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Old Jul 05, 2010, 01:23 AM
Scale Builder
United States, AZ, Litchfield Park
Joined Jul 2002
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EDF newbie dreaming of D-558

G'day gents,

I'm a die hard warbird fanatic, mainly second world war stuff, and I've never had any interest whatsoever in building jets. However, I've recently been bitten very hard by the "Skystreak" bug and am contemplating drawing one up. I've got lots of design experience and the airframe presents no challenges but the power system is a horse of a different color. I've got very little experience with electrics and none with ducted fans! I'd like the airplane to be large enough to carry the weight of flaps and retracts as well as a glassed/painted finish. If I may I'd like to run some rough numbers by you guys and get some input from those with a bit more experience than myself. I'm thinking 1/8 scale which gives a total length of ~51 inches and a span of 37.5 inches with roughly 350 square inches of area. There is room to increase the span/area a bit without detracting from the scale appearance as well. These are just preliminary number and are in no way set in stone. My questions are thus:

1. Does this sound feasible for a 90mm fan unit to haul around? If so, what kind of weight should I be shooting for? (Other than as little as possible!)

2. At this scale I don't think there will be any issues with inlet/outlet ducting but whereabouts would be the ideal location for the fan unit from an efficiency standpoint? As you can see the fuselage is your basic tube so I can probably situate the fan unit well aft if that is the best place and maintain the CG with proper placement of the radio equipment/batteries.

Feel free to offer any and all advice you will regarding the project as I'm open to all ideas. Looking forward to your replies!

Chad Veich

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Old Jul 05, 2010, 07:51 AM
Da' Cajun
Boogie_'s Avatar
Lake Charles, Louisiana
Joined Jun 2009
2,892 Posts
Wow! What a great subject to model.
I've only been into fans for the last year so I'm sure you'll get more educated replies.
But I think your on the right track.
My advice would be to locate your retracts first and design the wing around them. That is going to be the toughest fit. The fuse on this jet is large enough to place the fan and batts just about anywhere needed.
The exhaust tube should be as short as feasible.

There are some old Comet plans of this model. Might help in your drawing up your plans.


Julian
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Old Jul 05, 2010, 08:45 AM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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I have the book on the Skyrocket. I wanted to build one my self some time ago. Based on the size that you are planning a 90mm would do it, but a high powered 69-70mm fan would make life easier.
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Old Jul 05, 2010, 02:37 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
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I have always like this one too. Good luck with you design.

Eric B.
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Old Jul 05, 2010, 05:37 PM
Scale Builder
United States, AZ, Litchfield Park
Joined Jul 2002
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Thanks for the replies and the encouragement gents, much appreciated.

Julian, I have already downloaded the Comet plans for reference although I'm hoping to come across some accurate 3-views to base the design on. Although, quite honestly, the Comet plans look pretty good except for the flat bottom airfoils. As soon as I settle on the size (and airfoil) I will start looking for suitable retracts.

Kevin, I just purchased the book below on ebay. Is this the one you have per chance? I have not received it yet but hope that it contains some half way decent scale drawings. Two questions for you:

1. Why would the smaller fan make life easier?
2. Does this mean I could go larger for a 90mm fan? I'd like to make the bird as large as possible without breaking the bank and I think the 90mm is probably as big as I can afford at the moment.

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Old Jul 05, 2010, 05:43 PM
Scale Builder
United States, AZ, Litchfield Park
Joined Jul 2002
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Here is the reason I've become interested in modelling the D-558, Max Merckenschlager's turbine powered version. I had never even considered the Skystreak as a modelling project until I saw this picture. For whatever reason I find it very attractive and should be a very straightforward build.

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Old Jul 05, 2010, 08:32 PM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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Hi Chad,

Yes, that is the book that I have.

The reason I said smaller if you were doing it 51" long w/37.5" span would be the intake area for the fan and the wing loading.

The quick numbers I run sometime ago would put a 90mm fan at 80%+ sq in of intake area (FSA). This will work but when you factor in the area lost due to nose retract area you would probably be in the high to mid 70% FSA.

So a larger model would be better in terms of space for the fan, batteries, retracts and ducting. And finally the wing loading issue, a 90mm fan and motor can weigh in at 14oz where a 69-70mm unit can weigh in at 7oz. Factor in the increased battery weight for the 90mm versus a 69-70mm unit, you could end up with something that isn't very fun to fly .

BTW, the Comet plans that Julian mentioned are much better than whats in the book. The techincal data in the book is priceless...so it isn't a waste at all!
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Old Jul 05, 2010, 08:33 PM
IWC
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Old Jul 06, 2010, 12:22 AM
Scale Builder
United States, AZ, Litchfield Park
Joined Jul 2002
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Quote:
Originally Posted by Kevin Cox View Post
Hi Chad,

Yes, that is the book that I have.

The reason I said smaller if you were doing it 51" long w/37.5" span would be the intake area for the fan and the wing loading.

The quick numbers I run sometime ago would put a 90mm fan at 80%+ sq in of intake area (FSA). This will work but when you factor in the area lost due to nose retract area you would probably be in the high to mid 70% FSA.

So a larger model would be better in terms of space for the fan, batteries, retracts and ducting. And finally the wing loading issue, a 90mm fan and motor can weigh in at 14oz where a 69-70mm unit can weigh in at 7oz. Factor in the increased battery weight for the 90mm versus a 69-70mm unit, you could end up with something that isn't very fun to fly .

BTW, the Comet plans that Julian mentioned are much better than whats in the book. The techincal data in the book is priceless...so it isn't a waste at all!
Thanks for the info Kevin. I have been looking at the F-105 project going on elsewhere in this forum and that one is 65 inches long with a 37 inch span and using a 90mm fan set up. Based on those numbers I think I can go up to 1/7.5 scale on the Skystreak which would result in a 56 inch length and a 40 inch span. (Although I would probably stretch the wing out to 44-45 inches) I'm not a speed freak by any means and am only seeking scale like performance.

IWC, thanks for the link. I did see those plans in my Google searching but the Comet plans were a free download and look just as good, if not better, in the outline department. Plus, as somebody who draws and sells plans, I tend to have issues with guys who profit off of other people's work.
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Old Jul 06, 2010, 05:38 PM
deltas are cool
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Tehachapi ,CA.
Joined Apr 2006
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im sure a 90@ 80% would work ....a good high powed 70mm might get the job done too . the ducting could be made to "bend around" the nose tire .
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Old Jul 06, 2010, 06:58 PM
Scale Builder
United States, AZ, Litchfield Park
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Thanks for the info AIR SALLY. Now for the first of, I'm sure, many dumb questions just to get the ball rolling. The scale inlet provides for approximately 3032 sq/mm of area. Is that going to be sufficient for the 90mm fan? I can't seem to find any FSA approximations for the 90mm units. (I'm looking at the WeMoTec Midi unit) I fly only off of paved surfaces with plenty of runway so longer take off runs and slower climb outs are an acceptable trade off for higher top end speeds.
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Old Jul 06, 2010, 07:07 PM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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Chad,
The Midi is about 7.73 sq.in. of FSA or about 4987sq.mm. I think .

BTW, Wemo Mini is about 4.5 sq.in. and about 2903sq.mm.
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Old Jul 06, 2010, 07:25 PM
deltas are cool
AIR SALLY's Avatar
Tehachapi ,CA.
Joined Apr 2006
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my son (corsair nut) has a JHH F-86 with a 90mm in it ,the inlet is about 80-85 %fsa and it flies very well.are you doinf a 8s power system
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Old Jul 06, 2010, 11:50 PM
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St.Catharines, Ontario
Joined Jan 2009
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the WM400 MKII from Efflux RC is a 70mm fan with an FSA of 3,044mm squared. your intake would be ok for this fan. worth looking into at least. the WM400 is capable of powering some very large airplanes. check it out.

for a 90mm fan, you can go down to about 70-80% FSA with a straight inlet duct. do the math and see what number you come up with at those percentages. if it falls into that range, it'll work with a 90mm fan. just expect long takeoff runs.

Rich
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Old Jul 07, 2010, 02:33 AM
Grumman Ironworks :-D
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United States, VA, Newport News
Joined Dec 2006
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OHH the red rocket,always had a soft spot for vintage jets
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