Flap control horn clearance

Thank's for the help with the seat problem issue. Now for my current problem i am installing my fuselage stringers and the flap horn on the control mixer drops below the belly stringer. If i attach that stringer to the strut carry thru tube as the plans say. If i move it forward to the rear landing gear tube & run it over the strut tube i'm fine. But will that create issues down the road also if i do that i should use stand off tubes as i pass the cross members aft i've included some pics. Which hopefully clarify the situation. #1 shows the stated attach point, #2 shows the clearance problem, #3 the fish mouth attach forward, #4 the crossover of the srtut tube, #5 the resulting abundant clearance, #6 what are those tabs for? Again thanks in advance for the help. By the way this is a mod 3 with 4 1200 mods.Attachment 3140

Attachment 3141

Attachment 3142

Attachment 3143

Attachment 3144

Attachment 3145

Dutch,

You did not say which model you are building but the structure appears to be the same as the 5,6,7SS.

Another builder had asked me about the same question and I still had some file pics of my SS.

I believe your flap linkage rod ends may be extended too far. That causes the flap control horn to extend too far downward when full flaps are selected. Limit the full flap position by screwing in the rod ends a little more.

Pic 1 shows the clearance on the flap control horn with the belly stringer.

The belly stringer does, in fact, attach to the lift strut carry through tube. It is important the belly stringer be flush with the bottom of the carry through tube and not vertically centered on the tube.

Pic 2 shows the belly tube terminated in a channel “socket” welded on by the factory.

Your airframe does not have the welded socket. It’s just another one of those subtle changes. The socket required crushing the end of the tube in a vise to fit then safety wired and Hysol bonded. Yours can be safety wired and bonded without the socket. A piece of chafe tape will cover the wire when you cover.


The combination of rod end extension and belly stringer flush with the lift strut tube should fix your problem.

Do not install the belly stringer extending to the forward landing gear tubes as it will only create covering problems and limit access to some of the belly mounting tabs after covering.

John Pitkin
Greenville, TX

Thanks for the reply i should have been clearer this is a model 3 with a bunch of model 4 modifications i have the flap tube about as short as it will go. In picture # 1 i am holding the stringer even with the bottom of the strut tube and the horn still protrudes about a 1/4" below that point. Could the flap tube be too long? Also what are those tabs on the bottom of the strut & rear gear tube i don't mean the seat belt lugs but those sticking straight down. 2 on the strut tube & i on the rear gear tube.again thank's for the help i'm dealing with 2 sets of plans that sometimes seem to contradict each other

Dutch, those tabs sticking down from the lift strut carry through tube are attach points for an optional round bomb style cargo pod. I have a Classic 4 and understand your problem. Your 3 with 4 control system may be worse, I don't know.

I put a dollar patch...you will learn about those in the covering section if you are not familiar with covering... on the inside of the fabric and another on the outside of the fabric to reinforce it where the flap horn control push pull rod end contacts the fabric. I figured it was something to watch for wear on the fabric. During flight testing I put a stop on the flap handle to limit flaps to a maximum of 20 degrees. The rod end just touches the fabric there now and it has shown no wear after 100 hours.

Another thing to check. Pull on full flaps and then work the aeleron controlls stop to stop to see if that 4 inch long aeleron linlage consisting of two rod ends connected with a short threaded rod will touch the fabric. Mine does so I installed the dollar patches on the fabric there too.

Fior what its worth this same problem was discussed 16 years ago when I was assembling those same parts on my kitfox. You will...probably...be installing an inspection ring just forward of the lift strut carry through as an access to remove and replace the elevator control tube if ever needed so you don't want to run the stringer past the carry through tube.

My airplane (Model IV-1050) has an aluminum plate under the contact point to protect the fabric.

My project plane had a hole worn in the fabric at this point... :mad:

Dutch,
Over the years there have been several rod ends used in the control system. Some have different length barrel sections. This may account for your control rod assembly being a bit long.

Try adjusting the flap control rod as short as possible. Your picture shows about 3/16 gap between the rod end and the flap control rod before it will bottom out. I can’t see what you have at the other end. If the studs are too long to screw the rod ends all the way, you may shorten the studs ¼ inch.

If you still need to shorten the total length you can get shorter rod ends from ACS. Or… in the true home builder fashion you can cut about ¼ inch off the threaded barrel of the rod ends. Re-drill the witness holes to make sure you have adequate stud penetration.

Assuming you have 3/16 exposed on both ends of the control rod and you shorten the rod ends by ¼ you can pivot the control horn forward by 7/8 inch without ill effects.

This should not affect your total flap extension. What you lose by shortening the control rod will be made up when you adjust the vertical flaperon control rods.

Let us know how that works.

John Pitkin
Greenville, Tx