This is the article on flap design. That article began of page 556 of book 2 of my free Al's Models 4 DVD and ended on page 563. If you would like to see this flap article with its full size color pictures, you can go directly to the the shortcut link to download the the entire Al's Models 4 DVD by making one click on this link. The flap article is copied from book 2, pages 556 - 563. The direct download includes 190,000 words and 1601 pages of text and full size color photos. Again, there is no charge for the download. It's free.
Sorry about the confusion of my original post which stated that the article began on page 550. In fact it began on page 556. My problem was that the DVD is posted in PDF
format instead of Word which I prefer. The PDF files for the DVD didn't copy photographs. This necessitated copying and posting text appearing only between photographs. In any case, the link to the complete DVD is posted here, tested, and works as easily.
If you find the information on torque tube flaps compelling, there continues the article above with 29 pages of text ending on page 592 including 30 full size color photographs showing in detail, just how I build these torque tube flaps.
https://www.youtube.com/playlist?list=PLkHUtj971Jgk_k-N5vVQX_as0dcVexiz4Al
Flaps for Control Line stunt Ships
When I began to build stunt ships with flaps, they were typically finished like
other bare wood portions of the airplanes. Like everyone else, I silkspaned
and painted mine too. There was no awareness at the time that these
torsionally weak flaps were twisting, or “washing out”, under flight loads. We
were all getting away with weak flaps because we didn’t need better. The
wings were contributing nearly all of the lift necessary for satisfactory
performance.
Then I built the original Bearcat. With its large fuselage, it was simply too
heavy for its modified Nobler wing. To solve this problem, I cut into the
trailing edge of the wing beginning at the wing root flap horn and sweeping
forward 3/4” at the tips. The larger flaps solved the problem well enough for
the Bearcat to place 2nd at the 1969 NATs.
When I built the Bearcat III, The design included a molded fuselage and
larger flaps which included a swept hinge line. Sweeping the hinge line
allowed flaps of nearly double the area of earlier airplanes without changing
the shape or total area of the wing. The flaps totaled 25% of the wing/flap
area. Recognizing the increasing need for flap lift, the Bearcat III also had
the flaps stiffened by a finishing layer of glass and epoxy. The removable
wing Bearcat III also managed a 2nd at the 1970 NATs. Typically, today I use
about 25% chord flaps. This would be 3.3” on a 10” root chord and 2” on a
6” tip.
The Mustang V, the first light blue E2-S, also used flaps stiffened by using a
layer of glass and epoxy on the flaps. Logically, this seemed enough at the
time. In flight, however, the Mustang V was plagued by “hinging” In square
maneuvers. Tip weight was removed over a series of flights to trim out this
characteristic. I flew the 1974 NATs with no tipweight and very little line
tension squeaking into 3rd place under ideal light wind conditions. Finally, I
discovered the reason for the Mustang V’s curious lack of line tension. The
outboard flap was torsionally weaker and flexing under flight air loads. The
Mustang V was behaving like it was equipped with Palmer’s “differential
flaps” which added lift to the inboard wing to create a “roll out” moment on
each control application, both inside and outside. Palmer did it by making the
inboard flap movement greater than the outboard flap when the controls were
deflected. It was happening to the Mustang as the result of softer wood in the
outboard flap. That flap was “washing out” and not deflecting as much as the
inboard flap under flight loads. The problem was solved by adding area to
the outboard flap to balance the aerodynamic rolling moment and adding
tipweight for line tension in maneuvers. This was a vivid demonstration of the
comparative effect of flaps of different torsional rigidity on the same airplane.
E2-S also had a bit softer corner that I desired. With each new airplane stiffer
flaps improved lift but slowed the turns somewhat. Part of the evolution of the
molded Mustang series was moving the elevator hinge line forward to increase
pitching moment to restore the turn. Downward deflected flaps push up on
the trailing edge of the wing. This is an adverse pitching moment which
cancels some of the positive pitching moment from the elevators. To correct
the negative pitching moment, the elevators of the Mustang VI were widened
1/8” by moving the hinge line forward into the stab.
The NATs winning silver Mustang VI used matched quarter grain wood and
five layers of glass to make matched flaps. It also incorporated 1/8” wider tip
chord on the outboard flap. This was the first use of reduced wing asymmetry
and wider outboard flap tip chord at a time when nearly all stunt ships were
still using an inch or more of asymmetry and symmetrical dimensions of the
inboard and outboard flap chords. Reduced asymmetry was explained in the
“Evolution of a Thoroughbred” article published in the August 1978 issue of
Flying Models.
With a new found appreciation for flap torsional rigidity, the green Mustang
VII, built expressly for the 1978 World Championship. It had flaps with
matched wood, 5 layers of glass and epoxy and continued the original concept
of wider chord on the outboard flap tip. It was a good airplane which suffered
an untimely destruction. The Mustang VII was a lighter, more powerful
airplane but destroyed two days before leaving for the 1977 World
championships in Woodvale, England. I was practicing the seven minute FAI
time limit when the Mustang was hit by a gust in the overhead eight, rocking
the nose up, causing the remaining fuel to uncover the pickup. The engine
quit and the Mustang fell into the center of the circle, beyond any hope of
recovery. Snaggletooth had to replace the green Mustang VII but lacked
enough fuel for the stronger engine I was using. Its tank was built in and had
to be cut out and replaced with a larger version the day before leaving for
England.
When the Snaggletooth Mustang was updated thirty years later to bring it up
to modern standards using new engines and laser cutting, I wondered if still
stiffer flaps would add anything more to the original Snaggletooth’s
capability.
When I built the dark blue Snaggletooth II, I knew it was going to be heavier.
If I wanted to keep the original wing area it needed more lift from the flaps.
About this time, I saw where Howard Rush had built an airplane using
carbon tubes to stiffen the flaps on his airplane. I thought this was a
marvelous idea, and decided to give it a try. I built my first set of torque tube
flaps using commercially available carbon tubes.
When I built the Snaggletooth II, I didn’t appreciate just how much its
pitching moment would be reduced again by these unusually stiff carbon tube
flaps. Snaggletooth II was very pleasant to fly but had an intolerably soft
corner. This situation was aggravated by using the same size elevators as the
Mustang VI which were already enlarged from the Mustang V. Fortunately, I
was able to increase the pitching moment of the elevators by taping the
elevator hinge lines. The tape and a reduction of nose weight restored
Snaggletooth’s turn. This was a very convincing demonstration of the
effectiveness of stiff flaps. The redesign of Snaggletooth II, with its heavier PA
.65 and removable tank weighed almost 8 ounces more than the NATs
winning Snaggletooth but it turns as well, and the modern engine allowed a
very aggressive pattern. Now, I compensate for adverse pitching moment
from carbon torque tube flaps by designing still larger elevators, this time, by
adding area to the elevator trailing edges.
These flaps are several ounces heavier but I think that they increase the
weight that a given wing area will carry by at least 10%.
I used carbon tubes for the first time on Snaggletooth II with two layers of
cloth, thin wall 5/16” carbon tubes and 3/8”, 7 lb, quarter grain balsa. 5/16”
tubes are used because this is the largest diameter router bit available for my
Dremel Tool for routing leading edges.
When glassing my first carbon tube flaps, I found that they were noticeably
stiffer than the Mustang VII’s flaps of five layers of glass. The BBQB was
built before carbon tubes and used three layers of glass and epoxy. The later
Critical Mass and BBFB Bearcats use carbon tubes and three layers which
are tremendously stiffer. The Critical Mass turned out heavy and would have
been useless as a stunt ship without the exceptionally rigid flaps.
The tubes I have been using are .317”. I particularly liked these as they had a
much thinner wall thickness than I had been able to find at other hobby or
kite shops. They were also a lot cheaper at Mike’s Hobbies than the thicker
wall tubes. These tubes have a wall thickness which seems to vary somewhat
between .021” and .026”. Hobby shop and kite stores tubes were all thicker
with the thinnest being .035”.
My concerns with wall thickness were that thicker wall tubes are much
heavier but not much stiffer in torsion. Torsional rigidity is mostly from the
outermost layers. These are good tubes which have worked well in my
airplanes to give the desired flap stiffness at a reasonable weight. The second
time I bought these tubes, it was based on the experience of having used them
already with satisfaction. I still have ten tubes. These will last for the next
five airplanes.
A few years ago, somebody with a very good airplane attributed their success
to having 1/8” flap horns which didn’t wash out in strong winds, and the
stampede was on to 4” bellcranks and 1/8” horns. Our airplanes do in fact
pick up energy from the wind when maneuvering down wind. The extra wind
energy causes our airplanes to accelerate (fly faster), particularly in
consecutive maneuvers. As our airplanes speed up the air loads on the flaps
increase and there will be some washing out of our flap deflections.
Considering the airplanes typical of the time, the real need was for stiffer
flaps instead of 1/8” flap horns to minimize “washing out”. For awhile there,
some builders were even flirting with 5” bellcranks to avoid the dreaded
“Netzeband Wall”. This ignores the fact that 3” bellcranks and 3/32” flap
horns were, and still are, demonstratively adequate for well built and well
flown airplanes at the highest level of competition. This is further evident in
observing Classic airplanes with top notch maneuvering capability at VSC.
Their 3” bellcranks and 3/32” flap horns are certainly up to the job of
providing all of the lift needed for competitive maneuvers.
Well, how much torsional strength can be rationally justified? The Molded
Mustangs V, VI, and VII were designed as .46 powered airplanes. The final
two, the Snaggletooth VI and VII, flew with home made 60’s. The updated
Snaggletooth II is pretty much same airframe flying today with a Ro-Jett .76
and weighs 12 ounces more than the best of the original Molded Mustangs.
3/32” Horn wires were, and are, perfectly adequate for these airplanes. The
Mustang VI used a 3” bellcrank. The Snaggletooth II uses a 4” bellcrank.
With the flap operating arms are sized to give the same deflection with both
sets of controls. The most noticeable difference was the wider line spacing at
the handle when using the 4” bellcrank.
Yes, I do use 1/8” horns in the Millennium Cavalier and Saito powered BBFB
Bearcats and think these are proper applications too. In general, I think that
3/32” horns are appropriate for slightly smaller stunt ships using .46 - .60 size
engines and weighing 60 ounces or less. I personally use 1/8” horns when
building designs for .72 - .91 engines which have a design weight of 70 ounces
or more. Enough is enough. Proportionally sized control system elements are
all that is necessary to obtain all the performance desired. I doubt that any of
us would consider 3” bellcranks and 3/32” horns for 1/2A airplanes, and, of
course, we shouldn’t. But neither should we be overly impressed by extreme
technical assertions where supporting evidence is lacking.
Obviously, carbon tube torque tube flaps are a bitch to build, but after flying
the first set, I’ll never begrudge the necessary labor or be without them. I
don’t make these torque tube flaps because I’m obsessive-compulsive. They make my semiscale airplanes
competitive without overwhelmingly huge wings.
If you liked the article on flap design, here is just a contents of book 2 of my DVD.
Contents
Evolution of a Thoroughbred 1
Go for Broke 44
Mustunt IV 88
Snaggletooth II, Trim and Tanks 387
Rabe Rudder 457
Shock Gears for Stunt Ships 471
Snaggletooth Gets a New Cockpit 488
Appliques and Vinyl Stencils 502
Trouble Shooting an Engine Problem 519
The BBQB Bellcrank Mount 528
Jig Building with Laser Cut Parts 538
Flaps for Control Line Stunt ships 556
Aerodynamics for Control Line Stunt 593
Miscellaneous Photos 605
Al
Topic: Flap design. (Read 6608 times)
