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| thirsty Newbie  Joined: 22/06/2016 Location: AustraliaPosts: 34 |
The link I posted above talks about errors with these types of instruments when you introduce yaw which I'd never really considered. You really need this gauge during low slow turns where you are likely to be yawing (slipping/skidding) as this is where people kill themselves in light aircraft. |
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centrex Guru  Joined: 13/11/2011 Location: AustraliaPosts: 320 |
Please be aware that the table Paceman posted the measurement is in inches on a water manometer and is the distance between the two water levels in the manometer. My day job is the service and maintenance of gliders and every year as part of the annual inspection we have to check the calibration of the asi. This is carried out by attaching the manometer to the pitot and checking the asi in 10 knot increments up to the vne (velocity never exceed) of the glider generally in the 160Kt region. we also ensure there are no leaks in all the plumbing in the pitot and static systems. Hope this is of some use. All good fun. Cliff |
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Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
All excellent information Gent's..! I think that the better way to go with the AOA setup, is to use the pressure sensor approach, as it has no moving parts... But, first I need to get the airspeed sensor working... I will get the parts ordered soon, and get a starting setup going... I LOVE the easy way to test for and simulate airspeed by using a water tube..!  |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Just read your link Tony - much more authoritative, and some very good references at the end. I'll need to read it half a dozen times more to digest it properly but a good half hour scan through and ignoring the maths gives a better understanding, plus the photos are handy. Greg |
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| thirsty Newbie Joined: 22/06/2016 Location: AustraliaPosts: 34 |
Yeah it makes sense once you realise what's happening. Probably need to have a probe that can swivel to counter the yaw! |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Did some more digging and here's what looks to be the recent definitive work on a low cost AOA system. The work they did was using a 1 psi sensor (MS4525DO-DS5AI001DP, US$38 at DigiKey) but they were using faster speeds. Have a read. Low Cost Accurate Angle of Attack System Greg |
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| Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
Wow... That's some "detailed" information... The author, David Rodgers has a very good "scientist" like approach to his data gathering and had access to a wind tunnel to test and prove the accuracy of his system design...! Later actual flight testing was completed on his sensor and firmware.. (nice)  Most of the math work is over my little (8 bit) brain, but I hope we can create an MM clone... I do like the later parts, showing the setup, and need for the 4066 switching with 2 sensors having like addresses... Since it seems to function with the BMP180 already being used, it could be a good match for the AOA device... I still need to look over the code and somehow convert it to MM program code... Maybe the MS4525 sensors could be used for airspeed as well, but show in the BOM to be $42 each... ouch... But I do like the I2C interface part, but as Peter mentioned... you be payin for it..! 
Thank you very much Greg for finding this and passing it along to everyone...! Looks like this is going to be an expensive gauge, but needs to be done correctly and show accurate results to the pilot otherwise people will be hurt.. 
The next part cost will be the "probe" needed under the wing... Could be awhile before things get going... I need to get finished with the Altimeter and Vertical Speed gauges first... |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
I think it said somewhere in there that the regulations don't allow any mods to the Air Speed Indicator system and that the AOA system has to be totally stand-alone. The report is an official DOT (Dept. of Transport) investigation because of the number of accidents that already happen due to small aircraft usually not having AOA info available to the pilot. It says they officially now recommend AOA installation for small aircraft. You're definitely right though, it's gotta be done properly. The accuracy they quoted (tested 0.25 - 0.5 degrees) is very good, it surprised me. At least we know now the probe has to be mounted under the wing between 20% and 60% of the "local chord length" - that gives quite a lot of leeway for mounting it. The actual probe itself seems to be pretty straight forward so it's probably quite 'make-able' in your workshop. Still, as you say, plenty of time to think it over while sorting out the other gauges .
Greg |
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| Bill7300 Senior Member  Joined: 05/08/2014 Location: AustraliaPosts: 159 |
Great find, Greg. It has gone into my avionics reference library. Bill |
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| Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
Interface Board Update.... Added a few more bits... I wanted to try out the MPxx5004 pressure sensor that Peter suggested using and create an airspeed gauge.. Also added an RTC module to create a real time clock display. With the clock also keeping track of engine run time hours (hobbs meter) using the TIMER value and storing it with the VAR SAVE area... Engine "service" timers can also be added to the clock display and displayed as red, yellow or red dots to remind the pilot it's time to do something... Maybe also add a "flight time" display screen also... Anyway, for this to work right, I figured I needed to add a power fail detect circuit also, to give the micromite time to do a last second VAR SAVE command to keep a current store of the TIMER value... I didn't want anything fancy, so using the "kiss" method, I think I have something that should work using just two resistors (R8 and R9), diode (D2), and a "bigger" cap (C15), to keep the MCU powered up long enough... Power comes in on CON1, using the VIN pin. R9 just keeps trying to pull the VIN line down to GND... When power is removed, D1 will isolate the voltage left in the backup cap (C15) from the VIN line. the VIN line is routed to J1 pin 6 and returned to the MCU using a 5v input pin. D2 protects the MM pin by not allowing it to go much over 5 volts. R8 limits the current to a safe value... At power up, D2's cathode will be very near 0 volts, till the 5 volt regulator gets going... I wanted to check if this way of doing a power fail circuit is OK to use... Any feed back would be helpful...
Schematic attached... 2016-09-11_003355_Versa_Gauge_Interface_Board_Rev-B.pdf |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Hi Zonker - I'm no electronics expert so I won't try to comment on your power fail circuit. Here's some other info that may (or not) be useful though. Geoff did a 12v GPS car tracker project back in the Nov. 2013 Silicon Chip issue that needed needed power fail detect to keep the micro's 3.3v power up for about 2 secs (to write data to an SD card) and also to separately maintain the 5v GPS module data for about a week. Basically he used a mosfet and biiiig cap (4700uF,25v) for the micro supply and a super 'button' cap for the GPS. I built it and it works fine. You'd probably get away with a much smaller hold-up time and cap to do a SAVE or the option below though. PM me if you can't access the SC article. Another thought is could you use the small storage available on the RTC chip if you only need to keep the timer info. Greg |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
I would use a different approach. You can add a small FRAM chip to your design and write to it every second. No matter when power fails, you will have data of maximum 1 second before the power failure. You can write to FRAM as much as you want, it does not wear out, and it keeps its data without any backup battery needed. Just write to it and it will keep it. Microblocks. Build with logic. |
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| Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
Yes Sir..! You are correct... I guess I forgot about it..! I did an FRAM/RTC IC on the DIP-600-28 PCB... I guess there is no way of wearing it out then... Thanks Jean for the reminder..!! I will change the PCB design to include the DS3231 RTC and FRAM parts...  |
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