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Sep 18, 2018, 07:00 PM
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Build Log

Hobby King 1875mm B-17G Build Blog


Nothing special, but I thought that I’d do a build blog for the HK 1875mm B-17 that I recently purchased. I stumbled across the HK B-17 by accident. HK was running a sale, and I couldn’t pass up a great deal on a multi-engine warbird.

I’d have preferred to have gotten an OD Green B-17, but HK only had the silver in stock. I think that HK is discontinuing the B-17 and discounting their remaining stock to free up the shelf space. Glad to have gotten one.

I’m a warbirder, but not much of a bomber guy. So, I did some fast and furious research and decided to model the G Model I’ll Be Around of the 390th, on display in the Tucson Air Museum.


Mods planned:
- retractable tailwheel (FMS 1.4m Bf-109G Tail Wheel)
- G Model nose details (Freewing B-17G plastic parts)
- make the Main Struts more secure (replace grub screw with better fasteners, flatten spit on struts for grub screws, and probably drill/tap additional grub screw holes)
- Decal Set from Callie Graphics.
- weathering? Haven’t decided yet. I may leave it as a pristine B-17G, like the one on display in the Tucson museum.

This is how B-17 looks right now.



The guys over in the HK B-17 Discussion Thread have provided a lot of great information. A special shout-out to BornToLate and BRGT350 for being especially helpful!!!!
Last edited by Oxotnik; Sep 18, 2018 at 09:29 PM.
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Sep 18, 2018, 07:49 PM
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Painted behind the cowl. Used a Tamiya silver. Should have used aluminum. Don’t have aluminum paint handy. May redo this later in Aluminum.





Fail!
Last edited by Oxotnik; Sep 18, 2018 at 07:58 PM.
Sep 18, 2018, 08:36 PM
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Configuring the Receiver


My receiver will be a LemonRX LM0040 10-channel Diversity Antenna. My TX is a DX9.



Motor Configuration:
- Battery:
—Outer Engines Y’d and Inner Engines Y’d
- Throttle
—Left Engines Y’d and Right Engines Y’d

I’m configuring this B-17 to have differential thrust. To do so, I have created 4 mixes that have worked well for differential thrust on my Avios Bush Mule Float Plane. The left engines are on the Throttle Channel, and the right engines are on Aux4. I have configured the inside engines (‘inside’ being on the inside during a turn) to reduce thrust by 50% or 100% with the rudder. Three-position Switch G will goggle between (0) No Differential Thrust, (1) 50% Differential Thrust, and (2) 100% Differential Thrust. Here’re the snapshots of my mixes:











Since half of my engine throttles are on Channel 1 and the other half are on AUX4, I had to program Mix 5 to create a Throttle Cut for the AUX4 engines.



TX Switch Assignments:
A - Gear (0 Down, 1 Up)
B - Flaps (0 Full, 1 Half, 2 Retracted)
C - Rates (0 High, 1 Medium, 2 Low)
D - Timer reset (2 reset)
G - Differential Thrust (0 Off, 1 50%, 2 100%)

RX Channel Assignments:
T - Left Engines Throttle
A - ailerons (Y’d at the moment; I may do I dependent ailerons later)
E - elevator
R - Rudder
Gear - Flaps
AUX1 - empty ( I couldn’t assign anything. Not sure why)
AUX2 - empty
AUX3 - Gear
AUX4 - Right Engines Throttle
Last edited by Oxotnik; Sep 18, 2018 at 10:42 PM.
Sep 19, 2018, 05:42 PM
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Masking and Painting


This evening, I did a little masking and painting before I started assembly. I stopped by Hobby Lobby on the way home and picked up a can of Kryon Foil Metallic to fix what didn’t work yesterday- chrome-looking heat shield behind the cowls. I like it.



Next, I did the yellow markings on the nose, rudder, and right wing. I had some yellow paint defeat portions of the masking on the wing, but it’ll be easy to fix with silver paint.





Sep 21, 2018, 10:05 PM
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Finished assembling my wing and have connected all of the wing servos, gear, and motors to the RX for a system test. As I said in my second post (IIRC), I have set up my B-17 to have a mix for differential thrust. A switch throw will deactivate differential thrust. With differential thrust active, the left two engines will drop 50% when left rudder is applied and the right two engines will drop 50% when right rudder is applied. Here’s a short demonstration video of differential thrust in action.

HK B-17 Differential Thrust Test (0 min 49 sec)
Sep 22, 2018, 05:53 PM
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RX Installation


I had a spare channel on my 10-channel LemonRX and DX9; so, I gave each aileron its own channel. R Aileron is on Channel 2 and L Aileron is on AUX1. Doing so, however, did something funny to my gear channel, and the gear were deploying with the flaps. Went through the setup a zillion times, but I couldn’t isolate the gear from the flap switch. So, I showed the Gear Channel the bird and moved the Gear to AUX2. Flaps are on AUX3.

Here’re my final channel settings:
1 - R Throttle
2 - R Aileron
3 - Elevator
4 - Rudder
5 - vacant
AUX1 - L Aileron
AUX2 - Gear
AUX3 - Flaps
AUX4 - L Throttle
AUX5 - LEDs

since I couldn’t get inside the fuselage over the wing saddle to mount the RX, I decided to mount the RX on the hub of the wing. Since the 10-channel RX is rather large, I cut out an appropriately sized hole in the wing saddle to accommodate the RX. I bundled the servo wires so that they’ll be easy to slide through the hole and will allow the RX to also fit. Only the Elevator and the Rudder servo wires have to be mated to the RX during final assembly. I added 8” servo extensions into the E and R channels on the RX, to make it easier to connect the E and R servos during final assembly. The fit check was perfect. I’m happy with this setup.





Things left to do:
- Balance Props, then install them.
- Add I’ll Be Around decal set, which is in the mail from Callie
- Even though I have the FMS Bf-109 Retractable Tail Wheel, I’ve decided that I’m going to fly it first with the stock tail wheel, before installing the retractable tail wheel.
- The Freewing B-17 G-Model plastic pieces are still out of stock at Motion; so, she’ll be flying as a hybrid F/G model, until I can get the G Model details for the nose.
- Since the aft machine guns stick out so far past the rear of the plane, I’ve decided to leave them off for now. Otherwise, I think that they’d get broken off, probably before I got her in the air for her maiden flight.
Sep 27, 2018, 09:40 PM
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agedflier had made some mods to his B-17 and posted them in the HK B-17 Thread. I stole one of his ideas about enlargening the battery hatch. This will give me access to the RX through the battery hatch, if I need it. The foam was easy to cut cleanly. I made a rib using 1/8” birch plywood. I also made a tongue with the 1/8” plywood and stuck a 1/4” dia rare-earth magnet into the PW tongue and put a matching magnet into the bottom of the hatch. The hatch is tight and the magnets keep the hatch extra secure.





Sep 30, 2018, 08:20 PM
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Today, I modified the battery bay to accommodate a single 3S-5AH battery. To do so, I carved out some foam and cut a 3/16” plywood support to secure that battery. I can still run two smaller batteries as well.
Oct 01, 2018, 06:48 AM
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Quote:
Originally Posted by Oxotnik
Today, I modified the battery bay to accommodate a single 3S-5AH battery. To do so, I carved out some foam and cut a 3/16” plywood support to secure that battery. I can still run two smaller batteries as well.
That's a great mod on the battery tray. Im going to do the same with mine. At the minute I only use a 3s 3900mah lipo but my flight time is fairly short. I have four 3s 4250 mah that I would like to use but they wont fit in, so this mod of yours will be of great help. Thanks for sharing.
Oct 01, 2018, 07:05 AM
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Aidan, I got idea from borntolate Bob. You can see his battery bay mod in his build blog. He also posted a few pictures in Post 11140 of the B-17 Memphis Bell Thread. I think that I’m going to be happy with this mod.
Oct 03, 2018, 01:06 PM
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I maidened my B-17 late yesterday evening. I flew a single 3S-5AH battery. Differential Thrust was disabled. CG was right at 15" from the nose. During taxii and the takeoff run, I noticed that the B-17 felt really light in the tail, as the tail wanted to rise as soon as the throttle was nudged. After an aborted takeoff run, due to unexpected yaw, the second takeoff roll was very nice and the B-17 lifted easily into the air. I spent most of that first battery trimming out the right roll and dive. Once trimmed, she was a peach to fly and the landing was picture perfect.

For the second flight, I used a single 3S-5200 battery and pushed the back 1" or so of the battery over the rear wall of the battery bay, which pushed the CG back a half inch or so. I had to abort the first takeoff run again, due to mishandling the yaw. On the second takeoff run, I was so focused on controlling the yaw, that I didn't manage the tail properly and the B-17 lifted into the air too quickly, at a high AOA. I stabilized her and she flew fine, not needed much in the way of tweeks to her trim settings.

For both flights, I ran for 5 minutes, which included aborted takeoffs both times. So, the flight time was probably more on the order of 4:15. After each of the flights, I had 3.8v and 3.7v per cell, respectively.

After both flights, the battery and battery cables were almost too hot to touch. This is worrisome. So, I will probably start flying two 2200mah or 2600mah batteries. I also don't have sufficient cooling through the battery bay, since the G model chin turret blocks the air inlet hole. Will need a mod to increase the airflow through the battery bay.

Here is the slightly edited video of the Maiden and subsequent flights:
HobbyKing 1875mm B-17G Maiden & 2nd Flights (10 min 17 sec)
Nov 14, 2018, 11:10 AM
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Very nice work!

Bob
Latest blog entry: The XB-38 s/n 41-2401
Nov 14, 2018, 11:10 AM
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This past weekend, I had my third flight day of my B-17, usually getting 2 flights per day. As I have previously noted, I modified my battery bay so that I can fly a single 3S-5AH battery. After four of five flights with the single 5AH battery, it had begun to really worry me that the battery was getting too hot during flight, as the battery was almost too hot to handle. For the flights this past Sunday, I bought two GensAce 3S-2600-45C batts. I made two flights, one with the single 5AH battery and the other with the 2.6AH battery (two motors Y'd into each battery, not a true parallel power circuit). After the flights, the single 5AH battery was again very hot; however, the two 2600s were cool to the touch. I was amazed. I guess it makes sense that four motors pulling from a single battery is going to pull twice the current of two motors pulling from a single battery, but I did not expect it to make that much of a temperature difference.

So, if you're planning to get one of these HK B-17s definitely plan to run dual batteries, and not a single larger battery.

Further investigation/tasks planned:
- Solder together a parallel power cable, to see if that makes the two batteries run hotter. I don't think it will.
- Put an Wattmeter in the circuit (both single and dual motors), to see how much current each configuration is pulling.
- Check the IR of the batteries, especially the older 5AH battery.
Nov 14, 2018, 11:15 AM
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Quote:
Originally Posted by borntoolate
Very nice work!

Bob
Thanks, Bob! I've really been enjoying the B-17. I've never been much of a bomber warbirder, but flying a four-motored plane has been a blast.

Despite hearing reports that the B-17 will nose over on grass runways, I have not experienced that problem. I have not had to rake my Mains forward any more than they already were. I've also found that the stock props provide plenty of power for takeoff and flight. This plane is also a pussycat to land.
Nov 14, 2018, 11:21 AM
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And here’re the photos of I’ll Be Around with the J added to the right wing.


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