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The
Very Best In Aviation and Air Race News and Photography
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The "Intrepid" |
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Air Racing News
Archived Report
By Mark Kallio
Originally published 1998
>The "Intrepid"
intrepid
(in-trep’id) adj. Unshaken in the
presence of danger; dauntless. The given name of this
prototype aircraft also describes the efforts of one man:
Hal Dantone, and his vision to create an aircraft capable
of such speeds as to make all existing air racers
obsolete. A daunting task at best, Hal is going at it
with a passion similar to that which embodies the spirit
of Reno Championship Air Racing. Fans and
contestants alike have a love affair with the aircraft
and with speed. This entrancing mix provides the
driving force spurring the ongoing quest for more; faster
is definitely better!. We are poised at a point in
history when proven technology and innovative design can
come together to make a significant increase in speed for
piston-engine, propeller-driven aircraft a reality. Hal
Dantone’s The Aeronautics Company is
currently working on its entry into this arena. An
Unlimited Class racing aircraft, the Intrepid is
designed to shatter the world speed record of 529 MPH as
well as win dramatically at pylon racing.
| General
Design Specifications & Performance |
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Intrepid
Unlimited Air Racer
Speed: Max. = 590 mph.
Stall = 96 mph +/-.
Climb rate = 7,760 fpm @ sea level.
Weight = 7,100 lb. gross.
Wing loading = 33.1 psf.
Fuel flow = 300 gph at full power;
(35 gph of radiator cooling water.)
50 gph at cruise power.
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Just
how is Intrepid to accomplish this performance? By
making major reduction of drag and significant increase
in thrust as outlined below...........
Drag
reduction
a) Skin friction drag (by the use of composite materials)
b) Induced drag (with less weight)
c) Profile drag (by an almost 50% reduction in frontal
area)
d) Wave, Parasitic and Compressibility drag (by using
transonic aerodynamics.)
Increased
thrust
a) A counter-rotating propeller (to achieve the benefit
of an "area ruled" fuselage)
b) A "steam afterburner" ( Patent Pending)
that develops significant thrust from the radiator
exhaust.
c) In addition, directional thrust will be used to speed Intrepid
around the pylons quicker so that she spends more time in
faster straight-and-level flight and less time turning.
Those are the basic
concepts and design philosophy of the aircraft. But to
truly understand the project one must have specifics
relating to the design requirements; the problems and
solutions associated with them, availability of aircraft
systems and the reasoning behind utilizing those chosen
for the project. We present these individually as
described below.
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The
basic concept of Intrepid must start with a proven
race engine. Both radials and V-12's have long dominated
at Reno, realistically there are only a limited number of
aircraft powerplant choices available to select from
within the power ranges necessary to take you to the
winner's circle. This choice has generally been governed
by the type of aircraft used - in this case design
requirements dictate that choice. Hal states,
"Large
air-cooled radial engines
are not desirable for our purposes because their profile
drag (created by the width of the engine) negates much of
the power they produce."
A good example of this is
the Rare Bear, whose engine/wide fuselage requires
the use of a huge, three-bladed propeller (more than 12
feet in diameter) to attain 475+MPH speeds. This large
oversized prop must turn at a relatively slow RPM to
avoid having the tips go supersonic and lose efficiency.
She actually cannot use all of her available power as the
prop tips would go supersonic and thus the aircraft would
go slower as a result. Although this combination has
produced winning results, it is a limiting factor, which
Hal does not wish to be governed by.
The choices that
remain are that of the liquid-cooled engine category,
which are the Allison, the Rolls Royce Merlin and the
Rolls Royce Griffon engines. The Allison V-12, 1710 cubic
inch engine, is heavier and less powerful than it's
contemporaries, thus the Allison was easily ruled out.
The Griffon engine
is very similar to the Merlin engine, only larger. It has
2,400 cubic inches compared to the Merlin’s 1,640
with proportionately more power, weight and
cross-sectional area. As Hal states,
"Being
heavier and wider, its extra power is not worthwhile for
our application in light of propeller efficiency
limitations at the design speeds. Using the Griffon
engine would require the use of a fuselage as wide as a
stock Mustang and a prop to suit. This negates the
reduction in profile drag we are accomplishing on
Intrepid. The basic issue here is one of aerodynamics
versus brute strength and the answer is
aerodynamics."
This leaves the
illustrious Rolls Royce Merlin–9 engine,
the one chosen for this project. A proven performer, it
is a 1,640 cubic inch V-12 engine with a 2-stage
supercharger,  capable of 3,500 HP. Engine
builder Rick Shanholtzer has lent his expertise to the
Intrepid project, and has stated that he is willing
to build an engine with even more power if this becomes
necessary or desirable. One thing remains an undisputed
fact: using a Merlin-9 engine Strega has captured
the gold event for six of the last seven Reno races . As
Hal puts it,
"This reinforces
the selection of this engine for Intrepid, as it has more
than sufficient power to move this aircraft at the speeds
we desire to fly. The Merlin is small enough to design
around and achieve an almost 50% reduction in frontal
area and profile drag when compared to a stock P-51. The
Merlin engine is the clear choice."
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Hal's
next step was to design an aircraft around the powerplant
with significantly less drag of all types, an
aircraft designed to race! While Intrepid
will have reductions in every form of drag, only the
major ones will be discussed here.
Skin
friction drag is self-defining: both the amount
of skin area and the smoothness of the skin’s
surface cause it. Intrepid will have less
skin area than a P-51 and so will have less skin surface
to create friction. More importantly, she will be
constructed of composite materials which are inherently
smoother (and more naturally conducive to a laminar flow)
to reduce this type of drag.
Induced
drag is the drag caused by inducing or creating
lift: a situation common when rounding the pylons and is
proportional to weight. As a comparison, Intrepid's
target design weight is 7100 lbs. vs. Strega
who's reported empty weight is 7800 lbs. At this
point in the preliminary design phase, an accurate weight
of Intrepid cannot be given - but as Hal states,
"Intrepid
should and will weight less than 7800 lbs. - somewhere
between 400 to 900 lbs."
An accurate weight will be
forthcoming after structural and composite figures are
finalized.
Profile
drag is very important. While induced drag can
be described as relating to weight, profile drag relates
to size, specifically to wetted area of the
aircraft’s skin and to the size of the aircraft in
front view, the frontal area. In general, reducing
frontal area almost directly reduces profile drag. This
is a common method of drag reduction and is seen
frequently among the top finishers at Reno (smaller
canopies, shortened wingspans, etc.). Hal states,
"Intrepid’s
frontal area is only 54% - almost half - of that of a
Mustang. Stock Mustangs are 3’-8" wide and
7’-7" from the top of the canopy to the bottom
of the scoop: Intrepid will be 3’ wide and 5’
tall. Intrepid’s wing will be shorter and thinner,
with a newer airfoil that is more efficient at both
straight-and-level flight and while turning and pulling
G’s.".
There are many
considerations as to reductions of frontal area,
particularly in wing design. A thin wing is ideal for
racing, however the wing's internal area is generally
used to house the fuel, ADI (alcohol) and water (for the
radiator spray bars). Hal addresses this concern by
relocating the cockpit aft - and utilizing the area above
the wing for these stores. Also, the aircraft will
utilize what is referred to as "wet wings" to
supplement capacity. It should be noted that "Intrepid's
fuselage is actually longer than that of a stock
Mustang, so there is sufficient room for fuel, ADI, H2O
and the nitrous bottle in the belly of the beast".
The major reduction
of profile drag on the Intrepid will come from
the fuselage design. The elimination of the radiator
scoop plays a large role in this, and is discussed
further in the "steam afterburner" section of
this article.
Wave,
compressibility or parasitic drag is also of
major importance. This is the drag associated with the
speed of sound: it is important to Intrepid
because it starts over parts of the aircraft before or
below that speed. At subsonic speeds, air is
incompressible and molecules of air start to move out of
the way of the aircraft before it gets to them. At the
speed of sound, the aircraft is moving too fast for the
molecules of air to get out of its way before it gets to
them (the flow has become compressible) and they snap out
of its way in what is known as a sonic-boom producing
shock wave. The wave or compressibility drag associated
with this phenomenon rises dramatically at speeds from
Mach 0.7 to 1.0. To go faster requires either
substantially more power - or transonic aerodynamics such
as wing sweep and an area-ruled fuselage. Hal adds,
"That is why transonic aerodynamics
will be used to delay the onset of
compressibility drag for Intrepid, but in a manner to
ensure that they do not cause a handicap at slower
speeds. The wing will have some sweepback (probably 15
to 20) and an appropriate airfoil, and the fuselage
will have modified area-ruling."
Area-ruling, in
summary, is a means of handling fuselage-wing
interference as air speeds approach and go beyond
supersonic so that compressibility drag is reduced.
Area-ruling was intended for aircraft with short (low
aspect ratio) wings, rather than long (high aspect ratio)
wings such as Intrepid will have. Additionally,
area-ruling can have detrimental effects if the change in
fuselage area is abrupt and causes flow separation.
Collaborating with several other aeronautical engineers,
Mr. Dantone has area-ruled the fuselage using only a half
to one-third of the wing length and area, rather than all
of it. Hal explains
"This is an
engineering compromise to get the benefits of area-ruling
while creating no disadvantages at lower air speeds
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Intrepid’s fuselage will taper behind the engine and
above the wing, and the canopy is located at the point
where the fuselage should enlarge again aft of the wing.
The airfoil, wing and fuselage design will raise
Intrepid’s Critical Mach Number and Force Divergent
Mach Number (the speed at which she will hit the huge
increase in drag associated with compressibility) Result:
she will go faster before she hits the compressibility
drag increase that is limiting the ex-military aircraft
racing in the Unlimited Class."
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Radiator &
Steam Afterburner |
The
radiator scoop for the later model P-51’s was well
designed. While the scoop caused some profile drag, the
heated air exiting the exhaust expanded and created
enough thrust to compensate for the drag anytime the air
was heated above 170 F. The stock P-51 radiator is
large, being 14’ deep and 21" wide by 28"
high, and it was sized to cool the engine at 3800 RPM and
55 lbs. of manifold air pressure (MAP). Racers go to
4,000 RPM and 120+lbs. of MAP, so additional cooling is
required. Rather than add an even larger radiator, racers
added cooling water spray bars in front of the radiator
to accomplish the same thing. Spraying cooling water onto
the P-51 radiator is necessary but reduces the thrust
created in the exhaust because it also cools the air,
decreasing hot air expansion.
The question for
Hal was how to turn this disadvantage into an advantage.
By looking at the picture below you will see Mr.
Dantone’s design which answers that question. It
both
 reduces profile drag and in
theory will produce a considerable amount of thrust.
In summary, it takes the heat and momentum of the engine
exhaust and applies it behind the radiator to turn the
cooling water into steam, creating a large amount of
thrust in the augmenter tube in an effect similar to that
of a jet afterburner: a steam afterburner . The
term "Steam Afterburner" may be an oxymoron,
but it creates the right image. Cooling water enters the
augmenter tube and is flashed to steam by the engine
exhaust causing it to expand and create thrust much like
fuel which is pumped into the afterburner of a jet engine
where the heat of the exhaust burns it causing expansion
and thrust. As Hal describes it,
"The idea is to take the typical radiator design,
streamline its flows aerodynamically, and combine it with
an augmenter tube similar to the one used on the Convair
440."
The radiator is in the bottom of the fuselage directly
behind an opening with only a small adjustable scoop that
can be opened into the airstream. Inside the scoop are
two spray bars to spray cooling water onto the front of
the radiator. Behind the radiator, an augmenter tube
flows straight back to the tail. Just behind the
radiator, the hot, high velocity engine exhaust is
emptied into the augmenter tube causing a vacuum by
venturi effect: the higher the engine RPM, the stronger
the vacuum. That vacuum pulls air into and through the
radiator without the need for a scoop. Without the P-51
style scoop, there is less profile drag. Hal goes on to
explain that
"while the
necessity for a scoop is eliminated, there is the
probability that some amount of scoop will be beneficial
by creating a type of subsonic ramjet effect. An
adjustable scoop of about 8"opening 30 will be
used so that
a) maximum thrust through ramjet effect can be obtained
while racing, and
b) adequate cooling can be obtained at cruise power
settings"
.
The hot, high
velocity engine exhaust (at 1500 - 1700 F) mixes with
the air coming through the radiator, heating it and
causing it to expand thus creating thrust in a manner
similar to the P-51 exhaust, but with much stronger
thrust due to the hotter temperature. Note that this will
produce more thrust than the P-51 radiator even before
the introduction of cooling water. Of greatest import,
the engine exhaust also mixes with the cooling spray
water, turning it into steam and creating what could be a
"significant" amount of thrust. This extra
thrust is essentially free since the engine exhaust is
otherwise discharged to the airstream and the water must
be used to cool the radiator. Mr. Dantone also explained
that
"sizing the
exhaust, augmenter tube and exhaust nozzle to obtain the
maximum thrust is very important. It could be done
mathematically, but modeling is more accurate and
dependable. A professor in the Aerospace Engineering
Department at Texas A&M University has assigned this
problem as a project to two graduate students to build a
model and find the optimum sizes: their assistance is
invaluable and greatly appreciated. When they complete
this study, the predicted thrust from this steam
afterburner will be accurately calculated and
published".
Jet engines with
afterburners optimize thrust with exhaust nozzles that
constrict the area of the exhaust thus increasing the
velocity of the flow and the thrust created. The
augmenter tube will be approximately rectangular in shape
so that the aft portions of the side walls can be made
into adjustable flaps to perform this nozzle function.
Note that the end of Intrepid’s fuselage
(ref. to the 3D rendering) is an opening rather than
coming to the traditional closed point. This is the
exhaust opening, and the flapper valve nozzles are on
each side.
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This
is an idea developed and tested by NASA and the U.S. Air
Force to make its fighters more maneuverable. Unlimited
Class pylon racers spend 70 to 75% of their racing time
turning around pylons.
"With the steam
afterburner utilized on Intrepid, it became obvious
that directional thrust was a potential addition".
By adding
"deflectors" inside the exhaust stream that
angle up when the elevator is angled up, the thrust would
go back and up rather than straight back - thus pushing
the tail down, the nose up and the aircraft around a turn
faster.
"While Intrepid is
not designed to be a fighter, this extra maneuverability
will help it fly around the pylons quicker and return to
faster straight-and-level flight sooner. With more of her
time spent flying straight-and-level rather than turning,
Intrepid will fly even faster average lap speeds".
(For reasons of safety,
this directional thrust will be connected in such a way
that it does not work when pushing the nose down, just
when pulling it up.)
Propeller
efficiency is critical to the transfer of engine power
into speed. As a worst case, Intrepid will use the
same propeller used by Strega and the other P-51
racers, which are four-bladed, constant-speed propellers
of 10.5 to 11 feet in diameter. The design consideration
is having enough blade length and area to transfer power
into thrust without having such a long blade that the
tips go supersonic causing a dramatic fall-off in
propeller efficiency.
"A great advantage
of Intrepid’s reduced frontal area is that it makes
it feasible to trim the stock propeller to as little as 6
feet in diameter so that Intrepid will go faster before
the tips go supersonic".
This avoids the main
problem of Rare Bear with her brute power and
huge propeller.
Hal will work to
obtain an even faster propeller by using the new
propfan/unducted fan technology developed in conjunction
with NASA in the late 1980’s. The manufacturers that
worked with NASA are GE, Hamilton Standard and
Messier/Dowty Rotol. These propfans are expected to be 6-
or 8-bladed and of 6 to 7 feet in diameter. The propeller
hub would come from a commercially developed turboprop
hub with prop blades that will likely have special
airfoil sections and a shape for the tips to postpone, or
work efficiently at, supersonic velocities. Additionally,
NASA has awarded a grant to a small firm to investigate
supersonic tips on helicopter rotor blades: this may lead
to technology that may help Intrepid go faster.
"Propeller
research has not yet begun by The Aeronautics Company,
and one question to be answered is whether a
counter-rotating prop or a geared counter-rotating
propfan will be necessary to take advantage of the area
ruled fuselage. With a "stock" propeller,
Intrepid would break the world record, but we are not
going to settle for a stock prop and are all but certain
that a counter-rotating prop (similar to the one used on
Miss Ashley II) will be utilized during the initial CFD
(Computational Fluid
Dynamics)
analysis. The propfan
technology will be incorporated later after these tests
are concluded and the area-ruled design matrix is
finalized".
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Wing
design is very important to Intrepid’s
success and it is still undergoing analysis. The serious
challengers in the Unlimited Class have modified their
wings by shortening them, creating smoother finishes,
etc. The wing will have 15 to 20 of sweep and be
about 28’– 6" in width with a 5 foot
average chord. The airfoil will be laminar flow and
possibly supercritical. For initial test and development,
the airfoil will be a NACA 64A-410 with a 10% thickness,
a 25% flap that can be reflexed to -5, and 20 of
sweepback. The wing will have no twist and very little
dihedral. Intrepid will utilize a low wing so that
the landing gear can be installed with a ten-foot width
for stability, while providing ground clearance for a
ten-foot propeller in case one is used. As previously
mentioned, the wings will be of composite construction to
reduce skin friction drag and enhance laminar flow.
This
will make maintenance easier to perform and allow changes
in the future to keep Intrepid competitive. Intrepid
will have a steel tube frame from the firewall back to
the cockpit (and possibly to the tail) that will include
a pilot protecting cage and roll-bar. This tubular frame
will be covered with a composite skin. The wing and
horizontal tail will be affixed to the steel frame thus
allowing changes in location or angle of incidence to be
easily made.
"While designing
this aircraft to be right from the start, the history of
racing shows that being right means being able to change
- and alter - the aircraft as you learn and
progress".
Speed of maintenance can sometimes make the difference
in winning a race after a problem in a qualification
heat. A benefit of the modular type of construction
utilized for Intrepid is that maintenance can be
rapidly and easily performed, as well as alterations
easily made. With the steel tube frame and removable
composite skin, maintenance can be performed on the
flightline wherever the plane may be.
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Since
Intrepid is a racer, its cockpit will be
minimalist but sufficient to fly cross-country flights
and will stress pilot ergonomics. The pilot needs to be
free to concentrate on racing. A mutually beneficial
arrangement with an avionics company (such as Archangel
Avionics, Inc. which manufactures a general aviation
glass cockpit) will be investigated later in the design
and construction process. The cockpit is positioned so
that it enhances the area ruling of the fuselage and
allows the fluids which change weight during flight to be
positioned close to the center of gravity.
"Intrepid
has a two-seat cockpit for several reasons. From a
marketing standpoint, this will allow reporters and
sponsors to experience this aircraft firsthand and to
write about riding in a world record setting aircraft
that was 'made in the USA' with American ingenuity and
know-how. For cross-country flights, one seat may be
lowered or removed and the space used to carry an extra
fuel tank. This is a nuisance, but hardly a problem:
light single-engine aircraft have often used this method
to cross oceans successfully".
It is a far better option
than designing Intrepid larger and wider (with
more drag) to have enough room to carry more fuel in a
permanent fuselage tank.
The
F8F Bearcat, Rare Bear holds the world speed
record of 529 mph, but is already  maximized aerodynamically.
Her wide fuselage creates profile drag and wave drag,
plus it necessitates the use of a wide (or long)
propeller. Strega has won the Reno Air Races six
of the last seven years. Her shortened wing and
aerodynamic mods are a part of her success story. She has
also redesigned her radiator exhaust by placing side
doors on the exhaust exit, thus preventing vortex flows
at that point. The real competition, however, may be one
of two previously unmentioned aircraft, Miss Ashley
II and Shock Wave. Miss Ashley II
finished installation of a new engine just three months
before the Reno Air Races in September 1997 and did NOT
go "all-out" to win. Of even more significance, she has a
wing that is the outer wing portion of a Learjet: this is
a laminar flow airfoil with 13 of sweep. The wing is
the same approximate length as that of the Mustang, but
with a much wider chord. The presence of this aircraft
makes it obvious that designing Intrepid with
transonic aerodynamics was an excellent - and necessary -
idea. Shock Wave is a racer under construction.
The aerodynamics are not known, but it is known that she
is applying brute strength to the equation: she will be
powered by a 4,360 cubic inch, air-cooled radial engine.
Using a wide radial engine requires using a large (wide)
prop to move air around it. However, this width (or prop
blade length) can make the tips go supersonic, limiting
both the usable power and the obtainable speed.
"Considering
that Rare Bear cannot use all of the power of her 3,350
cubic inch engine due to propeller inefficiencies and the
drag of the huge engine, adding additional power to a
fuselage which will have to be so wide does not seem like
the best answer".
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In
the debate of aerodynamics versus brute strength, using
aerodynamics to reduce drag  seems
more beneficial and promising for all the reasons
discussed above. Intrepid not only maximizes
aerodynamics to reduce all forms of drag, but she will
also use several innovative ideas (propfan, the steam
afterburner and directional thrust) to increase her
thrust - and speed. Although it cannot be confirmed until
after CFD analysis, Intrepid has the potential to
be the fastest piston engine plane ever built. If she
comes close to fulfilling her design parameters, Intrepid
may so completely dominate this class of racing that Strega,
Rare Bear, Miss Ashley II and Shock Wave
may just have to compete among each other to see who
comes in second place.
The book is far
from closed on this chapter of the future of air racing,
there are many more pages to this story, and FlightLine
OnLine will be here to present them.....
all the time.... on line....
Story by: Mark Kallio
CGI
models built & rendered by Ian Hay 1998
For information on services available and
commission rates-
contact Ian at
render@elier.powernet.co.uk
The
AAFO staff wishes to personally thank:
Hal Dantone
aeronaut@flex.net
of The
Aeronautics Company
and
Ian Hay (3D Artist, pixel prodder and veteran video
vandal!)
for their contributions.
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