Designers advice please?

[Grant Webb]

Hi guys.

I'm seeking some advice/ideas as to how to approach the construction of a tightly rolled rear fuselage. My model is being designed with a wingspan of 1300mm, which gives a mid fuselage outer diameter of around 95mm, tapering down to just 35mm at the tail.

The full sized aircraft's fuselage was constructed using a F4U Corsair jettisonable fuel tank for the front half, with an aluminium alloy rear section.

Any guesses at to what the aircraft is?

I'm still in the early stages of drawing up my plan as can be seen from the attached picture, but need to consider my construction technique.

My preferred method in mind would be some form of built up balsa tubing, probably planked over formers, then covered with light weight glass for strength, but I am conscious of keeping weight down as the power source is to be a .30 four stroke.

Any ideas or comments greatly appreciated.

Grant W.

[Dave Hopkin]

Perhaps a Carbon fibre tube core with formers threaded onto it for the planking - light and strong

[Simon Chaddock]

Grant

As a long time 'lightweight' builder I would have no qualms using balsa planking over formers with the planks just thick enough to allow a bit of sanding to achieve a smooth circular profile.

Personally I would forget glass, you can always use a slightly harder grade of balsa if you are concerned at the strength.

As a stressed skin monocoque and it will be amazingly rigid and light.

DSK-1 Hawk?

[Tim Hooper]

I'd consider doing the front part as planked balsa over formers, and the rear section (where it goes to a conical taper) as rolled 1/64 ply to maintain the strength at the skinny rear end.

tim

[Robin Colbourne]

Hi Grant, That shape can only be a Piper Pa-8 Skycycle, or else the Carlson replica below.

If you wet it, balsa will form quite a tight tube. I once made an indoor rubber powered canard with a 1/32" balsa tube fuselage about 5/8" diameter. I used cyano and an activator to tack it together. You could laminate another layer on top which would prevent splitting.

As Dave says, a mandrel with threaded on formers is the way to go. A stand with a clamp at each end of the mandrel, will allow you to rotate and secure it as necessary.

Useful info on rolled fuselages here: https://www.rcgroups.com/forums/showthread.php?t=292963

Edited By Robin Colbourne on 19/11/2016 00:27:03

Edited By Robin Colbourne on 19/11/2016 00:41:17

[Grant Webb]

Thanks everyone for the feedback.

I'll see how I can use your ideas to nut out my approach.

Robin... you are spot on with your Piper Pa-8 Skycycle replica. Something about it really appeals to me as a lazy evening parkflyer.

I'm looking to utilise this as my first foray into CNC using a friends machine.

Cheers,

Grant W.

[Peter Miller]

I have quite a bit of information on the Skycycle. Some pretty good 3 views and a few good pictures in an article.

IT was only that vast canopy that put me off.

My own method would be round formers with 1/4" square longerons so you can join the sheet on the longeron.

On my B-17 I used a piece of 3" X 1/4 as s jig with the lower profileof the fuselage drawn on and slots cut to hold the formers. Once the top half of the sheet has been fitted you just crack it out of the jig and finish the sheeting/planking.

[Simon Chaddock]

Personally I favour carvel planking - as in boat building with no longerons.

Yes, each plank has to be carefully shaped but then it has to be anyway even with the stringer method but the end result is light and strong..

[kc]

Quite often conical fuselages are made on a horizontal or vertical crutch with half formers. Each half is done flat on the board and eventually joined after planking.

An alternative way could be a square box fuselage padded out with foam which is then sanded to shape and covered with glasscloth etc. A perfect taper shape could have the foam hot wire cut.

I seem to remember a model  ( reviewd by David Ashby? ) that had formers assembled on a horizontal steel or carbon rod which was later removed.

Edited By kc on 19/11/2016 15:33:13

[Grant Webb]

Thank you gentlemen for your input.

Peter & Simon, I'm thinking now maybe a hybrid of your two ideas, with longerons tapering away to nothing in front of the tailplane. My reasoning behind this is to provide some strength at the transitional point at mid-fuselage where the tail-cone joins with the "jettisonable fuel tank" forward pod (if that makes sense).

I do like the removable crutch concept as well and am considering the stepped crutch as used in TDM Model's Polikarpov I-16 to positively locate the formers length-wise as well. However I do have some 'T' section aluminium extrusion which might serve the purpose very well. But maybe a removable 22mm carbon tube would give more room for control push-rods later on?

Just for interest, below is my 3D model of B. Schoefeld's rubber powered model as published in the Sept 1946 edition of Air Trails Pictorial. I found the design floating somewhere around the web (Outerzone maybe?) and 3D modelled it as a proof of concept (test my ability to produce a CNC model). May yet build it as a rubber model too.

It utilises shaped scarf-joined longerons full length, maybe this could be used here too?

Cheers,

Grant W.

[cymaz]

You could adapt the Seagull Challenger kit....here and , as others have mentioned, extend the tube rear ward

[barry knight]

Hi there Grant. The description of how they built the original one is interesting.

For the aft section.

What about rolling a sheet of card into the conical shape you need. Have round disks of balsa each end for tempory support. Then cover it with greese proof paper.

You could then lay up tissue glass paper ( from hobbyKing) painting it between coats with poly urathane.Once set remove the discs and card cone.

Make the front conventionally with formers and stringers then join the 2!

[Grant Webb]

 

Well design progress is slow, but proceeding none-the-less.

However, I'm unsure about the airfoil on this aircraft and haven't been able to confirm what was used on the original.

I found another r/c model design which was described as using the NACA2415 section, so I have followed suit. I had previously used a Clark Y section, but that appeared too thin. But if you look at my screen captures below with the NACA2415 section and compare the wing thickness with that of the 3-views being used, it looks wrong/too thick.

Thoughts please????

 

 

I have used 2.5degrees of washout in the wing, have double checked it, but that too seems distorted.

Cheers,

Grant W.

Edited By Grant Webb on 08/12/2016 01:39:45

[Denis Watkins]

Without reinventing the wheel Grant and building this beautiful model with complex washout, similar model designs fly gently and sedately; namely the Seagull Spacewalker and the Astro Hog, and both wing types can be described as thick sections. Without chaps like you, we would learn nothing new, but crib some part of your design from stable flying designs.

Edited By Denis Watkins on 08/12/2016 06:57:53

[Peter Miller]

The airfoil section is USA 35B. That is from my collection of Skycycle material

[DaveyP]

I seem to remember in my dim and distant past making a conical fuselage for a glider.

I used thin balsa sheet which I doped on one side, the inside, left to dry then soaked in luke warm water it curl a treat

[Grant Webb]

Thanks for your advise, Denis. I'll try and take that on board.

Peter, actually I now seem to remember that airfoil was used. Don't know how I let that slip. Thank you for the prompt. Actually, looking at the flat bottom of the USA35b, it's very much in keeping with the photographs of the full sized that I have come across. Wing now being re-drawn.

Cheers,

Grant W.

[eflightray]

Posted by Grant Webb on 08/12/2016 01:37:29:

 

..........................................................

I have used 2.5degrees of washout in the wing, have double checked it, but that too seems distorted.

Cheers,

Grant W.

Edited By Grant Webb on 08/12/2016 01:39:45

 

I hope this doesn't detract from a great thread, but isn't the reason for so much wash-out is that we may tend to fly models too slow and want a 'safety net' ?.

I have built many models with no washout, just ensuring there is no wash-in, and just don't fly near the stall speed. 2.5 deg does look a lot in the picture, and surely must reduce some of the wings lifting ability.

Not all models are prone to the stall and death spiral, many will just stall with the wings level. Perhaps so much wash-out is still a throw back from free flight days.

What made you choose 2.5 deg. ?

Ray.

 

Edited By eflightray on 08/12/2016 10:59:34

[Timo Starkloff]

Washout is fine for that type of wing, but 2.5° sounds a bit too much. What is the length of your inner and outer wing rib?

Interesting plane, never saw that before.

Timo

[Peter Miller]

Actually I really don't think it needs any washout and certainly not 2.5 degrees.

I only use 2 degrees on my elliptical wings and they really do need it.