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Q-Talk 168 - Flipping the Board

Belly Board mod that rewards you with yet another GUARANTEED  .003 mph speed gain.

By Jerry Marstall

[EDITOR'S NOTE: This article is another "speed mod" provided by Jerry Marstall. You can read about more of his work in Issue 166. Thanks again Jerry!]

I mounted the belly board on my Tri-Q according to plans, hinged at the back to open to the front.  By designing the belly board to open to the front, the linkage was quite basic.  Simply release the handle and let the wind open it and simply pull the cable to close it.

 I have never liked this implementation for the following reasons:

  1. I wasn’t able to get the corners of the board to stay up tightly within the fuselage.  As a result,   I always felt that the airstream was probably pulling the board open a bit costing me airspeed/fuel.
  2. Whenever I opened the board it scooped up exhaust fumes – not good.
  3. If the retraction cable were to malfunction, it would fail to the open position requiring a timely landing before being asphyxiated or running out of fuel.

Hinging the belly board at the front for a rear opening addresses all of these concerns.

While it has always a concerned to me, I confess that it wasn’t enough of a concern to cause me to do anything about it over the 17 years my Q has been flying.   

It wasn’t until the 2014 Sun N Fun Q-form when Richard Kaczmarek of Fast Little Airplanes fame showed up and mentioned he had turned his board around to hinge at the front. That peeked my interest.  I thought, this should be simple enough.  I’ll simply turn the belly board around and somehow attach it to the front of the belly board cavity. 

The challenge became how to open the belly board in this configuration.  The original mechanism design for opening and closing would no longer do the job.  As I previously mentioned, in the original design, the belly board acts as a scoop with the air stream holding the belly board open.  A tug on the cable would retract it.  Hinging the board at the front meant the mechanism must now force open and hold the belly board into the air stream that is trying to force the belly board to close. 

To solve this problem, Richard installed a direct control tube to the belly board for deployment and retraction.  As usual, I chose to make things more difficult by wanting to retain my present control handle and cable linkage to the belly board.

Here is what I did.  I’ll address the easy step first - relocation of the hinge; turning the belly board around 180 degrees. 

1.  Remove the piano hinge from its attachment point on the fuselage by drilling out the rivets.  Do not remove the piano hinge from the belly board.

2.  Make a paper pattern of the hinge side that is opposite the belly board.  Make the pattern the exact size of the hinge leg and mark the center of each mounting hole.  Put it aside.  More on this later.

3.  Above I mentioned, “. . . simply turn the belly board around and somehow attach it to the front of the belly board cavity.” For this to fit properly, the belly board had to be originally cut as a perfect rectangle leaving you with a perfectly rectangular cavity otherwise the board won’t fit the hole when turned 180-deg.  Unbelievably, my belly board fit the hole nearly perfectly.  That isn’t my typical luck.  If yours doesn’t fit, you will have to modify the shape of the belly board to fit the shape of the cavity.  Suggest you modify the edge opposite the piano hinge so you won’t have to remount the hinge.

4.  A length of 1”x 1” aluminum angle is used to reattach the free side of the belly board piano hinge to the fuselage.  I used a hand-held hack saw blade to scratch out a slot in the foam the thickness of the aluminum leg (1/8”) between the bottom fiberglass skin of the fuselage and the foam that forms the fuselage shell.  This slot runs the length of the cavity and is parallel to the bottom fuselage skin.  The slot recedes enough to receive the base leg of the “L” of the aluminum angle.

5.  To attach the angle aluminum to the hinge, with the horizontal leg from the hinge slid into the slot you carved, position the vertical leg of the hinge (A) flush up against the aluminum angle (B), pushed tight against the hinge pin.  Clamp the vertical leg of the hinge to the vertical aluminum angle.  Make sure to open and close the belly board to ensure that it will.  Then drill each of the mounting holes for 1/8” pop rivets and rivet the hinge to the aluminum angle.

Flipping the board

6.  When satisfied with the fit, secure the belly board in the up position (good ol’ duct tape).  Retrieve the hinge template that you made in step 2.  Align the template with the hinge and tape it to the bottom of the fuselage.  Drill the 1/8” holes for the pull rivets through the template, bottom of the fuselage and horizontal leg of the aluminum angle.  If you want to countersink the pop rivet heads, now is the time to do it.   Then from the underside, pop rivet the horizontal leg of the aluminum angle to the bottom skin of the fuselage.  You now have a belly board that opens to the rear.

7.  The next challenge is to fabricate the mechanism to open and close the belly board. The problem is space.  There is only 1.5” between the bottom of the main gear bow and the top surface of the belly board when it is closed.  In this small space needs to be a mechanism that can force the belly board out into the wind stream at some predetermined angle.  I chose 80-degrees which required the trailing edge of the belly board to open 8.5”. I finally settled on a scissor styled mechanism.

Mechanism Right

I strongly suggest fabricating the initial mechanism components out of a material other than aluminum, such as 1/8” plastic, because you will probably make more than one before getting the geometry right.   Since I am not an engineer, there were no fancy formulas for me – lots of do and redo.

The final two scissor arms are constructed from 1/8” aluminum and are each approximately 3” long.

Since I chose to retain my actuation cable, I already had one pulley mounted in front of the landing gear bow.  In order to get a straight, horizontal pull on the scissors, it was necessary to mount another pulley beneath the original pulley.

Pulley Mechanism

 You guys with conventional gear will probably need a different solution.  A way of testing your mechanism design is to hold the belly board up with your hand and attempt to deploy it with your actuation control handle.  If the belly board cannot be forced down, your geometry needs modification to gain the appropriate power advantage.

8.  All that remains is a means of retracting the belly board out of the wind stream.  I installed two springs, one attached to each corner of the belly board trailing edge with the opposite ends attached to the seatback bulkhead.

Spring Mechanism

9.  Now for the fun part.  Pull/push your actuation handle and see if it deploys.  Now release the handle and listen for the belly board to slam shut.  I discovered that the actuation cable was hanging up on the bolt head that attached the lower scissor arm to the belly board.  That is why you see the “dowel” on the bolt.  It keeps the cable from hooking on the bolt head. 

All together

Flight Test:  When I first deployed the belly board, the nose quickly pitched up about 5-deg.  The original board was a scoop which pulled the nose down, now it seems to work as a lifting surface.  I simply push it over, allowing me to point the nose down for a good view of the runway. 

No more fumes, better runway view, less drag and if the mechanism malfunctions, it will fail to the closed position.  Why didn’t I do this years ago? Thanks for the push Richard.