Reply to post: Re: Control electronics

LOHAN ideas..

imanidiot Silver badge
Boffin

Re: Control electronics

Hmmm, spent quite some time typing a reply this morning, but this seems to have not been posted and/or disappeared.

My point is that with a baro sensor you don't NEED the GPS altitude indication and can just ignore it within the control algorithms. The sensor itself is not really that big, in fact, there are SMT components that can do the trick. (For example: http://www.sparkfun.com/products/9603, although this one only goes down to 300 hPa, I've seen models that go to lower pressures/higher altitudes) I'm pretty sure there's UAV controller boards already out there that simply integrate this thing on the board.

The biggest advantage from a pressure sensor it seems is not even in accuracy, it's in stability of readings. A GPS can jump up and down 10 or 20 meters in altitude in a few dozen seconds or even less. Some uav controllers simply don't like that sort of behaviour. It certainly makes reading the data more difficult.

@LeeE, a set of 3 gyros and 3 accelerometers, each aligned with the major axes of rotation/translation can more simply referred to as an Inertial Measurement Unit. Your smartphone is already an IMU. An IMU does not have to be "relatively massive". Even a smartphone can be used for inertial navigation, there's quite a few bot-building enthusiasts who have proven that. (And most smartphone IMU's are not that great/accurate) Problem with Inertial Navigation is cumulative error. The larger your moment-by-moment error, the larger your navigational error is going to be over time as errors stack up. Thus needing the GPS to correct the positional data. Inertial is a good backup for GPS black-out emergencies though.

LeeE is correct that ram-air-pressure ports on the airframe itself don't really work. Not because they wouldn't read a ram air pressure but because this pressure is dependant on the airflow around the model/wing. A variation in angle of attack alters the flow around the model and alters the pressure point, meaning a measurement taken in the boundary layer will not be accurately describing the "free-air" conditions of the air. (just get one of those airfoil simulator "games" and look at what the airflow around the leading edge does as the AoA changes)

This neatly segues into the question about static ports. Static pressure usually IS measured directly at the skin of the aircraft. And there's a very nice trick used when doing it. Most craft have several static ports along the fuselage (often symmetrically placed on either side of the fuselage) On gliders the usual pattern is 2 on the nose, 2 on the tailboom, sometimes another set further back.) The placement of these ports is done so that as the airflows around the aircraft vary with speed, the resultant pressure in the static pressure piping stays as close to the true static pressure as possible. All this can nowadays be determined with some fancy FEM flow-modelling. In the crotchety grandpa "in my days" times it was often a combination of designer experience, trial and error and dumb luck. Some older gliders use a so-called venturi tube, which is a necked tube pointed into the airstream, with the static port at an appropriate location along the venture to obtain the correct pressure. Those only worked correctly with the nose pointing exactly forward and within a narrow speed-band. Outside of those parameters they are not good to horrible.

This brings us to the pitot tube problem. Pitots are normally mounted on a stalk, outside the boundary layer of the adjacent skin. And then offset slightly forward of their mounting point. This is to eliminate as much turbulence from the pressure as possible and obtain a reading as true as possible. Angle of attack still has a slight effect on the measured pressure too.

Keep in mind that ram-air pressure as measured by a pitot is infact 2 distinct components, the static pressure plus the energy pressure of the still air being squashed into the fast moving pitot tube. If we were to take a tube, close the top and then drill some small holes axially on only one side, then point this out of the fuselage with the holes pointing back we can obtain a "suction pressure" that is comprised of the static pressure minus a component directly related to the speed of the craft (The negative energy pressure I was talking about) This suction pressure is not exactly as accurate as a pitot pressure, it'll be influenced by angle of attack and yaw angle more than a pitot would be.

Modern glider variometers (aka the fun gauge, indicating climb or descent, thus positive or negative fun) usually have a compensation for so called "stick-thermal". If a pilot in a glider pulls the stick back in still air (provided he has speed) the glider will climb, but in doing so lose speed. The old un-compensated vario's would indicated climb, possibly falsely leading a pilot to think he found a thermal. To eliminate this effect most gliders have exactly the arrangement I just described, with a pipe outside the boundary layer with some backwards facing holes or pipes. This suction pressure is then fed into a compensation bellows arrangement. (on mechanical vario's atleast, theres now also digital once that use solid state components) These bellows expand as the speed drops and temporarily expand the volume of the "reference volume" thermos-bottle of the vario, thus preventing the vario from showing a climb. This is called Total Energy compensation. The reason for using a negative pressure for this compensation and not the ram air pressure taken for the speed indication is because mechanically the negative pressure is easier to use for compensation. There's no reason other than the slight loss in accuracy and stability why it couldn't be used to take a speed measurement.

Now where is the "I'm putting on my cool pilot shades" icon?

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