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Author Topic: LPP Build  (Read 5368 times)
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Hepcat
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« on: August 02, 2008, 04:33:00 PM »

A few days ago a friend and I decided to build a Penny Plane each so that we could support the British Indoor Nats in September. I thought I would put up a few words and pictures about the build. This is not a tutorial; it is my first LPP and although I have enjoyed some indoor flying in the past I have been a dabbler, not a dedicated exponent. However I hope it will have some interest and perhaps even stimulate some discussion.

The rules for Limited Penny Plane limit the span and chord of wing and tail, the length of motor stick and boom, the diameter of the propeller and the minimum weight so most of the interest is taken out of the design side but I still have to build it so here goes.

I started with the wing ribs. I have used sliced ribs quite often but have never liked them because of the way they split along the short grains at the end of the ribs so I decided to try some laminated ones. I cut two pieces of 1/32 sheet and soaked them in water for about an hour. After wiping off the excess water I placed them together and used some pins to hold them in an airfoil shape as in the photograph. When the wood was dry I coated the faying surfaces with a very thin coat of PVA (white glue) and put them back in the pins until the glue was set.

The curved, laminated sheet is then stripped into 1/32 thick ribs using the simple stripper which is also shewn in the picture. (Always remember to measure twice and cut once – I didn’t, which is why I have another curved sheet in the stripper, being cut, which is one inch short!)

I don’t think I shall ever again make sliced ribs the ‘old’ way. During the soaking and sticking you can be doing something else, the actual making time is not much different and the ribs are more accurate and much stronger.

John
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« Reply #1 on: August 02, 2008, 07:10:37 PM »

The only part of the plan that is drawn out properly so far is the wing and that is what I am building first. Because of the rules aeroplanes don’t come much uglier than Penny Planes so I thought that I would at least give the wing a trendy, curved leading edge at the tip. A silly idea really because it is a lot of extra work for two simple parts! The tip LE is 1/16 square so I decided to take a piece of 1/16 x 1/8 balsa, soak it overnight, pin it down around a former and then, when dry, to split it down the centre with a 1/16 balsa stripper.

 For soaking over night I find it convenient to use a bit of 22mm plastic pipe (the same stuff I use for Coupe winding tubes). I fix a piece of plastics bag over one end with adhesive tape, fill with water, and pop in the strips for soaking. A crude bung is needed in the top otherwise the strips float with half their length poking out of the top.

The picture shows the strip pinned in place around the former. Pieces of wood are placed between the pins and the workpiece otherwise the pins will mark the wet wood.

The next thing I knew would be coming up was selecting the wood for the main spars. The LPP does not require the best of wood but no model should be built with rubbish and eyeballing alone does not do it. I checked the Stiffness Coefficient by the method in the second picture. For those not familiar the procedure is as follows. The sample piece of wood is weighed and measured as accurately as possible. The bottom end is placed on a weighing scale and pressure is applied to the top end. This causes the strip to buckle. The load is increased slowly and carefully and it is found that there is a point where the buckling increases without any further increase in the load figure on the scale. This figure is the buckling load and from this load and the dimensions of the strip the Stiffness Coefficient can be calculated. The SC is a measure of the quality of the wood to resist bending. The piece of wood in the picture registered an SC of 100, which is not the highest quality but is good and adequate for the LPP.

John
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Jun
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« Reply #2 on: August 02, 2008, 10:34:18 PM »

Hi John,

 Your rib fabrication method is very interesting. If you use a Simplex airfoil, it'll work for wings of different sizes and shapes, tapered or curved. Actually, it's similar to the method Wombat (Neil Dennis) uses and recommends, except that I think he uses a single curved sheet, not two laminated and glued ones. Must try!

Regards, Jun
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« Reply #3 on: August 03, 2008, 12:31:36 AM »

...I thought I would put up a few words and pictures about the build. This is not a tutorial... However I hope it will have some interest and perhaps even stimulate some discussion.

Hepcat

Thanks for posting your LPP build. Please keep on with the "few" words and pictures as you did in the above posts. "This is not a tutorial..."; well, I respect your words, but I`ll have a tough job trying to convince myself about your affirmation...
Waiting here with "some" interest...!

Respectfully
Julio
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« Reply #4 on: August 03, 2008, 10:27:55 AM »

Julio
Good to know you are still around. Thank you for the kind words.
Jun
That was a good thought to use Simplex sections on the ribs. It makes the method even more attractive because one could make some laminated, formed blanks for stock and then just pull one out of the drawer and cut a batch of ribs in a couple of minutes. I was aware that some people just heat-bend the blanks but with my workmanship I always worry that heat bent things might ‘unwind’ over time and that glued laminations should be more reliable.

There is little to say about constructing the wing centre section. A straight strip is pinned down in front of the LE and behind the TE and the spars are held against these with a few short bits of wood held by pins. The difficult bit is not building the wing but covering it! I elected to cover the centre section before adding the tips, which does make the centre section much easier.

I am covering the LPP with Ultra Film (‘Mylar’) which came from Ray Harlan 10 years or so ago. I cut the piece of film required and then roll it into a tight ball. Don’t forget when using film to keep the working area clean and free of blobs of cement and such like perils. It is probably easiest to work on a sheet of paper. I find it helpful to wash my hands in cold water and dab them reasonably dry but a little damp. For me this minimises the problem with static. For adhesive I used 3M Spray Mount repositionable.

I open the ball of film and spread it with my finger tips (although I have heard of others using a soft brush). The top of the LE is rounded off to match the curve of the ribs so after applying the adhesive the centre section is placed on the film LE first. Now I must confess that the photograph is a fake but I couldn’t hold the wing properly and take the photo at the same time, so I propped the wing in the right place with the scrap of wood. The wing is then rolled backwards until the TE is touching and then the whole centre section can be turned over for a final prinking. I trim the tissue with a fine blade, usually one broken out of a multi blade razor.

John
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« Reply #5 on: August 03, 2008, 09:19:39 PM »

Hi John,

I am about to try your rib fabrication method and I have an important question. When you laminate the two sheets do the grains run in the same direction or opposite, i.e. one sheet chorwise and the other sheet spanwise?

Jun
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« Reply #6 on: August 04, 2008, 12:08:27 PM »

Jun
The grain on both pieces of wood for the ribs is longitudinal (along the length of the rib). I recommend some care in the gluing. If the ribs are for outdoor use it probably does not matter so much but for indoor the glue can be a high proportion of the structure weight. I need to admit here that I was too skimpy and did not ensure complete coverage so that, after stripping, several of the ribs delaminated at one end and had to be rescued with an additional smear of glue. As mentioned before I used white glue (PVA). I have heard of wallpaper paste being used; I have used it successfully on laminated propeller blades but did not think to try it here. Any comments would be welcome.

In the photograph here the port tip has been attached and covered and work is just starting on the starboard tip. This is made easier because I left 1/32 sheet dihedral braces protruding from the centre section when it was built. I don’t know if these are visible in the photo’ but they are on the inner faces of the LE and the TE.

A point that I have not mentioned so far is that I dislike working with plastics covering sheets over the plan, to prevent the bits sticking to the plan. Instead I just rub the plan with a stub of candle anywhere a glue joint is to be made. That may sound like a lot of work but I think it near enough as quick as getting a cover sheet pinned on smoothly and is more pleasant to work with.

To cover the tips I did not use the Spray Mount because I find it easier to glue to film to the joint rib first without it clinging to the rest of the outline. I used contact adhesive thinned to watery consistency and applied it with a very small brush. After getting the film as I wanted it on the dihedral joint rib I tacked it to several points on the outline with little spots of glue and when all looked right I glued the whole lot down.

That is the wing about finished except for the fixings. That’s 0.63g so far. I am intending to use tubes on the wing and posts on the motor stick although most people do it the other way around and I am wondering why!

Onto the tail next. A bit more plan drawing to do so perhaps a couple of days to the next installment.

John
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« Reply #7 on: August 05, 2008, 07:38:51 AM »

Hi John...interesting build! I am wondering what you used for ccontact cement and how you thinned it?? I use UHU Por a lot but it would be great if I could thin it to apply with a brush.

thanks
Bernard
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« Reply #8 on: August 05, 2008, 01:36:44 PM »

Bernard
I do not know if ‘Contact Adhesive’ is a worldwide term but with us it is a thick, light yellow coloured, glue which I think is most commonly used for attaching plastics laminates to wood to form kitchen work tops. It comes in tubes and also in tins for large jobs. It is intended to be applied to both surfaces, spread with a spatula, left to dry for 15 minutes and then the two parts are brought together for an instant bond. The top brand in this country is Evo-Stik but I used a Wilko’s Contact Cement which is much cheaper. The usual thinner is Evo-Stik Cleaner which is basically Toluene. Being a solvent our stupid government makes this very difficult to buy so I often use ordinary dope thinners to bring a blob of glue down to a watery consistency. I do this mixing in the little aluminium foil dishes that little cakes come in - as I guess most people do!

On indoor models I apply the film as soon as possible after applying he glue. On outdoor models I often do the same but sometimes I leave the glue to dry and then reactivate with a warm iron.
John
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« Reply #9 on: August 06, 2008, 07:48:11 AM »

Bernard

The UHU POR adhesive is a foam friendly contact adhesive, similar but different than the one Hepcat (John) is using. I don´t know the recommended solvent that can be used to dilute this product, but if you still want to use it, and living in Germany, you can send a mail to UHU (Germany) asking for help. The mail is their web page, and if you explain your interest I believe you will receive a positive answer. If you go that way, please let us know if you have any info!

John

Please, can you give a brief explanation on how you control the quantity of contact adhesive sprayed (not thinned and applied with a brush) on the center section of your wing? I have read about modellers that spray a mist of adhesive to the air (with a light that let them see the mist floating in the air) and then move the wing structure through the mist to have enough but very little adhesive over the structure. I´m just interested in your method and advise.

Thanks!
Julio
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« Reply #10 on: August 06, 2008, 08:03:56 PM »

I´m just interested in your method and advise.

It took me more than eleven hours to realize that I misspelled the word "advice"; but also, and more important, I didn`t mention I`m interested in other members methods and advices too. Please accept my apologies, specially with the second issue.

Negative kudos Happy Hour...

Julio
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« Reply #11 on: August 07, 2008, 10:42:47 AM »

Julio
There is certainly no need to apologize for your English, it is better than what most so-called English speakers can manage!  How have you achieved such fluency?

I too have seen mention of spraying adhesive in a cloud and then passing the framework through the cloud but I keep wondering where the residual spray goes! After a few models I think my shoes would be sticking to the floor. I keep an old magazine with a large page size in my drawer of covering materials. I open it at a clean page and lay the framework on the page. I try to do one smooth pass across with the adhesive but I must admit I usually give a second in case I missed a bit and consequently finish with a little too much. I think it is difficult to develop a good technique because most of us only use the method a few times a year.

I have made the PP tail unit, apart from covering, but it is so simple that there is little of interest. The picture shews how I use a strip of balsa to hold the outside edges of the LE and TE in place instead of lots of pins. I have several straight strips in my scrap box that I have rubbed over with soap so that the framework cannot stick if the glue strays a little. Special care is always worthwhile when removing indoor frameworks from the building board. The LE and TE were the same wood as the wing spars but of about 0.05” x 0.04” cross section. Ribs were from the wing blank and just cut to length. Uncovered weight of the tailplane is 0.19g. The fins are from 0.035” sq material and each weighs about 0.025g. but that is getting towards the limit of my home made scales. Smiley

John
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« Reply #12 on: August 07, 2008, 06:37:12 PM »

Hepcat-John

Thank you for taking the time to share both your experience and knowledge; what a great combination indeed. Very kind from you and other kind members who also have and share that virtues.
I was also worried about the adhesive cloud, so I used exactly your method: one smooth pass and then (gggrrrr!) a second one, just in case. Feeling very guilty about the second...I have to build more often to get rid of that guilt.
When you say "it is so simple that there is little of interest" is when I get more interested. "The beauty of the simple" are the words that come to my mind.
Great build topic here with your LPP. Lot of good info.
Thank you again John.

Julio
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« Reply #13 on: August 08, 2008, 12:49:04 PM »

I covered the tail unit and am now thinking about wood for the motor stick and tailboom and I am sure that a lot of you will agree with me that it often takes longer choosing the wood than doing the build. Just to keep things moving whilst doing the thinking I decide dto make the propeller bearing.

This is the usual indoor type bearing with a pigtail at the rear so that the propeller shaft can be taken in and out. (IIRC I first met Julio through talking about pigtail bearings on SFA.) They are not too bad a job but sometimes they won’t seem to come out right and the first one I did today was one of those – it is on the right of the photograph. They are easier if the pigtail is formed behind the rear ‘leg’ but I prefer to have the pigtail in front of the leg. Also there seems to be no best order for doing the bends; whatever starting point you choose seems to leave some trouble at the end.

Today I tried something new. I bent a front bearing loop on a short straight piece of wire. I formed a pigtail on another piece of wire. I made a simple bend on each piece of wire and soldered the two together as shewn on the left in the photograph. I should mention that the wire is only 0.013” diameter and there was no need to add any solder (although I applied some flux), the ‘tinning’ on the iron soldered the joint at the first touch.

I found this procedure very much easier than doing the job as one piece and I shall use it again in the future. I think that cyano would probably join the parts with adequate strength because these items are glued into a slit in the front of the motor stick. Unfortunately I have no cyano at the moment to do a test. ( A modeller without cyano – must hurry and tell the King!)

John
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« Reply #14 on: August 12, 2008, 03:47:43 PM »

The tail unit is suspended below the tail boom on posts as shewn in the first picture. Apart from esoteric aerodynamic reasons which may (or may not!) accrue from this it does allow the incidence of the tailplane to be easily adjusted. This is accomplished by the tiny paper tubes, fixed to the leading and trailing edges, which can be slid up and down the posts.

This leads to two jobs that are not usually encountered in building outdoor models, rolling tissue tubes and rounding off balsa posts for them to slide on. The tubes are usually made 1/16 dia and are formed on a piece of 1/16 dia piano wire like the piece shewn in the foreground of the second picture. The label on the end is not just for the photograph. I have a box for short pieces of wire that will ‘come in useful’ in the future and if I toss it in there without a label it will take a lot of finding. After you have a decent piece of wire, the correct length, polished and with the ends deburred and radiussed there is no sense in losing it! There is also a short piece of brass tubing that will be explained later.

In the background of the picture are the parts for the ‘tool’ I use to round the posts. There are two pieces of balsa, with 400 grit sandpaper glued on, which are held together with a rubber band which sits in the notches at one end of the balsa pieces. The scrap of balsa sits between the other ends of the balsa pieces so that the two sandpaper faces are held at an acute vee angle. (I am sure I have seen a method like this mentioned in print some time ago but I can’t remember where so I can’t pass on the credit.)

To make a tube I use a piece of Jap tissue about 0.75 wide and 1 inch long. I think the tube comes off the wire easier if the grain runs lengthwise in the tube, so that is across the width of the tissue. Place the brass tube on the wire and glue the 0.75 edge of the tissue to the wire with thinned balsa cement as in the picture. Add more cement to the rest of the tissue and roll to form a tube. Immediately slide the tube off the wire, allow it to drop onto the bench and don’t touch it until dry. The usual recommendation is to slide the tube off the wire with a thumbnail but I have found that the piece of brass tubing is more successful at removing the tube from the wire without crushing. When the tube is dry it can be cut to length and it is probably safer to slide a piece of wire into the tube whilst the cut is made.

Wing and tail posts are usually made from 1/16 square balsa. To round them to fit in the tissue tubes it is usual to cut or sand off the corners to form a rough octagon and then to twirl the post between the fingers whilst applying sandpaper. The sandpaper can be held in a rough vee between the fingers but the set-up shewn is safer and more consistent for those who are not naturally dexterous. The tool is held so that the thumb and one finger prevent the post from dropping too deep into the vee as it is twirled. If it does drop too deep it will jam and break.

John
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« Reply #15 on: August 20, 2008, 02:58:15 PM »

At last we come to the most interesting part of any build – the propeller. Penny Plane propellers almost invariable use thin sheet balsa blades moulded on a former to give a twist along the blade. Often this is done on a cylinder (can, bottle or jug formed props!) but I prefer to use a former that gives me better control over the shape of the finished propeller. When going for a, so-called, True Helical Pitch it is often suggested that the former is made as if carving half of an ‘X’ blank propeller. This is simple in some ways but does require a large former. This is particularly the case if the propeller blade has a large blade width, as is common with indoor propellers, and especially so if the diameter is restricted as with a Penny Plane.

I prefer to make a former that has a constant width from root to tip and where the twist is apportioned from root to tip in such a way as to make a compact former. This is difficult to describe in a few words but will be apparent as the build progresses. Although I have talked loosely of the former going from root to tip the forming part only stretches from the 15% radius to the tip. It is rare that those hub portions of a propeller are shaped in any aerodynamically meaningful way and certainly, on indoor models, there is usually just a circular spar.

The first illustration shews the pieces of wood ready for assembly. The piece at the back is a 1/4” sheet base, made quite thick to prevent distortion. All the rest is 1/16” sheet. The triangular pieces on the left are identical; one goes at each end of the former and the acute angle on each is half the total twist used along the blade. The two large pieces, each with a curved edge, fit at either side of the form and the curve decides the distribution of the twist. It will be noted that the curves mate with each other so only one cut is needed to form both pieces. The curves do require a little calculation but it is very simple and takes only a few seconds on a tiny spreadsheet.

In the second picture the parts are assembled. One of the few times I use cyano! The top of the framework will next be ‘planked’ but there is a little sanding to do first. The curved sidepieces will have been cut with square edges and it is necessary to angle them slightly to suit the twisted top surface. A curved sanding block is necessary; I wrap some sandpaper around a pencil as in the picture. This is drawn along, keeping it at right angles to the centre line. Surprising though it may seem at first, the sanding bar will be in contact with both curved side pieces and the straight piece that runs down the centre line at all times.

In the third picture the top has been planked with 1/16 sheet with the grain running the short way, sanded, and then doped to resist moisture when the wet blade blanks are strapped on. The triangle in the foreground is placed on the extended base of the former when the propeller spar is being glued to the blades. The propeller shaft is fixed to the hypotenuse with masking tape, which ensures that the blade is fixed to the spar at the correct angle.

John
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« Reply #16 on: August 20, 2008, 04:53:20 PM »

i love your prop block mold that you made!! can you supply some eager beavers with the specs to get the right curve fir the sides??

regards
Matthew
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« Reply #17 on: August 21, 2008, 12:55:27 AM »

John

I am fascinated with this, your LPP topic. Liked every detail of your replies here. The tissue tubes for the tail posts in opposite sides of the boom...that knocked me down.

I have a couple of questions. A propeller thing; just checking if I`m doing right or wrong. Hope not to bother you or other members.

First question: Your third picture in reply #15 shows the finished blade form with a label. I watch the numbers on that label and de-brained myself doing maths. I took 12" as the propeller diameter; P/D (with a subindex?) is 1.98; for this P/D, Pitch gives me 23.76". The radius (R) of the propeller is 12"/2=6". Assuming the root of the blade could be at 20% of the propeller shaft, that is a radius (r) of 20%6"=1.2" from the center of the propeller.
Using the formula: Blade Angle= arctan (P/D/(pi(r/R)):

Blade angle at 20% from the center of the propeller= 72,39
Blade angle at 75% from the center of the propeller= 40,04
Blade angle at 100% from the center of the propeller (tip of the blade)= 32,22

Is this correct?

Second question: What is 3.75º? Washout?

Thank you for beeing so didactic and graphic!

Respectfully.
Julio
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« Reply #18 on: August 22, 2008, 03:18:03 PM »

Mathew
With regard to the curved sides of the propeller former. Can you use ‘Excel’ spreadsheets? If you can I can send a simple program that does all the necessary calculation; if not I will try and put together some equations that will do the job.

Julio
Your calculations are fundamentally correct but we have some slight differences because of ‘Pitch’. The term pitch is beloved of aeromodellers but is rarely seen in any book of propeller design because it is so imprecise. Fundamental in propeller design are the forward speed of the aeroplane and the rotational speed of the propeller and the vector sum of these forms a triangle where the hypotenuse is the airflow striking the propeller. If we think of the forward and rotational speeds as being constant (actually they are not because of inflow factors) we can visualize a helical path of the propeller through the air with a constant pitch. However for a working propeller the blades must be set at an angle of attack to the airflow so the pitch must vary all along the blade. There is actually no such thing as a ‘true helical pitch’ propeller!  So, after that wordy explanation, the figures on the label indicate the following:

The theta with the subscript 0.75 is the blade angle at three quarter radius. I made this 40 degrees which is the sort of figure found to be good on Penny Planes. This gives a P/D at three quarter radius of 1.98 but the P/D will vary all along the blade because I have used an angle of attack (alpha) of 3.75 degrees all along the blade.

This is a big subject which can’t really be dealt with in the middle of a build but I hope that is of some help.

John
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« Reply #19 on: August 22, 2008, 05:36:45 PM »

John

Thanks for your answers. Always very kind.

Julio
Your calculations are fundamentally correct but...

That gave me a warm feeling... but I can see I need some more homework. Trying to get closer to a basic level of workable and useful understanding...
On regards to your answer to Mathew, may I be included in the "enlightened" list if possible?

Again, thanks a lot for your help.

Julio
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« Reply #20 on: August 23, 2008, 01:57:27 AM »

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Can you use ‘Excel’ spreadsheets?


I am very limited..

regards
Matthew
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« Reply #21 on: August 23, 2008, 02:49:19 AM »

Mr B

I'd like to join the others in applauding your efforts on this subject. I've only ever built an Ikara Junior from a kit for sport indoor flying (although I did win a bottle of wine or two for longest flights at the Old Ford Set Xmas bashes - literally!), and your construction information is invaluable.

Many thanks

Peter
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« Reply #22 on: August 28, 2008, 10:54:09 AM »

I finished the build a few days ago but haven’t had time to bring the thread up-to-date.

Here is how I handled the propeller blades. After soaking in water I put them on the forming block, protected them with a piece of foam ceiling tile (the first time I had tried foam), and strapped everything firmly down with open weave bandage. I gave them about half an hour in the electric oven at 80 degrees Celsius.

Now I have to tell you I fouled up here and these instructions are not to be followed. The day I was making the propeller was the one day in the month when we had the use of the church hall for some testing and I wanted to be there with the PP. When I opened the bandage the blades were still wet. I resorted to drying them with a hairdryer and working them to shape over the former. The prop is more or less OK but it is not the way to do things. It is a long time since I molded any blades and I cannot remember the times and temperatures I used but I know I always left them on the former overnight. The foam was possibly a mistake because it seems poor at letting the moisture out.

I would really welcome some comments from others on moulding procedures.

The next picture shews the former being used to assemble the blades to the spar, which went much better! The blade is held to the block with ‘tabs’ of masking tape. The spar, with the shaft assembled to it, is held on the ‘setting’ triangle with another piece of masking tape over the propeller shaft. The setting triangle is raised with packing pieces so that the spar lies, without distortion, along the back face of the blade.

John
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Hepcat
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« Reply #23 on: August 28, 2008, 03:32:39 PM »

Some pictures of the finished Penny Plane. PPs are not the prettiest aeroplanes around and they should probably have called the class Penny Plain but at least they do appear to punch above their (not inconsiderable) weight!

Talking of weight; mine finished at: Tail Unit 0.31g, Wing 0.71g, Stick and Boom 1.35g, Propeller 0.70g giving a total of 3.07g. That leaves me 0.02g short which is probably best used up with diagonal braces from the wing spars to the wing posts.

Now, for once in my life, I am determined to put some proper fixings in the model box so that the fragile filly does not suffer travelling to and from the flying site.

I think that’s all for now.

John
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Sundance12
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« Reply #24 on: August 28, 2008, 03:52:37 PM »

Hi Hepcat

I have been watching your build with some interest for the duration which has resulted in a great build presentation. Even though it is a Penny Plain as you called it I think that it is a fine example. My experience with this type of airplane has been less than spectacular but it looks like you got a winner there. I hope that you continue to receive responses from this build and we get some other fine examples going in this type of model.

Thanks again for a great build,
Sundance12
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