Quote from Hangar45 Admin on December 14, 2017, 8:21 am

(Original thread started on 02-06-10 by Rand Mathews)

In the real Lear45, does the speed knob on the FGC double as the VS encoder also? I noticed that there is a VS button with a line pointing to the speed knob on the FGC so I'm guessing that this swaps it's function. Is this correct?

 

I went through the FSUIPC offsets and haven't found anything that links to that function yet. Has anyone come across this and has the correct offset?

 

(Posted by Alaxus on 02-07-10)

I am still scratching my head about this issue too. As far as I know the only way is through software, since I am using Phidgets. Eric, if you have some good ideas on this, I would be interested.

 

(Posted by Eric Tomlin on 02-08-10)

A while back I was asked to provide some detailed insight into airspeed management of the Learjet 45/40 as well as using IAS vs. Mach in the Learjet.

 

First, let’s have a good understanding of how the real aircraft works:

The Learjet 45 has airliner style avionic features, but has no airliner style auto throttle. Many folks are confused (including myself initially) when they see that the LJ45 has a SPEED function but learn that in the real aircraft it actually doesn’t control the throttle to maintain a dialed-in speed in the speed window. On the speed tape, once the aircraft exceeds mach .40 the speed window automatically changes from KIAS to mach and once that IAS window's value drops below mach .40 it reverts back to KIAS. Knowing this, then why would Learjet/Honeywell put a SPD knob on the FGC and allow the crew to adjust the value on top of the speed tape as a target speed? They put that ability there to control the aircraft's forward speed during climbs and descents, not for level flight. This gives the crew more options for maintaining airspeed for the climbs and descents over just dialing in a Vertical Speed (which as you know controls how many feet you climb or descend PER MINUTE, not how fast you move FORWARD per nautical mile). Let me give some examples.

 

A) The crew is cleared for takeoff and assigned 11,000' as their initial altitude. Naturally they want to get up past 10,000 feet as soon as possible so they can get above the airspeed restriction of maintaining 250 KIAS or less below 10,000'. Since they are able to climb like a bat outta you know where, they dial in a 5,000 FPM climb using the VS knob and the airplane gets them to 11,000' in 2 minutes or so. Once the aircraft levels off at the initial assigned altitude of 11k, they will have to retard the throttle back to a lower detent if they want to keep their airspeed manageable and below 330 KIAS (the max KIAS the craft is certified for). This is an example of using the VS hold button to hold a target vertical speed.

 

B) The crew has now been cleared from their initial altitude assignment of 11,000 feet to FL210. They want the airplane to climb at the prescribed best climb speed of 270 KIAS, so now they hit the SPD button and use the SPD selector to dial in 270 on the target window. Here's where the function of the SPD control comes into play to control the aircraft's speed. Upon dialing in the desired SPD value, the nose begins to lower so that the current level of thrust selected (which would be MCT, or Max Continuous Thrust) is capable of pushing the plane to the desire forward airspeed. Eventually the plane reaches its final cruising altitude and the thrust is reduced to the MCR (Max Cruise Thrust) detent. At this point, the aircraft will likely fly somewhere around mach .77 and .81 depending on several factors. If the aircraft begins to get close to an over speed situation then the crew can pull the throttle back to the next lower detent. However, note that in the real aircraft the spaces between each detent are not 'active' thrust zones; only the detent provides a set power based on ambient conditions. The space between MCR and APR in the TQ range are only active at the detents due to the DEEC calculations (which we don’t have at this time) but the range between IDLE and MCR is an active range and responds to where ever the crew places it (for taxi and approach/landing).

 

C) It's time for decent- you are not limited by any speed restrictions legally, but only by the aircraft's 330 KIAS max. You dial in 4,000' FPM decent and make sure the aircraft limitations are kept safe. Likely you will need to use Spoilers to keep the speed managed safely.

 

D) You are now closing in on 10,000' and must stay below 250 KIAS (at least here in the U.S.). You dial up 240 KIAS on the SPD dial and hit the SPD button. The aircraft's nose now pitches up enough to bleed off the extra speed you had from your 4,000' FPM decent. As the aircraft slows to the dialed in 240 KIAS the nose then pitches forward again to maintain the target speed. When you dial in 200 KIAS on the SPD dial, the nose will pitch up again in the decent to slow the airplane.

 

Hopefully this explains how speed is controlled in the LJ45 using the autopilot and no auto throttle available.

 

Making this work in our simulators-

How do we accomplish the SPD hold function in our simulators? The only way to do this is with a custom written Flight Guidance Controller software. This is something that Jason and I have on our To-Do list in conjunction with the FMS/CDU software because there are some elements that go hand in hand. Basically, if you can write a program (custom autopilot SPD hold) to cause the nose to pitch up or down to maintain a desired forward speed, you have the function available.

 

Hardware-

The 2nd challenge is that you must then make the hardware control the function properly by having the SPD hold button cause the SPD rotary encoder to control the SPD target window, but then have the same SPD rotary encoder functions to control Vertical Speed target values when you press the VS hold button. Pressing the SPD encoder should cause the current speed window alternate between KIAS and mach.

 

So in short this is something we can do with software (and I believe EagleSoft has done this with the Citation X v. 2 that was recently released) but it should be noted that I've never needed to use the SPD hold function. I simply set my target altitude, begin the climb or descent, and then manage my airspeed manually using a combination of the VS function and the throttle. Also, while it's not realistic to use the speed hold (which is using AutoThrottle) already built in to the default LJ, it is there for folks that don't care about that aspect of realism. However, I like the challenge of monitoring my airspeed and never use autothrottle in the LJ.

 

(Posted by DonnyRay Jones on 02-09-10)

Eric's discussion of how the system operates is excellent, but a little reminder on flight dynamics might be useful here: Thrust controls ALTITUDE, whereas PITCH controls airspeed.

 

If you hold a given pitch attitude constant, increasing the thrust will increase your altitude. If you hold thrust constant, changing pitch will change your airspeed. Yes it does.........

 

(Posted by Vince C. on 02-09-10)

Excellent explanation both Eric and DonnyRay. I don't think the hardware part is going to be difficult, because in my opinion, it is not a hardware related matter at all. It will be the software that recognize the VS status and then will recognize changes made on the encoder as VS changes and vice versa for altitude if not selected.

 

(Posted by Alaxus on 02-10-09)

I posted a message on the FSUIPC forum about this issue and a consensus was that it could be handled by FSUIPC, though I am still yet to figure it out. It could be that they didn't fully comprehend what I was asking. Any one here that has had a real good look at the macro functionality of FSUIPC including LUA scripts? This may be an avenue to look at.

 

I have just been playing around with the latest version of fs2phidgets (5.0.15). So far its looking like I might be able to do just this. I will let you know how it goes.

 

UPDATE:

With Alan's help from fs2phidgets, we have managed to get 1 encoder to drive the 2 offsets based on the AP mode.

 

(Posted by Eric Tomlin on 02-22-10)

Ah, very nice! That's awesome. Now, it seems to me that if that's possible with phidgets it should be possible other ways too!

 

(Posted by Ron Rollo on 08-16-17)

Hey Shane, this thread hits on your question about the single FGC SPD encoder controlling both the SPD function button and the VS function button. According to Alaxus back in 2010, he has figured out a way to get this dual function to work using a single encoder with software logic. We need to see if he can elaborate on this.

 

I do believe it will be addressed with software logic, not hardware which means no changes will be needed with the Project45 FGC design. Hopefully the future Flightdecksoft Jet45 FGC package will address this, if not LUA scripts or SOIC logic will take care of it. It can certainly be done!