Archive for August, 2009

I got the idea from a Model Airplane News web article.  It seemed simple enough and would add a bit of excitement and challenge to those high speed low passes to try and bomb a target.

The drop bombs are from Hobby-Lobby and are relatively inexpensive at a little over $10.00 a pair and each drop bomb includes a mounting / release mechanism.

The other item needed is a micro servo.  I used a Turnigy TG9 from Hobby King.  Not a bad deal for $2.99.

Size: 23 x 11 x 29 mm
Voltage: 3v ~ 6v
Weight: 9g / 0.32oz
Speed: 0.12 sec/60(4.8V)
Torque: 1.6 kg-cm

The first thing I did was find the ideal location on the wing to mount the bomb.  Preferably you’d want it at or just forward of the CG and in an area deep enough to allow for the depth of the servo with enough clearance between any ribs and spars.

Bomb Drop 006

Once I found the location, I cut out the hatch.  The size of the cut is up to you.  My VQ Hurricane has covering with panel lines and I simply used part of a panel section as my guide.

I used an X-acto with a fresh blade to carefully cutout the hatch.  I added 1/16 ply strips to attach the hatch mounting screws and provide support for the opening.  I cut a small hole through a balsa spar for the servo lead exit.

Bomb Drop 005

I assembled the bomb mount/release mechanism without using the included spring.  For this application, using a servo to actuate the release, the spring is not needed and just added stress to the servo.

The mechanism was mounted off to one side of the hatch to allow for the mounting of the servo using #2 x 3/8 button head screws.  Be sure to remove the screws and add a drop of CA to the holes in the wood to strengthen the threads.  Wait until the CA has completely cured before you re-install the screws.

Bomb Drop 004

To mount the servo, I CA’d a 1/4 inch balsa standoff to the hatch to provide clearance for the servo arm.  The linkage is made from thin connecting rod wire.  I attached the servo to the standoff using strong double sided tape.

Bomb Drop 011

When testing the completed assembly, be careful not to over actuate the servo.  The mechanism requires less than a 1/4 inch of travel to lock and release the bomb.

Note that unless you are using a servo reverser or dedicated receiver channels for each bomb drop servo, you will need to flip the servo mounting location 180 degrees from the above picture for the second bomb drop.  Otherwise the servos will operate in the opposite direction of each other.

I attached the hatch to the installed ply strips using #2 x 3/8 button head screws.  Again, remove the screws and add a drop of CA to the holes in the wood to strengthen the threads.

Bomb Drop 007

Bomb Drop 008

Look for upcoming video of the Hurricane making bombing runs.

Update: Flew the maiden and made several bombing runs.  Bombs drop great each and every time.  Sorry, no video of the flights yet.

Update: Video added.  Not much of the drop is captured though, but it’s all I have.

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