A few years ago I ran some MotoCalc modelling to determine the most efficient modeled setup with an 8" propeller and common motors, and the Great Planes Ammo 24-35-4875kv
, 3.0:1 gearbox 8x6 2 blade prop proved to be the best of the lot on hand. On 11.1V, it should draw about 33.6A while providing 31.5oz or static thrust. Static motor rpm was modeled at 38289, static prop rpm 12763, max speed 66mph, Pitch Speed 72.5mph. I've drawn up some other motors for use and located the matching 28mm/24mm gearboxes, each with variable ratio. If I was going to use a motor from Great Planes, I would choose the 24-35-4875 due to it's efficiency at 31mph cruising speed [battery amps 3.5A, motor 8.2A and long run time] with the available ratios and props. The other secondary options were the Ammo 28-35-5100kv (7-8" props) and Ammo 28-35-3900kv in a 3.3:1 gearbox. Tolerances and variances that come about in the real world will reduce the speed and flight time but the overall concept and comparison to other motors and gear ratios will hold true. The fact, however, it that I had other motors on hand that I wanted to try.
She's a pretty girl mocked up. Lets get on with this!
The first thing I did was remove the stock gearbox and motor. The stock motor was good for about 110 Watts of input power and 50 Watts of shaft power on the stock 9x6" propeller and 9.6V 1800mAh NiMH pack. The pack weighs a hefty 9.2oz. I'm looking for at least 200 Watts of shaft power on 8" props.
The gear drive comes apart in two sections to produce a nylon spur gear and brass pinion on a stock 2mm shaft, in 3:1 ratio.
I moved to the nose and re-drilled the 3mm mounting screw holes in the gear of the gear drive for the new brushless motor. I drilled as high as possible to keep the mesh tight. The next thing I did was install and glue the factory carbon fiber wing root, not shown, but self-explanatory.
The drive shaft was a bit too long and it held the propeller spinner out about 3mm from the cowl. I cut a couple mm from the shaft and repositioned the spur gear about 1mm forward. To do this, I loosely placed the gear and shaft in the gap of a bench vise and used a hammer to force the shaft in the direction required. It is really tight.
The propeller and spinner sat too far to one side, and too high. I used a power drill to re-drill the firewall holes downward and to one side to realign the spinner. The $15 dremel tool I recently picked up for my neighbor's Hobbico Skylane Cessna 182 Brushless Conversion
(another interesting build log in my blog) proved to be handy once again in reworking the motor shaft hole in the center of the firewall.
While I was in the area, I removed some material (shown in red) that was in the way of the steerable nose gear. I proceeded to drill the holes for the plastic mounts.
Notice both pinion gears mesh as 48 pitch, but the teeth on the stock gear are longer. The stock T-400 380 motor versus the Suppo H2223-4 4400kv, essentially the same size. The weight difference is about 5 grams. The difference in power limits is five-fold, although an 8 or 9" prop will not pull very high current.
Again, this 500 class heli outrunner is rated for 500-600W of input power with a built in fan. I also tested the steerable nose gear for fit.
I shimmed the rear of the gear drive with .010" plastic sheeting. The way I have it positioned as shown, it increases the downward and right thrust angle when mounted against the firewall.
I wanted to use an 8x6" APC propeller and the center hub of this prop is rather large. I used the dremel's 1" cutting/grinding wheel to carefully remove the obstructing material from the screw pillars on the rear spinner plate and some material from the inner hub. Do not modify the propeller or it will not only be out of balance blade-wise, by also laterally! The end procedure was actually a lot easier than it reads here. The prop now fits perfectly within the spinner.
I installed the motor and gear drive to prepare for the cowl. I drilled the stock brass pinion to 3.175mm and affixed it to the new motor shaft.
Fitting the fiberglass cowl for the steerable nose gear is easy. Simply set the cowl over the gearbox, align the pin stripes make a forward-running line where the nose gear rod extends out. The fiberglass is brittle but easy to work with. you can reinforce the inside with .015" plastic sheeting to prevent future cracking of the cowl. I recommend it.
Inside the fuselage, the motor and ESC fitted. This ESC is the Turnigy Trust 45A, and I have two of these and each one causes interference symptoms on 72MHz band radio systems such as aileron servos "ticking". It will NOT be staying in the plane for the maiden flight and is only shown here for build purposes. I recommend a Castle Creations, Jeti Advance or other quality type. Mystery (Hobbyking Blue Series) has proven to introduce zero interference on 72MHz even at long range over 1km on a 60mW transmitter, in my other aircraft. As it turns out, FM radio systems do not like some SBECs (switch mode battery eliminator circuit integrated into the ESC). If you ever encounter this problem of flutter or ticking servos, spastic throttle RPM or poor range in a range test, you have two primary options. Convert to using a 2.4GHz transmitter and receiver system, or stick to FM and replace the ESC with one that uses a linear BEC. It's also becoming more common today to install an opto ESC without
an integrated BEC, and simply buy or build a separate linear BEC. They only weight about 10 grams.
Centered! Much better than the factory fit.
Looking good from above. I may add a bit more offset to counter any torque steer during takeoff but that has yet to be determined. The cowl permits at least another degree or more and will actually align the spinner better. If you view the photograph of the gearbox again, notice the pinion position which also would align better with the gearbox offsetted to counter more torque steer. Funny how that all works out so proper.
Profile view, great. I could go half a mm lower, but I don't have OCD and thus it's very good.
The battery tray had been a point of contention for some time. I decided to raise it a bit to install the ESC under the removable battery tray to keep the lateral balance centered, as installing on the side made the fuselage side-heavy. I am strongly opposed to adding weight or using aileron to compensate, because the magnitude of compensation will change as an inverse square relationship to the airspeed. I used screws to make it removable so that I can change my setup in the future when it is required. Future compatible, kids!
Next, I extended the battery tray by 2.5" using 2mm plywood. Balsa wood will work just as well and you do not need to extend it this much, as only a very heavy pack will have to be mounted this far back. 1.5" to 2" extension should be perfect so most anyone and most any pack. Short, fat packs are the easiest to position and balance in the stock tray but longer packs may have to be further forward or rearward. My Nano-Tech 65-130C 1800mAh 3S 11.1V pack which only weighs 180 grams will balance the plane when sitting almost all the way forward. Larger packs nearing 2500-3000mAh can sit back further. I reinforced the underside with .050" carbon fiber rods, which are very strong yet weigh very little.
Battery tray installed, but looped velcro not shown here. The flat velcro on the bottom keeps the battery pack from shifting forward or aft, and the looped velcro stops it from flying around inside the aircraft.
Before I could go much further, I had to mount the vertical and horizontal stabilizers. I used a coarse sanding wheel on my dremel tool to remove a good portion of the Gelcoat finish for a strong adhesion. Prepare the control rods for the elevator and rudder but only glue and shrinkwrap the rearward steel rods with the factory bent ends. You will understand why, soon.
A soldering iron on medium-high heat made a defined sealed line in the Monocoat and made trimming really easy. This is done so that the glue can bond directly to the wood. My advice is to use a small paintbrush and coat the wood in thin CA and allow it to dry before epoxying or thick-CAin the stabilizers. My one observation of CA is that while it makes wood hard and difficult to break, after about one decade it can make balsa wood susceptible to breakage and brittle degradation. The control horn that you use for the elevator has to be mounted with the steel control rod in place before the stabilizer is affixed to the fuselage. The plastic horn has to be shortened on both ends so that it does not interfere with the near-by surfaces. You may wish to trim the curved hole in the tail so that the elevator can move the entire length without any interference. A hole is made in the center of the elevator and the horn and its opposite holder is glued tight with CA.
After mounting the horizontal & vertical stabilizers and letting the glue solidify, I prepared the control horn for the rudder. I used the utility knife to produce a hole through the control surfaces for the plastic horn that was parallel with the airframe. You can use the pin striping as a guide. I followed the instructions to drill two 2mm holes 16mm below the horizontal stabilizer (elevator) spaced 13mm apart. I then used a utility knife to complete the hole and pass the control rod through. This works out really well. Again, the steel control rod has to be fed through the hole and the horn is placed on the end, only then can it be glued into the rudder.
Now, this is why you do not glue & shrinktube the servo end of the rods. Once the battery tray has been installed with screws, the servos will not sit in the stock position and the control rods would be too long. Simply shorten the wooden rods as required but leave yourself with more than you think is needed. A utility knife makes it easy as pie.
Installing the aileron servos is easy but requires some level of craftsmanship, creativity and cutting with a very sharp utility knife and a bit of sanding. I installed a Hitec HS-55S analog servo in each wing. I didn't waste time making it flush, sinc the rear of the servo begins to protrude through the top of the wing.
Here, I revisit the carbon fiber rod and the wing root. You may glue the wings permanently to the fuselage or allow them to remain detachable. The factory method of attachment consists of this carbon fiber rod, and one small screw-in steel rod on each wing that the collets lock onto. I added two .050" carbon fiber rods drilled into the trailing edge of the wing to provide additional stiffness. No floppy wings allowed in my hanger.
The forward round hole toward the leading edge of the wing is never explained in the manual, but the long oval one near the middle portion of the airfoil is for your aileron servo. It narrows but extends out to the end, so that lights can be installed if you wish.