Author Topic: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine  (Read 80685 times)

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #390 on: May 27, 2019, 04:30:29 PM »
I don't understand what's going on in the last two pictures.  Also, we never needed turn fins on cats.  However, David Preuse thought that a turn fin improved the cornering on the XTR-21 tunnels we made.  No others ran it.

Lohring Miller

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #391 on: May 27, 2019, 06:44:13 PM »
I don't understand what's going on in the last two pictures.  Also, we never needed turn fins on cats.  However, David Preuse thought that a turn fin improved the cornering on the XTR-21 tunnels we made.  No others ran it.

Lohring Miller

The last two photos show the .020" thick cardboard drawing material used to make templates for cutting the .078" thick carbon fiber bulkheads. This became necessary to get a close fit for the bulkheads which are not exactly the same inside the hull. I posted photos of the right & left side. I don't know if a turn fin is needed for positive turning of a cat, but I intend to find out.

Jim Allen
« Last Edit: May 27, 2019, 06:47:32 PM by strictlybusiness1 »

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #392 on: May 28, 2019, 08:04:33 PM »
This is how I build the die set that will be used to disassemble & assemble three piece crankshafts. It will give an accuracy of total runout from end to end of + or - .0005". Final trueing of the crankshaft will be done in a four jaw chuck to a total runout of  + or - .0001". I true the crankshaft to this tolerance because it is mounted between 4 bearings, two on each side. Alignment of the four bearings is checked with a precision ground hardened shaft shown in the first photo.

Jim Allen

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #393 on: May 28, 2019, 11:21:22 PM »
These photos show what the adjustable weight system looks like after the front & rear bulkheads have been glued in place with JB Weld Quick. This system will also lower the CG of the hull because the weights are sitting in the lowest part of the hull. With a hull weight of 4.5 lbs., I can't wait to test this thing out!

Jim Allen

Offline lohring

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #394 on: May 29, 2019, 03:35:37 PM »
Adjustable weights!  I was never that fancy.  Sometimes when the water got really rough I would tape a wrench or two under the engine.  On monos a length of radio box tape on the bottom of the rudder also helps hold the bow down.  These days I usually play more with prop and rudder angles on heat race boats.  Ultimate speed trials do need a more sophisticated touch, though.  The boat looks really nice and I understand what the pictures I questioned show.

My die set only uses two posts.  Even though they are larger in diameter than yours, I don't get as good accuracy.  The crankshafts always need a little truing.  I got a true two cylinder crankshaft, though.  I just need to build the rest of the flat engine around it.  The ultimate would be a turbocharged 30+ cc twin with maybe 40 hp.  Don't hold your breath waiting, though.

Lohring Miller

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #395 on: May 29, 2019, 05:13:44 PM »
I have built several die sets with various plate thicknesses, pin diameters & pin spacings. The two most important holes in the die set, that hold the two crankshaft halfs, were held to very tight tolerances & alignment. Holding the crankshaft alignment tolerance of the three pieces involved to an amount of less than .0005" becomes very difficult; maybe impossible. Consider that it is critical that the two holes in the crankshaft halfs "MUST" be in alignment to one another & to the crankshaft itself. Without this, alignment will be "IMPOSSIBLE", no matter how good the die set is. I have four crankshafts, out of maybe six or seven, that pass the crankpin hole alignment test.

I'm very sure that the tolerances required in a smaller size engine become much more difficult to produce. This is especially true in a crankcase that has four bearings for the crankshaft. I also believe that the 12 mm (.4724") crankshaft diameter is to small for the HP & RPM that a 27 cc engine runs at. This comes from many years of building 15 cc engines that had a 15 mm (.5906") crankshaft diameter.

Jim Allen
« Last Edit: June 03, 2019, 01:14:10 AM by strictlybusiness1 »

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #396 on: June 05, 2019, 07:21:20 PM »
Lohring,

The simple adjustable weight system shown was added to an AC Slayer 49" Mono which uses a 35 cc Zenoah & a Performance Unlimited tuned pipe. The hull could not be balanced to the designed balance point, 17.5" from the transom, after adding the 3 " diameter, 11 degree baffle tuned pipe & muffler. This was after moving the engine, radio box, fuel supply, etc., as far forward as possible. By placing the newly designed weight cavity 16.5" forward of the designed balance point, a single 4 ounce brass weight balanced the hull. The cavity, made from a 2.000" ID X 3.350" deep tin can, which is epoxied in place, could hold three .750" thick weights with a maximum OD of 1.760". These removable weights are sitting on the same size .625" carbon fiber rod used on the cat's weight system. This rod is threaded into a glued in PVC base plate with a 58/11 nylon thread, & it is removable. The machined PVC cap is located on the ID of the .625" OD carbon rod & the ID of the tin can cavity. Radio box tape is used to seal the machined PVC cap.

We will be testing the system this Friday & Saturday, 6/8 & 6/9, 2019 at the Suffolk, Va site.

Jim Allen
« Last Edit: June 05, 2019, 07:34:31 PM by strictlybusiness1 »

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #397 on: June 06, 2019, 03:32:58 AM »
I decided that clamping the weights was much better than using a set screw to hold them in position. However, this process was much more time consuming. Notice the wider slot cut opposite the clamping screw side which has a thinner slot. Doing this makes the brass piece flexible & therefore it is easily clamped to its carbon fiber tube.

JA

Offline strictlybusiness1

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Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #398 on: June 13, 2019, 07:58:02 PM »
The adjustable weight system is now complete with the addition of a 5/8-11 nylon bolt, a 5/8-11 nylon nut & a hard 1/8" thick rubber retaining washer. Finger tightening of the 5/8-11 nylon nut against the hard rubber washer, locks the carbon fiber rod mounted between the two bulkheads, securely in place. A few more pieces & I can get back to the engine work.

Jim Allen