SEN-445 July 25 2000
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- Category: Archive 2000
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News and Reports 2000 - second half
SCAT Electronic News 25 July 2000 issue 445
Table of Contents
=================
More on B7406F etc. - King
> Beginner or the "not-so-serious"? - Linkosalo
Strain the paint ? - Martin
Differential Wing Settings - Brush
Woebbeking airfoil, the background - Montes [and Woebbeking]
Wobeking airfoil - Magill
More on B7406F etc.
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Author : This email address is being protected from spambots. You need JavaScript enabled to view it.
Martin Dilly's experiences are interesting and point up the typical diversity
in experiences of different modelers. I am sure he is right about the
effect of that TP section. I have seen it and it is a very odd shape, as he
describes and probably works in just the way he suggests. Maybe we should
look again at it. However I think it would produce very high TP drag
although, with typical tiny Stab sizes, today, this may not be much of a
penalty.
The construction he used, (Sheeted top surface) would normally tend to cause
even more problems with the airfoil, so the stab may indeed have been a big
contributing factor. Also the V dihedral quoted in the 1st piece in SCAT may
also have contributed to the bad stalling characteristics described there.
There are obviously so many more factors, other than the airfoil to take into
consideration, as always.
Regarding structure, it may be worth pointing out that the original idea for
the construction for B7406F, (As used,without turbulator, in the Stuttgart
tests and by Ex World F1B champ,Loffler, who was I believe, one of the first
to use it), was quite specific and as follows:
A full depth spar at the max top surface, (ca 35/40%) and a load of riblets
forward of this spar. It may well be that the riblets were very helpful in
promoting just the right type of 3D turbulation needed to make the section
work well.
Peter King
> Beginner or the "not-so-serious"?
===================================
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> FAI and "not-so-serious" are a contradiction in terms. If you are not going
> to take FAI modeling serious, Well then you won't mind getting your BUTT
> kicked by the people that do take their flying serious. Do we not have
> enough Slow, Nostalgia, Old time, Straight tow, events to satisfy the
> Beginner or the "not-so-serious"?
My apologies for using a term that gives raise to such strong emotions!
What I meant with that is that there are people like me, who fly F1B with
all bells and whistles, but would like to fly glider every now and then,
as it is so much different from wakefield. It would be no use to fly "full
house" gliders, though, as I'll never have the time to practise
sufficiently. I do mind have my butt kicked, and would therefore like to
have a way to compare my results to the others, not just fly inferior
models for fun.
Here in Finland we do not have slow, old time or straight tow classes to
choose from. Only F1A and F1H.
-Tapio-
Strain the paint ?
==================
Author : This email address is being protected from spambots. You need JavaScript enabled to view it.
hello,
perhaps you can help me.
what is the best way to strain floquil model paint?
i am painting ho railroad cars and i think that my brush is clogging due
to paint that is not throughly mixed, but more likely needs to be
strained.
Thanks for your time!
KEN MARTIN
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Differential Wing Settings
==========================
Author : This email address is being protected from spambots. You need JavaScript enabled to view it.
Hi Roger,
Please include this in your next Scat Electronic News:
Thank you,
Al & Tina
In response to Bill Bogarts hypothesis on differential wing incidence
benefits, I would like to announce the "Nova Star;" which incorporates
a timer actuated mechanical mechanism, that actually adjusts a
controlled optimum decalage power/glide pattern.
This F1B design by Andrey Burdov of Russia is the first commercially
produced model with this mechanical miracle. I have named it the "Dual
Decalage & Differential Director." (DD&DD). With this device,
optimum climb cruise altitude can be obtained by individually
controlling both left and right wing panels in both high climb decalage
and normal glide decalage. This mechanical marvel is assisted by the
constructed special warps, designed into the Nova Star wing panels.
I suspect, no expect, and predict that other producers of commercially
available F1B's will include a DD&DD device in the next couple of years.
Wing wigglers will undoubtably be eclipsed by the Nova (new) Dual
Decalage & Differential Director.
The Nova Star is available through:
Al Brush/Star*Link
25500 Aquila
Sun City, CA 92586
909 301-9975 (email) abrush@inland@net
Woebbeking airfoil, the background
==================================
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Roger, due to the interest of the discussion generated in this forum on
the Wöbbeking airfoil, I thought it would be useful to provide a
translation of part of a long article that appeared in Thermiksense
1/2000, "F1A Sport- not yet a new class, but an idea around well-known
models", by Gerhard Wöbbeking. In this article Wöbbeking addresses the
broader issues of the choice of wing and stab profiles for the new F1A
Sport category that has been proposed to FAI, but not yet ratified. This
translation addresses only his remarks on the stab profile, but there is
much interesting material concerning the choice of wing airfoils. My
thanks to Jean Wantzenriether for his help in this regard.
Choice of a stab profile
In what concerns the stab, the Chopp profile can hide very well its
defects. It can, nevertheless play some games with modellers without
much experience, when zoom launching at a high Re regime, this profile
experiences a sudden increase in lift precisely when one expected the
lift to decrease. The result is, in the best possible case, a failed
attempt, in the worst, a dive to the ground.
A stab of this type was used in my first glider inspired by the
Russians, with a circular tow hook of Lepp design. It was a flat bottom,
ClarkY profile of about 8% thickness. Towing was terrible, the model
dived severely when the turning circle was small and it was necessary to
work continuously to recover the lost height. The stab profile seemed to
react too strongly to small variations of angle of attack (AOA) on tow,
and inversely, to react too weakly to large variations of AOA when
gliding in turbulent air. But at least it stabilized better with its
aspect ratio of 6 than the first stab tested. For that first stab,
following a well-known trend, I had chosen the small AR of 4 and covered
the stab with mylar, a combination that led to a miserable performance
in the typical weather conditions of North Germany.
After a discussion with Reiner Hofsäss in 1982, I tested some new
profiles which ideally should have reacted weakly to small variations of
the AOA, and provide a strong correction in the case of large variations
of AOA (flight in strong turbulence). I defined certain design criteria,
and the geometry of the airfoil developed from them almost by itself.
The required profile should then:
1) Have an advanced highest point in the mean camber line. Such profiles
have polars without hysteresis loops. I wanted to avoid that at all
costs, and be certain that the profile would work continuously without
separation of the flow.(Hysteresis is a special behaviour of the flow
over the top of the profile. After a separation at large AOA, the flow
does not re-attach until much smaller AOA have been reached. Profiles
with extended hysteresis regions must be handled with special care).
2) Have a small lift gradient (ie, lift derivative dCl/dalfa)
3) Have a large radius of curvature of the nose of the profile, similar
to the ClarkY 12 % thickness. Hans Doermann, among others, had chosen
this characteristic for his F1B "Quo Vadis" after many tests. I had the
idea that the flow about a well rounded nose profile shows this
astonishing broadening of the range of lift, as shown by the polars of
similar profiles in Schmitz book ("Aerodynamik des Flugmodelles"), even
for values of Re of the order of 20,000, typical of a stabilizer.
Does the curvature of the top rear of the profile bring about a real
increase in lift? That is not very probable. Again we return to Schmitz.
His Goettingen 625, with an extreme 15% thickness provides the same
positive lift at a 10 degree AOA as the negative lift at -10 degrees. As
it is a very cambered profile, with 6% mean camber, and a flat bottom,
one would have expected at negative AOA a much weaker negative lift.
On my special profile I could thus finish the top of the profile by a
straight line from the point of maximum height to the rear, as in the
case of the Goettingen 625. With an airfoil of 8% thickness one would
obtain the same lift as for a 12 % thick airfoil (the ClarkY), and of
course, with much less drag. This was a combination that has worked very
well with HLG of solid balsa construction. In their case it works well
for conditions of highly variable speeds and AOA. Airfoils used in these
HLG have a straight rear top profile after the highest point, this
design of the top part of the profile also advanced the highest point of
the mean camber line as specified in (1).
The success of this Wöbbeking 2.5-20-8 is due to Andreas Lepp. Andreas
tested the profile and verified that the bunting became more
predictable. He then equipped several of his models with the new stab,
and won the 1989 WC in Argentina, then in 1991 the European title in
Hungary. Others have also made use of the profile, for example Victor
Stamov in F1A, and Alex Andriukov introduced it in F1B.
In my experience, when flying my F1A "King's Air" in calm conditions,
the new stab profile did not increase sufficiently the pull on the
towline. As it was simpler to test another stab profile than a new wing
profile, I sought a more cambered alternative. The increased camber
should give a zero lift not at -1 degree, but around -2 or -3 degrees.
When towing, this modification should have helped the wing, by forcing
it to develop higher lift, and thus slow down the model. This profile
existed prior to my researches of 1983: it is the Wortmann M2 with a 5 %
camber. It performed well in the "King's Air" model, and I will try it
again on a new glider.
It goes without saying that I was not alone in experimenting with these
stab characteristics, nor to seek increased tow line tension. Stamov has
verified that a turbulator thread on the nose of the profile helped the
towing characteristics, without other negative side affects. In summary,
the Wöbbeking 8% would be the answer to our search for a stab profile
for an F1A Sport model.
(Both the Wöbbeking 2,5-20-8 and the Wortmann M2 are illustrated in the
original article. No coordinates are provided as Wöbbeking recommends
to scale the profile in a photocopy machine to the desired chord)
Wobeking airfoil
================
Author : This email address is being protected from spambots. You need JavaScript enabled to view it.
Hiya,
Regarding Peter King's thoughts on Wobeking airfoils, I've been using this
airfoil for F.1.A:-) tailplanes for a while now, and have come to these
conclusions:
1. It does not seem to affect the "zoom" part of the trajectory at all, i.e
it's "nuetral", unlike my old airfoils...with these, the thicker the airfoil
was, the more nose-down and flatter the zoom seemed to become(disreguarding
things like c.g postition e.t.c.)
2. The Wobeking seems to be very forgiving when trimming the model's glide,
i.e you can adjust things a fair way before something nasty happens.
3. The above mentioned point can be a problem if not watched..one time at a
contest, flown in VERY calm conditions, i knew my F.1.A:-))would do around
3.10-3.20 in "still" air, and it was only doing 2.30-2.40...i found that the
tail incidence screw had moved on the journey down to the contest, so i
started upping this screw each time, and in the end, the model was back to
it's usual 3.10-3.20!
Funny thing was, the model LOOKED as though it was gliding fine, but it was
actually under elevated....the eyes can decieve sometimes!
And so, my new trimming technique now comes from a very old piece of advice:
"Raise the trailing edge of the tailplane untill the model definatly stalls,
then back off a little"
I guess measuring the model's trim with a stop watch would be a good idea
too.
4. My dad also reckons that my models with Wobeking tail airfoils, seem to
stay more nose up on circle tow, when going downwind...however, it's hard
for me to notice this from my veiwpoint!
5. With the above points in mind, i like this airfoil, and now use it
almost exclusively on my F.1.A tailplanes.
P.S The first time i saw this feature on a glider was on Andres Lepp's
F.1.A's, but I'm sure it dates back alot further than this.
Thermals,
Jason Magill
New Zealand
..........................
Roger Morrell
