Force on trim tab

[whee]

I'm thinking the method I used to mount the end of my push/pull cables in the elevator isn't strong enough. I'm trying to guesstimate how many pounds of force would need to be applied to the trailed edge of the trim tab to approximate the load placed on it during flight. What's your SAWG?

[Battson]

You can calculate an estimate pretty easily by rearranging the Bernoulli equation, plus some simple vector / moment calculations.

But.... that would be hard work.

The practical engineer in me says - theory be damned, it's just one small part. Make it 4 times stronger than you need it.

Ok - I just ran some numbers an easier way.

My electric servo can exert roughly 4.0 N (say ~4 kg or ~8 lbs) at the end of the trim tab, with some allowance for friction. That much force would be vastly more than is required.

[whee]

Huge thanks Battson! I was worried there for a minute that I'd have to dust off the textbooks. I would like to make it way stronger than needed but I'm also going have have 50hp less than Z; if I want to kick his ass in a stol comp I need to watch the weight

We've decided to modify the weak point by moving the cable connection on the trim tab to the inside edge like Pat has on his. Hopefully that will reduce the moment that is twisting the elevator rib to the point it doesn't twist.

[corefile]

Huge thanks Battson! I was worried there for a minute that I'd have to dust off the textbooks. I would like to make it way stronger than needed but I'm also going have have 50hp less than Z; if I want to kick his ass in a stol comp I need to watch the weight

We've decided to modify the weak point by moving the cable connection on the trim tab to the inside edge like Pat has on his. Hopefully that will reduce the moment that is twisting the elevator rib to the point it doesn't twist.

Is that a (potential)problem in the stock configuration? Twisting of the elevator rib? Didn't Pat change more than just the connection location - thought I read he completely changed to a servo'd trim tab vs. the fixed via the plans? Whee did you move to a servo system as well?

[whee]

It is not a problem with the stock system. I created the problem when I modified the trim system. I have since fixed the problem.

The stock system has servoing trim tabs. Pat's systems, which I kinda followed, makes it so the trim tabs don't servo when the elevator is moved.

[Battson]

It would be great to see some pictures of your system Whee.

[whee]

I just tore the plane down to finish weld and prep for paint. I'll look and see if I have any pics of the trim system.

[Nev]

whee Hows your pitch trim system working out ?

[whee]

whee Hows your pitch trim system working out ?

I really like it. IMO it makes the controls well balanced. There are some flaws that could be eliminated on another revision but they aren’t a big enough deal for me to tear into the plane to change them.