Wednesday, 5 April 2017

A simple way of Calculating a Tuned Pipe Length

I managed to acquire a brand new in box OS61VR.

There is very little information on this engine on the OS archive, in fact I cant find any reference to it. I have a choice of a number of tuned pipes I can use  - one came with it, but it looked rather small compared with the other 60 sized pipes I have so it may be the wrong pipe. The supplied header also appears to have been used as a hammer - it was very badly creased so I ordered a new one from Just Engines.

They also had no information on this engine.

It  has a large inlet and exhaust port. This suggests that the engine will be rather powerful and probably very thirsty as well.

Now, forgive me if I am teaching you to suck eggs here, but I need to set the pipe length and this can be calculated using the following formula.

Pipe length (in inches) = Exhaust Timing (in degrees) * 1650 / Desired RPM.

I am hoping to get 12000 rpm on a 12x8 prop (in the absence of any other information).

The exhaust timing is calculated by placing the engine at TDC, turn the crank (in the direction of rotation) and look for the piston crown just appearing through the exhaust port (at the top). Note the angle on the timing disc. Continue to rotate through BDC until the piston crown disappears again on the compression stroke - i.e. you have timed the exhaust open window.

I made up a simple timing disk and attached a pointer using a jubilee clip as shown.


Using this technique, I measured this engine to have an exhaust open timing of 145 degrees.

Substituting the numbers above,

Pipe length = 145 * 1650 /12000 = 19.9375". The pipe length is measured from the centre of the piston crown to the baffle in the pipe where the diverging and converging cones meet.

There becomes a point where the exhaust timing becomes too long for a tuned pipe to become effective. A longer exhaust open time will give the user an easier handling engine but the benefits of the tuned pipe diminish - reducing to zero when the timing increases towards about 175 degrees.

Monday, 3 April 2017


This is a short post  that links the downloads that I have made available.

Mystic 40

Mystic 40 plan and and cut parts layout

Kwik Fli 3 for a 40

Kwik Fli 3-40 plans and cut parts layout 

Kwik Fli 4 for a 40

Glass Slipper Slope Soarer

DH87B for Indoor Scale

I have been asked to take part in the indoor Scale Nationals at the end of March 2017. Probably being rather overambitious but I have taken up the challenge and will try and build a 1/14 scale DH 87B Hornet Moth.

G-AELO is the target aircraft that I am going to try and replicate,

(image Wikipedia)

I started work on the plans in 2015 - it was originally intended as a Free Flight Scale model at 1/12 scale (31" span) - something I may attempt to start and complete again in the future.

I have simply scaled down the original CAD drawing I produced and I am using this as the primary source for the airframe structural layout. As the CAD drawing is currently incomplete, it is likely that I will have to rework substantial parts of the model. Hopefully not too much rework though.

G-AELO is also one of the featured aircraft in the book "De Havilland Moths in Detail" by Stuart McKay.

This will simplify getting the supporting documentation needed for the competition. By the way, the book is an excellent read, a great deal of information provided in a manner that is very interesting with some superb photography to support the text.  Makes you feel part of the story. Highly recommended.

This is the plan that I am currently working to. It's traditional Stick and Tissue construction but with micro RC gear fitted. The Scale Maestro Danny Fenton is also giving me lots of guidance and for that I am exceedingly grateful.

The plan was tile printed from PDF and the A4 sheets taped up then trimmed. A very low cost way of producing a plan

There is not much actual build information on the plan, its really just a parts placement plan so ideas will be tried (and many rejected) as the model is being built. I'll also be trying out some new ideas as well

A couple of evenings in the shed and one side is almost completed. The cling film is to protect the airframe - not the plan.

The second side is built directly on top of this side (separated by cling film) and then joined together using cross members with measurements taken directly from the plan. That curved top elevation will be fun to achieve.

The fuselage sideas are jigged upright and then a few selected spacers added. The whole lot is then weighed down carefully and the glue allowed to dry thoroughly. The taper behind the wing is straight and very simple to achieve. The taper in front of the wing is curved and different top and bottom and far more challenging.

Very fragile in this state, I cant imagine how many spacers I have to secure. Glue is weight and is kept to a minimum, sometimes a little more is required.

The front fuselage spacers added and work started on the fin.

I used a laser dye transfer method. I am trying to get the appearance of a scale structure. The method is quite simple, laser print the shape with ribs etc. Flip the print face down on the balsa with the grain running in the correct direction and then rub a little cellulose thinners through and the image is magically transferred onto the wood.

This has a 1/32 balsa core hollowed out around the 1/32 doublers in the fin area then a significant amount of wood is removed from the rudder, leaving mainly an outline and the 1/32 square ribs. The whole lot is then sanded down to a streamline.

It sits in the top of the fuselage with the small tongue located in a slot in a 1/32 fill sheet. Weight of the fin and rudder is 1.44gms

The nose block was manufactured from 1/2" very light sheet balsa. A suitable sized washer - the same diameter as the spinner was tacked into place and gave me something to sand to.

Its not quite the correct shape yet, but its not a million miles out either

And this is what it looks like. The 3 curved upper front deck formers are slightly too high and need to be reduced by a couple of mm.

The cockpit was fabricated and installed before the fuselage could be covered.

I was really quite pleased with that console as a first attempt

Cant claim any credit for the pilot. He is actually slightly too small in scale and I did have to provide legs and paint him (now known as Cedric).

And installed..

Wing construction is fairly traditional for indoor models, comprising of wing ribs created by wrapping a preformed (sliced strip) for the top and bottom over internal spars. I a not entirely happy with this method of construction. While it is undoubtedly very light (the wing panels weighed 8 grams each), I found it rather too fragile for my ham fistedness.

Ailerons were built with the wing then separated when ready for covering then hinging

A sort of bare bones shot.

The flight radio gear was procured from Micron UK, comprising of a micro Rx and servos and a park-fly motor, gearbox and prop.

Carbon push-rods used for the aileron linkages out to traditional bell-cranks. Horns fabricated from 1/32 plywood.

I found covering very difficult. Any construction errors were magnified by the resulting creases and it took many attempts to get it to a state that I was half happy with.

 The model was covered in red and silver Esaki Jap tissue. The orange roof panels were from Orange cellophane from a brand of Jaffa Cakes.

Windows frames cut from a very thin baking tray and glazing from the base of a cake tray.

Tailplane and rudder

Interplane struts are located using a bent pin through a small loop in the correct place

Lettering and logos were produced in Photoshop then transferred back into CAD to make sure that the size was correct then printed onto transfer paper then finally sprayed with car lacquer to make them waterproof, before they ere placed in water to separate them from the backing

When finally finished, it looked like this

A very interesting project. I am not entirely happy with the wings as they have bowed slightly and may redo them but I think its not a bad representation of the original.

Monday, 9 January 2017

DHC-1 Chipmunk - wing

After about a 18 month break, I have restarted work on the Chipmunk. The Fuselage has been parked for the time being and I have started work on the wings. Nothing too challenging at the moment, it is simply a case or reigniting my enthusiasm, getting a bit of progress and hopefully, I'll now see this through to completion.

As before, it will have to fit in with whatever else is happening in the Shed, but now I have got the irritating bits of the design out of my system (it's scale accuracy), I feel much happier and I'll now try and get this completed and with a moderate amount of scale detail. Like all large projects and eating elephants, its best to approach the task, one bite at a time.

The parts kit is almost complete (centre rib and 2 1/16 ply brace/doubler ribs not present - but that's not a problem) although the balsa wing rib wood is rather hard and prone to splitting. Still quite usable but I am not sure how it will survive a hard arrival.

Lots of clamps used to make sure the joints are nice and tight. The number of braces in the centre are in my opinion rather excessive but that is what we are stuck with. The wing certainly wont fold in flight. It was actually far more difficult to get the two panels to align correctly surprisingly. It took me 3 attempts to get it to where I am happy with it.

 Progress so far has been slow but the upper wing sheeting has been added. I can how start work on the aileron push-rods and the undercarriage. A bit of belt and braces while work progresses on the overly delicate Hornet Moth

I was never happy with the original method of retaining the torsion bar undercarriage legs which used tinplate straps binding the legs and then screwed to a hardwood block. The centre section is so over engineered - this looks like a potential weak point.

My approach has been to implement much larger beech blocks and cut a deep channel. Aluminium plate saddle clamps will hold the leg in place without risking damaging the wing skins  which sit below the torsion bar.

The anti twist stop will use a metal saddle clamp - bolted through the wing ribs

 Hobby King Oleo legs grafted onto the end of the torque rods. These are designed to be clamped. Its quite an easy task to drill out to 4mm to fit (interference) with the 8swg (4.1mm) diameter rod.

M3 hole drilled and tapped all the way through and cap head screws used to clamp the legs in place. A small flat will be ground on the legs to prevent them twisting

The sprint tension is too tight. The spring needs to be reduced in tension so that the leg compresses by about 6mm with (guessing) about 1.5kg load on them. I don't want to soften the spring, simply reduce the preload - the easiest approach is to incrementally reduce the spring length.