Comment by Andrew Rabbitt on February 25, 2012 at 9:27pm

@ John and Gary, according to my hand calcs and X-plane simulations, flying at extreme altitudes (>FL1000) is not a difficult problem and with the winds up there (I think) at quite low speeds, relative to the jetstream winds, and with TAS at ridiculous rates (300-400mph) it's the best place in the whole mission to try to fly.  You can make some incredible mileage to help prepare the aircraf to deal with jetstream headwinds down lower.

@Alan, I am working on revisons to the APM flight controller to enable it to change the descent rate depending on the headwind conditions.  As you'd probably know, best range speed is higher than still-air when there's a headwind and lower when there's a tail wind, so since APM knows which direction the wind vector is, I can use that to ramp my target speed/descent rate accordingly.

Additionally, the only telemetry I'm planning on having at thsi stage is a Dronecell on board which I am planning on SMS'-ing data as and when network coverage becomes available at regular intervals with position fixes and direction/speed info, with the possibility perhaps of SMSing back an alternative landing site instruction

Comment by Andrew Rabbitt on February 25, 2012 at 9:32pm

@John and others, check out my previous blog post on my X-plane simulation at FL1000.  This is quite early stuff and I've had a load more success now, with completely stable HIL control from a simulated zero-speed release at FL1300 and more.

Comment by Hans Miller on February 26, 2012 at 1:52am

You do realise that you will be without gps above 60000 feet?

Comment by Andrew Rabbitt on February 26, 2012 at 2:17am

Hans, well yes, maybe that will be a problem, but it is very device dependent.  I have yet to find out whether the uBlox or other DIY-drones-sourced GPS systems allow operation above 60k feet.

The 'regulations' that specify this have an 'and' clause, whereby above 60k feet AND 999kts, so I am quite sure I won't achieve this condition, however many GPS manufacturers treat this as an OR condition, but some don't.  I need to find out more on this issue, however I could probably make my system GPS independent if necessary.

Comment by Andrew Rabbitt on February 26, 2012 at 2:19am

Monroe - more info coming soon, I promise!

Comment by Andrew Rabbitt on February 26, 2012 at 2:42am

More regarding the COCOM GPS altitude limits.  I found out here that the uBlox 6-based chipsets are rated to 50km and are available here relatively cheaply.  I'd buy one from DIYDrones Store if someone there would kindly integrate this module into APM! (hint hint...)

BTW, the Mediatek 3329 that I have will not function over 18km according to the datasheet - this is the 10Hz one on offer at the DIYDrones Store.  An earlier uBlox 5 that I have, may work, according to that previous link, however it is only 4Hz.

Comment by John Rambo on February 26, 2012 at 5:52am

Andrew, if you're up to catch a JetStream, why are you aiming for 120k feet, instead of 50k? I did also a check on your link on x-plane simulation, but missed the pressure data on the screen - you should double check there's a standard ISA atmosphere model implemented.
As for GPS restrictions - uBlox5/6 and SIRF-III (EM-406,407) are ok to use, but I wouldn't go for SIRF - they're bit more expensive, and 30% of them come with faulty antenna connections (at least those sold by sparkfun).

Comment by Andrew Rabbitt on February 26, 2012 at 3:02pm

John,

I'm not trying to catch a jet stream, but it will play a big influence on the return-to-launch.  It will go downwind during the ascent and it will need to go upwind during the descent, so in effect it will be fighting the jetstream.  If it can reposition itself upstream during the high altitude sector, then it has a good chance of returning to the launch site.

Comment by Larry Grater on May 9, 2012 at 3:44am

Andrew,

Great stuff above and sounds like you're making good progress.  I also noticed the X-Plane issue with using TAS.  Great to know that this has been fixed.  IAS is directly related to QBAR and much more useful for scaling autopilot gains.  Typically gains are scheduled with both Mach and QBAR.  The former is true because XCP and control effectiveness vary significantly with Mach above 0.75 ‘ish.  That's not to say that there might not be issues with local compressibility affecting both the aero and maybe IAS depending on the airframe. 

I initially thought that I wanted to attempt to capture the A/F at low Qbar and work the gain scheduling problem pretty hard.  However, I've decided to head off in a different direction utilizing a very stable airframe and drogue (commanded dual release system).  I still don’t fully understand what APM is doing with scaling and you’ve provided the spot to look in the code.  I think there’s also a forum thread on this topic by Doug.  Another potential advantage of not turning on APM outputs before the A/F is "flying" could be a potential to saturate the sensors due to rates.  Of course you have to think hard to find the forcing function. 

I'll attach a handy spreadsheet that does a few flight envelope calculations, computes Mach number, desired turning radius, and has a few A/P and guidance design notes.  This is very simple “motherhood” aero stuff, but helpful to understand what it takes to keep the A/F at an IAS where it wants to fly.  The aero loading is obviously related to QBAR and we’re trying to keep that fairly constant (unless scheduled for guidance reasons). I’m like you and worried about dynamic divergence of wings and surfaces.  I may balance the control surfaces.  Flight Envelope

I'm working in X-Plane 9 as well and I'm also having stability issues above 40k ft or so for representative IAS.  I don’t know if this is X-Plane or APM related.  I could be both.  X-Plane does have issues with small models though there is a frame rate setting that helps a little. In any event, the gain scaling problem needs to be solved.   

I'm doing drops with a handy X-Plane plug-in that allows reset of the state vector.  This might help speed things up a bit for simulation. Position Aircraft

Regards,

Larry

 

 

 

Comment by Larry Grater on May 9, 2012 at 3:58am

I have an earlier post where I've solved the 60k navigation issue.  There are a few receivers that will work.  I'm using a Venus GPS running 5 Hz NMEA data. My surrogate A/F is stock APM2 and the flight A/F will be APM2 minus GPS + Venus GPS. See post for integration.  I'm also using a BigRedBee APRS with a Lassen IQ.  I understand that the Lassen has a lower update rate, but have not verified or checked to see if the rate can be changed. 

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