By Fred Culbertson
I have always had a special interest in WW II fighter aircraft. This dates back to my first encounter with the Mustang flown at the first airshow I attended with my father in 1968 at McCoy Air Force Base (now Orlando International Airport). Although the SR 71 and the F-4 Phantom seemed to be the latest technology paraded through the sky that day, I would never forget the howling silver 4-bladed warrior making its low passes. I was hooked for life.
For years I dreamed about Mustangs, and as a teen in the early ’70s I built and flew several replicas, but there was always something missing—that sound: a combination of 3,000rpm 4-bladed chop mixed with a supercharged 12-cylinder Merlin.
I have flown the Byron Original P-51 offered in the ’80s, and as a 4-bladed replica, it was the era’s best (and only) 4-blade kit. In November of last year, I found a Byron Mustang Kit and decided to bring it up to current technology, but I was having several issues with its light construction and reduction-drive vibrations.
I was on the Internet looking for different engines and power combinations for my Byron, when I stumbled across AeroWorks’ newest offering— their ARF 1⁄5-scale Mustang. I knew of their reputation for large aerobatic birds, but a Mustang? I couldn’t believe what I was seeing.
After several phone calls to AeroWorks and a quick sale of the Byron, I was in the club. We needed the uncovered version for our project and paid $1,585 for this production kit. My Aeroworks Mustang arrived December 29, 2009. I was like the kid in “Christmas Story” getting my new Red Ryder BB gun. When I opened the boxes, I swear I heard a choir in the background!
I confess that I fly, test and video sponsored aircraft, but this bird is mine. My partners in this insane mission, Abe Williams and Dave Collins, were involved in the selection, build, testing and flight of this project and share a common desire to make it work. Without their hundreds of hours and dedication, this would take another year to get off the ground.
As we put the bare-wood parts on the table, you could tell how much effort was placed on the construction and detail of every flight surface. This was well thought out, and I couldn’t wait to see what it looked like put together. Mission 1: Use the rest of my wife’s left-over adhesive Christmas tape and assemble the parts for some well-earned dream time. I purchased the uncovered version because my plan was to cover it in bright shiny Flite-Metal, with rivets and decals and logos, oh my.
This was the first time I had ever seen a preinstalled sliding canopy. You can opt to install a servo, or manually slide the canopy open. It was a very nice touch, and I am sure most will take advantage of this. Pic1A
As we played with the new plane all taped up on the bench (while making little Mustang low-pass noises), I was very happy to see the retracts, air lines, retract servos, actuators and valves already installed. The first time we charged up the retracts and cycled them, we found out that the sequencing valve to operate the main gear and doors was already programmed.
We noticed our air system had a few leaks and only held air for about 10 minutes, so we introduced some air conditioning compressor oil (very light oil) into the actuators to get the seals wet, and we were in business. We also added some oil to the sequencing valves and are now holding 120psi. After an hour, we are around 80 to 90, and we get five good cycles (done in succession) from a full charge.
I’ve seen lots of RC Mustangs in my time, but the only ones I have seen flying with a 4-blade prop was the Byron, and I never found a reduction drive I was comfortable with. I was determined to find a way to fly this bird with a 4-blade prop. It just wouldn’t be the same without it.
The AeroWorks Mustang was designed for the DA 50, and I have seen some electric conversions that are very fast, but this called for a little more power to swing the scale 24-inch prop system needed to carry this off. This was a job for the DA 85 8.5hp with an operational 1,200 to 7,500rpm.
We selected the Solo Prop 4-blade system for our Mustang. It isn’t a cheap setup, but when you open the box, you know where your money was spent. Having the ability to adjust your pitch to govern rpm and thrust was paramount to the success of this mission and would not have been achieved without this key piece of equipment.
The DA 50 setup was intended to be installed with standoffs, but the 85 bolted directly to the firewall. We added a 24mm extension to the front of the 85 to get the exact main shaft length to clear the installed cowling for the prop and spinner installation.
Our next project job was to prepare the surfaces for our Flite-Metal (F-M). I am not a very good painter, and there isn’t anything on the market (in paint) that could replicate the metal skin better. This is the longest part of the build. We needed to fiberglass all of the wooden surfaces, and our final product needed to have a paint-ready surface.
After preparing the aircraft for the Flite-Metal, I found the installation of it to be a little time-consuming at first, but after I had finished the first wing, I was in the groove and completed the top surface on half the wing in a few hours. Once the product is installed, you can polish it to a mirror-like finish (as it is real metal), or scuff it with a Brillo pad to get a weathered look.
The only problem I encountered with the Flight-Metal was its high adhesion factor. This is great in normal circumstances, but if you need to remove a panel because of a poor installation, it will bring any primer or filler underneath the surface with it when you remove F-M from your project. F-M comes with a small spiked wheel you roll across the surface to give your project those cool rivets, but be careful not to press too hard, as it can penetrate the fiberglass underneath.
The F-M was the perfect touch for our project and gave the Mustang a great look. Not bad for a bunch of guys lacking any serious painting skills.
The time had come to get serious on our electrical setup. This Mustang has an average build weight of around 24 to 25 pounds, and we knew the addition of the DA 85, fiberglass, Flite-Metal and the Solo Prop was never factored into these numbers. Once the aircraft had been completed, it weighed in at 27.4 pounds. I was thrilled with this number, and I knew we had the power to pull it off.
We selected Hitec’s 7985MG servos as recommended by AeroWorks. We needed seven for flight surfaces and added Hitec’s 5645MG servos for throttle and choke. In our opinion, if you invest in a project such as this, the electronics is no place to skimp, and I use Hitec equipment in all of our aircraft. I realize everyone has their own favorite equipment package for hundreds of reasons, but I have never lost an aircraft to servo failure since I switched, and that’s throwing helicopters into the mix. It’s just my own personal comfort factor.
We also added the Smart-Fly Power System Eq6 that has the ability to provide 7.5 amps continuous with 15A peaks backed up with three Fromeco 2600mA lithium-ion batteries. This would give us enough redundancy and ignition power to handle our needs, and to be honest, I was looking for a little more drool factor. It is overkill for this project for some, but having the ability to individually adjust servos’ EPA and rates separately from the radio is a great feature. It also helps with my pucker factor, knowing I won’t lose my warbird to battery failure.
I love the Hitec Aurora 9 system; it was the perfect match for this project. My reason for this radio selection was its ability to give me onboard aircraft-battery voltage information with audible alarm if my voltage drops to an unsafe level. It is also the most program-friendly system I have ever used. The touch-screen programming usually requires just a few pushes to change anything I want, and its ability to program any switch on the transmitter, or change any receiver channel to any function I want, is very convenient when setting up a model of this caliber.
The hardware included in the kit for these installations was nothing short of top shelf. Honestly, I would have never purchased separately what came with the aircraft. Adjusting each control rod is made so simple, and you don’t sacrifice strength for the option. The control horns look like they were made by a jeweler with the install points drilled for the screws, with installed hardwood backing plates at every point.
The last change needed was the addition of a 32-ounce fuel tank instead of the included 22. If you plan to play with a fire-breathing dragon, you need to be prepared for the additional fuel request. Desert Aircraft claims the 85 will drink 2.2 ounces per minute at 6,000rpm. That may be true for a 2-blade sport aircraft, but this bad boy guzzles 3.6 ounces per minute. Glad we brought the fuel truck because dead-sticking 700 man-hours of work with more than $5,000 invested with months of fibs to my wife explaining were all these parts came from is not on my bucket list.
At last our new shiny Mustang was completed and ready for some engine test. If you will follow in our footsteps and successfully fly a 4-bladed design, this is where you want to pay close attention. In our efforts to resolve blade speed, thrust, blade diameter and destabilization rpm, we found the following.
We started with a 24-inch blade design, as this was the closest diameter for a 1⁄5-scale Mustang. For the record, I am not an expert on 4-blade systems or claim to have much intelligence even considering taking on a project such as this, but I will give you my best effort in explaining our success.
We were concerned about blade destabilization, meaning at some point on the rpm scale a 4-blade prop will start to lose thrust. This is created because the distance between each individual blade is obviously half that of a 2-blade, and the air between these blades will become unstable if you try and run any propeller too fast. Four blades will destabilize at around half that of a 2-blade.
The problem with available engines on the market is that most nitros run in the 1,400 to 1,500rpm range, and gas motors are closer to 7,500. Through our research we found the destabilizing rpm for our setup. At 6,500rpm, we had 27 pounds of static thrust; at 6,800rpm we had 1.5 pounds static.
When setting up a Solo Prop, you are adjusting more pitch to govern your full-throttle rpm the engine can create. For example, we could dial in enough pitch to keep the engine at 3,400rpm at full throttle, but this would create way too much strain on the e
ngine and more likely cause it to overheat.
We started with a 24-inch prop, and we were not creating as much thrust at our desired static thrust experiments. On further investigation, we discovered (this means we actually read the information provided by DA on the 85) that this engine was most successful running a 26- to 27-inch 2-blade or a 24- to -25 3-blade.
After the light bulb popped on in our heads (which happens much slower these days with about half the wattage), we decided to go with what was already proven technology and move our 4-blade to 23 inches. Again, once we installed the new 23 in our system, I heard the choir singing in the background again. Talking about blind hogs finding acorns; this was the epitome of dumb luck, but I will gladly take it.
Our optimum setup for the DA 85 was losing the extra mass of the 24-inch system and installing the 23 with about 12 degrees of pitch, giving us 5,600rpm with 37 pounds of static thrust and most likely an unloaded in-flight engine rpm of 5,800 to 6,000. This gave the engine enough rpm to run cool at full throttle, while providing us with about 30 pounds of in-flight thrust at full throttle. This is a calculated guess, but we have tons of performance in the flight video on our 27-pound airframe.
It was time to put this beast in the air. After two throws of the prop with the choke on, it gave us a pop. Choke off and one more throw, and life was created. The four blades with their yellow tips were looking as real as it gets.
As warbirds go, you have to be on the rudder rather quickly, and we had concerns over the torque factor on takeoff. I only gave it about 1⁄3 throttle, and in 30 feet we were in the air. I wasn’t expecting that, but it was flying, so I just let it climb out and retracted the gear. At full throttle on low passes it is, to say the least, very intimidating. The DA 85 on the Pitts muffler (if you want to call it a muffler) drew attention from the next county, but the howling of the 4-blade wasn’t going to be outdone.
After a few flights and some relaxation exercises, I was starting to enjoy the ride instead of concentrating so much on flying the airplane. There wasn’t much trim needed in the aircraft, but what really shocked me was dropping the flaps. The aircraft never pitched or gave any indication that the flaps were extended.
Landing a 4-blade aircraft is not your average approach. Cutting throttle to idle is like throwing out a drag chute, and it will fall out of the sky if you let it. Only ¼ throttle was needed with about a 30-degree angle of attack, and it floated down the runway like a trainer. I couldn’t believe how easy this aircraft was to fly, land and taxi. I did throw in around 5 degrees of tow on the main gear, and it really helps the tracking.
Videos of the flight can be found on the Aeroworks website on the Mustang page under 4-bladed DA 85, or directly on YouTube at http://www.youtube.com/watch?v=RlHdJQL4AGE
All the guys at AeroWorks, DA, Solo Props, Flite-Metal and Fromeco were very patient with us, and we appreciate all the hours of conversation provided to make this a success. This was without a doubt the most exciting project I have been involved with as it was a real community effort.