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Jul 21, 2015, 10:06 AM
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Thread OP
Build Log

De Havilland DH106 Comet 1 Prototype


Hi Everyone,

I have been asked to look into building a large model of the DH106 Comet 1 Prototype and considering power arrangements at the moment, either turbine or EDF.

I have absolutely no experience of EDF so could do with some help.

The model is 1/10th scale so a 3.5m wingspan and body length of around 2.75m, I have to keep weight down to under 20kg with a loose target flying weight of 19kg in mind which I think I can do with a fibreglass fuse and lots of carbon fibre.

I have done some provisional measuring and think a 120mm fan will fit within the confines of the inner nacelle.

So using the specs on the Jetfan 120 ECO on 10S it should be capable of achieving 6Kg of thrust so x 2 gives 12kg of thrust which is way over the top for a 19kg model

However the scale intake and length of the exhaust tube required will rob a lot of performance I am sure the root chord of the wing for a Jetliner is huge which will be great for slow flight but at approaching 900mm for combined intake and exhaust length I am sure this will have a bad impact on performance?

Any advice on fan choice and general help would be great

Ken
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Jul 21, 2015, 05:12 PM
Classic jets rule
AIR SALLY's Avatar
the JF 120 needs a big tail pipe ......I believe 4" or right at 100 mm to work. big project with a lot of challenges like the wing spar for starters I had a book about this plane ...really a sad story for a great looking and awesome performing air plane.
Jul 21, 2015, 05:22 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
Wasn't it the square windows , that lead to it's demise ??
Jul 21, 2015, 08:05 PM
Classic jets rule
AIR SALLY's Avatar
yep such a simple thing ....one reason you don't see that any more on pressurized birds
Jul 21, 2015, 10:51 PM
Classic jets rule
AIR SALLY's Avatar
theres some good vid programs on youtube that show how they figured out the cause ...a manufacturing defect. also some good vid of R/C comets flying both EDF and Turbine powered.
Jul 21, 2015, 11:15 PM
Registered User
Passenger in one from Dallas to Mexico City back in 1965. Nice ride.
Jul 22, 2015, 12:10 AM
Official Boat Bum
Eddie P's Avatar
Hello Ken,

There are a few design particulars to the Comet that will make for a very interesting model.

Have you seen the Comet 4 in operation and up close that has flown at all the major British events and have also flown at some of the European ones too?

Also, what are the construction methods you plan to use? At that size you may come in heavier than your estimates after all the equipment goes in but then again perhaps not if you have some very light weight methods in mind.

Obviously two high power 90mm units would produce a certain amount of thrust but as an alternative you could consider four medium power units as a way to get a certain higher amount of "installed" thrust if your weight adds up as you build. You would not have to use four units if you keep her light... but if you build in space for four ducts / equipment bays, you could have the option.

I'm building a four engine airliner of 1/16 scale (3.6m fuse and 2.8.m span) and have run a lot of numbers on various particulars you may find interesting. It's in my blog.
Jul 22, 2015, 12:51 PM
Registered User
Thread OP
Completed the basic wireframe drawing today, the fuselage itself is very easy to draw as its circular for most of its length.

Ran the figures through a CG calculator so I could position the fans, the drawing shows 120mm fans which are marginal, I would worry about putting 4 x 90mm fans in due to the weight of the batteries but it would make construction easier in terms of dealing with the spars and what a noise it would make any opinions?

I will build in two spars made from 2mm carbon fibre sheet which will have circular section for the ducts

I can start adding detail now starting with the spar arrangements themselves

Any more thoughts on the ducts which you can see are very long and also power systems, is this doable with EDF's

Construction details -

Wingspan - 3.619m
Incidence - 1'
Dihedral - 3'
Airfoil - modified ClarkY with slight undercamber and lengthened
Tailplane dihedral - 11'
Fuselage length - 2.765m

Two piece fibreglass fuselage split at TE of wing
Outer wings split/plug in at change in TE
Carbon fibre sheet spars
Wings will be built up balsa with fibreglass sheet stressed skin

Battery and avionics compartment in forward section of fuselage
Last edited by heli_madken; Jul 22, 2015 at 01:00 PM.
Jul 22, 2015, 05:21 PM
Official Boat Bum
Eddie P's Avatar
It's completely doable, no doubt. Four 90mm fans are not an issue for weight, but please if you will state your inlet opening dimensions. I know they are not round, they are oval. But some general dimensions would help decide what you can get away with in terms of two large fans or 4 smaller fans. If you decide to "bifrucate" or split the ducts in exhaust for one fan to feed two smaller outlets you will lose about 20 percent of your installed thrust from an already present "installed" thrust figure. Smaller motors and ESC's may not add so much more weight for 4 motors but your ducting efficiency gains with 4 fans may over ride what you think you may be gaining in only 2 larger fans.

Essentially what you are designing will fly well at about 0.6:1 thrust to weight. The challenge will be what sort of runway will you fly from, and are there obsticles near by: that may require greater thrust for the takeoff and initial climb only. In flight, no worries at 0.6:1.
Jul 22, 2015, 05:28 PM
Classic jets rule
AIR SALLY's Avatar
may want to make molded spars so you can use unidirectional C/F tow to lay it up with . a spar cut out of a C/F cloth laid up sheet might not be the best way to go. are there any pics of the full scale spars ? be interesting to see how they did it .
Jul 22, 2015, 06:30 PM
Official Boat Bum
Eddie P's Avatar
See this thread for some general build ideas in a lighter "built up" foam medium. Same idea just different materials and sizing.
https://www.rcgroups.com/forums/show....php?t=1898550

The Buccaneer has some similar design challenges in that ducts pass through the spar system. Though they do have a bit more beef to work with, the idea is similar to the Comet.
Jul 22, 2015, 06:35 PM
Official Boat Bum
Eddie P's Avatar
Here is another good article about the British RC Comet IVc, mentioned earlier. Some liasing with this builder may generate some helpful photos too.

http://www.modelflying.co.uk/news/ar...comet-4c/13857

Below is an interesting cutaway of a Comet II. The spars carry over and under the inlet ducting. The generous wing area will ensure this flies like a butterfly, compared to most airliners with insanely high wing loading.
Jul 22, 2015, 07:53 PM
Classic jets rule
AIR SALLY's Avatar
that's why I suggested a laid up spar with unidirectional Tow ,so you can have the tow go around the duct cutout and have some of the tow extend in board and out board from there . it would be super strong doing it that way .and yes a lot of wing area and a super clean design ....the brits used this configuration on a few different jets (engines buried in the wing ) seem in the usa they are all hanging on pods. I guess pros and cons to both . if you blew a engine with the engine in the wing that might just be the end. there have been a few engines in pods blow but not much other damage happens ...some have fallen off too which is not good ,but..... neither is a fire in the wing .
Jul 23, 2015, 04:34 AM
EB-66C Team Member
J Morgan's Avatar
Ken,
I noticed that you did not ask me for help. Welcome to the "know nothing about EDF" club. I have nothing for you.
And I hope you know that since you have wetted the appetites of the sharks in this forum, you will have to do a build thread on it.:roll eyes:
It will be a beautiful build.

J
Latest blog entry: A2D Skyshark at 1/8th scale
Jul 23, 2015, 07:43 AM
Oh no, not again!
jhspring's Avatar
Wow, another Ken thread! Whatever you do will be pure gold. The Comet is/was a beautiful bird, it will be exciting to see you bring this one to life. I think I have about as much EDF experience as John Morgan, but from a design standpoint, it would appear that the weight penalty for 4 x 90mm fans will be minor, and the smaller diameter will be must useful when designing the wing and spar configuration. FWIW.

Regards,
Jeff


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