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The "Intrepid"
Air Racing News
Archived Report
By Mark Kallio
Originally published 1998


The "Intrepid" Future Unlimited Class Air Racer

>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
click here for larger image (640x480) 62 k

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.

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.

Engine

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 radialRare Bear's Huge Radial Engine 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, Rolls Royce Merlin enginecapable 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."

Aerodynamics

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 . 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."

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.

Directional Thrust

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

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".

Wings

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.

Modular Construction

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.

Cockpit

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 Competition

The F8F Bearcat, Rare Bear holds the world speed record of 529 mph, but is already Rare Bear - Reno 1997maximized 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 moreMiss Ashley II - Reno 1997 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".

Conclusion

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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