Ok, here is another problem Ive spotted in the mind field of hardware issues and for people who continually bang their heads trying to fix them through PID tuning. .
If you have a housing that is covering over all of your electronics such as a plastic container,.. then when you descend or ascend quickly the pressure inside the container changes and gives your barometer an false reading.
For example if you descend quickly the pressure increases inside the container making the barometer think that you are descending faster than you actually are. So the APM will try to compensate this out and try to increase the throttle and this probably cause a bouncing behavior some people are finding happens .
Some may argue that the barometer cant be this sensitive, but I beg to differ because the pressure variation it can sense for the pressure variances in altitude that it works off is very fine. The pressure change inside the container even for horizontal flight will be enough to for it to sense the changes of pressure in the container.
This also includes pressure zones around the incoming and out going air stream around the props if they are to close to the container or even if you have an open center frame the pressure zone around the props can possibly still affect the barometer
I'm considering some fixes for this such as a tube that has a H tube branch at its end and that sits above or below the center board.
I would be interested to know what others view and ideas are on this. .
Basically an airspeed indicator works by having ram air on one side and static air on the other side, if you have a natural variation in air pressure from whatever reason it generally affects both RAM and static air pretty much the same so they both cancel each other out, so since you are reading differential pressure it will tend to be quite accurate.
In a full size light plane which uses a baro for an altimeter (which you adjust to field altitude before take off), the size of the static air space inside the airplane cabin tends to dampen short term air pressure differentials, also a standard manual altimeter has considerable built in mechanical dampening.
That having been said in a light plane, 10 or 20 or even 30 feet is generally of no consequence and is considered well within acceptable parameters of error.
In fact, normal barometric air pressure changes and site differences often throw an altimeter way more off than that by the time the plane lands.
And although mid flight corrections are sometimes possible, they are rarely undertaken.
I didn't realize that SONAR was primarily used below 10 meters.
From looking at the specs on the SONAR sensors that are supported, it does seem that they are only specified to about 16 feet for reliable detection.
John, I'm full agree with your analisys.
This is a problem difficult to solve on a multirotor because we have quite no opportunity to put a vent port in a good position to handle flying conditions (or lateral wind gusts). The pressure reading in hovering and no gusting wind it's not a problem
Do you experience higher or lower pressure or the multirotor try to grow or drop in forward fly ?
BTW MK and DIJ perform a solid altitude hold in gust
As to MK and DIJ, any idea how?
My problem is not with side gusts but up and down drafts (and gusts) caused by terrain causing what I believe is genuine local air pressure changes which are interpreted by the barometer as altitude changes and which Alt Hold tries to compensate for generally inappropriately with undesired consequences.
At higher altitudes this is generally not much of a problem because you are out of the effect of terrain caused wind anomalies.
I'm not trying to be contrary, but I think you should maybe at least consider that you're overestimating this effect and what you're seeing is not due to the wind.
I don't believe there should be any real pressure problems from the prop wash like you describe. If you're shielded from actual buffeting from the air stream you should have no problem.
It would be easy enough to test this. Just try an experiment with fans. Very simple.