Conference vmszoo::rc

                      
    A plane with a symetrical airfoil generates its lift with a certain
    angle of attack. Even a flat surface generates enough lift if moved
    through the air with an angle of attack. The more angle the more lift,
    the higher the speed the less angle - but always possitive. Look at
    those pattern planes when flown inverted, they certainly have their
    tail hung down a bit on inverted. Due to the high speed the difference
    is not that much though.
    The basic explanation of generating lift on a flat bottom airfoil for a
    long time was "the air on the top surface has to move faster than on
    the bottom thus generating a negativ pressure according to the
    Bernoulli equation. The slower air on the bottom generating a higher
    pressure - this difference in pressure IS the lift."
    Not too long ago people started to calculate the lift on a real big
    bird. The result was that it could never fly - but it flew like a
    champ. Slowly they recognized that the Bernoulli equation was not all,
    there is annother component that generates a cirqulation of the whole
    air around the wing, from front to top around the trailing edge against
    the airflow on the bottom back to the front and so on. This way the
    basic lift gets amplified. This cirqulation also generates the
    turbulences on the wingtips that (I think) are called the induced
    resistance (as translated from my native language).
    The theory of this circulation can be watched when holding a teaspoon
    half in a cup of coffee. Mov it quick across the surface like an
    airfoil and stop. Then watch this little spiral (expl.?) at the
    trailing edge of the airfoil - aeh I mean spoon. People use this to
    demonstrate that there must be some kind of circulation. If the airflow
    - aeh coffeeflow would be the same at the bottom than at the top, what
    would then generate this spiral (?) ?
    
    This whole lift thing is a bit more complicated to explain...
    
    
    Bernd 

dave,

you are right. a symmetrical airfoil does not provide lift,
as long as it does not have an angle of attack.
with the angle of attack you get differences for the path
going over the upper side and the bottom side, and then again
you have your lift. wings are mounted onto the fuse with
a given angle of attack, so when the fuse flies normal
(level flight it is called?) you have lift.

with the above said it is clear that a symmetrical foil does
also work, i.e. provide lift, in inverted flight. of course
you need again the same angle of attack as in normal flight.
because the wing is mounted with an angle of attack for normal
flight, the fuse must fly with the double of the angle of attack
in inverted flight, i.e. the tail is "hanging down"

you will notice this effect very strong on non-symmetrical
airfoils, a clark y for example. on this foils you need a
large angle of attack for inverted flights.

i encourage you to read through the notes regarding foils,
especially note 1113. look at 11.783 for more pointers.

of the books mentioned, i have the book from martin simons,
i consider it a good one for the start.

cheers
joe t.

    Now let me see, who's been drinking to much coffee??? :-)
    
    Forget the section, just add a bigger engine.....    :-)
    
    2 days to the WRAM show and counting............................
    
     E2.

    This is a litle off the subject, but related to understanding how
    airfoils work.  The previous 2 replies make sense to me, particularly
    when thought of in the context of powered flight.
    
    When a plane is pulled through the air by a motor the air flow over
    the wing, be it symmetrical, flat bottomed, flat plate, whatever
    generates lift and the plane rises.  A certain amount of airflow
    must be sustained over the wing, either be adding power or dropping
    the nose or the plane stalls(stops flying).
    
    Where I remained somewhat confused is how airfoils with gliders
    work.  All gliders, regardless of airfoil, sink unless they are in
    lift(thermal or slope).  It is my understanding that if two gliders,
    one being a sleek F3B model and the other an OLY II, were released
    in no lift air, at the same altitude, and both flew in a straight 
    line at minimum sink trim, the F3B ship would cover a lot more ground
    and probably have a greater sink rate than the OLY II.  So for 
    arguments sake, lets say the F3B ship covered the same distance
    but only stayed up half as long as the high lift/high drag OLY II.
    
    Both ships are sinking, but at different rates.  Assuming the two ships
    weighed the same, I guess you can conclude that the flat-bottomed
    Oly II airfoil generates more lift than the RG15 airfoil on the
    F3B ship.  But neither airfoil generates enough lift to sustain
    level flight, hence they are both flying in a nose down attitiude.
    I have seen polars showing glider airfoils generating lift at negative
    angles of attack.  
    
    If the two ships now enter the same thermal, would they both climb at
    the same rate?  If not why?  
    
    
                                                           Regards,
    
                                                           Jim 
    

    
    Had to read your note multiple times to understand what you want :-)
    
    In general the sink rate is measured in m/s (take inch/minute or miles
    per second). I would not compare an OLY II with a sleek F3B ship and 
    assume they cover the same distance ...
    Lets say plane A is built by Jim (leightweight) and Plane B by Eric
    (lead slead). Plane a has a sink rate of 0.5 m/s whereas plane B has a
    sink rate of 2 m/s. (Because Eric's trying to compete with his Panic
    against Jim's Alcyone ) 
    
    Now this mild thermal lifts off the ground with 1.5 m/s. Plane A will 
    raise with (1.5  m/s - 0.5 m/s = 1 m/s while plane B 's still sinking 
    with (1.5m/s - 2 m/s = - 0.5 m/s).
    
    ...and because Jim's might better thermal anyway  :-)
    
    Clear ? 
    

    Re: -1
    
    Bernd,
    
         I am not clear yet.  Let's assume that plane A(F3B design)
    has an L/D of 24:1 and a sink rate of 2 m/s.  Plane B(floater)
    has an L/D of 12:1 and a sink rate of 1 m/s.  Both planes are
    released at 100 meters altitude and fly in a straight line in
    no lift conditions.
    
    Plane A is on the ground in 50 seconds and has covered 2400 meters.
    Plane B is on the ground in 100 seconds and has covered 1200 meters.
    
    This is a hypothetical example, so let's use these numbers, even if
    they are a bit dramatic.
    
    I may be wrongly assuming in the above example that plane B's airfoil
    is generating more lift, which is why it stays up twice as long.
    What is you comment on this assumption?
    
    I have read that F3B type aircraft can actually outclimb the more
    traditional thermal designs in lift.  Is this true?  If so why,
    given that the F3b designs have a higher sink rate?
    
    My question is how is it possible for a plane with a higher sink
    rate to outclimb a plane with a lower sink rate in constant lift?
    
    
                                               Regards,
    
                                               Jim 
    
         

Outclimbing a floater type is not unusual. The quoted sink rate 
conditions are for "dead" air. Once you introduce a thermal you're 
looking at how efficiently the plane uses the rising air. While Bernd's 
numbers might be true, the same parasitic drag that causes the lower 
sink rate in the floater will also cause it to climb slower in the 
same conditions. The slipperier the plane the better it will react 
to the thermal. The more efficient the plane, the more efficient the 
plane will be in using the lift available.

To look at it a different way, Newton proved that a feather and a cannon 
ball will drop at the same rate IN A VACUUM. The feather has more 
parasitic drag and will drop slower in air due to air resistance. We 
won't get into the thermalling capabilities of the cannon ball 8^) The 
parasitic drag is always a force opposite to the force acting on the 
plane. In the minimum sink case it is opposing the force of gravity 
making the plane drop slower. In the case where there is lift (assuming 
that it is a net positive force when combined with gravity) the drag is 
keeping the plane from climbing as "efficiently" in the same manner.
All this is regardless of the airfoils used. This would be true in the 
case where the two planes have the same airfoils but have drag induced 
by poor finishing, fillets, hinge gaps, etc.

The F3B-type is going to the field expecting that there is sufficient 
lift to be found such that he can offset his dead air sink rate over the 
floater. He also has the added advantage of covering twice as much 
distance (in Jim's example) so he has twice the likelihood of finding 
the lift and being able stay in it further downwind (use it longer). 
There have been several times when I've seen others in lift off field 
and known where to go but also known that my L/D wouldn't let me get 
there.

    
    Jim B., you are assuming that an F3B type of plane has a higher sink
    rate, is this really true ?  I doubt it. If you release a floater type
    plane and a F3B type plane at the same hight in dead air I'd guess the
    difference of duration must not be worse for the F3B plane. 
    
    On the other hand I would not say that an F3B type of plane outclimbs a
    floater. One example: Say a paraglide (1.5 m/s sink rate, L/D ~ 5) is 
    your floater, and a hang glider (1.0 m/s sink rate, L/D ~10) is your
    F3B type of plane. Which one would you say climbs faster ?
    
    Wrong, its the paraglide, because it can make the much tighter turns
    and thus better centering the thermal (multiple own experience). 
    
    What do we learn from this ?  Always compare apples with apples....
    
    
    Bernd

    Dave,
    
    You might want to check with your trainer.  He can explain.
    
    Don't ya just hate it when your trainer catches ya.
    Just kiddin'
    
    John

    
    This is good stuff, I'm hearing alot about angle of attack, but where
    does the angle of incidence fit in or has it and I didn't catch it.
    
    Bruce
    

The angle of incidence is a preset amount of difference between the 
stab and the wing. If you consider that the plane is flying along a 
line through the stab and engine then the angle of incidence = the 
angle of attack. If you put in up elevator, the stab is pushed down 
and the plane rotates around the center of lift (pressure) and the 
angle of attack is increased. The angle of incidence is fixed on 
the plane, the angle of attack changes as the nose/tail move up and 
down.

    
    I think this kind of discussion is great, and most of us can learn
    something from someone else, I cannot encourage this kind of discussion
    strongly enough!!
    
    Lets see more of it, I'm learning from it!
    
    Richard
    
    Now how can increasing weight (ballasting) be beneficial to duration?
    
    8*)

Bricks fly slower due to the extra drag of the rough surface 8^)

The main reason that some ships fly better ballasted is the airfoils
used. There are several airfoils which need to fly faster to get 
into the range of their best L/D ratio. When you plot the efficiency
of the airfoil at various speeds (Soartech 8) you find that lift and 
drag are not linear. Ballasting doesn't always help reduce the sink 
rate but there are airfoils that you can tune to specific speed 
ranges and the models using them will fly best at these speeds. 
With my Alcyone I found that even though the plane is heavier than 
Sal's custom built job (72 vs 58 ounces) he only beat me by 11 
points (6 rouunds, my landings were pretty poor) and I seemed to be 
able to get further out and look for thermals over a larger area.

Even the Predator that Dave Walter designed seems to like to be a 
little heavier and move faster. I was constantly trying to fly too 
slow. Once I trimmed it with a little more down trim it moved 
faster and actually flew flatter. This is just a simple S3014. My 
15 ounce model seems to fly better than the 12 ounce one that I 
built and it's sturdier too

    
    In stronger winds you like to add balast to you floater type of plane
    if you want to move forward. I watched several Gentle_Ladies_type of
    floaters hoovering in strong wind and not beeing able to look for lift.
    With balast you might be able to penetrate in this cases and go look
    for lift - increasing duration with added weight (sounds contrary,
    doesn't it ? )
    
    Bernd
    

    I started flying gliders with a couple of flat bottom airfoil planes. 
    As I got better and wanted to fly in higher winds I stumbled on a great
    way to make a flat bottom airfoil with turbulator spars penetrate the
    wind.  I ran a strip of 1/8th inch trim tape between the two turbulator
    spars on the top front third of the wing.  The result was that the
    plane would not float anymore but I flew it in 20+ knot slope wind and
    it penetrated without adding balast!
    
    Will

    One interesting thing to keep in mind is best L/D and minimum sink
    are not usually obtained with the same trim setting.  Typically
    best L/D trim results in a faster glide(and descent rate, of course)
    than a ship trimmed for mimimum sink.  The glider will stay up
    longer when trimmed for minimum sink, but will cover less ground
    than when trimmed for best L/D.
    
    
                                                          Regards,
    
                                                          Jim

    The effect of increased weight on the performance of a full scale
    high performance sailplane(ASH 26E) was illustrated in Wil Byers
    column in the 9/92 RCSD.
    
    When the wing loading was increased by 44.5% , the sink rate only
    increases about .1 meter per second. "The speed of the ship increased
    however, from 85km/h to about 105 km/h.  Not a bad price to pay for the
    ability to cover ground at 20 km/h faster I'd say.  So for the ability
    to cover a great deal more sky, the soarer doesn't increase its sink
    rate very much and its odds of finding lift increase substantially." 

    All wings ultimately work by directing air at a downward angle from the
    trailing edge of the wing.  All of the other characteristics or
    phenomenon generated by the wing in the creation of this downwash,i.e.
    tip vortex, center of pressure, spanwise flow etc are the result of the
    wing influencing the mass of air through which it passes.  If you think
    of the amount of air a wing influences as it passes through the medium
    at (insert speed here) its huge and it makes it much easier to
    understand how the air could support a plane.  If you ever want a
    graphic and eye opening example of the downwash effect, go for a ride
    in a sail plane and ask the pilot to box the wake then ask the pilot to
    fly through the wake.  This will allow you to visualize where the down
    wash is, then be bounced around in the glider as you fly in it!
    
    Will

The non-symmetrical airfoils do still generate lift by lowering the
pressure over the top of the wing. The top molecules have farther 
to travel so the same number are farther apart creating lower pressure.

RE: -.1

Actually .19 is correct for all wings, at least according to recent studies. I
read an article a year or so ago (in Scientific American, I think) that talked
about NASA's shock and dismay upon discovering that the lift produced by the
Bernoulli Effect was not sufficient to fly an aircraft. They were doing some
kind of lift analysis and discovered that the standard explanation of why planes
fly - the Bernoulli Effect - wasn't in fact correct. While the B.E. does indeed
generate lift it is insignificant when compared to that created by downwash as
explained in .19. It is this downwash that is the major component of lift. The
B.E. is real and explains why flat bottomed airfoils lift changes with speed, it
just isn't all, or even a major part of the equation.

Well, the Princeton wind tunnel tests used in Soartech 8 seem to imply 
that airfoil has a non-insignificant part of it. Changing the airfoil 
while maintaining the crosssectional area can have significant effects 
on the lift generated at the same airspeed and AOA. In real life 
unpowered tests you find that the parasitic drag getting reduced also 
allows you to take better advantage of the inherent lift in the foil. 
This is why a tripped E205 does so much better than an untripped.

    I am confused by the term "downwash" from the trailing edge of the
    wing.  Could you clarify?
    
                                                              Regards,
    
                                                              Jim
    

    In the example of "boxing the wake" behind a tow plane the location of
    the turbulent air is behind and below the tow plane. The airfoil is a
    refinement of a surface that deflects air downward at the trailing
    edge.  I think someone mentioned that even a flat plate or a door will
    "fly" if you propel it through the air fast enough with a sufficient
    angle of attack to the relative wind.  The downwash I referred to is
    the result of deflecting the air downward at the trailing edge of the
    wing.  Thats why I titled the note "for every action" (theres an equal
    and opposite reaction).  If you force the airmass downward, somthing is
    forced upward.  I don't know the exact numbers, but you know the
    atmosphere of earth has mass and a measurable weight per volume unit.  
    The amount of air that is forced downward by an airplane wing is a
    surprising one if you calculate it.  When you do calculate that number 
    it is easy to see how a multi ton airplane is kept up.  The calculation
    is a function of volume of air affected times speed of the wing for
    subsonic flight.  
    
    Will

The way I think about all of this (which I think matches the "moderen" 
theory based on NASA'a recent research) is this:

*ANY* shape of airfoil will generate lift given the correct angle of attack.
As examples already given, a flat plate does generate lift.  The REAL 
difference in airfoils comes into play when talking about DRAG.  It's
the Lift vs. Drag relationship that makes one airfoil better than another.
Imagine a standard plank of wood that is 2 inches thick and 8 inches wide
by 4 feet long.  If you move this through the air with a positive angle
of attack it will generate lift.  However, the drag will be huge compared
to the relatively small amount of lift.

Now, think about how the plank is generating lift.  The Bernouli effect
doesn't count since it's the same length of travel across the top and 
the bottom of the "airfoil".  The lift is generated simply because of
the (positive) angle of attack and the INCREASED air pressure of all the 
poor little air molucles squished up underneath our "wing".  This is where
the downwash comes from - the wing is pushing the air down to generate lift.

Now, how about a real airfoil.  The only difference is that the shape has
been designed to reduce the amount of drag for a given amount of lift.
The best airfoils generate the minimum amount of drag for the amount
of lift required (equal to the weight of the aircraft).

The above ideas also explain how a flat bottomed airfoil can fly upside
down.  If the Bernouli Effect were the only factor this would not be
possible.  Note that the drag generated by an upside down flat
bottom airfoil will be greater than right side up.  [ Glider tip:
If you don't have flaps or spoilers but need to come down faster, fly
upside down. ]

Also, a wing that has stalled does still generate some amount of lift 
(although less than before stall).  The catch is that it generates MUCH
more drag which causes the aircraft to slow down, reducing the lift even 
more until the plane falls out of the sky.  If the aircraft is designed
properly, this will cause the angle of attack to be reduced and the wing
will come out of the stalled condition.

Now back to the original topic:  Symmetrical airfoils are designed
to produce the minimum drag either side up for a given amount of lift
but they do require a positive angle of attack to generate lift.
(Just like my plank airfoil does.)  However, you don't get something for
nothing.  The drag is greater for a symmetrical airfoil.

                       _____
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    Re: -1
    
    Dan,
    
       Everything you said in the last note makes sense, except your last
    statement that "symmetrical airfoils generate more drag".  I asked 
    this question early on, because I have seen no polars for symmetrical
    or semi-symmetrical airfoils.
    
    I had assumed that symmetrical airfoils generated less drag(and less
    lift) which is why they are used on glider stabs, heavy lift slope
    soarers, and cross country gliders(E374, SD6060).
    
    It seems that most of the winning slope racing ships use F3B airfoils
    (RG15, RG14, SD7003 etc) rather than the semi-symmetrical foils(E374,
    SD6060).  Is this because the F3B airfoils have better L/D?  If so
    why have many of the successful cross country designs use the SD6060
    or E374?  Can anybody with Soartech #8 comment?
    
    
                                                    Thanks,
    
                                                    Jim