7/27/2019 Regen Soaring http://slidepdf.com/reader/full/regen-soaring 1/36 Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 1 Pelican Aero Group Regenerative Soaring The Next Regime of Low-Speed Flight J. Philip Barnes April 2007 Update In his 1926 landmark text, the famous British aerodynamicist Hermann Glauert suggested we “consider the case of a windmill on an aeroplane.” Although Glauert offered no specific application thereof, he knew the airborne turbine would one day find important applications. In 1998, American engineer Paul MacCready offered an application of the airborne turbine, introducing “with caution” the concept of regenerative soaring . Here, an aircraft incorporates energy storage, a propeller, and a wind turbine, or dual-role machine thereof, to propel the aircraft and regenerate stored energy in updrafts. Today, it is my pleasure to share leading-edge discoveries about this next regime of low-speed flight. We will demonstrate the feasibility of an “entire flight without fuel,” including self-contained takeoff and climb, cruise, regeneration, and landing on a full charge. To begin our study, we first review and expand upon the principles of classical soaring. Then we extend these new methods to evaluate the feasibility of regenerative soaring. We show that the flight performance of a “regen” is not only sustainable, but competitive with that of a sailplane, while adding the regen-unique capabilities of self-contained thrust for takeoff, climb, and cruise, and climb. Finally, we preview the additional advantages of “solar-augmented” regenerative soaring.
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Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 1
Pelican Aero Group
Regenerative Soaring The Next Regime of Low-Speed Flight
J. Philip Barnes April 2007 Update
In his 1926 landmark text, the famous British aerodynamicist Hermann Glauert
suggested we “consider the case of a windmill on an aeroplane.” Although Glauert
offered no specific application thereof, he knew the airborne turbine would one day
find important applications.
In 1998, American engineer Paul MacCready offered an application of the airborneturbine, introducing “with caution” the concept of regenerative soaring . Here, an
aircraft incorporates energy storage, a propeller, and a wind turbine, or dual-role
machine thereof, to propel the aircraft and regenerate stored energy in updrafts.
Today, it is my pleasure to share leading-edge discoveries about this next regime of
low-speed flight. We will demonstrate the feasibility of an “entire flight without fuel,”
including self-contained takeoff and climb, cruise, regeneration, and landing on a
full charge.
To begin our study, we first review and expand upon the principles of classicalsoaring. Then we extend these new methods to evaluate the feasibility of
regenerative soaring. We show that the flight performance of a “regen” is not only
sustainable, but competitive with that of a sailplane, while adding the regen-unique
capabilities of self-contained thrust for takeoff, climb, and cruise, and climb. Finally,
we preview the additional advantages of “solar-augmented” regenerative soaring.
Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 12
Pelican Aero Group
“Generation-X” Regen
Our rationale for the design of “Generation-X” begins with our intent to minimize in-flight aerodynamic
interference between the windprops and airframe, while also providing self-contained and robust
ground handling by the pilot alone. Thus, the counter-rotating windprops, which allow steering on the
ground, are kept aerodynamically clear of the airframe via twin pod installations.
The windprops are arranged in a pusher configuration, whereby the sudden rotational flow impartedby the blades cannot impinge on the leading edges of downstream lifting surfaces which otherwise
would suffer interference and induced drag penalties. If necessary, pod-boom trailing-edge blowing
may mitigate any adverse affects of the pod-boom wake on windprop operation.
Windprop noise is dramatically reduced by the selection of multiple blades at the smallest diameter
which meets requirements for climb thrust and cruise/regen efficiency. The windprop speed control
and motor-generator units, housed and air-cooled in the pods, are relatively close to the fuselage-
enclosed energy storage unit to minimize line losses and to mitigate aft center-of-gravity trends.
The wing design incorporates downward-pointing winglets with integrated tip wheels, the latter
required regardless of wingtip configuration. The winglets, which develop aerodynamic thrust in flight,
are somewhat elevated above the ground via tip dihedral. Such elevation is enhanced as the wingflexes upward under steady lift load.
Finally, the system enjoys the simplicity of fixed geometry for the windprops and their installation.
Retraction or folding mechanisms are not required, and as illustrated later herein, the windprops can
simply “pinwheel” when neither the propeller nor turbine mode is used. A parallel study of a “constant-
speed” windprop (actuated blades) yielded 40% greater max-capacity regen power, but did not offer
gains in efficiency for any operational mode. Uniform fixed pitch was selected for our study herein.
Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 18
Pelican Aero Group
Load Factor and “Clean” Sink Rate
Airspeed, v ~ km/h
50 60 70 80 90 100 110 120 130 140 150
dz/dt ~m/s
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
g-Load, nn
1.0
1.2
Sea level25 kg / m
2
A = 16
1.4
1.6
Min SinkMax L/D
dz /dt = - nn (d / l ) v
Load factor drag/lift airspeed
Lift Coefficient
c L = nn w / (qs)
Drag polar ⇒ c D
d/l = c D /c L
To relate the normal load factor (nn) to sink rate and airspeed, we first
recognize that the lift coefficient (cL) includes the load factor as shown in the
formula at the upper right. The drag polar then provides the drag coefficient,
and the ratio of drag-to-lift ( D/L or d/l )* is then equal to the ratio of drag-to-lift
coefficients (cD/cL).
Now we can calculate the still-air clean sink rate, [nn(d/l)v], the latter clearly
proportional to load factor. For example, the aircraft in max L/D glide (1.0-g)
sinks at 0.75 m/s at 85 km/h airspeed. However, the aircraft turning at 1.4-g
sinks at 1.25-m/s at 100 km/r airspeed. The left-hand tip of each curve
represents operation at max lift coefficient, and the maximum of each curve
represents minimum-sink operation.
Finally, we note that this graph addresses “clean sink rate.” When the
windprop system is added, the regen aircraft will fall more quickly throughthe thermal, whereby (dz/dt) is no longer equal in magnitude to the clean
sink rate [nn(d/l)v].
* Note on notation:
Most of our charts and notes herein implement a suggested nomenclature philosophy using lower-
case letters to represent dimensional variables, and upper-case letters to represent dimensionlessgroups. For example, lift, drag, and their corresponding coefficients would become (l,d,L,D). Until such
Here we plot isolated windprop efficiency versus a “speed ratio” (S) for two uniform-pitch
windprop designs sharing the same diameter and climb thrust, but with the high-speed
design having two blades and 14-deg blade tip angle (i.e., relatively low pitch), and the low-
speed design having eight blades and 30-deg blade tip angle. In either case, propeller
efficiency has the traditional definition with shaft power in the denominator, whereas turbine
efficiency follows Glauert’s definition with shaft power in the numerator. Since for turbine
operation both torque and force change sign, turbine efficiency remains positive. Note thatturbine efficiency is not subject to the “Betz Limit” of a ground-based wind turbine using a
different definition of efficiency.
We next notice that for either design, peak efficiency is comparable for both operational
modes, indeed slightly higher for turbine operation. We also notice that the efficiency is
plotted versus a speed ratio (S) which has been defined for the blade tip. This speed ratio,
proportional to advance ratio, S = J / [π tan (βtip)], applies to both propeller and turbine
operation, while also illuminating the essential principles of pinwheeling operation. When the
speed ratio is near unity, we have pinwheel operation with zero torque and zero efficiency.
As speed ratio is reduced, we have propeller operation, but if speed ratio is increased, we
have turbine operation.
Finally, we plot the force coefficient (F), again versus speed ratio (S). This force coefficient is
referenced to windprop disk area and flight dynamic pressure (q). Such characterization,
together with the formula in the blue box, allows us to easily relate installed thrust-to-drag
ratio (t/d ), aircraft drag coefficient (c D), wing area ( s), windprop radius (R ), number of
windprops ( N wp), and climb rate (dz/dt ). Regardless of operational mode, installed thrust (t )
includes the normalized change in drag (∆d /d ) due to windprop system addition. For climb in
still air, both (t/d ) and (dz/dt ) are positive, but these two terms are negative when the regen
“falls” relative to a balloon-based observer rising with the thermal.
Total Shaft power, τω~ kW 29.5 3.50 0.00 -1.36 -2.58
Energy storage rate ~ kW -36.9 -4.12 0.00 1.16 2.07
* incl. trim drag (< 0, turbine mode)
( ) p s st
nt s
/ e: power shaft prop / e:Watts~ power shaft turbine
d
t v
l
d n z u z w / e: s / m~rate storageenergy specific
ηε−=τωεη−=τω
ε⎥⎦
⎤⎢⎣
⎡−=−−=
••
•••
Here we have applied the Regenerative Soaring Equation (with related
formulas) to compute the performance parameters of the regen in each of its
operating modes. The table distinguishes the various rates (dz_/dt ) while also
showing the applicable sign conventions. Table entries at lower left indicate
how the propeller climb mode exercises the system capacity. Note that
efficiency is significantly degraded in both climb and max regen.
Thrust/drag ratio (t/d ) varies from +6.33 to –1.01 as operating mode varies
from climb to cruise, then to pinwheel (t/d = -0.10), max-efficiency regen, and
max-capacity regen. Whereas the aircraft climbs at (dz/dt ) = 4.0 m/s after
takeoff, it falls through the thermal (relative to an observer in the thermal) at
2.06 m/s during max regen. Also, the total specific energy rate is –5.4 m/s in
climb, and +2.18 m/s in max-capacity regen.
For max-capacity regen, a ground-based observer sees the aircraft climbingat the rate (dz o /dt ) = 1.66 m/s, even though the aircraft is falling at 2.06 m/s
relative to the thermal. Although we have included max-capacity regen here
for study purposes, only max-efficiency regen provides competitive flight
performance. Of course, max-capacity regen often proves useful, including
landing descent where the energy storage rate is approximately 2 kW,
enhancing the chances of landing on a full charge. The latter condition will
strongly depend on the distance from the last thermal to the landing site.
Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 28
Pelican Aero Group
Climb in the Thermal ~ Ground-observed ~ dzo /dt
1.0 1.5 1.6 1.7
Max efficiency Regen
1.0 1.6 1.70
3500
Sailplane
/dt ~ m/s
1.0
0.5
0.0
3000
2.02.5
1000
1500
2000
2500
1.1 1.2 1.3 1.4
1.0
0.50.0
2.0
1.5
1.0
1.5
1.1 1.2 1.3 1.4 1.5
Load Factor ~ nn
dzo
zo ~ m
1.0
1.5
2.0
2.0
1.50.0
2.5
1.5
1.0
500
1.00.5
0.0
0.5
0.0
0.5
1.0
2.22.6 m/s
0
O p t i m
u m
1.0
Here we have applied the foregoing models and methods to calculate and
plot, versus load factor and elevation, contours of ground-observed climb
rate (dz o /dt ) in the thermal, for both the sailplane and regen. The sailplane
obtains a maximum climb rate of 2.6 m/s turning at 1.4-g around 1500-m
elevation. The regen, shown at the right, climbs more slowly because it is
storing energy during the climb.
We will assume that for both aircraft, the interesting part of the thermal
extends from 500-m to 2500-m elevation. The dashed curve represents the
optimum (minimum time-to-climb) “trajectory” in terms of load factor versus
elevation, indicating 1.5-g turns at low level and 1.1-g turns at high level.
The white contour for each aircraft represents flight at fixed elevation. The
regen could undertake “equilibrium regeneration” at either 200-m or 2700-m,
but at those elevations the thermal has little to offer. Thus for the mosteffective strategy, the regen climbs in the thermal as it regenerates. This is a
fundamental result, not anticipated at the outset of our study where we had
anticipated equilibrium regeneration would be a typical operational mode.
Flight Without Fuel - Regenerative Soaring SAE 2006-01-2422 (sae.org), J. Philip Barnes 29
Pelican Aero Group
“Total Climb” or Total Energy Rate ~ dzt /dt
0.0
0.5
1.0
1.5
2.0
2.0
1.5
1.00.50.0
2.5
1.0
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Max efficiency Regen
0.0
0.5
1.0
1.5
2.0
2.0
1.51.00.5
0.0
2.5
2.5
1.5
1.0
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.70
500
1000
1500
2000
2500
3000
3500
dz t /dt ~ m/s
Sailplane
Load Factor ~ nn
zo ~ m
O p t i m
u m
2.62.5 m/s
Next we plot the total climb rate, or rate of change of total specific energy.For the sailplane (where dz o /dt = dz t /dt ), this is the same data as just shown,
but with different colors. But for the regen, the rates “dz o /dt ” and “dz t /dt ” are
distinct due to the energy storage feature.
Note that the regen gains total specific energy at almost the same rate as
the sailplane. The peak rate is about 2.6 m/s at 1500-m. The optimal energy-
-load-factor trajectory for the regen is represented by the dashed line.