28 MODEL AVIATION
C
by Dan Berry
WHEN MARVIN MACE designed the original A/B
Marval in the mid-1980s, he integrated the experience
and success of his good friend the late Mark Valerius
and Mark’s successful Rum Runner model series. The
Marval design’s name is a combination of their first
and last names.
As years have passed, the Marval’s evolution has
benefited from Marvin’s extensive success,
culminating in a model that shows the influence of two
of the country’s top AMA Power fliers in the last two
decades.
The design’s initial success was as a “locked-up”
(no auto surface) airplane that climbed nearly straight
up, with a transitional counterclockwise roll into a glide
that literally hunted for lift. The Marval was so good
that it was named the Large Gas Model of the Year in
the National Free Flight Society’s (NFFS’s) 1993 Ten
Models of the Year competition.
The latest iteration—the Super Marval—has shown
its versatility as an auto-surface bunter, to the extent
that the NFFS named it the Large Gas Model of the
The 1/2A Marval’s diagonal ribs combined with the upper surface sheeting and
carbon-fiber caps on the wing provide excellent strength with minimal weight.
The Marval’s planform reveals the slim box fuselage that
tapers in a straight line from just in back of the wing to the
tail. The covering is UltraCote Lite, which stays stable and
is unaffected by humidity and moisture.
06sig1.QXD 4/24/08 2:13 PM Page 28
June 2008 29
Dan powers his Marval with a Russian VA engine.
The spacer engine mount allows the VA to be
placed as far forward as is required to balance the
model.
The 1/16-inch spring music-wire skid mounts to a
plywood insert just in back of the firewall. The fuel
is cut off by a Texas Timers Micro I timer.
Photos by Larry Kruse
The 2007
AMA Nats FF
Classic Gas winner
Dan Berry with his 1/2A Marval,
which won 1/2A Classic Gas (for
models without auto surfaces)
at the 2007 Nats.
Year in 2007. The design is not lacking in heritage or bloodline.
My experience with the Marval began approximately five years ago,
with the locked-up A/B version, and I had initial success with it on the
contest circuit. Marvin’s sage advice and tutelage augmented that success.
My current A/B Marval and a C-size Marval with a Nelson .36 engine
on the front have been so competitive that I decided to build a smaller
version of the design for 1/2A. It was covered with tissue in its original
iteration and flew well enough last win 1/2A Classic Gas at the 2007 Nats.
The 1/2A Marval, along with its larger siblings, brought home the overall
Classic Gas championship.
Of primary concern in building a smaller version of the model was
maintaining the design’s integrity while making sure the weight was kept
as low as possible. The result was a 1/2A model that spanned 52 inches
and had 270 square inches of wing area, yet weighed only 6.2 ounces,
finished and ready to fly. It will be important to your airplane’s success to
keep the overall weight at or close to that target.
In the locked-down mode the DT line loops over the
4-40 nylon bolt on the top of the stabilizer and is held
in place with the stabilizer-hold-down rubber bands.
In the “pop-up” mode, the plywood platform on
the bottom of the stabilizer, the DT line routing
tube, and the 4-40 nylon incidence bolt are visible.
06sig1.QXD 4/24/08 1:11 PM Page 29
30 MODEL AVIATION
Texas Timers’ button-type viscous timer is set up using Gene
Smith’s “mousetrap” arrangement to take direct pressure off the
timer. The marks denote the length of timer runs.
The carbon-fiber capstrips give the high-aspect-ratio wing its
required strength. Those capstrips run full chord on the
bottom of the wing but only halfway down on the top
of the wing.
The pressure tank tucks up into an opening in the bottom crossgrained
sheeting just aft of the firewall. The propeller is carbonreinforced
fiberglass, to handle the high-rpm loads.
Some features, such as the “box” fuselage, sheeting on only the
top of the wing LE, the use of carbon-fiber rib caps, and an optional
adjustable motor mount, are incorporated into the smaller version to
keep the weight as low as possible, yet not sacrifice the Marval’s
well-earned reputation as a model that can take some knocks and
keep flying, contest after contest.
CONSTRUCTION
Construction usually moves more rapidly if the model is
“prekitted” before pinning or gluing anything. Wood for any sheet
components should be 6-pound balsa and even lighter for the
stabilizer ribs, if available.
Because this is not a beginner’s airplane, I’ll dispense with most
“stick-to-stick” instructions. I’ll touch on those things that will
ensure appropriate alignment and structural integrity.
Fuselage: One of the first decisions you need to make is what engine
you will use in the model. Whether or not you employ the optional
engine spacer mount will determine where the firewall blind
mounting nuts will go and how the whole nose area will be set up,
depending on the engine’s weight.
The goal is to have the completed model balance at the indicated
CG without adding dead-weight ballast to either the nose or the tail.
Of the two, because of the long tail moment it will be less of a
problem (and potentially less weight) to add ballast to the tail.
To help in the engine-selection process, I’ve provided a list of
power plants you could use in the Marval and their respective
weights. As the photos accompanying this article show, I’ve used the
VA engine in both of my models. They required the optional engine
spacer mount.
Unless you use a Cyclon, chances are that you will need the
spacer mount and some adjusted thickness of the spacer after the
airplane is completed to arrive at the correct CG with the engine
installed. If you do decide on a Cyclon, you will be well served to
make the wing a full geodetic structure as far as the diagonal ribs are
concerned.
With the Marval’s high aspect ratio and the Cyclon’s additional
speed and horsepower, it is imperative that the wing does not flex in
the slightest. The additional diagonals, coupled with the carbon-fiber
rib caps, should head off the problem before it emerges.
When construction begins, it’s crucial to build a straight fuselage.
Since balsa parts have a tendency to stress-relieve even several days
after being cut from a sheet, it’s best to cut the blank for the fuselage
top roughly 1/2 inch oversize in width (1/4 inch on each side).
Let the blank sit for a couple days while you’re working on some
other phase of the model. Then you can draw a longitudinal centerline
on the underside of the blank and cut the pylon and rudder slots.
You can build the fuselage upside-down on the fuselage top, with
Type: FF 1/2A Classic Gas
Skill level: Intermediate to expert
Wingspan: 52 inches
Flying weight: 6.2 ounces
Wing area: 270 square inches
Length (without engine): 33.25 inches
Stabilizer span: 19.375 inches
Stabilizer area: 80.75 square inches
Engine: 1/2A (.049 cu. in.)
Construction: Balsa, plywood, carbon fiber
Covering/finish: UltraCote Lite recommended
Other: Engine cutoff and DT timer
06sig1.QXD 4/24/08 2:15 PM Page 30
Dan launches the
Marval at the correct
angle: almost straight
up, angled to the right
of the wind, and with
the right wing tilted
down slightly.
Engine
Weights
The first decision you need to make if you build
this model is what 1/2A engine you will use. The 1/2A
Marval was designed with an optional engine spacer
that is custom-made depending on the weight of the
engine.
Here are some popular engines and their
respective weights:
• Cyclon .049: 68 grams
• Cox TD .049: 47 grams
• Stels .049: 45 grams
• VA .049: 44 grams
• AME/Norvel .049: 38 grams
As I mentioned in the article, the goal is for the
completed model to balance at the indicated CG
without adding dead-weight ballast to either the nose
or the tail.
Because the Cyclon is the heaviest of the engine
choices, it will need a shorter spacer than the lightest
engine choice: the AME/Norvel. The nice thing about
the spacer is that it will allow you to fine-tune the CG
by either shimming or sanding the engine mount’s
layered material. MA
—Dan Berry
the former locations lightly marked on the centerline. The bottom
of the fuselage is planked with cross-grain balsa from just in back
of the firewall to approximately 1/2 inch in front of the rudder. The
pylon and rudder must be lined up square with each other in all
respects as they are installed.
The stabilizer platform can be tilted (right-side high) so that the
stabilizer angle is roughly half the wing dihedral angle, for starters.
Finish the completed fuselage with your favorite fuel-proofing
method—as long as it’s lightweight!
Flying Surfaces: The stabilizer presents no special problems,
other than it should be kept as light as possible and won’t tolerate
warps.
Add the soft-balsa tips after construction is complete, and drill
a hole to angle the 4-40 nylon bolt backward approximately 10°
into the 1/4-inch-thick center rib. I drilled the hole, put a few drops
of cyanoacrylate in it, and then tapped it for the nylon bolt after the
cyanoacrylate cured.
The bolt can be removed until after the stabilizer is covered. It
is at an angle for leverage purposes.
In rigging the DT, loop the DT line over the bolt head and then
attach the stabilizer to the fuselage with rubber bands. The rubber
bands will allow the stabilizer to pop up in DT mode and keep the
DT line from coming off.
The wing is fussier but well worth the trouble in terms of
lightness and rigidity. Frame up all four wing panels flat on the
building board by pinning down the LEs and TEs and adding the
full-depth ribs, the top and bottom spars, and the half ribs. Do not
install the diagonal ribs, dihedral and polyhedral ribs, top sheeting,
or shear webbing at this time.
To arrive at the correct dihedral and polyhedral angles, block
up each panel to its correct height and then carefully sand in the
angle, much as you would on a Hand-Launched Glider wing. The
difference is that you do not have a solid surface to work with, just
a bunch of sticks poking out, so the word “carefully” comes into
play.
Once you are satisfied with the fit, glue all four sections
together and add the dihedral and polyhedral ribs.
This next step is where the wing’s strength begins to show up.
Pin the left main panel flat and add the 1/32 sheeting from the LE to
the back of the main top spar. Notice that the sheeting is only on
the top and that the wood needs to be as light as you can find.
The rib diagonals are next. For now they do not need to be
blended into the airfoil; it’s okay if they’re slightly “tall.”
Move to the right panel and prop up the LE 1/8 inch for the
required washin. The LE and bottom spar should be supported with
small incremental wedges along their entire lengths to avoid
putting a bow in them when the 1/32 sheeting is added.
The diagonals are next. During this process the left main panel
is in the air over the building board and must be supported evenly
and level with two 1/8 blocks under the LE and TE.
Once the right panel is dry, move back to the left tip panel. Pin
it to the building board with a 3/32-inch shim under the rear of the
most outboard rib to set in the required washout. During this
operation the rest of the wing will be in the air and must be
supported appropriately. The 1/32 sheeting and diagonals will build
strength into the wingtip.
Follow the identical procedure with the right wingtip, including
the 3/32-inch shim for washout, while you continue to block up and
support the washin in the right main panel.
Add the rear 3/32-inch top spar across the entire span of the
wing. Lay a straightedge along the existing main rib slots to mark
the diagonal ribs, and notch the ribs to accept the spar. Once the
spar is installed, you can use it as a sanding guide to bring the
diagonals to the same shape as the main ribs.
Complete the wing structure by adding the gussets at the
polyhedral joints, the 1/20-inch vertical shear webs between the
main spars, and the plywood wing joiners. The gussets at the
polyhedral joints are installed on top of the wing, providing a
surface for attaching the covering. Those gussets will eliminate
annoying wrinkles in the covering that typically show up near the
TEs of polyhedral joints.
06sig1.QXD 4/24/08 1:16 PM Page 31
the air,” without the need for the panels to
be pinned down.
To see if all the capstrips are stuck down
as they should be, hold the wing up to one
ear and gently twist it back and forth. You’ll
be able to hear the strips that pop up and
then see those that came loose or were never
glued down properly. Once all the capstrips
are locked in place, sand the entire wing
with 220-grit sandpaper.
Go back to each rib and vertically sand
the caps to their final width. Use an emery
board, or similar small sanding block, to
bring the 3/32-inch width down to the
finished 1/16-inch rib thickness.
Cover the flying surfaces with an iron-on
covering such as UltraCote Lite, traditional
tissue and dope, or Polyspan Lite. Stay away
from the heavier iron-on coverings; they
will add nothing but weight.
Flying: The 1/2A Marval must balance as
shown on the plans. I hope that little or no
weight will be needed. If it is, do not
hesitate to add it. This model is quick out of
the blocks and won’t tolerate being out of
balance for very long.
First flights should be under full power
over the proverbial tall-grass field we all
have, but with no more than a two- to threesecond
engine run. The correct launch angle
is nearly vertical, with the right wing tilted
down a bit and the model facing to the right
of the wind.
Do not launch the Marval to the left of
the wind! A right turn with the right wing
dipping indicates too much stabilizer
incidence. Reduce the incidence with a half
turn of the stabilizer incidence bolt on
successive flights until the model remains
nose-up with a slight turn to the right.
If the power pattern looks safe, increase
the engine run to four to five seconds. You
may need to add a small 1/4 x 1/32-inch
tapered balsa wedge to the left side of the
rudder to get the model to climb vertical and
begin its counterclockwise roll induced by
the right-wing washin. Add or subtract
rudder shims as required.
Once the power pattern is repeatable and
satisfactory, you can tighten the glide turn,
if necessary, by adding 1/64 plywood shims
under the left wing. Shimming the wing
instead of the stabilizer is more precise and
doesn’t affect the power pattern.
For optimum glide, you can add a small
amount of clay or lead tape to the tail to get
the slow, nose-up, thermal-hunting glide
that is the Marval’s trademark.
Good luck with your version of Marvin
Mace’s classic! Address any
correspondence or questions about building
and flying the 1/2A Marval to me. MA
Dan Berry
1827 Hot Springs Hwy.
Benton AR 72019
[email protected]
Sources:
Texas Timers
(423) 282-6423
www.texastimers.com
Kitting It Together
www.kittingittogether.com
FAI Model Supply
(570) 882-9873
www.faimodelsupply.com
34 MODEL AVIATION
!"
# $!"
#%&
'()
*+
*
,&
-
*
.+
*
www.pspec.com
NELSON Engines and Accessories.
ULTRATHRUST Muffl ers.
4-Stroke Muffl ers, Elbows, & Twisters.
Pylon Racing and Performance Parts.
06sig2.QXD 4/24/08 11:52 AM Page 34
