There's No Place Better - EMRR! EMRR Rocks!
the basic, real and invariable nature of a thing2!

Guests On
   myEMRR
The Best & Worst of Plastic Model Conversion
Featured Story:   All  Previous  Next 

By Chan Stevens

PMC

With the 2006 EMRR Challenge likely to include a plastic model conversion (PMC), I thought it might be helpful to share my experiences with this very unusual event.

To be honest, I had always been a little intimidated by PMC, despite the fact that in most local and regional competitions, I had generally done fairly well on craftsmanship events, and roughly 2/3 of the scoring for PMC is craftsmanship-related. I just thought the complexity of fitting a motor tube and recovery system into something like a plastic jet plane, and figuring out how to turn the canopy into a nose cone was a little more than my rocketry skills could handle.

In the summer of 2003, though, I got hooked on the NARTREK program, and quickly progressed through the bronze, silver and gold levels, and started picking away at the advanced. That same summer, our local rocket club decided to include PMC as the main event in their fall contest. I begrudgingly decided I might as well try a PMC, figuring I could kill two birds with one stone (NARTREK advanced/PMC and the images/contest/NAR points). Plus, with the contest only likely to draw 3-4 entries, I figured I'd be a lock to at least place 2nd or 3rd.

For my first venture, I decided to go with something that someone else had already figured out, hoping that would be reliable and relatively easy. I hit the Qualified Competition Rockets website (http://www.cybertravelog.com/qcr/) and bought an F-104 Starfighter kit. One of the things I quickly discovered about PMC is that NAR rules restrict entries to jet planes or rockets, so the field was narrowed a bit.

F-104The QCR kit arrived about a week later, and I was ready to begin. The kit came with two sets of instructions-one for the plastic model, and one with notes about the conversion. I found the notes a bit confusing, especially since I had no PMC experience, and relied quite a bit on emails to/from Ken Brown at QCR for help. Most plastic models are not stable for flight, and so nose weight must be added for the conversion. This kit included lots of lead sinkers and clay, which I packed as tightly into the nose as I could. For a motor, I was able to leave off the tail cone piece from the kit, leaving enough room to get a BT-20/18mm tube inside the model. Since there was a lot of slop, I had to epoxy in tiny wedges in order to keep the motor tube centered.

Recovery is probably the hardest part about PMC. Since there's not exactly a nose cone to pop off, you have to create some type of deployment mechanism. One of the most common with jets is to saw the plane in half, usually just behind the cockpit. Then, you can insert a coupler into the cockpit area, and line it up with the motor tube going back. A major part of the craftsmanship score hinges on how well (and seamlessly) you pull this off, as a model that looks obviously hacked and doesn't line back up right isn't going to fare well in front of the judges.

Once I had the basic conversion done, I proceeded to finish off the model. This included building and bonding lots of detail parts (the more the merrier, as they help the degree of difficulty score). By the time I was done, I had something like 16 missiles, 100 decals, and had used over 12 different colors of paint. The finished model was beautiful, and after static judging I had an insurmountable lead and was virtually a lock for first place.

Unfortunately, at NARAM this event is nicknamed "plastic death", as so many otherwise beautiful models tend to crash in rather spectacular fashion. I was fairly confident, though, having used a kit rather than risk my own design. I loaded in the C6-3 and set out to the pad.

Once my F-104 got off the rod, it almost immediately arced over and headed out on a horizontal plane. This apparently is not unusual for jets, but not exactly what I expected. After cruising about 50 feet, though, my pilot lost control, and the F-104 went flopping around completely unstable, crashing into the ground while still under thrust. Many of my beautiful details had been destroyed by the impact, including the canopy.

Tip #1 for PMC-bring lots of duct tape. You're allowed two flights, and due to a quirk in the scoring, damage points (deducted from the flight score) are for each individual flight, not cumulative. What this means is that if you manage to put a broken model back together, and it flies OK the second time, unless there's additional damage caused by the second flight, there are no damage points despite the duct tape and missing details. I patched my F-104 back up as best I could, and added a little more nose weight (at that point, I couldn't fit in any more, and was concerned about the model weight versus the tiny thrust of a single C6-3).

Flight #2 was only slightly better than flight #1, though this time with the added weight the pilot kamikazed into the ground and the plane literally crashed and burned. I had to stomp out a small brush fire when I got out to the impact crater. After about 30 hours of conversion work, I had fallen from a lock on first place to DQ, not even flight points for the event. I was so upset I took the pieces back to my car/prep area, deposited them in front of my tires, and drove the car over the remains. Then, just to make sure I had done the job, I backed over them a second time. I was so frustrated by this event that I'd sworn I'd never mess with it again.

Well, never is sort of a long time, and I'd also set a goal for myself to be somewhat competitive at NARAM-46, and hopefully place in the top 10 when NARAM-47 would return to my home state, and PMC was on the slate for NARAM-46. I was facing another wonderful PMC experience.

This time, I did quite a bit of research before starting again. I found some excellent tips on George Gassaway's site (http://members.aol.com/GCGassaway/images/contest/PMC.htm), as well as a yahoo group devoted to PMC exclusively. Through the research, I learned a number of valuable lessons, the most important of which are:

  1. Always build a prototype first. Before devoting all the time and effort into detailing and finishing a PMC, build just a basic version first to prove out the stability and conversion strategy. I found that Hobby Lobby conveniently has half-price sales every 6-8 weeks on plastic models, so I waited for a sale and loaded up on 2 copies of 3-4 different models I was considering.
  2. Pay careful attention to the thrust alignment. I didn't realize how big a factor this was, but if you look at the alignment of jet wings versus the typical motor tube positioning in the tail, you'll find the thrust is not parallel to the wings. This is hazardous to the flight performance.
  3. The best rule of thumb for stability of a jet is to make sure the CG is at least forward of the leading edge of the wings. My first PMC had a CG roughly a third of the way back from the leading edge.

For my second PMC attempt, I selected a 1/72 scale F-14 Tomcat. The main reason I went with the Tomcat was that it was small/cheap, looked fairly easy to convert with (2) 13mm aft engine nacelles, and had ducts under the wings that I felt I could get a couple of parachutes into. The conversion did prove to be fairly easy, as I was able to bend a couple of 13mm tubes up from the nacelles all the way to the ducts, so I wouldn't even have to cut the model for deployment-I could just slip the chutes down the end of the tubes for ducted ejection.

NARAM-46 1

I built the prototype in two evenings, and took it out to the field the following weekend for a test flight. I stuck a couple of A10's in, and crossed my fingers. The prototype flopped around totally unstable, and crashed under thrust. Damage was minimal, so I decided to rework it for another attempt. The model has wings that can be deployed forward or swept back, and I had opted for the forward deployment. After some debate with my fellow flyers, we decided swept back might be more stable, so flight #2 went swept back. Slightly better, but same result, though this time the damage was substantial.

Back to the laboratory/drawing board, I tried adding a little more nose weight. I was also concerned about the deployment, since I hadn't gotten a single chute out on either flight, though the ground could have been a factor. After packing in as much clay as I could fit, I went back out at our next club launch for another attempt.

Flight #3 showed traces of stability, though clearly not enough power, as the extra nose weight was more than the pair of A10's could handle. It crashed to the ground during the delay phase, and the model was basically destroyed at that point. So, having built a prototype and destroyed it, what had I learned? I seemed to have mastered the crash flight, but still couldn't get a stinkin' PMC to fly.

At this point the NARAM deadline was approaching, and I was down to my last F-14 kit, so decided to proceed boldly ahead with the same kit rather than try a new/unproven design at NARAM. Not that my Tomcat was exactly a proven design…

For my NARAM entry, I went all out on the detailing, which was incredibly difficult on such a small scale model. The night before turn-in, I was practically going blind applying decals that couldn't even be read except with a magnifying glass. The effort paid off, though, as I was astonished to see I was in second place after static judging, quite a feat for a first-time NARAM competitor. My friends were confident I was going to take a trophy for the event, though I had to remind them I'd never actually gotten a qualified PMC flight on any model ever in my life.

Naturally, the day we were to fly was the windiest day of the week, so PMC's were going to be raining down all day. As I checked in my model, the RSO gave it a close examination, and after an awkward pause, asked if I'd flown a prototype of the model. I said "yes sir, I flew several prototype flights first". That was enough to get me to the pad, where I anxiously awaited my turn to fly.

NARAM-46 2Remarkably, my Tomcat was stable, rolling over to a horizontal path about 75 feet up, then nosing down after burnout. Even with a pair of A10's though, the altitude wasn't enough to cover the 3-second delay, and the model crashed into the ground, popping the chutes out while on the ground. It landed right at the feet of the RSO. Yikes. He suggested I take out a bit of weight, tape/glue it back together, and give it another shot. I did so, and flight #2 managed to get a little bit higher, deploying the chutes about 10 feet above the ground. It still crash landed, though the RSO invoked a popular PMC ruling, saying "we'll let the ground decide" on the qualified flight. This means that he'd allow the flight to be considered qualified, though I'd likely get a high damage score wiping out most of the flight points. Since I had already had such a high static score, though, I was able to hold on to second place for the event. To top it off, I also won the "Best Midwest Qualified Flight" award for the first flight. This award is given at NARAM for the most memorable crash of the meet. I managed to become the first person to ever win both the Best Midwest Qualified Flight award and a regular trophy for the same event.

Fast forward to the summer of 2005, and I was getting ready for another PMC for NARAM-47. This time, I was bound and determined to learn from my previous efforts and sail through with a good flight. I decided to go with another Tomcat, though this time went with a 1:32 scale, which would offer enough room for a pair of 24mm motors. That should eliminate that pesky lack of altitude problem. The conversion was basically the same, though business travel at the last minute prevented me from building a prototype in time to fly, so I had to jump straight in to the final version. I again went all-out on the detailing, building an absolutely spectacular model, NARAM 47and dumping at least 40 hours into the project. As an afterthought, the morning of the turn-in, I decided since the decals were fairly fresh, a clear coat might be helpful to protect them. I sprayed on a Krylon clear, which attacked the clear plastic cockpit cover, resulting in a disfiguring frost-like finish (and masking the intricate detailing I'd done for the cockpit and instrument panels). At that point, I felt like I was doomed. As it turned out, though, I was about 4th in static, as the judges felt the canopy was only a minor blemish on an otherwise wonderful model.

I had packed in a lot of steel shot and epoxy as nose weight, and made one other serious corner-cutting mistake by not weighing the model until it was returned to me the night before flight day. My model was a staggering 32 ounces, well above what a couple of D12's could lift. I chipped away at some of the nose weight, taking out as much as I thought I could spare while still keeping the CG just ahead of the wings. As I checked in, a couple of friends from my club surveyed my PMC, and when I told them I was flying on D12-3's, they said "Oooooh, bad idea-you want to go with zero's". I thought about it, and decided I'd be better off with 3's than run the risk of shredding the mylar chutes with a zero delay deployment. Besides, the guys are nicknamed "The Ballistic Berry Brothers" and had set the field on fire earlier that week, so how seriously could I take their advice?

Crash HelmetAs I loaded my Tomcat on the pad, the RSO (wearing a crash helmet to commemorate the day of Plastic Death) was concerned about the weight versus thrust, so cleared all spectators from the flight path. I then raised the paddle signaling to launch the rocket. It headed up beautifully straight, but agonizingly slowly, then turned over to a horizontal flight path, in the opposite direction as we'd cleared. Once the motors burned out, I began the infinitely long 3-second count, and noticed that the Tomcat was headed back down to earth fairly quickly. It was also headed towards the row of vendor tents and trailers. Just after I counted to 3, the parachutes deployed, though just a split second before the Tomcat attempted a "carrier landing" onto the trailer of one of the vendors. Still clipping along at a good speed, the Tomcat exploded in a shower of plastic as it penetrated the sheet metal trailer of Performance Hobbies. After some debate and pleading, the RSO qualified the flight, again deciding the ground would be the judge. The flight did, after all, deploy the chutes and had the trailer not been under it, the recovery would likely not have been too bad.

Getting ReadyWith the qualified flight, I managed to place second in the event again. Again, I won the Best Midwest Qualified Flight award, though this time it was not just for the same event as a regular trophy, but for the same flight. I also became only the second back-to-back winner, a distinction I have no intention of repeating.

What I've learned is that PMC is an extremely difficult and challenging event, and that it takes lots of practice and experience to get it right. You can't exactly build one of these things in a simulator and try it out in a vacuum. Read, study, and practice, practice, practice. Preferably a safe distance from people and property (my NARAM flight wound up costing me over $400). As much as I hated it, and swore I'd never do it again, I find my thoughts drifting back towards plastic models, as the challenge and excitement keeps calling to me. To paraphrase Dickens, it is the best of events, it is the worst of events…

Copyright © 2019 by RocketReviews.com