Author Topic: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine  (Read 129379 times)

Offline Allen Smithee

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #105 on: November 17, 2014, 02:42:36 PM »
DU may not be so expensive as it is effectively a waste product from the current nuclear industry. It is used for shielding isotope X Ray cameras and also as ballast weights in some aircraft.

Many years ago DU was used for ballast weights in aviation (mainly for mass-balances and inertial dampers in control systems where the balance arm had to fit into the smallest possible space) but civil aviation went through a programme of eliminating DU after scares about contamination after a couple of crashes, most notably El Al Flight 1862 at Amsterdam in 1992. The concern isn't over radiation - DU (like plutonium and many other heavy metals) is extremely chemically toxic (swallow or inhale a few grammes of powdered plutonium and the poisonous effects will kill you long before the radioactivity can do any damage).

Military aviation had been eliminating DU from aircraft for many years prior to this due to concerns over the toxic hazard to maintenance crews working on battle-damaged aircraft, and these days it's extremely rare to find any in use as ballast in a military aeroplane. You still find it in some kinds of cannon rounds, of course.

AS
Quidquid latine dictum sit altum sonatur

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #106 on: November 17, 2014, 03:01:30 PM »
Very good information to be aware of Allen. Even though it is not written in Latin, the information could be profound in some situations!

JA
« Last Edit: November 17, 2014, 03:07:29 PM by strictlybusiness1 »

Offline Allen Smithee

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #107 on: November 17, 2014, 04:13:17 PM »
"I have been led to understand" (if you catch my drift) that there are two F-16s and an A-10 which suffered AAA damage to DU components during GW1 which were buried in the desert rather than repaired for this very reason.

AS
Quidquid latine dictum sit altum sonatur

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #108 on: December 09, 2014, 03:37:09 PM »
The dedicated fixture in the first two photos is used to grind crank pin holes in through hardened crankshafts. A Themac tool post grinder is used in the lathe to do this. The fixture is accurate to .0002" squareness in two axes for 6.000".  I have already stated the mechanical properties of the AISI S-7 tool steel to make these parts. The fixture could also be used to grind a one piece crankshaft's crank pin & it assures that every crankshaft is the same. The second fixture uses the hole on the left side to hold & machine the crankshaft's counter balance area when used in the milling machine. The hole in the right side has the finished ground crankshaft's OD as it's ID. It holds the crankshaft to allow the crank pin hole's ID to be ground, after mounting the fixture to the lathe face plate with dowel pins. Sometimes a Yuasa air turbine gig grinder is used for this operation.

The making of a machined retainer, with rollers, that can survive in very high HP, high RPM engines WITHOUT FAILING!

Jim Allen

Offline petertha

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #109 on: December 23, 2014, 02:43:13 AM »
Hi Jim. Sorry for the dumb question. I read through the entire post a few times & still at a loss to visualize your method to finish the chromed liner. You mentioned a few points that caught my eye & attached (what I think) to be a fixture related to the operation. But how exactly is it used? 'Grinding' (to me) conjures an image of a powered rotating abrasive tool brought against the part. Is there a motorized aspect to this? What does the business end of the grinding tool look like? How do you preserve the pre-chromed taper angle during finishing? Thanks in advance! -Peter

(reply 3)
- No lapping compounds of any kind, including non-embedding types, should ever be used to fit a piston in ABC or AAC engines
- Cylinders are machined with their taper amount & then chrome plated with approximately .0015" thick chrome/ side
- The cylinders are then ID ground
(reply 35)
- Cylinders in nitro engines such as the Nelson are honed on a Sunnen honing machine to give a fine finish in a short amount of time.
- Cylinders in my .90 cu in engine are ground & will have an even better finish than honing



Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #110 on: December 24, 2014, 02:33:04 PM »
Definitely NOT a dumb question! I'll review the incomplete explanations given & explain in detail how this can be easily done in the lathe with a tool post grinder. The accurate setting of the compound's angle can be done without the use of a sine bar & gauge blocks.The photos show the grinding of a tapered split collet's seat after setting the compound with a "taper master". A similar procedure is done for grinding the chrome plated tapered cylinder ID.

Jim Allen
« Last Edit: December 28, 2014, 05:10:55 PM by strictlybusiness1 »

Offline dieselpilot

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #111 on: December 28, 2014, 04:15:43 AM »
Jim, how much balance do you shoot for? It seems like most production model engines are pretty light on counter balance, but don't seem to suffer much.

Greg

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #112 on: December 28, 2014, 02:29:21 PM »
Greg,

It is impossible to balance a single cylinder reciprocating engine's forces. However, a partial balance can help greatly in reducing second order forces in a single cylinder engine. Some miniature engine builders have been able to use Ricardo's formula for balancing single cylinder engines. 1/2 the total reciprocating weight, which is the piston, wrist pin, "C" clips, upper end roller assembly if used & the upper half of the connecting rod, plus the total rotating weight, which is the bottom half of the connecting rod & any lower end roller assembly, should be on the crankshaft's counter balance. This works on engines that have a "hanger type piston", similar to that shown in the photo, because there are no piston bosses & a very short wrist pin. Some engines with hanger type pistons also had windows in the piston's circumference which further reduced the reciprocating weight. Many of these engines used a single ring or a Dykes type ring with chromed bores & steel roller type connecting rods.

In my engines, the piston bosses, long wrist pin length, steel "I" beam connecting rod, & the bottom end roller assembly make the use of Ricardo's formula impossible. The counter balance weight becomes much to heavy for very high engine RPM's (26,000 to 35,000) & will cause severe vibration. Through trial & error, I found that 33 to 35% of the total reciprocating weight, plus the total rotating weight, will make the engine run smoothly in it's operating range, 12,000 to 32,000 RPM.

It should be noted that a heavier crankshaft counterbalance does increase the crankshafts overall inertia & this will make the engine run smoother in the high rev range because there will be a decrease in the engine's cyclic speed variation. The cyclic speed variation which is reduced by the greater inertia, gives more effective time area numbers at high RPM's. This is clearly demonstrated in several SAE papers on the subject & dynamometer tests.

This explains in part why production model engines are using light weight counter balances. They also have a problem with their short connecting rod center distances. The connecting rods in my engines contain no bushings & no roller assemblies in the upper ends, which allows the connecting rod to be lengthened to it's maximum length. A 2 to 1 ratio; the connecting rod's center distance compared to the engine's stroke is considered optimum. My connecting rod itself may be heavier, but the reciprocating weight is much lower ( smaller piston bosses) & the piston slapping is reduced to a minimum.

Jim Allen
« Last Edit: December 28, 2014, 04:42:37 PM by strictlybusiness1 »

Offline Roger B

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #113 on: December 28, 2014, 05:47:46 PM »
As always thank you for posting all this knowledge and experience on here. Much is outside my current experience but there is a lot here to learn  :ThumbsUp:  :ThumbsUp:
Best regards

Roger

Offline dieselpilot

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #114 on: December 28, 2014, 11:16:56 PM »
Thank you for your reply. I understand the fundamentals of balance, I was just looking for some empirical data so that certainly helps. Based on some CAD models of RC engines, I found a few to be about 20-25%. Balance comes in different flavors for single cylinders and some of the resulting feel depends on engine mounting orientation and rigidity, and also the ratio of the imbalance forces vs the mass of the entire engine. As you say, less reciprocating mass always wins.

Greg

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #115 on: December 29, 2014, 01:28:57 AM »
Greg,

Rigidity is very important, especially for engines with cantilevered type crankshafts. If you were to put a strobe on the front end of an engine turning 30,000 RPM, you would be surprised at the large amount of movement, in a circular path, that is taking place, even with the engine bolted down to a very solid metal mount. Some engines, such as the Nelson FAI speed engine,  have an additional set of mounts on the front end to prevent this movement. I use a hardened steel front end & a machined crankcase on my engines to eliminate this unwanted movement. The photos shows the 6 mounting bolts used on the Nelson FAI engine. The engine also uses a balanced flywheel to counter balance the very large induction window in the crankshaft.

JA

Offline Niels Abildgaard

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #116 on: December 29, 2014, 06:37:43 AM »
Hello Jim Allen

Thank You for a most fantastic thread with explanations and pictures.
I am a little obsessed with replacing all vibrating single cylinder two strokes with 90 degree ,common crankcase twins.
Is there anything in the racing rules, You obey ,that makes this illegal?

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #117 on: December 29, 2014, 01:58:38 PM »
Niels,

Engine classes are determined by displacement,  not by the number of cylinders, either common crankcase or geared. A 90 degree, common crankcase twin could be made to run with very little vibration. I think another consideration would be the wide width of such an engine.

JA

Offline Niels Abildgaard

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #118 on: December 29, 2014, 03:18:19 PM »
Jim

The interesting thing here is mass.The flywheel and crank can be half mass  of a single.Port areas are up,piston speed less for same rpm if geometry ,ie bore/stroke ratio is the same and the charge air pumping is done by two resonant exhausts instead of one.To See if this kind of reason is valid it could be fun to scheme one as competitor to the configuration You have now,being State of art I would say.
Have You ever considered releasing drawings to us two stroke lovers?
The real stumbling block is to make a non overhung crankshaft that stands rpm on the far side of 500 rpm and still allowing this enormous flow of air and fuel.I will try to reread the whole thread and see how much dimensions You have given already.
It has been a wonderfull mental journey until now.
A happy new Year to You and all Your admireres.

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #119 on: December 29, 2014, 03:45:59 PM »
Hi Jim. Sorry for the dumb question. I read through the entire post a few times & still at a loss to visualize your method to finish the chromed liner. You mentioned a few points that caught my eye & attached (what I think) to be a fixture related to the operation. But how exactly is it used? 'Grinding' (to me) conjures an image of a powered rotating abrasive tool brought against the part. Is there a motorized aspect to this? What does the business end of the grinding tool look like? How do you preserve the pre-chromed taper angle during finishing? Thanks in advance! -Peter

(reply 3)
- No lapping compounds of any kind, including non-embedding types, should ever be used to fit a piston in ABC or AAC engines
- Cylinders are machined with their taper amount & then chrome plated with approximately .0015" thick chrome/ side
- The cylinders are then ID ground
(reply 35)
- Cylinders in nitro engines such as the Nelson are honed on a Sunnen honing machine to give a fine finish in a short amount of time.
- Cylinders in my .90 cu in engine are ground & will have an even better finish than honing

Peter,

To begin, I will describe a method used to set the compound accurately & quickly without the use of gauge blocks & a sine bar. The "taper masters" shown in the photos are machined with a length that is at least 2X the actual length required, but sometimes 20X the actual length required. Each, marked for location, "taper master" can be reset while being held in a precision grade collet & indicated at both ends for concentricity. A .00005" indicator is used for this operation & the setting of the compound's angle. A Themac tool post grinder, turning at 26,000 RPM, is mounted to the compound to grind the cylinder's ID taper. The grinders stone is diamond dressed after mounting.  The fixture, mounted in a collet,  allows for precise centering of the cylinder by indicating the cylinder's OD.

The photos show the same procedure which in this case is done for precision split collets used to clamp a cable drive to the engine's flywheel. The compound is set with a "taper master" as described in the procedure above. Again the stone is dressed after mounting the tool post grinder. Since the ID of the collet is much smaller then the ID of the cylinder, the grinding stone's RPM is raised to 40,000. I prefer to use a Themac tool post grinder because it is the only tool post grinder that allows precise centering of the grinding stone's center, with its unique rotating mounting of the spindle.

Grinding gives a superior finish to the chrome that cannot be achieved with honing, even if the cylinder is finished honed with 95 grit stones.

Jim Allen
« Last Edit: December 30, 2014, 12:29:38 AM by strictlybusiness1 »

 

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