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Old Oct 25, 2011, 07:27 PM
Bypass Ratio = Infinity
scatsob's Avatar
United States, NC, Jacksonville
Joined Apr 2006
1,653 Posts
I just emailed Tru-Turn asking them for a quote. The image I attached was to help them visualize what I am looking for. I fully expect them to say they can do it but it will be $500, as it is their 4.5" spinners are all over $100.
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Old Oct 25, 2011, 07:37 PM
Tow Dog.
Pilatuspc12's Avatar
United States, CO, Hotchkiss
Joined Oct 2006
1,651 Posts
OK,
well we can start a Spinner "Kitty" for you.

Randy
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Old Oct 25, 2011, 09:58 PM
Scale Builder
United States, AZ, Litchfield Park
Joined Jul 2002
2,444 Posts
Scatsob,

Gene Barton is cutting down spinners for a friend of mine in exactly the same manner as you are seeking. He might be worth talking to. He has a pretty good list of spinners so he just might have what you need. Worth a shot anyway.

http://www.genebarton.com/
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Old Oct 25, 2011, 10:43 PM
Rangers Lead the Way
Joined Mar 2010
1,950 Posts
Quote:
Originally Posted by scatsob View Post
TT- I might be misunderstanding you, but I need two backplates, one for each of the props. I dont understand what you mean by groves.

On a related topic, I got an answer back from Zinger, and I quote:

"all of our spinners are p51 style,we can not make what you are talking about."

Well, I never said I had a problem with the shape....and I never really asked them to "make" anything. I just asked if they could cut the spinner at the point where the front portion had a 4" diameter. I replied and asked if they would just sell me the 4.5" spinner and just the backplate from a 4" spinner. Well see I guess.
I'm such a dork...totally forgot you were doing the contra. Now the issue as I see it is the sections of the spinner that go between the blades for the rear backplate / prop, as these are part of the spinner shell. How will you deal with that? You would have to somehow attach these sections to the backplate. The only solution would be TIG welding. Other solution to the whole thing would be to have a shop fab you up a billet backplate with the prop blade grooves cut in.
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Old Oct 25, 2011, 10:51 PM
Rangers Lead the Way
Joined Mar 2010
1,950 Posts
You could send Zinger a pic of the full scale contra but my sense is their custom work is limited to cutting grooves in spinner blanks and assembling multi blades. Tru-turn's quotes seem ridiculous to me. I'd just take it into a machine or welding shop. We have dozens of machine shops in Socal that could do billet and welding work needed to get this done at a fraction of the cost Tru turn wants.
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Old Oct 26, 2011, 09:00 AM
got any foam to bash?
Tom Hunt's Avatar
Lake Grove,LI, NY
Joined Aug 2000
5,322 Posts
Quote:
Originally Posted by V2rider View Post
I think I've read on here somewhere on these setups you want to run a higher pitch on one motor than the other. 16x10 and 16x8 would probably be a good match.
The aft prop must either:

turn faster at the same pitch
turn the same RPM at a higher pitch

or you will be VERY disappointed at the speed of the model.

2" pitch difference on a 16 prop is not going to hack it. you need more like 4.

The whole purpose of a prop is to accelerate the airflow through the disk. That is the job of the front prop. If the aft prop adds nothing to the flow, then no increase in velocity through the rear prop will be had. Static thrust will go up (if you're interested in a 3D spitfire!) but the speed of the air will not rise any through the second prop. If fact, the second prop (if the same pitch) will go into "windmill" mode very quickly after take-off and contribute nothing but drag there after.

Hate to burst your bubble scatsob.... but been there, done that.

Tom
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Last edited by Tom Hunt; Oct 26, 2011 at 10:14 AM.
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Old Oct 26, 2011, 03:40 PM
http://roys-rc-tv.webs.com/
Basingstoke Roy's Avatar
United Kingdom, England, Basingstoke
Joined Oct 2005
448 Posts
That not Strictly true, Sorry!

With out getting into reams of theory, there come a point ware swirl increases the effective AOA of the back blades. at which point the front blade start wind-milling. Contra props are not simple.
Have also been there done that.
See post http://www.rcgroups.com/forums/showt...246407&page=77 item 1143

Peak O/P of 1.6KW. 755 Watts off the front blade & 852 off the back, flying with same pitch. Ended up with more on the front.
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Old Oct 26, 2011, 06:09 PM
got any foam to bash?
Tom Hunt's Avatar
Lake Grove,LI, NY
Joined Aug 2000
5,322 Posts
Quote:
Originally Posted by Basingstoke Roy View Post
That not Strictly true, Sorry!

With out getting into reams of theory, there come a point ware swirl increases the effective AOA of the back blades. at which point the front blade start wind-milling. Contra props are not simple.
Have also been there done that.
See post http://www.rcgroups.com/forums/showt...246407&page=77 item 1143

Peak O/P of 1.6KW. 755 Watts off the front blade & 852 off the back, flying with same pitch. Ended up with more on the front.
Is that data static (you qoute "take-off") or at flight speeds? not clear and it makes a BIG difference.
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Old Oct 26, 2011, 07:09 PM
Bypass Ratio = Infinity
scatsob's Avatar
United States, NC, Jacksonville
Joined Apr 2006
1,653 Posts
From the looks of his thread those numbers are in flight. I will just start with that I have for props and go from there. Honestly I'm not looking for the fastest e-spitfire, just something different thatís fun to fly.

I got another reply back from Zinger on my question about just buying a back plate and they again said no. Either they are really bust or their CS sucks because they have not been helpful at all. Oh well, I guess I will just buy a 4.5" spinner and 4" spinner and do the modification myself. It will still end up being the cheapest option.

It also looks like I am just going to fix the motor I have and go from there. I will start posting pictures of the motor as I begin to fix it, hopefully starting this weekend. First order of business will be ordering a new shaft from McMaster Carr, the one in there is too short and I will probably use a collet adaptor for the front prop. That will be easiest for adjusting distance between the props. This weekend I will also start JB Welding the gaps between the magnets to make sure they donít fly off in flight. I may also try the epoxy putty they have now because it will be much easier to work with Third will be to sand down the rear stators a little so the rear rotor no longer rubs on them. Someone in another thread mentioned that the motor they received had a stator assembly that was not concentric to the shaft but that is not the case with my motor. Well see I guess, it will be fun regardless. I still cannot justify spending $360 on the Himax motor, if it comes to that I will just do one of the new Turnigy SK3 motors on 12s.
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Old Oct 27, 2011, 10:52 AM
Tow Dog.
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United States, CO, Hotchkiss
Joined Oct 2006
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Well, Vario Prop has a nice 5 blade prop for your Griffon powered Spitfire.
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Old Oct 27, 2011, 05:02 PM
http://roys-rc-tv.webs.com/
Basingstoke Roy's Avatar
United Kingdom, England, Basingstoke
Joined Oct 2005
448 Posts
Good luck with the motor repairs, I've had similar with their AEM CR28M motor.
but once shorted it's been a good little motor. It's a shame their build quality lets them down.

I think the thing with contra motors is you've got to be prepared for a fair bit of DIY. Just focus on how much cool factor your have when it's done.

Can also recommend Vario Prop.
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Old Oct 27, 2011, 07:00 PM
Bypass Ratio = Infinity
scatsob's Avatar
United States, NC, Jacksonville
Joined Apr 2006
1,653 Posts
The cool factor is what I am going for here. I have been in love with the later Spitfire variants for a long time, especially the contra prop ones, and I havent seen one done on this scale yet. It’ll be fun tinkering with the motor too. Today I ordered the shaft material from McMaster Carr along with some thrust bushings. I’m not going to do any ordering on the spinner until I know the motor will work. I also need to get some servos for the stabs to see if my crazy idea of cramming them in the stabs will work. Batts are on their way so when I get them in I can start fiddling with the balance.
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Old Oct 27, 2011, 07:04 PM
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TripleW's Avatar
Prescott, Arizona
Joined Feb 2010
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I wonder if one of the magnets was not glued in it's proper position.
Since HK has a reputation for loose magnets you might make sure they are all secure. Good Luck
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Old Oct 27, 2011, 08:37 PM
Tow Dog.
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United States, CO, Hotchkiss
Joined Oct 2006
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Good Idea!
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Old Oct 28, 2011, 03:20 PM
Bypass Ratio = Infinity
scatsob's Avatar
United States, NC, Jacksonville
Joined Apr 2006
1,653 Posts
Quote:
Originally Posted by TripleW View Post
I wonder if one of the magnets was not glued in it's proper position.
Since HK has a reputation for loose magnets you might make sure they are all secure. Good Luck
What do you mean by wrong position? Some are a tiny bit closer or farther apart from one another but not enough to worry about. I am definitely going to epoxy the magnets in before running, and attempt to balance them. A good hand balance should suffice.
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