HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Dec 19, 2012, 03:52 PM
Ken Stuhr
Seattle, WA
Joined Aug 2005
308 Posts
Motor

I ask because I hate 2 blade props where more blades are scale-ish.

I cite the example below:

Martin Baker MB-5
Span 42"
Spinner dia ~3.5"
Props MA 10" 3blade
Motor Maxx CR 2816
AUW ~3#
Batt 2x3s2100

The props are still a bit smaller than scale, but they work ok for my sensibilities. I blame martin Baker for using too big a one.

The props would overamp the motors at max throttle so I am careful, but it can be done. Since you are using 11" 2 bladers, could you go to 10" 3 bladers? I think you've got the bigger motor anyway. A little overpowered isn't it? I mean, I get even a bit more than scale looking speed with mine.

Granted, I installed a cooling duct that takes all the belly scoop air and runs it into the engine room, through and around the motors, from which it travels aft and out the wing and out the wheel wells. This is probably necessary given the loading the props put on the motors, but so far, fine. I seldom have flown it over half throttle.
flyboyken is offline Find More Posts by flyboyken
Reply With Quote
Sign up now
to remove ads between posts
Old Dec 19, 2012, 07:07 PM
Registered User
Bill Smudge's Avatar
United States, KY, Bowling Green
Joined Jul 2011
1,075 Posts
Hey Mark,

Great looking build you've got going!! I expect it to be a real beauty.

Bill
Bill Smudge is offline Find More Posts by Bill Smudge
Reply With Quote
Old Dec 19, 2012, 07:37 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Ken,
This ship is hand launched, so three bladers are not going to work. Besides, if they aren't the right size, then they aren't "scale" anyhow . Two bladers are more efficient, cheaper, and more readily available in varying pitches and diameters.
Part of the issue is finding Reverse Rotation (LH) and RH 3 blade props in appropriate diameters and pitches as well. The article I posted from NASA suggests that there be pitch differential ( + on the rear prop), but no reason for diameter differential other than noise reduction.
The motors I am using call for a 12-6 and 11-8 to run at 35 amps max. I want more speed, less climb, so I dropped down to 11-8 and 11-10 (or 10-10, depending on performance) This fits the formulae for keeping amp draw the same, and increasing my top end. After all, PM does nearly 500 mph.
Now, one thing I think you might be missing, is that reduced throttle does not reduce amp draw. A motor pulling too many amps wide open is pulling too many amps at any throttle setting, just not as long. Part throttle also generates more heat on the ESC than wide-open power.
Now, I had a look at your motor specs, and they are calling for 10-5 and 9-7 props, as two bladers (which are more efficient than three). Following the rule to drop an inch diameter and add inch pitch when going from 2 to 3 blades suggests to run the proper props, you should have 9-6 and 8-8 3-bladers, a long way from your setup....so I have no idea how you are keeping your draw under the 20 amps specc'ed out for those motors.
You have a fine looking ship, for sure. I think I'll stick with the props I have until I can run some amp draw tests and find out where I'm at. I need to be under 35 amps per motor...

William, thanks. I lost my mom in Sept, so I use modelling as a form of therapy..

Now, back to the mancave to work on PM...

Mark
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 20, 2012, 08:23 AM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
How did you work out your spinner Ken?
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 20, 2012, 05:03 PM
Falling with style!
Rick Ludtke's Avatar
USA, WA, Freeland
Joined Nov 2010
226 Posts
It just occurred to me that your 3 inch spinner could probably be used attached to the forward prop as per normal. The you build a second "spinner" for the rear prop out of fiberglass that is a constant 3" diameter from from to back (not tapered like a spinner), you could mount it to the aft prop using a spare backing plate. A fiberglass buildup of mill fibers and cabosil could be placed within the perimeter of the "spinner" that could be later drilled for scews to mount it to the backing plate. The only tricky thing is maintaining balance.

Cheers
Rick
Rick Ludtke is offline Find More Posts by Rick Ludtke
Reply With Quote
Old Dec 20, 2012, 06:45 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Kinda what I was thinking, I bought a 3" GP , more or less to get the aluminum backplate. I am going to cut the spinner where it needs to be, and see if another one will be the right diameter for the front. If not, I was thinking glassed foam. I did that on my Bf109 for MAN, and on my Forward Swept Wing P51S, and both worked great. So light ther are no balance issues at all....

Working on the rudder, and top hatch tonight.
Mark
Mark
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 20, 2012, 08:22 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Did some fin/rudder work. Had to re-cut the top of the fin after looking at a few pics. It has a more squared-off trailing edge.
Cut hatch floor and formers, they look good. Will start planking that tomorrow.

Mark
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 21, 2012, 05:21 AM
71% of the world is runway . .
Bart83's Avatar
Nederland, ZH, Oostvoorne
Joined Jun 2009
1,779 Posts
Great project !!!


Good lick with the rest of the build.


Cheers,

Bart
Bart83 is offline Find More Posts by Bart83
Reply With Quote
Old Dec 23, 2012, 12:26 AM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Planked the top hatch. Needs front nose piece yet, will be light contest balsa. Small rear turtledeck will be very light balsa as well.
I love planking, I look at it as a challenge to do without any filler.
I'll get to nose block and rear deck Sunday. Starting to look like PM!
Made a two-piece spinner from a 3" White Great Planes unit, and a 2 1/4" Black GP unit. Front is too pointy, so I'll more than likely make one from foam and glass it...

Mark
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 23, 2012, 08:40 AM
Registered User
thomanie's Avatar
Trondheim, Norway
Joined Jun 2003
2,806 Posts
I bow before thy - o' great stripper... wait, now that just sounds plain wrong!

She's looking great Mark!

T
thomanie is offline Find More Posts by thomanie
RCG Plus Member
Old Dec 23, 2012, 01:49 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Thanks, Thomas.
I'm making progress on the front part of the hatch, and I made the turtledeck. Slows you down when you have to use Titebond on all the parts that will be on the surface and need sanding...
Mark
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 23, 2012, 09:32 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
Hatch is looking good, have to sand out the fillet on the scoop. Turtledeck glued on, fin/stab fairing done.....front scoop is funtional, pulling heat from the motors , past the ESC's,and out the back.
mrittinger is offline Find More Posts by mrittinger
Last edited by mrittinger; Dec 23, 2012 at 10:52 PM.
Reply With Quote
Old Dec 25, 2012, 07:48 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
I'm at 56 ozs with a 3s5000 pack, minus Rx, 4 servos, and 2 esc's.

That leaves 10 ounces for those goodies , pilot, and finishing. Thats cutting it close, but I could always drop a few ounces with a foam spinner, and another few by going down one pack size to about 4400...hoping to stay under 66 ounces RTF to get a 25 ounce wing loading.
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Old Dec 25, 2012, 08:24 PM
Or current resident
glewis's Avatar
USA, FL, Tampa
Joined Jul 2002
4,832 Posts
You sure build some cool models man. This is another one!
25oz loading? How do you launch it, fire it out of a cannon? LOL.

Merry Christmas

Glenn
glewis is offline Find More Posts by glewis
Reply With Quote
Old Dec 25, 2012, 08:33 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
9,026 Posts
25 is the highest I can throw, at least at this age
With nearly 800 watts, I should be OK. I do wish she were lighter, but there are two motors, two esc's, etc. With wanting to do a metal type finis, it has to be all sheeted surfaces as well.
Probably the most weight is the motors and battery, they account for 22 ounces alone!
mrittinger is offline Find More Posts by mrittinger
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Reno Racer "Precious Metal" mrittinger Electric Plane Talk 49 Feb 15, 2007 06:36 PM
Discussion LEG 48" Precious Metal Reno Racer: Spinner color? PDX Slope Pilot Slope 19 Aug 27, 2006 08:23 PM
Precious Metal Pics so far : ) mrittinger Electric Plane Talk 92 Aug 31, 2003 09:45 PM