Thread Tools
Dec 31, 2018, 07:57 PM
Father by day, hacker by night
JornWildt's Avatar
Thread OP
Quote:
Originally Posted by balticS2
I see how the length of slot and its relationship with the trunnion will determine the retract angle. On a servoless retract there are start and stop switches to limit the travel. How do you see this working with a servo? Just use the travel controls on the tx?
A normal servo is not well suited for this kind of thing - it may not be possible for it to get to a position where it can "relax" and stop the motor. A retract servo is better - it can only rotate 180 deg, nothing in between, so you can neither slow it down or set any kind of end stops - but it allows for some slack in the end, so it won't start again immediately if the arm rotates a tiny bit. This also means it is mandatory to use a servo arm that generates exactly the amount of travel (at 180 deg) needed for the retract. No travel control here.

On the other hand, the V2 retract design does not require precise end stops - there is as much room/slack as you want to build into the trunnion. And since it will lock perfectly in the end stops, and thus not put any continual load on the servo, it may very well be possible to use a normal (heavy) servo - I don't know. It would also be possible to slow down a normal servo, but that would draw much more current since it would need to hold the weight/torque of the landing gear in the temporary stops needed to slow the movement (a spring somewhere could help here).
Last edited by JornWildt; Dec 31, 2018 at 08:04 PM.
Sign up now
to remove ads between posts
Jan 05, 2019, 09:58 AM
Father by day, hacker by night
JornWildt's Avatar
Thread OP

Retract done :-)


After some fiddling with the FB board geometry and quite a bit of staring blankly on the result, trying to figure out how to do the "push-pull arm", I finally came up with something that should work - it feels solid, there is very little play in it and the retract angle is exactly 84 deg.

I still need to drill mounting holes and take off some of the material from the mounting lip to save some weight - but that will have to wait until later.

Next step is some more planning of the wing layout and then sheeting the foam cores.

The single retract (without the strut) weights 22g. I believe it is comparable in size to this one https://hobbyking.com/en_us/alloy-ba...s-model-1.html - and that weights 65g! The plastic version https://hobbyking.com/en_us/mechanic...long-pair.html has a shipping weight of 75g/pair - so that is something like 30g / piece.

All in all it seems like I end up with something useful
Jan 05, 2019, 06:02 PM
Aka Magical Moonstone
balticS2's Avatar
Thanks for explaining Jorn.

I suspect someone in China is following your thread and has a small army turning your IP into a merchandable product

Alec
Jan 05, 2019, 07:00 PM
Registered User
Looks great Jorn!
Another hurdle crossed in record time...
David
Jan 06, 2019, 11:40 AM
Father by day, hacker by night
JornWildt's Avatar
Thread OP
Quote:
Originally Posted by balticS2
I suspect someone in China is following your thread and has a small army turning your IP into a merchandable product
Well, for once it goes the other way around - I have mostly been copying what the Chinese normally produce
Jan 06, 2019, 02:34 PM
Father by day, hacker by night
JornWildt's Avatar
Thread OP

Wing planning


I am trying to identify stuff that needs to be done before I sheet the wing - drawing everything in CAD helps, but there is still a bit of guessing since I have never tried to make a foam core sheeted wing.

So here is the grand plan:

BEFORE SHEETING (not much)
- Sand to correct airfoil shape at the tip.
- Route surface channels (top and bottom) for some spruce spars (1).
- Dig out the foam at the wing root between the two spars, to be able to insert a plywood dihedral brace later on.

AFTER SHEETING (a lot!)
- Add solid balsa leading edge to the wing and sand to shape.
- Cut out flaps (with a angled cut such that the bottom gap is forward of the top gap - drawing added later).
- Insert hinge blocks for flaps.
- Sheet all the flap edges with balsa.
- No outboard flap servo box needed.
- Cut out ailerons.
- Insert hinge blocks for ailerons.
- Sheet all the aileron edges with balsa (and do some work to curve the leading edge of it).
- Cut out aileron servo box.
- Add solid balsa wing tip and sand to shape.
- Carve out room for wheels and landing gear.
- Bury false ribs in the foam to attach the retract to.
- Melt internal routes for servo leads with a hot brass tube or piano wire.

It will be a single piece wing without a central wing spar tube (mostly because of the dihedral).

Aileron servo will use internal (hidden) push rod to move the aileron. Ailerons will be hinged underneath the wing.

Flap servos will be sitting inside the fuselage and push the flaps down by a rod insert in the in of the flap (much like the mosquito flap actuator inside the nacelle). Flaps also hinged underneath the wing.

If anyone can find something that I need to do before sheeting the wing then please let me know :-)
- So far I am a bit worried about the servo lead routes. Is it doable after the wing has been sheeted?

(1) The wing should be solid enough for flying without spars, being sheeted with both balsa and glass, but I am not so sure about the forces from the landing gear.
Jan 07, 2019, 12:20 AM
Registered User
Usually foam wings have top and bottom sheeting and are already fitted with LE to overlap or the other approach was to then square cut front and rear to fit LE and sub trailing edge for the ailerons and flaps. Typically sheeting would carry to TE in areas not requiring a platform outline change or a Aileron or flap. Often sheeting and fiberglassing is done in single step vac bagging the wings.

Cutting the slots for sub rib bulkheads for gear and channel for wiring is done before fiberglassing.

You can get there different ways and vac bag setups are easy to rig up using a metal coffee or food can with an aquarium air pump serving as a vac pump. Caulk seals the bag and paper towels serve as breather cloth. Mylar sheeting is still used and a heavy sheet of plastic is used as the bagging material.

Sharpened large brass or metal tubes can hollow your wiring channels and you can always use a wire rotary tool to carve foam out of wheel wells if you do not mind the mess. You can remove all the foam leaving the balsa top skin if you like and need the room.

How does JMorgan go about it?
Have you seen his wings lately, hotwiring the wing from sheet s structure?

David
Jan 07, 2019, 02:01 AM
Registered User
I think that the largest load problems the structure will see are the point loads from the undercarriage. These loads need to be diffused into the main wing structure. I would suggest a strong rib outboard of the undercarriage beams extending from LE to main spar and a ply skin structure above and below the beams to diffuse the loads into the skins. Nice looking model, John
Jan 07, 2019, 02:04 AM
Father by day, hacker by night
JornWildt's Avatar
Thread OP
Thanks for the tips David. I have seen JMorgan's sheet boxes and may even try it for the stabilizer.
Jan 07, 2019, 02:15 AM
Aka Magical Moonstone
balticS2's Avatar
Hi Jorn

I don't know if John (RPFJ) is following along yet, but he has provided me with a lot of useful advice in this area.

What we are building with this method is a stressed skin wing. It is helpful to think of the balsa and glass skin as a tube being held in optimal orientation by the foam. (Most of, I would suggest) the flying stresses are carried spanwise through the skin. So you want, as far as possible, to avoid chordwise cuts to the skin which will disrupt the spanwise flow of stresses creating stress risers and potential for failure.

For this reason, I would rout the foam and insert the chordwise bearers for the undercarriage before skinning the wing. Rather than burying them in the foam, I would leave them in contact with the skin as this will help to distribute the landing loads.

I rout the foam and insert plywood frames to screw the servo covers before skinning. A dremel tool with the routing attachment is a great help for this. https://www.rcgroups.com/forums/show...2&postcount=26

J Morgan's method for cutting holes for cable runs and spar tubes works very well. https://www.rcgroups.com/forums/show...0&postcount=23 I would worry about my ability to maintain accuracy on a very long run out to the ailron servo pocket. I would tend to cut a slot (with a soldering iron) and insert the wire before skinning. Or inset a paper tube the wire can be run into afterwards.

Alec
Last edited by balticS2; Jan 07, 2019 at 02:21 AM.
Jan 07, 2019, 02:49 PM
Registered User
Quote:
Originally Posted by JornWildt
The single retract (without the strut) weights 22g. I believe it is comparable in size to this one https://hobbyking.com/en_us/alloy-ba...s-model-1.html - and that weights 65g! The plastic version https://hobbyking.com/en_us/mechanic...long-pair.html has a shipping weight of 75g/pair - so that is something like 30g / piece.
In My experiences Hobby Kings' shipping weights Do Not usually reflect actual item weight.
Often they are Grossly exaggerated. Presumably for shipping cost reasons.
I ve had bits that were 1/3rd the weight on MY scale in their as shipped packaging .
Sometimes reading 'reviews' gives a better weight number
Jan 07, 2019, 08:58 PM
EB-66C Team Member
J Morgan's Avatar
Jorn,
Like John said, the important thing is the landing gear mounts. When you land, the gear puts a down force toward the front, so the gear mount would try to rotate down at the front. Important to tie the mount bearers into the LE for one solid point to resist that down force. Make sure you have ply sub spars that are long enough to spread out the load. I like to go out at least 3"-4" each side of the mount with ply sub spars to mount the gear to and the sub spars are full depth.
Forget about embedding spruce spars in the wing for strength , waste of time and adding weight that you don't need. The wing when sheeted is a stressed skin that will carry all you need. Ply dihedral brace ( or two short ones, front & rear)) is however a good idea plus a strip of glass over the middle seam.

J
Jan 08, 2019, 01:06 AM
Father by day, hacker by night
JornWildt's Avatar
Thread OP
Thanks for all your input.

I’ll add plywood half ribs for the retract mount before skinning and sand them flush with both the leading edge and the foam surface, such that they get glued to the sheeting.

The spruce spars goes out. Even if there is no inner spar tube? Or maybe I should just melt a route for as long a piece of carbon tube I can find room for with the dihedral and glue a single carbon rod in as a single light weight spar.
Last edited by JornWildt; Jan 08, 2019 at 03:05 AM.
Jan 08, 2019, 03:05 AM
Father by day, hacker by night
JornWildt's Avatar
Thread OP
Alec, how is it going with that Meteor?
Jan 08, 2019, 06:35 AM
EB-66C Team Member
J Morgan's Avatar
Quote:
Originally Posted by JornWildt
Thanks for all your input.

Iíll add plywood half ribs for the retract mount before skinning and sand them flush with both the leading edge and the foam surface, such that they get glued to the sheeting.

The spruce spars goes out. Even if there is no inner spar tube? Or maybe I should just melt a route for as long a piece of carbon tube I can find room for with the dihedral and glue a single carbon rod in as a single light weight spar.
Yeah, instead of dihedral brace or two, just route a channel for a carbon tube and glue it in. Doesn't have to be too long though, 6" total length or so will do it.


J


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Build Log Scratch built 65" De Havilland Mosquito Scale 1:10 - build log JornWildt Scale Kit/Scratch Built 1230 Oct 23, 2019 04:08 PM
Build Log 1/10 P-47 Thunderbolt: scratchbuild noob Gary27 Scale Kit/Scratch Built 20 Sep 11, 2016 10:35 PM
Build Log Scale Build-Off 5 - Scratch built 49.5" P-40N mrjoshua Scale Kit/Scratch Built 79 Apr 08, 2016 04:57 PM
Build Log The P47D-30-RE Thunderbolt Bubble canopy 1/5 scale spikes 1954 Giant Scale Airplanes 244 Dec 03, 2009 08:25 PM
Discussion Atomic RC micro P-47 Thunderbolt build log Joe 1320 Parkflyers 38 Jun 01, 2008 12:02 PM