Logo
Builders' Plan Gallery  |  Hip Pocket Web Site  |  Contact Forum Admin  |  Contact Global Moderator
September 17, 2019, 04:05:49 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with email, password and session length
 
Home Help Search Login Register
Pages: [1] 2 3 4   Go Down
Print
Author Topic: Comet 24" span Hellcat - CAD redraw and build  (Read 4937 times)
0 Members and 1 Guest are viewing this topic.
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« on: September 15, 2016, 12:20:35 PM »

I have recently completed redrawing the Comet 24" span Hellcat. There are some very good Hellcat plans available so why mess with the old Comet version? That particular kit goes way back in my history. Very early in my modeling life, started around 7 or 8 years old, I came across the Comet Hellcat. At the time it was too expensive and seemed far too complicated for me. Later in life I did buy a kit but for some reason never got around to building it. All these years later I happened to pull that old kit out of my storage area and said I think it is time to finally scratch the itch that occurred so long ago. As I examined the plan and parts I could see things that did not look quite right or that accumulated experience said might need adjusting. Hence my redraw of the plan and parts.

My changes to the original plan and parts were not extensive. Mainly adjusting the fuselage formers to better reflect the scale cross sections, to allow for stretch winding, make the landing gear removable for flying, and other minor structural adjustments for strength and lightness.

I have been able to start the build from my redrawn plan and parts. The fuselage is well underway and so far the parts are fitting perfectly. When I have the model finished I will place the plan and parts drawings on my website for free download.

Here are a few images of the build so far along with a 3D sketch of the structure showing some of my modifications.

Paul Bradley
Attached files Thumbnail(s):
Comet 24" span Hellcat - CAD redraw and build
Comet 24" span Hellcat - CAD redraw and build
Comet 24" span Hellcat - CAD redraw and build
Comet 24" span Hellcat - CAD redraw and build
Logged
strat-o
Gold Member
*****

Kudos: 7
Offline Offline

United States United States

Posts: 378



Ignore
« Reply #1 on: September 15, 2016, 01:56:42 PM »

This is great, Paul.  I remember building and completing this kit when I was 13.  I like how you have been able to re-engineer some of these old Comet designs.

Marlin
Logged
Sky9pilot
Titanium Member
*******

Kudos: 50
Offline Offline

United States United States

Posts: 2,733


WWW

Ignore
« Reply #2 on: September 15, 2016, 01:57:34 PM »

Fantastic Paul...thanks for sharing this and the progress looks great so far!
Sky9pilot
Logged

Best Regards,
Sky9pilot
    "If God is your Co-pilot, consider switching seats!" 
http://www.stickandtissue.com/cgi-bin/yabb2/YaBB.pl
http://freeflightarchive.com/index.php/homepage-2
skyraider
Platinum Member
******

Kudos: 93
Offline Offline

United States United States

Posts: 1,282

WWW

Ignore
« Reply #3 on: September 15, 2016, 02:24:03 PM »

Way to go Paul!  Now I don't have to do it.  I've reworked a few of the Comet's
myself,  Dauntless, F4u Corsair & Avenger and a few more. Likewise I did the
same as your doing, correcting the formers to assume the actual fuselage shape.
Granted, its no easy task but these old comet's need some new life. Glad to see
I'm not the only one doing it.

Skyraider
Logged
Konrad
Titanium Member
*******

Kudos: 38
Offline Offline

United States United States

Posts: 2,162


Measure twice cut once



Ignore
« Reply #4 on: September 15, 2016, 04:42:06 PM »

Love the Hellcat!

I have one still under repair. I think I have a thread over at Stick & Tissue site.

What I didn't like about the Comet kit was that it used 3/32 stringers. I've found that cutting the Comet kit for 1/16 stringers allowed a better fuselage shape, greater strength and less weight.

Will  your download be in the form of a laser cut file?

All the best,
Konrad
Logged

Cut it twice and it's still too short!
Modelace
Gold Member
*****

Kudos: 37
Offline Offline

United States United States

Posts: 765



Ignore
« Reply #5 on: September 15, 2016, 05:43:15 PM »

Konrad: Please explain how replacing 3/32" stringers with 1/16" will result in "Greater strength" Huh
Logged
Konrad
Titanium Member
*******

Kudos: 38
Offline Offline

United States United States

Posts: 2,162


Measure twice cut once



Ignore
« Reply #6 on: September 15, 2016, 07:32:04 PM »

Geometry.

The more of the material that is on the outside of the shape the stronger the shape. More of the load bearing material further apart means a stiffer structure. Also in this case there is more surface area in contact between the bulkheads and stringers (more glueing area).

I think you might be under the impression that the number of stringer was held constant, this is not so. With the 1/16 stringers you would double the number of stringer and still have less wood in the model, than if you used half as many 3/32 stingers (such as originally used by Comet). The cross section of wood in a 3/32 stringer is larger than the cross section of two 1/16 stringers.

For example 0.093752 =.00879 in2, and 0.06252 x 2 =0.0078 in2. So two 1/16 stringer will have 89% as much wood a single 3/32 stringer. This is where the weight savings comes from.
Logged

Cut it twice and it's still too short!
tom arnold
Gold Member
*****

Kudos: 29
Offline Offline

United States United States

Posts: 686


Casper Wyoming



Ignore
« Reply #7 on: September 15, 2016, 08:22:40 PM »

The Comet Hellcat is the worst airplane in the world. It is a terrible, terrible model. You drool over it, lovingly build it, trim it out, fly it and giggle for joy to see the sun shining through the tissue, and then one day it flys away. It goes up, up, up into the delirious blue and disappears forever. It never writes, never calls, never comes back and smashes your heart. It is a terrible, terrible, terrible model and they all do that.
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #8 on: September 15, 2016, 09:54:13 PM »

Thanks for the support guys. Tom, you sure are correct about our models being heart breakers when they depart the field never to be seen again. The Comet Hellcat looks to be especially prone to breaking hearts.

Konrad - your point is well taken about using 1/16" square stringers doubling their number. I have found that the weight savings can be offset by careful selection of the wood used to strip the 3/32" square stringers. Typically I have to use a higher density wood when making 1/16" square stringers so they can be handled during and after assembly. I thought about doing what you have suggested but decided that the added work of doubling the number of stringer notches was in the end not worth the effort. For the curved areas increasing the number of stringers does indeed provide a better representation of the full scale airplane, but for this model I did not think that would be necessary. My goal is to fly it in FAC WWII combat so a better scale representation of the full scale aircraft is not as important.

I did get a little bit more work done on the fuselage today. I made a minor modification to the wing mount plates. I intend to extend the leading and trailing edges from the root rib so they will pass through the wing mount plate. This will hopefully make it easier to have both wing panels at the same incidence angle. The kit version has only the spar extend through the wing mount plate and is glued to a fuselage former. That can lead to some misalignment of the wing panels. It does not take much to have a real trimming issue. To allow for my extended LE and TE I cut corresponding holes in the wing mount plate. Oh yea, I also added a spar box to the former that supports the spar to provide better strength between the spar and fuselage joint.

I also added balsa plates where the motor peg will go. The kit shows the old method of anchoring the rear of the motor with a strip of wood behind a former. No way to easily replace the rubber motor or stretch wind the motor. In addition to the motor peg supports, I also added balsa filler blocks below the fin. The kit depends on the top stringer for this and does not really provide for a clean way to support the tissue above the stab. The filler blocks on each side of the fuselage below the fin add very little weight and really clean up the area for the tissue. Along with the fin filler blocks I also added a stab support plate. This makes it much easier to secure the stab and also provides a better way (in my opinion) for supporting the tissue below the stab.

Konrad - I forgot to answer your question regarding a laser cutting layout being available when the plan is posted to my web site. I do have all the parts laid out for laser cutting. I did not use laser cutting for this model. I have the parts also laid out for iron-on T-shirt transfer paper. I wanted to verify that layout is correct so I used iron-on transfer paper to transfer the parts to balsa for this build and hand cut the parts. The laser cut file is available and includes the necessary offsets to compensate for the laser kerf. I will make it available upon request. I would post it to the web site but I have found there are a few individuals who might use it for commercial purposes without permission. I would certainly be happy to share the laser cut file with a fellow modeler.

I have included a couple of pictures showing these simple modifications to the original kit structure. Next will be shaping the nose block and getting it installed. I can then get to work on the flying surfaces.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
« Last Edit: September 15, 2016, 10:05:27 PM by PaulBrad » Logged
Konrad
Titanium Member
*******

Kudos: 38
Offline Offline

United States United States

Posts: 2,162


Measure twice cut once



Ignore
« Reply #9 on: September 15, 2016, 11:01:36 PM »

Paul,
Thank you for the offer! As I have so much on my plate, I'll have to wait on making my own (from your files) Laser cut kit.

I agree with the idea of internal LE &TE spreaders. Without these any cartwheeling will crush the fuselage. I added these to my Hellcat and am very thankful for the life they added to the model.

Thanks,
Konrad

P.S.
On my Hellcat I found that using a shorter than stock motor actually increased flight time as I need a lot less nose weight. Motor weight was the same but I added some pitch to the prop, to extend the run time.
Logged

Cut it twice and it's still too short!
Glenn (gravitywell) Reach
Gold Member
*****

Kudos: 34
Offline Offline

Canada Canada

Posts: 795


It ain't pretty, but its me!




Ignore
« Reply #10 on: September 15, 2016, 11:26:50 PM »

Old Comet plans......"Betterfied!"  Grin
Logged

Glenn Reach
The only time you want Guillow's balsa, is when its worth its weight in gold!
Mooney
Gold Member
*****

Kudos: 30
Offline Offline

United States United States

Posts: 697


Whenever possible, fly with the sun at your back



Ignore
« Reply #11 on: September 16, 2016, 07:39:58 PM »

Funny stuff, Tom.
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #12 on: September 16, 2016, 10:11:36 PM »

A bit more progress today. I was able to get the flying surfaces framed up and sanded. The wing panels need to be cut apart so the dihedral can be added. I was also able to get the nose block installed and sanded. I am using full balsa blocks for the wing tips rather than the 1/16" balsa plate called out in the kit. The balsa block tips look more realistic and are much easier to cover. Two 1/16" top sub spars were also added to he wing to improve torsional strength and to help reduce covering sag. The stab and fin use 3/32" square stock and have been sanded to a symmetrical airfoil cross section.

For the nose block I realized I did not quite get the oil cooler inlets shaped correctly. I elected to use what I had at this point. The the parts drawing has been corrected.

Next will be adding the wing dihedral and then the prop assembly. Hopefully I can get the Hellcat covered next week.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Logged
p40qmilj
Platinum Member
******

Kudos: 29
Offline Offline

Canada Canada

Posts: 1,853


love that P40Q



Ignore
« Reply #13 on: September 17, 2016, 03:34:01 AM »

 Grin pul did u put decalage in the tail?

jim Grin
Logged
backyardbalsapilot
Bronze Member
***

Kudos: 1
Offline Offline

United States United States

Posts: 32



Ignore
« Reply #14 on: September 17, 2016, 08:43:17 AM »

Looks good. What are you thinking about for coloring?

I think I'll have to build this one after my comet p-40...
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #15 on: September 17, 2016, 12:12:54 PM »

Jim - The model has a fair amount of positive incidence in the wing. For that reason the stab is per the kit at zero incidence. The wing incidence is per the kit plan.

Backyard - I am currently planning to use the tri-color scheme that was used by the Navy during the war. I will be using inkjet printed tissue for the color and markings. I have attached a reduced size copy of the color three view I am planning to use.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Logged
voc
Bronze Member
***

Kudos: 0
Offline Offline

Netherlands Netherlands

Posts: 20



Ignore
« Reply #16 on: September 17, 2016, 03:48:03 PM »

The Comet Hellcat is the worst airplane in the world. It is a terrible, terrible model. You drool over it, lovingly build it, trim it out, fly it and giggle for joy to see the sun shining through the tissue, and then one day it flys away. It goes up, up, up into the delirious blue and disappears forever. It never writes, never calls, never comes back and smashes your heart. It is a terrible, terrible, terrible model and they all do that.

I have to admit this one made me smile.
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #17 on: September 17, 2016, 10:27:30 PM »

My shop time was more limited today. I did manage to get the dihedral added to each wing panel.

I plan on working on the prop assembly next. For this model I am using a 3D printed 7" diameter three blade prop. I have been developing 3D printed rubber power props for the past year and a half and have been very happy with the results. Without making this a prop discussion, I can say that my best results so far are with props using a hypotwist pitch distribution. I read a good paper on the National Free Flight Society web site in their technical library on the subject of hypotwist props. I have found that a prop with a P/D of 1.2 out to the 3/4 radius point and then a linear P/D progression out to 1.8 at the tip really works well on my models. I built a rotating arm dynamic test rig and found that the 1.2 to 1.8 hypotwist prop performs slightly better than a helical pitch prop with a P/D of 1.5. The hypotwist prop has better acceleration and the same duration as the helical pitch prop. The prop for the Hellcat is using the 1.2 to 1.8 hypotwist pitch distribution.

I am including a picture of the model's framework assembled. I am also including photos of the prop during the 3D printing process and how it looks in the raw printed form. Supports are generated by the 3D printing software to support the blades as they are being printed. The supports are then peeled off after the prop is printed. They come off fairly easy. Post printing sanding of the prop is required to get rid of support residue and to in general clean up the blade surfaces. I use ABS when printing props and that plastic sands quite nicely. Now that the prop has been printed I need to do the post print sanding clean up.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Re: Comet 24" span Hellcat - CAD redraw and build
Logged
backyardbalsapilot
Bronze Member
***

Kudos: 1
Offline Offline

United States United States

Posts: 32



Ignore
« Reply #18 on: September 18, 2016, 01:45:13 PM »

I've never seen anyone 3d print propellers before, but if it works, why not?

I like your choice of a color scheme. How difficult is it to get the inkjet templates right?
Logged
OZPAF
Palladium Member
********

Kudos: 180
Offline Offline

Australia Australia

Posts: 5,109



Ignore
« Reply #19 on: September 18, 2016, 07:47:43 PM »

Using 3d printing to produce small props sounds interesting Paul. Hypotwist pitch distribution also is something I will have to check.
The September EAA magazine has some good info on finishing ABS plastic 3d printed articles using acetone. It apparently can lead to very smooth finishes.
John
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #20 on: September 18, 2016, 10:38:18 PM »

Backyard - The inkjet printing layout for the Comet Hellcat is a bit tedious but not overly difficult. Not surprising, the fuselage represents the major challenge due to the compound curves. The flying surfaces are pretty straight forward. I have an initial layout completed and just need to print the components to see what revisions may be necessary. For reference I have included a snap shot of the left fuselage side. I am planning to cover the fuselage in segments using the formers as the separation points. This will result in a 1/16" overlap at each former. Hopefully the overlaps will not be too noticeable.

John - It is possible to use acetone to smooth out the surface of ABS 3D printed parts. There are a variety of techniques. I have not messed with that but probably should just to see what I can achieve. It may do a nice job of making the surfaces of a prop look more like a commercial prop after sanding the 3D printed prop.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Logged
PaulBrad
Gold Member
*****

Kudos: 36
Offline Offline

United States United States

Posts: 342


Topic starter
WWW

Ignore
« Reply #21 on: September 20, 2016, 03:36:10 PM »

Looks like my progress will be delayed a bit. While working on the fuselage covering I discovered that one of my ink cartridges had run out of ink. I thought I had replacements on hand but made the unhappy discovery that the color that had run out was out in my printer supply bin. Bummer. I have to get some replacement ink cartridges before I can get the covering printed for the Hellcat.

I did manage to get the individual fuselage panels fine tuned so they will fit the model and line up properly. I was printing just the panel outlines for the most part so very little ink was used during my fit tuning process. When I printed the first set of full color panels I noted that the color was just a bit off. That is when I discovered the empty ink cartridge. At least the set I printed allowed me to confirm that all of the graphics will line up properly when the individual panels are applied to the fuselage.

I have attached a photo of the left side panels that I was able to print. One of the panels has been cut from the sheet in the photo. The remaining panels were also cut out for my graphics alignment verification.

Paul Bradley
Attached files Thumbnail(s):
Re: Comet 24" span Hellcat - CAD redraw and build
Logged
Prosper
Platinum Member
******

Kudos: 55
Offline Offline

United Kingdom United Kingdom

Posts: 1,425



Ignore
« Reply #22 on: September 21, 2016, 08:36:40 AM »

Good stuff, Paul, very interesting especially the propeller and tissue printing.

Stephen.
Logged
SBlanchard
Silver Member
****

Kudos: 10
Offline Offline

United States United States

Posts: 154



Ignore
« Reply #23 on: September 21, 2016, 10:12:38 AM »

This is really great Paul. I am looking forward to the finished build and the availability of the plan!

Steve
Logged
Don McLellan
Titanium Member
*******

Kudos: 63
Offline Offline

Canada Canada

Posts: 2,371




Ignore
« Reply #24 on: September 21, 2016, 11:31:45 AM »

Hi Paul,

Very interested in the printed propeller and how it stands up to flying (well, actually landing).  Do you have to paint or otherwise protect the propeller from UV?

Also, beautiful printed tissue!!

Don
Logged
Pages: [1] 2 3 4   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines Valid XHTML 1.0! Valid CSS!