Radio Control Soaring - 2009/05

THE WING IS the thing in RC Soaring, and
one of the most respected authorities in RC
sailplane design is Dr. Mark Drela of the
Massachusetts Institute of Technology
(MIT). He has been active in FF and RC
model aircraft since childhood. He’s the guy
behind the AG series of airfoils that is used
on a number of the latest super-gliders.
Mark was kind enough to answer a few of
my questions.
LE: It seems as though every few years
there’s a new “in” airfoil. Could you
perhaps describe the performance
differences between a contemporary airfoil,
such as the one on the Supra, with those from
the past, including the ubiquitous 7037?
MD: The Supra AG4x airfoil series are
significantly thinner and have less camber
than the SD7037. The SD7037 section was
designed when some 120-inch gliders pushed
80 ounces or more.
The AG4x series is geared for lower Reynolds numbers, which
made them a better match for the lighter gliders, which started to
come into vogue in the early 2000s. It turns out that this newer “light
+ low camber” approach has resulted in a wider overall speed range
than the older “heavy + high camber” approach.
Another major step was to use a significant variation of airfoils
across the span. The Supra wing-root AG40d airfoil is 8% thick and
is similar to the MH32. The Supra wingtip AG43d airfoil is 6.5%
thick and is very nearly the same as a DLG [discus-launch glider]
airfoil. This makes the wing perform significantly better than if one
airfoil is used across the entire span.
The previous airfoils were also typically designed for some
neutral flap setting, and a camber/reflex flap was then typically
added by the wing designer almost as an afterthought. In contrast,
the AG4x series was a priori designed for use with a camberchanging
flap, with the contours designed for both the high- and
low-camber flap positions.
Incidentally, this latter approach has been the standard practice
[[email protected]]
Radio Control Soaring Lee Estingoy
Dr. Mark Drela shares his airfoil-design theory
This rough sketch of airfoil shapes shows how the two model airfoils are noticeably
thinner than the full-scale airfoil at the bottom.
Today’s Supra TD model sports
airfoils that Mark Drela designed
specifically for the wing and
airspeed characteristics of the TD
flight envelope.
Dr. Mark Drela’s work with airfoils is a major reason for the
outstanding performance of today’s Thermal Duration sailplanes.
in the design of full-scale glider airfoils since the ’80s, so it’s really
nothing new.
LE: I have a loose grip on Reynolds number and wing-chord
concerns. Smaller wing chords offer unique design concerns.
I’m curious to know your thoughts about Scale sailplanes. Some of
the smaller models of high-performance gliders, such as the Nimbus,
wind up with some terribly small chords at the tips. Is there a sweet
spot for chord for Scale models, given their usual operating speeds?
MD: Airfoils for scale sailplanes are tough to design, for several
reasons.
May 2009 117
05sig4.QXD 3/24/09 1:02 PM Page 117
118 MODEL AVIATION
• The high aspect ratios make the structural
demands quite severe, so the airfoils
naturally want to be thicker than what the
aerodynamics alone dictates.
• It’s not clear what the peak loads are,
unlike in a TD [Thermal Duration] glider,
where the peak load is simply the known
winch load. So on a Scale glider, it’s
difficult to make a rational aero/structural
tradeoff.
• Suitable airfoils do not “look right” on an
RC Scale glider, at least to me. If matched
well to the Reynolds number, they are
noticeably thinner and have their max
thickness point farther forward than on the
full-scale glider. I don’t know if this really
matters to most RC scale glider fliers, and it
may not be a big deal.
On the issue of small-chord tips, that’s
actually less of a problem than one might
think at first. The trick is to make the airfoil
progressively thinner towards the tip, as on
the Supra. Then one can always add
washout to quash any tip-stall tendencies.
The resulting airfoil will especially “look
wrong” as in the point above, but such a
wing will perform well and have good stall
characteristics.
The Supra wing uses a touch of washout
to get the same effect. The amount of washout needs to be carefully
chosen via a vortex-lattice method like AVL [a program for the
aerodynamic and flight-dynamic analysis of rigid aircraft] of
arbitrary configuration or something similar. If overdone, the highspeed
performance will suffer, as is well known.
LE: Your model designs all seem to stress lightweight building
techniques. However, many modelers seek to put ballast in their
models to modify performance on windy days. Could you shed some
light on the compromises that are involved in adding weight to
sailplanes? What exactly does adding ballast achieve?
MD: I’m not sure it’s possible to define “optimum ballast” for XXX
mph wind, which is really best in all situations. It also depends on
what type of lift is present. I very rarely use ballast in either DLGs
or TD gliders, even in serious wind. I instead fly more
conservatively and simply don’t allow myself to get too low when
flying downwind of the field.
I take the penalty in penetration performance to still be able to
react to and work very tight and fleeting puffs of lift, which are
common in windy weather. Other people prefer to ballast up and
live with the increased turning radius. Both extremes and anything
in between can be made to work, so there’s no magic ballast
formula.
LE: The conventional wisdom out there is that some airfoils “like”
to carry weight more than others. For instance, the 7037 airfoil
seems to do very well if the sailplane is a bit heavier, such as an
electric. This probably has something to do with the speed at which
the model is flown—back to the Reynolds number again, right?
MD: Yep. Compared to the AG40d, the SD7037 likes more weight
largely because it’s designed for a larger Reynolds number.
LE: Where are we headed with airfoil design for models? What can
we look forward to in the way of improvements, if anything?
MD: Tough to say. One big issue is construction accuracy and
variability, especially with built-up or [vacuum] bagged models.
This influences transition and especially high-speed performance, in
ways which we don’t really know how to quantify. And even if such
variability could be quantified, it’s not clear how one could make
better airfoils with the information.
So I guess it’s fair to say that I don’t know what is the best next
step in airfoil design.
Visit YouTube (www.youtube.com) and search for “Decavitator” to see its world-recordholding
run. If Dr. Drela’s work with sailplanes isn’t impressive enough, that will make you a
believer. John S. Gill has faithfully re-created this model.
LE: Competition sailplanes seem to have grown in the past years.
Two-meter and 100-inch Open-class sailplanes gave way to 3.2-
meter and now 3.7-meter gliders. What is the benefit of having the
larger wings? What are the costs of having such larger wings?
MD: Pros:
• More range (in terms of actual glide range and visual range).
• Higher launch (up to a point—assuming the winch is not maxed
out).
• Can handle stronger winds better.
Cons:
• They are less maneuverable, especially in small low-level thermals.
• There’s more airplane to haul around.
• They crash harder.
LE: What’s on your building table now?
MD: Nothing right now. In the past year I’ve been far too busy with
work. I’m also at the point in my RC “career” where early-Christmas
gliders occasionally show up on my doorstep. (Thank you, RC
Builder and Maple Leaf Design.)
It’s hard to get motivated to build from scratch when you get
such really first-class hardware with no building work required.
LE: If you were a sailplane or Soaring coach, what suggestions
would you have for the new pilots?
MD: When timing for relative newbies, the most frequent
suggestions I make are:
• Detect lift or sink and watch carefully for “uncommanded motions”
of the glider, both in roll and pitch. The glider not only rolls away
from lift, but also pitches up and balloons when flying into lift.
• Follow the lift as it drifts with the wind.
• When struggling against the wind to get back to the field, always
maintain a good positive ground speed, at least half of the wind
speed, say. Never, ever let your ground speed drop to zero—then
you’re just losing altitude for nothing.
LE: What’s your favorite model of all time? Is there any one
sailplane you thoroughly enjoy taking to the field for an afternoon?
05sig4.QXD 3/24/09 12:22 PM Page 118
MD: Either the SuperGee II or the Bubble
Dancer, depending on the conditions. The
SG2 is most fun in tight and puffy
thermals, with lots of wind shifts.
The BD is most fun for floating around
in low wind with really weak high up,
such as late afternoon or at sunset. When
there’s lots of strong lift everywhere, and
staying up gets easy enough to get boring,
I really like to fly the BD from hand
throws.
Professor Drela obtained his Bachelor of
Science (1982), Master of Science (1983),
and doctorate (1985) degrees from MIT.
He is currently the Terry J. Kohler
Professor of Fluid Dynamics at the MIT
Department of Aeronautics and
Astronautics. He joined the faculty there
in January 1986.
Mark’s primary research interests are
in low-speed and transonic aerodynamics
and computational aerodynamic-design
methodology. He has developed a number
of computational aerodynamic design/
analysis codes that are currently being
used in the aircraft and gas-turbine
industry.
He has also developed tools for
analysis and design of control systems for
highly aeroelastic aircraft. Mark teaches
aircraft-design fundamentals, external
aerodynamics, and fluid mechanics of
boundary layers at the undergraduate and
graduate levels.