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Flap Pushrods

jim miller

Well Known Member
My flap pushrods are the threaded aluminum type. In re-installing the flaps
and ailerons the flap control rods seem to require a slight bend as the flap
and flap lever connect points are lateral to each other. They will connect if
not bent but are at the extreme of lateral movement for the rod end bearings. Is this common and is bending OK?

Thanks
Jim Miller
 
Do NOT bend flap pushrods!

The flap pushrods carry more load than any other control in the airplane. Flaps are the only control surfaces that extend into the wind-stream and stay there.

The pushrods operate in total compression and must not be bent.
 
flap pushrods

concur - don't bend the flap pushrods. (IMHO: the aluminum tubing is too light for that duty - I fabricated mine out of 4130 steel.) Now, I think I know what you are describing, and I think I encountered the same or similar problem. In any event, I solved *some* problem by taking the flap assy actuator arms (that extend into the baggage area) and bending them slightly inwards, towards each other. I forget exactly why I did that (it was 25 years ago!!), but have been flying with it all along, no problem (will supply photos if you like). Now, if the ball of a rod-end bearing is turned to the extreme of its range, then as long as the through-bolt does not contact the rod-end-case over the full range of motion of the assy, you're ok. Check that first before bending anything.

- Steven
700+ RV3 hours
(2 more yesterday!)
 
Hex Tubes

(IMHO: the aluminum tubing is too light for that duty - I fabricated mine out of 4130 steel.)

When I bought my -6, I found the flap pushrods were slightly bent. As has been discussed previously here, I installed the aluminum 'hex' tubes Van uses on the -9, Van's part VA256. Much more robust.

-jon
 
Thanks all for the replies. I definitely won't bend. The plans show a alternative method to build these rods using a long 1/4" aircraft bolt
and female rod ends. Has anyone tried these instead of the aluminum
tube? The original builder put 200 hours on the plane and my rods do
go on but just seemed to be near limit of lateral movement. The flap
connect point is outboard of the Flap handle/actuator arm but will connect.
Probably no problem but I may later build the steel/bolt pushrods if
better/stronger.

Thanks
Jim Miller
 
Like Steven, when I built my -6, I felt that the aluminum tubing was marginal in this application. I also fabricated mine out of 4130 tubing.
19 years later, so far so good.
 
Rod Failure

Have there been any reported problems with the aluminum ones?

Dave

At least one flap rod has failed in flight: http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20010105X00030&key=1

The rod failed on landing, leading to full left flap w/no right flap. Plane cartwheeled, no injuries. The failure was apparently brought about by contact between the rod and the skin, weakening the already not-so-strong tube.

Here's a colorful and entertaining five page thread on this: http://www.vansairforce.com/community/showthread.php?t=11522

-jon
 
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The stock tube is more than adequate. The only problems I have heard are due to poor fabrication, like the above mentioned incident.
However, the hex tubes are an easy upgrade so why not.
 
I did a quick back of the envelope (that means, rather informal) analysis of the tube and it looks like it would take considerably more than 200 pounds on the manual flap handle to yield these flap link tubes, and that's more than the FAA requires for the control stick (they don't explicitly have a force for the flap control) on certified planes. As long as it's straight, it won't come close to buckling at that load.

I checked the Euler buckling strength, the axial yield strength and the thread shear-out strength.

I didn't check anything else. Just the tubes, folks. And they seem to be strong enough.

Dave
Old Aerospace Stress Guy
 
I did a quick back of the envelope (that means, rather informal) analysis of the tube and it looks like it would take considerably more than 200 pounds on the manual flap handle to yield these flap link tubes, and that's more than the FAA requires for the control stick (they don't explicitly have a force for the flap control) on certified planes. As long as it's straight, it won't come close to buckling at that load.

I checked the Euler buckling strength, the axial yield strength and the thread shear-out strength.

I didn't check anything else. Just the tubes, folks. And they seem to be strong enough.

Dave
Old Aerospace Stress Guy
Ya, Van's Engineers aren't stupid. I was told they went away from the tube to the hex rod primarily due to builder variance. Evidently not everybody is capable of tapping a hole properly. There is not a lot of edge distance and if you ream out the tube at a slight angle, you could get in trouble I guess.
 
I may later make some out of 4130 tube. Another related question on
how to adjust the flaps. In the zero position what is the reference in relation
to the wing? Do I use a straight edge to align with top or bottom of wing?

Thanks
Jim Miller
 
After further trial fit I still don't like the extreme lateral position of the rod
ends even requiring force to put the nut on the lower rod end that attaches
to the flap. The two flap control arms need to be twisted to a more outboard
position which I would not want to do unless off the plane and heated. Here
is what I plan to try. Make new 4130 flap push rods but rather than one
piece tig weld (By a excellent welder) two shorter pieces laying side by side
with one inch overlap. This will give me 3/8 lateral shift but still remain
straight. and if properly made and tapped the 3/8 OD .083 4130 tube
welded as described should be much stronger than the alum tube and solve
the lateral alignment problem. Opinions welcome.

Thanks
Jim Miller
 
My opinion is that the original design has been working great for many years and thousands of airplanes. If something doesn't work properly, find out why, and fix that.

Don't modify where modifications are not needed.
Especially in the control systems and the fuel systems.
 
Mel
I have the proper space between the flap and aileron meaning the flap
is in its normal position. The flap control arm connection point is basically
straight up and down. If it was twisted very slightly to point a little outboard
the flap control rods would not require as much lateral deviation. I can put
the rods on and with a little force bolt them up and they work OK. I have
three options. Bend the control arms, operate under tension, or mod control
rod. The plane has flown 200 hours the way it is. But in reassembly I noticed
one side where the control rod goes thru fuselage skin the skin had been rubbing the arm, was cracked which required stop drilling. Both flap control
rods had evidence of paint scraping off.
 
I've inspected in the neighborhood of 500 RVs as an A&P and DAR.
I have not seen this problem. Something is not right.
 
Mel
I think I have it fixed. I used a small piece of Square 1/2 inch steel to
go inside the ear on flap control lever and two 1/8 pieces on the outside
and using a c- clamp straightened where the upper end of the control rod
connects. It had gotten slightly bent for some reason. When I retried the
rod it hooked up in an acceptable manner. I am still going to use the 4130
and make new standard control rods from it. The weight difference is slight
and it takes the aluminum out of the picture. Even from the plans the aluminum tube rod is an option over using a long 1/4 inch bolt and female
rod ends. Thanks for the advice.

Jim
 
More on rigging

I may later make some out of 4130 tube. Another related question on
how to adjust the flaps. In the zero position what is the reference in relation
to the wing? Do I use a straight edge to align with top or bottom of wing?

Thanks
Jim Miller

I'm a bit late to this party, but I highly recommend building a female airfoil
template out of plywood to assist with rigging flaps and ailerons. This is
made by laying out a full size wing cross section on a piece of 1/4" plywood
or similar. Use the dimensions shown on the wing rib plans. Cut out the
interior portion of the profile and separate the upper and lower halves
at the wing leading and trailing edges. Then put the two halves back
together with "hinges". I made hinges by fastening small plywood
scraps across the fore and aft joints in the plywood templates. The
scraps are glued to the upper template and "pinned" to the lower
template with an AN3 bolt. This allows you to easily assemble the
template around the completed wing and will precisely locate the
trailing edges of your flaps and ailerons. It can also be used when
drilling the flap brace and aileron hinge brackets to the rear spar.

This is much more accurate than depending on the wingtips as an
"in trail" reference for your control surfaces. Ideally, you would
use the template to align the ailerons first, and then "adjust/modify"
the wingtips to get their trailing edges into alignment. I credit
this technique for helping my RV-3B fly straight with no need for
aileron trim. :)

- Dan Benua
Tech Counselor
Repeat Offender
 
The original kits for the 6 actually had a full scale wing section drawing you could place on your piece of plywood and cut out the template.
I do not know how you could possibly align the aileron and flaps without the template on a kit that was not matched drilled.
 
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