TruTrak ADI
 

I thought this might be of interest to anyone considering the TruTrak ADI. I asked several times on this list about what this device actually indicates and got lots of conflicting answers. Even Trutrak reps seemed to have trouble explaining it. Well I just powered one up in the palm of my hand and I have the definitive answer.

Regarding Pitch:
It does not detect pitch, it detects pitching. Once the pitching stops, the device returns to an indication of level flight, even if you're pointing uphill/downhill. However, the device also measures vertical speed via a static air connection and uses this to maintain an up or down pitch indication. For example, you nose up to climb. The gyro detects the pitching and indicates nose up. Once you settle into a stable nose up attitude the device is held in a nose up indication by the vertical speed detector.

Regarding Bank:
This thing doesn't measure bank at all, nor does it measure yaw; it measures yawing and indicates it via bank. That is to say it will only detect when you are turning around the vetical axis. Once the turning stops, it returns to indicate level flight. For example, you could put the plane in a slip and, since you're not actually turning, it would indicate wings level even though they're not. Incidently, this is exactly what a turn indicator does.

Having said all this, I still think this thing is a bacon-saver and think it's the perfect piece of backup equipment for my Day VFR/Aerobatic mission.

Steve Zicree
RV4 Wiring and playing with gadgets

Is it possible that it works differently in flight than it does in your hand? For instance, does it get data from a GPS to help it detect turns? Is the groundspeed signal from the GPS used at all?

It's designed to work without GPS, so I have to assume all the attitude functions are independent of GPS. It does use the GPS to determine heading (track) though.

Steve Zicree

Another theory...
 

OK, here's my latest theory on how this works...I have no more knowledge of how the ADI works than any of you, so this is just an educated guess.

The pitch angle it displays is supposed to be "gyro-enhanced vertical speed." What I think that means is that a rate gyro provides the high-bandwidth data, and a vertical speed sensor provides the low-bandwidth data.

Having worked on similar systems, this sort of makes sense...electronic VSIs have to be averaged (low-pass filtered) heavily to avoid noise. Thus they don't show rapid maneuvers well. The rate gyro, on the other hand, can show rapid changes but not steady-state pitch angle...it has a bias, so you have to apply a washout filter (i.e. block the DC value) to avoid drift.

So, if the unit is sitting on a bench, the altitude is obviously not changing. It'll just show the pitch change information.

Does it have a pitot pressure input? To turn VSI into pitch angle, it needs to divide vertical speed by TAS (which it can calculate knowing static pressure and pitot pressure, at least close enough). If it has TAS, then turning turn rate into bank angle is straightforward.

(So this is a long way of saying that I bet the behavior Steve Zicree observed is right...it'll work different in flight, but it's fundamentally a rate-based instrument.)

That's pretty much how Chuck from Trutrak explained it to me at OSH. You could hold it in your hand and give it some pitch, and it would show a climb or descent for a little bit before settling back to a "level" indication. In the airplane you'd have additional baro data to feed it to sustain the climb/dive indication.

These look like neat boxes, and the price is pretty good, but I think I'd want to spend some time flying one before I committed to using it as my primary ADI. Luckily I have a ways to go before I'm at that point. :)

mcb

Is vertical speed really what a pilot would want this instrument to display? I don't think I would want that combination. So to sum up what's been said before, the unit displays vertical axis angular rate and altitude rate. Its sensors seem to be a single rate gyro and a static pressure sensor.

I don't understand why TruTrak hasn't just put out a full attitude indicator. The sensors required to do this are pretty cheap these days. Here's what you need:

3 rate gyros
3 accelerometers
1 inertial speed reference (pitot-static is ok, or GPS. Good for tame flying without this, for example if GPS fails)

The sensors required for this combination can be had for well under $1K.

This is the sensor combination I use all the time at work for autopilots, and it provides a very reliable attitude solution.

The arcsin of vertical speed/TAS is flight path angle. Flight path angle differs from pitch only by the angle of attack. If you assume the angle of attack is small (and it's never more than 12 deg or so for the typical GA airplane) then displaying flight path angle and calling it "pitch" is not too far off. More importantly, it would be an adequate reference for steady-state climbs, descents, and straight-and-level. What it would NOT show is the fact when you're holding altitude at low airspeed, the pitch is higher than at high speed.

How do you remove drift? Cheap rate gyros (and accels) have significant bias. When you integrate that bias, you get an erroneous angle indication. Remember, the ADI is designed to help a human pilot fly, not a automatic flight control system. An autopilot can be made to ignore bias more easily than a human. Some AHRS use a magnetometer as source of long-term stability.

I'm told by the folks at Trutrak that the pitot input is only used to provide a low speed warning. It's not used for any of the attitude display functions. After fooling around with the unit I feel that it will work well to help a guy stay straight and level, climb, descend, shallow turns, etc. On the other hand, I think it could be just about worthless to get out of a severe upset, since it doesn't really tell you what the plane's attitude is. For example, a plane pitching forward while in a tailslide would actually show a nose down attitude. I have one not for IFR use, but rather as a bacon saver should I ever stumble into thick haze or similar.

Steve Zicree
RV4 finishing

Yes, I'm just questioning whether pitch and flight path angle are close enough for the situations a pilot could encounter. Especially in maneuvering flight, I'm curious how accurate this instrument will be for even the flight path angle estimate that it's supposed to produce, let alone as the pitch angle reference that pilots are used to.

For attitude only, the accelerometers give you a low frequency attitude estimate, once you remove the centripetal acceleration using the rate gyros and inertial speed measurement. This gives the bias estimates that get applied to the rate gyros and then integrated up to get attitude. You can use a magnetometer (properly leveled) to get your heading bias, but it's not necessary for attitude.

I'm not an expert by any means but I'll tell you what I experienced flying with Jim Younkin in his RV-9A. I could maintain + or - 50 feet in elevation by using the ADI as my only flight reference. I must have flown for 5 minutes or so and the altimeter hardly moved. Then he made me sick as a horse flopping the plane in every possible attitude and the needles centered back immediately. I didn't have anything even remotely as accurate when I was flying Cessna's. I can't wait to fly my own RV-9A. I even spent 4 hours debuuring wing ribs today.

JIm Wright RV-9A 90919 Arkansas