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Elevator Trim Tab sizing.

Posted By:
David Flugum
Homebuilder or Craftsman
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#1 Posted: 1/21/2011 13:46:37

Are there any basic calculations or rules of thumb for sizing an elevator trip tab?  I am planning on building a Hevle Classic Tandem Fly Baby and I would like to add a trim tab to the elevator but am not sure how to size it. 

Thanks,

 -Scott



David Flugum
Homebuilder or Craftsman
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#2 Posted: 1/23/2011 20:05:38 Modified: 1/23/2011 20:06:47

How about this for a design.  The Trim Tab is ~ 4-1/4" (25% of cord), 10-1/5" wide and deflects ~ +-30 degrees.  Too much, not enough, any thoughts?

 
Basic Elev Trim.jpg



Arthur Armour
Homebuilder or Craftsman
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#3 Posted: 1/27/2011 04:31:17

OK, while NOT an aerodynamicist, my observations would suggest the following;

The hinge line of a trim tab needs to be regarded as the point of leverage that the tab has over the control surface, so the further aft the better.

At the same time, the area of the tab, as a percentage of the total control surface area will dictate the amount of deflection required of the tab to have an effect.

The tab you depict in your drawing would be considered 'adequate' but would require  a lot of the 30º deflection to work and would cause a bit of 'Trim Drag' at it's edges near the fixed parts of the elevator.

A better solution would be to push the hinge line back to 20% of chord and make the tab incorporate two ribs and maybe even include the inboard section as well.

Have  a look at a Cessna 150.....

Just my thoughts,

Arthur.



Go Fast, Turn Left !!
Brent Bunch
Homebuilder or Craftsman
15
Posts
2
#4 Posted: 1/27/2011 19:37:40

I'm not a rocket scientist either but, I suggest using a proven system. Go with the Piper trim like they have on the J3 cub. The rear of the horizontal stabilizer is attached and the front of it moves up and down via a jack screw. It's a little less drag since it eliminates the trim tab. It also eliminates play needed in the trim tab cable. No extra cutting and fitting of fabric covering either.

Good luck

Brent



bbunch
David Flugum
Homebuilder or Craftsman
10
Posts
0
#5 Posted: 1/27/2011 20:04:55

Gents, Thanks for the ideas.  I like the idea of shortening and widening the trim tab.  I doubt I will go inside of the 1st rib but expanding one rib in the other direction would be easy.

Brent, I have a short wing (PA-22/20-150) similar to the one in your avatar (see my avatar) and it has the jackscrew elevator trim.  I like that system but this is my first build and I am trying to keep the modifications of the design to a minimum and only what is necessary.  In this case the single seat Fly Baby does not have a trim system and the Hevle Tandem version really does need it with the PIC flying from the back seat.  The Hevle plans do not include details of how they added trim so it's one of those things I need to figure out for myself.

Thanks again,

-Scott

 



Ron Blum
Young Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
13
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#6 Posted: 1/27/2011 21:45:28

Scott:

Here's a picture of one with the tab being fairly clear in the picture.  As an aerodynamics person, I doubt it travels very far.  Tabs are always a compromise between too little (rarely) and too much (fairly common).  It also depends on if you make it electric or mechanical.  Mechanical ones can't runaway (and fail to the faired position).  Electric ones are easier, but can you (or the airplane) handle the failure modes (too quick to the stop or stuck somewhere ... like full travel/ stick forces and G-loads).  Remember the only job of the tab is to relieve control force (elevator hinge moment from not being faired).  If the force is always present or abnormally high, I would look at the stabilizer/elevator relationship at different speeds (and CGs since you mention that as a factor).

http://www.airplane-pictures.net/image16064.html