Variometer system for soaring flight

Apparatus for use in a glider for indicating the rate of vertical air mass movement. The apparatus includes sensing means for detecting both glider airspeed and acceleration orthogonal to the plane of the wings. This information is used to modify the output display of a conventional variometer to eliminate the effects of drag.

FIELD OF THE INVENTION 
This invention relates generally to aircraft instrumentation and more 
particularly to glider borne equipment for the detection of rising and 
sinking air. 
Motorless soaring flights by gliders can be sustained for many hours and 
tens or even hundreds of miles can be traversed in the process. Such 
flights are made possible by a very sensitive and fast altitude rate 
sensing instrument called a variometer. This instrument aids the pilot in 
the critically important task of detecting rising and sinking air. 
Unfortunately, much of the time the information provided by the 
variometers in common use today is not interpretable in terms of air 
motion due to frequent and significant, pilot and atmosphere induced wing 
load factor changes. 
The background and limitations of presently existing variometers, and 
proposed improvements, are discussed by applicant in an article in 
SOARING, Feb., 1990, entitled "Total Energy Reconsidered", page 25, which 
article is by reference incorporated herein. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved variometer which more 
accurately reports the rate of vertical movement of the air mass the 
glider is passing through. 
In accordance with the present invention, wing load factor changes are 
sensed and their effect on the glider's performance calculated, and then 
eliminated from the signal driving the variometer display to thus provide 
more useful information to the pilot. 
In a preferred embodiment of this invention, sensing devices are provided 
to detect both acceleration of the glider in a direction orthogonal to the 
plane of the wings and speed of the glider. Information from these sensing 
devices in conjunction with computed and/or stored glider flight 
performance characteristics data is used to alter the output of the 
variometer. 
Further objects and advantages of this invention will be apparent from a 
consideration of the drawings and the ensuing description thereof.

BACKGROUND 
A glider flying through still air will descend at a rate determined by it's 
aerodynamic qualities, wing load factor and speed. Minimum still air sink 
rates for gliders in common use are in the range of 100 ft/min to 300 
ft/min. Sink rate typically increases with load factor and/or speed. When 
a glider flies through a moving air mass it's net motion is the algebraic 
sum of the gliders motion and that of the air mass. Thus, when a glider 
flies in rising air, or lift, where the ascent rate of the air exceeds the 
still air sink rate of the glider, the glider climbs. Of course, the 
converse is true, when the glider flies in subsiding air, sink, it's 
descent rate is increased. 
Generally, lift is sparsely located but intense, whereas sink abounds but 
is weak. Lift is sometimes organized in a predictable fashion, but more 
often it is randomly located. Human senses cannot detect regions of lift 
or even accurately report a glider's descent or climb once altitudes of a 
few hundred feet are reached. Detection of lift and descent or climb, is 
accomplished by an instrument well known as a variometer. 
The basic variometer is a rate of climb reporting device similar to--but 
faster and more sensitive than--the rate-of-climb instruments common on 
powered aircraft. It senses changes in the barometric pressure of it's 
environment, and reports them as climb or descent. An improved variometer 
version in common use on gliders utilizes airspeed as well as barometric 
pressure to report changes in the "total energy", kinetic +potential, of 
the glider. The total energy type variometer displays changes in the 
gliders ability to sustain flight rather than simply changes in it's 
altitude. Those changes are conventionally expressed as climb or descent, 
e.g. feet per minute. 
In order to sustain flight the soaring pilot endeavors to fly rapidly 
through regions of sink, and to slow and/or circle for climb when regions 
of lift are encountered. Locating and flying in lift is the primary 
strategy. Variometers have proven essential to the task but those in 
common use are imperfectly suited in that they report glider motion not 
air motion. In terms of the vertical air motion information needed, they 
are strictly accurate only in steady-state accelerationless conditions 
because the air mass data they communicate has been transformed through 
the dynamic characteristics of the glider. 
In some common flight circumstances variometer information is so confounded 
by high load factor maneuvers as to defy interpretation by the pilot. An 
example is the important situation of entry into lift from cruising 
flight. In sequence: the glider happens into lift; the pilot perceives 
that fact via the variometer; he then immediately pulls up to slow and 
circle for climb. Both the abrupt pull up and the sharp turn to start 
circling contribute to cause load factor to increase markedly. The 
increased load factor, in turn, increases drag to cause a greatly 
increased glider sink rate during the initial period of entry into the 
lift. 
The pull-up/turn induced degradation of the glider's performance occurs 
during the most critical instants of the first encounter with lift. The 
variometer's report of the glider's load factor induced transient loss of 
total energy obscures the strength and location of the lift which the 
pilot is attempting to find and pull into. After initially observing lift 
on the variometer the pilot pulls up and turns only to see sink through 
most of the pull-up/turn maneuver. He is blinded as to direction and 
intensity of the lift he is passing through and so is unable to optimize 
steering to center on lift. 
A similar obfuscation of critically needed atmospheric lift /sink 
information takes place in straight cruising flight. In a typical such 
situation the variometer indicates lift has been encountered so to secure 
maximum climb benefit from the lift the pilot pulls up sharply. The 
pull-up increases load factor and drag increases. The variometer now 
indicates heavy sink and the pilot is confused. Of course a similar but 
reverse sequence occurs when sink is encountered and the pilot seeks to 
minimize loss of altitude by pushing over to speed through it. Such 
maneuvers are important to flight optimization but are difficult to 
execute because of the misleading variometer responses. 
Yet further difficulties with the variometers in common use are encountered 
when gliders are flown in turbulent conditions. The turbulence results in 
frequent random accelerations of the glider. The consequent rapidly 
shifting wing loading can result in radical gyrations of the variometer 
display. In such conditions variometers provide little interpretable 
information regarding general air mass movement. 
FIGS. 3 and 4 graphically illustrate the limitations of state of the art 
variometers. In FIG. 3A a glider transitions from "level" gliding flight 
at one altitude to "level" gliding flight at a higher altitude. FIG. 3B 
shows the load factor effects of the glider's trajectory. In FIG. 4 the 
effects of load factor and speed changes are plotted on a multi-load 
factor polar diagram. Note that in this example during the pull-up process 
positive acceleration occurs and the locus 1 shifts from the 1G polar 2 to 
the 2G polar 3 with a consequent large increase in sink rate. The converse 
occurs during the push-over. FIG. 3C summarizes the sink rate consequences 
of the glider's maneuver as interpreted from FIG. 4. Large excursions in 
sink rate as B to E and F to I of FIG. 3C illustrate the problem addressed 
by this invention. Such large changes tend to obscure the air mass motion 
information the pilot needs. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In order to facilitate an understanding of the manner in which the 
invention may be practiced and before considering particular systems in 
accordance with the invention, attention will be given to some physical 
properties of a glider in flight. 
In normal flight the sink rate of a glider flying in still air is 
determined by the resistance to movement through the air of the glider. 
That resistance is called drag. Drag can be regarded to consist of the sum 
of two principal components: 
(1) Induced drag , D.sub.i and 
(2) Parasitic drag, D.sub.p. 
Induced drag is largely a reaction to the lift of the wings. It takes the 
simplified form 
EQU D.sub.i =K.sub.i (L/V).sup.2. 
Where 
K.sub.i is a constant, 
L is the load factor on the wings and 
V is the aircraft's airspeed. 
Parasitic drag is the consequence of pushing the aircraft through the air. 
It takes the form 
EQU D.sub.p =K.sub.p (V).sup.2. 
Where K.sub.p is a constant. 
Total drag consists of the sum of the induced and the parasitic drag 
components as 
EQU D.sub.t =D.sub.i +D.sub.p =K.sub.i (L/V).sup.2 +K.sub.p V.sup.2. 
Via conservation of energy sink rate can be deduced from the drag 
resistance times the speed as 
EQU SW=VD.sub.1. 
Where 
S is the sink rate and 
W is glider mass. 
Thus 
EQU S=V/W[K.sub.i (L/V).sup.2 +K.sub.p V.sup.2 ]. (1) 
The foregoing is the polar equation for the curve 1 in FIG. 2. Note that 
sink increases generally with speed, but it increases as a function of the 
square of the load factor term. The present invention is directed 
primarily toward minimizing or eliminating the effects of load factor (L) 
variations on the variometer display. 
The curve 1 of FIG. 2 is called the "polar" of the glider and depicts 
glider performance versus speed. Each glider has a polar unique to that 
glider. The exemplary polar depicted in FIG. 2 represents the polar of an 
actual modern high performance glider. The shape of the polar predicts the 
quality of the flight performance of the glider such as the minimum sink 
rate 2 and speed 3 and best glide rate 4 and speed 5. The shape of the 
polar is determined by such factors as surface smoothness, streamline, 
wing platform, airfoil etc. 
The shape of the polar of a given glider can be estimated with usable 
accuracy via either of the following two sets of readily measurable flight 
properties of that glider: 
Minimum Sink rate and speed (2 and 3 resp. of FIG. 2) or 
Best glide rate sink rate and speed (4 and 5 resp. of FIG. 2). 
The mathematics of the polar curve are such that minimum sink occurs 
approximately at that airspeed where the value of induced drag is three 
times that of the parasitic drag, (D.sub.i =3D.sub.p). Similarly, best 
glide ratio occurs approximately at the airspeed which results in equal 
values of induced and parasitic drag, (D.sub.i =D.sub.p). Using this 
information and knowledge of the form and factors of the polar equation 
(1), the following equations are derivable: 
EQU S=S.sub.Min /4[3L.sup.2 /(V/V.sub.Min)+(V/V.sub.Min).sup.3 ] and (2) 
EQU S=S.sub.L/D max /2[L.sup.2 /(V/V.sub.L/D max)+(V/V.sub.L/D max).sup.3 (3) 
where 
S is the glider's calculated sink rate at airspeed V, 
S.sub.Min is the glider's measured minimum sink rate; 
V.sub.Min is the glider's measured airspeed at S.sub.min ; 
S.sub.L/D max is the glider's measured sink rate at best glide rate; 
V.sub.L/D max is the glider's measured airspeed at S.sub.L/Dmax ; 
L is the load factor on the wings; and 
V is the glider's airspeed. 
Equations (2) and (3), above, are used in this invention to mathematically 
"model" glider performance. Other useful models equivalent to (2) and (3) 
are known and are derivable. 
Attention is now called to FIG. 1 which illustrates, in block diagram form, 
a system in accordance with the present invention, for devising an 
acceleration corrected total energy variometer. In FIG. 1 a conventional 
electric variometer 1 senses static barometric pressure and from it 
develops the sink or climb rate, S, of the glider. S along with airspeed, 
V, are supplied as inputs to a conventional total energy calculator 3. The 
output, S.sub.TE, of the total energy calculator is a quantity 
representing the glider climb/descent rate information conventionally 
displayed, via meter, to the pilot in modern gliders. The balance of the 
equipment illustrated in FIG. 1 is directed to enhancing the 
interpretability of the total energy information by diminishing or 
eliminating the effect of the load factor(L) variations. 
As illustrated in FIG. 1, the total energy signal S.sub.TE is supplied to a 
differencing device 9 along with a signal selected by an optional switch 
8. The output of the differencing device is supplied to an analog or 
digital display 10 for pilot monitoring. The switch 8 is not essential to 
the invention but does provide selectable alternate information displays 
which in practice could prove useful to the pilot. As illustrated in FIG. 
1, three alternate displays are possible depending on which signal is 
selected for input to the differencing device 9. 
(1) In TE position the selectable input is grounded so the pilot's display 
10 communicates simple conventional total energy variometer information. 
(2) In the ACTEV (acceleration corrected total energy variometer) position, 
the selectable input is S.sub.D, component of sink due exclusively to 
glider drag. Thus the pilot's display 10 communicates S.sub.TE -S.sub.D 
which in steady state is glider climb/descent rate due solely to vertical 
air mass movement with all aspects of sink due to glider drag having been 
stripped away. 
(3) In the ACTEV.sub.SMIN position, the output of the differencing device 9 
is S.sub.TE -S.sub.D +S.sub.MIN where S.sub.MIN is that sink rate due 
exclusively to glider drag which would occur if the glider were flying at 
it's minimum sink speed, V.sub.MIN. The pilot's display 10 then 
communicates the climb or descent rate which would occur for the glider 
flying in that air mass at minimum sink speed. This information might be 
useful to the pilot flying at high speed cruise between regions of lift. 
Referring still to FIG. 1, the term S.sub.D which corresponds to glider 
sink due exclusively to glider drag, is developed by the DRAG SINK 
CALCULATOR 6. Variable inputs to the DRAG SINK CALCULATOR 6 are supplied 
by the LOAD FACTOR SENSOR 4 and the AIRSPEED SENSOR 2. The LOAD FACTOR 
SENSOR 4 is typically an accelerometer device arranged to sense and report 
accelerations in the direction orthogonal to the plane of the wings of the 
glider. The AIRSPEED SENSOR 2 is typically an aircraft pitot type device 
similar to those in common use on aircraft of all sorts. Constant inputs 
to the DRAG SINK CALCULATOR 6 are supplied by the adjustable GLIDER 
AMETERS 5. These are in the nature of screwdriver adjustments. They 
would be adjusted prior to flight and, typically, when the variometer is 
installed in the aircraft. They would be adjusted to correspond to one of 
the two sets of information: S.sub.MIN and V.sub.MIN or S.sub.L/DMAX and 
V.sub.L/DMAX which, as described previously, can characterize the polar of 
the glider. Operation of the DRAG SINK CALCULATOR is illustrated in 
greater detail in FIGS. 5A and 5B. 
The S.sub.D -S.sub.MIN term of FIG. 1 is developed by the difference device 
7 from the inputs S.sub.D and S.sub.MIN both of which are in turn 
developed by the DRAG SINK CALCULATOR 6. 
The DRAG SINK CALCULATOR 6 of FIG. 1 functions to compute Glider sink, 
S.sub.D, using one or the other of the polar equations: (2) or (3). FIGS. 
5A and 5B illustrate alternative implementations of the DRAG SINK 
CALCULATOR 6. 
In FIG. 5a equation (2), is implemented using the minimum sink glider 
parameters S.sub.MIN and V.sub.MIN. Sink components due to induced and 
parasitic drag for 1.0 G are computed by the DRAG FACTORS CALCULATOR 3 
using the minimum sink parameters 1 and airspeed 2. Sink due to induced 
drag is calculated as 
EQU S.sub.DI @1G =3S.sub.MIN /4(V/V.sub.min). 
while sink due to parasitic drag is calculated as 
##EQU1## 
Load factor 4 is squared in the ACCELERATION CALCULATOR 5 and multiplied by 
S.sub.Di @1G to develop 
EQU S.sub.Di =3S.sub.MIN L.sup.2 /4(V/V.sub.MIN). 
The induced and the parasitic drag terms S.sub.Dp and S.sub.Di are then 
summed at 6 to produce the glider's sink rate due to drag, 
##EQU2## 
The DRAG SINK CALCULATOR of FIG. 5B is similar to that of FIG. 5A except 
that the Maximum L/D parameters, V.sub.L/Dmax and S.sub.L/Dmax are used. 
As a consequence the equations solved differ somewhat. The result, 
however, is equivalent. The output is again glider's sink rate due to 
drag, but in this case 
EQU S.sub.D =S.sub.L/Dmax /2[L.sup.2 /(V/V.sub.L/Dmax)+(V/V.sub.L/Dmax).sup.3 
]. 
An alternate method of developing glider sink rate due to drag is 
illustrated in FIG. 6. In that method, glider polars are previously stored 
in a non volatile memory 4. The appropriate sink rate is read out as 
needed using load factor and airspeed combined as a memory cell address. 
Operation of the system of FIG. 6 is as follows. The POLAR FILE CELL 
SELECTOR 3 develops a cell address using airspeed 1 and load factor 2. The 
DRAG POLAR FILE memory 4 is loaded by placing the LOAD/READ switch 5 in 
the LOAD position. Then as airspeed 1 and load factor 2 are incremented 
through their ranges, an S.sub.D value appropriate for the increment and 
for that glider is supplied at the S LOADING INPUT 7. The loading process 
would typically take place when the variometer instrument is first 
installed in the glider. For normal operation of the instrument as a 
variometer the LOAD/READ switch 5 is placed in the read position. In 
flight, airspeed 1 and load factor 2 are supplied by sensors to the POLAR 
FILE CELL SELECTOR 3 which in turn develops a cell address with which to 
access the DRAG POLAR FILE 4. The DRAG POLAR FILE, in turn, retrieves the 
appropriate drag sink rate S.sub.d from the selected cell. 
It is noted that this invention can be implemented in a variety of 
equivalent technologies including digital electronic, analog electronic, 
mechanical, electromechanical and pneumatic as well as in combinations 
thereof. Similarly, numerous alternative sequences of calculation and 
arrangements of components can be used to implement this invention, all 
intended to be encompassed within the scope of the appended claims. 
Although the description herein has been directed to the subcategory of 
aircraft commonly referred to as "glider", it is recognized that the 
invention more generally applies to any winged aircraft in non-powered 
flight, or in powered flight if supplied energy is taken into 
consideration.