BRS-6 General Installation Guide - CAFE Foundation

Since performing even a small series of flight tests with each aircraft is usually
cost prohibitive, our prescribed aircraft attachment strength minimums is
normally calculated from rated or ultimate loads obtained from these deadweight
drop tests. Due to drag differences of a small, dead weight (usually
pallets with barrels or lead-filled steel slugs) vs. a disabled aircraft with a
significant surface area, the ultimate loads from drop tests will most likely be
conservative. This may lead to fairly conservative strength requirements that
some home-builders or manufacturers may have trouble meeting. Additional
engineering work may have to be done to distribute these loads onto more
points on the aircraft or optimize harness lengths.
Now, after reading the above, if you are thinking that you should use the
parachute size (or manufacturer) that offers the lowest ultimate (or opening)
load, please consider this:
The magnitude of an ultimate load is relative to what you are trying to
accomplish. You can put a larger parachute onto your aircraft and, due to a
longer time required to fill and inflate the canopy, opening loads will be lower.
However, a longer opening time equates to increased altitude loss before
complete inflation…not something you want if you are only 100-200 ft. in the
air. A lower descent rate and opening load will do you no good if you hit the
ground before it inflates.
In automobiles, the faster you want to stop, the harder you need to hit the
brake pedal. In parachutes, the quickest opening possible, given the
deployment conditions, comes from the smallest, hardest-opening canopy.
However, now your aircraft will have to be built to withstand the higher loads
and the higher descent rates and this is not possible in some cases.
Therefore, it is a tradeoff, and BRS designs canopies to optimize a combination
of opening time, load, and descent rates, not just one aspect. Aircraft builders
need to choose the parachute that will most likely survive the deployment and
lower them down at a safe rate of descent. They must then design their
attachments as necessary.
2. Aircraft Structure Analysis
Aircraft are designed to fly. They are designed to take their greatest loads
vertically through the aircraft structure, as these are the lift loads. The
requirement to resist excessive drag loads is much less. Unfortunately, that is
exactly what an inflating parachute puts onto an airframe: significant drag and
force in a direction most likely never designed for.
During the initial deployment sequence, the force vector coming from the
inflating parachute (directly inline with the parachute riser) is always with the
relative wind. The initial position of the parachute riser could then very easily
be nearly parallel (