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Multidisciplinary
Design, Analysis, and
Optimization Branch
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EDUCATIONAL ACTIVITIES: THE NASA AEROQUIZ
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Week of 1/3/00:
Q:
Beginning in the 1940s, aerodynamicists designed supersonic vehicles
with sharp leading edges and slender body shapes. A pointed, slender
body traveling at supersonic speeds has a weaker shock wave ahead of it
and lower drag than a blunt body flying at the same conditions. The
pointed nose of the German V-2 rocket and the sharp edges of the
Lockheed F-104 are excellent examples of these kinds of designs.
However, many supersonic vehicles do indeed have blunt bodies. The reentry
bodies of ballistic missiles and most hypersonic aircraft are blunt.
Why would designers want extra drag?
A:
They usually don't. But they don't want their vehicles to melt, either.
By blunting the body, more heat is able to be absorbed by
the air around the body instead of the actual body itself.
This can't occur if the shock is attached and close to the
body. By letting the air absorb the heat, the body can stay
cooler, which is important for hypersonic vehicles.
(Of course there is a trade off between the amount of
blunting and increase in drag for hypersonic cruise
vehicles).
Congratulations to Lael vonEggers Rudd.
A blunt body is able to reject the heat generated by friction
with the surrounding air much better than a sharp-edged vehicle.
The faster a vehicle flies, the harder it is to keep it cool.
It can be shown that the temperature gradient (and the heat transfer)
of the air next to the vehicle's surface is much lower for a
blunt shape than it is for a sharp one. A blunt vehicle does a
better job of rejecting the heat of friction back into the flow.
- The Aeroquiz Editor.
Week of 1/10/00:
Q:
The "spoiler" wings often seen on the rear of many cars are intended
to increase stability and traction at high speeds by adding a "downforce"
over and above the weight of the car. In other words, they're upside-down
wings that provide a downward force instead of lift. Formula One racing cars,
for example, have wings that provide huge amounts of downforce at typical
racing speeds. If spoilers work on race cars, they must also
help to improve performance on Mom's minivan. Right?
A:
No, unless it is mummy Schumacher driving a very special minivan.
An aerodynamic force is not much felt at low speeds, for one, and I
suppose the compared proportion of the aileron's surface to the total
"wetted" and frontal areas of the vehicle is much less favourable to the
minivan than to the F1. So, the minivan's aileron is almost purely aesthetic.
Congratulations to Nicolas Cousineau.
On most cars bought off the showroom floor, spoilers are primarily a
visual styling cue added by the manufacturers to increase sales.
A few high-speed sports cars have genuinely functional spoilers.
And sometimes cars that have only modest power but are driven fast
are given functional spoilers. Audi's new TT, often driven on the
Autobahn, where speed limits are, well, not very limited, is an example.
But keep in mind that even the spoiler on Dodge's Viper GT2 is
aerodynamically trimmed to a neutral position.
And the new 174 mph Corvette C5 has no spoiler at all.
Spoilers on your average econobox cars (especially front wheel drive
econoboxes) do nothing.
- The Aeroquiz Editor.
Week of 1/17/00:
Q:
For structural support, early aircraft dating from the Wright
Flyer to those of World War I and beyond were equipped
with interwing support wires. These wires, perhaps surprisingly,
were a high source of drag on these early multiwing airplanes.
Anthony Fokker surprised the aeronautical world with his Dr-1 --
the famous triplane flown by the "Red Baron," Rittmeister Manfred
Freiher von Richtofen. Fokker confounded conventional aircraft
design practice by designing the Dr-1 without any interwing wires.
Partly because of this, the triplane had a zero-lift drag
coefficient of only 0.032: among the lowest of all wartime
airplanes. How was Fokker able to build a structurally sound
triplane without any interwing support wires?
No one got the correct answer. The question stands another week!
- The Aeroquiz Editor
Week of 1/24/00:
Q:
For structural support, early aircraft dating from the Wright
Flyer to those of World War I and beyond were equipped
with interwing support wires. These wires, perhaps surprisingly,
were a high source of drag on these early multiwing airplanes.
Anthony Fokker surprised the aeronautical world with his Dr-1 --
the famous triplane flown by the "Red Baron," Rittmeister Manfred
Freiher von Richtofen. Fokker confounded conventional aircraft
design practice by designing the Dr-1 without any interwing wires.
Partly because of this, the triplane had a zero-lift drag
coefficient of only 0.032: among the lowest of all wartime
airplanes. How was Fokker able to build a structurally sound
triplane without any interwing support wires?
A:
The common belief by Allied aerodynamicists during the early
part of WW1 was that a thinner aerofoil produced lower drag.
Fokker was using a thicker wing section (with a much more
effective "I" beam main spar), allowing each wing of his triplane
to carry its share of the loads independently of the others.
Because the new section was known to be efficient (See progress
made using the Eindeker), Fokker knew he was on to a winner.
Congratulations to Andrew White.
As Andrew noted, conventional wisdom at the time was that thick airfoils
would produce high drag. This effect is not as bad as designers thought
as long as the shapes are streamlined! Fokker designed his Dr-1 wings with
a remarkably high 13 percent thickness. This gave him enough internal wing
volume to design the wings using cantilevered supports and eliminate
the interwing wires. The thick wings also performed better than
the thin wings. The high lift properties of Fokker's thick wings
gave the Dr-1 extremely high rates of climb and enhanced
maneuverability.
- The Aeroquiz Editor.
Week of 1/31/00:
Q:
It's the week of the Super Bowl and time for a question about U.S.
football! Oakland Raiders star punter Ray Guy booted 1049 punts over thirteen
pro seasons for a whopping 44,493 yards with an average kick of 42.4 yards.
Only three of his punts were blocked. Historically, kickers were afterthoughts
in the draft. But that changed in 1973 when Guy became the first punter
to be drafted in the first round. No one had a sweeter "swing" than Guy,
who would kick the pig so high that other teams accused him of using
helium in the ball. And owner Al Davis wanted them to think that he did.
But seriously, would helium make Guy's footballs significantly lighter?
A:
I calculate a weight savings of about a tenth of an ounce.
Congratulations to Chris Snyder.
So do I. Using regulation football dimensions (about 11 inches long and
21 inches around the middle), I get a volume of about 129 in3.
Using a pressure of 10 psig and good fall football weather of 518 degrees
Rankine, I get a weight savings of 0.13 ounces, making a 14- to 15-ounce
football about one percent lighter. I leave it to you to decide its
physical and psychological significance!
- The Aeroquiz Editor.
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