star mylar reeds

i thought i saw some on ebay a while back, how come bernie doesn't make them, if i had a sheet of mylar i would make them myself.

If Bernie make star mylar reeds , sure I will buy some.

I have a couple in my engines. They run nice. I honestly can't tell the difference between an oblong reed and a star reed for sport flying.

Phil

If I were to make an un-educated guess... the "star" reed has a symmetrical shape. When the crankcase "de-compresses"; I would think that the "star" reed pulls away from the seat with a more equal pressure; spread-out around the E-clip/holder. The intake-charge also enters theoretically... through 4 radial points; equally spaced at 90 deg. apart... as compared to the semi-circular reeds' 2 points of entry at 180 deg. apart.

When the crank-case "pressurizes"; and the reed "seats"; there's no difference in the action between the two styles... (sealed is sealed...)

Does this matter? It "is" simply a metering valve, with one purpose... and both styles work well.

"If"... the intake charge where to take a "direct path" to a symmetrical combustion chamber; I'd be willing to bet that the "star" reed would perform better. That's not the case with our Cox engines. The intake fuel/air charge takes a 90 deg. turn; while also being influenced by the rotation of the crank and connecting rod, on it's way to the intake ports.

This concludes my un-educated opinion...

roddie, im just going off what Paul Gibeault said, but what you have said makes sense, he also says that the star reeds work better with e-clips, wonder why? also does anybody know where i could get some raw mylar sheet at?

looperdude wrote:.......also does anybody know where i could get some raw mylar sheet at?

An old 3.5" floppy disk will give you a source of two thicknesses of mylar.  There is a clear thicker mylar that was used to apply wiper pressure to the liner in the diskette housing -- peel the liner away from the plastic case to reveal it.  The diskette itself may also be used and is of a much thinner material.  You can run with or without the oxide layer on the diskette --- a cotton swab and acetone will wipe the oxide away leaving a completely clear plastic.

I've been using the wiper material, but many folks use the diskette as well.

There is a sticky thread at the top of the forum that covers making reeds.

pkrankow wrote:I have a couple in my engines.  They run nice.  I honestly can't tell the difference between an oblong reed and a star reed for sport flying.

Phil

We did some tests on both before and the results were basically a wash between the star-shaped and more common rectangular ones. While it might be interesting from a nostalgic perspective it wouldn't make sense to manufacture any as the die would be much too expensive to produce something that produces essentially similar operating results.

Matt

I have read, and I am sorry I don't have a source, that the oblong reed is supposed to have more open area compared to the star reed.  I am unsure how this conclusion was obtained.  

I would expect the calculation would be the exposed edge area underneath the bowed out reed.

I would also expect that when the total area becomes greater than exposed area at the venturi area the gain of MORE area becomes more moot.  Yes, more area would mean less restriction.

Phil

How far does the reed bow out anyways?

Phil

pkrankow wrote:How far does the reed bow out anyways?
Phil

At 16K rpm, the reed pulses 267 times per second.  Given that a single pulse involves the reed moving away from the reed seat then back again to seal, it means that the mass of the reed is moved 534 times a second.  Consequently, the bow is likely significantly less than the thickness of a thin mylar reed.

If the mass of a star reed is less than that of a rectangular reed, theoretically, you could drive the star to a higher rpm before reed float became a problem.  Secondly, it would seem that a very thin, flexible reed could be pushed into higher rpm ranges due to less mass and the ability of a flexible reed to flutter only around the seat without having to lift the entire reed, i.e. reed ends, off the reed housing.  Consequently, the inertia of the reed mass and the lag in opening and closing being tied to crankcase vacuum/pressure are probably the limiting factors then comparing performance of the reedies to the TD's.  Given that piston/cylinder combinations can be switched between rotary and reed valve engines, it comes down to breathability.

I took some thickness measurements, with a precision vernier/caliper on the reeds that I have.

"Newer" (oval) style reed:

"steel" = .002"
"mylar"=.005"
"teflon"=.010"

"Older" (star) style reed:

"copper"=.001"
"mylar"=.005"

All I know is I would like to find one reed that can move freely without having to shave down the circlip.

Ron

looperdude wrote:roddie, im just going off what Paul Gibeault said, but what you have said makes sense, he also says that the star reeds work better with e-clips, wonder why? also does anybody know where i could get some raw mylar sheet at?

Cox mylar reeds (oval and star) measure .005" in thickness. Here's a link to buy a sheet. CHEAP... you'd have enough for a lifetime!

http://www.plasticareinc.com/Merchant2/merchant.mvc?Store_code=P&Screen=PROD&Product_Code=I8900

Cribbs74 wrote:All I know is I would like to find one reed that can move freely without having to shave down the circlip.

Ron

I've never heard of that, what do you shave,

Jim

The circlip itself. The reed is supposed to be able to move 360 degrees once clipped in place. Rarely does that happen without me having to massage the circlip.

Ron

Cribbs74 wrote:The circlip itself. The reed is supposed to be able to move 360 degrees once clipped in place. Rarely does that happen without me having to massage the circlip.

Ron

Really? What kind of performance boost do you get from that?

Jim

Mark Boesen sez-
High-Power REED VALVES:
The original Cox copper/beryllium reeds tend to 'float' at
~17,000 rpm and so they are not optimal for top performance. In a reed valve engine, top
performance is only attainable by using the clear Mylar reed. I find the cross shape better in
tanks that use a 'G-Clip reed retainer wire and the rectangular ones better in the tanks
using the nylon retaining cap. This last production change allows reed engines to be on par
with “Tee Dee’s” in performance! My test bench results indicate that 24,000+ rpm is
achievable for steady-state running with such reeds.

What about 'other ' reed materials and shapes? Cox engine designer Larry Renger prefers
the Cox stainless steel reed over the Mylar reed. My main concern with the steel reed is
that it wears the anodizing right off the mouth of the venturi tube, although the rpm seems
to be the same. I've tried other reeds made of thinner steel, floppy disc material, etc. and so
far haven't found anything better. One Australian made metal reed was indeed 300 rpm
faster, but it broke away after only a few minutes of running. Teflon reeds may or may
not work as well. I've not found them to be any faster, and sometimes worse.

I dropped a wire drill bit in the venturi of a baby bee tank and measured .035 deflection.

Phil

[/quote]Ron mentioned having to "shave" down his E-clips; to get his reeds to move freely 360 degrees. I don't think that they were "meant to"... and this could possibly be compromising their proper sealing and hence... "operation". The old "Copper" reed @ .001" may have had the clearance to do that... but Cox didn't design thinner wire E-clips for their thicker mylar reeds... did they?[/quote]
Wether they are meant to or not they don't run so well when pinched. When Cox changed to the newer style holder it allows quite a bit of movement of the reed. 360 degrees and in and out.

I could very well be mistaken, but the mylar reeds came out right around the time as the newer reed holder.

Without trying to sound too pompous I shaved the clip in my tach race engine so it was able to rotate and it didn't hurt the way it performed.

Ron

Wire retaining clip on a brass star reed. The deflection was measured at the point the reed touches the cross wire, which was about the force required to hold things steady to measure, so almost no extra force. The reed is strong enough to hold the wire drill bit without deflecting.

I ran some numbers last night while exhausted and came up with the reed seat area of a cylinder between the deflected reed and the seat is about 3x the area of the venturi constriction, using the drilled out number of 5/64 for the venturi. Deflection assumed the maximum .035 inch.

Even using a very simple projected area the oblong and star reeds allow at least 2/3 this area to be open to the case.

Unless things get drilled out very oversize the constriction will be the venturi as it should be, and not the reed. However this is before assigning flow values to all these various cross sections. The flow impinging on the reed disrupts it greatly.

I did not measure the thickness of the reed itself.

Phil

ive been drawing up some different shape reeds on paper, i was just wondering if a tri-tip star reed would be any better than a regular 4 tip star reed, hopefully you guys will understand what im saying.

Ron mentioned having to "shave" down his E-clips; to get his reeds to move freely 360 degrees. I don't think that they were "meant to"... and this could possibly be compromising their proper sealing and hence... "operation". The old "Copper" reed @ .001" may have had the clearance to do that... but Cox didn't design thinner wire E-clips for their thicker mylar reeds... did they?

Wether they are meant to or not they don't run so well when pinched. When Cox changed to the newer style holder it allows  quite a bit of movement of the reed. 360 degrees and in and out.

I could very well be mistaken, but the mylar reeds came out right around the time as the newer reed holder.

Without trying to sound too pompous I shaved the clip in my tach race engine so it was able to rotate and it didn't hurt the way it performed.

Ron

Roddie,

Somehow I managed to modify your original post to respond to your writing. In doing so I ended up erasing all that good info you wrote. I must have hit "edit" as opposed to "quote"

I am terribly sorry for the mistake and it was totally un-intentional. I gave you a couple rep points in an attempt to make up for my blunder. Sometimes being able to edit others posts is not a good thing. Again very sorry. I tried to restore it back, but no dice.

Ron