Flight Training - June 2020 - 42

The CAFE
Foundation
was begun
more than 30
years ago to
evaluate
a broad array
of aircraft
technologies,
and it came
up with
its own
mathematical
formula to
judge aircraft
efficiency.

(Speed
times
2.25/
Fuel
Flow
times
Payload)

Slow airplanes that fly at the low end of the speed
spectrum have an advantage in speed per horsepower
because they pay a relatively light drag penalty. Doubling airspeed, for example, brings a fourfold increase
in drag. That's why a low-tech Luscombe Model 8F
Silvaire built in the 1950s can deliver a top speed of
120 knots on just 90 horsepower-or 1.33 knots per
horsepower.
Another useful and familiar performance
measure from the automotive world is miles per
gallon, yet few manufacturers use mpg because
it's seldom flattering to aircraft. A Cessna 172S
Skyhawk, for example, gets about 15 statute miles
per gallon in cruise. That's better mileage than an
H1 Hummer (12 mpg on the highway), but nothing
to write home about.
Miles per gallon comparisons for aircraft improve
in larger models when you consider seat miles per
gallon. And those get downright impressive in modern airliners (a Boeing 777ER can get more than 73
seat miles per gallon at 500 miles per hour.) Miles per
gallon is spectacular for a few ultra-efficient experimental category piston aircraft; race pilot Klaus
Savier of Lightspeed Engineering gets up to 100 mpg
in his Burt Rutan-designed canard aircraft.
The total cost per mile per hour is a calculation
that aircraft owners do their best to avoid thinking
about-especially when it comes to speed upgrades.
In general, we're drawn to airplanes that go fast, and
we're willing to pay for accessories that look good
and add speed, especially when those two attributes
go together.
An aircraft's Carson Speed is the indicated
airspeed that delivers the most speed for the least
practical fuel consumption-yet it's not marked on
any airspeed indicator. The concept was spelled out
in a 1980 scientific paper by aerodynamicist B.H.
Carson, who called his theoretical number the "least
wasteful way of wasting fuel."
What pilots must know to determine the Carson
Speed for any piston airplane is its best-glide speed,
or VG. Add 32 percent to that, and that's the Carson
Speed. Flying at the Carson Speed is an aerodynamic
bargain because it represents a 32-percent airspeed
increase (over VG) yet fuel consumption rises just 16
percent.
For example, a normally aspirated Beechcraft
Bonanza G36 (with the landing gear retracted) has
a VG of 110 knots. Add 32 percent and the Carson
Speed is 145 knots indicated airspeed (KIAS). Climb
to the maximum altitude at which the Bonanza
can maintain 145 KIAS in level flight (typically
between 9,000 and 12,000 feet) and you'll be flying
as efficiently as possible in terms of speed and fuel
consumption.
Flying faster than the Carson Speed imposes
steep penalties in terms of fuel consumption and

FLIGHT TRAINING JUNE 2020

range. Flying at a slower indicated airspeed saves
relatively small amounts of fuel in percentage terms.
Piston aircraft engines are measured in terms
of horsepower and fuel consumption in gallons
per hour. A piston aircraft engine's specific fuel
consumption, or the amount of fuel it consumes to
produce a single horsepower, is surprisingly uniform.
In general, an air-cooled aircraft engine uses about
0.06 gallons of avgas per horsepower when properly
leaned in flight. A 100-horsepower aircraft engine, for
example, typically burns about 6 gallons of avgas per
hour at full power.
But engineers have found some clever ways to
extract more power and greater efficiency. Heavierthan-standard pistons are one way to increase an
engine's power output. Heavier pistons provide greater
force on the spinning crankshaft and propeller-an
advantage to air racers looking for more speed, or
long-distance fliers seeking greater flight efficiency. Bill
Harrelson, an earthrounding endurance flier, equipped
the Continental IO-550 engine in his Lancair IV with
heavy pistons for his record-setting distance flights.
Electronic ignition systems also provide more
power and greater fuel efficiency due to their variable
timing. Unlike traditional magnetos which always fire
at the same point in the engine's operating cycle, variable timing accounts for changes in manifold pressure
and air density at high altitude. Electronic ignition
allows for an optimum air/fuel mixture in each cylinder, and more powerful spark plugs ensure that all
the fuel in each combustion chamber is burned at the
correct time for the most possible power.
Relatively new Austrian-designed Rotax engines
have shown major improvements in the power to
weight ratio of piston aircraft engines. A 100-horsepower Rotax 912 engine, for example, weighs about
140 pounds-or 50 pounds less than a Continental
O-200D which produces the same horsepower.
Both are four-cylinder engines. But Rotax uses far
higher rpm (5,500 compared to 2,750), a propeller
gear reduction drive, and both air and liquid cooling
(rather than air alone) to produce the same amount
of power from smaller cylinders.
Rotax engines dominate the light sport aircraft
market, and the company has begun producing larger
engines that could bring major changes to larger
airplanes. The company's turbocharged 915iS engine
produces 141 horsepower and is making its way into a
growing number of both experimental- and standard
category aircraft.
Aircraft design has always been an imperfect art
of balancing competing interests. Comfort or speed?
Range or payload? Appearance or utility? Taking a
close look at the hard numbers will show the designer's priorities.
dave.hirschman@aopa.org

Flight Training - June 2020

Table of Contents for the Digital Edition of Flight Training - June 2020