Locating the Center of Gravity

The next challenge is to establish the centre of gravity for the vehicle. It is sometimes necessary to find a way to shift the centre of gravity, when changing from flight mode to car mode, and vice versa.

Basic engineering for the automobile calls for a center of gravity (and the driver) to be located half way between the front and rear wheels. The purpose, of course, is that this is where the car provides the best ride.

The locating of the center of gravity in the airplane must be done for different reasons. It is usually slightly ahead of the center of lift. The reason for this is that in a power-off situation, the craft will gently nose down and keep flying. If it were behind the center of lift, the aircraft would nose up and eventually stall. It is usual to place the pilot at the center of gravity too, not to give him a better ride, but because the weight of the pilot is a variable. When located at the center of gravity, he or she may be light or heavy and still be accommodated with a minimum of trim adjustment. The center of gravity must also be slightly ahead of the main gear (on a tricycle landing gear) to keep the tail of the aircraft from falling on the ground when parked. As well, the center of gravity may not be too far ahead of the main gear. If it is, the aircraft will not rotate for takeoff. As you can see, there are some basic incompatibilities here that must be dealt with.


Dimensions

The dimensions of the vehicle must also be considered. The automobile has limitations on size depending, first of all, on an allowable road width of 8 feet. After that, size depends on whether it is a luxury automobile, a compact car, or something in between.

It is the span of the wings that normally takes up most of the width of an aircraft, but actually, it is possible to build an aircraft with no wings at all. Such an aircraft could be built using vectored thrust, ducted fans, and rotors. These are, of course, Vertical Take-Off and Landing aircraft.

If you insist on having a wing, it is possible to build one with a span of only eight feet to accomodate the width limitation imposed on road vehicles. However, performance may be affected in a variety of ways. The span can be increased by using folding or telescoping wings that may be extended, or by using attachable wings. There are advantages and disadvantages to all of these possibilities.


Control Systems

Control systems in autos and aircraft are markedly different.

Aircraft are controlled in three axes - roll, pitch, and yaw - and the most common control system to manage these requirements is a wheel on a yoke. The wheel is rotated to the right or left to control roll about the axis through the length of the aircraft. The pilot pulls or pushes on the yoke to control pitch, so that a push forward moves the nose down, a pull back pitches the nose up. Yaw, which is movement to the left or right, is controlled by the rudder pedals. In normal flight, however, an aircraft is turned by using inputs in all three axes. Usually it is gently rolled to the left or right, causing induced yaw. The rudder pedals are then used to co-ordinate the turn. In addition the pilot may pull back on the yoke to keep the nose up. In a sharp turn, the pilot may add a fouth element of control by adding power, to compensate for the loss of lift when an aircraft is rolled sharply.

The only directional change available in a car is yaw and this is achieved by turning the steering wheel.
Pitch may be induced in the car by directing it up or down a hill.

Roll may be induced by directing the car along the side of a ditch.
A dual purpose vehicle must have all of the capability required by a car and an airplane. It is desirable to achieve this without having the floor of the machine cluttered with controls. The thought of having a gas pedal, a brake pedal, two rudder pedals, and perhaps a parking brake pedal at the drivers feet is a very unpleasant prospect.

Vehicle Use

How the vehicle is likely to be used is also a major consideration in designing dual-purpose transportation. The average use of a privately owned, general aviation aircraft is somewhere around fifty hours per year. It is not uncommon for the same pilot to use his automobile fifty hours a month, and in some cases he will use it fifty hours a week. Accordingly, a truly practical flyable and drivable vehicle must be comfortable to fly, and also it must be as comfortable to drive as a normal factory-produced automobile.


The Market

A normal production run for an automobile is at least two hundred thousand units, and it can be as high as eight hundred thousand units. Development costs and tooling make such high production runs essential for economic viability.

The market for aircraft is considerably different. The latest year in which figures are available for factory built aircraft is 1998. In that year some 2,200 general aviation aircraft were produced. This figure does not include homebuilts which might add several thousand or so, worldwide.

The introduction of a truly viable roadable aircraft would modify the demand for general aviation aircraft, but the degree of improvement is impossible to predict. It is certain that the demand for cars would still greatly outpace the demand for aircraft. Clearly, for an auto manufacturer to be interested in producing a dual-purpose vehicle, the inventor must come up with some way to accommodate such an imbalance in demand.


Aesthetics

It has been said that Truth, Beauty and Contact Lenses are in the eye of the beholder! It is a given that everyone has their own idea of what is beautiful. It is probably fair to say that in the designing of aircraft and automobiles, the product will be easier to sell if it has attractive lines.


Convertibility

In any proposed design, it is expected that it will be necessary for the operator to make changes of some sort to convert from Flying Mode to Driving Mode and back again. It is essential that such changes be easy and convenient. It is particularly essential, if the design allows for attachable and removable components, that they are included and available for use at any time. A pilot must be able to fly part way to his destination, then be able to continue on by road if the weather has degraded. He can not do that if something he needs has been left behind at his home airport.


Human Engineering

Having dealt with the technical aspects of the development of a flyable / roadable vehicle, it is important to deal with the human side of using such a product.

One of the sound basic premises of design is called the KISS Principle. (Keep It Simple Stupid). However, if the simple technical answer creates difficulties for the operator, it will meet with out-of-hand rejection.

For instance, this could happen if a car driver is asked to steer his automobile with a joy stick. It is probably possible to do and would likely simplify the design of a dual-purpose vehicle. However, many people would be uncomfortable without the steering wheel in their hands. To them, this design would be totally unacceptable.

A similar problem could arise in the combining of the control systems of a car and a plane. It is conceivable that it would be possible to use the steering wheel as a method of yaw control while in flight. This would eliminate the rudder pedals and would seem to be a simple way to eliminate "control clutter" on the floor boards. Roll and Pitch control could be handled in another way, such as a joy stick, and technically, this would be easy to do. Engine power could be controlled by the gas pedal on the floor.

However, currently licensed pilots flying today's general aviation aircraft ordinarily use the wheel of an aircraft for roll control. They do not use a wheel for yaw control. Drastic changes in the established, familiar control systems presently in place could be dangerous for such a person. Remember, it is the established pilot who initially will be the prime market for any new design.

The human needs must be met in any new concept.


Unit Cost

There are those who would buy a "roadable" simply for the pleasure of owning such an unusual vehicle, and for them, the price will not be a major consideration. It is to be expected, however, that the size of this market will not be particularly large.

After that, the potential purchaser will consider his options. Would he be better off to simply buy a car, and buy an airplane, and put up with the expense and inconvenience of finding some kind of local transportation when he gets to his destination? Whether a prospective purchaser buys a "modular" or an "integrated" design, the family still has to travel to the airport to change into flight mode. Perhaps they might just as well transfer the baggage, and climb into a regular airplane.

This factor sets the parameters for the pricing of a new dual-purpose vehicle. It should be a figure comparable to the cost of a new car plus the cost of a new airplane. To be really competetive it should be significantly less. For any individual purchaser, the size of the proposed car, and the size of the proposed airplane introduces a host of variables into the mix. However, the bulk of the dual-purpose market will probably be for vehicles that carry a family of four people.

Pricing now becomes a real challenge. For the development of a new automobile alone, it is not uncommon for an automaker to spend millions of dollars. These costs must be amortized over the production run that follows. The development costs of the airplane must be added to that.

None the less, to be truly competitive, any dual-purpose machine must be saleable at a price less than the combined cost of a comparable aircraft and a comparable automobile.


Safety

It is impossible to stress too strongly the need for safety in the design of roadable-flyable vehicles. Serious accidents have already impeded progress in this field. The makers of the ConVairCar, a very promising aircraft, gave up in 1947 when it crashed. It crashed because the pilot ran out of gas! The was nothing demonstrably wrong with the aircraft. Any plane that runs out of gas will come down! However it gave the appearance of a safety issue, and the event was sufficient to terminate the effort. There have been other similar incidents. The designers of a successful roadable-flyable must aim for ultimate safety, and that includes pilot training. The world media is waiting for the next attempt to fail so they are able to say, "I told you so - those things are so unsafe."

The most successful roadable-flyable will be the one that is so gentle and forgiving that anybody's grandmother or grandfather who wants to fly it, and who can qualify for the license, can do it without hurting themselves or anybody else. To those who want to experiment in this field, please design safety into your contribution.