Stick flutter on VNAV approach

[Andy A]

I've been playing around with the VNAV approach option on the Skyview recently.  It's pretty cool. It will intercept a faux "glideslope" and provide vertical and lateral navigation from the final approach fox all the way to the runway.  Today when I started my descent down the glideslope I noticed stick flutter and porpoising at an airspeed of about 115 knots indicated.  It feels like an autopilot issue to me.  I slowed down to 100 knots and - 500 fpm rate and it went away.  Could it just be a trim problem?  Any recommendations? 

[coppercity]

Might also check if there are airspeed limits set for the pitch loop.  Most have an upper and lower airspeed limit, maybe you were bumping up against that upper limit.

[Andy A]

Thaks for all the advice.  I'll see if any of these items help.  I agree 115 knots is probably too fast if I was actually doing a real rnav approach, but when I am out playing around it sure makes the distance go by faster.  The autopilot is the only thing that seems a little finicky to me on this plane.  The Cessna 310 we previously has was rock solid on autopilot at any speed I used.  

[FlyingMonkey]

My AP has a lower limit set to 60kt, but if I get below about 70kt it gets wallowy and wanders around the course line.  You could have a similar issue at the upper end; there may be some AoA or force limits above/below which the AP has trouble interpreting what's going on.

[Andy A]

I spoke with the guys at Dynon.  They think I need to adjust the Pitch Sensitivity srtting on the autopilot.  The range is 0 to 24.  The default is 10.  They said to increase it to 15 and see how it does. 

[Andrew Lane]

You mentioned a VNAV approach option, is this within your skyview map menus? Or is this through a connected garmin nav device?

[Andy A]

VNAV is available on the Dynon Skyview if you are in the expert autopilot mode. 

[WmInce]

36 minutes ago, Andy A said:

VNAV is available on the Dynon Skyview if you are in the expert autopilot mode. 

What exactly do you mean by "VNAV?"

Are you talking about it's use with a certified IFR GPS navigator?

 

[Andrew Lane]

3 hours ago, WmInce said:

What exactly do you mean by "VNAV?"

Are you talking about it's use with a certified IFR GPS navigator?

 

That’s my question too...I’ve heard that SkyView can follow vnav paths from an IFR certified god navigator (like a 430w or the new 175, 355, 375) but does it have any sort of VNAV capability on its own? 

[Andy A]

The skyview will basically do a "watered down" GPS approach.  You have to set the glideslope angle (most are about 3 degrees) and a minimum descent altitude and the autopilot will provide vertical guidance to the runway.  When selecting the destination, first select the airport, then select the runway.  My airport is KMAO and the runways are 4 and 22.  Once you select the runway, the direct to location will be .22 if you are using runway 22.  The skyview will provide lateral guidance to the runway heading and vertical guidance based on the glideslope info.  To get this to work, you must be established on the final approach heading before getting to the RNAV Final Approach Fix.  Its not approved for IMC, but it sure does a better job of following the approach than the IFR Certified Cessna 310 that we used to have.  Just search "Skyview ILS" in YouTube.

This approach can be done with the skyview system only.  It does not have to come from an IFR certified GPS.   To legally do an approach in IMC conditions, you would need the IFR certified GPS, but if you are just out playing around, the skyview does a pretty good job.