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Dec 19, 2012, 04:52 PM
Ken Stuhr


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.
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Dec 19, 2012, 08:07 PM
Registered User
Bill Smudge's Avatar
Hey Mark,

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

Dec 19, 2012, 08:37 PM
Gravity sucks.
mrittinger's Avatar
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 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...

Dec 20, 2012, 09:23 AM
Gravity sucks.
mrittinger's Avatar
How did you work out your spinner Ken?
Dec 20, 2012, 06:03 PM
Falling with style!
Rick Ludtke's Avatar
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.

Dec 20, 2012, 07:45 PM
Gravity sucks.
mrittinger's Avatar
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.
Dec 20, 2012, 09:22 PM
Gravity sucks.
mrittinger's Avatar
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.

Dec 21, 2012, 06:21 AM
71% of the world is runway . .
Bart83's Avatar
Great project !!!

Good lick with the rest of the build.


Dec 23, 2012, 01:26 AM
Gravity sucks.
mrittinger's Avatar
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...

Dec 23, 2012, 09:40 AM
Registered User
thomanie's Avatar
I bow before thy - o' great stripper... wait, now that just sounds plain wrong!

She's looking great Mark!

Dec 23, 2012, 02:49 PM
Gravity sucks.
mrittinger's Avatar
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...
Dec 23, 2012, 10:32 PM
Gravity sucks.
mrittinger's Avatar
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.
Last edited by mrittinger; Dec 23, 2012 at 11:52 PM.
Dec 25, 2012, 08:48 PM
Gravity sucks.
mrittinger's Avatar
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.
Dec 25, 2012, 09:24 PM
Or current resident
glewis's Avatar
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

Dec 25, 2012, 09:33 PM
Gravity sucks.
mrittinger's Avatar
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!

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