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Cox Engine of The Month
March-2019
Jason_WI's

"Cox Pee Wee .020 with 3D printed horseshoe backplate"

More info on this engine!


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Pee Wee Horseshoe back plate

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Re: Pee Wee Horseshoe back plate

Post  getback on Tue Mar 05, 2019 8:29 am

They look really good Jason , Looks like a bit of cleanup after the print but is nice and smooth product . Very Happy
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Re: Pee Wee Horseshoe back plate

Post  Jason_WI on Tue Mar 05, 2019 11:51 pm

1/2A Nut wrote:How much does that assembly weigh with engine?

Both the horseshoe and the venturi mount weigh 24g with black prop, screw, and starter spring. The tanked Pee Wee weighed 27g. So about 3 grams less than a standard Pee Wee.

Not much weight savings.

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Re: Pee Wee Horseshoe back plate

Post  Jason_WI on Wed Mar 06, 2019 2:23 am

Shortened the tube in the back to allow more clearance for the needle valve from the firewall. Filed in some notches in the mounting legs to allow clearance for the needle and the fuel tube. I’m going to add that feature and beef up the other side of the notch to add strength.

This is a 7 minute run. The syringe sucks as a tank since the fuel height affects the draw. I’ll have to mount a decent tank so I don’t have to mess with the needle during testing. Nothing melted on this longer run. Too bad the TD .020 fine thread needle valve assemblies from Ace RC are hard to find. That would be an awesome addition to this setup. I have one I can try.

Cox 25% super power fuel and a black 4.5x2 flexible prop.  



A pic after the run.



Set off the smoke alarm this time.
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Re: Pee Wee Horseshoe back plate

Post  Marleysky on Wed Mar 06, 2019 11:49 am

3:23am? Man, that’s well after last call and you’re doing test runs of a 020 in the house! I mean, there’s nothing wrong with that, I am impressed with your work and willingness to work into the wee hours to accomplish amazing projects! Keep it up!
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Re: Pee Wee Horseshoe back plate

Post  Jason_WI on Wed Mar 06, 2019 11:53 pm

I wanted to see if it would run with an 80 TPI NVA from a sure start backplate. It runs but the needle setting is very touchy which is expected. Same prop and fuel as previous runs.

Video posted earlier today than well......... today. Smile


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Re: Pee Wee Horseshoe back plate

Post  1/2A Nut on Thu Mar 07, 2019 12:38 am

Need to back off with the cell phone camera the static noise is messing up the audio app readings for rpm.
Back away and use the camera's software zoom feature so the mic is not so close need some clean audio
for future reference.

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Re: Pee Wee Horseshoe back plate

Post  getback on Thu Mar 07, 2019 10:11 am

what app R U using again Brad , Thanks
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Re: Pee Wee Horseshoe back plate

Post  1/2A Nut on Thu Mar 07, 2019 10:42 am

'RPM Gauge" for android phones.

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Re: Pee Wee Horseshoe back plate

Post  1/2A Nut on Thu Mar 07, 2019 10:43 am

Has presets for 2 stroke / 4 stroke and multi cylinders / low rpm and high rpm set to
what you expect to get out of the engine most times 30k is enough for TD engines.

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Re: Pee Wee Horseshoe back plate

Post  ticomareado on Thu Mar 07, 2019 10:49 am

Suggestion for next Pee Wee printer project:

Do a scaled down copy of a Kavan Baby Bee tank extender for the Pee Wee.

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Re: Pee Wee Horseshoe back plate

Post  Jason_WI on Sat Mar 09, 2019 12:34 am

ticomareado wrote:Suggestion for next Pee Wee printer project:

Do a scaled down copy of a Kavan Baby Bee tank extender for the Pee Wee.


That would be like the SR-71......leaks fuel on the ground till takeoff.
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Re: Pee Wee Horseshoe back plate

Post  Jason_WI on Sat Mar 09, 2019 12:37 am

Here is another run video. Zoomed in and the phone is further away from the prop.

Also the venturi mount failed in the 3rd run. I think vibration broke the mounting feet off.






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Re: Pee Wee Horseshoe back plate

Post  1/2A Nut on Sat Mar 09, 2019 1:14 am

18,087 rpm much better =)
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Re: Pee Wee Horseshoe back plate

Post  getback on Sat Mar 09, 2019 6:33 am

1/2A Nut wrote:'RPM Gauge" for android phones.

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 Thanks Brad , will get it . Jason i guess that material is too tight ,ridged , stiff too early but you get it has no flex lol


Last edited by getback on Sat Mar 09, 2019 6:36 am; edited 1 time in total (Reason for editing : add on)
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Re: Pee Wee Horseshoe back plate

Post  aspeed on Sat Mar 09, 2019 7:17 am

I don't want to be the one to say it, but a machined mount from aluminum would be the way to go. Somewhat like the Surestart. Maybe some kind of air intake coming out the side for a throttle? Maybe not.  It would be more solid and stronger IMHO.  As a speed guy, an aluminum pan would always be good for a few revs too.  Having said that, maybe a fillet on the back tabs could be designed in on the next one.
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Re: Pee Wee Horseshoe back plate

Post  GallopingGhostler on Sat Mar 09, 2019 12:46 pm

Jason_WI wrote:Here is another run video. Zoomed in and the phone is further away from the prop. Also the venturi mount failed in the 3rd run. I think vibration broke the mounting feet off.

Nice going Jason, your experimental proof of concept Thumbs Up indicates great potential with a few minor changes, IMO. Airplane

It shattered because of concentrated stress point being the near 90 degree angle of the mounting foot. This is why the British Comet jets were discovered to burst and lose cabin pressurization after so many hours of flight. The passenger compartment windows were rectangular. The window vertices at a sharp 90 degree angle in the aluminium frame and aircraft skin provided a high stress concentration point. The constant high frequency vibrations combined with expanding and contracting by cabin pressurization and depressurization coupled with fuselage bending by flight loads lead to material fatigue and weakness. After a couple blew apart in mid-air killing all, the aircraft were grounded.

The entire industry was benefited by the retrofits required to mitigate. This is why all subsequent airliners by all manufacturers now have oval shaped windows.

I remember the DC-9 cabin pressurization depressurization test when I worked at Douglas Aircraft in the 1980's. It had already passed one life time (20 years flying), when the after test bulkhead blew out. They repaired it, went another life time. Then the retired the test.

It's why you don't see DC-9's and derivatives (MD-80's, MD-90's) becoming convertibles in the air. Douglas prided themselves on their structural designs. (Not saying there were not other weaknesses; 1999, an Alaska Air MD-80 crashed when its elevator screw jack fatigued and failed.) It's why you still see an occasional DC-8 flying, now re-engined with turbo fans (basically ducted fan turbine engines). That aircraft was considered to have an unlimited lifetime. You don't see any old Boeing 707's flying.

As is, I gather that you had a very small cross sectional area to carry the weight of the engine (shear) and its bending moment (engine CG times distance to engine bulkhead, in in.-oz. or its metric equivalent). I'm thinking that if you were to move the mount legs inward slightly to a straighter position, provide a radius in lieu of the sharp inner corner near where engine bulkhead screw mounts, and may be even a triangular web outboard of the screw to strengthen the 90 degree at the foot, might be enough to mitigate point fatigue. This will increase the amount of material at the foot to carry the vibration load.
YMMV (Your Mileage May Vary). Popcorn
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Re: Pee Wee Horseshoe back plate

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