|Nov 20, 2007, 05:33 PM|
Build Off II - 1/6 Scale de Havilland DH. 88 Comet - Design
Yes Yes, I know, a month or so ago I started a design thread of a Fiat biplane and nothing came from it so far. It will come someday, but this beasty will go first.
And I also know that I got a Mig-3 and Heinkel He 100 design ready to be send to Charlie and waiting to be build but that has to wait untill after this too
And last, I also know that I still got to clean up the Macchi plan and cut files. It will be done before Xmas. Because then the next Build off contest will be running, and therefore I need a design. (Oh yeah, and Dionnel wants his Macchi kit under the tree )
So a sixth scale Comet it will be(come).
Span will be 2.23m or 88 inch;
Lenght will be 1.47m or 58 inch.
My aim is to get a flying weight of 6 kg. The plane will be entirely build of wood. Covering glass cloth for the wing and fuselage. On the tail I am not sure yet. On the accurate drawings I have, lines have been drawn to simulate the tail structure comming through the covering. However I can't see this on the photos I found so far. I don't know if I will make it a planked tail or an open structure that is covered with something. Suggestions are welcome!!!
Functions will be Throttle, ailerons, elevator, rudder, flaps and retracts.
Power will be provided by two outrunners
powered by a 3s lipo pack each. A rule of thumb we use overhere says that you need 150watt/kg to do a ground start with a plane and have a little surplus. I aim at 6 kg so I'll need 900 watts. 900/2 = 450 watt peak for each motor. That will be 450w/11.1v = 40.5A peak. That seems reasonable and I can push it a little further if needed.
Since I got very positive comments on my cad stuff in the other contest and with the Macchi, I thought to share the design process.
First, I got some very good drawings from a frend.
I scanned these drawings. The biggest scanner I've acces to is on school, but this machine can only handle A3 pages. So I had to scan parts of the plan and photoshop them back together. You must take care that you scan all parts you need and that you do this with the same scanner configuration. Else you might get different size's on the different scans.
Then, when the parts I needed from the drawings were back togheter, I used it as a background in Rhino. I started tracing all the lines I needed to get a model. For tracing the drawing, I used the line and curve functions. The round curves are drawn approximatly right. It is nearly impossible to draw them exact from the first time. So when I have something that looks like it from far away, I turn on the control points for the curve and drag them arround untill the curve is good enough for me. Also I try to draw the curves with a minimum of points. This will simplify things when I'll start making surfaces with these curves.
Also I draw only half a plane. The other side will be mirrored. (This will not work on a BV 141 )
I've also drawn a landing gear unit in Solidworks allready (I was a little bored a couple of evenings ago ). It is derived from a landing gear for a big scale sailplane. It will be servo operated (on the upper axl) and it is self locking. I plan on making this from aluminium.
Enjoy the first screen shots.
|Nov 20, 2007, 08:03 PM|
5 miles from the geographical center of Pennsylvania
Joined Aug 2005
using the scans as a background is a good idea. I'm still learning Solidworks, I designed a fairly complex machine in Solidworks, but nothing as complex as a plane. It would be great if you can walk us through the process.
|Nov 21, 2007, 08:47 AM|
I worked a little further.
When all the lines I needed were traced, I hided the background image. Then I started rotating all the lines to get them in the desired view. A good help with this is the group option. You can group several lines togheter, and then you can handle this group as if it were one line. This way it becomes easier to rotate and move the whole thing.
When all lines are in place, I started with the Comet's flank. This flank is straight and makes it very easy to make. I made a surface using 'extrude curve straight' and the outer fuselage contour in the top view. Then I took the lines which define this straigt flank in the side view and I did the same. Doing so I got 2 intersecting surfaces. I trimmed one with another and the one I didn't need anymore went to a special layer to where I put all the stuff I don't need anymore. I allmost never delete. There is always a chance you need it once more.
Then, when the flank was drawn, I started with the top of the fuselage infront of the cockpit. First, I duplicated the upper edge of the flank. Then I used 2 outer cross sections in the part I wanted to make. I only need the top curve of these cross sections. These tops were adjusted so they fitted to the edge of the flank and to the contour curve.I adjested them by dragging the control points arround.
When that was done, I splitted the contour curve and edge curve with the cross sections. Then I selected the cross sections, the contour and edge in between them and I used the 'curve network' tool. Voila, part of the top was finished. The same method is used to create the belly and the front part.
|Nov 21, 2007, 12:26 PM|
Fuselage model finished
With the same 'curve netwerk' tool, the back and the complete end of the fuselage were also created. Sometimes it is much easier to work with the edges of a surface you just made, instead of redrawing and readjusting all the curves.
When that was done, the little piece under the cockpit was drawn. I took half of the front edge of the back and half of the aft edge of the top of the front. The two other lines came from the top edge of the flank and a line I drew between the two first edges. This little surface was then trimmed with an extruded surface that I made from the cockpit outline in the side view.
All the surfaces were then joined to one. Now I had one half fuselage. This half was then mirrored to get a full fuselage.
After this I did the nose cone. This little surface was lofted using the two half circle edges of the fuselage and the contour as a third curve.
Then the cockpit. This was made again with the curve network tool. Each half is build with two surfaces. Curves came from the edges, and the intersection of the trim surface and the front top surface (see, If I had deleted this trimming surface I would have had to draw it again, now I just made that layer visible and used it.)
Then I cleaned up a bit to make the last screen shot.
|Nov 21, 2007, 03:55 PM|
After the fuselage, the wing
First I made a surface by extruding the center wing line in the front view to the back. On this surface, the curves of the wing's top view were projected. Then, when this was cleaned up a little, I imported 2 airfoil files. A naca 2412 for the root and a naca 2415 for the tip. Normally I use only one airfoil, but scince the comet has very narrow tips, I thought I better use a more lifting airfoil near the tip.
When an airfoil file is imported, this must be checked first. With the 2412, the curve did something strange so I had to fix this first. Then the curves are scaled, rotated and dragged untill they are in place. When scaling, amways take care to do it the right way. Else you can have a totally different airfoil.
When that is done, its time to make a wing. with the airfoil curves I split the wing contour curves in pieces. To make the surface, I use the 'sweep two rails' tool. The leading edge and trailing edge are the rail curves, the airfoil curves are the cross sections. When this is done, you have a big pieces of wing.
Then the tip. Here I first drew a little help line to split the tip contour curve. Then I use the sweep 2 rails tool again. The railcurves are the 2 tip contour curves, the cross section is the Airfoil curve.
That is the tip. Then I mirrored the whole thing to have a look at it
When it was mirrored I also rotated the whol wing 1.2° to get the angle of attack right.
Then I made everything red to see how it looks allready
|Nov 21, 2007, 04:03 PM|
Most of the stuff wil be done in Rhino since I master solidworks not that good enough... (....yet; hopefully )
|Nov 21, 2007, 09:31 PM|
United States, VA, Arlington
Joined Dec 1996
Tom - don't know how far in your research on this fabulous aircraft has gone, but there are two in this colour scheme.
The Shuttleworth Collection at Old Warden, north of London, has the restored and airworthy original, but there was also a US built replica. One of the few - an major - differences is that the original was built to go as fast as possible and thus had its wing built with no washout, to achieve minimum drag in cruising flight. This grounded her for years, as her old 'home' airfield was closed and Old Warden's runway at that time was not long enough for this aircraft's demanding handling on landing and take-off.
The US replica has some washout built in to the wings and I recall reading that it is operated from a smaller grass strip than Old Warden had to start with. I unearthed some shots of this 'version', to get you thinking some more, perhaps.
Also - it doesn't have to be red! There were also dark green and black versions built for the London - Melbourne air race.
Idle thoughts - for my last three years in England, I lived on "Melbourne Drive" a couple of miles from RAF Mildenhall, where that race started from. Also, back in the early 1990s, Keith Shaw used to fly his model 88 - slightly smaller than yours - at a lot of electric meets. By all accounts, and I've seen one video of it, it was a reliable and solid performer with good flying manners. If you look up Ken Myers 'AMPEER" newsletter, there's some archived articles by Keith Shaw that use his 88 as an example about twin electric models
|Nov 22, 2007, 02:43 PM|
Nice cockpit foto.
I think I'll do the 1934 G-ACSS. I just love the red. The green one is also cool but I miss the white line on the side. I havn't seen any pictures of the 'Reine Astird' so maybe....
I do know that there is an American replica but I didn't know the wash out thing. If you look at the black magic picture above and then to the flying pictures on the site that Dereck posted you can clearly see the difference.
I'll think I'll put a little wash out in it, just to make it easier to fly.
|Nov 23, 2007, 02:41 AM|
I'm glad that I gave a little spark to you so that you are now also verry interested in the DH-88 Comet Racer. For those who don't know the plane here are a few site's were you can find history and pictures.
|Nov 23, 2007, 06:51 AM|
I suppose in a way there's a lot of the Comet in the DH Mosquito..
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