|Dec 09, 2012, 12:47 PM|
Hobby King P-38 Version 1 Maiden and Build Notes
It took about three months of off-again/on-again build effort, but I finally maidened my HK P-38. Winds were light, but a little gusty, and sometimes cross to the road I use as a runway. I still have some finishing touches to apply, but since the planets aligned such that I had no family obligations and the weather was good, I headed out to my flying site to get it in the air. I was there by myself, so no pictures of the airplane in flight. I had several notes regarding my build, though, and thought I would go ahead and give the full rundown now. I’m hoping in the next few weeks I can get some formation-flight photos with my Lightening and my buddy’s Banana Hobby F-35 Lightening II.
CONFIGURATION/MODS: It is a version 1 from Hobby King that came with mechanical retracts. All-in-all, I was pleased with what I saw in the box. No damage or major warpage, paint looked pretty good, all parts were there, and it even came with a carbon spar in each wing (though the spars were not connected to each other. My build took about three months because of other projects I had on-going, holiday obligations, a car crisis (ran into a ferrule hog that caused about $4200 in damage!), work conflicts, and of course, weather. There were also the complications of having to fix and modify the model. My mods included installation of E-Flight servoless retracts, 2 1/4-in. foam main wheels, 1 ¾-in foam nose wheel, installation of a nacelle-to-nacelle carbon fiber rod wing spar, installation of a carbon fiber spar in the horizontal tail; installation of rudder tail-skids, and unintended replacement of the ESCs (more on that later).
CONTROL SET-UP: I set up the pushrods to the next-to-furthest-out holes on the control horns and servo arms. This resulted in the following control throws: Elevator, +/- 20mm; Rudders, +/-20mm; Ailerons, +/-18mm. I programmed my radio to 75% dual rates on all surfaces, and 25% expo. Center Of Gravity ended up at 75mm from where the curve on the inboard part of the wing leading edge begins, where the leading edge meets the cockpit pod. Add another 10mm if you are measuring from the front of the joint where the wing fits into the cockpit pod. This CG position is just about on the front of the trough that the wing carbon fiber spar is in (the one that came installed in the kit). I didn’t try to ballast to the recommended position in the manual, as I felt like the recommended CG was too far forward anyway. See my post 5362 at this link for more detail on my CG analysis: http://www.rcgroups.com/forums/showt...28276&page=358
POWER: I used a Turnigy 3S2200mah 35C battery, weighing 186.2 grams. It’s about a year old, and has had 66 charge cycles. When I ran my power numbers, the battery wasn’t quite fully charged. It had a static charge of 11.72 Volts. With that battery, and the stock props, here are the full-throttle numbers my power meter gave me: Dynamic voltage, 9.48V; 29 amps; 250 Watts. Measured Static Thrust was 2.5 lb. Airplane total weight with battery is 3.3 lb., resulting in a power ratio of 75.8 W/lb, and thrust-to-weight ratio of 0.82. Performance with a fully charged, new battery would probably be notably better. Even so, this puts my plane at the low end of the Sport category. I get about 4 1/2 to 5 minute flights and have 3.7 volts per cell left. I have my timer set for 4 minutes, and start making my way to the runway when the timer beeps. Batteries are pretty warm at the end of a flight. Spec weight from the manual is 2.5 lbs. I really wonder if that number is achievable? Maybe completely stock with the gear taken out… I’d like to hear how heavy everyone else’s Lightenings are…
FLIGHT CHARACTERISTICS (maiden): Okay, so maybe all the grief was worth it. Smoothest, least-anxious maiden I’ve ever had. Even with the moderate power and heavy weight, it is a sweet-flying warbird! I still contend the manual’s recommended CG is way too far forward. Check my CG calculations in my previous post. My experience seems to confirm those findings. The airplane still felt clearly nose-heavy, though not too bad, even with my further-aft CG position. Nose had to be trimmed upward for normal flight, and it really noses down when I pull the power off. I feel like I could easily ballast the tail to get it back to the 15% static margin position and it still be solidly-stable, but that would just add more weight.
The airplane felt like it had plenty of power for scale flight. Acceleration was not breath-taking, but more than adequate for keeping me out of trouble when I would get too low and too slow. It won’t do too many vertical rolls, but it does climb easily.
It has plenty of aileron effectiveness as far as I’m concerned. Even at 75% dual rate, full-throw aileron rolls just looked too fast for a P-38. The dynamics of having those two booms and engines out from centerline make some maneuvers look a little awkward (such as quick roll-reversals), with the inertia swinging the movements out of axial at times. Part of that is probably due to being nose-heavy. I wonder if counter-rotating props would take care of some of that? Turns required a slight amount of rudder coordination, along with up-elevator. If you want to watch the plane drop, just pull the throttles back and make a sharp bank. I think the plane has great handling characteristics, but they could be improved considerably with a further-aft center of gravity.
Landings are awesome. I had to hold the nose up and give it the slightest amount of throttle on the approach. With throttle all the way back, the props create more drag than if they were free-wheeling, so just a crack of throttle keeps it gliding reasonably. It didn’t need a flare at landing as much as just needing to keep the nose from dropping as speed bled off near touchdown. I was afraid of banging the rudders on landing, but the airplane seemed to want to stay level once in ground effect, and anything more than a modest flare would cause it to head upward while decelerating. That being the case, landings were fairly flat, and once the mains touched the ground, the plane pretty much planted itself solidly on the ground. No porpoising tendencies or pronounced wing drops at all.
BUILD NOTES: This forum has been awesome in setting my expectations and helping me to address the airplane’s shortcomings. The most frustrating thing about the model is that it could have been SO MUCH BETTER with the slightest effort by the manufacturer. Of course, that would likely have made it a more expensive purchase, too, but come on! Do the canopy frames really have to look like they were spray painted by hand without any stencils or masking? Does the gear really have to be a throw-away? My kit appeared to be a conglomeration of spare parts. The invasion stripes on one nacelle were crisp white-and-black decals. The stripes on the other nacelle were painted on, and the “white” was more of an ivory color. Likewise on the wing halves, except the wing with the decal evidently had a tear in it that had been painted over to fix it. The spar on the bottom of one wing was painted silver, while the spar on the other wing was not. The green anti-glare paint was only on the removable hatches that go on top of the nacelles, and on the battery hatch. None on the nacelles or cowlings themselves. A few other beefs with the paint: Anything yellow had to be repainted. The spinners were horrible, with runs and drips and scuffs. They had to be sanded smooth, as well. The yellow on the cowlings and the prop tips was just a faint misting. All of the yellows and greens had to be repainted. The canopy was absolutely terrible. It looked like the silver had been sprayed on free-hand. It was such a mess, I had to take two nights to polish it all off. To the manufacturer’s credit, this paint was solidly stuck on. I didn’t think I would ever be able to get it all off. I used my Dremmel tool and some chrome polish to get it off. I tried every solvent I could find to get that paint off the canopy: Mineral spirits, acetone, alcohol, polishing compound. The only thing that would come close was the chrome polish, applied with the Dremmel polishing wheel and a good amount of pressure. It was very tedious work. After I got the paint off and polished and cleaned the entire canopy, I used electrical tape that I had primed and painted chrome to make the canopy frames.
While I had the canopy off, I decided to take care of the pilot with the scary white eyes. I took some thinned black paint and wiped it over his face, and then wiped him clean. That made him look a LOT more realistic – and toned down those headlight eyeballs.
Aside from those areas, the paint was actually pretty good. Very little chipping or poor adhesion of the base silver. One funny nit – They had the wing tip nav light colors reversed. The left wing “lights” were green, and the right wing’s were red.
So here are my mods and build comments…
First to go was the gear. I had an unused set of E-Flite retracts that were looking for an airplane. They may be expensive, but they are the best, and they just work – no muss, no fuss. The real gut-check was carving inside the wheel wells. Installing the gear wasn’t hard, just nerve-wracking as I was trying to be very careful not to cut in the wrong place or carve out more than necessary. I have to say that the wheel well doors are terrific. The hinges hold them tightly closed, and they stayed firmly attached even while I was Dremmeling and carving. Once I got the gear in, I used some pushrod wire to fashion attachments to the struts to push the doors open, and keep them from catching on the E-Flight axle collets, during retraction and extension. Same idea as the original gear.
One knock on the E-Flite nose gear is that you can’t easily move the steering mechanism from one side to the other. I’m not sure if you can do it without breaking it. So, I installed the steering servo on the starboard side, where the gear steering linkage was located. This meant that I had to somehow reverse the direction of either the rudder servos or the steering servo, since left rudder made for a right steering command. The simple solution was to take the steering arms off the rudder servos, rotate them 180 degrees, put them back on the servo, and reverse the rudder direction on my transmitter. One benefit of doing this is that it eliminated any interference of the rudder pushrods with the elevator pushrods.
Next two mods were to add carbon fiber spars to the wing and the horizontal tail. The main purpose of putting the spar in the horizontal tail was to straighten out the slight bowing of the foam. Turns out that it helped, but didn’t completely get rid of it. I am glad I did it, though, because it seemed to make the entire aft structure of tail booms/rudders/tail more rigid. It wasn’t until I was ready to put the horizontal tail into the rudder slots that I realized there was an up-side and a down-side to it. Of course, I had put the spar in through the top side. I sprayed some scotch tape with chrome paint and used it to cover up the spar. For the wing, I used a 16-inch long, 1/8th inch drill bit and eyeballed what I thought would maintain the wing dihedral when the two halves were put together. In order to get the straight spar to be level through the wing dihedral, the drilled hole ended up being very near the upper wing surface near the wing root. The bit tore a little of the surface in that area, but I covered it up with tape. I poured a lot of epoxy into the hole, slathered epoxy over half the spar, and pushed it into the hole. I had to wipe some of the epoxy that ended up coming up out of the insertion hole, but I was assured that any voids between the spar and the foam had been filled with epoxy. That’s probably the heaviest way of doing it. Probably could have done the same thing with Gorilla glue at a lot less weight. I did the same thing on the other wing when I assembled the wings and cabin pod. I was wishing I had four arms and hands for that process!
Another hopefully great mod was installing wire tail skids on the rudders. I took some stout music wire and bent one end into a curve to match the shape of the foam “tail skids”, and sharpened the other end of the wire so that I could push it about an inch into the foam. I used CA to glue the long end into the foam, and epoxy to glue the wire to the foam skid, and fill the gaps. Then I painted it with the Short Cuts Chrome.
The Greatest Convenience Modification Award is a tie between two hatches. I installed a spring latch from Hobby King on the back of the battery hatch, and replaced the thick foam front tab at the front with a slice of plywood. That holds the battery really secure, and the hatch cannot come off. The front tab had to go, in any case, as there wasn’t enough room for the battery with it there. The other hatch was one I cut out of the bottom of the cockpit pod, along the molded-in panel lines, as was suggested in some previous posts. I liked it so much, rather than gluing it back when all the wires were connected, I put magnets on it so I could easily position and access the receiver and antennas. I would consider this a must-do mod.
Replacing the ESCs was not in the original plan, and I’m embarrassed to relay the story. Hopefully others can find some humor and caution in the story: When I finally had my airplane together and ready to do an installed electronics check, I discovered a problem during motor run-up. I was holding the airplane on the ground, but when power was applied, there was a major yaw to one side. I quickly figured out that I had counter-rotating props. That’s not a good thing when the props are designed to turn the same direction. I thought, “No problem. All I have to do is switch two of the ESC wires.” Well, it was late, I was tired, I was in a general bad mood, and wasn’t thinking clearly. Instead of swapping the wires between the ESC and the motor, I opened the nacelle hatch and swapped the black and the red ESC power wires, thinking those were the wires between the ESC and the motor. It wasn’t long before I started smelling the “oops-you-just-fried-your-ESC” smell, and quickly disconnected the battery. The battery was pretty hot, and the ESC was clearly baked. Normally, there would have been a Dean’s plug on those wires, so I would have caught myself. Not so on this twin, which has bullet-connectors on the extension cables to the battery. Fortunately, I had a pair of 30-amp Detrum ESC’s in my parts bin. I quickly switched them out with the originals, and was back in business. I’m not sure on the fate of the battery yet, though. It takes a charge, it balances, and it provides the same power and duration as before, but is sure seemed hot after flying with it the next time I used it. (Follow-up note: Indeed, the battery is no good. Using that battery now, the motor will spool up to full power for a few seconds, but then it just runs down to where the props are barely swinging. I discovered that when I tried flying with it. It produced great power at the pre-flight run-up, but as soon as I turned downwind after takeoff, it lost almost all power. The motors still pulled a little, but not enough to maintain flight. I was fortunately high enough that I was able to turn her back into the runway and do a not-quite-deadstick-but-almost landing. Just a reminder, boys and girls: Static voltage on your volt-meter can be a lot different than dynamic (under load) voltage!)
FUTURE MODS: I may cut the foam cross-member out of the back of the battery bay to allow pushing the battery further back, and allow for a larger battery. (Posted by WestTexasKing: If you do this mod, velcro is required, or possibly a rod you can insert to stop the battery from sliding rearward.) I need to figure a way to vent the battery compartment. It's not good to have the battery tightly wrapped in a foam beer cooler when you are pulling a lot of amps. I might put a couple of small intakes on the sides of the nose to let air in, and direct it up to the battery bay. A fun mod would be to hang some bombs on the wings where the fuel tanks go. I think it would be really easy to put a servo in the fuselage bay where the receiver is that would allow in-flight bomb release. Also, I think it is a matter of time before I have to put new motors in. If a prop blade ever gets broken, or a motor shaft gets bent, I don’t think I will have any other option due to the lack of available parts. I would really like to change the nose art as well, if I can ever find something that historically goes with the rest of the paint scheme. I really did not enjoy painting the airplane, although I think it made a big difference in how good it looks.
WIRING: The wiring for this airplane looks pretty intimidating when you start looking at the wad of cables. It really is not as bad as it looks. All the connections are numbered, and there is plenty of room for positioning and routing. Just make sure everything moves the right way once everything is connected and you have your transmitter programmed. My kit did not come with any 3-way connectors. I had one that came with my E-Flite retracts, and I used two "Y" connectors in place of a 3-way for the rudders/steering servos. The wiring diagram with some tips is attached below.
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|Dec 10, 2012, 10:27 PM|
|Dec 15, 2012, 10:07 PM|
You replaced the gear and retained the orientation of the main struts on the outside. Is there a structural reason?
Was the gear leg length an issue as the gear seems very short compared to the real thing. Wheel wells too short?
Thanks, good looking model.
|Dec 16, 2012, 01:50 AM|
I left the struts to the outside because that is the side the door hinges are on. The strut pushes the door open, and if the struts were on the opposite side, the doors would catch on the wheel during retraction and get crumpled.
The gear on mine are a longer than what were on the original model, and the greater diameter of the tires makes the overall gear length longer as well. I wanted to retain the existing gear doors, as they are really nice, but use larger tires that looked more realistic. I carved into the front end of the wheel well bay so that the strut was as far to the front as I could get it, and it still be covered by the door. The front of the retraction mechanism actually extends slightly forward of the door and the extended strut, but it is below outer mold-line of the boom so the door doesn't have to cover it. I also carved out the back side such that the wheel, when retracted, is actually further back than the back edge of the door. As the gear extends, the arc that the wheel follows means that it moves forward, so I set the gear such that the arc of the wheel path just clips the point where the back of the door is when closed. A shorter way of saying all that is that when fully retracted, the wheel is below mold-line and slightly aft of the back edge of the door, and when fully extended, the strut is right at the front edge of the door. This results in overall greater length of the gear, when compared to what came with the model originally.
I had to do quite a bit of carving to get the large diameter tires to fit in, and to get the servoless retract mechanism deeper into the wheel well. I also had to make little cuts into the ledge that the doors close against so that the wheel collars could pass through.
Having done that on the nose and main gear, the main gear actually ended up being a little too tall. The length of the nose gear is more constrained than the mains, so the mains ended up making the back of the airplane a little higher than the nose. I ended up moving the axles on the mains up a little higher on the struts so that the airplane would at least sit level. I could have carved a little less out of the back of the main wheel wells. The airplane would look a little more scale on gear if the mains were shorter, but I didn't want to drag the tail on landings and takeoffs. What I ended up with isn't perfectly scale, but looks a lot better than the tiny wheels of the original model.
If you don't care about the landing gear doors, then you can make the gear a lot longer, but I think you'll still be constrained at some point by how tall you can make the nose gear.
Part of what makes the gear look taller on the real airplane is that instead of having one large door hanging down on one side of the wheel well, there are two, narrower doors hanging on both sides of the wheel well. Each door looks longer and narrower than what is on the model. That makes the wheels look larger and the struts longer relative to the doors, on the real airplane.
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