Bistormer 60" (A Barnstormer with more ribs)

[Danny Fenton]

I always have trouble sanding aliphatic, Superphatic and most of the pva glues. Titebond is not too bad, but I find CA easiest to sand, must be just me because everybody else I know says differently Whatever floats your boat I say

Been musing over the next part to build, tempted to start the fuselage, but there are stil decisions churning over in my mind, so it might be wise to crack on with the lower wing while my neurons work on the fus

The wing is going to be almost vanilla. I will add ailerons as per plan, they look large enough. I am going to use a single servo and bellcranks because nobody ever does them and you may want to see how I do them? I am also going to top hinge the ailerons rather than use a central hinge that is shown on the drawings.

Cheers

Danny

[Barrie Dav 2]

Danny, the wings will be ok 'Almost vanilla' but not almost banana. Vanilla pods are generally straight (ish).

[kc]

Danny's photo of his bandaged thumb is the best advert for not using glass headed pins! Presumably the Als pink pins are plastic headed.
I am glad you agree 1500 watts is far too much. Could be an error? 100 watts per pound is considered usual for reasonable aerobatic performance on monoplanes, but do you reckon to allow any extra for the extra drag with biplanes Danny?

[Danny Fenton]

Thanks Chaps, the thumb was a result from cutting sheet balsa, Never been so foolish before. Held the wood down with my thumb on the cut line Really slowing me down...... never had a glass head come off but I only use pins in soft balsa.

Cheers

Danny

[Danny Fenton]

Hi KC I am pretty sure that this will have enough power, remember I just want it to fly like a 20's biplane, the emphasis will be on slow flying. If you go bigger on the motor the weight sneaks up. One of the factors I use when choosing a motor is the weight, often you will see a similar spec of motor but 200 grams heavier! I would expect 60 watts /lb will fly it but it would be marginal and no extra for getting out of trouble.

You are right the extra drag would be a factor on something draggy like a Pitts Special, but for slow aircraft it doesn't appear to matter much.

Cheers

Danny

[kc]

I have found 50 watts per pound adequate for flying a vintage style model but a little marginal for takeoff when the grass is long. So 100 w per pound seems right for a Barnstormer or Bistormer - a biplane with no draggy bracing wires etc.
Those scalpels cut skin just like they were made for it!

Peter Miller's designs very often uses single servo ailerons with bellcranks. His way of putting both piano wires from ailerons into a brass tube together with a U shaped rod to the servo arm seems a very good idea. I believe he uses a RadioActive brand servo mount which helps mount the servo from the top. A good photo of this setup in the latest RCModelWorld February 2014.

Edited By kc on 24/01/2014 12:54:08

[Danny Fenton]

Hi KC thanks for that, I had missed that pic in RCMW, I love the way Peter has charred the balsa around the servo bay when soldering, brilliant! I like Peter's designs

Cheers

Danny

[cymaz]

Danny , stick a laser engine in. It is crying out for one. You wont regret it

Edited By cymaz on 24/01/2014 13:46:34

[Concorde Speedbird]

Yes, it does have foam wings. It is the best flying aeroplane I have flown, stall is a non-event, so I wouldn't be worried about the Bistormer being well over 6 pounds. The Bistormer is more of a slow flying aeroplane though, rather than the aerobat Domino.

CS

[Danny Fenton]

LOL CYMAZ I am recovered now.... and the Laser moment has passed.....

Thanks CS, I have no doubt it would fly well at 10lbs but I don't want that I want a floater and with a symetrical wing it wont slow down as well as Chris's Moths.

First of all I worked out where the piano wire was going to run from the centre section out to the belcrank, and made suitable holes in the ribs involved. A Permagrit round file is superb for making the neatest of holes, forget drills in the scenario.

I was asked previously of my exact process, well here you can see a rib being positioned, checked for exact upright then CA'd into position, before moving on to the next bay. Don't forget the root rib is not vertical!

Considering the innacuracy of the wooden template this time I made one from card using old school protractor and ruler. It isn't held in place for the picture as I have only two hands, but you get the idea

Two extra trailing edge ribs have to be made up per aileron, and also notice the small section of 1/16 under the aileron leading edge, I have used this to keep it stable for when I saw the aileron off

One extra riblet at each end of the aileron.

I was really unhappy with this bit, not a clever solution, but unless I have missed something I can't see any other way this is meant to happen. I have added a block per bay to build up the trailing edge spar to the the upper skin. With hindsight I shouls have slotted in some 3/16 x 3/16 and webbed the gap. But these blocks seem to be how the plan shows it. Any input guys, have I missed a trick here?

The aileron was then seperated from the wing panel with my razor saw and set aside for later.

The wing panel had a trailing edge fitted, I haven't shown it as I showed it on the upper wing, but the sheeting is chamfered to fit.

Nearly there. In this shot you can see that I have added the spar webbing. Because I have done it now it is really easy to get a really tight fit against the ribs. Perhaps BEB could put me straight but to my mind if the joints are not close with regards webs then a lot of the D box strength is lost

The last thing tonight was to fit the false leading edge.

Cheers

Danny

[john stones 1 - Moderator]

where you supposed to cut the 3 inner ribs

and put a full strip in ?

seems an odd method

[Danny Fenton]

Hi John according to the attached notes those blocks get added above the spar and in each bay. I haven't made it very clear and the joint is good so perhaps you cannot see it but the rear spar is intact all the way from root to tip, I have simply added blocks to bring it up to the top of the rib
Cheers
Danny

[john stones 1 - Moderator]

no you were clear enough Danny

I didn't engage brain before typing

[kc]

Danny you questioned whether the webs need to be a tight fit against the ribs. This is not conclusive, but it's interesting to note that the Avicraft Mini Frantic has only small bits of webbing. Each rib has a only a bit of web material glued either side of the rib , with a large gap until the next bit. Presumably to save weight but even these small bits of web stiffened the wing a lot.
My instinct is it doesnt matter if the web touches the rib but it's important to have it firmly glued to the spars all the way along. Doubtless the engineers amongst us will quote the apropriate theory! (It's quite common to see steel girders with large cutouts in the central rib and this is much the same)
Anyway as always the definitive answer could come from a practical test to destruction on small test pieces. Remember Alistair Sutherland's RCMW article on balsa Warren girders compared to T truss girders etc tested until they failed? Very informative.

[Danny Fenton]

Hi Chaps, we all do it John, me more than most

Thanks KC I do not recall the article in question, can you enlighten us on Alastair's findings?

Managed to get both wing panels to the same point, in between trips to stagecoach and swimming for my little 'un

The rib cap strips were actually done after the leading edge sheeting, its just the leading edge was fitted using Titebond and the capping with Zap, hence the pins

Cheers

Danny

[kc]

Alasdair Sutherland's practical tests revealed that the T triangle (similar to the Barnstormer etc rear fuselage) was about 50 percent stiffer than a Warren girder and only 7percent heavier. He proved that gusseted square structure was nothing like as stiff as the T triangle or Warren. He also tested how the strength improved with covering by film or by tissue. All this was in RCMW Aug 2002.
I would like to put a copy of this here on Modelflying but RCMW are the competitors so I wont to avoid problems!
My point in mentioning this is that Alasdair did simple tests with balsa sample structures to prove what was best for balsa models. It's almost the same as the webbing spars discussion but not exactly. So the field is open for someone to do a test on spar webs -touching the ribs and not touching etc-- in a practical but scientific way. Maybe a project for engineering students to investigate?
Anyone who is interested in this sort of balsa structures to test engineering principles might also want to read the book Basic Structural Behaviour via Models by Barry Hilson pulished by Crosby Lockwood in 1972. This shows other balsa structures being tested to destruction.

Someone built a model ( almost certain it was the great David Boddington) and used tape or ribbon in a double diagonal manner (rather like the herringbone struts used in roofing)instead of webbing the spars. He said it increased stiffness remarkably.
So to sum up I believe that its not essential for the webs to actually touch the ribs but of course it would be better.

Edited By kc on 26/01/2014 12:28:43

[Danny Fenton]

Thanks KC, It is an interesting area for sure. I have responded via a PM in more detail as KC very kindly sent me a scanned copy of the article by Alisdair.

Not much to show, the lower wings have followed the same sort of construction as the upper ones. I promised to show how I do the belcranks, probably much the same as everybody else. We will see how they stand up the test of time.

These are the component parts, I very carefully remove any bur from the plastic parts

The hole in the plywood is made with a quality 2.5mm drill bit. The screw is then threaded through the ply wood until it is snug. The Nyloc nut is then fitted over the top until every thing is done up but still free.

a simple Z-Bend is all that is needed, but the hole must be a good fit!

Next up is the small centre section which I will lash together next.

Cheers

Danny

[Nigel Day]

'Lash together'

One man's lash is another man's really neat piece of work.

[Danny Fenton]

Thanks Nigel

Cheers

Danny

[Biggles' Elder Brother - Moderator]

The webbing acquires no significant strength from touching the ribs as such, webbing adds strength via two geometric routes;

Firstly it couples the upper and lower spars with a member that is extremely stiff in bending about the chordal axis - ie a sheet of balsa "end on".

Second because the grain is vertical this stiffness is even further anhenced by a resistance to buckling under load which it might display were the grain spanwise.

The complicating factor here is that what maximises the effect of webbing is maximising its area - the bigger the area of webbing we can create the stronger the wing will be and the lower the stress levels will be in the wing for any given deflection. Now of course the absolute maximum area of webbing would be achieved by completely spanning each bay with a webbing piece - then the webs would indeed touch the ribs. But this would be maximally strong not because the webbing is in contact with the ribs, but because the area of the webbing was maximised! In fact there would be no detectable difference in strength between a wing where the webs did contact the ribs and one where the webs were 99.9% of the bay width, but just fell short of actual contact.

If fact it could be the case that contact is sub-optimum because it would introduce another deformation mechanism that is not necessarily desirable - ie the bending wing would start to "compress" the webs between the ribs. While balsa (like most woods) is very strong in compression, its even stronger in "edge-on" (or in-plane) pure bending. To prove that to yourself take a small piece of balsa sheet - you can crush it comparatively easily, but you will not bend it "in-plane"! In well installed webbing the joint with the spar would probably fail before the wood in bending "in plane".

BEB

[Danny Fenton]

Thanks BEB well that clears that up! However BEB makes no mention of the power of OCD this makes the webs fit perfectly. I CANT make gaps
Cheers
Danny

[Tony Bennett]

OCD has its good points then, cos you produce some excellent models.

[Danny Fenton]

Thanks Tony, you don't do half bad yourself

"lashed together the centre section. Used a different approach again, this time I pinned the spars and ribs straight onto the board, then the 1/4 leading edge, webbing then sheeting. Finally lifting the assembly from the board and sheeting the leading edge underside section.

It suddenly dawned on me that there was no provision for wing attachment. So a quick scratch of the noggin and look at the plan, a couple of dowels set into the leading edge, and some blocks in the trailing edge should sort that. Good job I didn't sheet the undersides yet

Next up is to notch the ribs adjacent to the webbing and epoxy the dihedral braces into place.

Cheers

Danny

[Nigel Day]

This is a bit like watching golf on the telly. On the rare occasion a pro shanks it, it comes as a shock.

[Danny Fenton]

Thanks Nigel, in my enthusiasm to get the wings joined I have overlooked the servo! doh! Another affect of modelling at the end of a day Anyway I wanted to have the top sheeting on to keep some rigidty in the centre section during wing joining, so its not really wrong. Just don't be surprised to see a hole appear in the centre section upper sheeting through which to mount the aileron servo. Hopefully more later.

Cheers

Danny