Phil Kent Cub

[Flying Squirrel]

Thanks RR, yes I thought the Pete Miller panel might be rogue but it does have the same reference number as some parts on the laser cut balsa so thought I'd mention it.

 

I'm now sure you are right about the 'outer' sheeting, I actually found a short YT video of a Sig 1/6th build that had this 'outer' sheeting, I did see it on the CSA of F2 but hadn't given it enough thought as that nose section appears to be built separately and didn't consider that the panels would be infilled and then sheeted over.

 

Yes, not for a beginner that's for sure! I naively thought as my Seagull P47 Kit went together pretty well I'd be OK but alas, I'm struggling! 

 

That said, I hope to get it finished, even if it means bugging you guys for help! Although there are a few builders at my club it doesn't compare to the wealth of knowledge on here.

 

Thanks!

 

[kc]

My opinion of the wing root/ cabin top area is that it probably needs a 3/16 lite ply ( or 1/16 birch plus 1/8 balsa ) rib to the shape shown on fuselage side.   2 liteply plus 2   1/16 birch should be drilled exactly the same keeping drill really vertical to avoid errors.    the 1/6 ply are for the wings roots, the liteply is attached to cabin top.   Also there seems to be 2 balsa ribs inside the centre section which support the sheeting.

There is no indication of whether both left and right wing joiners go into same brass tube or not.  I assume both left and right go into same tube.   These tubes need to fix onto something strong across the fuselage, so perhaps 1/8 birch ply would suit.  This ply with tubes needs to be connected firmly to fuselage.  As I said before they need accurate alignment and probably fix one with a drop of glue and ensure its correct before fixing the other one too then bound with thread & completing glueing with epoxy.    I would check alignment - both square to fuselage and at same height- with a long bit of 16SWG wire and sighting with a straight eye.   Fuselage flat on a board and measure height of wires and distance back to fin each side using thread.  Also of course check wing incidence with tailplane but that should come out OK  with a correctly built fuselage.   Get it right with just one blob of glue holding tubes in case needs removing and refixing.  

Just my view!

[kc]

When you have the centre section fitted then consider how the bolts can be fixed.   The go horizontally into wings into tapped blocks in the wing halves.  ( ply or hardwood can be tapped easily with a taper tap - quite cheap- or just a bolt filed into a taper at tip )   Wood is hardened with thin cyano after tapping then tapped again.   Maybe the thumbscrew type bolts could be used as its difficult to line a screwdriver up.   M6 would seem suitable or 2BA.

Best to work all this out early and fix a hatch or something so fingers can tighten bolts.

The plan shows the 16SWG piano wire joiners with about 6mm turned up into the wing presumably to make them captive.    I reckon the balsa holding these joiners should only be glued into the wing at a late stage when wing is fitted together with struts and slight ( or none?)  dihedral achieved.   Then glued in and boxed as per plan.  Must be in parallel or wing halves won't slide in!  

Wing struts might be structural so should be carefully made with firm attachment points.

[Flying Squirrel]

Brilliant, I'm pretty sure I understand so thanks for being detailed, I may sketch it first just to make sure I've interpreted correctly. Definitely some good points which I've noted.

[kc]

CubConstruct1.pdf

This is my idea of the wing centre section construction on the plan with my ideas on how the joiner tubes are fixed to ply crosspieces.    Ply is shown blue, while Liteply is hatched, tubes are gold.  Balsa is purple.

Edited February 10, 2023 by kc

[Flying Squirrel]

Thats amazing thank you!

[kc]

Having looked at the plan a bit more there are no indications of where the servos should be mounted, so you need to work that out at an early stage, also how the aileron servos will be mounted in the wing and how the servo leads will go thro the wing.

One tip that is worthwhile is to draw a line on the plan from servo arm to the elevator horn, on both the side view and view from above.  Same with rudder. This enables one to plot where the pushrod will come out and make suitable slots.   Also to work out if the hole in F4 will clear the pushrod ( if servos are low it will work - if high it needs a bigger hole in F4. )

 

If you are happy to build the plug in wings and that you can align them, then proceed.   However if you feel that is all a bit difficult then I will suggest a simple way to make a one piece wing using dowel and wing bolts as in many other planes.

[Flying Squirrel]

I had thought about a one piece wing and the servo locations, in fact that's probably my next job. My plan was to have a simple ply servo tray accessible from the functional doors, about the height of the lower door hinge but thought a one piece wing would make this simpler.

 

Aileron servos, I  planned to use micro servos here and mount them in a pocket with a cover, the article says just cover over them but I think I'd rather some kind of access, hadn't given much thought about the cabling though.

 

I do worry about the strength of the fuse were the wing joins, unless I'm under estimating the strength provided by the 1/8th Ally tubes at the front.

 

Here's were I'm at, I didn't restart it, just a bit of hacking here and there with my new found confidence and I'm much happier. I've not fixed the window 'frames' yet as I wanted to see how they fit against the sheeting, I also snapped off the front uprights but they were so fragile I think I'll just put a strip of spruce or something in.

 

 

 

Edited February 14, 2023 by Flying Squirrel

[kc]

First of all I think you need to ensure the fuel tank can pass by the servos and servo tray - assuming there is no other hatch.

 

Second if you do intend a one piece wing with dowel and bolts then you should consider putting in the former at this stage.    The dowel cannot really go at the front of wing as there is just glazed area there, so former to take dowel needs to go further back.   This is convenient as it leaves the glazed area to be part of the fuselage not wing.   Wing centre section is therefore less chord than wing panels ( as in many models)   Wing spars need to be left longer than panels so they meet in middle of centre section.  

My sketch shows where the former might go ( in my opinion)

CubFormer.pdf

[kc]

That sketch omits  the front glazed part and the former replaces the 3/16 upright ( you removed that already.)   It should strengthen the fuselage as it connects both horizontal longerons.

 

The former could be like this drawing and it needs to be high quality ply with grain along the vertical legs as its narrow.  Could be painted matt black so it does not show too much through windscreen.   Or it could reinforced with spruce or carbon strip.  The former is drawn at the same taper as F4 - narrows slightly at top.

 

All just ideas - not proven!

CubFormer2.pdf

[Flying Squirrel]

I had planned for the fuel tank to be 'front loading' accessible by removing the engine.

 

I understand most of this but a bit lost by 3/16 upright that I removed?

 

Thanks

[kc]

Your photo today showed lack of vertical part in the cabin area while the photo 30 Jan showed it - perhaps it was part of the door.    I assumed it had been removed!    Anyway the former I suggested would go in place of the front vertical upright  and in fact goes where the wire wing joiner is shown on the original plan.

The former I drew could be narrowed down if the opening doors are fitted although the 'pillars' would still need to be wide enough to have some strength.

Edited February 14, 2023 by kc

[Flying Squirrel]

Ah yes, I'm with you, yes it was part of the door structure but there's an upright to go in on the left hand side as the door is only on the right. I  like the idea of a former in that area as, like I said' it all looks a bit weak. I'm surprised there are no formers between F2 and F4 other than F3 which does very little, just seems like there should be one in the cabin area.

 

Time to sit and study the plans again and make some decisions, much easier now I understand a bit more. Thanks.

 

[kc]

You could also look at the Great Planes Cub Plan here on Outerzone which uses a similar idea for the dowel area.    That plane is a bit chunky compared to your plan!    The Phil Kent plan is more scale and seems lighter built especially the wing which has very little sheeting.

 

A one piece wing would not use the piano wire joiners but have the spars meet in middle and have a birch ply joiner right across centre section and out to the rib beyond start of aileron ( Jury strut fixing area ) . Joiner to be tapered at outboard end and main spar joiner would be longer than rear spar joiner.

[RottenRow]

My model of the Brazilian version of the Cub (called a Paulistinha) has a full-height ply fuselage former exactly where KC shows his suggested one to be. Most model Cubs have something similar, in the case of the model that you are building, Philip Kent has wanted to leave the cockpit area open (as in the full-size). My model is about 72" span (not a Clipped Wing version) and has a one-piece wing, with dowels at the front and a couple of screws at the rear into a 1/8" ply plate. It is not an accurate scale model, but I have flown it in a few of the BMFA's Flying Only scale competitions.

 

I have added a liteply 'frame' inside the top of the cockpit area, as this area is very weak and has broken on more than one occasion when a gust of wind has got under a wing during take off and caused a wingtip to hit the ground.

 

I'll go and take a couple of photos of it and post them shortly.

 

The two crossed aluminium tubes inside the windscreen on your plan are just to represent the visible tubular structure of the full-size and will add little if any strength.

 

I used to have a Black Horse Cub (awful model), that also had the same wing mounting method.

 

Brian.

[RottenRow]

Photos as promised in the previous post.

 

 

 

 

 

Hope they are self-explanatory.

 

Brian.

[Flying Squirrel]

Very helpful, thank you.

[Flying Squirrel]

I feel lazy asking this but having searched on here and google I'm confusing myself.

 

I'm just setting the horizontal stab but there is no referrence to incidence. Happy to try and figure it out but whats the refferrrence point, is it thrust line?  

[RottenRow]

Yes, it'll be the line shown on the plan as long dashes and dots, running from the crankshaft centre at the nose to half way down the tail wheel bracket.

 

From zooming in on the rear section on my screen, the tailplane appears to be exactly parallel to this line on the plan, ie incidence of 0 degrees.

 

Brian.

[Flying Squirrel]

Thank you RR, I thought it may have been. That'll be the very same line that I confused with a stringer when I started!

[kc]

The wing on the Phil Kent design is quite thin especially at TE,  therefore wing bolts would need to be near the rear spar and not as far back as RR's model ( in my opinion)   Wing bolts need to go into a firm part of the wing!

[Flying Squirrel]

7 minutes ago, kc said:

The wing on the Phil Kent design is quite thin especially at TE,  therefore wing bolts would need to be near the rear spar and not as far back as RR's model ( in my opinion)   Wing bolts need to go into a firm part of the wing!

These snippets of info help SO much, even bits I thought I had my bonce around like linkages that yu mentioned earlier. I'm not sure if to do a two piece or single wing yet, I think I may put the former in that you suggested but to suite the two piece. I guess I have three main considerations...Easiest to build....easiest to fit/maintain radio gear etc and ........easiest to repair!

[Flying Squirrel]

Well, despite lifes best efforts I have made some progress, fus is best part done, with two peice wing version canopy cover, rough servo positions and control line routes, empanage built. Quite happy in the main and a few new skills aquired..

 

 

 

However, despite me thinking about the wings while working on the fus and thinking it'll be pretty straightforward, now the time has come to build them I'm struggling!!

 

I'm ok for the main part but not the ailerons, although I now have the confidence to kind of ignore the plan a bit and crack on I would like to understand the designers intent a bit more as I'm sure it may help in future builds.

 

There's quite a lot of what appears to be conflicting info between the various plan details and the article that I'm hoping somebody can clarify for me, couple of specific points..

 

The ribs are full ribs and there is mention of 'cutting out' the ailerons after, I can't see how this is possible with the aileron spars that need to be installed?

 

There are small 'aileron ribs' as part of the laser cut pack - not sure where these go?

 

The general build up seems confusing to me, I can't understand what the 'sheet trailing edge' and aileron spars are? The plan mentions 3/8 x 7/16 triangular balsa capping....whooosh.....there is some triangular stock that carries the airleron 'cover' but thats like 1/4 maybe?

 

Anyway, if you had the patience to read all this then I thank you and hope somebody can help me understand!!

 

Some of the relevant detail....

 

 

 

 

 

Wow, long post, sorry!

 

 

[Flying Squirrel]

One example of what I find contradicting, these pics are from the article that came with the plan and are apparently of the same thing....

 

Note aileron built seperately?...

 

 

...And, no aileron or needs to be cut out??

 

[john stones 1 - Moderator]

Just variations on a theme FS, the method of getting there is just personal choice, either build the wing then cut out the aileron section, which you'll then have to face for hinging, same with trailing edge of wing you cut out, the extra ribs are likely for the end of aileron section. Or build ailerons in as you go, results the same.