VMAR DHC-6 Twin Otter with roll, pitch and yaw gyros
This blog is dedicated to my build and flying experience with this Twin Otter.
Motors: Turnigy 2213, 1050kv, 16A cont, 19A peak, 56g, 27.6x32mm, 3.175mm shaft
Props: APC Electric 9x6, or 10x5 for payloads. 10x5 is approx. 10mph less than the 9x6, but higher thrust at low speeds.
ESC: Turnigy Plush 30A
LiPo: One 3s 5000 for 15 minutes, or one 3s 2200 for 6 minutes or when carrying payloads.
Aileron Servos: Align DS415M
Elevator and Rudder Servos: Hobbico CS-17
Steering Servo: Futaba FP-S133
Aileron Gyro: Spartan Quark (headlock gyro)
Elevator and Rudder Gyros: Turnigy TG380 (rate gyros)
Tx/Rx: Futaba 7c with 617FS
I was looking for a new plane, something with a high wing and something with tricycle gear because I've been flying low wing tail draggers for too long now. I was thinking a twin would be nice since I've never really got into those since I started flying 14 years ago. I used to Skydive out of an Otter so I always wanted one of those, but the kits were either too small, too big, or too expensive. I was browsing thru RCGroups classifieds and stumbled upon this kit. The guy listed it less than an hour before I saw it. I quickly Googled the plane to find out more about it and wondered why I never heard of it. I guess VMAR is international, so I thought the seller was international too and shipping would be a killer. I came back to the RCG page and saw the seller lived 15 minutes from my house! A quick PM and this plane was all mine. I drove over to pick it up today. Refer to the pictures below for the following comments...
Pic 1: A picture of the box before I opened it up.
Pic 2: Everything was packaged quite nicely. Each piece is in its own plastic bag, and the bags were all sealed by a hot wire. Delicate pieces were near foam, and you can see the bag of confetti that was protecting the windshield.
Pic 3: Here's everything laid out. Hardware was grouped and stored in separate bags and each bag had a label describing the contents. After handling all the big pieces I was surprised at how light everything was. I think the wingspan here is 63" and flying weight should be around 3.5 pounds. I have a 62" Corsair that comes in at 11 lbs! They did a great job keeping this plane light, light, light. The detail on this plane is amazing. The covering is VMAR's specialty. They use a printed layer with a clear layer on top of it. The printed layer is detailed down to the rivets! The wing struts (shown next to the landing gear) actually have an aerodynamic airfoil cross-section. Just to the right of the struts are the 4 dummy exhaust stacks.
Pic 4: Looking down thru the battery hatch, here's the cockpit. How's that for detail?
Pic 5: Again, looking down thru the battery hatch you see the battery tray with a velcro strap included. The nose gear pushrod is already assembled in place. That green thing you see is a 30 amp fuse (just like the one in your car) for the battery lead which is already installed. I'm not a fan of this fuse. I'll probably take it out or replace with a higher capacity fuse. I'd like to get more than 30 amps out of my 5000 mAh battery.
Pic 6: I like this hatch for the nose gear installation.
Pic 7: The battery hatch lid already has a spring loaded lock lever on it.
Pic 8: I see this as a potential problem. The two elevator halves are attached by that thin "bridge" of balsa. This is not okay since the kit calls for only one elevator control horn. I'll either do a double control horn setup or I'll join the two halves with a steel joiner wire.
Pic 9: Seatbelts!
I'm going to get this plane built up and weighed and then decide whether or not to use my two Frio 10 motors on it.
Cool....where did you get it?
See the first paragraph above... I believe RichmondRC carries these.
I find it nearly impossible to get any information on VMAR products. I would like to learn more about them, and your post here is great. I think this is the first time I have seen a VMAR kit from box top to the insides to laid out parts. Thank you!
It looks great, BTW. :)
Thanks! I worked on it more today. I'll get some pics soon. I'm hoping to maiden this weekend or even earlier. I'll have all the weights, power specs, etc listed here soon.
Pic 10: Fuselage is all built up, minus the electronics. Rudder and elevator are hinged and glued in place. The elevator came preglued, so after giving it a good tug I decided the hinges were good enough. The rudder does not come glued in place, but it does come installed in place with the hinges already installed. This is because you must remove it in order to slide the horiz. stab. into the fin. The hinges are pretty cheap. They frayed on me, so I was not able to reinsert them. I replaced them with my trusty CA hinges I use on everything else.
Pic 11: Here's the power setup. Frio 10 motor rated to 150 Watts. Apollo 25A ESC. 9x6 2-blade prop. CS-12 micro servos shown on the ailerons. I wanted two 8x6 3 blade props, but couldn't find a right and left version of them. I'll be doing counter rotating props even though there's a lot of discussion saying there is not much difference on these small planes whether you use counter rotation or not.
Pic 12: These are the control horns supplied in the kit. The rudder control horn is shown here. They are held in place by only one screw. I don't know if I like this so much, but I tugged and twisted them and they seem to be quite strong. I'll give it a try. On the rudder horn shown here, I used CA to hold it in place along with the screw. The screw provided with the rudder control horn was not long enough to penetrate all the way thru the rudder, so I had to replace it with a standard servo mounting screw I had lying around.
Pic 13: I went ahead and used my more trusty control horn for the elevator. I also used CA for this one too. I never did install a joiner wire for the 2 elevator halves. Again, I've applied enough force and twisted the two elevator halves apart from each other and it holds up well. I figured that if it decided to split in the middle, it will not be too significant since I'll still have control over one half of the elevator.
Pic 14: Here's what the Otter currently looks like, sitting on top of my car...lol.
Misc: The pushrods supplied with the kit are plastic tubes with threaded clevises on each each. These are quite flexible, so I've replaced them all with the usual steel wire with one threaded clevis at one end, and a z-bend at the other end.
Hoping to maiden this weekend. I'm still awaiting my order from Tower Hobbies to arrive in the mail. I'm getting a 10A switching BEC, a new receiver, another prop, and 3 more CS-12 servos for the elev, rudder, and nose wheel.
Hey flyer, guess who?
I'm sure glad your building this guy since it sat on my shelf for so long. I'm watching with anticipation. It looks like you have a thorough build going.
PM me when your goona fly, I’d like to be there.
Thanks much, John
That rudder control horn is indeed unusual. I suppose they did it that way to save weight, IE one less screw. And once the linkage rod is attached, it would not be able to rotate out of alignment.
Same with the pushrods. Plastic is so much more lighter than steel rods. Of course nowaday there is also carbon fiber rods, and the old standby Golden-Rods.
The twin is looking great. Hook up a spring loaded, servo actuated sky diver! :D
John, sounds great, I'll let you know.
Kmot, you have no idea how long I've spent looking up RC skydivers :) I'll have to see how the Otter flies first, and then decide if I want to put more weight on it.
Pic 15: The wiring schematic for the plane.
Pic 16: Wing connections completed last night.
I installed the steering servo, which was quite fun. There is a preinstalled servo tray, and a preinstalled pushrod. I had hooked up the pushrod to the gear already and today when I went to put the servo in, I noticed the tray was on the right side of the fuselage while the pushrod was on the left! I made a quick new tray and installed it on the proper side and got everything connected.
I finished up the 6 pin connector for the fuselage. I was so nervous when I powered everything up for the first time. I thought I was going to short something. I've never used a BEC before and I've never made my own connectors. When I plugged in the battery, everything went 100% as planned! I first did all the proper servo reversing and subtrimming, then I programmed the flaperons. After that, I calibrated the ESC's and ran the motors at idle just to make sure they spun in the proper direction. The propellers were not tightened, so I did not increase the throttle at all. I'll save that for tomorrow.
If you look at Pic 5 above, you can see the battery tray with Velcro strap. I had to modify the tray a little so my LiPo now sits with the forward end of the battery where the Velcro strap is shown in the pic. Of course, I moved the strap back too. This was to get the CG right.
Speaking of CG, I learned a new trick from this month's Model Aviation magazine. I had 2 pins laying around, you know, the ones with a little sphere on the end of them. I just stuck one under each wing, right where the CG is and that makes it so easy to balance the plane. Your fingers know exactly where to go each time. You can do it with your eyes closed!
All I have left to do is balance the props, verify consistent RPM throughout the throttle range, recheck the CG, recheck all connections, recheck all connections, and recheck all connections.
And yes, I will be taking weight and power data and posting that here as well.
Now if only those 30 MPH winds will die out so I can go fly already....
Pic 17: DeHaviland zulu, foxtrot, papa, at parking, ready for taxi.
Pic 18: Another angle.
Pic 19: I'll put a mesh screen on here in the future to keep debris out.
Pic 20: RPM vs. Total Watts. It's a pretty nice match. The Y-cable setup works out alright and I won't need to put the ESCs on seperate channels for mixing.
Test results: 315 watts, 27 amps, 7560 & 7530 RPM at full thtottle. This gives me a pitch speed around 43 MPH. 3.8 pounds gross takeoff weight means I'm at 83 watts/lb for performance. Wing loading is approximately 22.5 oz/sq.ft, so this model should fly fairly well at slow speeds.
Successful maiden flight today! Thanks to Herb for taking the photos. The first flight needed lots and lots and lots of trimming, but after all that trimming the plane flew great. I flew around for 2-3 minutes and landed. Further inspection of the plane showed that the wing is slightly warped, and so is the elevator. That explained all the trimming.
I also had to make sure to fly at one throttle setting since it seems I needed different trims for different speeds. This is probably due to the fact that the RPMs are not perfectly matched throughout the throttle range.
I trimmed the Otter for 75% throttle as that's where I felt comfortable. It flew great at half throttle as well.
Landing was nice and easy compared to my other planes. The Otter came in nice and slow and didn't seem to have any stall tendencies for as slow as I got. The plane flares nice for touchdown on the mains. My Otter came to an abrupt stop on the first landing, and I made a comment, "Oh, it has brakes too". I guess from takeoff, the nuts on the axle tightened against both main wheels. I fixed this for the next flight.
I took the Otter up for another 5 minutes, did a few circuits to get the hang of it, and also did a few touch and goes. It's nice having a plane that flies as slow as this one does. It gives me plenty of reaction time. For final approaches, I focused on keeping the speed down so that I had a little bit angle of attack on my decent. A little touch on the elevator right above the ground made the flares look nice.
I completely forgot the wing struts at home, so I'll have to get them on there next time I go out, and take some more pictures :)
Pics 21 thru 25: Various shots of the Otter's maiden flight.
Pic 26: My fleet. Twin Otter, Super Sportster (modified with retracts, lights, and the Mustang look), Top Flite Corsair.
Hey I found you :) ...
The pictures turned out ok, no ? Where's the video ?
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