Using "IAS" for maintaining pitch in cruise flight

[porterjames]

My Dynon autopilot maintains altitude within plus or minus 20 feet. The cycle is up 20 feet and then down 20 feet. Each "up" cycle includes a loss of airspeed and RPM and a gain of airspeed and RPM going down. In very smooth air it isn't too noticeable but in turbulent air the changes in attitude and airspeed are much greater.
Quite by accident I discovered that using the "IAS" mode works great for use in either smooth or turbulent air in maintaining cruse altitude/pitch.
Once at cruise altitude set the altitude bug well above or below your desired altitude. Then select "IAS" just as you would if you wanted to climb or descend. Then set the airspeed bug to whatever your airplane will normally cruise at. Set your power at whatever you normally cruise at and then adjust the airspeed bug or power to maintain that altitude. Even in turbulence I was rarely more than within 200 feet of my desired altitude. Primarily due to me not paying attention over a 10 or 15 minute period. The gains or losses are small and mommetary. Most of the time altitude was within plus or minus 50 feet. The airspeed is maintained very accurately , much more so than during the plus or minus 20 feet using altitude hold.
The reason to set the altitude bug well above or below your desired cruise altitude is that if you ever intercept it during the ups or downs the autopilot will capture it and cancel the "IAS" mode and go to altitude hold instead.
Obviously you can still use the "NAV", "TRACK", or "Heading" mode in addition.
Something I wasn't able to determine is if this method results in a faster airspeed in cruise. I was more concerned about getting to my destination so I didn't do any comparison runs.

 

[Raymo]

Sounds like your static pressure may be fluctuating. You can try tweaking the pitch sensitivity and gain.

Start with gain set low; increase sensitivity until happy. If that is not enough, lower sensitivity and increase gain slowly.

As mentioned in other threads, gain is a multiplier of sensitivity. You should not have high gain with low sensitivity, IOW.

 

[porterjames]

The issue as I see it is not that the 20 feet altitude cycles are a problem. Trying to maintain altitude while hand flying with great precision in turbulence would result in the same increases and decreases in airspeed.
Just as in hand flying, the most efficient way to deal with turbulence is to accept the increases and decreases in altitude and simply attempt to hold the nose level. That's what the autopilot does in IAS mode. Looking at the downloaded profiles the graphs are obvious when comparing the difference between the two modes.

 

[Raymo]

Exactly. Altitude is directly related to static pressure. If it is fluctuating, the AP will try to maintain the same static pressure by increasing or decreasing altitude. In IAS mode, static pressure is not in play.

 

[porterjames]

Again, this is off the point but it seems to me that fluctuating pressure indications cannot be cured by an autopilot adjustment.
I flew the airplane again and made a more accurate calculation to see if flying in IAS mode is advantageous over the altitude hold mode. The altitude changes were in fact 10 feet above and 10 feet below the selected altitude. My previous 20 feet estimate was linked to the "20" being the next numeric above and "80" the next number below the "000". The indicator was actually between the two- 10 feet.
I found that the difference was a speed advantage of 1.8 Knots in using IAS to maintain cruise flight. Also there was a small fuel savings but I couldn't calculate it precisely.
The altitude excursions between the highest and lowest was about 150 feet.
Using altitude hold the altitude excursions between the highest and lowest was about 20 feet (10 up and 10 down). The TAS ranged from 83.9 kts to 95.6 kts.
Obviously lots of wasted energy used in trying to maintain altitude.
I am pretty happy with the function of the Dynon equipment. My previous Cirrus SR20's STEC autopilot had a similar 10' range of accuracy, but the combination of increased power and a constant speed propeller kept the differences less noticeable. Of course my STEC didn't have an IAS mode.
I can't imagine ever doing a long cross country again without using IAS as a primary cruise mode.
I suggest you try it.

 

[Raymo]

I use IAS for climb to altitude but the AP automatically stops the climb at the altitude I program in for cruise.

If adjustments do not correct your altitude excursions, I would look into a solution to correct the static pressure issue. Sometimes a small dam in front of the port helps stabilize the pressure. Maybe there is a forum for you aircraft where they have seen this issue before?

 

[porterjames]

Only fluctuation is from the rising and descending air columns that the airplane fly's through thereby causing the altimeter/ADAHRS to sense the change in pressure altitude and cause the autopilot to command a climb or descent.
The reason the airplane climbs and descends when in the IAS mode is that those same rising and descending columns of air cause the airplane to go up and down.
This is not a hardware malfunction or autopilot performance problem.

 

[Raymo]

You should be able to tune it to handle the rising/falling air. Mine holds very well but I typically cruise at 155-160 KTAS.

 

[porterjames]

There you go, that 5 knot difference is the autopilot climbing and descending to keep up with the fluctuations of updrafts and downdrafts.
Do you download your stored flight information from the Dynon and analyze it?

 

[65xx]

I fly x-country every summer and use the autopilot extensively. On one summer x-country some years back and not paying adequate attention to the panel, things didn't feel or sound right. A quick glance at the panel and it was obvious, the aircraft had enter a mild downdraft and the autopilot was trading A/S for ALT. I was down to 85 kts and slowing. I immediately added power and accelerated to normal cruise speed until I exited the downdraft and reduce to normal cruise power.
My point is this, would not IAS HOLD attempt to do the opposite and trade ALT, up or down, for IAS. I would assume altitude excursions of plus or minus 200' would generate a call from Approach or Center, if getting radar service. Not to detract from the above, but IMHO, Dynon AutoThrottle would be wondermous thing and first on the block. Dan from Reno

 

[Raymo]

There you go, that 5 knot difference is the autopilot climbing and descending to keep up with the fluctuations of updrafts and downdrafts.
Do you download your stored flight information from the Dynon and analyze it?

Sorry, no. That is the difference between 7.5 GPH or 8 GPH. Properly configured/tuned AP keeps it steady on altitude with very minor fluctuations to deal with up/down drafts, which it does well.

I will admit, it took me a number of XC flights to nail the tuning.

I have downloaded the logs but haven't started poking at them yet.

 

[porterjames]

When you do analyze the logs, I would be interested in the altitude hold accuracy- how many feet above and below your autopilot holds altitude. Also the TAS range while the autopilot is holding altitude. Particularly data recorded during some turbulence.