I have constantly struggled with keeping the cylinder head temps in the normal range during climb.  Oklahoma summers are hot which require a faster climb out speed and lower rate of climb to keep the rear cylinders below 400 degrees.  I also have to pull the prop rpm back to 2350 during climb to keep the CHT from bumping 400 degrees.   In cruise all my CHT's run around 365.  I do have GAMI injectors which helped to even out the temps at cruise.

I have read several posts on the forums about the differential pressure in Vans RV10 cowling and problem with the design of the cowling fro proper cooling.  If you look at any other airplane that is running the Lycoming IO-540 engine, the cowling has cowl flaps.

I did some research and found Nick Nafsinger on the Matronics forum. The problem with the RV10 is the size of the mufflers and the location relative to the cowling.  There is not much room for a cowl flap to be installed incorporating side fences to direct the air out of the cowling due to the mufflers position.  I used some of Nicks ideas in my RV10.

DESCRIPTION:
The cowl flap has a foldable side fence when retracting back into the cowling.  A delrin block serves as a guide for folding the side fence away from the muffler.  Actuation is via a electric Pololu concentric actuator with a 2" travel.  The actuator is mounted in the forward tunnel area in the cabin and is run by a switch next to the throttle quadrant.  A Blue Max cable with a 3" travel connects the actuator through the firewall to a control tube mounted on the engine side of the firewall.  The cowl flaps were designed to open 29 degrees in the open position.  They are mounted on the bottom of the cowling.  The composite material on the cowling in the cowl flap area was removed and then a 6 layers of glass were applied to strengthen the area around the cowl flaps.

Pictures and drawings are available on this web page showing the installation.
-- Click pictures for larger image --
DRAWINGS:
I did quite a bit of drawing and re-drawing the cowl flap system trying to get the correct operation in the RV10 cowling.  Below are some of the layouts.  Also included is the wiring diagram.  The cowl flaps are operated by SPST LED lighted switch that is located in the console beside the throttle quadrant.  The switch controls the electrical cowl flap relays.  The cowl flaps are either open or closed.
cowl_flap_layout

cowl_flap_layout

actuator_lever

actuator_lever

folding_guide

folding_guide

wiring

wiring

cowl flap sides

cowl flap sides

THE ACTUATOR ASSEMBLY:
Nick had is actuator mounted inside the cowling.  Even though he said he wasn't having any problems with the heat, I decided to mount the actuator on the back side of the firewall inside the cabin.  I have the actuator for the Ram Air System mounted in the forward tunnel area and found there was also room for the cowl flap actuator.  I mounted the Actuator on a 17"x3" aluminum plate.  The mounting plate also includes the attachment of the push pull cable going to the cowl flaps.  The relay wiring was attached to the aluminum plate.  The aluminum plate was mounted on the left side wall of the tunnel.  You will note there is a 1/2" spacer where the actuator attaches to the plate.  A 1/4" bolt was used for the attachment.  Brackets were made to hold the other end of the actuator in place.  Brackets were also made to attach the push pull cable.  The actuator was a 2" travel Pololu actuator.
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PUSH-PULL CABLE ROUTING:
You will probably notice that there are 2 different cables (black and blue) showing in the pictures.  The original black cable had a larger  bend radius and also was not heat resistant.  The Blue Max cable replaced the original cable as the cowl flap installation progressed.  The Blue Max cable has a 3" bend radius and is also heat resistant. The Blue Max cable was purchased from TEWCO in Wisconsin.  Part number Blue Max 795-6-V-TT-132.  The cable is 11ft long with 3/16" threads on both ends.

The cable attaches to the actuator plate and curves under the fwd passenger seat through a lightening hole in the
F-1042R bulkhead side panel forward door seal bulkhead.  It then travels up behind the F-1068B-R panel and out the top of the firewall.  The firewall location is just right of the center point.  From there is curves to the top of the engine mount.  The cable is clamped to the upper engine mount tube and down towards the cowl flap.  The threaded cable attach fitting is clamped to a lower cross tube and the nuts are threaded tight against the clamp.  The cable is then attached to the control arm in cowling.



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COWLING MODIFICATION:
Since the cowl flap was going to attached directly to the cowling, I decided to remove the inner composite layer and leaving the outside cowling glass.  I place a cardboard layout of the cowling on the inner side of the cowling.  Mark the area and then using a dremel carefully cutting the traced outline in the cowling.  I then used a 36 grit sanding disk and removed the composite material inside the cowling. 

After removing the composite material, I used a combination of glass beads and epoxy to close out the exposed composite material left in the cowling.  I then laid up 7 layers of bi-directional glass in the trimmed area with a layer of polyester peel ply as the final layer.  This provided the beefed up mounting area needed to attach the cowl flap.  The peel ply left a surface that did not need sanding when glassing any attachment to the lay-up.

I made a fiberglass splash of the outside of the cowling in the cowl flap area.  This was to be used if I needed to lay-up a new cowl flap that matched the outer cowling.  This was not required, but the splash did come in handy when adhering  the cowl flap to the cowling lay-up.  Using a cutting disk on the dremel tool, I carefully cut the entire cowl flap lay-up from the cowling.

I covered the inside and outside of the cowling with aluminum attached with spray adhesive.  This provided a parting agent that could be covered with wax.  This prevented any of the epoxy/flox adhesive mixture from attaching to the cowling.  I scuffed up the the aluminum flap with 80 grit sand paper and glued the flap to the trimmed out layup.  A 3/16" wood strip was later bonded to the cowl flap layer in the cowling for attaching the hinge.
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CONTROL ROD:
The control rod transfer the travel of the push-pull cable to the cowl flap.
The control rod was made from  5/8" x .065 wall aluminum tubing.  The actuator levers were made from 3/16" aluminum plate.  The actuator levers were welded to the control rod on opposite sides of the spacers.  3/4" x .049 wall aluminum tubing is used as a spacer between the actuator lever and the flanged bushing inside the pivot block.  These spacers keeps the control rod from moving side to side and also prevents the control rod from digging into the delrin pivot blocks.
I originally thought the control rod would need to be split in order to install it through the engine mount.  This is the reason for the screws in the rod shown in the pictures.  This was not required and the split and screws can be left out of the assembly.
control rod drawing

control rod drawing

control rod assembly

control rod assembly

route thru engine mount

route thru engine mount

control links

control links

completed assembly

completed assembly


PIVOT BLOCKS:
The pivot blocks are made out of 1" thick delrin block.  A bronze flanged bushing is placed in the block to position the control rod.  A #6 set screw was inserted in the top of the pivot block to contain the flanged bushing.

Reinforcement plates were made out of a 3" x 3.5" .05 aluminum sheet.  The reinforcement plate is attached to the back of the firewall with #8 screws and nutplates.  The pivot blocks are attached to the firewall to the reinforcement plate with AN3--40A 3/16 bolts.  1/4" nutplates were attached to the reinforcement plates to secure the bolts.

The pivot block interfered with the outside oil cooler flange.  The flange was trimmed back to allow clearance with the pivot block.
pivot block drawing

pivot block drawing

reinforcement drawing

reinforcement drawing

reinforcement plate and pivot block

reinforcement plate and pivot block

back side of firewall

back side of firewall

oil cooler trim

oil cooler trim


GUIDE BLOCKS:
The guide blocks guide the inside cowl flap fence and folds it under the engine muffler.  The guide block is made from 1/4" Delrin and supported to an aluminum angle that is attached to the cowling reinforced area.   A guide pin was made a 1/4" bolt 3" long that had the head removed and the threaded end was installed in the folding fence of the cowl flap.  Several guide blocks were drawn and made before the correct angle of the fence could be achieved.

One problem that cropped up after the initial block was made was the force from the air stream pushed against the extended cowl flap, the cowl flap would retract about a 1/4"of an inch from its fully opened position.  This was caused by any play in the push-pull cable.  After securing the cable with clamps inside the cabin, the movement was minimized.  When the cowl flap would retract from the wind force, the inside fence would fold following the guide block.  To prevent the cowl flap side fence from folding, a 1/2" notch was placed at the end of the travel on the guide block.  This prevented the fence from folding when the cowl flap would slightly retract from the airflow in flight.

A full scale of the guide block drawing was printed and glued to a board with spray adhesive.  A hole was drilled at the center point of the guide radius and a bolt was inserted thru it pointing up.  A router was also attached on a separate board.  A line was drawn from the center of the router bit 12" out to the pivot point shown on the drawing.  The guide blocks were cut to dimensions shown on the drawing and placed over the position on the drawing.  Small 3/16" blocks were screwed into the board to hold the delrin block in place.  Using multiple passes, the router was guided over the delrin block increasing the depth as it was moved until the router bit was completely thru the delrin block.

Flipping the board with the drawing over, after removing the router from its board, I was able to position the router with stop blocks.  The delrin block was also positioned with stop blocks.  This allowed the 1/2" long notch to be routed out of the delrin block.
guide block drawing

guide block drawing

router pivot plate

router pivot plate

curved cut

curved cut

extended slot

extended slot

flap retracted

flap retracted

flap extended

flap extended


COWL FLAP:
The cowl flap was made from .032 aluminum sheet.  The full scale drawings were glued to the aluminum and cut out on a band saw.  The stationary side fence and bottom of the cowl flap were made from one piece then bent at a 90 degree angle.  The hinged side fence is a separate piece.  An aluminum angle pin attachment was trimmed and riveted to the the hinged side fence to allow the attachment of a 1/4" guide pin.  The guide pin was made from a 1/4" AN bolt with the hexed end cut off.  Two nuts were used on the threaded end of the pin to attach it to the pin attachment.  The final length was determined after the cowl flap and guide bock were attached to the cowling.  A circular cutout was made in the center of the hinged fence for clevis clearance.  The hinge for the cowl flap is the oil door hinge sold by  Non Stop Aviation
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INSTALLATION:
The following pictures show the cowl flaps installed in the cowling.  The location of the cowl flaps, keeps it hidden from the side when retracted.  At first I was only going to adhere the cowl flap to the cowling's cutout with an epoxy/flox mixture.  After I had them installed I decided to include 4 #6 countersunk screws and nuts hold the assembly together.  The console cover around the throttles was sent off to be wood grained.  The cover shown is temporary.  The switch to the left of the throttle quadrant is a lighted switch for the cowl flaps.  The switch on the right operates the ram air induction.

not visible when closed

not visible when closed

OPEN  position

OPEN position

CLOSED  position

CLOSED position

after paint

after paint

console switch

console switch


THE RESULTS:
Before the cowl flaps were installed I would have to climb the RV10 out at 115kts to keep the #5 & 6 CHTs below 400 degrees.  I would also have to use a power setting of 21"/2300rpm.  This power setting and speed would only allow about 600 fpm climb.  Today May 13, 2013 the temperature in Tulsa was 90 degrees.  I decided to do a comparison takeoff and climb with the cowl flaps.  I had already done the initial test with the RV8 flying along side to relay back the movement of the cowl flaps during flight.  Using the power settings I used during the first flights, I took off with the cowl flaps closed to simulate what I had used in the past.  Takeoff with full power, Ieft the MP at full throttle (26") and pulled the RPM back to 2500.  Climbing out at 95 kts I was achieving a1300fpm climb.  Going thru 1000' the #5 CHT was indicating
400 degrees and the AFS engine monitor was squawking about the high temp.  I had to pull the power back to 18"/2300 rpm and increase the speed to 120kts to get the temp to go below 400.

I went back to the airport and landed to a full stop.  During the taxi back for takeoff I opened the cowl flaps.  Not allowing the engine to cool down I used the same power settings as before but with the cowl flaps out, I was able to get the same climb results at 95kts BUT the #5 & #6 CHTs were showing
375 degrees.  I continued the climb to 3000' feet and the CHT never got above 380. 

Designing and coming up with the cowl flaps took me 3 months of work, but I needed to do something so that I could fly the RV10 with a normal climb rate in the summer time.  I had always felt that the existing cowling would not be able to cool the big Lycoming engine during the Oklahoma summers.  During winter flying I would still have high CHTs on the back cylinders (before the cowl flaps) during the climb.

Update 08/03/2013:  After many flights since the installation of the cowl flaps, my high CHT temperatures having been running in the normal range of 350-375 during the climb to cruise.  I am now also able to cruise at 2500 prop RPM if needed for speed.  Even with a flight during a 100 degree day, the temps remained in the normal range during climb.