SE5a CONSTRUCTION BEGINNING TO . . .

[Guest]

may sound stupid but what is aft centre of gravity

is it just the centre of gravity of the plane?

[Greyhead]

Iain

Firstly there is no such thing as a stupid question, only a question you don't know the answer to.

The centre of gravity (CG) is the point at which the aeroplane physically balances, it is always marked on the plan and it is very important to get the model to balance correctly. A model that is nose heavy will usually fly reasonably well but a tail-heavy model can be literally unflyable. The CG is always some distance in front of the aerodynamic balance point to ensure stability.

On all the planes I know there is more model behind (or aft) of the CG than in front, in the case of the SE5a there is 10" from the CG to the nose and 34” from the CG to the tail so it needs 3.5 times the weight in the nose to balance weight at the tail. The length of the nose is calculated to allow the weight of the engine to balance the plane but as our engines tend to be light compared with the full size counterpart it is important to build as light as possible behind the CG to minimise the “dead” weight required in the nose.

So to actually answer your question, to read it correctly “aft centre of gravity” needs that next four words added to make sense which are “build up of weight”.

Hope that all makes sense!!

Grahame

[Greyhead]

Before I start on the construction of the fin itself I want to be sure that the tail plane incidence adjuster will work so I’ve made that first.


The fin post is a carbon fibre tube through which the adjuster rod, a snake inner, slides; the 2 attachments for the bracing wires are made from closed loop adapters and brass wire.





The lower attachment has a 2mm cap head bolt that will do the actual adjustment by screwing in or out of a length of snake inner firmly fixed to the fin post; I used snake inner for this as it acts like a large “Nyloc” nut so won’t alter the incidence as a result of engine vibration.


Any adjustment required will be made using an Allen key through a small hole in the base of the tailskid fairing.





This is the position it will be fitted in, at this setting the adjustment is about central and can be altered approximately 7mm either way.

[wonwinglo]

Grahame this little feature is so important,and it surprises me that more modellers do not use it,no matter how well calculated the tailplane incidence nearly always needs adjustment to enable the model to fly properly.

I really like the idea of the snake inner put to use in this way,by the way this material also makes ideal cowling screw fastener inserts,you just push a flared piece through the wood and attach with a dab of cyno,the flare can be made with a hot metal butchers skewer quickly pushed into the snake inner,another flare on the outside takes care that it is not pushed through the wood,the job is best done during construction of the model.

Full sized aircraft utilise a screw jack arrangement for ground adjustment by the engineer.

[Greyhead]

The fin is made using “core and half ribs”; for those not familiar with the method it is a simple way to make strong but light structures, especially if the outline is “curvy”. It is my preferred method even for simple shapes such as the SE5a fin, rudder and elevators; the tail plane may have to be a fully built up construction because of the cable and pulley elevator linkage, I’m not decided as yet.





To start the core is cut to the shape of the finished item, the usual material is 1/16th balsa but I’ve heard of the use of thin ply for extra strength or even Depron if weight is an issue. The positions for ribs etc. are marked then one side constructed whilst the core is pinned to the building board, thus ensuring a straight structure. After leaving plenty of time for the glue to fully dry the process is repeated for the other side.

[Greyhead]

The rudder uses the same method of construction. There’s no separate trailing edge, the core itself does that job; the covering is fastened purely to the edge of the 1/16th core, this works OK because all the edges are eventually taped.





The top and bottom edges have 1/8th x 1/16th balsa added. The balsa is soaked in ammonia for a couple of hours, the pieces for the other side are soaking in the plastic tube, the inner edge is pressed with the back of the scalpel blade to help it bend then it is pinned in place and allowed to dry before gluing.





With the lightening holes cut and the edges sanded all that’s left is the hard points for the hinges and that’s another problem.





I’ll have to make my own hinges to go around the tail incidence adjuster; I could use a commercial hinge for the top but then they wouldn’t match. If I’ve got to work out how to make one I might as well make them all, including the elevator and aileron hinges, happy days!

[wonwinglo]

Without a shadow of doubt this is by far the best type of construction for the tailplane,elevators,fin and rudder,the method is tried and trusted whether working on small scale flying models or large models such as Grahames S.E.5A.

I am not sure that I would have personally bothered to remove those lightening holes as the weight saving is so negligable as related to the extra strength of leaving them untouched,but this is such an individual choice.

The sternpost/rudder hinges will need some thought but I am sure with your ingenuity you will think of a way around it,there is sometimes no easy way out ith these things ? if you are thinking of making the hinges then beware of metal fittings which can cause 'music' and the harmonics cause problems,I have seen models crash because of metal to metal joints,just thought that I would mention it Grahame.

[Guest]

so far it looks lovely, keep it up, cant wait to see it when its done!

[Greyhead]

Barry

My philosophy is to save weight wherever possible, but not to compromise strength; I’d rather have a heavy model in one piece than a super light model in a bin bag! I’ve used “core an half ribs”, with holes, on several models and haven’t had a structural failure yet; I hope that’s not going to be a case of “famous last words”.

As for “metal to metal contact” that’s really a misnomer, it’s when metal to metal make and break contact that problems arise. Wheel collets firmly attached to the undercarriage leg cause no problem, but if the grub screws work loose and vibration causes intermittent contact then radio interference could well be the result. If a design calls for say 2 pieces of metal to slide over each other, which might cause problems, one way to over come it is to join the 2 pieces together with a flexible wire that allows the individual movement but ensures there is always electrical contact.

[wonwinglo]

The bonding wires are used on full sized aircraft all over the airframe,miss a tree out and see the static build up on the aircrafts radio system,as you rightly say it is chatter between metal that can cause a problem and not metal to metal perhaps I should have phrased that better,not as big a problem as it used to be but still relevant.

I think the core method is perhaps the overall best system for thin surfaces,the advantages being that once built you cannot see the core itself,and once more it is so much easier to construct,I even use a 1/32 core on my small electric models now having proved the system many times.

[Greyhead]

As the build progresses I’m always thinking in advance, sometimes many months in advance, and one thing that has occupied my mind for a while now is whether or not I should make the tailskid steerable. From a purely practical point of view it isn’t necessary as it is only really used whilst taxiing, which for some reason is frowned upon by our club committee; on the take off run the rudder is effective enough on its own. It is also more complicated, with hinges and linkages to organise, not to mention the extra weight, so all things considered I decided against it.


The problem was I was thinking about it “in isolation”; now I’ve come to actually designing the part it is obvious that because of the tail plane incidence adjuster the skid has got to be separate from the front section and will need some substantial fixings, so they might as well be hinges. The best laid plans of mice and men etc.





The construction is a variation on the core method; in this case the core is a 1/16th ply frame with tongues to locate it in the fuselage, 1/16th ply doublers at the rear where the hinges will fit and 1/16th balsa sides.

[Guest]

As the build progresses I’m always thinking in advance, sometimes many months in advance, and one thing that has occupied my mind for a while now is whether or not I should make the tailskid steerable. From a purely practical point of view it isn’t necessary as it is only really used whilst taxiing, which for some reason is frowned upon by our club committee; on the take off run the rudder is effective enough on its own. It is also more complicated, with hinges and linkages to organise, not to mention the extra weight, so all things considered I decided against it.The problem was I was thinking about it “in isolation”; now I’ve come to actually designing the part it is obvious that because of the tail plane incidence adjuster the skid has got to be separate from the front section and will need some substantial fixings, so they might as well be hinges. The best laid plans of mice and men etc.



The construction is a variation on the core method; in this case the core is a 1/16th ply frame with tongues to locate it in the fuselage, 1/16th ply doublers at the rear where the hinges will fit and 1/16th balsa sides.

I'm afraid I have to say Greyhead that I would never consider a comittee's preferences when designing or manufacturing a model, I make mine for myself and no-one else.

Let them frown upon what they want, they really need to be putting thier efforts into more constructive pursuits.

[wonwinglo]

Here,here,from my past experiences those on the committe at clubs are the less informed on modelling matters and seem more concerned on restricting members than encouragement,they like to make comments to make them feel big because their knowledge is inadequate ! nuf said,we think alike.

Build what you like,how you like and fly it in the manner befitting the real aeroplane and you cannot go wrong.

[Greyhead]

The tailskid is made from 1/8th ply with the hinges from brass tube and strip. To avoid the dreaded “metal to metal contact” the hinge pin is a snake with the outer attached using the time-honoured method of stitching with button thread. The top hinge has a washer soldered to it to act as a bearing





Here it is temporarily in position.





The tailskid shroud will be from litho plate and will hide the stitching.


With a steerable tailskid there may well some taxiing going on (when there are no committee members about!!)

[wonwinglo]

You make good use of the snake material Grahame,it is worth buying just for the alternative uses that it can be put to.

The skid/steerable unit looks very businesslike,there is nothing better than starting the model in the pits and then to taxy out in such a scale fashion,after all you never see a real aeroplane being carried out to the runway,same on landing turn off the strip and taxy back again,but this alone requires the skill to perform it,scale judges love things like this.