[humptybump]

I can't tell if it is technical ignorance on my part or market speak in the product materials. I hope an engineer or two here can help.

A product such as the TruTrak Gemini PFD has capabilities which include: Velocity Vector Display, Bank Angle Display, and Standard Rate Turn Indicator.

How do it compare to traditional gyroscopic instruments ? And under which circumstances is solution better than the other?

How do the above two solutions then compare to something with an ADAHRS ?

 

[Ironflight]

I can't tell if it is technical ignorance on my part or market speak in the product materials. I hope an engineer or two here can help.

A product such as the TruTrak Gemini PFD has capabilities which include: Velocity Vector Display, Bank Angle Display, and Standard Rate Turn Indicator.

How do it compare to traditional gyroscopic instruments ? And under which circumstances is solution better than the other?

How do the above two solutions then compare to something with an ADAHRS ?

Last question first Glen....the ADAHRS is a way of measuring attitude and air data - any EFIS has to has one, and it has nothing to do with how the data is displayed to the pilot.

Your first three questions are about specific methods of displaying data to the pilot.

A Velocity Vector is a way of displaying which way the aircraft is actually moving relative to the background - there is nothing comparable in traditional instruments. A Bank Angle Display would, I guess, be comparable to an old-fashioned artificial horizon, showing the angle of bank relative to the horizon. A turn rate indicator is comparable to the turn rate needle on a T&B - it tells you how fast you are turning (yawing).

The FAA has a great Handbook (FAA-H-8083-6 - it is downloadable as a pdf)on flying technically Advanced Aircraft - this would be a good place for anyone transitioning to glass cockpits to start.

 

[Dynon]

Last question first Glen....the ADAHRS is a way of measuring attitude and air data - any EFIS has to has one, and it has nothing to do with how the data is displayed to the pilot.

A device which only wishes to display a velocity vector does not need an AHRS, and in fact uses completely different data.

An ADAHRS is an "Air Data Attitude Heading Reference System". If you don't display Airspeed, or Altitude, you are just an AHRS. If you don't do magnetic heading, you're an ARS.

The "A" inherently means attitude, but a velocity vector need know only two things, vertical speed and turn rate. Vertical speed and turn rate are not things that are displayed on a pure attitude display at all, so you can't just display one as the other. There are significantly less sensors needed to make a velocity vector display work than a full ARS. In fact, you can make a very functional velocity vector display with just a GPS.

In my biased words, this is how I would describe the difference:

An attitude display is what you are used to in every training aircraft you have flown. It shows where the nose of the airplane is pointed relative to the horizon. It works if you are stopped or upside down. If you are in a high AOA situation and have the nose above the horizon, but the aircraft is descending, it will show you as nose up.

A velocity vector display shows you where you are going, not where you are pointed. If you are upside down and not climbing or turning, it will show you level. If you are doing a flat turn with the rudder, it will show you banked. If you are in a stall with the nose above the horizon but descending, it will show you nose down. If you place it on the desk on it's side, it will show you level because you are not turning or changing altitude.

Some velocity vector displays do enhance their display with gyros, so that in the short term, they are showing you the angle of the aircraft relative to the horizon, but in the long term they are still showing you the motion. This gives them a more immediate response to the motions of the aircraft since a sudden change in angle does not always mean a sudden detectable change in something like vertical speed. This makes them much more useful in stabilizing an aircraft.

They are both functional displays, and in cruise, coordinated flight they often are indistinguishable, but in unusual attitudes and other situations they are vastly different, and a pilot using them should be aware of their differences since they are sometimes displayed in very similar manners which does not make it obvious what they are showing you.

As a side note, many full ADAHRS EFIS systems also display a velocity vector as an overlay to the standard attitude display.

--Ian Jordan
Dynon Avionics

 

[gasman]

A product such as the TruTrak Gemini PFD has capabilities which include:
Velocity Vector Display, Bank Angle Display, and Standard Rate Turn Indicator.

How do it compare to traditional gyroscopic instruments ?
And under which circumstances is solution better than the other?

How well can you trust one over the other?

 

[Rainier Lamers]

If I may chime in here.
The question is definitely not easy to answer. You are trying to compare two different things.
You may be making the assumption that the velocity vector is somehow a replacement for an AHRS with the same functionality. It is definitely not - as Dynon pointed out quite correctly.

Now I'll try and answer your question as best I can - our EFIS systems allow you to use either so I have good experience with both systems.

We call the "velocity vector" a "Flightpath" display since in our normal AHRS based mode the velocity vector is displayed as "VV" symbol on the screen. In this case the display shows your attitude and the "VV" shows where you are going.
Now, if you don't have an AHRS you could interprete the "VV" and assume that you are flying an aircraft that generally does balanced banks in a turn. From this and knowing how fast you are going and how fast you are turning (you can get that from a GPS or even airspeed and compass) you could assume a bank angle and display that. As long as you are indeed flying a balanced turn that is just fine. If you are however slipping through the turn or are indeed doing a flat turn - then the indication is wrong as the assumption is wrong.
Pitch is different - here you show a pitch angle that is not attitude but the actual angle you are going up or down (based on speed and vertical velocity).

The clincher is that either method is good when used by a properly trained pilot that interprets what his display is showing him. You can use the "Flightpath" display perfectly to say, decend blind in a cloud. As a pilot you know well when you are approaching a stall so you don't need the pitch angle for this and one can even argue that flight path is better - it shows you you are going down while the AHRS could give you an impression that you are going up (of course you would fix that problem by cross checking with your other instruments, the VSI or altimeter comes to mind).

"Flight path" or "velocity vector" looses its functionality when you are upside down. You can argue that traditional AHRS systems are typically limited to bank angles of 60 degrees anyway so you're not loosing anything.

AHRS is the ultimate but it has shortcomings as well and interestingly, these oppose the typical velocity vector method.
As a result of this, I tend to recommend to our customers that have two panels in the aircraft to set the pax panel to "Flight path" display. This way you have the best of both Worlds.
Flightpath never drifts, never tumbles, never exceeds maximum gyro measurement rates but needs fairly normal aircraft motion.
AHRS shows real attitude and modern AHRS shows any attitude including upside down.

Again, they are two very different methods and show you very different information - both are equally usefull if you know how to use them. Know the limitations of either and you are going to be just fine.

Rainier

How well can you trust one over the other?

 

[LAMPSguy]

Forgive me

If I am improperly assuming.

I am thinking this sounds like "old school" partial panel flying.

If your ball is not centered, using heading for wing attitude can be a little off. Similar with a climb. So it is not worse, just a different method of interpretation. My problem with it is if you plan to use it in IMC, especially if it is a backup instrument. I have always thought that a backup instrument should be simple to transition to. If you are having to use it, especially in IMC, it means you had a failure and you are already having a bad day!

 

[Brantel]

Good info on the technical difference above.

You can see the real world example of normal use here:

http://youtu.be/KtlLPJzRXtg
Sorry for the crappy shaky iPhone video....

You can see the way both handle simple aerobatics here:

http://youtu.be/dXpPeOvcuT8

The Gemini is a hybrid and TT is the only one that can tell you exactly how they work since their algorithms are proprietary but they are based on air data and gyro information. They can work without GPS input just fine however the ground track indicator will revert to free gyro mode with no GPS so it will precess just like a mechanical DG and it must be tweaked to keep it on the correct heading. The Gemini displays gyro based bank and pitch information in the short term and slowly convert to turn rate and vertical speed (velocity vector) in the long term. TT has stated in the past that in the future they will have a firmware update that will enable a user selectable option of their standard velocity vector display vs a traditional attitude indicator display or ARS.

The Gemini is limited to the amount of pitch and roll it can handle before it will tumble but so is the traditional mechanical attitude indicator. You can see this limitation in second video above when the airplane is rolled. Notice that the Gemini gets its bearings and recovers in a few seconds. Also notice how while the Dynon does not tumble but it does end up lagging the aircraft's true attitude relative to gravity. It quickly recovers however. It should suffice to say that the real world limitations of the Gemini on pitch and bank never even come close to impacting normal sport flying. Aerobatics will tumble it but they will also tumble most certified mechanical AI's.

Typical marketing tactics are to highlight the bank and pitch angle limitations of devices like the Gemini while stating that full blown ADAHRS based systems do not have those limitations. My argument is that in situation that is likely to kill you like a tight diving spiral or spin, most ADAHRS based systems will not keep up with the real situation either just like the Dynon does not keep up with the simple roll displayed in the second video above.

Now as was stated above there are some unusual flight attitudes that will be displayed on the Gemini differently from those of a traditional AI. it is important to understand those differences. TT has done several things to help the pilot stay out of those situations like providing a low airspeed warning and a slip/skid indication. They also have done a pretty good job in their algorithms to minimize these false indications.

The bottom line is that the Gemini while different can make a very usable IMC backup. I have said many times that if you cannot keep the greasy side down with the Gemini alone, you are not qualified to be flying as PIC in IMC anyway.

 

[Brantel]

So it is not worse, just a different method of interpretation. My problem with it is if you plan to use it in IMC, especially if it is a backup instrument. I have always thought that a backup instrument should be simple to transition to. If you are having to use it, especially in IMC, it means you had a failure and you are already having a bad day!

The real world performance of the Gemini type display in IMC by a qualified pilot suggest that the display is no harder to use than the traditional display even with the differences stated above. Some even argue that it is easier to use.

Now get all bent out of shape and that is where you need to understand the differences....but one could argue that if you allow yourself to get into the situations where the differences would matter, most likely no ADAHRS or AI or panel full of gyro's is going to help you.

Most weekend aviators in normal flying will not be able to tell the difference in a blind flight test.

 

[LAMPSguy]

That's the ticket

Most weekend aviators in normal flying will not be able to tell the difference in a blind flight test.

That is exactly what I am trying to figure out. I am so used to having so many redundancies in my "work aircraft" but that is because of the type of flying we are expected to be able to do.

I like to think that when I get my plane built I will stay away from the scary IMC I have had to fly in in the Navy, but I want SOME ability to fly IMC...then my mind goes back to those redundancies...vicious cycle.

But, I fly with true dual redundant GPS/INS systems and even THOSE have failures so I guess nothing is perfect!

 

[Brantel]

I guess nothing is perfect!

You got that right! The more I learn about all of these systems, the more I believe that!

 

[humptybump]

I am not suggesting this thread is complete but I wanted to extend my appreciation for the contributions thus far. It has been very informative.

In order of posts ...

Paul, thanks for the intro and the suggestion of the FAA publication (link for those interested). (Addendum: the publications does not cover VV and spends most of its content on enroute and approach navigation)

Ian (on behalf of Dynon) and Rainier (on behalf of MGL), you guys are class acts. You have added to the dialog without even dropping into market-speak or dissing your competition. (these are rare in my day job). Many thanks.

Brian (end-user for TruTrak), I appreciate your first hand knowledge and the real-word videos. (perhaps we should pass the hat to help get you some videographer training )

Nick and Warren, you are asking the questions I am thinking so lets keep going.


I was trying to treat them all as different implementations with the same result. They are not. My take away is that there are differences across all the solutions and learning what each does dramatically increases their usefulness and their value in day to day flying and in critical situations. I guess it's time to hit the books !

 

[Brantel]

I
Brian (end-user for TruTrak), I appreciate your first hand knowledge and the real-word videos. (perhaps we should pass the hat to help get you some videographer training )

Sorry, it was very rough that day and a hand held iPhone in one hand, stick in the other and eyeballs outside.....well you get the idea.

 

[humptybump]

Brian, no worries on the video. It is useful all the same.

In the second video, there is a point where you initiate a turn from level flight. The Gemini and the Dynon are in agreement during the level flight but the Gemini appears to not detect the turn. Can you explain what is show in the frame grab here ...

Addendum - from the link in the next post ... there was a firmware update to address "horizon spinning or locking up". So the screen grab is before that fix.

 

[Brantel]

Yes, that is what this is all about:

http://www.vansairforce.com/community/showthread.php?t=92462

 

[REHughes]

Velocity Vector Definition

but a velocity vector need know only two things, vertical speed and turn rate.
--Ian Jordan
Dynon Avionics

I would disagree with this characterization of the term "Velocity Vector". In the history of display development, "Velocity Vector" is a term used interchangeably with "Flight Path Marker", the choice varying with whatever company manufactured the HUD or Panel Display. The ability to display the Climb/Dive Angle (CDA), either in reference to the surrounding Air Mass (Air-Mass Referenced VV/FPM) or to the Earth (Ground-Referenced VV/FPM), is an absolute requirement. This enables the symbol to be conformally referenced to the outside World in see-through or synthetic visual displays.

If the vertical movement of the little airplane in a display is scaled as a vertical-speed value, as in the Trutrak ADI, you cannot characterize the symbol as a Velocity Vector, as it does nothing to define the flight path in terms of a Climb/Dive Angle. Further processing of GPS and/or Air Data output is required to produce a satisfactory angular flight path solution, and I presume that this is what the Gem display will present as a Velocity Vector.

 

[humptybump]

After a bunch of reading and sitting at my desk with various objects (a pencil, a die cast airplane, and a foam ball) I am starting to visualize how the different systems behave and how they are used in primary flight. You can digest from the prior posts that

• the various methods are not interchangeable
• you can safely control an airplane with the various systems if you understand what they are telling you

As it relates to pitch information, I used the foam ball to understand the velocity vector display (VV). My foam ball does not have a nose or a tail. All it can represent is that it is going up or down relative to its surroundings. I like the term "flight path" since that has a clear visualization for me. I can see that it would be helpful to have an angle of attack (AOA) display to compliment the information from the VV. I would also want a "ball" to show coordinated flight.

Another 'hat tip' to Brian for the video. It showed me that in most flight conditions, I can have confidence to fly the airplane with either a VV based solution or an AHRS based system. It's rare you get to see multiple solutions operating side-by-side.