Isn’t it called a trim tab?
It seems a bad idea to me, given an engine failure, to go head-in to find a rarely used autopilot setting. Then at some point have to remember to disengage it. The l/d curve isn’t so sharply inflected that precisely regulating your speed will buy you a big advantage in glide distance. Adjusting your speed for winds might.
Surely the risks are those of stall/spin at low level where you’ll probably want the autopilot off anyway. I gather heavy control forces and/or an annunciator help.
I think any autopilot which can do both axes allowing lateral to be not selected at all would be a very bad idea. Easy to forget that a lateral mode was unselected and commence a gradual roll while trim goes crazy. Why would anyone want this?
kwlf,
the newer digital autopilots will hold the speed very precisely and on the DFC90 the setting is very simple: Press IAS and turn the knob to Vbg … if it was not already set.
Also these autopilots will never stall the airplane, because of their “envelope protection” features.
As Ultramad has already stated the PSS60 add on does this. I have it and he is right.
So why keep asking the same question, is there one out there?The question was answered.
Set up VS for whatever airspeed you want and it will do that ie. hold airspeed similar to if you trim the airplane manually. It does not however do IAS where you punch in a number.
As Ultramad has already stated the PSS60 add on does this. I have it and he is right.
However, if the PSS60 is part of an S-TEC autopilot system (60-1 or 60-2), then you cannot engage a vertical mode without also engaging a lateral mode. I believe the statement is still true: an autopilot capable of lateral and vertical modes will never let you engage a vertical mode without also engaging a lateral mode.
IMHO IAS hold is of limited use in GA pistons, due to their low performance. And if you set an IAS low enough to “last” in a climb to a reasonable altitude, say FL150, say 95kt, you will overheat the engine low down because 95kt is far too slow for the power then being developed (IO540 case).
Not true in a turbocharged piston Peter. I always used it in the Mirage for the climbs at best rate then cruise climb.
I assume it is equally useful in an SR22.
And you can always use cruise climb speeds down low then lower it for performance as you get higher.
Certainly not Peter. IAS hold is one of the best things and I use it all the time for climbs. It’s the only climb mode that works, VS is too unstable with a bit of weather and our low performance aircraft. A VS control loop is far less stable than an IAS control loop which is rock solid. For descents I use VS. Both modes require manual throttle operation unless you have an electronic wastegate such as the SR22T.
I basically do my climbs by adjusting the IAS knob to yield the desired CHTs because — as with pretty much all high performance piston planes — climbs are CHT limited.
I never do climbs without IAS mode. Even in the SR22 NA it works perfectly.
If the temps go up I just dial in a higher speed but it’s almost never necessary.
Like Achim I use IAS for climbs and VS for descents, with the Vnav feature of the 430.
However, if the PSS60 is part of an S-TEC autopilot system (60-1 or 60-2), then you cannot engage a vertical mode without also engaging a lateral mode.
It often isn’t, it’s often used as an altitude retrofit for a completely different lateral only autopilot. In all of these installations I’ve seen it is possible do engage each autopilot separately.
Not that I consider this an advantage, really.
