A Step in the Right Direction

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Blow a blade off that prop at 30000rpm and it may just give you a fail in life!! Other than that it's an A. If you are game enough to run that prop NEVER STAND OR HANG ANYTHING IN THE DIRECTION LINE OF THE BLADES if it sheds a blade off that prop it could kill you or small animals! Seen a guy cop a shed blade from a ducated fan engine in the 80s and it entered his cheek and come out the top of his head! It was nasty! If a Dr wasn't flying that day it would have killed him
 
We threw a blade on a gas engine. It went through the wall and the imbalance wrecked the engine. A ducted fan is probably better. They are designed for high rpm. At the least, run a high quality molded carbon fiber prop.

Lohring Miller
 
It's a 6x4. I'm kinda scared of the thing. I know Master airscrew are only rated to 19k or so for the smaller props. The first attempt with only the small set collar spun a 7x4 loose. Yes, I do have a kill switch set up on the radio. Since I'm breaking the motor in, I'm not trying for any records. Waiting on another 7x4.
 
This has been great for getting some faith in the head temp telemetry sensor. Seems like a good way for a rookie without a coach to know where the needle should be. I also have a ball valve on the water. Just don't try to adjust it while running, it gets hot! Another one learned the hard way :)
 
Ducted fan props and shroud should work? But depending on your wallet? Cut down balanced larger props may also work? But like Lohring said if it goes bad? It goes bad in a hurry that kill switch finger better be your trigger finger and just as quick as a gunfighter or it may wreck the whole setup including the engine (I really hope not!) But hope for the best and prepare for the worst! Murphy was a prick!
 
Grayson,

The photos show a test stand built in 2017 which is used for both nitro & gas engines. The test stand uses various pitches & diameters of Falcon brand carbon fiber propellers as engine loads. The test stands base is a 36" long X 5" wide X 1" thick piece of aluminum jig plate. The test stand weighs 30 lbs & can be mounted to a special table to prevent any movement during testing. The test stand has reliably tested engines at RPM's beyond 32,000 many times! Some things to notice are the double ball & pin universals used to eliminate any misalignment problems; the Fromeco TNC tach which is permanently mounted directly behind the propeller; two dual bearing blocks that can adjust the preload to the 4 ball bearings used to support the hardened propeller shaft; the propeller shaft locking device which allows positive locking of the 3/8-24 propeller lock nut assembly; all propellers balanced in a magnetic balancer; etc.

I will give more details when time permits.

Jim Allen

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Ducted fan props and shroud should work? But depending on your wallet? Cut down balanced larger props may also work? But like Lohring said if it goes bad? It goes bad in a hurry that kill switch finger better be your trigger finger and just as quick as a gunfighter or it may wreck the whole setup including the engine (I really hope not!) But hope for the best and prepare for the worst! Murphy was a prick!
A BV ducted fan would be safe up to 30,000, providing a shroud was used. The photo shows the 3/8" thick aluminum shroud used on a typical torque cradle type test stand.

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I had seen Jims test stand before starting this project. Love it. Unfortunately I only have wood working tools.

I looked for u joints and shafts, but I didn't find anything other than old stock Dumas shafts. So I went with 1/8 scale setup. 32" 1/4 shaft. About 2" vertical drop. After a couple of runs I can see the temp for the strut getting warmer like 112 degrees. Time to regrease.

I made extra side rails for the motor mount. I have a set fitted for my OPS 67. If it mounts on 4" rails, I should be good to go.

Thank you Jim for the prop source info.

I went with a pusher prop thinking of a boat. Also that the shaft would run cooler that way. After looking at the gap between the thrust washer and strut and considering why it's there, maybe a puller prop would be okay. Any thoughts on this?
 
Grayson,

"After looking at the gap between the thrust washer and strut and considering why it's there, maybe a puller prop would be okay." I think this would be okay with the front bearing taking up the propellers thrust load. Also a puller prop probably would make the stuffing box tube run cooler. I believe the smallest size carbon fiber prop available would be a 17 X 6. Start by cutting the props diameter down to 8". Each time you reduce its diameter the prop must be carefully balanced again. I cut mine down in 1/2" steps to provide for load changes. Any out of balance or non concentric rotating parts will cause many unsolvable problems!

JA

NOTE: No miniature size universal I know of will survive at 25,000 or 35,000 RPM & 6 HP. However, properly designed & machined ball & pin universals can survive at RPM's beyond 35,000. For the size engine you're running, a 5/16" ball with a 5/32" pin will suffice.

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Good for you Grayson! Nice setup.

I had a blade come off an OPS 67 years ago at about 24K, made a nice dent in the pavement. After that I used the biggest props I could find (Master Airscrew IIRC) and cut them down.

I'm going to build a similar run in stand/inertial dyno this winter using a Byron Fan unit (pusher) that will be convertible to an inertial dyno down the road.
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Do it Terry!

There are many things you will discover with a good test stand. I wanted to determine the RPM limit of a typical AISI S-7 steel, "I" beam, roller connecting rod. The .80 cu in engine used was dyno tested at 38,000 rpm. After about 1 minute running time at WOT the rod failed completely! The carburetor used had a .750" bore. The exhaust timing was 198*; the transfer timing was 130* & the boosts 125*. The inverted drum opened 30* ABDC & closed 66* ATDC, for a total duration of 216*. The two stage pipes tuned length was 8.75 in & the fuel used was 65% nitro.

Jim Allen

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Do it Terry!

There are many things you will discover with a good test stand. I wanted to determine the RPM limit of a typical AISI S-7 steel, "I" beam, roller connecting rod. The .80 cu in engine used was dyno tested at 38,000 rpm. After about 1 minute running time at WOT the rod failed completely! The carburetor used had a .750" bore. The exhaust timing was 198*; the transfer timing was 130* & the boosts 125*. The inverted drum opened 30* ABDC & closed 66* ATDC, for a total duration of 216*. The two stage pipes tuned length was 8.75 in & the fuel used was 65% nitro.

Jim Allen

38K with a 90 motor? As the kids would say: "that's sick"!!!
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And that was a seriously though conn rod. Jim's test bench ready to eat more radical motors! Some people have all the fun...

I'm going to try to talk to Falcon tomorrow and find out about a 1/4 hub. If I can't do that then I'm going to need a custom prop nut.

I already have a couple of ideas for a jig to chop the prop.
 
Grayson,

The Falcon carbon fiber propellers come with a 1/4" hub hole. I remachined this hole to .312 minus for a tight fit on the hardened prop shaft. The reason for doing this is to ensure that the hub mounting hole is square to the back side of the propellers hub. I also made sure that the blade tips are running in the same plane before reboring this hole, using the back side of the propeller hub as the mounting surface. Balancing is done after boring the hole. The hub washers are 7/32" thick hardened steel. The propeller shaft is mechanically fastened only to the two bearings in the front bearing block. Notice the split in the side of both bearing blocks which allows the radial play to be adjusted. I reduce the radial play to reduce the axial play to the minimum. Notice the second block adjacent to the bearing block. It locks the propeller shaft which allows for maximum tightening of the propeller shaft nut.

Jim Allen

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