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Author Topic: Pressure tap Cox reed engines  (Read 1567 times)

Offline Larry Renger

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Pressure tap Cox reed engines
« on: November 11, 2009, 01:06:26 PM »
Thought I would separate this from the model's thread.

I made .004, .006 and .008 restrictors.  Just a 1/2" length of aluminum tube, scored around the ends for better hose retention and swaged in the middle over the appropriate size wire.  These can be inserted into the pressure line and exchanged for experimental purposes.

I tried running the setup this morning and was getting a run on the .004, but the engine was vibrating.  I changed the prop to get one in better balance.  I also wanted to try the .006 restrictor.

At that point, I couldn't get the engine to do more than run the prime.  HMMM!  Turns out that the tap in the crankcase had come out!  JB Weld next time. 
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Robert McHam

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Re: Pressure tap Cox reed engines
« Reply #1 on: December 23, 2009, 07:06:00 AM »
This does deserve a separate thread but you need the pictures here too!

Just wondering if there has been more to report since your last post.

Robert
Crop circles are simply open invitations to fly C/L!

Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #2 on: December 24, 2009, 12:04:24 PM »
The little restrictors I mentioned work just fine on the AP Wasp.  I haven't gotten back to the reed valve engines, but will when I get the Blackhawk Bullet done, as that is the power source I plan for the model.
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #3 on: December 27, 2009, 05:58:25 PM »
Here are photos of the restrictors and the pressure taps.  This is 1/8" soft aluminum tubing.  The wire is .010 steel, the end has been deburred.  Use round nose pliers, or you either won't get enough crushing or you can cut the whole thing in pieces.  As you see, I am experimenting with a couple of different places to tap the case.

Note that I have scored the ends of the tubes by using a #11 blade.  This helps retain the pressure hose.  I have had it blow off without the scoring to provide traction.
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #4 on: January 03, 2010, 07:30:22 PM »
Got some runs in today and an actual flight.  Not there yet by a long-shot, but there is some hope.  I tried the .004, .006 and .008 restrictors, with the performance and stability improving as I went larger.  I'll make .010 and .012 size for future tests.  With the smallest orofice, I couldn't get fuel draw enough to run at all.  The .006 would surge at first then go lean, open the needle and recover, but surge and go lean again.  Never did hit a happy setting.  The .008 seemed stable enough to try a flight, but it again didn't prove stable enough for stunting and would do outsides OK, but died on insides. 

More as I learn....
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Mark Misegadis

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Re: Pressure tap Cox reed engines
« Reply #5 on: February 05, 2010, 03:44:58 PM »
Larry,

Pressure: What about surgical stainless steel tubing. The kind that is used for needles? There are various sizes that can be purchased. The Drag Racing slot car guys use if for Frame Rails and its very tough. Probably a better idea once you have narrowed down the size that your after. Just a thought.


Is that a Brass flywheel on that engine?  If so that will make a difference on its own as to how that engine will run. A friends father who flew control line years ago and now it part of a group that machines engines from scratch suggested flywheels on Cox engines. I've thought about it and recently noticed that the Cox Super Stunter had one.


Mark

Offline minnesotamodeler

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Re: Pressure tap Cox reed engines
« Reply #6 on: February 05, 2010, 06:56:08 PM »
I picked up a tip from...someone, can't remember now who or where, of using threaded inserts for restrictors:  I cut about 3/16" length of 2-56 threads and worked it up inside the fuel line...air follows the threads around for pressure but fuel, coming the other way, tends not to make it; sort of a semi-one way valve.  Pressure can be adjusted by the length of the threaded portion. Have installed one but haven't run it as yet; does seem to work as advertised.
--Ray 
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Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #7 on: February 06, 2010, 01:36:59 PM »
The brass item on the front of the engine is for noseweight.  Not my favorite thing, but workable.  The aluminum tubing forms over wire to make a perfectly round orofice.  Brass doesn't, and I don't expect that stainless would be even that good.  I want to be able to precisely control orofice size, and my current system does just that by varying the wire size.  I just need to get back to testing. 
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #8 on: February 15, 2010, 05:59:16 PM »
I got a decent run on the engine yesterday!!   #^  It looks like the .010 restrictor is about right.  The needle is touchy, but that is no surprise.  Also, I was running 10% nitro fuel, as I had forgotten to fill up on higher powered stuff.  Now for flight testing.
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline Bill Heher

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Re: Pressure tap Cox reed engines
« Reply #9 on: February 24, 2010, 05:55:59 PM »
Larry,

Did you run both versions of the pressure tap locations shown in the photo?  Was there any difference between the 2?

Did you put the one in the extrusion rail / screw boss because there was more material there to hold the tube?

OK - last one - What if you drilled the hole right at the end of the installed case screw thread depth, and had the screw lock it in place mechanically + the JB weld.
Bill Heher
Central Florida and across the USA!
If it's broke Fix-it
If it ain't broke- let me see it for a minute AMA 264898- since 1988!

Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #10 on: February 25, 2010, 10:33:40 AM »
Both locations seem to work, no problems with blowing out since I am using JB weld.

I still am not able to get a good run in the air.  If anyone else is working on this, I would love to hear how you are doing.
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline John Rist

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Re: Pressure tap Cox reed engines
« Reply #11 on: February 25, 2010, 01:40:42 PM »
Both locations seem to work, no problems with blowing out since I am using JB weld.

I still am not able to get a good run in the air.  If anyone else is working on this, I would love to hear how you are doing.
I guess I am missing something here.  I assume you want pressure on the fuel tank to get a consistent run throughout the varring G-force of a stunt pattern.  I have run TDs on pressure with great sauces. As I understand it the TD uses the rotation of the crank to time the pressure making sure it always positive.  I have also tapped the back plate of bigger engines for pressure.  But in this set up I used a check valve to maintain positive pressure. Would you not need a check valve in your set up.  And why would you want to restrict the amount of pressure?
John Rist
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Offline Robert McHam

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Re: Pressure tap Cox reed engines
« Reply #12 on: February 25, 2010, 04:00:23 PM »
I now understand the problem here! However I don't know how to fix the problem.  ???

I was explaining to a friend how the reed valve works the other night and I see exactly how this fits together with thiis problem.
When the piston goes up it creates a vacuum in the crank case which unseals the reed valve and sucks the fuel air mixture into to the crank case. It needs all of this vacuum and the volume as well to get enough of this in there.
When the piston goes down it creates pressure in the crank case which seals the reed valve and forces the fuel air mixture past the piston via the cylinder port/s. It needs all this pressure and volume to achieve this properly it seems.  This last statement very well may not be true for the Tee Dees can run using crank case pressure and do so successfully. More on this in a moment.

By 'stealing' some of this pressure the reed may not seal completely nor deliver its full volume of fuel air mixture into the cylinder as it should.
Attached is a picture of a Cox twin cylinder that can never be made to run because it will never create any vacuum in it's crank case so long as the pistons are alternating. On the other hand, if the pistons were to made to fire at the same time and down stroke at the same time, there would be vacuum in the crank case as it should have

When you tap a Tee Dee engine, the difference is this seal is made by the crank shaft! As for the volume, I mentioned before, this doesn't seem to hurt a tapped Tee Dee in the least so I think the problem is in the reed/seal department.

Larry, the RR1 is rather a valuable engine and I have always wanted one but have not been able to afford one. Do you think the rotor seal in this engine would work better? I think so.

Robert
Crop circles are simply open invitations to fly C/L!

Offline John Rist

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Re: Pressure tap Cox reed engines
« Reply #13 on: February 25, 2010, 10:45:28 PM »
I don't think a pressure tap steals much pressure from the crankcase especially if you use a check valve. once the pressure in the tank equals crankcase pressure very little air is needed to maintain the pressure.  You only have to replace the volume of fuel used on that revolution.  The big problem is tanks air leaks and getting it started in the first place.  I have found that an electric starter works best with a pressure tank setup.  It allows the pressure to build so the engine will start.
John Rist
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Offline Larry Renger

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Re: Pressure tap Cox reed engines
« Reply #14 on: February 26, 2010, 10:40:41 AM »
The problem I am having is not to get the engine to run, it is to get it to run through a full tank without having to fiddle with the needle valve.  It runs great when it runs.  The restrictor is used to prevent generating so much pressure in the tank that the needle is too sensitive.  Also, you don't want any back flow of the pressure when the crankcase is at negative pressure while drawing fuel and air.  Since the net pressure in the case is positive over the cycle (gotta be, or you would never deliver fuel to the cylinder) the key is to smooth out the cycle.  Thus a restrictor.  I will try a check valve.
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!


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