The present invention relates to parachutes and more particularly low altitude parachutes for use in both high velocity deployments as well as low velocity deployments.
The recovery of personnel resulting from low altitude ejections from military aircraft has become an increasingly difficult task. For many military applications the operational velocities are becoming greater causing the recoverable payloads such as ejection seats and ejectable capsules to become heavier due to the addition of protective equipment. The heavier payloads typically require larger parachutes so that they may be successfully recovered. The larger parachutes, however, take longer to inflate which may result in premature ground impact if deployed at low speeds and low attitudes.
Parachute opening speed is proportional to its deployment speed. This generates two major concerns which the recovery system must address. The two major concerns occur at the velocity extremes of the operational envelope. The first concern, as discussed above, is premature ground impact at low speeds due primarily to slow inflation. The second concern is the excessive opening forces that are present at high speeds.
At low speeds, the airflow surrounding the parachute is often too slow to fully inflate a large parachute in a relatively short time. This is due primarily to the inherently poor opening characteristic of standard parachutes which wastes or does not effectively utilize the available inflation air. There is often very limited time in which to inflate the parachute before ground impact. Thus in the low speed scenario, a major design goal is to speed up the inflation process of the parachute by capturing much of the available inflation air.
The opposite is true in the high speed regime, where the airflow velocity is sufficient to quickly inflate the parachute. The concern, however, is that the drag force caused by the parachute is proportional to the square of the deployment velocity multiplied by the effective drag area of the inflated parachute. Thus at high speeds when the parachute inflates very quickly, the drag forces can become excessive. If not controlled, these drag forces can exceed human tolerance limits or otherwise cause failures in the parachute, which is often equally fatal to the crew members. Therefore in the high speed regime, the design goal is to slow down or control the inflation process of the parachute in order to avoid excessive drag forces. Alternatively, one can attempt to control the drag forces by controlling the effective drag area so as not to exceed tolerable limits.
The related technology for speeding up the inflation of parachutes include a variety of techniques. The conventional methods for speeding up the inflation of a parachute include; reducing the permeability of the fabric or the porosity of the canopy or both. Also, one can close the vent of the parachute which prevents the trapped air from escaping. Yet another conventional method for speeding up the inflation of a parachute is to sew inflation pockets on the canopy around the outside of the skirt. These inflation pockets aid the spreading of the canopy by generating greater eddies of air around the parachute and contributing a small spreading force. These conventional methods, however, do not increase the opening speed of the parachute enough to warrant their use on low altitude military ejection applications.
One method currently used on some ejection seat parachutes to speed up inflation at low speed is a pull down vent line. This technique is also commonly known as pulling down the apex of the parachute. In this method a line attached to the vent area of the parachute is pulled down prior to deployment. When the parachute is deployed at low speed, the pulled-down vent forces the air entering the parachute to open the skirt quickly, allowing for a faster inflation. When the parachute is deployed at high speed, the pull-down vent line simply breaks allowing for normal inflation. The increase in opening speed of the parachute at low velocities is accomplished because the opening shape of the parachute is altered. However, by altering the shape of the parachute, the distribution of forces on the canopy are also altered. Altering the opening loads presents additional problems to those involved in parachute design.
Another way of speeding up inflation sufficient for use on ejection seat parachutes is with the use of a spreading gun. This gun explosively throws outward metal slugs that are attached around the perimeter of the parachute skirt. This spreading force acts to open the skirt very quickly. At high velocities this fast opening of the full canopy is undesirable because it can cause excessively high opening forces.
A third method used in this field for speeding up inflation is to use a cluster of smaller parachutes instead of one large parachute. While it is true that smaller parachutes will inflate faster, the increase in weight and complexity of such a system is viewed as a severe drawback.
On the other hand, the related technology for delaying the inflation of a parachute so as to avoid excessive opening forces is staged deployment systems. One such staged deployment system is a reefed parachute. This system employs a reefing line that prevents the parachute skirt from fully opening until a timed pyrotechnic line cutter severs the reefing line allowing the skirt to fully open. This type of staged deployment system distributes the opening forces in two or more smaller peak forces instead of one large peak force. Reefed parachutes and other staged deployment systems are not typically desireable during low velocity deployments. This is because any delay in inflation of the parachute can result in premature ground impact.
The present invention is a single parachute system which can recover a payload throughout virtually the entire performance envelope of military aircraft. That is to say, the regulated area parachute disclosed herein is capable of safely and successfully opening in both low speed situations as well as the high speed situations. The present invention is a significant advancement in recovery system technology which addresses the age old problem of minimizing the parachute inflation time while also minimizing the opening forces. This new parachute is especially effective in solving the problem of low altitude recovery of manned ejection seats and ejectable capsules.