By Eric Ahlstrom

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Installation

Fitting the system requires that there be no hard obstructions above the path the pilot and seat will take when the handle is pulled. Breakable Plexiglas of ~6mm thickness should ideally make up the entire overhead canopy. The .24" thickness is equivalent to the compound curve stretched 3/8" material used by most highly modified Unlimiteds. The thicker bullet resistant canopies of the stock warbirds would need to go to thinner material or use an external means of canopy breaking.

Shoulder and foot clearance is a major issue for some Unlimiteds, especially the ones modified with low profile canopies. The solution here is to hinge the side areas next to the canopy to fold out as the side of the Spitfire cockpit did. Allowing this to happen in an automated fashion with the seat on its way up is challenging, but not impossible. The aircraft that would have the hardest time fitting a clear path for an extraction system are also the ones that the pilot can’t get out of easily in a manual bail out.

The seat manufacturer has stated that assistance in these integration areas would be available. While the seat manufacturer doing the integration work would be costly, design assistance and advice is much cheaper and there is no lack of creative engineering among race plane builders.. In reviewing the restrictions of the three dominant warbird designs, no show stoppers were seen by the seat engineers.

Operation

PES operation in aerobatic (and potentially air racing) aircraft is simple. When the handle is pulled, a gun fired catapult blows the seat up on an extension pole to clear the airframe. The canopy is broken by the seat and the remains stay in place. This saves the time delay originally allotted to canopy jettison and clearance. The parachute system fires off the top of the seat frame on a rocket after canopy break and during the seat’s upward travel. The chute opens as it reaches maximum extension at about the same time as the seat reaches the top of the rail. This is possible because the parachute pack is accelerated to much higher G loads than the pilot could stand.

The parachute rocket provides the means to open the parachute quickly without losing altitude or using heavy and expensive seat rockets to raise the entire system to altitude. In this manner, the ejection envelope actually extends to lower altitudes and airspeeds than most military fighter ejection seats. The inflated parachute pulls the pilot off the seat frame, which insures that the frame will not injure the pilot.

Time from handle pull to clearing the airframe is less than a quarter of a second. Chute opening starts within another few tenths of a second since the chute has been flying to extension from the moment the seat breached the canopy. This is fast enough to save a pilot from under 100 ft. with some amount of sink rate. The most important factor to increase the chances of a safe ejection is altitude. The second is sink rate. The parachute can only decelerate the pilot once it opens and any sink rate represents a loss of altitude between the time you pull the handle and the time the chute deploys.

Safety of the system on the ground will require discipline and vigilance equivalent to that used around military aircraft. However, the PES has no rockets and is lighter than a typical military seat. If accidentally actuated, it will not fly to several hundred feet and come crashing down. The possibility of a ground actuation injuring personnel is still present, and safing the systems against over exuberant race fans is an issue.

The Solution?

A PES is not a cure-all, guaranteed, you-can-get-out-of-anything-alive system. Like any other piece of flight equipment, it has a performance envelope. Within that envelope it is a system that can save lives that would otherwise be lost. Near or beyond its limits, a PES may not save the pilot. We have to look at crashes that have happened in the past and are likely to happen again where we would want a PES and see if the performance envelope lines up well with the crashes.

The first versions of this technology were used in the last generation of prop driven attack aircraft, the T-28 and the Skyraider. The latest versions are being applied to aerobatic aircraft and helicopters. In both cases the PES can separate the pilot from the airframe and open his chute under most, not all, bail out conditions. Crop dusters are another good candidate since many ag aircraft accidents involve clipping the airframe at the end of a run and having two or three seconds before the aircraft hits the ground. Even so the question remains: is the PES right for air racing?

Only pilots and owners can answer that. With Unlimited Gold contenders worth millions and quarter million dollar engines blowing every other year, the expense is not hard to justify. Although the maximum ejection speed of the SKS-94 is less than the top speed of Silver and Gold class racers, it is higher than the speed of most fatal accidents. With a system size small enough for aerobatic aircraft, a PES could fit into almost any air racer; with some effort, perhaps even Formula 1 and Biplanes. At 50 lb. including the seat, the weight impact is a non issue for Unlimiteds and Sport Class racers. For a cost of $30k to $40k, the pilot gets a US certificated system that requires no maintenance for ten years that could have saved the majority of race pilots lost in the sport.

The weight, cost, and performance of the PES may be just what air racing needs to take the next step in pilot safety.

Eric Ahlstrom

About the author:

Eric Ahlstrom is president of Star Aerospace LLC, an aerodynamics and aerospace systems consulting and manufacturing firm. He and several other ex Douglas and Boeing engineers are helping bring the peace dividend to air racing and general aviation. The principals have decades of experience in a wide variety of motorsports, including motorcycles, hydroplanes, drag racing, off road rally, and circle track. Star Aerospace LLC markets a line of high performance modifications for general aviation aircraft and is constructing its own line of kit aircraft. An Unlimited racing program is in progress and details will be published when the design phase is complete.

Author’s note:

"The desire for ever improving safety equipment has to be balanced by the practical limitations of weight and cost. If racers are required to spend so much on safety systems that no one can afford to race, then the rules will have saved the pilots and killed the sport. If the mandated systems weigh too much then they won’t fit the majority of today’s racers, neglect the smaller aircraft classes, and stifle future designs. I have had my share of uncontrolled excursions and have found that the term "safe racing" is almost an oxymoron. I have only recently been fortunate enough to round the pylons at speed, though I have years on the track in a number of different motorsports and understand that risk is part of what it takes to push the edge. Like air racing, I raced in sports where "crash structure" was limited or just was not possible. Like the helmets, clothing, and life jackets I wore, parachutes and now ejection seats are a technology whose time has come. Accident dramatizations have been included to help the reader understand what a pilot goes through during a potentially fatal emergency. Most of the accidents listed in these articles resulted in major or fatal injuries to the pilot. It is the opinion of the author that some, perhaps most of these injuries could have been prevented with the pilot extraction system technology described." -Eric Ahlstrom-

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