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ELECTRIC PITCH TRIM ON AN RV-4

R

RV-4

Well Known Member
Hello Gents

I'm in the planning phase of installing an electric pitch trim on my -4 and I was wondering if anyone has ever installed the trim servo where the trim handle is normally located ( on the left side by my left leg ) using the usual trim cable as supplied by Vans.

I just replaced the cable and it is very smooth but wonder if the trim servo has enough power to operate (pull & push ) the cable and make the trim tap move??

I seem to recall someone here doing just that.

Inquiring mind like to know

Thanks

Bruno
[email protected]
 
Digging yourself a hole!

I doubt this has ever been done for several reasons. You will
Be adding weight, more places for the system to fail and design difficulties. Why would you do this when the electric trim works fine as designed?
 
RV-4 said:
Hello Gents

I'm in the planning phase of installing an electric pitch trim on my -4 and I was wondering if anyone has ever installed the trim servo where the trim handle is normally located ( on the left side by my left leg ) using the usual trim cable as supplied by Vans.

I just replaced the cable and it is very smooth but wonder if the trim servo has enough power to operate (pull & push ) the cable and make the trim tap move??

I seem to recall someone here doing just that.

Inquiring mind like to know

Thanks

Bruno
[email protected]
Click to expand...

I suspect your -4 is already built? It can be done and I've considered doing the same with mine. However, I've become accustomed to my manual trim and the immediate response it gives me. On the plus side, electric trim will let you fine tune a lot easier. More knowledgeable people on this subject are sure to pipe in.
 
Eh?

Yes my 4 is built and flying with electric trim and it works fine, bu what has that got to do with it?

Keeping the cable but installing the trim motor push it means you have more failure points in the system, and rather than saving the weight of a cable you keep it. The only issue with a 4 is that it is weight sensitive. Look at the other threads here recently about removing fuel to take passengers.

So, I ask again why not just install the standard electric trim rather than trying to reinvent something that has been proven over many flight hours.

I have flown Spring loaded trim (tailwind) and found it always ran out of enough movement to trim and forget in the approach phase. Also trim wheels....... Oh what a pain!

What I find with the standard trim system is that one ore two taps on the button are all that is neaded for cruise speeds. Where it really gets used is on approach to slow the aircraft and hold the nose attitude.

Rather than trying to modify in a way that will increase weight its is better to try to loose it!
 
WAM120RV said:
Yes my 4 is built and flying with electric trim and it works fine, bu what has that got to do with it?

Keeping the cable but installing the trim motor push it means you have more failure points in the system, and rather than saving the weight of a cable you keep it. The only issue with a 4 is that it is weight sensitive. Look at the other threads here recently about removing fuel to take passengers.

So, I ask again why not just install the standard electric trim rather than trying to reinvent something that has been proven over many flight hours.

I have flown Spring loaded trim (tailwind) and found it always ran out of enough movement to trim and forget in the approach phase. Also trim wheels....... Oh what a pain!

What I find with the standard trim system is that one ore two taps on the button are all that is neaded for cruise speeds. Where it really gets used is on approach to slow the aircraft and hold the nose attitude.

Rather than trying to modify in a way that will increase weight its is better to try to loose it!
Click to expand...

Steve, you make some valid points but I think the previous poster was referring to the original post, not yours. Just a point of clarification. Note the quote above his post. :)
 
If you don't want to hack into the elevator for the trim motor there is another option.

Use a shorter cable through the same tail spar holes and mount the trim motor on the deck under the forward end of the tail fairing. no mechanical modifications needed to the horizontal tail of elevator.

This will still be a weight savings since the bulk of the weight is in the long manual trim cable.

Several early builders adapted this approach. Not my pic, but shameless stolen off the web - it's a -6 but the idea is the same.

901jh_02aug05_012.jpg
 
Pat Hatch said:
Steve, you make some valid points but I think the previous poster was referring to the original post, not yours. Just a point of clarification. Note the quote above his post. :)
Click to expand...

Thanks that makes sense now.


Even if it is built and flying I would probably still try to get a double whammy electric trim plus saving a bit of weight, but the other option shown is a good idea. It saves trying to cut a hole in the elevator and fitting the doubler inside.
 
Electric Pitch Trim

First of all:

Thank you everyone for your imputs, they are very appreciated.

Steve: I get your point about keeping the system simple and I've been doing that for the last 16 years I've been flying my RV-4 but as I fly the a/c 99% of the time alone, I won,t loose sleep if the a/c weights a pound or two more and I would also like to use to autotrim option of my Skyview autopilot..:)

Gil: I really like your idea of mounting the trim servo in the tail section and I've been thinking of doing just that but I don't know where I could find a cable this short and that is the reason why I'm planning on installing the servo in the front of the a/c.Any idea where a cable like that might be available?

If you have more pictures of this set up I will appreciate having them

Thanks

Bruno
[email protected]

P.S; Found the cable I need..AC Spruce have them ( Found the info on Randy Lervold RV-3 web site)

Cable is : ACS #05-16936, A-950

That's the way I will do it
 
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RV-4 said:
.....

Gil: I really like your idea of mounting the trim servo in the tail section and I've been thinking of doing just that but I don't know where I could find a cable this short and that is the reason why I'm planning on installing the servo in the front of the a/c.Any idea where a cable like that might be available?

If you have more pictures of this set up I will appreciate having them

Thanks

Bruno
[email protected]

P.S; Found the cable I need..AC Spruce have them ( Found the info on Randy Lervold RV-3 web site)

Cable is : ACS #05-16936, A-950

That's the way I will do it
Click to expand...

It also looks like these guys can custom make a cable to your length.

http://www.midwestcontrol.com/series.php?id=197

I found mine as surplus many years ago....
 
Yes I have seen one RV-4 years ago that had the electric trim servo in the cockpit but the way Gil describes would be a lighter and nicer solution.

RV-4 said:
Hello Gents

I'm in the planning phase of installing an electric pitch trim on my -4 and I was wondering if anyone has ever installed the trim servo where the trim handle is normally located ( on the left side by my left leg ) using the usual trim cable as supplied by Vans.

I just replaced the cable and it is very smooth but wonder if the trim servo has enough power to operate (pull & push ) the cable and make the trim tap move??

I seem to recall someone here doing just that.

Inquiring mind like to know

Thanks

Bruno
[email protected]
Click to expand...

az_gila said:
If you don't want to hack into the elevator for the trim motor there is another option.

Use a shorter cable through the same tail spar holes and mount the trim motor on the deck under the forward end of the tail fairing. no mechanical modifications needed to the horizontal tail of elevator.

This will still be a weight savings since the bulk of the weight is in the long manual trim cable.

Several early builders adapted this approach. Not my pic, but shameless stolen off the web - it's a -6 but the idea is the same.

901jh_02aug05_012.jpg
Click to expand...

az_gila said:
It also looks like these guys can custom make a cable to your length.

http://www.midwestcontrol.com/series.php?id=197

I found mine as surplus many years ago....
Click to expand...
 
Bruno,
I have just what you are looking for. It's a relatively short cable - like a throttle cable - that is connected to the plate that goes under your left elevator and connects to the trim tab horn in the normal manual trim fashion. The other end of it connects to the Mac servo which will resides on the aft deck below the vertical stabilizer. This keeps the servo weight out of the elevator and maintains that original balance that it had when it was built. I originally bought it for my first airplane, RV6, when I was building but then changed my mind. I'll take a picture of it today and email it to you if you want to see it.

https://im1.shutterfly.com/procserv...o=1&ps=1&px=233&py=350&r=1&rx=233&ry=350&sa=0


https://im1.shutterfly.com/procserv...o=0&ps=1&px=350&py=233&r=2&rx=350&ry=233&sa=0
 
Last edited:
RV4 trim servo

My servo and relay were mounted just forward of the bulkhead anchoring the throttle and mixture cables. The actuating rod went through the bulkhead and attached to the manual trim lever. Doesn't really matter how you choose to mount the servo, the fact is that the servo has the power to move the lever and thus actuate the trim tab.
 
I think you will lose precision if you have the servo forward actuating the cable. You will get all of the lost motion in the entire system from front to back. The shorter you make the cable the better off you will be, with the ultimate obviously being the servo in the elevator.

Lost motion could also be described as slop. The servo pushes the cable end from point X to point Y which moves the tab from angle A to angle B. Then if the servo moves back to point X, the tab won't quite go back angle A because the cable flexes between the points where it is anchored, there is friction, the cable ends all have play in them which adds up. On the other hand if you have the servo at the elevator it will be rock solid.

The weight is less than the hamburgers you are going to eat on those Saturday flights. The precision to me is the issue.
 
sblack said:
I think you will lose precision if you have the servo forward actuating the cable. You will get all of the lost motion in the entire system from front to back. The shorter you make the cable the better off you will be, with the ultimate obviously being the servo in the elevator.

Lost motion could also be described as slop. The servo pushes the cable end from point X to point Y which moves the tab from angle A to angle B. Then if the servo moves back to point X, the tab won't quite go back angle A because the cable flexes between the points where it is anchored, there is friction, the cable ends all have play in them which adds up. On the other hand if you have the servo at the elevator it will be rock solid.

The weight is less than the hamburgers you are going to eat on those Saturday flights. The precision to me is the issue.
Click to expand...

I agree in theory. However I did not find that to be the case in practice. My RV4 had the forward located servo pictured in my previous post. My RV8 has a plans installed servo in the tail. I felt no difference in the precision of movement between the two systems.
 
Electric Pitch Trim

Thank you all for the really good info, it is really appreciated.

Gil: Thank you for the link.

Gary; The more I think about it, the more I will mount the servo under the vertical fin and use a shorter cable.

I checked the length today and it should be 36'' or less.

Scott; How long is your cable and how much would you like for it ( I don't need the aluminum plate though ) PM is fine.

Thanks again

Bruno
[email protected]
 
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