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Bob Axsom

Well Known Member
I fly our RV-6A in Cross country air races that have some extremely tight turns as well as some mild ones. Before a race I do a lot of preparation work and have a race checklist with a page for each leg. One of the key items is the heading outbound after the turn at the end of the current leg. The objective is to stand it on it's ear and get moving in the new direction as precisely and as quickly as possible (high speed in the wrong direction is worse than inefficient). Anyway I had a RC Allen DG that was drifting badly so after living with it for a while I bought a new Sigma-Tek DG and the function was so good it was near perfect. OK get the picture?

I raced in the Rocket 100 at Taylor Texas last month with my nearly new DG. Turns 5 and 6 were less than a mile apart and each one was on the order of 90 degrees. Because of the wind I planned very high rate of descent between these two turns in order to come out under the headwind on the way to turn 7. I was really relying on the DG for the rollout because the Pictorial Pilot heading display goes blank even on standard rate turns and these are way beyond standard rate (very high bank and using elevator to bring it around). Here is the point of all this lead-in: As I am making turn 6 the DG looses its stability (tumbles?) and I just have to guess at the rollout point (which turned out to be an undershoot) and the DG was indicating some random heading after I was cruising level to turn 7. I reset it of course but I had this feeling that it was not performing as well as it had before. Last week I had to put the plane in for the 2 year static system and encoder check for IFR operations. After it came back I made a test flight of 0.7 hours and it seemed to me that the DG was drifting similar to the old RC Allen unit.

Is this a normal failure mode for this kind of operation? What is the actual internal failure? What can be done to prevent recurrence while retaining a vacuum driven DG?

Bob Axsom
 
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High G loading

Continual g loading is hard on any gyro. Perhaps consider a digital like the RC allen, or keep the vac system for your IFR cert, but add a quick panel like an Extra 300 to quickly remove the AI and DG. In their place(also on a matching quick panel you might carry in a gym bag for racing) consider an MGL exteme EFIS with a lightning fast 50 channel GPS or a Dynon with a digital heading and solid state inertial sensors., or perhaps an Aspen EFIS like they put in the Extra 300 touring model. Try your thread in the glass panel section for fresh ideas there.
 
I sent an e-mail to Sigma-Tek

I sent an e-mail to Sigma-Tek to get their opinion on this. I will post their response when received if possible.

It has been suggested that this would be a better fit in the "Steam Gauges" section. Management do as you wish with this.

Bob Axsom
 
Called Sigma-Tek today

I sent an e-mail to Sigma-Tek earlier and there has been no response yet. This afternoon I called the company and after explaining the problem/questions to the lady I was forwarded to her by the operator, I was told that Jim Hearn is the cognizant engineer and he is out today. He should be in tomorrow and he will be the one that answers the e-mail. More to follow...

Comment: I am surprised that there has been so little response here.

Bob Axsom
 
Bob Axsom said:
I sent an e-mail to Sigma-Tek earlier and there has been no response yet. This afternoon I called the company and after explaining the problem/questions to the lady I was forwarded to her by the operator, I was told that Jim Hearn is the cognizant engineer and he is out today. He should be in tomorrow and he will be the one that answers the e-mail. More to follow...

Comment: I am surprised that there has been so little response here.

Bob Axsom
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Bob, I suspect the lack of response is due to mechanical gyros being pretty much off the radar of most RVers these days. They are rarely installed or even considered anymore since few have any interest in a vacuum system in their planes. The new digital avionics offer so many advantages over mechanical....so much so they are no longer considered experimental.

Hope you have success getting your instruments serviced, but at some point you may want to seriously consider going digital. You will never look back. :)
 
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I am thinking about it

One exposure that I have is a digital readout on the Pictorial Pilot. Even in a standard rate turn the display blanks until I settle out on a heading. I need something that is analog, continuous and preferably not dependent on an electrical system. I've had total electrical failures in hard IFR - not good. But, I may have to move in that direction.

Bob Axsom
 
Bob Axsom said:
One exposure that I have is a digital readout on the Pictorial Pilot. Even in a standard rate turn the display blanks until I settle out on a heading. I need something that is analog, continuous and preferably not dependent on an electrical system. I've had total electrical failures in hard IFR - not good. But, I may have to move in that direction.

Bob Axsom
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Bob,

The Dynon does not blank out even during steep turns. This little puppy (with built-in backup battery) would be a great addition to your panel:

http://dynonavionics.com/docs/D6_intro.html

Not only would you have a turn and heading indicator that works all the time with no error or precession, but a complete backup 6-pack!

Come on, join the 21st century. ;)
 
Sam You Have My Attention

I just spent a large amount of time, effort and money modifying my panel to accommodate an SL-30 and indicator while retaining my old Terra VOR/LOC/GS. It pains me to go through that again but your reference is impressive for price, size and function because of your personal recommendation. I will study it.

IMG_4279.jpg


Bob Axsom
 
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Bob,

Thread highjack!!!!!!!! Maybe I should post somewhere else or PM you....

Have you ever thought about winglets for the 6A?
 
Bob Axsom said:
I just spent a large amount of time, effort and money modifying my panel to accommodate an SL-30 and indicator while retaining my old Terra VOR/LOC/GS. It pains me to go through that again but your reference is impressive for price, size and function because of your personal recommendation. I will study it.



Bob Axsom
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Bob, just replace either your TC, attitude indicator, or VSI with the Dynon, it is sized and drilled to fit a standard 3 1/8" instrument hole. It most likely will slide right in without having to mod your panel. Hookup is simple--tee into your pitot and static lines, and connect a simple connector. The biggest time investment will be installing an external magnetometer in the tail of your plane and running the cable to it. The device has a magnetometer built into it but you will probably need the external magnetometer to get best performance. Lots and lots of info on this install in the forum archives and on Dynon's forum. I suggest you also purchase the optional wiring harness because it will save you time and effort making the the installation almost plug-n-play. :)
 
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I'm a pretty good wire and plumbing guy

I'm a pretty good wire and plumbing guy - that kind of thing is easy for me. The flange footprint may cause some problems but my first thought would be to install it as a direct replacement for the DG. Mounting the box in the tail is a piece of cake as is running the wires. The part that requires thought and planning is the installation in the panel including all of the stuff I have in back of it.

Hijacker - Winglets just look like more drag potential to me. I have tried a set of flat metal tips which were not as fast as the 3" streamlined tips I developed but I do plan to reinstall them and mount some test tip plates in the coming year. A simple winglet configuration could be installed as well. I don't expect much from the effort except knowledge.

Bob Axsom
 
I think the D6 is a good choice for you as Sam has suggested. We have one in mind for our biplane.

I saw a special on an airline last week that really bragged about the better performance offered by winglets on their airliners. They may be effective only at higher speeds....I don't know.

Good luck with the D6. I have the Dynon Flightdek 180 in my RV. Couldn't be happier and no suction required.
 
Bob Axsom said:
I'm a pretty good wire and plumbing guy - that kind of thing is easy for me. The flange footprint may cause some problems but my first thought would be to install it as a direct replacement for the DG. Mounting the box in the tail is a piece of cake as is running the wires. The part that requires thought and planning is the installation in the panel including all of the stuff I have in back of it.

Hijacker - Winglets just look like more drag potential to me. I have tried a set of flat metal tips which were not as fast as the 3" streamlined tips I developed but I do plan to reinstall them and mount some test tip plates in the coming year. A simple winglet configuration could be installed as well. I don't expect much from the effort except knowledge.

Bob Axsom
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Bob,

Download the product brochure and print out the actual size photo of the front of the D6. Use it to determine if it fits your panel.

http://dynonavionics.com/downloads/Brochures/Mini_Postcard_Catalog_Web.pdf

Since you are thinking in terms of a Dynon, be sure you check out the D10A before making a final decision:

http://dynonavionics.com/docs/D10A_intro.html

I find the TAS, winds aloft, and density altitude features to be worth the difference in price. You would probably like the HSI as well.

No, I'm not getting a commission....... :D
 
Bob Axsom said:
One exposure that I have is a digital readout on the Pictorial Pilot. Even in a standard rate turn the display blanks until I settle out on a heading. I need something that is analog, continuous and preferably not dependent on an electrical system. I've had total electrical failures in hard IFR - not good. But, I may have to move in that direction.

Bob Axsom
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I guess I'm not quite sure what you mean by "blanks". Are you speaking about the display limit of the horizon mask or the DG window?
 
Three dashes until I roll out of the turn

TrutrakTech said:
I guess I'm not quite sure what you mean by "blanks". Are you speaking about the display limit of the horizon mask or the DG window?
Click to expand...

When I roll into a significant turn the digital readout changes to three dashes instead of a three digit number until I roll out of the turn then it quickly tells me which way I'm going again - very good but unsuitable as a rollout indicator in low altitude high speed racing. Since my old RC Allen DG was drifting badly at all times I decided to "upgrade" to a 4000B Sigma-Tek DG. It "burped" during a critical part of the race last month in Taylor, Texas and now brand new it seems to be drifting similar to the RC Allen DG did after several hundred (500+) hours. My thinking is this is a typical failure mode for mechanical gyros subjected to racing forces - although I don't have confirmation of that from Sigma-Tek yet. Probably nothing basically wrong with anything for normal flight environment but I need a solution for my operating environment.

Bob Axsom
 
Bob Axsom said:
When I roll into a significant turn the digital readout changes to three dashes instead of a three digit number until I roll out of the turn then it quickly tells me which way I'm going again - very good but unsuitable as a rollout indicator in low altitude high speed racing. Since my old RC Allen DG was drifting badly at all times I decided to "upgrade" to a 4000B Sigma-Tek DG. It "burped" during a critical part of the race last month in Taylor, Texas and now brand new it seems to be drifting similar to the RC Allen DG did after several hundred (500+) hours. My thinking is this is a typical failure mode for mechanical gyros subjected to racing forces - although I don't have confirmation of that from Sigma-Tek yet. Probably nothing basically wrong with anything for normal flight environment but I need a solution for my operating environment.

Bob Axsom
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More than likely what is happening is you are temporarily losing your GPS signal due to the orientation of your aircraft and the location of your antenna. There is no limitation of the device for not displaying the ground track in a high bank. The Pictorial Pilot does not have a magnetometer in it for a backup heading indicator, therefore you'd see the three dashes indicating no GPS signal and the autopilot is disengaged.
 
Bob Axsom said:
Comment: I am surprised that there has been so little response here.

Bob Axsom
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Bob:

I also have an RC Allen electric Artificial Horizon (and the DG). It has never worked all that well (good enough for VFR) in the five years I've had it, but lately it is also starting to tumble when I do a steep turn.

I'll be interested in what you find out as I'm trying to decide what to do about repair or replace with something else. I have been thinking of the TruTrak Gemini unit when it comes out next year. For about a thousand dollars the basic unit should provide the AH and DG function in a single unit and it fits the same hole (3 1/8) as the RC Allen unit. NOTE: It costs about that much just to have RC look at their unit.

Of course, once you start changing panel items, you start thinking of other upgrades and suddenly you're talking real money.

One concern I have is that some of the digital horizons are really rate instruments and not true artificial horizons but for VFR flying it shouldn't make a critical difference. You might consider the TruTrak unit in your search.

Please post what you find out.

Regards,

Chris
 
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Bob - I've been reading your posts for awhile now listening to how you tweak every detail in order to gain just a fraction of a knot.

After seeing your panel - which looks very nice BTW - I think you might have at least a knot stored in the weight of all those gauges and wiring.

Upgrade to a Dynon Skyview and a Vertical Power VP-50 and you'll have all the functions you need with considerably less weight.

Just a thought :)

Ken
 
Heading ref

RV max roll rate is about 140 deg/sec IIRC -? slightly higher at race speeds . I think this is within the normal rate limits for attitude for D-10A, 6 etc. Suspect hdg limits are similar but don't know lag figures; bet Dynon could give u good info on this. If lag were consistent you could probably anticipate with your skills. Let us know how this works out for you. Bill
 
I don't know the cause but it happens every time

TrutrakTech said:
More than likely what is happening is you are temporarily losing your GPS signal due to the orientation of your aircraft and the location of your antenna. There is no limitation of the device for not displaying the ground track in a high bank. The Pictorial Pilot does not have a magnetometer in it for a backup heading indicator, therefore you'd see the three dashes indicating no GPS signal and the autopilot is disengaged.
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The antenna is an Apollo SL-60 antenna that is mounted on top of the fuselage externally. It happens every time I make a real turn. I do not believe it is GPS loss of signal given this information. I do not know the processing that is done in the SL-60 upstream of the serial data output going to the Pictorial Pilot. It is reasonable to assume that the heading data fields in the output are invalid during the turn and the Pictorial Pilot is not receiving heading data to display so it goes to the three dashes. I have never seen the format of the SL-60 output - I assume it is in packets with headers, trailers, checksums, etc. This clearly is not an antenna blanking or loss of signal problem. As you indicate, it is not a Pictorial Pilot problem either but it is at least a small inconvenience for me and something I have to allow for (read find another means for determining instantaneous heading) to be competitive.

Bob Axsom
 
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The Story from Sigma-Tek

I talked to Jim Hearn, Engineer at Sigma-Tek and we talked very candidly about my situation and application. I took notes but I could have missed or misunderstood some things - anyway here is my progress report:

- Somewhere between 55 and 60 degrees in the 4000B directional gyro, one of the gimbals comes up against a hard stop where a condition called Gimbal lock occurs and the Gyro will tumble. This is not immediately fatal but it will eventually cause mechanical gyros to fail as indicated by increasing drift rates due to mass shift balance and/or increased friction in the gimbal bearings. The gyro speed is 22,000 rpm. The operational drift tolerance is 4 degrees in 10 minutes. In production the objective is zero drift but the reality of cost and income sets the acceptance limit at 2 degrees in 10 minutes. The technology exists for 360 degree freedom of roll as in jets but the cost is prohibitive for general aviation.

Test method suggested by Jim is this:

- Start up the plane and allow the gyro to come up to equilibrium speed (say 10 minutes) then taxi out to the runway, line up and set the gyro to an exact index mark on the compass card. Takeoff and fly twice around the pattern with nothing more than standard rate turns, then land, line up exactly the same way as in the before take off check and recheck the indicated heading. If the drift is more than 4 degrees, the DG is degraded and needs to be serviced.

I am to call him afterward with the results and I will report back here.

Bob Axsom
 
I did the flight test

The drift after two large standard rate turn patterns the drift was 3 degrees. That is within the operational tolerance Jim Hearn specified.

Bob Axsom
 
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