I start to realize it is a more complex matter than I thought. However I still don't understand why the solid state devices cannot self-calibrate in a second while still on the ground before taxi, just using earth gravity acceleration signal, via their internal accelerometers (equivalent to straight and level flight). I mean, once you managed to squeeze an accelerometer and a gyro into a phone, once you managed to create an algorithm for an App to use the signal from those instruments to simulate an AI, how difficult could be to make the system self-calibrating using the earth gravity at the inception of the App (you don't even need the plane to stand on flat ground while you do that, provided is not moving/turning/accelerating) ?
However I still don't understand why the solid state devices cannot self-calibrate in a second while still on the ground before taxi ...
They can and do! But once you are flying, vibration from the engine, turbulence and accelerations of the aircraft will quickly ruin that calibration. Furthermore, to make the display more stable, all these programs contain some amount of dampening that will "pull" the gyro back to level display once the measured accelerations are steady again. The solid state sensors can not distinguish between unaccelerated level flight and steady balanced turns.
Just try it out: Take your "old" iPhone 4 (how old must I feel if you call an iPhone 4 old ;-) ) with you when you fly next time, fix it somewhere on your dashboard and fly one or two standard rate turns at constant altitude!
understood. thanks all a lot for the useful info. I'll go for a real spinning gyro.
The solid state sensors can not distinguish between unaccelerated level flight and steady balanced turns.
But... nothing can. It's a physical impossibility. It's like detecting the wind speed and direction purely from within an aircraft.
One has to bring in something which "reaches out" outside your immediate frame of reference, to the next frame of reference outside that. That "something" is GPS. Or airdata (constant heading + constant altitude = assumed S&L flight).
However I still don't understand why the solid state devices cannot self-calibrate in a second while still on the ground before taxi, just using earth gravity acceleration signal
An accelerometer measures acceleration, not a steady gravity force. For that you need a level detecting device, like in the electronic spirit levels, and crude versions are in (some) phones and (more expensive) cameras.
I honestly don't know whether/why anybody has/has not done an app for an Iphone (or any other smartphone; there are many similar models now) which combines the levelling sensor with the accelerometers.
Maybe they have, but in my tests with the Ipad2 and the app I posted about on the other thread, I found it drifting off in just a few minutes. It was a joke.
What is certain is that without a levelling input into the algorithm, the accelerometers will drift off too rapidly to be of use for IMC flight.
It can't be that hard to do, because the old gyro AIs will drift too. What keeps them workable is the erection process, which in turn relies on the flight being "mostly straight and level". You can fly in IMC for hours if necessary with these (or drive an autopilot with them) so clearly it works.
I spent allot of time trying to find an economical...but safe and reliable backup glass attitude gyro. I use a ipad2 for moving map and the iPod touch 5 gen for a backup attitude indicator. The l3g4200d gyro in apple devices has allot of drift. As explained in a earlier thread all the solid state gyros do. Many of the low cost backup gyros like the dynon d1 require gps backup also.
After much trial and error I came to the conclusion that the best gyro program out there if you are using the apps store at apple is a program called pocket horizon pro. I tried all the others and this is the only one I would trust my life to. If you want to see how well it works go to you tube...type in pocket horizon attitude indicator and you will see about 7 short videos I shot in my longeze. Don't waste your time and money on any of the others...like I did. With present technology this one keeps the apple gyro in line using waas gps. Don't try using the internal gps in many apple products. It does not have waas capability. Get a bad elf or a dual electronics....the bad elf has better sensitivity. I have about 340 dollars invested and the pod is permanently mounted on the panel. The last thing to say on this subject is to read and follow the instructions closely on pocket horizon. Otherwise, you will have problems. The guy that wrote the program is a truly gifted software engineer. His program also does other things....check it out....for ten bucks ....about the price of one gallon of av gas in Europe....you can't go wrong.....zuke
I have worked, together with two sw engineers, on developing an attitude indicator for the iPhone fir 2 years. In short:
Any iphone from 4 upwards does NOT only have accelerometers but REAL MEMS technology gyro chips, in principal the same technology as in any glass cockpit
These GYRO chips do work fine on the ground but they are not reliable in flight and actually nobody until now has developed the right filters to make the iphone a reliable backup ai. We tried to correct the ai with the help of the accelerometers and with GPS. It worked a little, but i would never have trusted any of it. We not found a good solution to perfectly eliminate the drift.
The only way we could get a reliable ai was using the Levil AHRS, and even that is far from perfect.
We stopped the project after two years although we had a nice prototype running that looked like a miniature Aspen glass cockpit. I will post some pictures.
By the way: an ACCELEROMETER chip like it is also found in every iphone cannot show attitude in space under accelerations. Therefora all apps that use the acc. chips will work fine on the ground - but never in flight. But from iphone 4 upwards all have GYRO chips too. Only that they are not precise enough and their drivers are not good enough for flight. Of course we tried combining the gyro data with information from the GPS and the acceleromters. Does not work well, the GPS is too slow and the iphone accelerometers have a limit of 2 g.
A professor and two informatics students worked on it for 2 years ( i developed the idea, features, design ... and supplied 3 iphones)
Forget it! ;-))
I have not looked up the actual devices used in the Iphone but aren't they all the same type of device i.e. "accelerometer" = "MEMS" = "GYRO"?
No phone has rotating gyros or fibre optic gyros, obviously. All they can have are the standard MEMS chips.
These do measure acceleration. The measurement includes the 1G gravity vector but that's OK and that is probably how they sense the static orientation. I am sure they don't use the proper liquid level sensing principle used in these.
The I3G4200 is a standard MEMS device.
There is another technology which uses a micromachined silicon tuning fork but looking at the data sheet, it isn't clear whether that device uses that as well. It does clearly say it detects acceleration around all three axes.
With a decent GPS receiver it should be possible to do a "gyro instrument" display which is an enhanced version of the "instrument panel" which you get on say a Garmin 496 which does it all just with GPS data only
I don't know if "zuke" is the actual developer of the program 
but maybe somebody has found a way to make it work. After all, as I say, Garmin do it almost acceptably just with GPS.
OTOH Garmin probably have a position update rate of 5/second, and a superb GPS receiver, whereas the GPS in IOS devices is really for finding your nearest Big Mac. You do need a decent position update rate and a very low 3D position jitter to make this work.
Hi Peter,
no G1000, Avidyne Entegra or any other AHRS system has any moving or rotating parts either. The GYRO chips based on MEMS technology nevertheless deliver information about their psition in Space, just like a mechanical or (lazter) Laser Gyro.
The ACCELEROMETERS on the other hand can only measure accelerations. MOST of the silly attitude indicators in the app store are based on te accelerometers, simply becasue their non flying programmers do not understand the difference. These AIs will show no turn in flight.
The BETTER ones in the appstore (and i have bought them all :-)) are based on the gyrosocopic chips of the iPhone and DO show a turn, but they all drift away, and nobody has yet made one that doesn't.
Until the iPhone 3G/3GS the iphone only had accelerometers, but from the 4 model it also has gyroscopic chips and – and therefore i theory a perfect attitude indicator just like in an Aspen or Avidyne or Garmin glass cockpit should be possible.
The proble is that these gyrsoscopic chips were a) not developed for high g and speed eniviroments and not the radii airplanes fly - but mostly for 3 D computer gamaes. Also in a glass cockpit or even i the small AHRS boxes like the Levil (i have one here) the gyrsoscopic chips are of higher quality, they drift less and less vulnerable to vibration.
With a fast enough GPS receiver you could SUPPORT the gyroscopic chip in the iphone just like a Turn Coordinator in a STEC ("rate based") autopilot can fly a plane level - knowing that the wings ARE level when there is no turn. Also in my Cirrus the TC provides a check signal for the PFDs attitude indicator. If the PFD says "left turn" and the TC "right turn" a "AHRS TC miscompare message" will come up.
We (seem to have) found out that vibration was the biggest problem.
LOOK HERE! http://www.ifixit.com/Teardown/iPhone+4+Gyroscope+Teardown/3156/1
or better, here: http://www.memsjournal.com/2011/01/motion-sensing-in-the-iphone-4-mems-gyroscope.html
And, before somebody asks: Forget he iPhone compasss (that's the third kind of chip in the iphone) in an airplane. It does not work with all the electronics around (did not work in 2 Cessnas I tried and my Piper Warrior)
I can see different Iphone models have different types of MEMS accelerometers, but
no G1000, Avidyne Entegra or any other AHRS system has any moving or rotating parts either. The GYRO chips based on MEMS technology nevertheless deliver information about their psition in Space, just like a mechanical or (lazter) Laser Gyro.
The ACCELEROMETERS on the other hand can only measure accelerations. MOST of the silly attitude indicators in the app store are based on te accelerometers, simply becasue their non flying programmers do not understand the difference. These AIs will show no turn in flight.
suggests we have our wires crossed 
because nothing can independently give you a position in space. That's a physical impossibility. It's like the debate we had about an aircraft "feeling" the wind.
All that one can measure, with a free platform, is acceleration.
If you integrate acceleration, you get velocity, and if you integrate velocity you get position (or pitch/roll/yaw attitude, etc).
The only way to get position directly is to use something that reaches across the frames of reference e.g.
The only way to get a position using inertial navigation (measurement of acceleration on the 3 axes) is by starting off at a known position (which is why e.g. the gates where airliners park at a big airport have the lat/long on a big sign, though I am sure that stuff is in the FMS database anyway) and then integrating the accelerations.
MOST of the silly attitude indicators in the app store are based on te accelerometers, simply becasue their non flying programmers do not understand the difference. These AIs will show no turn in flight.
This one does
but since it has no GPS input it drifts, and needs to be manually levelled when the wings are level.
The solid state sensors can not distinguish between unaccelerated level flight and steady balanced turns.
But... nothing can. It's a physical impossibility.
Am I just misunderstanding what you're saying - because I can't see why it cannot. If you've got a sensor that is sensing your turn rate, then you can differentiate a steady state coordinated turn from straight and level flight. The iPhone surely has enough sensors to be able to both sense turn rate, roll rate and pitch rate at the same time (although I'll readily accept the particular sensors they use might not be good enough for doing it in flight!)
Sparkfun Electronics are selling modules that are good enough for automatic control of radio controlled helicopters (that vibrate a lot more than a typical GA aircraft) for about $100 or so, so surely the hardware can't be that expensive to do the job. I suspect the cost of these standalone attitude indicator devices for GA (even the non certified ones) is that the non recurring engineering costs of writing the software can't be spread over very many units rather than the cost of the sensors.
