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Progressive switch for electric trim

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enginerdy(at)gmail.com
Guest

Posted: Mon Aug 30, 2010 7:07 pm Post subject: Progressive switch for electric trim

Bob, list,



I was reading through a thread on Van's Air Force today and it got me thinking about the approaches to elevator trim control. There seemed to be two camps:

People who use (on)-off-(on) type switches who have problems with sensitivity,

and people who have some kind of high/low motor speed switch, either manually controlled or airspeed controlled with a Vertical Power (or similar) module.



 	  Quote: 	
		  From a usability versus complexity standpoint, I was imagining a progressive-speed switch, implemented as a 5-position switch:


	


(second-up) : fast down trim

(first-up) : slow down trim

center : trim stop

(first-down) : slow up trim

(second-down) : fast up trim



A quick search has not turned up any toggle switches of this kind, which may be the real problem with this approach. Have you seen this approach elsewhere, or have any part number suggestions?



Thanks,

--Daniel

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 31, 2010 4:32 am Post subject: Progressive switch for electric trim

At 10:05 PM 8/30/2010, you wrote:




Bob, list,



I was reading through a thread on Van's Air Force today and it got me 

thinking about the approaches to elevator trim control. There seemed 

to be two camps:

People who use (on)-off-(on) type switches who have problems with sensitivity,

and people who have some kind of high/low motor speed switch, either 

manually controlled or airspeed controlled with a Vertical Power (or 

similar) module.



 >From a usability versus complexity standpoint, I was imagining a 

progressive-speed switch, implemented as a 5-position switch:



(second-up) : fast down trim

(first-up) : slow down trim

center : trim stop

(first-down) : slow up trim

(second-down) : fast up trim



A quick search has not turned up any toggle switches of this kind, 

which may be the real problem with this approach. Have you seen this 

approach elsewhere, or have any part number suggestions?



     I's aware of no COTS (commercial off the shelf)

     switch with this characteristic. There have been

     lever operated, sping loaded to center, wafer

     switches but those go back a lot of years. I

     don't think they were a catalog item.



     I've spent a significant portion of my career

     working on motor driven trim systems for aircraft,

     mostly bizjets. I did get to put the first microprocessor

     controlled trim system on Roy Lopresti's M30 prototype

     at Mooney. This was a proof of concept controller that

     adjusted the servo gain scale factor on a servo/anti-servo

     tab. The goal was to provide a constant stick-force/G

     response irrespective of IAS.



     Initial flight tests were encouraging but the M30

     program was scrapped for a host of reasons. We've

     had a number of discussions about trim systems here

     on the List. A search of the aeroelectric.com website

     using the words trim and speed will produce a number

     of hits. But in particular, may I suggest you review

     the article published at:



http://www.aeroelectric.com/articles/Trim_System_Failures.pdf



     The "ideal" trim controller would adjust the motor's

     speed linearly over the full range of max to minimum

     depending on the airplane's IAS value. After

     crafting the trim speed controller for the Lear

     55, I wrote and delivered a paper at an SAE

     convention where I hypothesized a number of

     possibilities for future designs. Two features

     of the paper talked about a way to control trim

     speed from a single processor in a manner that

     precludes a runaway in spite of worst case software

     failure. The other was a proposal to adjust motor

     speed based on the value of pitot pressure (IAS).



     Obviously, there's a huge difference between a

     performance of a Lear and an RV. A larger difference

     exists between the product development budgets

     for the two aircraft.



     Setting the "ideal" system aside, what operating

     characteristics might offer 90 plus percent of

     everything we'd like the trim system to do.



     An airplane spends MOST of its flight time in

     stable flight at max IAS. This configuration

     suggests a very slow trim rate that allows

     fine tuning of trim for holding altitude.



     A small percentage of flight time is spent

     in approach to landing where IAS is on the

     order of 90-100 kts and some degree of

     flaps are deployed. Here the trim speed needs

     to be faster to make timely adjustments to

     establish the desired IAS.



     On the Lears, the high speed trim was 4x faster

     than low speed trim.



     I've published a suggested schematic for a

     two speed trim system for small aircraft

     at:



http://www.aeroelectric.com/PPS/Flight/Trim/Two_Speed_Trim_2.pdf



     This is modeled after the system flying on

     30 series Lears where speed commands come

     from a switch on the flap system. With flaps

     full up, low speed is commanded. With flaps

     extended more than 10 degrees, high speed

     is commanded.



     I'm confident that a system crafted after

     this model will meet design goals for

     crafting a very utilitarian pitch trim

     system. The neat thing is that speed

     selection is automatically controlled by

     flight configuration. I could get this

     system qualified on a TC aircraft with

     a minimum of bureaucratic fuss.



     The controller should be fitted with

     screwdriver adjusted trim pots for speed

     selection. Over a series of flight tests,

     you can optimize system performance to

     meet your design goals.

   Bob . . .

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marcausman

Joined: 08 Feb 2007
Posts: 70

Posted: Fri Sep 03, 2010 4:15 am Post subject: Re: Progressive switch for electric trim

Automobiles now have a similar switch for electric windows. Pull (or push) the switch a bit to move the window, then pull (or push) it past a detent to automatically raise or lower the window.

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_________________
Marc Ausman
http://www.verticalpower.com "Move up to a modern electrical system"
RV-7 IO-390 Flying

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