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$100 Scratch Been There Done That DH106 Comet Build

Old 11-25-2014, 06:08 AM
  #1  
Bill G
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Default $100 Scratch Been There Done That DH106 Comet Build

This will be a second build of the Dehavilland DH106 Comet at 33" park hand toss size for a single EDF55 fan, since I've never gotten around to flying my larger one. A Don's Wicked EDF55 setup will either be purchased, or likely pirated from my smaller Saro A1. It may provide the incentive to give that thing a toss and try flying it first, since the bow is to shallow to take off maybe anything other than glass.

The scale will be cheated a bit for added wing chord, and to make the necessary flow area requirements for the fan. So far I have the basic center wing section fabricated, with 100% FSA intake area and a bit over 90% exit area, flowing through the rear wing section, which was built from a thin basswood frame to maximize flow area. Off top of head it should be feasible, with an AUW of roughly 22oz and thrust of 11oz, considering a good bit of loss. There will obviously be no bells and whistles such as lighting and retracts like my other one, to minimize weight.
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Old 11-25-2014, 06:38 AM
  #2  
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I'll add you to the list Bill. It'll be great to see a Comet fly. The competition is getting stiff! Good luck!
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Old 11-25-2014, 08:50 AM
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As usual Bill never takes the easy route.This will be intresting, I will be paying close attention to the ducting. Will it be vacu-formed? No, wait...I will just watch and learn....

The awesomeness that has already been done, just to give you an idea what Bill is doing...
http://www.wattflyer.com/forums/showthread.php?t=64321

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Old 11-27-2014, 12:47 AM
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Bill G
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Thanks Xmech and Bronco

Originally Posted by BroncoSquid View Post
As usual Bill never takes the easy route.This will be intresting, I will be paying close attention to the ducting. Will it be vacu-formed? No, wait...I will just watch and learn....

The awesomeness that has already been done, just to give you an idea what Bill is doing...
http://www.wattflyer.com/forums/showthread.php?t=64321
There won't be any molded parts, but there will be a good bit of formed plastic sheet for the exhaust ducting. The ducting is one of the things I want to clean up a bit, over the last one. The exits tubes on this one will be faired into the wing with V-cuts, with the idea to move the opening area as far back as possible. On the last one, the exit ducting had the required cross section area, but the airflow had to snake around a bit to exit the exhaust tubes. I want to straighten this path as much as possible this time. Also installed are airflow diverters behind the intake openings, to direct the airflow smoothly into the openings in the wing formers. I just finished making these from plastic sheet part, then lightening them from 3.2 gms to 2.1 grams, by cutting much of them away and replacing with light cardstock laminate. At this size, I can lighten things a bit over the last one.

Pic1: The nacelles are formed with sheet balsa, but instead of installing a ton of framing to make the shape as on the last one, this time only 2 formers were used per side to frame the high point of the nacelles. After sheeting, the sheet was then backed with stringers and a few balsa blocks, pushed outward while gluing in place to help form a smoother curved shape. Much simpler and lighter than having to frame everything perfectly, prior to sheeting. The nacelles will probably use some blue foam also, for the bottom nacelles and rear portion of the top nacelles, blending into the exhaust tubes.

Pic2: The grain of the forward part of the bottom sheet was ran to form the curve between the nacelles. There is a thin basswood divider used to join the next portion of sheeting, where the grain will be ran spanwise for strength. The laminated balsa on the top of the "eyeglasses" front wing former adds a lot of strength to it. For the bottom nacelles, I will likely use blue foam since there is a much lighter curve in the front area than on the top wing surface. That will not add as much strength to the former as all the laminated/sculpted balsa on the top of the nacelles does. For that reason, only a short length of the sheeting runs with the grain chord wise. This is just enough length to form the feature where the dips between the nacelles blend upward into the LE, and most of the bottom sheeting will run span wise directly behind the front portion of the sheeting.

Pic3: Airflow diverters added behind the intakes which will direct the airflow into the openings in the wing formers. The hard balsa center framer will be used for the wing hold down points.
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Old 11-27-2014, 02:16 PM
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quorneng
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Bill
An bold interesting project!
I do wonder if you are not making the ducting problem ever harder going down in size - for a given air velocity the boundary layer against the duct wall remains the same thickness regardless of duct size. The smaller duct proportionally bigger are the losses.

The main advantage of going smaller is it improves the strength to weight ratio of the airframe. The problem here usually is making things light enough for the loads they actually have to bear. "If it doesn't break its too heavy!"

I shall watch this build with interest.
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Old 11-30-2014, 08:21 AM
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Bill G
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Originally Posted by quorneng View Post
Bill
An bold interesting project!
I do wonder if you are not making the ducting problem ever harder going down in size - for a given air velocity the boundary layer against the duct wall remains the same thickness regardless of duct size. The smaller duct proportionally bigger are the losses.

The main advantage of going smaller is it improves the strength to weight ratio of the airframe. The problem here usually is making things light enough for the loads they actually have to bear. "If it doesn't break its too heavy!"

I shall watch this build with interest.
Thanks
The main reason for the smaller size is so that I may actually fly the darn thing this time around. The larger one would require a field half the size of Heathrow, with a paved runway. The frictional losses are proportionately greater with the smaller ducting. I am constantly arguing with myself on that exact thought, that if it doesn't break it's too heavy. By nature I tend to prefer reasonably robust builds. Items such as single CF rod ran through the wing panels greatly enhances strength, but it's dead weight that a project like this cannot afford. I'll have to omit those types of reinforcements.

Using near contest grade wood will considerably help. The two main factors that have reduced weight substantially in my more recent builds is the use of light 1/32" sheet, and Coverite Microlite covering. The 12oz AUW was excellent for the P26 I just finished, versus the common 16-20oz that we see with standard weight iron-on coverings and 1/16" sheet for a similar model. I still think it's the best kept "secret" that hasn't caught on with many, for whatever reasons. This model will also not have long flight times, granted it flies. Recent high discharge lipos can provide 30A and greater from cells as small as 800mAh. Not long ago I would have needed somewhere in the 1500mAh range or greater, to provide the anticipated power level required for this fan size. I'm going to attempt to use this battery that I have on order http://www.hobbyking.com/hobbyking/s...dProduct=59093 as well as a 56mm Hyperflow/3790kv Ammo EDF I bought from Don of Don's RC.

Pic1: Diverter walls added in the exhaust ducting, eliminating the "dead" corner zones.
Pic2: Old round cell used for the cardstock exhaust pipe forming mandrel.
Pic3: Yellow foam (similar to blu cor) and filler used to form the rear portion of top wing nacelles. The bottom wing surface nacelles will be a bit easier to fabricate and shape. The outer wing panel main spars and wing formers have been cut, with a bit of a cheat on the outer wing chord for added area.
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Old 12-01-2014, 04:24 PM
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Gotta love grainy, enlarged drawings used for plans, where all the lines and curves need to be sketched in. The parts cutting method is one I've used for several years, which works well and is fast. The wood is placed under the plan, and the part outlines are scribed with a dull exacto knife. This places a visible cutting guide line on the part, while not cutting through the plan, or requiring separate part templates.

Pic1: Fuse keels and formers cut using contest grade wood, with the exception of the top and bottom keels. Still have keel and stringer notching on the formers to be completed. Bottom wing pan nacelles sculpted.

Pic2: Top wing pan nacelles sculpted. Looks like the fan should be here today, so I'll have something to experiment with.
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Old 12-02-2014, 08:25 AM
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Found a molded clear plastic part in my stash that was perfect for the exhaust cover. The 56mm Hyperflow fan was test run before installing the cover, but I'll have to test it again for thrust figures with the cover installed. Given the weight of everything now, the remainder of the aircraft will need to be kept light, using light grade wood. The body of the aircraft with stringers and wing saddle formers installed will just clear the fan assembly.

Pic1: Exhaust ducting cover in place. Corners at the cover to wing seam were rounded for better flow. The front fan mounts are 1/16" ply, with the rear mounts made using 1/8" hard balsa.

Pic2: Ramps installed using 1/32" sheet for smoother airflow into the fan. The 40A ESC is a bit heavy, but I'm not a fan of "just big enough" ESCs with EDF. 1/16" balsa stringers were added to the top of the front wing panel at the center opening, to allow for rounding underneath for smoother airflow.
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Old 12-04-2014, 04:15 AM
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The fuse is now framed up and ready to sheet with light 1/32" sheeting. The antenna looking structures made from stringers in the fuse ends are used to set and hold the fuse stringer curves. Without this framing the curves would not only be a bit flat, but tend to flatten more when applying the sheeting.

A small mod was made to the rear of the duct cover, which was a bit of an oversight given that I already build one of these. Without tapering the duct at the rear, the fuse would require a large faring behind the point where the wing meets, which would be needed to flare in the wing saddle. The saddle would be far too wide at the rear to properly blend into the fuse at the wing TE.
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Old 12-06-2014, 01:27 PM
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Pic 1: Stab made from light grade 3/32", with the exception of a hard balsa main spar which was an odd size between 3/32" and 1/8". The stringers were custom cut to accommodate the varying thicknesses. The dihedral was formed by making a relief cut in the center, and then gluing firm while holding a dihedral gauge in place.

Pic2: A small amount of added area makes a massive difference in the ducting. I've noticed a big difference with every mm gained from porting things out a bit. Since this photo was taken, the ducting was hogged out a bit further. There was an oversight in that the wing panel tapers from the center former to the next former, which reduced the ducting area. That issue has been corrected by removing a bit of wing sheeting from both the top and bottom wing panel and patching it over to regain the inner ducting area, then blending into the wing surfaces with filler.

Pic3: Stab fitted in place, which actually went better than expected.

Pic4: Wing panels completed with partial sheeting on the top and bottom. Started sheeting the fuse, where the compound curved areas will be done first, and then the straight tube section.

Pic5: That was a fun time. Forgot to open up the center wing formers for the aileron cable harness, before installing the fan. I had to get in there with a sharp, round tip file and poke holes to open up rectangular slots that the connectors would fit through, while holding the center wing section up to a spotlight so I could see inside the wing. Fortunately the contest grade wood made this much easier. Fishing the harness through was also a bit of fun.
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Old 12-06-2014, 02:57 PM
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Have you run up the fan to get an idea of the thrust?
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Old 12-07-2014, 04:14 AM
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Originally Posted by pmullen503 View Post
Have you run up the fan to get an idea of the thrust?
I need to get it on the scale, but it's now lifting the 9oz center pan assembly out of my hand with some to spare, so probably about 10oz. At this point it's all looks doable. At one point I was ready to resort to an ugly cheater exhaust. A bit of exhaust rework resulted in massive changes. What I did was take the cleanest flow path and cut away the sheeting in that area on bot the top and bottom wing panel, increasing the inside height by 2mm. 1/32" sheeting patches were added to the outer surface of the wing panels, to gain the inside area. It really amounts to more like 4mm, since I cut away the thin basswood strips that formed the rear airfoil section and then laminated them to the outer surface of the wing panels, so that they would still be continuous spars as the structure is really needed and they can't be eliminated. The benefit now is that the inner sheeting surface is now flush with the spars, since the sheeting patches that were added to the top and bottom wing surface were butted against the thin basswood spars. Before the mods, the spars were blended in with filler on the inner sheeting surface, but still restricted area and created friction as ramps.


With the slightly raised areas tapered in and blended with sanded filler, they are barely noticeable and the thrust gain was ounces. The inside of the inner exhaust tubes was also trimmed to match the new, increased area. The first picture below shows the area gained by opening up the most direct flow path. The inner duct on the right used to look like the outer duct on the left. Done again I may have just made a wing center section that was more balloned like a fully symmetrical section, but that would have it's drawbacks also in that it would have thickened the entire center section and changed the scale appearance. With the localized mods, only the area needed for better flow was opened up a bit. The second pic was taken after the mods, showing how the areas reworked after blending in the raised patches are basically unnoticeable.

The 20oz AUW still looks doable, in that all the weight of this plane is in the gear/batt. The wood used is really good grade that I collected for a while. Without the heavy EDF gear, this thing could probably have two 10gm outrunners installed with one in each of the outer exhausts using 4.5" props and have more than ample power.
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Old 12-07-2014, 02:27 PM
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Lots of fun fitting the wing saddle, which was a good task to get out of the way, as well as sheeting the fuse with all the relief cuts needed at the tail. Put everything on the scale including the receiver and battery at 16.5oz. With Microlite covering and efficient painting a 20oz or better AUW should be doable. The rear wing hold down fixture is already fitted, and the front wing will use a pin locator. About the only framing weight to be added is the battery door, and it won't add much weight considering the heavy, hard balsa top keel section will be removed before adding the door.
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Old 12-07-2014, 06:19 PM
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Bill
That fuselage looks very nice indeed.
And I agree when building small and very light it certainly does show up just how inefficient EDFs are - and smaller ones particularly so!
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Old 12-08-2014, 12:58 PM
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Originally Posted by quorneng View Post
Bill
That fuselage looks very nice indeed.
And I agree when building small and very light it certainly does show up just how inefficient EDFs are - and smaller ones particularly so!
You bet! I'm convinced this model would be sub 14oz and easily fly with 2 10gm outrunners and 5" props. I have a similar sized Guillows DC3 that easily flies with small outrunners of around 18gms and weighs almost a whopping 20oz.

Going over the specs I'm thinking this model has a decent chance, especially considering how marginal small models are. The model should have a wing loading of around 15oz/sq-ft, which seems to be the point where required power starts to hit the point on the curve where it sharply increases. Along the same lines, over 15oz/sq-ft is where ease of flying and hand launchability sharply decreases. Small models are definitely non-linear beasts in those respects. The large center pan should be a big help, with nearly 1 sq-ft in area. Overall the wing area is 1.32 sq-ft. I'm hoping this will behave like the GWS F15 I had, which was by no means light but was a real floater with the large center body/wing configuration.

Pic1: Wing blended in front to meet the fuse.

Pic2: Wing hold downs in place, with a hard wood dowel in the front and #4-40 blind nut in the rear. A small portion of the hard balsa center keel helps capture the rear hold down plate, as well as being gusseted. The inner intakes each have a hole on the inside that allows air to flow into the cuts in the front wing saddle fuse former. When I did the area calcs the wing center openings were a bit over 100% FSA, but a bit extra flow area doesn't hurt, especially flowing more directly into the fan.

Pic3: Battery door fitted with Dubro flat pin hinge in front, and slide latch in rear. The hinge fits into a slot that is cut into a 45 degree angle in the top fuse keel, for solid mounting.

Pic4: The battery door slide latch has a closing spring, to ensure the door stays shut.

Pic5: The outer wing panels are attached with effort put into ensuring that the incidences are matched. Cap strips were added to the former tops at the sheeting edge and then tapered, creating a smooth blend from the sheeting to the formers and avoiding an ugly step in the covering. Plastic covers were added to the fronts of the farings that blend the wing into the fuse. They will be painted leaving a small clear circle in the center, simulating the inner wing lights. The next job is fair in the leading edges of the outer wing panels to the nacelles, also using clear plastic to simulate the landing light covers. It would be nice to have real lights like my last Comet, but they weigh.
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Old 12-08-2014, 05:48 PM
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quorneng
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Bill
I would agree 15oz/sqft is quite acceptable for hand launching. I have had no problem with my Q400 at that but of course is also has available a thrust loading better than 1:1 if required.
Although not launched on full power in the first video you can actually hear me give it a quick squirt as it did not look like it was going to climb away safely.

No such luxury with a 50% thrust ratio.

Do you know where you are going to hold it? It does not look like there are any natural hand holds around the CofG on that smooth underside!
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Old 12-08-2014, 11:22 PM
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Bill G
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Originally Posted by quorneng View Post
Bill
I would agree 15oz/sqft is quite acceptable for hand launching. I have had no problem with my Q400 at that but of course is also has available a thrust loading better than 1:1 if required.
Although not launched on full power in the first video you can actually hear me give it a quick squirt as it did not look like it was going to climb away safely.

No such luxury with a 50% thrust ratio.

Do you know where you are going to hold it? It does not look like there are any natural hand holds around the CofG on that smooth underside!
I've tossed models like this before, although it is a bit awkward to grip the model around the fuse behind the cg and maintain grip. You really have to watch for the proverbial nose dive while doing that though. My Ansaldo bipe was really tricky to launch in that respect. There's always the ungainly center wing gripping keel option like my Alfa FW190 has. This wing has a 1/8" hard balsa center keel, so there's some reasonable structure to attach it to.

Speaking of cg, I've been examining a few similar models and debating over where I'll initially place it. My findings were always that the sweep does not move the cg as rearward as the geometry would entail, presumably due to the inefficiency of the outer wing panels due to diminishing chord length. I thought about slamming together a cardboard chuck glider, but those seem to produce aft results.
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Old 12-09-2014, 01:01 AM
  #18  
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Catapult? Especially with a low thrust, probably a little nose heavy on the maiden airplane. You're up to flying speed almost immediately.

Otherwise, a skillful assistant to handle the toss so you can concentrate on the sticks.
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Old 12-09-2014, 02:13 AM
  #19  
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I wish I had a skilful assistant!
Fortunately I am left handed but fly Mode 2 so I atleast have aileron and elevator straight away.
A chuck glider certainly helped me confirm the CofG on my Depron Concorde.
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Old 12-09-2014, 07:04 AM
  #20  
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Yep, being a lefty really helps with hand launching. I've taken video and flown at the same time. Even more favorable is that I use scissors with my right hand, and feel more comfortable with my right thumb than left thumb for working the sticks. Works out really well.

Below is about what I'm thinking for CG setting. Based on past swept wing models, they've been more forward than some would think. The blue line shows about where I have in mind. The battery bay allows for setting a reasonable distance on either side of the line. The thrust may be better than what I've seen so far. I've been testing with an old 4-cell that I believe has seen better days, noticing that it drops off quickly and may likely never be reaching the level that a new lipo would produce. I'll have to charge the new battery, or one of the two other decent 4-cell lipos I have on hand and test the setup. Best would be to use the new one that I plan to fly with. After the exhaust cleanup/rework the performance improved notably. Anything over 10oz and possibly ending up with an AUW somewhat under 20oz would really help.

What I found with chuck gliders is that you want to move the CG a bit forward of what they show. My theory is that super light gliders will both tolerate flying more aft, and may actually want to fly aft since the flat plate wants the positive incidence created by a slightly tail heavy condition to produce lift.

Covered the tail feathers and am moving on to other little tedious parts such as ailerons and then the nacelles. Gotta love covering. The little oddball stringer in the photo below worked really well for correcting a slight washin dip in the right wing panel. It was wedged in, and then when the wing was flexed, it would move closer to the plate structure that surrounds the aileron servo. I thought, aha, if I flex it like that and then glue it in place, the wing will hold the new shape, which worked well.
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Old 12-11-2014, 12:14 AM
  #21  
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My test battery was shot. I finally tested using the new Turnigy lipo purchased for this project and it was night and day difference. No worries about this thing flying now. Probably a good thing that I used a weak lipo for testing, as I may have not bothered to rework the exhaust ducting, which was quite an improvement also over the initial result.

To window or not to window. Well windows weigh, but I'm not a fan of painted on glass, so thin plastic plastic windows shouldn't be much of a penalty. There's a big temptation to just glue a strip of plastic over the window line, but individual round windows will be a weight saver. Covering the nacelles actually went well, although tedious. Makes the remainder of the job look like something that could be done blindfolded. Also added were light 1/16" balsa spars which join the main spar stubs to the rear main spar, at the next former outward. Too much of a strength and rigidity gain to pass up, for the small amount of added weight.
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Old 12-11-2014, 12:55 PM
  #22  
quorneng
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Well done on doing all those windows although the earlier Comets did not have so many.
I chickened out on my Q400 and took the minimum weight option.
I expect I will do the same for the Canberra.
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Old 12-12-2014, 06:14 AM
  #23  
Bill G
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Originally Posted by quorneng View Post
Well done on doing all those windows although the earlier Comets did not have so many.
I chickened out on my Q400 and took the minimum weight option.
I expect I will do the same for the Canberra.
This is a Comet 3. I initially was planning to build an early Comet model, but the inlets would not look correct, given the early Comet's distinctive small, oval inlets. The Comet 4 versions have even more windows! Being a Comet 3, it makes the chosen Dan Air livery incorrect, although I doubt many will notice. Speaking of windows, it was quite a job installing 30 windows, and then cutting small discs for masking.

Almost finished with the covering job now. To keep painting as light as possible, the scheme chosen will have a silver wing, with a white fuse and tail feathers. The last build had silver on the lower fuse half which looks nice, but weighs. A light coat of gloss white paint will be applied to hide the balsa grain that shows through the covering. It still ends up lighter than a heavier covering like Parklite or Ekonokote. The v-stab was first striped with red Microlite covering, with the red then masked over and painted white, ending up with a weight of 2gms. With the white on white painting, the masking can run a bit over the red striping without being noticed after painting which made masking easy. Masking for red painting would have wanted to pull some white paint, and the red edge would not have been as clean, so the method had it's benefits. This is the general scheme chosen below, although it may not have all the markings:
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Old 12-12-2014, 12:22 PM
  #24  
quorneng
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I wouldn't worry as I doubt many people would be able to recognise the difference between a Comet 3 and 4 as the one flying example was only shown to the public at Farnborough in 1954 (BOAC colours) and again with shorter wings in 1958 (BEA colours).
It did however have a surprisingly long life being retired in 1973 after blind landing system development.
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Old 12-13-2014, 06:57 AM
  #25  
Bill G
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Interesting info on the Comet Quarneng. It is really something that Dan Air used these through the 70s.

Getting close with the paint work finished. The wing was painted with Top Flite silver I've had for a while, but probably the last time I'll use it on covering. It adheres to covering well, but the metallics are difficult to apply without getting what is known as "modeling" in the grain. I've been using the Rustoleum bombs that Wal Mart and Home depot sell, which seems to be really good paint for applying over covering. I weighed everything in two batches afterward, and if my math isn't wrong the AUW is under 18oz which is as good as I could ask for.

I guess I'll cut out the Dan Air London graphics, for the second time in life! The little tail emblems were first printed, then enhanced with red marker and black pen to clean up the appearance. Next they were covered with a layer of clear tape, cut out, and then glued to the v-stab using GWS contact cement. The stuff works really well in that you can rub off what little glue smears out and it will basically ball up like wet snowballs as it collects itself, and then simply can be brushed away with your fingers.
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