Propeller Cupping????

[dwilfong]

Jim here is a running log of a ABC2818 in my SGX with a 1.01 in it full race setup.

this was a death run as you can see from the temp. could not cach up to the pipe with the needle.

with 300000RPM behind this all I can say is WOW. Give it a spin I thing you will like it.

On the next build I am setting the pipe for 260000RPM will see if it will live.

David

David, exactly what failed in the motor? How can you run the engine at it's mechanical limit & expect it to survive? No amount of fitting, exotic timing, exotic fuel, special pipe, etc. will mean anything until you find a way to make the engine mechanically, 100% reliable at the RPM you want to run at. With all highly stressed mechanical devices there must be a cushion built into the design.

Jim Allen

Hole in the billet piston.

that is why I started the piston fit thread.

It was my own fault that it happen. got caught up in the moment and did not feed it enough fuel.

Also cut the crow to much to set the timings was at .100

I am still new to boating and have much to learn about control with the needle.

I still Have my dumb a$$ moments.

David

 

[Jim Allen]

David,

Go to http://gallery.intlw....php?cat=10141; page two; The high price paid for engine building; fourth picture. Read carefully what is written below the picture & we will discuss this phenomenon of CMB engines melting holes in pistons. I think it was Joe W that said maybe the problem is fixable. I'm sure he is correct!!

Jim

 

[Jim Allen]

Mark,

E-mail your address & I'll check on the postage.

Jim

Mark,

I'll put the book in the mail today.

Jim

 

[MarkTibeau]

Mark,

E-mail your address & I'll check on the postage.

Jim

Mark,

I'll put the book in the mail today.

Jim

Thanks Greatly, Jim!

 

[hugh hargett]

Mark there is also a great discussion on prop cupping on Jims Boat dock go check it when you get the chance. They are truly sharing there.

 

[MarkTibeau]

Mark there is also a great discussion on prop cupping on Jims Boat dock go check it when you get the chance. They are truly sharing there.

Thanks for letting me know. I'll check it out.

Mark.

 

[Jim Allen]

There is an article printed by HydroComp, Inc. which was presented at the propeller/shafting symposium, Sept 1997. Its title is "Small Propeller Cup: A proposed Geometry Standard and A New Performance Model". This is a very simple, straight forward reading on 3 pages, about propeller cup, how it can be measured & how it effects propeller performance.

Jim Allen

 

[Jim Allen]

One way to determine if the propeller being used on the boat is the optimum, is to have the ability, if possible, to test additional props with the same shape, the same blade area & the same blade rake, but different leading edge & trailing edge pitches.

Since all cast propellers have their blades permanently fastened at their roots, to their hubs, with some helical angle, determined by the pitch of the blade, how does the modeler change the pitch from the blade's root out to the blade's tip? Is this impossible or is there a simple method which allows the twisting of the entire blade, except at the root, with corrections made at the blade's root by carving & grinding?

Jim Allen

 

[dwilfong]

Jim I was told of a way to change the whole blade at the same time.

take a square container and fill it with bondo then take the prop covered in STP with a drive shaft made of a 1/4/' bolt and drive dog. slip this in the bondo and when set twist the shaft.

Have not tried this yet but sounds like it would work good.

David

 

[lohring]

One way to determine if the propeller being used on the boat is the optimum, is to have the ability, if possible, to test additional props with the same shape, the same blade area & the same blade rake, but different leading edge & trailing edge pitches.

Since all cast propellers have their blades permanently fastened at their roots, to their hubs, with some helical angle, determined by the pitch of the blade, how does the modeler change the pitch from the blade's root out to the blade's tip? Is this impossible or is there a simple method which allows the twisting of the entire blade, except at the root, with corrections made at the blade's root by carving & grinding?

Jim Allen

Now you are forcing me to reveal the "secret" involved in raising the G! hydro record from under 100 mph to 106 mph with basically the same engines. At first we just bent the whole prop blade like described by David and ran more rpm. That got Mike Bontoft's Quickdraw powered rigger to 110 with around 23,000 rpm, but Zenoahs couldn't develop useful power much over 19 - 20,000 rpm. The problem with this approach is that the blade at the hub is actually running at a negative angle of attack. That results in big slip numbers on the outer blade pitch. Then Mark Grimm ran a custom high pitched prop by Joerg (the 140 mph electric hydro record holder) Mrkwitschka and got the record over 100. Mike came back the next year with a standard 2170 that he cut the blade at the hub from the leading edge almost through the trailing edge. He then bent and re-brazed the blades at a higher pitch. That was good for around 106 mph. Mark saw this and built some similar props to recapture the record.

 

Now the question is how much progression do you need from the leading edge to the trailing edge and what should the leading edge pitch be? Should this progression come suddenly as cup on the trailing edge, or should the pitch gradually increase? How does rake enter into all this? What actually matters?

 

Lohring Miller

 

[hugh hargett]

Not to be funny but Jims been using pithching blocks for over 40 years. With Ed Kalfus he casted custom props about that long ago too. Really need to FULLY check out his gallery. The mans got some stuff.

Hugh

 

[dwilfong]

One way to determine if the propeller being used on the boat is the optimum, is to have the ability, if possible, to test additional props with the same shape, the same blade area & the same blade rake, but different leading edge & trailing edge pitches.

Since all cast propellers have their blades permanently fastened at their roots, to their hubs, with some helical angle, determined by the pitch of the blade, how does the modeler change the pitch from the blade's root out to the blade's tip? Is this impossible or is there a simple method which allows the twisting of the entire blade, except at the root, with corrections made at the blade's root by carving & grinding?

Jim Allen

Now you are forcing me to reveal the "secret" involved in raising the G! hydro record from under 100 mph to 106 mph with basically the same engines. At first we just bent the whole prop blade like described by David and ran more rpm. That got Mike Bontoft's Quickdraw powered rigger to 110 with around 23,000 rpm, but Zenoahs couldn't develop useful power much over 19 - 20,000 rpm. The problem with this approach is that the blade at the hub is actually running at a negative angle of attack. That results in big slip numbers on the outer blade pitch. Then Mark Grimm ran a custom high pitched prop by Joerg (the 140 mph electric hydro record holder) Mrkwitschka and got the record over 100. Mike came back the next year with a standard 2170 that he cut the blade at the hub from the leading edge almost through the trailing edge. He then bent and re-brazed the blades at a higher pitch. That was good for around 106 mph. Mark saw this and built some similar props to recapture the record.

 

Now the question is how much progression do you need from the leading edge to the trailing edge and what should the leading edge pitch be? Should this progression come suddenly as cup on the trailing edge, or should the pitch gradually increase? How does rake enter into all this? What actually matters?

 

Lohring Miller

Lohring

Scott told me about this cut and weld prop Mark came up with. He is also the one that told me about twisting the prop in bondo.

If you look at the numbers on the ABC 2920 thy are wrong. the prop was miss casted and it is much more than the published numbers.

It has more pitch and progression than the 3020.

This will be the next test prop.

I have found that rake will suck the prop down at high speed and pull the front of the boat up too much resulting in blow off with a lite set up.

15 to 17 deg seams to be the sweet spot. 20 is to much in my set up. will pull up nice but it will blow over.

David

 

[Tom Foley]

David,

we will discuss this phenomenon of CMB engines melting holes in pistons. I think it was Joe W that said maybe the problem is fixable. I'm sure he is correct!!

Jim

Yes , open the combustion chamber up , feed it fuel and MAKE A PISTON WITH A THICKER DOME LIKE THEY USED TO BE !

 

[dwilfong]

Jim here is a sample of the 2818 prop I am running stock pitch and cup.

This is how I map the prop.

The % are from the point where the blade meets the hub out to the tip.

The Deg are from the trailing edge to the lead in.

This is how I get the measurements.

Now how do you determine the progression? in 5 deg increments? or the average of 10 deg at different sections? or 15 Deg at different sections.

As you See there are many ways to interpret this info.

So what matters?

With this mapping you can break the prop down to any segmentation you want for interpretation.

David

 

[hugh hargett]

http://www.hydrocompinc.com/knowledge/publications/PropellerCup.pdf

 

[Jim Allen]

Jim I was told of a way to change the whole blade at the same time.

take a square container and fill it with bondo then take the prop covered in STP with a drive shaft made of a 1/4/' bolt and drive dog. slip this in the bondo and when set twist the shaft.

Have not tried this yet but sounds like it would work good.

David

David,

The reason I posted the two questions that were asked is not because I did not know the answers, but to elicit answers from other boaters who had valid information which should be shared & not kept a secret.

In my photo album dated July 25, 2005, under the title "Propeller Details" are sample pitching blocks, with explanations, developed & used by myself & Ed Kalfus in 1973. All pitching blocks had two dowel locating pins. Propellers could be easily twisted after they were annealed. A correction of the hub insertion angle was required. Also shown is one of the silicone rubber molds made from a steel master. Waxes made in the molds were sent to an investment casting house. We used C-20 beryllium copper & 17-4PH stainless steel for props. Both materials were hardened, which gave very thin, sharp, unbendable props. Each propeller with a particular pitch had a particular hub insertion angle. Doing this made it easily possible to make propeller blades with true helical pitch, & therefore true pitch, from the blades root out to the blades tip.

Jim Allen

Note: I did not cut any C-20 beryllium copper blades to be brazed because temperatures exceeding 1300 deg F could cause the beryllium to leach out. Beryllium copper alloys cannot be hardened without the presense of beryllium, typically 2%.

 

[Jim Allen]

Jim here is a sample of the 2818 prop I am running stock pitch and cup.

This is how I map the prop.

The % are from the point where the blade meets the hub out to the tip.

The Deg are from the trailing edge to the lead in.

This is how I get the measurements.

Now how do you determine the progression? in 5 deg increments? or the average of 10 deg at different sections? or 15 Deg at different sections.

As you See there are many ways to interpret this info.

So what matters?

With this mapping you can break the prop down to any segmentation you want for interpretation.

David

David,

Look carefully at the two pictures on the right hand side. The top blade in the photo has 0% progression. The leading edge pitch, the middle of the blade pitch & the trailing edge pitches are are all the same, 21". When I attempted to use blades pitched this way, in a surface piercing setup, they had very high slip numbers.

The bottom blade has a total progression amount of 21%. 23"-19"=4"; 4"/19"=.2105 or 21%. What Andy Brown was saying in the post that I previously quoted is; the progression amount changes with the type of hull; the amount of blade area; the blade shape; the blade rake & the amount of drag the hull in question has.

The pitch gage I use measures the progression amount directly, at any radius point, from the leading to the trailing edge. This cannot be done with this type of gage design unless a minimum arc of 72 deg can be swung at one time. The Prop Tracker, prop gage, can read in an arc of 360 deg. (http://www.proptracker.com/contact.asp) The gage I use is this same design in reverse. The propeller is stationary & the mounted arm is rotating on the same 1/4" OD center post that the prop is mounted on. At the base, fastened to the center post, is a stationary drive dog. Since my blade is mounted on a stationary drive dog, readings taken at any radius, after rotating the blade must start & stop at the same points (blade orentation) & at the same depth (blade rake angle).

Jim Allen

 

[Jim Allen]

"Yes , open the combustion chamber up , feed it fuel and MAKE A PISTON WITH A THICKER DOME LIKE THEY USED TO BE !"

I do not disagree that this will solve the problem of burning a hole through the piston crown. However, what is really the problem with this large bore engine? NO LARGE BORE ENGINE CAN RUN AT A HIGH COMPRESSION RATIO WITH HIGH NITRO AMOUNTS WITHOUT ADEQUATE PISTON COOLING! Since the piston really only touches the liner intimately at the top of the stroke, how is the piston to be cooled? If the main transfers intake geometry has not been designed to direct the incomming charge across the piston crown, there will be no piston crown cooling! The result will be what has been demonstrated with holes through piston crowns when using 50% nitro fuels. I wonder what would happen if these engines were to use the 65 or 80% nitro fuels that I used?

Suggested reading; https://www.intlwaters.com/gallery/index.php?cat=10141; page two; the high price paid for engine building; fourth picture. Read very carefully.

Jim Allen

 

[Tom Foley]

Yes Jim your gallery is amazing with all you've designed and created over the years . I simply can't imagine the hours you've put in the shop over the years . Don't want to get away from your thread on cupping to dwell on the CMB piston deal just saying that very little changed over the years except for bore size then all of a sudden the holed pistons start showing up 2 or so years ago . Look under the dome and instead of material thickness remaining constant following the dish in the dome , they milled it off flat underneath making the dome thinner causing the problem to show up more readily . I run 60 % -18% oil in my 101 and have only had this problem once on an EVO II 90 in a Twin trying to bring it back on one engine . I have two pistons being fixed at my shop right now .

 

[Jim Allen]

Yes Jim your gallery is amazing with all you've designed and created over the years . I simply can't imagine the hours you've put in the shop over the years . Don't want to get away from your thread on cupping to dwell on the CMB piston deal just saying that very little changed over the years except for bore size then all of a sudden the holed pistons start showing up 2 or so years ago . Look under the dome and instead of material thickness remaining constant following the dish in the dome , they milled it off flat underneath making the dome thinner causing the problem to show up more readily . I run 60 % -18% oil in my 101 and have only had this problem once on an EVO II 90 in a Twin trying to bring it back on one engine . I have two pistons being fixed at my shop right now .

As I have already said & demonstrated many times, the problem is not the thickness of the piston's crown. The piston shown with the metal melted out happened on the first .80 cu in engines first dyno run. Its piston, made of Mahle 138, was more than 125" thick. This same problem occured in Frank Garzone's .60 cu in speed plane engines. Frank was forced to cast his own pistons, with a reinforcing web, to give a thickness in the crown of .180". The piston was not holed, but there was metal melted out of the center on almost every run. The engine used 70% nitro-20% oil-10% proplylene oxide & a mini pipe.

Jim Allen