PRELIMINARY DESIGN OVERVIEW

The following is best enjoyed if the reader has first viewed * Project History and *video clip
The designs are under continual revision; as examples, the front bumper and elevator were revised.

Contents of this page are [click on * to go to]:

BACKGROUND

Over a span of many years, the StrongMobile design has evolved through an
iterative dialogue process with the intended users.

The design is for an aircar that will be used for frequent regional trips, that is,
about three day trips per week for about ten years. A standard trip is defined
as a one-day 400 mile radius combination of road and flight modes that consists of 12 phases:



The graphic shows cumulative minutes on the left abscissa.

The standard StrongMobile trip is defined as a one-day 400-mile radius combination of road and flight modes in phases:
1. [ 7:30] Pre-planning in road mode and inspection at home;
2. [ 8:00] Drive to home airport;
3. [ 8:30] Transformation from road to flight mode (pre-flight checks);
4. [ 8:35] Flight to destination airport and refuel (140 knots true/161 mph for about 420 nautical miles;
(With an average wind at cruising altitude of 6,000 feet of about 20 knots, the actual groundspeed for a round trip would be
about 155 mph, for a 30-knot wind, GS would be 148 mph, and so on.)
5. [11:40] Transformation to road mode;
6. [11:45] Drive to final destination;
7. [12:15] (Lunch) Conduct business or recreation for a couple of hours;
8. [14:15] Drive to destination airport and refuel;
9. [14:50] Transformation to flight mode;
10. [14:55] Flight to home airport;
11. [17:55] Transformation to road mode and refuel; and
12. [18:00] Drive to home by 18:30.

THE STRONGMOBILE IS IDEAL FOR TRIPS WITH MULTIPLE DESTINATIONS AND TRANSFORMATIONS,
SUCH AS A ROUND ROBIN WITH TWO OR THREE DESTINATIONS..

Many trips that cannot be done within schedule and budget constraints
with non-aircar technology may be made easily with StrongMobiles.

For longer flights, such as overnight, or 3-day trips, insert refueling stops between steps
4 and 5 and again between steps 9 and 10. The fuel tanks can hold 40 gallons, for an
endurance of five hours at a 8 gal/hr burn rate.

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THE MAIN PROBLEMS

The main problem to be solved in aircar design is seen to be a matter of integrating both flying
and driving functions into a single unit so that each function individually may be relatively
beneficial to the operators. This problem may be reduced to three areas:

  1. lightening and streamlining the definitive automobile so it can be practically flown; and
  2. providing both flight and road propulsion systems and means of transforming; and
  3. configuring the definitive airplane so it will fit into garages and on roads to fit into a box that is
    less than 8' wide, 7' tall, and 20' long by stowing the wings, eliminating the open operating propeller,
    and providing automotive suspension and steering and brakes and a road drive.

The first area is resolved by three major features.

  • Limitting the payload to two or four persons, since most trips involve only these, rather than burdening
    performance with provisions for more passengers. Some features such as power windows,
    power seats, and air conditioning may be omitted in the prototype
    and reserved for user accesories or options.
  • Power brakes and steering will be incorporated for safety;
  • streamlining with such features as belly and wheel covers; and
  • construction of lightweight aluminum alloy and composites.

The second area is resolved by using a ducted fan and rear-wheel road drive:

The third area is resolved in the Dragon by:

  • using a lifting body-fuselage to provide about one-third of the lift ;
  • folding the wingtips and swinging for stowing the wings in the body;
  • folding the tailplane for stowage within the width restrictions, and;
  • folding the aft fairing, which also acts as a bumper, to reduce length and
    provide for an automotive rear end with lamps and license plate display.

The aileron design uses spoilers, rather than the usual trailing-edge panels.
This is similar to the B-52 and B-1 and Vought "Kingfisher" spoiler ailerons.


This provides for coordinated turns in flight and frees up the trailing edge for flaps.
The spoilers are warped to avoid the reverse region of operation.


With the WASP ailerons, the wing design calls for full-span flaps to increase lift.
This provides for increased lift during take-off while the aircar is level.
(Read the "Engineering Calculations" page in the section about "Rotation".)

The wing slat is considered to be important because many accident investigation reports blame wing
stalls during landing approach as a common cause that can be easily reduced or eliminated.
The action of plain versus slatted wing is illustrated.
The left diagram describes *stalling action . The right diagram illustrates the effect of slats *for STOL

Most of the pre-flight may be usually done while in the operator's hangarage at the beginning
of a trip and most of the post-flight inspection likewise after trips, out of the weather, cold or hot,
or precipitating.

The additional weight of the road features is accepted as a trade for acceptable
drivability, durability, and life-cycle cost. As an example, aircraft brakes and tires were reported
by an aviation manufacturer to be inadequate for rigorous road use;
further, factors such as tire size and wear, steering, and ride were considered.
For automotive suspension, an additional 100 pounds were factored into the design.

You might be wondering what a purely airplane version of a Magic Dragon would be like. You can imagine removing the road drive from a StrongMobile, along with the transaxle. The removal would avoid the cost and weight to the tune of several thousand dollars and about 300 pounds. The bumper-tail would be unnecessary, so that could be simplified to save or avoid more cost and weight.
The wing could be simplified to eliminate the stowage and be made fixed; this would also eliminate the wing bay stowage door magic. Ditto the tailplane tip magic.
The fan clutch can be replaced with a simple direct drive.
Automotive things such as lamps and license plates and outside mirrors would be eliminated.
So, the end result would be a quieter, safer, and very nice airplane.

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DRAGON DESIGN


The main features of the Dragon are:

  • The automobile style body provides easy entering and egressing and comfortable seating.
    The body-fuselage will make a substantial contribution to lift in flight, but not while driving on the road.
  • The engine is to be muffled to automobile standards;
  • The ducted fan system also provides space for the front wheels so the overall system blends into the body.
  • The size of the wing is decreased via Warp Action Spoiler Plate, "WASP", ailerons
    that allow the use of full-span flaps. This is similar to Rex Beisel's design for the Navy/Vought OS2U
    "Kingfisher" of the late 1940's, the B-52, the B-1 and the current Angel airplanes.
    During road operation,
    (1) the wingtips are folded in under the main wings,
    (2) the main wings are swung back,
    (3) with the flaps overlapping, and stowed in the rear of the body. During flight, the
    wing stowage bays will be covered by folding cover panels. This stowage arrangement
    provides for low side-wind effects and a low center of mass for stability.
  • The front grill is made of a low-drag thin wire grill.
    The front bumper is swivel-mounted as of 11/05/09, as a change to the side-mounted nerfing bars.
  • The rear bumper and fairing is swung up to streamline the afterbody for flight.
  • The "pi-tail", named from its resemblance to the Greek letter pi when viewed from front
    or rear, extends the automobile styling trend for rear-end spoilers for improving braking.
  • The Dragon will use Commercial Off-The-Shelf automobile suspension, including wheels,
    brakes, and steering, instead of aviation technology, to withstand the rigors of driving; and
  • The Dragon design may use an off-the-shelf transaxle with engine adapter. A key design
    problem is the Power Take-Off, PTO, from the engine, for the mechanical road drive.
  • A center keel was designed in to carry fore-and-aft loads, with conventional side rails.
  • The design uses off-the-shelf coil springs and dampeners for the wheel suspension.
    Similarly, COTS steering and braking are to be used.

    The use of standard automobile steering for taxiing, take-off, and landing is expected to
    provide a much easier and safer operation and eliminate the annoyance of trying to
    learn how to steer with foot pedals. The inventor notes that the airplanes he flew for thousands of hours
    for the USAF, Convair twin-engine trainers, had separate nosewheel steering wheels.

Variants of the Dragon may include added accessories such as air conditioning, autopilot,
anti-icing systems, cruise control, semi-retractable wheels, and so on.
The designs are continually being reviewed and revised and refined. As an example,
the empennage might appear nicer if the folded tailplane were narrower.

A Shorter Take-Off and Landing, STOL, version is shown below for those who would prefer the STOL faster climb,
and higher altitude performance. Take-off roll may be further shortened with an afterburner in the duct, described
in * THRUST AUGMENTATION.

Furthermore, since half of the survey responses indicated a requirement for more capacity, a four-seat
"stretched" version was conceived; this would be practical with the mechanical road drive with a rear transaxle.
The two-place design may be "stretched" to increase seating capacity and, if needed, with canard surfaces added to
assist in rotation for take-off.

For the ultimate concept, a six-place limousine/cargo version.

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DUCT AND FAN


The propulsion system is considered to be the riskiest area of the project development. Although there has been some
research by NASA with contractors, there has been none with the split duct design. In particular, there has been
zero research on a split duct boosting wing lift design, so verification of the design is considered to be crucial.
An advantage of the duct is that the fuselage sees only free-stream velocities, except along the sides, reducing overall drag.
An easy read that explains ducted fan design may be seen at * Ducted Fan Design
The front end of the engine is restrained by a four-point stabilizing harness to maintain the fan position and provide for
a close fit in the fan ring. Whenever the engine must be removed, the grill, spinner, fan, and harness are removed.
The elliptical intake has an area of 167% that of a circular intake of the same diameter, the "capture area".
The plan of the duct walls is quasi-ogive. The flow is presumed to exit and follow the contour of the fuselage-body to the
rear by Coanda effect. The material is composite Fiberglas for the outer walls and aluminum alloy inner walls.
The inner walls are made so that they may be removed for engine servicing; the gills would be removed before
extracting the inner walls to the exits. Soundproofing may be applied to the walls if desired.

The fan has nine blades with a four- to six-inch chord and pitch of about 30 degrees at the tips to 60 degrees at the
roots, relative to the plane of rotation. The hub will be solid aluminum alloy and the flades are of 0.125 inch stock.
The mockup is shown at * Fan.
The engineering model vehicle may have adjustable, changeable flade mounting flanges. The duct has a nominal 36-inch to
38-inch diameter with constant cross-section except for reliefs at the forward bottom to accomodate the suspension springs
and power steering mechanism and the engine cowling. Production Dragons may use a design criteria based on bird strikes.
The intake area is 5 feet by 3 feet, for an area of (22/7)(15/4)=12 sq.ft.= 1,700 sq.in.
The cross-sectional area at the fan hub plane is 904 sq. in. for the 36-in. diameters,
then expanding to the exit planes to 1,017 sq.in., with a velocity of about 150 mph for the static case.
Assuming a horizontally-opposed engine similar to the Subaru or Porsche, the area decreases at the engine cowling to 466 to 668 sq.in.,
increasing the local velocity somewhat. IF a rotary engine is selected, then this does not apply.
The fan may be adapted or custom-made COTS by manufacturrers such as * AeroComposites.com.


The NASA PAVE people say that they favor an controllable pitch fan for their Tailfan aircraft;
such a design may be cost-effective for StrongMobiles.
The result is a car that shows only a minimum of airplane in the road mode and an efficient airplane design in the flight mode.

The ducted fan provides a significant reduction in noise levels, as shown in the example below that is shown
by NASA PAVE office in support of their Tail-Fan project.

The enclosed fan is expected to reduce the noise level from about 107 pndB to about 70 pndB. A conventional automotive
muffler will be used.

The fan is designed to be 36" in diameter; however, the elliptical maw intake with the broad sides is expected to act
as a bell mouth to increase the effective diameter significantly. The fan blades ("flades") are expected to see a
variation in their angles of attack as they traverse the ellipse, with the angles decreasing at the sides with increased
forward velocity components. This variation may induce a vibration load on the flades; however, the
state-of-the-art in composite blade technology indicates an unlimited fatigue life and ameliorates concerns.

Some responses indicated a preference for a fan clutch; a design concept is illustrated for satisfying this desiradatum.
With a clutch gear, the duct exhaust doors are unnecessary; however, without the fan airflow for cooling the engine,
a separate electric cooling fan system is needed and will be built into the prototype.
.
Some operators may wish to have a capability to disengage the fan in the event of engine failure to reduce drag.
Conversely, some operators may wish to shorten the take-off roll by engaging the road drive with the fan drive.

THRUST AUGMENTATION


Some operators may need a shorter take-off capability. Therefore, a design concept of an afterburner
in the duct may be made to provide thrust augmentation. The pilot may switch on the ignitors and
simultaneously open the hinged flame-holders and then switch on the fuel pump.
Note the two segments are designed to provide a cool ambient flow back to the wing.
The afterburners will likely be loud for this "Fire-Breathing Dragon".
Depending on the reasons for powerplant failure (fuel exhaustion or electromechanical), the augmenter
might be useful as a back-up propulsion system.

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THE MAGIC TRANSFORMATION PROCESS

The "Magic" process consists of several operations:

  1. Turn off engine;
  2. Open wing stowage bay doors;
  3. Unlock wings (electric solenoid)
  4. Spread wings (spring-loaded COTS electric actuator);
  5. Lock wings (electric solenoid);
  6. Unlock tips;
  7. flip wingtips up (electric motor);
  8. lock wingtips and lower take-off flaps (~15-20 degrees);
  9. close wing stowage bay doors;
  10. unlock tailplane tips;
  11. raise tailplane tips (electric motor);
  12. lock tailplane tips;
  13. unlock front bumper and rear bumper-fairing;
  14. flip up front bumper andbumper-fairing;
  15. lock front bumper andbumper-fairing; and
  16. engage fan clutch gear (engine switched OFF).

The design calls for several other conversion concepts:
The tailplane joints have cover panels hinged to the main tailplane.
The outside rear view mirrors may be detachable to reduce drag.
(The desired solution will be to make them retractable automatically.)

After wing spread and lock, the tip and tail and bumper-fairing unlocking, activating, and locking may be done concurrently.


The "Magic" in the design is the use of simple automatic conversion systems. The bumper-tail is shown above with
the motor and cables. The wingtips, tailtips, and wing stowage bay doors have a similar design scheme.

The "Magic" mode conversion system will be electrically powered and actuated. The wiring for the locks will
be ordnance wiring, i.e., twisted and shielded. The system will have a main power switch, so that operation may be
enabled and disabled by the operator in the event of malfunction.


The wing is attached with a simple hinge to the main body beam and front wing spar.
The aileron control rod and flap control shaft automatically engage when the wing is spread.


The automatic wing transformation may be accomplished with a piston and cylinder or preferred motor drive screw shaft
and nut or a rack and pinion arrangement. The actuators may be COTS.

The wing root fairings will protect the control mechanisms in the road mode and pivot
on inboard hinges to the flight position on the leading edges. They are hinged at the
inboard front edge to the body and have slots in their rear surfaces for retention pins on the
wing leading edges to slide in for automatic non-powered conversion.
Tailplane protective fairings may also be provided.


The sketch of the aircar wing section shows how the leading edge can be opened up for maintenance access, similar to the
Beechcraft Bonanza design, with continuous ("piano") hinges top and bottom.
The stabilizer and elevator center panels and outer panels are hinged together to provide for locking when stowed.
The wheel wells will have internal housings to protect mechanisms from slush.
The Pitot static sensors may be mounted under the main wing halfway outboard, a la Piper Warrior.


The Warp Action Spoiler Plate, "WASP" aileron dynamics are shown above.

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WHEEL RETRACTION AND EXTENSION

Calculations for economic analysis are discussed in *calcs

There are three options for the wheels: (1) fixed; (2) quasi-retractable; and (3) retractable.

A quasi-retractable wheel version may be done by simply raising the wheels 6" with
motor and cable technology. The suspension spring rate of 200 ppi provides 6" of travel
between no-load and double static load. Therefore, with a retracted load of 1200 lb per wheel,
they can be raised 6". This would reduce flat plate area by one square foot, for a drag
reduction of 7% and cruise speed increase of about 12 knots. A simple technique is to
limit the downward travel to about 0.7 g loading, or about 2" or 490 lbs.; this rise of 4"
from full downward deflectionmay provide an increase of 8 KIAS and is the standard
design for the TACRA survey.


Full retraction of the wheels with covers is considered to be desirable for some users.

There is a slightly more complex problem for the rear wheels,
due to a need for intruding into the wing stowage bay. The intrusion may be facilitated via
doors in the bottom of the wing bay so that the wheels may be raised about 4" more and so
be fully retracted, as shown below in the cross-section.

The wheel wells may be covered. The covers are conceived as slipper doors that slide
back and forth over the wheel wells on tracks, powered by motors with pulleys and cables.

The extension will have a manual back-up system in case of electrical system failure or
malfunctioning of the motors. The manual back-up system will consist of a pilot-operated
lever connected to cables that are connected to the slippers and wheel uplocks.

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PRIMARY CONTROLS

The primary controls consist of: