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Hot Wings

Grumpy Cynic
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And, in case FBW power is too high, why not go for servo-tabs? Easily an order of magnitude less power-demanding.
In many cases that would be a preferred method. In the particular "thought experiment" being worked on servos in the control surface would be a problem due to having to run wire to them (flexing at the hinge point), aft weight due to the tab, and simply finding room for the servo (250mm chord 800mm span 13% thick). A push/pull cable running from a remote servo to the tab might also be an option but there is still flexing at the hinge and the aft weight. A typical RC servo could probably handle a tab.

Moving on with this thread on FBW:

There have been several references to using off the shelf servos for FBW systems here on HBA. One of the draw backs I see with using them is that they all require constant power to maintain position. To get around this one obvious solution is to put a worm gear somewhere in the drive train. The problem with this is that if the servo were to fail with the control surface fully deflected either the other control surfaces would have to have enough power to offset the failed surface or there would need to be some sort of clutch in the drive train to allow the failed surface to return to neutral. This adds complexity and another failure stream. Both options add weight.

The constant power use isn't really a problem for a conventional plane that has fully redundant power sources but for a glider or ultralight it gets to be a huge concern. Any thoughts on how to get around the constant power drain to hold a deflected position? If you were to design a clean sheet FBW actuator for a very light plane what might it look like?
 

Jay Kempf

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For that you need to know the frequency response of the autopilot and of the control surface actuators. Autopilot freq response is probably a programmable setting to suit to each aircraft but the manufacturer should be able to give you an idea. For the actuators you can get that data from their spec sheets. The highest low cutoff frequency gives you the most actuations per unit of time possible on that system. For amplitude assume full deflection -- this gives you a worst case scenario.

Then you have force * distance = work, work * speed = power and you should reach a ballpark figure.
Ooooh, that's good, and straightforward.
 

Pops

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If you want FBW Servos ---- Firgelli Technologies - Micro Linear Actuators

I am using a small L-12 series for my trim tab on my little singe seat Super Cub. Just a servo with end point limit switches. Stroke-- 30mm, gear ratio--210 /1, 12volt, weight-- 43 grams, power-- 45n@2.5 mm/s. Backdrive force-- 150 N. Price $80. Use a DTDP rocker switch. Works great.
My trim tab is a piece of honey- comb from Rockwell Int surplus store when they were making the B-1 bomber.

If you want to RC servo for an auto,etc, they have the servos.
Dan
 

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Pops

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Forget to add. I used to have a homemade autopilot in this aircraft using giant scale RC servos powering trim tabs. Worked great,(roll and pitch) with electric trim. Just got tired of the autopilot having all the fun and took it out, when I had about 50 hrs on the autopilot. I could have sat in the aircraft with a RC transmitter in my hand and flown the airplane but never did. That would have been FBW :)
Dan
 

Hot Wings

Grumpy Cynic
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That would have been FBW

Dan
But would you have trusted it if there wasn't a full manual backup already installed?

20% duty cycle on those L-12's is fine for a trim tab, or maybe even an autopilot in an already trimmed plane but it's way out of my comfort zone if there were nothing between the stick and the elevator but a bunch of wires.


With 150N of back drive force have you noticed any creep of the trim setting over time?
 

cavelamb

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But would you have trusted it if there wasn't a full manual backup already installed?

20% duty cycle on those L-12's is fine for a trim tab, or maybe even an autopilot in an already trimmed plane but it's way out of my comfort zone if there were nothing between the stick and the elevator but a bunch of wires.


With 150N of back drive force have you noticed any creep of the trim setting over time?
Excellent questions.

I'd add, what does it weigh compared to a couple of cables?
 

autoreply

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In many cases that would be a preferred method. In the particular "thought experiment" being worked on servos in the control surface would be a problem due to having to run wire to them (flexing at the hinge point), aft weight due to the tab, and simply finding room for the servo (250mm chord 800mm span 13% thick). A push/pull cable running from a remote servo to the tab might also be an option but there is still flexing at the hinge and the aft weight. A typical RC servo could probably handle a tab.
Not at all. Hinge it like an anti-servo tab and put the actuation point on the airframe, not on the control surface, where normally the anti-servo tab would be fixed. Save a single push-pull tube to your tab and an order of magnitude less energy required there is no difference to a fully actuated control surface.


As for redundancy, the wiffle tree approach?
 

Hot Wings

Grumpy Cynic
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Not at all. Hinge it like an anti-servo tab and put the actuation point on the airframe,

Well Duh! It's comforting that I'm still able to miss something obvious. :hammer:

I'd still like to use the surface fully stalled as an air-brake but a second higher powered servo might work.........:think:


As for redundancy, the wiffle tree approach?

Redundancy is not really needed for this particular "thought experiment". The small surface is one of 4.
 
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