Hobby rocketry flights are meant to be flown near-vertical, with maybe a small angle away from
the flight line for safety. Theoretically, your rocket is pointed up and keeps going up. If you're
doing an airstart, you want the motor to fire as close to vertical as possible, to prevent the rocket
from going horizontally under power. If you've ever had to chase an errant two-stager, you
know what we mean.
Barometric altitude computes the vertical distance from the ground, whereas the measurement
derived by integrating the accelerometer is actually the distance that the rocket has traveled,
independent of path. If the rocket is going "up" and straight, the two should be fairly close, if
your rocket has tilted over or for some other reason isn't going straight then they will deviate
significantly from each other. It's natural to have them deviate a bit from each other since they're
from two different sources and derived differently, however if they deviate by more than 10% or
15% that probably means that the rocket isn't pointed "up".
The Deviation percentage allows you to specify how far the barometric and accelerometer
derived distances can differ, if you have this turned on and they deviate more than this
percentage then the channel won't fire. Although not as accurate as a full 9-DoF IMU, this is a
pretty decent indication if you're not pointed "up". If you want to use it, we recommend that you
start with about 15%, you can get the actual deviation percentages from the Flight Detail graph
to refine it later on, by comparing the FAlt (filtered barometer altitude) with the FAccelAlt
(filtered accelerometer altitude) parameters.
Note that the barometer data tends to be fairly "noisy" due to a number of factors. A fairly
aggressive noise filter is used prior to comparing to the accelerometer data, but there are some
flight scenarios that tend not to work very will with this approach. For example, a very high
thrust and short burn motor (i.e. CTI's VMax series) can boost the rocket faster than the pressure
in your AV bay can equalize with the normally optimal vent holes, causing the barometric
altitude change to lag begin the accelerometer-computed distance. If your vent holes are too
large (for example, you have multiple access holes for a Schurter screw switch drilled into the
switch band), the baro data may be so noisy that it's almost useless for this purpose. If you do
have large screw switches, we recommend that you cover up the holes with tape before flying,
and drill the appropriate sized vent holes (don't use the screw switch holes as vent holes).
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