It has been said there are two types of model airplanes… those that have crashed and those that WILL crash…

Well, if you follow this site on Instagram or Facebook then you know that the inevitable happened!  My competition Mirage IIIRS crashed!  I was at an event back in October last year and for whatever reason, the fan wasn’t making the power it typically does during takeoff.  The takeoff roll was sluggish and instead of aborting and taking home a perfectly fine airplane, I forced the airplane off the ground thinking it would be ok once airborne.  Unfortunately, that wasn’t the case and I couldn’t get enough altitude to get the speed up and the airplane ended up landing in a few large bushes.  I walked up to the airplane expecting the worst and found it upside down but thankfully mostly in tact.  I was very fortunate!  The most damaged area was the nose section being broken but thankfully still connected to the airplane.  I later discovered that the lack of thrust was likely due to a combination of tired batteries not holding voltage under load and a partial blockage in the exhaust duct being caused by some loose tape that I was unaware of.  Thankfully it was repairable and so I thought it would be a good opportunity to discuss some basic fiberglass repair.  Oh, and the lesson here??

LESSON OF THE DAY: If your airplane isn’t doing what you expect it to, abort and troubleshoot!

The most impacted area from the crash was the nose.  Though technically still attached to the fuselage, it was pretty “crunchy” and loose.  Both sides of the nose were cracked just forward of the inlets below the canopy.  The right side took most of the damage which is the side I’m guessing impacted the ground first (the canard was also cracked on this side too).   Also, the whole front camera screen and pitot tube were gone from the very front and nowhere to be found.

The first thing to do was to re-set the nose to get it back in place.  This was done simply by piecing the bits back together on either side and using thin and medium CA to glue it all back.  This was simply to hold it in place so that a more structural glass repair could be done.  With it all tacked back together, some 2 oz cloth was cut and placed on the inside of the broken area through the canopy hatch area.  Note that I clamped a couple hardwood rails to the nose to ensure that the nose remained straight during this process.  From there, I used a laminating epoxy to wet out the cloth while it was in place in the fuselage.  Since I had it available, I used the resin system that my folks use for their fuselage layups (not a readily available resin).  A good and easily accessible option is West Systems 105 with 205 or 206 hardener.  I know a number of folks who use this resin with excellent results.  The nice thing with the West Systems is that there are different hardeners available that allow for a faster or slower cure time depending on what you need.  Now, to apply the resin, I used a cheap disposable paint brush (at 50 cents, we spared no expense!) to wet out the glass cloth while the cloth sat in place over the area needing repair inside the fuselage.  In applying the resin, less is more as you don’t want to apply too much.  It should be applied as necessary simply to wet out the cloth…remember, more resin = more weight!  The cloth should change color but not be overly glossy (glossy areas are typically resin pooling).  To soak up any excess resin, I simply applied some toilet paper to the wet cloth, tapped it with the resin brush and peeled it away.  From there, we let it cure and come back to start the exterior clean up work!

To do the cosmetic repairs, I used USC Icing which is a lightweight finishing putty to fill all of the affected areas from the crash.  A sanding block was used to sand the contours and smooth it all out once the filler was cured.  I usually have an assortment of different sized sanding blocks with different grit sand papers to do this work.  There’s no one size fits all, but usually I resort to about a 6″ to a 12″ sanding block.  A few passes were required to get the contours smoothed out and cleaned up.  Once I was happy with that, then I locally primered all of the effected areas with Evercoat Automotive Primer.

The wing repairs were pretty simple.  The leading edges were all chewed up along with a couple of the pylons.  To repair the wing leading edges, I first hardened up the exposed balsa wood with some thin CA (necessary so the Icing filler will stick to it).  From there, I simply filled  all of the affected areas with Icing Filler and sanded it all to match the wing leading edge contours again.  From there a couple light coats of primer were applied focalized in those areas and it was ready to touchup the paint.

Matching the paint can be difficult in these repairs.  Since I built and painted the airplane, I have plenty of paint to spare!  If you’re repairing an ARF or something someone else built, then it’s probably best to have paint matched and mixed.  Even having the paint on hand, the colors do tend to change tones slightly from sun exposure and whatnot (using a non-yellowing clear coat is critical to avoid a good portion of this!).  So, to blend it all, I decided to repaint the entire nose.  Also, localized areas around the inlets and the wing leading edges I sprayed using a wider spray pattern to effectively create more of a ‘blend’ on the paints.  Thankfully, the shades all matched up very well and any tonal differences really just look like touched up panels and areas that you would get on the full sized airplane.  The most difficult to blend was the underside light color based on the weathering that had been done.  So, using some creative washes, I blended the underside colors as much as possible.  Though not perfect, it’s certainly good enough as the full size is never perfect either!  Note that sometimes it’s better to touch up the paint by masking off areas.  Since I was dealing with larger areas and solid colors, I opted to go with the wider spray to help it all blend.

With everything all touched up again (including reapplication of some dry transfers) and finalized I finished it all off by re-applying a clear coat over the entire airplane.  Instead of localizing the clear to just the repaired areas, I applied it to the whole aircraft to help blend it all back together which it did.  It gave the airplane again a nice uniform finish like it had before the crash.  Note that I don’t apply a heavy clear coat, it’s mostly just a dusting since it’s really only applied to even out the paint finish and protect the weathering work that’s been done.  For clear, I use a non-yellowing lacquer satin.

Troubleshooting the fan was a must.  There’s no reason to fix the airplane if we don’t understand what happened!  The E-Turbax fan and Neu motor had been bullet proof for many years and so understanding what went wrong was paramount.  Upon pulling the fan from the airplane, the first thing I noticed was that there was some tape hanging inside the duct.  You see, for ESC cooling, I like to tape the ESCs to the top of the exhaust ducts exposed in the airflow as this helps keep them extremely cool during operation.  I typically tape on the outside, but at one point I had placed a piece of tape on the inside of the duct across the ESC.  Well, unknown to me, it was now hanging well into the exhaust flow.  At first glance, I wasn’t too concerned.  I ran the system on the bench which showed decent numbers.  However, while running the fan in an instant the tape started fluttering inside the duct and I saw over a 1 lb decrease in thrust and the tone of the fan changed radically.  In remembering what occurred when it crashed, the fan didn’t sound right on the roll out which must have been the tape fluttering.  Very interesting!  Also, the batteries were old and seemed to be struggling.  So the verdict was that the combination of the tape fluttering in the duct with the tired batteries not putting out the power they should have caused enough of a performance decrease to make the airplane power marginal.  Ultimately, there was nothing wrong with the fan or motor which is good!

Since I had the fan out of the airplane, I thought it would be worth installing and trying a JetFan 120 in the airframe.  The motor used was a Neu 1917/1Y (590Kv) which based on the specs would give about 16 lb thrust at about 90 amps using 12 lipo cells.  In bench testing these numbers proved to be about right.  With the same tired batteries from the crash, I was seeing about 15.5 lb thrust at about 85 amps.  This was good enough for me, so the fan was installed into the airplane and has since proven to be an awesome power system for the mirage.  With the lower current draw, the flight times have significantly increased which is ideal for competition and the overall performance is actually slightly better…what really seals the deal is the sound!  It’s the most turbine sounding EDF installation I’ve heard!  Note that the diameter of the fan is slightly less than the E-Turbax which the inlets were designed for, so I CAD modeled an adapter ( available through my 3D Printed Products Page ) that I then 3D printed on my Robo3D.  Also, the recommended exhaust for the fan is 4″.  Based on the afterburner lights I have in the airframe, I was limited to 3.75″ exhaust diameter which has proven to be fine thus far.  In ground testing, I didn’t see any difference in current draw running the fan without the exhaust or with the 3.75″ exhaust which told me that the it wasn’t struggling to much.

Here’s a quick flight video to give an idea of the JetFan 120 (note the sound!)

For comparison, here’s a vid with the original power system.

Next time we’ll talk about building detail parts such as pitot tubes and antennas as I lost a few in the crash and thus had to re-build them.  Until next time, I’ll see you at the field!

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