couple of years ago.
    
    
                             DESIGNS
                             *******
    
    The dominant design at this meeting was the swept wing.  Spans were
    around 2.8 meters being constructed of glass vacuum bagged over foam
    for torsional rigidity.  The AXOLOTL design flown by the LOGO team
    uses 6 control surfaces on the wing to givw a progressive washout in
    the launch fap and crow braking positions.  The center flap can also
    be used in conjunction with the aileron for thermalling.  The setup
    is complex, but does givee a widened speed range.  The E.T. design of
    Christian Hanke uses a similar control layout but employs a long boom
    with a central fin/rudder.  The advantages of this were apparent on
    the speed course where the model tracked very true, unlike the fin tip
    models that appeared to be a bit twitchy.  The disadvantage seemed to
    be in thermalling, it looked like it had to be worked at, whilst the
    tip fin wings were very happy in thermal turns.  Both models used a
    modified SD7003 airfoil, an unusual choice since it has a pitching
    moment that would require some reflex to overcome.
    
    Unverferth flew his CO5, a development of the long running CO series.
    It was 2.9 meter span, 9% root to 10% tip, 15 degrees sweep, MH45
    airfoil.  The model performed well and looked easy to fly in the hands
    of the designer.
    
    The SPIN OFF design by Behrens and Tollmein was nice and simple.  It
    uses 4 control surfaces and was a consistent performer.  The model
    weighs 2.2 kg, spans 2.9 meters and uses MH45 airfoil.  The root is
    220mm, the tip 200 mm and the sweep angle 20 degrees.  The anhedral is
    very small only being 16mm at the center.  The washout is also small at
    0.8 degrees, it starts at half span.  The loadings on the model are 35
    gms/sq dm dry and 40 grms/sq dm ballasted.
    
    Anton Gabsch flew his competition Horten wing.  This was an interesting
    model, very simple, just an elevon at each tip.  It was very quick,
    though.  It was sen later in the day performing fast sweeping low
    passes with a Keller electric motor installed.  The speed of the model
    was such that it had you thinking about whether a wing might be the
    answer for pylon racing.
    
    The ASPERGILLIS was apparently the winner last year in the hands of one
    of the Kowalski's but it did not do so well this year.  Like all plank
    designs it handled nicely.
    
    Landing accurately was a problem for all the designs.  Airbraking on a
    flying wing is difficult at model sizes so the approach has to be flat
    and long, frequently with a dumped landing.
    
    
                              CONCLUSIONS
                              ***********
    
    Progress has undoubtedly been made and if it continues at this pace we
    may well see the swept flying wing matching the performance of
    conventional models.  For thermal soaring these wings were perfectly
    adequate in the good conditions that prevailed and given a powerful 
    pulley launch they would be competitive(in F3J, I presume J.B).
    
    
                              RESOURCES
                              *********
    
    One thing is certain about studying flying wings, if you can under-
    stand the complexities of the tailless model you will have no
    difficulty with conventional or canard models.
    
    TAILLESS TALE - Dr. Ing. Ferdinando Gale - B2 Streamlines, P.O. Box
    976 Olalla, WA 98359-0976, USA.  This is a first class reference book
    that deals with the historical and technical side of wings.
    
    FASZINATION NURFLUGEL - H.J. Unverferth - Verlag fur Technik und
    Handwerk, Baden Baden, Germany, 1989.  This book, lke most of the good
    material in this field, is in German, but it has good illustrations to
    make the lessons clear.  This book will give you the practicalities of
    model flying wings.
    
    NURFLUGEL - R. Horten, P.F. Selinger - H. Weishaupt Verlag, Graz,
    Austria, 1983.  If you are into Horten wings this is essential reading.
    
    NURFLUGEL-SEGLER FERNGESTEUERT - R.H. Werner et al. - Neckar Verlag,
    Villingen Scwenningen.  A bit dated now but worth getting to see what
    has been tried before.
    
    
    
    

    A big thankyou to Kay..
    
    	I received, a few days ago, eight back issues of RCSD in the
    infernal mail. The flying wing coverage by B^2 is excellent. I am
    seriously contemplating subscribing.
    
    Thanks Trev