The
Design
Of
The
Das D-8 Zanonia
submission
for the
The
Royal Aeronautical Society, UK
International
Light Aircraft Design Competition 2021/2022
By:
Prodyut Kumar Das, MTech, IIT Kharagpur
Mechanical Engineer (Retired)
Email: prodyut.das7@gmail.com
Prasenjit Das
Iss.7 This is an edited version and contains only the first five chapters as they are of general interest. Departments/Colleges .
Zanonia; A plant also known as Alsomitra
macrocarpa native to the East Indies and of the Gourd family whose seed has
wings that lets it fly considerable distances. The name is twice appropriate to
the proposed aircraft which is, like the Zanonia, tailless and also because the
“seed” is an idea of an electric powered utility aircraft.
Introduction
If we go strictly by
the specifications then the winning formula for the R.Ae.S 2022 competition
would be an aircraft like the Gossamer Condor flying at a modest speed, the
logic being that a lightweight structure would allow for the maximum payload
whilst a ten per cent reduction in speed will result in a thirty increase in
range the aircraft being judged as an equal weightage product of speed range
and payload. Before one can triumphantly write Quad erat demonstrandum to such an approach one will hear the voice
of Sir Sydney Camm imagined to be rasping with sarcasm of ancient wisdom,
“follow the specifications exactly and you are a dead duck”.
The reason for
circumspection to a “strictly by Maths” approach lies in the Rules itself. It
is looking for a practical, well thought out, serviceable aeroplane. The
Gossamer Condor would win by the formula but it would probably wreck itself on terra firma in the first storms of the
Monsoons. There is also a limit to slow speeds. Even in the equatorial regions
road transport is improving and aircraft cruising speeds are intimately
connected to average surface transport speeds, my estimate being three times
faster than the average surface transport speed is absolutely the minimum. Over
much of rural India an average intercity speed of fifty kmph can be expected
(including the necessary breaks for essential “Chai”) so a cruising speed of
150 kmph is about the minimum acceptable the rest being dependent on the “as
the crow flies” factor of the route flown. In hilly terrain this can be
significant.
Given the great deal of
“fuzzy” logic involved and the fact that the two members of the team are not
qualified Aeronautical Engineers it was decided that it is best to believe that
in light Aviation the actual amount “Engineering” is quite slender and would,
like a column of similar proportions, buckle easily unless buttressed by paying
attention and obeisance to old wives tales, superstitions, folk wisdom et.al.
In other words the aircraft was to be designed by “playing it by the ear” and
the process of evolution of the design would be to “Think, then look around and
think some again”. Some of the team’s native superstitions –for want of a
better word but “superstition” describes the perhaps lack of a strong
scientific basis- are listed below though not in any order of importance.
Beliefs
and Superstitions
Aerodynamically there
appears to be a “boundary layer” at the low speed of flight aviation which
makes the introduction of aerodynamic refinements less productive than one
would normally expect. Costs of “hi-tech” aerodynamics are guaranteed but the
contribution always somewhat disappointing. To exemplify comparisons showed
that the Fokker D VII is really not so inferior in aerodynamic performance as
compared to the typical latest American light planes. Conjecture how much
performance the latest all plastic streamlined retractable wonders would show
if compelled to use the BMW III for power! One expects their general
suitability & “efficiency” over time would be more “bio-degradable” when in
the field. So-
1. Not
all the latest is best. One remembers a time when any light plane without the
Whitcomb GAW 1 aerofoil was considered antediluvian- Piper for example put it
on the Tomahawk but it is less fashionable nowadays. It is high school physics
that a high lift aerofoil will have a high pitching moment but aviation,
particularly American Light Aviation, the world leader, has its proneness to
occasional mass hysteria.
2.
Composites are by now not “new” having
been around for about fifty years and the prospects of a 20% reduction in
structural weight are mouth-watering. However between I K Brumell’s “SS Great
Britain” – amongst the first of big iron ships- and the Titanic is also about
fifty years when people discovered that low temperature fracture could be
disastrous! The point is that no matter how widespread the use or how tested the
new material there are will be Engineering “Huns in the sun” (politically
incorrect phrase from a less inhibited era!) which bite us just as we think we
know everything there is to know. The view of the team was that incorporation
of composites whilst as unavoidable as taxes, must be given another twenty
years to be considered safe; for the present it is enough if the design should
be progressively convertible to
greater use of composites. Given the
3. Despite
the views at 2 above composites have been used as an unstressed shell to
enclose the aircraft’s space frame fuselage structure. Given that a rectangular
section is best to reduce the interference drag between a high wing and the
fuselage which must in any case have a large and rectangular cargo compartment
for versatility as a utility aircraft, the main reason for using a Glass cloth
and foam shell is to reduce the “boat tail” or separation drag at the rear. The
foam shell changes section from rectangular just aft of the TE to a circular
cone near the propeller. One is reminded that in the nineteen thirties and the
mania of streamlined locomotives of LNER’s A4 designs a rival railway (could it
be GWR?) rather cheekily “stream lined” their locomotives merely by putting
streamlined fairings at the rear of the various fittings at the top of the
locomotive.
4. The
real resistance to the aircraft is not aerodynamic or gravity but the Bankers
and Financiers. To these people there is very little to choose between one
design and the next, the rule being any design that requires large funds is
bad. An aircraft no matter how brilliant and streamlined will remain a bunch of
yellowing paper if it requires large amount of capital to start building. The
Financial inertia of the design MUST be low; the ability to be “scratch built”
is thus an important attribute to the design.
5.
The design of an electric aircraft
requires a “total” approach in that it is quite different from a liquid fuel
aircraft – much in the way a component being converted from metal to plastic
can actually be changed quite significantly.
The
first pipe dreams
With these above
beliefs in our thinking there was considerable searching of data –mainly on the
internet but also amongst the rusty air frames of flying club’s junk yards. It
was surmised that the basic problem of electric flight- the heavy aggregate
weight of the propulsion system is similar to that faced by the pioneers. The
ancient monoplane aircraft- the Bleriot, the Bristol Type 10 the Deperdussin
Monocoque and even the Demoiselle were looked at with a view that with
judicious injections of technology – cantilever or propped cantilever wings,
modest amounts of streamlining, less “bird like” aerofoils etc. could be
persuaded to come up to perform the job but it was soon realized that these
aeroplanes like the Gossamer Condor were quite fragile. Being Old Reactionaries
aeronautically it was with considerable regret this line of persuasion was
abandoned.
At this point we had a
look at the Avia B 9. It seemed a very good basis to begin with. The
performance (pl. see Appendix1) was within the required range and it had the
charm of a large-scale model. Looking rather like a grandsire of the Evans
Volksplane (IP issues, anyone!), it promised the Frog Junior Scale series
(Spitfire, Mustang etc) of boyhood’s simplicity of building and thus low
capital expenditure and even made us think of simple composite construction
using 1” foam and FRP “sheets” replacing the 1/16” thick sheets of balsa of the
models of boyhood if you get the idea. The obvious tidying up of the engine
cylinders being replaced by the electric motor, re looking at the undercarriage
and the boxy rear fuselage rounded off with dorsal and ventral turtle decking
would improve wetted area and drag by a fair amount. The idea was that as a
classic “two hole” er (1920s term-self-explanatory), the front “hole” would be
used for passenger when required or else a cover- carried in a separate
compartment in the fuselage- would be in use to bung the hole and the
compartment would be as a baggage compartment.
It promised a very neat and versatile solution to the present problem.
Again, with great reluctance we turned away from the solution and the reasons
for rejecting the Avia B9 as a basis are as follows.
(The top picture is taken from the Kovo factory’s model
kit. How do we acknowledge this?)
1. The
low wing is fabric covered and would not stand up to the kind of “handling” (or
perhaps “footling” in this case!). People’s feet going through the fabric or
denting the skin is expected to be routine. With a cantilever wing the aircraft
would turn into a Messerschmitt M 17/ M 23 or a Klemm 25. Their problem would
be – in addition to fragility and lack of any wheel track- in the case of the
Messerschmitt – they would be underpowered when taking off at ISA +20 and soft
soil airfields –the Delhi Flying Club used the Klemm in the 1930s and found it
sluggish in the heat. In the case of the Messerschmitt M 17 one would not know
where one was going - those being the days of wooden planes and iron men….!
2. Apart
from getting in the way, the use of struts- much recommended for reducing the
wing weight- is also a negative point.
Though the Cranfield University’s technology demonstrator was examined
with admiration for its astuteness we refrained from going that way w.r.t. to
struts. They may be nicked in service resulting in their failing and the wing
coming off. The Twin Pioneer crash in Iran ages ago took the life of the then
Hunting Aviation’s Chairman is remembered and one hears occasionally of such
wings coming off due to strut failure. If that happens in highly Industrialized
Countries it is best avoided in Equatorial Regions. Whilst struts are
acceptable for “large” aeroplanes like the Turbo Porter or the Beaver they
would doubly be a nuisance in a small aeroplane during loading/unloading and
prone to accidental damage of the kind that no one notices.
A
second set of rules
These preliminary
excursions on the very early flying types led to crystallizing the concepts
with some further set of rules or parameters thought to be desirable to the
point of being essential.
1. The
aircraft would have to be scalable by which it was meant that the technology
developed and the layout could be “expanded” to much larger aeroplane rather on
the lines of a Folland Midge to the Folland Gnat.
2. The
aircraft would have to be a cantilever high wing design for two practical reasons.
Firstly, such a layout would keep the wings out of harm’s way during ground
handling and the high wing gives an uninterrupted lift field.
3. The
design should have a “Lego brick” type of design so that major changes in
individual sub-assemblies can be relatively easily made without major
modifications of the other parts. For example, a change in wing chord should
not require a major redesign of the fuselage.
4. Given
the reports of the battery pack proclivity to catch fire it was felt desirable
that the battery pack should ideally be quickly jettison able, preferably even
whilst flying without affecting trim and should be quickly replaceable. The
Hunter gun pack or the IAF’s Canberra B.(I) 58 belly pack came to mind as the
way to go for the battery pack.
5. Compared
to the last century, today pilots are being weighbridge-ed at 100 kgs. The idea
of such people clambering over the sides to get in as was common then is
probably dangerous - for the airframe certainly if not for the pilot. It was
decided both the pilot and the utility compartment would have separate car type
entry doors.
Freezing
the concept studies
The
Blohm & Voss 141
With those emerging
ideas the problem was again discussed and the asymmetric layout of the Blohm
& Voss was studied and since an all-round field of fire was not an
essential requirement the tail boom was attached to the cockpit nacelle and the
engine and battery pack would be an easily accessible “pack”. Some sketches
were made including one with a semi elliptic wing made from glass fabric over
foam outboard panels and a metal centre section but perhaps mercifully the idea
was not proceeded with.
The
Electric Fox Moth
Mr. Hillman’s idea of
cheap aerial transport seemed attractive because the concept was expandable,
and met most of the team’s requirements and A.E Hagg did provide remarkable
aerial locomotion for four fare paying passengers on the power of a 120 hp.
Gypsy. Our idea was to have a Fox Moth type of fuselage but the 600 kg MTO
meant that the cabin would seat only two and instead of a biplane one could do
with strut braced monoplane gull winged a la Pulawski P11 “Jedenstanka” and
attached to the fuselage at the upper longerons at right angles to the fuselage
top rounded decking to reduce the interference drag. The wing would be based on
the Wittman Tailwind in concept - parallel chord with the L.E swept inwards o
reduce the interference further i.e., a relatively small area about 13 sq. mts.
to the Fox Moths 22 sq. mts. but with split flaps to make it work harder. The
side view of the proposal study is included and shows how nicely the struts and
the wires of the original biplane managed to hide the good lines of the Fox
Moth’s fuselage. Somewhat regrettably the aircraft’s performance did not happen
to be the best and had to be discarded. The other reason is for not pursuing
was that whilst the utility compartment was easy to access the idea of a 100 kg
pilot heaving himself over the side from the ground would mean earning that
person’s eternal curses about stupid designers not knowing about real people
who have to earn a living flying the aircraft.
Finally, Mr. Hillman and De Havilland’s Mr. A.E Hagg success did not
solely depend on the Fox Moth being a “cheap aeroplane” or the appropriateness
of its technology; success depended as much on Mr. Hillman’s own shrewd
assessment of what went into the cost of a ticket and how he could pare it down
the bone. Though the “Hillman formula” is now a century old it may still be
worth a respectful study.
Enter
the Flying Wing
It was at this stage of
our musings the Flying Wing swam into our ken. It is difficult to remember now
how the wing came in but the initial reaction – probably a Pavlovian- was
negative despite the fact that there are thousands of tailless hang gliders
flying every day. There were many impassioned espousals matched by equally
scornful dismissal between the designers. Further study showed that it was a
case of giving a dog a bad name. As JW Dunne and later GTR Hill had
demonstrated over a hundred years ago the Flying wing was inherently stable and
safe and the Hortens had demonstrated that it was quite dramatically efficient.
What gave the Flying Wing a bad name was that it was pushed into areas where it
was less suitable flight. Paradoxically the Horten Brother’s ruthless search
for efficiency led to high aspect ratio wings- aspect ratios Hurel- Dubois
would be careful to tread - and the suppression of the rudder for the purity of
the “Nurflugel” concept made the handling whilst somewhat unusual going by Eric
“Winkle” Brown’s reports though
it must be mentioned this was the report on the Horten IV with an AR of 21.3!. The
fact that despite its obvious efficiency the problems of handling disposable
loads e.g. bombs and the CG shift with fuel burn led to the general disinterest
in flying wings.
Another interesting
observation was that the flying wing generally excited its designer to get rid
of the fin so that all the weight, drag and wetted area of the structure
supporting the empennage could be avoided. This usually led to the fins being
mounted on the tips of the wing. Unfortunately, in terms of formulae for stress
and strain this was “a flag pole bending” case i.e., the maximum deflection at
the tip is about sixteen times the deflection of a simple supported beam of the
same length and load. That would have led to wooliness of the control and
generally lag and uncertainty of control response. It is something to wonder
how much all this played a part in the crash of the Armstrong Whitworth AW 52.
Remarkably, if we recall
the work of Lippisch, “not at all an
Engineer” (!) who quickly stumbled onto the fact of having a separate fin did
much to “tame” the flying wing. In fact his final “Delta IV c” and the DFS 39
designs turned out to be very well behaved and the ME 163 Komet – whatever it’s
other animosities (due to the use of corrosive fuels) towards the lodger in its
cockpit, it behaved impeccably in terms of its stability and handling. Even the
DH 108 accidents were more due to the risks of transonic flight with subsonic
aerodynamics and loads on structures not designed for transonic flights. In
terms of handling and agility at lower speeds the DH 108 was quite acceptable.
The
Flying Wing Reassessed
Looked at
dispassionately the Flying wing concept is ideally suited for electric aircraft
just as the so disdained biplane has remarkable advantages for certain niches
like sports and light aviation aircraft. The main drawback of the flying wing
lay in the fact that its “tail volume” was limited. How despite such a
limitation it was applied to the Chance Vought Cutlass fighter is something that is a bit of mystery to this team. The
fact that at high transonic speeds the aerodynamic centre shifts backwards
meant it could not be considered very favourably by designers for supersonic
aircraft. The other problem with liquid fuel propelled flying wing is that as
the fuel burns the CG moves backwards and that makes for stability problems.
The result was that
because of its unsuitability for military applications the flying wing was broadly
“tar brushed” as a genre despite its obvious efficiency and suitability for
certain subsonic applications. In an electric powered aircraft the CG shift due
to fuel burn does not occur and the rather small stability margin is of no
consequence. Indeed, as in hang gliders it is possible to incorporate in the
design to use the considerable mass of the battery pack to achieve longitudinal
and roll stability.
A self-assessment of the proposed design
Old Pilots/ Aviation
people being conservative to the point of being superstitious, the Tailless
configuration was initially the cause of much acrimonious debate between the
two team members. The point of debate being that there would be consumer
reaction in selling the product. It gradually dawned on the dissenting member
of the team that perhaps for electric flight the tailless configuration, like
the biplane, is a very snug little niche for the task at hand with much to gain
in performance and project capital requirements whilst returning a better than
conventional performance.
Given the increasing obesity of pilots and people
the separate car style entry doors are a good feature. The use of a space frame
allowed us to maximize internal volumes and the use of a foam stabilized skin
allows the introduction of a reasonable amount of streamlining.
There are a few innovations (almost auto suggested
by the layout and the propulsion system):
i)
The use of an external battery pack to
“quick change” at the station to reduce turnaround times.
ii)
The use of the weight (of the battery
pack) to help trim the aircraft - as Lilienthal or Pilcher did so long ago-and
thus avoid the drag and weight of trim tabs and trimming flaps as Lippisch used
on the ME 163.
iii)
The use of a load cell system with
appropriate software to ensure that the aircraft is correctly trimmed on the
ground or else the aircraft motor will not start. This could be useful in case
the pilot is “bold” rather than “old.”
Though
the team is reasonably pleased with the effort we realize that there are
problems which we have not discussed but which needs us to further worry about.
Some of them are:
a) The
use of a welded steel tube structure may cause problems in manufacture outside
of the US and India. This is because of the availability of qualified welders.
A semi monocoque structure airframe may also need to be looked at particularly
at the initial stages of the project when insufficient funding and lack of
sufficient field experience may not permit a direct transition to composites
manufacture. The initial comparisons between the Bjorn Andreasson designed
Junior aka Bolkow Junior and the Wittman Tailwind leads us to believe that the
change to a semi- monocoque will not seriously affect the weight estimates the
slightly higher empty weight of the Bolkow Junior being due to a higher
standard of equipment fit e.g. the flaps in the Bolkow being electrically
operated instead of the manual handle operated job in the Tailwind.. It seemed
to the team that the Bjorn Andreasson Junior was a brilliant “conformal
transformation” of a traditional steel tube and wood construction into a sheet
metal manufacture. This is not “copying” which is often used pejoratively but
actually “standing on the shoulders of giants”.
b) The
present design is very much a “technology demonstrator” and thus it needs to be
both inspecting able and modifiable. The method of construction particularly of
the fuselage will permit both.
c) The
electric motor allows great flexibility as its torque rpm characteristics are
excellent when compared to the reciprocating engines. The present motor
propeller combination is to be considered in a sense provisional. The standard 115 h.p. light plane
engine was taken as a ”firm base” and the torque of the engine at the operating
horse powers and r.p.ms was set as the initial requirements to meet. From that
it came out that only an electric motor having such and such torque would do
the job.
d) We
have chosen the EMRAX 268 series only because it had a fairly comprehensive
manual and the design confirmed that there was a thrust bearing to take the
axial load of the propeller and it has a peak torque of 500 N.m. against a
required peak torque of around 320 N.m.so that there is a considerable margin
should the prototype wants more urging to get off the ground; the “overweight”
of the more powerful engine- by ten kilos- being an acceptable penalty to the
aim of getting the Bankers to get to witness the first flight on the planned date. Imagine the shattering
of confidence if the first flight is postponed because the Chief Designer is
suspected to have added the date to the power required calculations! OF Course
such liberties cannot be taken with the I.C engines with their poorer Power to
weight ratios. Experimenting with various
propeller diameter /pitch and rpm combinations will need to be
done. The ideal in mind is to have the largest propeller turning at the slowest
sped that will give a prop wash just twice the flight velocity if one goes,
again by high school physics- of V=2v . Various other motors e.g. Siemens will
be examined during the detail design phase.
e)
Another area that requires further
is the location of a Cardan shaft of composite manufacture. Coupled with a
reduction gear box - adapted from a Rotax 914 UL gear box- it may help us
achieve a lighter propulsion system. A more certain benefit by moving the motor
and drive closer to the CG (as in the Sopwith Camel!) would be to improve the
“handiness” of the aircraft.
f) Due
to the limited time available to iterate through varying fuselages length and
evaluate the directional stability the fuselage length has been kept at 5
meters. It is felt that this can be pared down to somewhere between 4.8 to 4.5
meters depending on maintaining directional stability.
The Battery Pack
Having read about the problems of new
technology batteries faced by Boeing et al. we realize that in our navigation
chart the Battery design area is marked “Here be Dragons”. There are problems
to be expected in terms of ground handling of the heavy batteries, safety,
cooling and emergency procedures to be followed in case of a fire whilst
keeping things as simple as possible. It is with these considerations in mind
that the following design decisions were taken.
i)
The Battery pack was chosen to have a
power rating of 40kWh. By the rules that would weigh in at 200 kgs when
equipped and fitted. This was between a good balance between the range being
obtained the range also being influenced by the speed at which the the aircraft
flies, the amount of margin for the freight/payload and the cabin space
available for the same. Given all that the 40k.Wh pack seems a good compromise.
One incidental advantage of the external pack concept is that it would allow
the use of different pack capacities to suit the range payload leg to be flown. The battery pack is externally mounted even at
the cost of slightly higher drag. This would benefit both replacement and
cooling.
ii)
Though a “drop tank” shape was initially
envisioned this was quickly replaced by a flat pack. A circle packs the maximum
area in the least perimeter whereas battery cooling demands the maximum
perimeter per volume if possible. The drop tank had a deeper core making it
more difficult to cool the centre line cells. An estimated 3400 cells would need
to be packed.
iii)
The pack’s dimensions are 70 cms. width
i.e. the same width as the cabin, 14 cms deep to allow seven stacks of cells
and 86 cms. Long using the Panasonic NCR18650GA cells. Though the battery pack
could be made smaller using “nested” rather than max. diameter to max. diameter
packing the latter has been used to provide bigger cooling air packages and
the pack is slightly inclined to allow
for a slight “chimney” effect. The
front and the upper rear of this pack has fixed GRP skinned Foam fairings with
air inlet hols for the pack fore and aft to streamline the rectangular box.
Cautious checks showed that the R.Ae.Soc. Limitations of 0.300 kWh./kg. Bare
cell and 0.200 kWh/kg when packed can be met using the Panasonic cells. The
weight of a 40kW.h pack works out to be around 200 kg and this has been used
for our estimates. Going by the very dense packing and the relatively higher
drainage rate in the RR racer aircraft and the much lower drainage or discharge
rate of this aircraft it is possible the simpler arrangement proposed should
work.
iv)
If the above battery shows tendencies to
overheat i.e. around 85 degrees C the
use of a “Meredith Radiator” reputedly used on the Mustang and which actually
gave a small amount of thrust is be examined. The cells are arranged in what
can be described as a “double walled “Bandolier” to give the required voltage
and feeding into a common bus bar. Between each row of cells there would be a
passage of air about 8mm and every cell would have air flow on both sides. This
would need the pack length to go up to about 1100mm. CFD studies followed by
rig testing would be needed before implementing the bulkier solution.
Scalability
One of the requirements of the design is that it should be scalable
because electric flight technology is expected to improve in every way. The
present design is a Technology demonstrator.
Once all the niggling problems of a new technology are understood and
cleared the basic design can be used to create a larger can be easily at an
appropriate time to a four/six seat design of similar configuration. Beyond
that one can think of a “Manx Cat” version of the Miles Aerovan using two or
three engines of similar power.
Our design was highly praised for its innovativeness. The competition for 22/23 is for an all electric Aerobatic aircraft. We have started work on the design and it looks like a winner. Since the competition is now being oriented for Engineering Colleges and Universities any college/ University wanting to participate can if they so wish contact either of the two designers (contact address given on the cover) to discuss. The college/Institutions must have students who enjoy doing this kind of work and have some running experience of stress analysis, fluid dynamics and computer modelling software. Students interested and willing to "research" in electric propulsion will be welcome.
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