MGL Avionics User Forum

Hi,

I am testing my new MGL setup and have some issues with the SP7. I couldn't find anything similar on this forum, I hope people can help me out.

While testing during a test flight when I bank (+/- 30 degrees or more) the virtual horizon starts to bank but then quite quickly levels back off again while the airplane is still at a bank angle. When returning to level flight again the virtual horizon first steeply banks at way to much angle in the opposit direction and then also quickly levels off. So basically during turns the virtual horizon is indicating a level flight.

I wonder what could be done about this.

I installed the SP7 pretty close to the center of rotation in my airplane (Europa XS) and in such a way to prevent vibration as much as possible. I am not sure what is used for speed, but I have a challenger efis + ibox v2 installed so I assume that is used as the source. The SP6 is installed at a place where the least amount of metal or wires are, but I am not sure if the SP6 is used by the SP7 when there is GPS available.

Any tips are welcome and appreciated, thanks.
Johan.

Your SP-7 should be getting velocity information from the EFIS. It uses GPS ground speed or true airspeed (the latter is preferred as ground speed is "contaminated" by wind) - if none of that is available the SP-7 will assume a velocity.
The SP-7 uses the velocity to work out centrifugal forces acting on the SP-7 based on rotation rates.
This in turn is used together with the actual G-forces measured to figure out where the planet is.

So - first thing to check is if the SP-7 LED is giving you a double flash - if it is it means it is getting data from the EFIS.

If that is the case then it's tome to look at how the SP-7 is mounted. If it is mounted in a way that it is shaking or affected by vibration (which is a fast movement) the acceleration forces cannot be measured properly and the SP-7 will start using a different method which is OK in more or less straight and level flight but in a turn it has no chance to follow what is happening. A typical bad mounting place that will result in your observation is underneath the seat on the actual aircraft skin - mentioning this as an example as this is sure fire way to get the SP-7 to give you wrong attitude in a turn - here the prop wash beats the skin like a drum and the SP-7 sees your aircraft's attitude plus a constant variation of this. Yes it's tiny movements but very fast - far in excess of what your gyros and accelerometers can measure and the calculated attitude goes out fast.

Mount the SP-7 on a sturdy airframe member such as a wing spar box or similar. Method of mounting that works really well - self adhesive velcro tape. This allows easy repositioning while being firm enough not to allow the SP-7 to flop around in the bumps while still being of a nature so engine vibrations can't get to the SP-7. Basically you want the SP-7 to only see the "big" movements of the airframe without being contaminated with any other movements however small they may be - as they are not helpful.

Be very aware of mounting the SP-7 on a shelf - I have seen very nice shelfs that I though looked sturdy and fit for purpose only to find out in flight they are anything but - just by placing my hand on the SP-7 and varying the engine RPM. You can quickly find resonances where the effect of vibration gets greatly magnified.

thanks for the detailed reply.

My iEFIS is showing correct airspeed and true airspeed as well, but I will check the double flash LED on the SP7.
I will also check the mount of the SP7 or otherwise move it around and do some test flights to see if I can get any better results.

Will update this topic when I have some results,

Thanks!

As a test only, I screwed the SP7 to a large piece of wood and put that in the baggage compartment behind the rear seats. This is a little behind and above the center of rotation, but still very close (~30 centimeters), and connected the device using an extension cable.

During some tests the SP7 seems to works much, much better. I am not sure if it's 100% of what it can or should do, it feels like it sometimes is a bit slow on leveling out the last few degrees when leveling out the airplane, but it does not level out anymore during a turn as it did before.

Due to bad weather I was only able to do some limited testing, but I will try a few other places in the aircraft to see what would give the best results, and then finally move it to there permanently.

Are there any other tunables or calibrations, besides zero'ing, that can be done or looked into on the SP7 ?

Regards,
Johan Postema.

Basically what you need to achieve - the SP-7 must move and rotate exactly as your aircraft's frame does. Any movement however tiny that differs between the two immediately creates a measurement error as these tiny relative movements tend to be much faster than the gyros and accelerometers can resolve. The SP-7 is a "strapdown" AHRS as are effectively all modern systems but it is also very light which makes mounting so important. If you ever have a chance to see an older type of strapdown AHRS in an airliner avionics bay pay some attention to the almost extreme efforts spent in mounting arrangements to isolate the gyros from the airframe while at the same time ensuring the (heavy) gyros do not move in ways not in line with the airframe itself). The SP-7 needs the same treatment but due to its very light weight it tends to be much easier.

In my personal experience with it I find mounting it using self adhesive velcro tape onto a very solid structure of the aircraft (like a wing spar box) does the job well. The Velcro is very good to isolate high frequency vibrations while the solid surface of the wing spar box has virtually no independent movement from the airframe (essentially the rest of the airframe really flaps around the wing spar box in any kind of turbulence. One tends to think of an airframe as a solid structure but in reality it is quite flexible and deforms all the time in particular in turbulence.

The worst places to mount the SP-7 are any areas directly onto the aircraft's skin (which tends to beat like a drum from propwash and airflow) or shelves which tend to have bad resonant frequencies. Obviously mounting locations that are suitable vs unsuitable vary a lot between different aircraft. If you are mounting it in a glider reasonably well it performs very good indeed - some of the smaller jet engined aircraft are also excellent platforms. Most other aircraft are anything inbeween very good and very poor platforms. In all cases the SP-7 can be made to work very well but the effort required to get it there may differ. The same goes for the SP-9 - while this contains rather expensive ring gyros and thus is very good at canceling out many of the effects of vibration you still have to pay the same amount of attention to the mounting location to prevent limiting its performance.

A good test is to take your aircraft in reasonably calm conditions and perform two 360 degree rate 1 turns (so the total maneuver will take 4 minutes) and then level out. The horizon must return immediately to the correct bank and pitch attitude. If you find there is a delay it means the SP-7 has not been able to follow your maneuver exactly.
One of the more difficult manouvers for an AHRS to follow (and thus a good test) is the phugoid test - in a straight line pitch up and down to +2 and -1 G limits and repeat this for at least 5 cycles (kind of like the "vomit commet" profile) - upon straight and level your pitch must show correct attitude.