Elevators
The
RV-7 has two separate elevators, each linked to its own elevator control
horn, bolted together and supported by a central bearing. The structure
of each elevator is quite different, due to the fact that one has a
trim tab and the other has not. Elevator construction is similar to
the rudder in that the skins need to have stiffeners attached. Otherwise
it is very different in that there is a single folded skin for each
side, and there is the complication of the trim tab, trim servo (or
cable) mount and accompanying reinforcement structures.
1.
Identifying Stiffeners
The
elevator stiffeners come packed as seven lengths of angle aluminium,
pre-punched with holes dividing them into the appropriate lengths. What
the plans do not tell you clearly is that there are subtle differences
between left and right sides. Count the stiffeners in the images below.
Yep, there are seven on the right hand side, and eight on the left!
Hence, the middle stiffeners are shorter on the LHS,and have to be trimmed
accordingly. The appropriate dimensions are shown below in "Note
1". You have to be really careful here. It would be quite easy
to snip the angle in the wrong direction and end up with a load of useless
aluminium angle of the wrong dimensions!. First I cut out the stiffeners
so their lengths were correct, and I placed similar stiffeners together
to check dimensions and numbers.
seven lengths
of angle
|
different
stiffeners marked
|
The plans
show stiffeners
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arranged on
each side
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how to trim
stiffeners
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stiffeners
rough cut
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2.
Cutting stiffeners
Once
you understand how the stiffeners are organised, you can lay them out
to make sure you have them in the right orientation. There are small
notches in the surface without the holes. These tell you where to start
the taper. The taper finishes at the end of the stiffener. I have laid
a ruler close to these notches so you can see how they all line up.Look
at the high mag view to see the notches aligned to the left of the ruler.
The taper goes from these notches to the right. In the next image I
have used a (blunt!) "Sharpie" pen to draw a line where the
cuts will be to form the taper. Notice that the small stiffeners (H)
have no taper, as per the plans. These are positioned in the area forward
of the trim tab, and so do not need to be tapered. The stiffeners were
trimmed with the Wiss snips (which are great!), the blue plastic was
removed, and the edges of each were smoothed with the Scotchbrite wheel.
It takes a while to do all this. Finally the stiffeners are clecoed
in position on the inside of each skin, and a #40 drill is used to drill
out each hole to its final dimension.
notches
aligned
|
high
mag view
|
marked
up
|
stiffeners
trimmed
|
ready
to cleco
|
drilling
skins and stiffeners
|
3.
Deburring, dimpling, priming, riveting stiffeners to skins
This
is repeated for both skins, taking care to use the right stiffeners
on each side. Then what seems like a zillion 3/32" holes each need
to be deburred and dimpled. It is difficult to get the C-frame tool
dies near the most aft stiffener attach holes. So I used the Avery "Pop-rivet"
dies and my trusty pop-rivet gun. Basically the pop rivet gun pulls
on a nail and squeezes the dies together. Minimum space is required
inside the structure. The results were good - I am glad they included
this tool in the Avery empennage tool kit. All stiffeners and both skins
were treated with 1) detergent/Scotchbrite pad rinse and light scuff;
2) Alumiprep bath or painted on; 3) Alodine 1200L anti-corrosive chromate
finish. Finally all stifferens and both skins were given a very light
spray coat of Sterling 2-part Epoxy primer/sealer. The back-rivet set
was used to fasten stiffeners and trim brace plate to the skins.
stiffeners
clecoed
|
pop-rivet
type dimpler
|
alumiprep
& alodine
|
gold
alodined stiffener
|
Right stiffeners
done
|
Left Stiffeners
& trim brace
|
4.
Brake and trailing edge bend
The
next step is to bend the trailinge edge. The image with the front spar
in position shows just how much bending is required - the top skin must
align with the upper flange of the spar. This is performed with a "brake"
made of two hinged pieces of straight dressed timber, separated by a
gap of about 1/8" (3mm). On the advice of another builder (Bob
Barrow), I made my brake wth the two pieces of timber hinged at right
angles to each other, rather than parallel, as in the Vans instructions.
The bending surface will be the 2" width of the upper length. This
means that I can se the surface angle being bent and hopefully ensure
the bend occurs evenly and right where it is needed. I bought a length
of 3mm diameter carbon rod at a hobby shop. This will be taped into
the angle to form a minimum curvature template. I tried bending a piece
of scrap first. I found that, if the angle is too sharp, the Aluminium
work-hardens and starts to split. It is obviously very important not
to over-bend and end up with too "sharp" a trailing edge.
The final image shows the end result - a trailing edge diameter of about
3/16".
Trim brace
outside view
|
Front spar
in position
|
Brake and
skin
|
Brake construction
|
Ready, rod
in place
|
after bending
- enough?
|
5.
Counterbalance ribs and control horns
The
next sequence involves the counterbalance weights. These are attached
to the outer ribs of both elevators by means of counterbalance ribs
and skins. The first image shows the right elevator skeleton clecoed
together. the next image shows BOTH (L&R) counterbalance weights
and the ribs and skins which support them. In the next image, one counterbalance
is fastened into its skin/rib assembly (with clecoes). The two small
holed at the front are use to locate a #12 drill which bores holes for
the bolts which attach these weights in position. Next the control horn
is positioned with clecoes and match-drilled. Once everything is drilled,
dimpled, alodined and primed you are ready to tivet it together. Be
careful NOT to dimple the upper flange of the inbord spar of the left
elevator. This is where the piano-hinge is to be attached. The spar
flange holes are countersunk to accept the dimpled skin holes
above, and line up snugly with the flat hinge plate below. You need
care here. Don't try to dimple the hinge. Study
Drawings carefully.
Skeleton
|
Counterweight
bits
|
Counterweight
assy
|
Control horn
|
6.
Skeleton and counterweights
The
sequence of construction is to first assemble the elevator skeleton
and control horns, riveting together the spar and the end ribs with
the horn. Next the counterbalance skin is riveted to the main skin by
two key flush rivets which would otherwise be inaccessable after final
assembly. Then the skeleton is slid into the skins, holes are lined
up and final finishing checked before final assembly. I found the counterweights
needed to be trimmed slightly to fit easily between their skins and
rib flanges. I just removed enough so that everything fitted together
snugly. The fibreglass tips need to be trimmed so their forward edges
fit neatly around the weights and between counterweight and counterweight
skin. These tips are just a fraction too long, but final trimming should
be made AFTER final assembly.I have decided not to bother with filling
the minor gaps between glass and aluminium. I will just rivet the tips
and and use microballoons to close off where necessary.
skeleton
on skin
|
cleco every
3rd hole..
|
..and rivet
in-between
|
Trim weights
|
Trim glass
tips
|
Fit tips
and weights
|
7.
Trim Servo Mount
There
is very little information given in the instructions about the trim
tab and trim servo construction. I have the Ray Allen Electric Trim
option, and these notes relate to this construction. The trim tab is
driven by a servo which is mounted on a plate which attaches to the
lower left elevator by seven screws. These mate with seven nutplates
which are riveted to a reinforcement plate which in turn is riveted
to the lower elevator skin. Belatedly, I now know that these 7nutplates
should be riveted to the reinforcement plate BEFORE it is riveted to
the skin. Since I had not done this (I followed the instructions!) I
had to use my backriveting set plate, placed judiciously so that it
did not overlap the skin (see first picture below). The plate here abuts
the overlap between skin and reinforcement plate, leaving zero gap between
plate and flush rivet head. I had to move the edge of the back-riveting
plate around to do this for all platenut rivets (second and third picture
below). It worked out OK (see "Mounting plate in position"
below). Finally, before trimming the theaded rod which actually moves
the trim tab, I set up the servo with its switch and LED indicator ("testing
servo operation" below) so as to find the "middle" position
for the servo (see indicator LEDs in last picture). I will try my best
to trim the rod so that the trim tab is in neutral position (lined up
with elevator trailing edge) with the servo in this middle position.
back-riveting
mounting nutplates
|
One nutplate
ready to rivet
|
nutplates
in position to rivet. Note backplate
|
Mounting
plate in position
|
testing
servo operation
|
8.
Trim Tab Blues
My
first real criticism of the RV kit is the lack of information about
building the trim tab. I botched mine and will probably replace it if
I get a chance. The problem is that you are told to bend the trailing
edge before attaching control horns and tab spar. Once the skin is bent
there is no room at all to get a bucking bar or rivet squeezer inside
the tab. I ended up having to waste a heap of MK- flush pop rivets attaching
the trim tab spar to the bent skin. I also managed to over-bend one
of the side tabs, cracking the Alclad skin. I cut this away and replaced
it with a small "riblet", but this turned out to be a little
too wide, so the tab does not line up with the rest of the elevator
as cleanly as I would like. Next time I would try riveting the spar
and control horn to the lower skin BEFORE bending the trailing edge,
and use a bigger radius on my bending blocks.
9.
Final Assembly and leading edge folds
RTV,
Proseal or other goo has to be spread between the stiffeners at the
aft angle of both elevator skins. This is to inhibit vibration and skin
cracking which apparently has occured without this measure. I used 'blue"
automotive gasket RTV as suggested by Dan Checkoway. Then most of the
skin rivets can be either squeezed or shot. I shot most of them by myself
and only had to drill out a couple. I cleco every third hole, and use
rivet tape to hold rivets in position and protect the skin surface.
Once complete, clecoes and removed and the remaining holes completed.
I rolld the leading edges with a 1" steel shower curtain. These
were more difficult than the rudder, even though I rolled them in stages.
Once the elevators were attached to the Horizontal Stabilizer, the expected
interference was observed between counterbalance sections and HS skin.
I marked out and removed the offending Alcld, using a drill and small
hacksaw to remove all but 1mm of material. A small file was used to
get the final smooth surfaces. The result was two freely swinging elevators.
The left elevator is a little HEAVIER than it should be (it falls down.
not up). I will need to use some of the right counterbalance material
to add a very small amount of lead to the the right. I guess I must
have removed too much lead on this side (or it may have been too much
primer). It is only a few grams.
ready to
rivet
|
cleco every
third
|
rivet tape
protects
|
leading edges
rolled
|
the usual
problem
|
how's it hanging?
|
Elevators
and Empennage basically complete 18th August 2005!
10.
Drilling elevator control horns, and adding fibreglass trims
(30th
April 2006) After
months in storage I dragged out the horizontal stabilizer and attached
the elevators. After confirming clearances and distances were to specification,
I used a 1/4" drill placed in the central bearing to mark where
to drill each horn. The instructions recommend using a piece of tubing
as an alignment tool to drill, but the 1/4" drill worked well and,
operated carefully by hand, did not damage the bearing surfaces. The
elevators were removed and drilled where marked. Image 2 shows the AN4
bolt (with the wrong large washers in position - the correct plated
washers are now in place). The elevator horns are individual weldments
- the apparent slight misaligment is correct, it is due to slight shape
differences between weldments. Next I will need to drill the holes where
the pushrod attached to the elevators - will devote a lot of time to
this to ensure correct alignment. The third image shows a heat gun used
to expand the shape of the elevator counterweight edge trim, in preparation
for pop-riveting to the elevator trims in place.
(31st
July 2006 - 3 hours) I have been too busy doing other stuff. First
time back in th workshop for 6 weeks so i decided to srill out the elevator
control horns for the AN3 push rod/control rod bearings. The relevant
drawing is in image 4. You align the elevators (image 6), find the aft-most
control horn, and drill a #30 hole 1/2" from the front and 3/8"
from the lower edge (image 5). Attaching elevators and other controls
is made very easy by using the excellent little devices from Avery shown
in images 7 and 8. They consist of a small piece of 3/16" rod (AN3),
bent and threaded on one end. The thread screws into an Aluminium handle.
When threaded in place, it is very easy to fit this little piece of
rod into the rod end bearings attaching the surface - in this case,
attaching elevators to horizontal stabiliser.
Align
elevators in preparation to drill control horns
|
Centre
bearing (wrong large washers)
|
Heating
fiberglass to get the correct shape
|
Elev
contrl horn drawing
|
Measured
ready to drill pushrod connect
|
Aligned
again
|
These
gadgets are great
|
The
AN3 hook screws in place
|
The
first image below shows one of these devices fitted in place in an elevator
bearing bracket on the horizontal stabiliser. The next image shows the
same view, but with the elevator in place, and the rod device manouvered
into position through the gap in the rolled leading edge of the elevator.
With the aluminium handle in place, this takes only seconds to do, compared
with minutes if AN3 bolts were used. Of course the bolts will go in
place eventually! Once the elevators are positioned, the pilot hole
is drilled in the aft-most control horn (left in my case). I then made
a block of hardwood which slid snugly between th elevator horns. Using
a perpendicular drill-press, I drilled this block with a #30 3/32"
drill. This block and the resulting hole were used to guide a #30 drill
through the first hole and across to the other elevator control horn
to drill another pilot hole. These hole MUST be square, otherwise the
elevators will not be aligned in final assembly.
It temprarily attaches the control surface hinge
|
Like
this - easy to disassemble
|
Here
is my hardwood spacer for drilling contrl horn
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Here
are both pilot holes drilled
|
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