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nickpike

Using autothrottle to fix speed, using throttle position to set pitch

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Hi all,

 

Anyone know if this is possible?

I set autothrottle so the speed is controlled. I would then like to move the throttle and be able to read the movement. I know the sim var throttle position will remain fixed. Is there some way to read the throttle movemt so I can get this to control the pitch?

Just a thought.

 

Cheers

Nick

 

 

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It's not difficult Nick - I've done this recently. What I did was to create the throttle gauge so that the bitmaps are independant of the throttle_vars. Moving the throttles by x-percent increased the throttle_vars by x-percent and vice-versa. When the ATS is latched in, I read the throttle_vars on the system and apply them to the throttles. The result is that as the autothrottle system throttles up and down, the bitmaps follow it.

 

Hope that makes sense...

 

-Dai

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Hi Dai, I'm not sure I understand you. Maybe I could explain a little more.

 

I'm trying to do the following. On my Tristar, if you press IAS, it needs to fix the speed. If the pilot advances the throttles, the pitch increases to hold the speed constant and the aircraft climbs. Likewise, if the throttles are reduced, the aircraft descends with constant speed.
I have tried all sorts to get this to work. The ATS is useless to hold speed because this makes the physical throttles useless, that is, you move them but nothing happens as the throttle settings are controlled internally by the ATS. So I need to find a way to hold speed, but allow the throttles to be moved and affect the pitch value.

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And now you know why developers that have FLC-type autopilots use custom code in a .gau file. :wink:

 

There is no way to get the autothrottle to do as you desire, I can assure you of that.

 

A FLC-type autopilot requires lots of math to track and adjust the aircraft's pitch to hold the desired speed be it a value in knots or mach.

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You have two choices only

1. Set IAS on the AP moving the throttle will either increase or decrease rate of descent.

2. Select autothrottle and adjust IAS by adjusting the numbers in the AT.

 

That's the way a Tristar is flown! Autopilots of that era were different to the modern ones with full RNAV/VNAV control. Throttle adjustments were manual. That's why aircraft like the VC10 had a second set of throttles at the Flight Engineer's station, thus freeing up the pilots to do their stuff.

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Nick's problem also goes further. On a TriStar if the pitch exceeds eight degrees than the AP breaks out of pitch mode and starts climbing.

 

Nick - Ed is right. I also mis-interpreted your initial post. You are going to have to build a complete feedback loop that looks at your throttle position (not the throttle vars), your pitch and your airspeed. You then have to take the result and pass it back to the FS ATS and to your elevators to place the aircraft in the attitude you want.

 

I'm sorry I can't go further with this and supply code but I'm also working on an L1011-500, so I know it can be done.

 

-Dai

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Interesting. That is not how I understand IAS mode.

 

From the L1011 PRM: "IAS - Holds indicated airspeed existing at engagement by varying pitch of the aircraft and disengages auto throttles if in use."

 

I hardly use IAS mode in my Tristar as after selecting t/o thrust I climb to V2+10 watching both the flight director and ASI + bugs. Before climb thrust is set I use V/S and reduced the vertical speed (this can be done with or with out the autopilot engaged (if without ap follow the fd pitch)…..I normally engage the A/P at this point as I have three mens jobs to do)

 

In my 747-200 standard procedure would be (you could do the same for the Tristar) select IAS mode on reaching V2+10 and your flight director would show you whether to pitch up or down to hold said indicated speed.

 

Before climb thrust is set use V/S to reduce vertical speed ,on reaching 250 knots (below FL010) you can select IAS again. This would then adjust the pitch of the aircraft to hold 250knots at climb thrust.

 

On my Tristar at the moment I hit VNAV on reaching 250 knots as my VNAV is working very well and will automatically change the speeds from 250 to 320/330 and .83 as per altitude. 

 

So there is no auto throttle is use with IAS mode... just set the throttles (t/o, clb, idle thrust) and IAS will adjust the pitch of the aircraft to hold the indicated speed at the time of engagement.

note: In descent one may want to keep the engines above idle thrust (e.g. for anti-ice) so an increase in a manual input of engine thrust will result in a lower vertical speed if in IAS mode.

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There is a XML FLC gauge that was written by a guy called Bob available in the library here called LightLevelChange it's basic but works very well. I have use it modified on many aircraft for IAS controlled climbs and descents and VNAV. It comes with instructions on changing values for weights and speeds. Worth a look

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