DHC-4 Caribou | 73" Span

[Pieliker96]

Main Gear Doors

Bevel both sides of the gear door hinges and reinforce with hot glue. Make sure to transfer the indicator lines to the backside of the part. Make sure you don't bevel the outer score line on the wider side. Wrap the gear door piece around and glue it into place - just make sure not to glue the doors themselves. Cut the doors free from the outside using the indicator lines. The last remaining foam bits should be small pieces of material with the corners cut off - these attach to the aft end of the door skin to somewhat close the gap between the flap and the rest of the skin.


This next part is quite tedious but does work well enough. Install two hooks at the front of the doors where they won't hit the wheels as they retract - I just used control horn wire embedded into the foam. The hooks should be below the hingeline such that they pull the doors open when tension from a rubber band is applied. A similar setup can be added towards the rear of the door with fishing line that catches the strut as it falls in and sinches the doors closed. Adding a taught-line hitch to this is a good idea for adjustability. There's a lot to play around with in regards to the geometry of what's going on here, and it can be fairly finicky to get right. Don't glue anything in until you're sure it works.


That's it, you're done! Best of luck on the maiden flight!

 

[Pieliker96]

Well so much for getting the plans done in a month - they are out now, though, along with the build guidance in the three previous posts in this thread:

PLANS!

If you find anything wrong with the plans or lacking in the build guidance, let me know and I'll do my best.

 

[cyclone3350]

Well so much for getting the plans done in a month - they are out now, though, along with the build guidance in the three previous posts in this thread:

PLANS!

If you find anything wrong with the plans or lacking in the build guidance, let me know and I'll do my best.

As someone who works from his onw drawings, I can state that it would be a lot of work to post them so that others can build from them. Thanks for taking the time. BTW AWESOME JOB ON THE BUILD!

 

[Ligbaer]

Well so much for getting the plans done in a month - they are out now, though, along with the build guidance in the three previous posts in this thread:

PLANS!

If you find anything wrong with the plans or lacking in the build guidance, let me know and I'll do my best.

I maybe will try to build one in March
(Maybe)

 

[Pieliker96]

I was finally able to replicate something resembling the problem on the ground. I swapped the motors side to side prior to testing. At specific throttle settings, when thrust reversing was engaged, the right motor would continue forwards while the left would reverse properly. My working theory is that interference in-flight, either from motor switching noise or external sources, would trip the reversing signal, which only the left motor responded to, but not the right. This is consistent with my observations of more throttle (getting out of the small throttle ranges where it does happen) fixing the problem, but not necessarily the large roll moments I observed while power-off. I've since disabled the braking function on the ESCs, removed a ground loop in the wiring, and ground-tested without issue. We'll see if it happens again tomorrow.

 

[Pieliker96]

Had another flight today, had another hard landing, it still has the problem on the same side. At this point I'm suspecting faulty solder joint(s) or bad ESC. If that doesn't work I'll do whatever it takes to get this thing to stop trying to crash itself - rebuild the entire wing, double-cross my fingers and toes, sacrifice a small mammal - whatever it takes.

 

[Scotto]

Had another flight today, had another hard landing, it still has the problem on the same side. At this point I'm suspecting faulty solder joint(s) or bad ESC. If that doesn't work I'll do whatever it takes to get this thing to stop trying to crash itself - rebuild the entire wing, double-cross my fingers and toes, sacrifice a small mammal - whatever it takes.

Was that still with the arduino connected? I know absolutly nothing about them, but I wonder if there is a wire that intermittenly loses connection would the arduino get screwy?

Plans look great btw. Thanks for putting all the extra effort in for us.

 

[Pieliker96]

Was that still with the arduino connected? I know absolutly nothing about them, but I wonder if there is a wire that intermittenly loses connection would the arduino get screwy?

Plans look great btw. Thanks for putting all the extra effort in for us.

It is, however it's only running the flaps and cargo doors at this point. I've seen the left motor stop in-flight before and presume that's the issue. It could be asymmetrical flap deployment as well. When I'm confident enough for another flight I'll take the arduino offline and tape the flaps up just to make extra sure they're not causing the problem.

Plans look great btw. Thanks for putting all the extra effort in for us.

Cheers

 

[L Edge]

Could one of the fixed (not extending or retracting) blown flaps be fluttering while flying causing a yaw problem which kills the one engine?

Got a camera that you can mount on the tail while flying to evaluate?

 

[IjadPronics]

Waouw!!!... its a nice project. I will start to build one in April... btw awesome on builds!!! rmaf malaysia

 

[Incader]

Such great build plans!!
I am putting this one my list.
Will try and build it sometime later in the year!

 

[L Edge]

It is, however it's only running the flaps and cargo doors at this point. I've seen the left motor stop in-flight before and presume that's the issue. It could be asymmetrical flap deployment as well. When I'm confident enough for another flight I'll take the arduino offline and tape the flaps up just to make extra sure they're not causing the problem.


Cheers

???

 

[Pieliker96]

???

I've got a replacement ESC in-hand, going to see if that fixes it. I'll also electrically and mechanically disable the flaps, as well as having a camera on the tail to see what happens. It should be buttoned up and ready to go in the next week or so - fingers and toes crossed!

 

[L Edge]

I've got a replacement ESC in-hand, going to see if that fixes it. I'll also electrically and mechanically disable the flaps, as well as having a camera on the tail to see what happens. It should be buttoned up and ready to go in the next week or so - fingers and toes crossed!

Yeah, can't wait to see your approaches for the "wheelbarrow". Watched a training film on how to do that on the real one. It impressed me that the wing has an +3 AOA and the power settings so it helps the pilot make the best approach to the ground and be comfortable.

 

[Pieliker96]

Update: New ESC did not fix the issue. Got a gopro as far back on the tail as I could - it's not the motors or the flaps. Turned it around and saw the tail feathers working fine - the only thing that's left is the aileron servo(s). Belly-landed the first time to save the retracts, then did some ground-effect hops to try and recreate the issue with less risk of planting the plane into the ground, no dice - though I was able to get some nice high-alpha ground effect practice in. Future flights will take a look at the ailerons to see what they're doing.

 

[L Edge]

To possibly get rid of your electronic gremlins and save your bird, do a simple receiver only of (thr, ailerons,elevator, rudder) minus all the other fc stuff such as gear or flaps.
Remember you can fly with rudder and elevator(trainer) or do throttle and elevator(single channel) like the old days if you are up in the air.
Oh yeah, did you do a range check on your plane?

 

[Pieliker96]

Here's onboard video of some test flights to try and diagnose the issue. Broken retract mounts and a broken motor mount have been repaired, and it's ready for another flight.

We're going to figure this out.
Observed behavior: plane banks hard left without pilot input. This typically occurs when pulling back on the elevator, though even at relatively high power settings, low bank angles, and low elevator deflections. It always breaks to the left, regardless of bank direction.

Tested explanations:

- Arduino flaperon control would intermittently fail and drive the ailerons into an uncommanded position.
However, the problem persisted with the ailerons running directly off the RX, and ailerons were observed via onboard video to respond appropriately in flight: counteracting, not initiating, the rapid banking as it occurred in flight.

- Electrical interference would inadvertently reverse the left motor in-flight, or the left ESC or motor was faulty.
However, the problem persisted with the reversing functionality disabled, stayed on the left side when motors were swapped, and persisted after the left ESC was replaced. Additionally, both motors are observed to produce power and maintain RPM via onboard video and audio, even when the problem is manifesting itself.

- Asymmetric flap deployment or fluttering of flaps in flight causes the rolling moment.
This has been observed to not be the cause via onboard video. The flaps stay in their faired position and do not deflect when the behavior is exhibited.

-The wing twists aeroelastically during flight and deforms as to stall the wing.
Onboard video shows the wing remaining quite rigid in-flight. Significant wing twist was observed however, due to the aft position of the main gear and a particularly rough landing.

-The wing has wash-in or other adverse aerodynamic twist.
While the wing was designed and built as a completely flat wing with no aerodynamic twist whatsoever, both have - either through construction or settling and warping of the foam over time - acquired a few degrees of wash-out, as observed by comparing straightedges at the wing root to the wing tip.

Incompletely explored explanations:

-The plane loses connection and/or goes into failsafe during the event.
The plane's fuselage passes a range test - a range test has not however been performed with the wings installed. The failsafe was set to pitch up, deploy the flaps, retract the gear, and bank to the left, however the flaps or gear were not observed (via onboard camera) to start to deploy during the incidents. After the failsafe was set to neutral controls with a tad of back-elevator, the problem persisted.

-The rudder deflects uncommanded during flight.
The rudder is wired directly to the receiver and is observed via onboard cameras to behave fine on short hops in ground-effect.

-The tail deflects aeroelastically during flight.
The tail attachment is not exceptionally torsionally rigid - the tail shakes around visibly during taxi. Short hops in ground effect have not observed any significant deflection, though the tail has not been observed in flight.

-Power / Signal connectors coming undone in-flight due to inadequate wing constraints
The wings can, with some effort, move around 1/2" fore and aft at the tip, even when locked. Additionally, the gap between the wing and the fuselage can potentially widen due to flight loads, causing intermittent connections which cause the roll. In theory, fixing this would be as easy as shimming the connectors.
From what I can see in the footage, the gaps do change a tad during takeoff and landing, but remain around constant in-flight, though variable lighting makes it difficult to discern. On the ground, the maximum extent of the wing flex is not sufficient to fully unplug wires and effect functionality. In any case, the flight controls or propulsion on both wings seem to be doing what they're being told, though we have no real good way of making sure they're always connected in-flight.

Unexplored explanations: (If you have one that isn't on this list or the one above, tell me!)

-Tip-stall: The outer, unblown portion of the left wing stalls first.
As a pilot, it doesn't feel like tip-stall. It happens even at high speed and relatively low bank angles with what I would consider to be relatively gentle pitch inputs. The airplane has flown fine during short hops in ground-effect with the tail nearly dragging on the ground, and the angles of attack I'm reaching in flight don't seem that extreme.
The footage seems to tell otherwise, with a somewhat sharp and significant pitch input (perhaps exacerbated by the slightly aft CG with the camera) followed by a violent roll. It happens to the left regardless of bank direction, though only sometimes. Another supporting factor is that the problem seems to be less of a factor on calm days, where the induced angle of attack variation due to gusts is lower.

I find it hard to believe that this thing is really stalling due to what I'd consider the most mild and benign of maneuvers.

Next flight (hopefully tomorrow), I'll put some tufting on the outboard left wing and see what's going on there. If it is stalling, vortex generators may be in order.



I've also released a correction to the build instructions, specifically regarding the upper cargo door and modifications to keep it from over-deploying and ripping off.

 

[L Edge]

You may have a bad joint(s) that is causing problems. Run your video at .25 speed and watch the right wing joint where you plug it in.

The right back wing joint will not only open and close in width, but changes up and down in angle. Shows up in takeoff/landing as well as flight. So could you check it there is slop in the fuse or wing joints . Point to watch is the right wing trailing edge before flap.

Might be an angular twist. Had that one in a Formula 1 going 180mph+ and took me three exciting flights to figure that one out.

 

[Tench745]

Here's onboard video of some test flights to try and diagnose the issue. Broken retract mounts and a broken motor mount have been repaired, and it's ready for another flight.

We're going to figure this out.
Observed behavior: plane banks hard left without pilot input. This typically occurs when pulling back on the elevator, though even at relatively high power settings, low bank angles, and low elevator deflections. It always breaks to the left, regardless of bank direction.

Tested explanations:

- Arduino flaperon control would intermittently fail and drive the ailerons into an uncommanded position.
However, the problem persisted with the ailerons running directly off the RX, and ailerons were observed via onboard video to respond appropriately in flight: counteracting, not initiating, the rapid banking as it occurred in flight.

- Electrical interference would inadvertently reverse the left motor in-flight, or the left ESC or motor was faulty.
However, the problem persisted with the reversing functionality disabled, stayed on the left side when motors were swapped, and persisted after the left ESC was replaced. Additionally, both motors are observed to produce power and maintain RPM via onboard video and audio, even when the problem is manifesting itself.

- Asymmetric flap deployment or fluttering of flaps in flight causes the rolling moment.
This has been observed to not be the cause via onboard video. The flaps stay in their faired position and do not deflect when the behavior is exhibited.

-The wing twists aeroelastically during flight and deforms as to stall the wing.
Onboard video shows the wing remaining quite rigid in-flight. Significant wing twist was observed however, due to the aft position of the main gear and a particularly rough landing.

-The wing has wash-in or other adverse aerodynamic twist.
While the wing was designed and built as a completely flat wing with no aerodynamic twist whatsoever, both have - either through construction or settling and warping of the foam over time - acquired a few degrees of wash-out, as observed by comparing straightedges at the wing root to the wing tip.

Incompletely explored explanations:

-The plane loses connection and/or goes into failsafe during the event.
The plane's fuselage passes a range test - a range test has not however been performed with the wings installed. The failsafe was set to pitch up, deploy the flaps, retract the gear, and bank to the left, however the flaps or gear were not observed (via onboard camera) to start to deploy during the incidents. After the failsafe was set to neutral controls with a tad of back-elevator, the problem persisted.

-The rudder deflects uncommanded during flight.
The rudder is wired directly to the receiver and is observed via onboard cameras to behave fine on short hops in ground-effect.

-The tail deflects aeroelastically during flight.
The tail attachment is not exceptionally torsionally rigid - the tail shakes around visibly during taxi. Short hops in ground effect have not observed any significant deflection, though the tail has not been observed in flight.

-Power / Signal connectors coming undone in-flight due to inadequate wing constraints
The wings can, with some effort, move around 1/2" fore and aft at the tip, even when locked. Additionally, the gap between the wing and the fuselage can potentially widen due to flight loads, causing intermittent connections which cause the roll. In theory, fixing this would be as easy as shimming the connectors.
From what I can see in the footage, the gaps do change a tad during takeoff and landing, but remain around constant in-flight, though variable lighting makes it difficult to discern. On the ground, the maximum extent of the wing flex is not sufficient to fully unplug wires and effect functionality. In any case, the flight controls or propulsion on both wings seem to be doing what they're being told, though we have no real good way of making sure they're always connected in-flight.

Unexplored explanations: (If you have one that isn't on this list or the one above, tell me!)

-Tip-stall: The outer, unblown portion of the left wing stalls first.
As a pilot, it doesn't feel like tip-stall. It happens even at high speed and relatively low bank angles with what I would consider to be relatively gentle pitch inputs. The airplane has flown fine during short hops in ground-effect with the tail nearly dragging on the ground, and the angles of attack I'm reaching in flight don't seem that extreme.
The footage seems to tell otherwise, with a somewhat sharp and significant pitch input (perhaps exacerbated by the slightly aft CG with the camera) followed by a violent roll. It happens to the left regardless of bank direction, though only sometimes. Another supporting factor is that the problem seems to be less of a factor on calm days, where the induced angle of attack variation due to gusts is lower.

I find it hard to believe that this thing is really stalling due to what I'd consider the most mild and benign of maneuvers.

Next flight (hopefully tomorrow), I'll put some tufting on the outboard left wing and see what's going on there. If it is stalling, vortex generators .

I love meaty troubleshooting problems like this to really sink your teeth into.
You're doing a great job at identifying and testing possible failures.

I thought maybe the elevator may be flexing, but the severe rolling that happens at the 5:05 mark makes me think differently. You're actively banking in a right turn. There is no aileron or flap movement which would initiate a left roll and if the wing stalled in a right bank typically the plane should roll right, not left. I do see that it's only after you give some up elevator to pull through the turn that the left roll initiates.
My thoughts here are that either the rudder occasionally deflects significantly to the left, you have asymmetrical thrust happening, or the left wing stalls before the right.
It looks like you perform a stall shortly after that failure, around the 5:28 mark, and the stall breaks straight ahead. Can't tell for sure if that was a stall or just a pitching up then down.
Similarly, the uncommanded roll at 6:10 shows no aileron deflection, no flap deflection, no wing flex... nothing, except that the nose raises rather abruptly just before each of the rolls to the left. I don't see any significant yawing before the roll left which makes me think that it's probably not a rudder or asymmetric thrust issue.

Based on what I've seen in the video your left rolling tendency is not an airframe problem, except possibly having too much elevator throw.
The motors, rudder, elevator, ailerons, and wings all seem to do exactly what they're told when they're told and nothing more.
Every left roll is preceded by an upward pitch input and so I would guess that you are stalling the left wing and possibly starting a left-spin. It would be informative (though risky) to let the left roll continue and see if it develops into a spin or is simply a stall that breaks to the left for some reason. If it is a spin that means there is uncoordinated rudder involved.
Worth remembering that in a 60degree bank at constant altitude, the plane will be feeling 2G of force, which increases your stall speed. If you aren't adding power in the bank to compensate, you are more likely to stall.

 

[Baron VonHelton]

My Nieuport 11 "Bebe" Shot out of my hand & made a hard left as well, lawn darting & messing up the cowling.
The only thing I noticed is that my top wing was not perfectly straight from top view, it was slightly tilted to the left.
It's possible this is why it veered to the left & crashed.