Lear 35 SPD mode?

[ryanbatc 12,774]

How to set the airspeed in SPD pitch mode with the autopilot?  Or does it just climb at current airspeed?

[ark 1,288]

26 minutes ago, ryanbatcund said:

How to set the airspeed in SPD pitch mode with the autopilot?  Or does it just climb at current airspeed?

I think it is helpful to understand the basics of how the AP speed (SPD) mode works. Since there is no autothrottle function in the Lear35, the only way the AP can control climb or descent speed is by using the elevator to control pitch and thus the vertical speed (VS). When the SPD button is engaged, the AP notes the current VS which it now uses as the reference VS for holding the target IAS. If the IAS should increase above the target IAS, the AP increases the VS by adding an appropriate increment to the reference VS. This now becomes the new reference VS. This causes pitch to increase and IAS to drop since the thrust (throttle position) has not changed. Likewise, if the IAS drops below the target IAS, the AP subtracts an appropriate increment from the reference VS. This now becomes the new reference VS. This causes the pitch to decrease and the IAS to increase.  So it is very important that the initial reference VS be 'valid'  -- that is, the climbing or descending a/c should be stable and in trim at the target IAS with the throttle set when the AP SPD mode is engaged. If this is not the case, the reference VS is not valid, and the AP continually 'hunts' to find a VS that results in the target IAS. It is recommended and most common (and safer) to use the AP SPD mode when climbing, and the Vertical Speed (V/S) mode when descending.

Al

Edited March 17, 2019 by ark

[ryanbatc 12,774]

I understand how FLC/IAS mode works.  This plane is a little different in that there isn't a set speed for the autopilot to pitch for.  I fully understand there is no auto throttle in the plane though.  So basically it just pitches for current speed you're saying.  If I increase power it should nose up and decrease should nose down - correct?  VS mode would be separate and would lock a current vertical speed depending on what pitch I was at when I engage VS mode,  It would not change regardless of my power setting, unlike SPD mode.

[ark 1,288]

3 minutes ago, ryanbatcund said:

I understand how FLC/IAS mode works.  This plane is a little different in that there isn't a set speed for the autopilot to pitch for.  I fully understand there is no auto throttle in the plane though.  So basically it just pitches for current speed you're saying.  If I increase power it should nose up and decrease should nose down - correct?  VS mode would be separate and would lock a current vertical speed depending on what pitch I was at when I engage VS mode,  It would not change regardless of my power setting, unlike SPD mode.

Correct. 

In SPD mode the pitch will adjust to try and maintain what the IAS was at the time you engaged the mode.

In VS mode the pitch will adjust to maintain what the VS was at the time you engaged the mode. If necessary the a/c will pitch up to maintain the VS and if the throttle is set, the IAS will drop -- possibly leading to a stall which is one reason why the AP VS mode is typically not used in climbs.

Al

[ryanbatc 12,774]

Excellent.

[bpcw001 22]

I might add that while using IAS hold is the preferred mode in climb, V/S for climbing can be mandated in strong gusty and changing winds. In such conditions, IAS hold mode can lead to huge pitch oscillations resulting in bobbing motions as the aircraft chases IAS. Pretty uncomfortable for pax and pilots alike.

Edited March 22, 2019 by bpcw001

[rightseat 277]

I'd also throw in that you should try to stabilize the aircraft near the speed you want to climb at, and only then engage SPD mode. Otherwise you should smell simulated puke coming from the cabin... 😉

If you engage SPD mode as the Lear is accelerating at a fair clip, you can get pretty big oscillations until it stabilizes, so I try to nail my desired speed with VS mode and then set SPD for the rest of the climb. 

🙂