What is the preferred elevator & throttle control laws for Fly By Wire
Poll - how do you want FBW to work?
Jason Short came up with a great flight mode, FLY BY WIRE, in ArduPilot 2.5. This mode differs from STABILIZE. In STABILIZE the autopilot constantly tries to return the plane to 0 pitch and roll, while you give stick inputs that tell the plane to do something else. It feels a bit like you are arm wrestling against the autopilot sometimes. With FLY BY WIRE if you move the right(*) stick half way to the right then the plane will bank half of the maximum amount defined. As long as you hold that stick position the plane will hold that bank. If you center the stick, the plane will go to zero bank.
* - All references to right stick assume a mode 2 RC setup where the right stick controls pitch and roll.
That is all great! You are not arm wrestling the autopilot. You are telling it what roll you want and it is giving you exactly that roll.
But what about throttle and elevator? Jason had a scheme where if you moved the right stick then you would get both a pitch and throttle response. In 2.5.1 I have been trying an alternate scheme involving both pitch and throttle increasing or decreasing together. I am convinced that these schemes are not the best. But what is? It really comes down to user preference.
I think the two best alternatives are:
I can't seem to get the poll to embed - please go here
Jason Short came up with a great flight mode, FLY BY WIRE, in ArduPilot 2.5. This mode differs from STABILIZE. In STABILIZE the autopilot constantly tries to return the plane to 0 pitch and roll, while you give stick inputs that tell the plane to do something else. It feels a bit like you are arm wrestling against the autopilot sometimes. With FLY BY WIRE if you move the right(*) stick half way to the right then the plane will bank half of the maximum amount defined. As long as you hold that stick position the plane will hold that bank. If you center the stick, the plane will go to zero bank.
* - All references to right stick assume a mode 2 RC setup where the right stick controls pitch and roll.
That is all great! You are not arm wrestling the autopilot. You are telling it what roll you want and it is giving you exactly that roll.
But what about throttle and elevator? Jason had a scheme where if you moved the right stick then you would get both a pitch and throttle response. In 2.5.1 I have been trying an alternate scheme involving both pitch and throttle increasing or decreasing together. I am convinced that these schemes are not the best. But what is? It really comes down to user preference.
I think the two best alternatives are:
- Have the throttle remain under manual control and have the right stick up and down control pitch angle. This setup is great for tuning your servo PID gains (both roll and pitch), and is a really fun way to fly.
- Have the elevator maintain a constant airspeed and have moving the right stick up and down increase or decrease throttle from a cruising throttle. This setup is probably the ideal setup for new pilots. You can hand the transmitter to anyone and they can fly. Can't stall because airspeed is controlled. Want to turn, move the stick to the side. Want to go up or down, move the stick up or down. Of course they still have to deal with what up and down is and what left and right are while coming and going, but if you let go the plane will always take care of itself.
I can't seem to get the poll to embed - please go here
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Comment by Ryan on March 21, 2010 at 6:52pm -
Throttle control altitude, and elevator control speed
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Comment by Ryan on March 21, 2010 at 6:55pm
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Also, as you said - aileron stick control the bank angle preferably with rudder mix in (5% - 10%) for coordinating the turn
Aircraft should go back to neutral flight when all stick positions are leased.
We generaly call this mode "stearing mode"
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Comment by Ryan on March 21, 2010 at 6:57pm
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I also recommend limiting the bank angle to 30 to 35 degrees only. This will help greatly in controling the altitude hunting (airplane goes up and down all the time to maintain pitch and altitude when the bank angle is too steep)
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Comment by Doug Weibel on March 21, 2010 at 6:58pm -
Currently the firmware is 3 channel, so no rudder mix. When we get to ArduPilot Mega we will do that. However, it won't be a mix, it will be coordinated turns based on IMU accel vector.... Better than a mix ;)
Please link through to the poll. Link is above.
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Comment by Doug Weibel on March 21, 2010 at 7:01pm
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Max bank angle is a user parameter. I agree that 30 to 35 is a good limit in most cases. Max pitch (up and down separately) are also user parameters.
With an appropriate airframe and good tuning you don't have to limit to shallow turns. I am currently running 60 degree max bank. Getting things tuned for such aggressive turns is not easy, but can be done.
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Comment by Ryan on March 21, 2010 at 7:07pm
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That's even better!
IMO, real fly-by-wire is when all control remain the same as in full manual mode (rudder, throttle, aileron, and elevator) while the autopilot works to maintain the aircraft stability. Such as when you bank the plane 30 degree Left and let the stick back to center, the AP will maintain the 30 bank angle untill you command a Right bank to return the plane to neutral.
During this time, all control surfaces are working to maintain airspeed and attitude in level Left turning flight until the pilot apply additional command such as Pitch Up which means the aircraft should now be in a Climbing Left Turn flight condition, and etc...
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Comment by Doug Weibel on March 21, 2010 at 7:23pm
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I hear what you are saying about "real" fly by wire. However that scheme is not as beginner friendly. IMO it is better to have the plane go to 0 pitch and bank when you let go of the stick. If you want to maintain 30 degree bank you just hold the stick over that amount..... It is really nice to fly that way and not have to make constant adjustments....
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Comment by Dave Whittington on March 21, 2010 at 7:48pm -
Just for an alternative idea, airliner fly by wire modes would include a mode where the pitch axis stick would command a climb/descent rate (feet per minute) where center stick is zero vertical velocity - not really altitude hold but close. A little more complicated version would be the command of flight path angle - similar to the above but really works great with FPV+HUD so that a velocity vector (flight path angle) symbol can be placed over an object if you want the aircraft to aim directly for the object - particularly good for the arresting wire on an aircraft carrier. This makes a nice harmony with the bank stick being a commanded roll(bank) angle which indirectly causes a paticular heading rate of change (at a particular airspeed). This combination is generally thought of as the "perfect" airplane - even with manual throttle. Of course, even more perfect is throttle being a commanded airspeed.
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Comment by Doug Weibel on March 21, 2010 at 7:57pm
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I had been thinking about the throttle stick being commanded airspeed.... I like the idea. It is a bit more advanced, but has possibilities
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Comment by bGatti on March 21, 2010 at 9:35pm -
I Think Dave Whittington is on the right track. The real measure (IMO) of fly-by-wire is in it's performance in landing, in particular in allowing for a slow speed, high throttle landing with roll stability - beyond what can be achieved manually. So as the airspeed command (throttle) drops, at neutral altitude, the plane would high alpha into a full hover. To prevent roll destabilization, FBW should sense the roll authority, and prevent airspeed from dropping into wing-stall territory. To prevent stalling, I think you react to low airspeed by pushing the stick forward (which explains why plugged pitot tubes crash planes.) ?
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