How Personal Air Vehicles Work

By: Kevin Bonsor & Katherine Neer
Springtail EFV-4A during a free hover in October 2003. See more jet pictures.
Photo courtesy

When the automobile first rolled onto dirt roads in the 19th century, it allowed us to go places we'd never been and let us get there in a matter of hours instead of days. Unlike the train, which confined people to stops and schedules, the car gave us an amazing sense of freedom. Today, our cars still give us great freedom -- but in many cities, that freedom is tempered by the annoyance of heavy traffic. There's nothing quite as frustrating as being late for work or an important event because of gridlock.

Jet Pictures


If you enjoy unique takes on aviation, then you should check out the paragliding article, video and images at Discovery’s Fearless Planet to learn more.

Wouldn't it be great if you could get around without worrying about traffic? With a personal air vehicle (PAV), it might very well be possible. In this article, HowStuffWorks looks at the technology behind a current PAV, the Springtail EFV-4B.


Springtail Basics

Springtail EFV-4A
Photo courtesy

­The Springtail Exoskeleton Flying Vehicle developed by Trek Aerospace wo­rks something like the Harrier jet, lifting off vertically. But instead of jet propulsion, the Springtail uses ducted propellers to lift the aviator off the ground. The EFV-4B is the most recent model of a series of several prototypes.

Once strapped into this personal air vehicle (PAV), the engine will turn the overhead duct fans to provide adequate thrust to propel you into the air. The Springtail EFV-4B is 8.3 feet (2.5 meters) high, and operators should be between 5 feet 4 inches and 6 feet 6 inches (163 to 198 cm) tall and weigh between 115 and 275 pounds (52 to 125 kg) for maximum maneuverability and safety.


Once airborne, you can zip over treetops at a top speed of 113 mph (182 kph) for 184 miles (296 km) on a 12.3-gallon (46.6-liter) tank of gas before refueling. Average cruising speeds are around 94 mph (151 kph). Because it can climb as high as 11,400 feet (3,475 meters), there's the possibility that the Springtail would share its air space with other small aircraft. However, according to the folks at Trek Aerospace, it's ultimately meant to fly at an altitude of around 400 feet above ground level, moving around 90 mph. The compact size of the Springtail will allow it to land on an area roughly the size of two compact-car-sized parking spaces.

Now, let's take a look inside the Springtail.


Inside the Springtail

Springtail EFV-4A during a free hover in October 2003
Photo courtesy

Four major components of the Springtail might someday enable it to fly us to work or the local movie theater:

  • 118-horsepower rotary engine
  • Drivetrain that drives the ducted fan blades
  • Counter-rotating ducted fans (two)
  • Fly-by-wire control system

At the heart of this patented vehicle is a small 118-horsepower rotary engine that uses 100-octane aviation gasoline (AVGAS). If necessary, it will run on 91-octane and there's a Springtail engine that actually uses diesel. A rotary engine is an internal combustion engine, like the engine in your car, but it works in a completely different way than the conventional piston engine.


Like a piston engine, the rotary engine uses the pressure created when a combination of air and fuel is burned. In a piston engine, that pressure is contained in the cylinders and forces pistons to move back and forth. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational motion that can be used to power a vehicle.

The pressure of combustion is contained in a chamber formed by part of the housing and sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons. The rotor follows a path that looks like something you'd create with a Spirograph. This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as the gases expand, and then expel the exhaust.

Springtail EFV specifications sheet
Photo courtesy

The Springtail's engine is connected to a system of drive shafts, universal joints and gear boxes, which drive the counter-rotating ducted fans. Earlier models sported fixed-pitch fan blades (as opposed to variable-pitch), meaning they are rigidly fixed to the central rotating fan hub at a set angle. Fixed-pitch blades helped to reduce the number of moving parts at work on the earlier models. However, the downside of fixed-pitch blades is that they cannot be adjusted during flight. Being the latest model, the Springtail EFV-4B now features several technological upgrades and improvements -- including variable-pitch blades. Another major improvement is the Springtail's new control system.

Let's take a closer look at that new system.


Fly-by-wire System

Springtail EFV-4A flight test held on March 16, 2005
Photo courtesy

­A pilot could operate the original XFV prototype using two hand-control grips and ­control arms, and by shifting his or her weight from side to side. However, after applying that operations concept in the wind tunnel at the NASA AMES Research Center back in 2000, the testing team discovered that kinematic (body) movement was not going to allow the pilot sufficient control of the craft.

Currently, the Springtail uses fly-by-wire controls. In theory, this system is somewhat like the drive-by-wire systems designed for concept cars such as GM's Hy-wire. The operator controls the vehicle using two joysticks, one for each hand. The left joystick controls the rpm of the ducted fans (the altitude control). The right joystick controls forward and backward vehicle speed, left and right turns (roll), and turning the vehicle on its vertical axis (yaw). This is also known as a three-degrees-of-freedom control device.


As the operator uses the joysticks, his or her commands are fed to an onboard computer system. The computer system interprets this information and moves the ducts, control vanes and other control surfaces so that the vehicle moves to accommodate the operator's commands accordingly. The impressive onboard computer also gives the Springtail a sort of autopilot function. The operator can enter GPS coordinates for the vehicle to follow so that the Springtail's onboard computer system will "drive" him or her to the programmed destination.

In addition to regular flight movement, like a helicopter, the Springtail can hover in a stationary position. Hover-time really depends on wind conditions and altitude, but the average time is approximately two hours.

In the case of any catastrophic failure, the aircraft would automatically deploy a parachute to safely bring the craft and pilot down. In the event of initial main parachute malfunction, there's a back-up parachute for the pilot. It's designed to deploy automatically after the pilot has unbuckled himself from the Springtail and pushed away from the machine.

So, we've discussed the craft and its core technology, but does it work? Who's going to use it? And, how much does it cost? Let's find out.


Who Will Use the Springtail?

This Springtail EFV-4B could be used by the military, homeland security, police officers, emergency response teams and even commercial businesses.
Photo courtesy

­As mentioned earlier, the core technology behind the Springtail has been subject to­ numerous wind-tunnel tests at the NASA AMES Research Center. The initial prototype, the SoloTrek Exo-Skeletor Flying Vehicle (XFV) completed more than 70 manned missions and made its first successful manned test flight on December 18, 2001. During initial flight-testing of the XFV, the machine was tethered to a crane, in hopes of avoiding costly damages. In November of 2003, the Springtail completed its first untethered mission - it was airborne for approximately 50 seconds and moved approximately 60 feet. Trek Aerospace reports that almost 200 tethered test flights and approximately 20 untethered tests have been completed so far.

While the company has not said when they will market the vehicle, it reports that there are applications for military and private use. Initially, it's likely to be used by paratroopers, who would use the Springtail to fly into battle. Other possible applications of the Springtail, as presented on the Trek Aerospace Web site, include but are not limited to:


  • Military applications such as search and rescue missions, surveillance and reconnaissance
  • Homeland security applications such as emergency response and rescue, routine police routes, firefighting and other security and safety measures
  • Commercial and industrial applications such as commuter travel, tourist travel and delivery services

Although Trek Aerospace hasn't said when the Springtail might be available to the public, the price is projected to be comparable to that of a high-performance sports car.

For more information regarding the Springtail, personal air vehicles and related topics, check out the links on the following page.