Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

ducted fan motor

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Old 07-31-2003, 02:28 AM
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Windwalker
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Default ducted fan motor

I found a rear exhaust K & B 7.5 ducted fan motor and I want to prop it, but I'm not familiar with these and have some questions-So naturally you extreme speed demons came to mind...

1. the engine has an O-ringed exhaust adapter that bolts to the engine with a straight aluminum pipe that fits over the O-ring, (the header I assume) but nothing holds the pipe on (like screws, spring, etc.) Is this right? What will keep it on at 20k rpms?

2. I don't have any planes that will accomodate a straight header pipe-Is there a company out there that makes an S-pipe to route it around the fuse?

3. Several MA and APC props have been test-run on it and although there wasn't enough light to get a good tach reading, some sounded like they were spinning so fast it was scary. What's the max rpm MA and APC props can handle safely?

Any help with this would be greatly appreciated--Windwalker
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Old 07-31-2003, 03:45 AM
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Default K&B 7.5

Windwalker,
I have been using the both the 7.5 and 6.5 versions of this motor for 10 years. Parts are tough to find now that the company was bought out.
The exhaust adapter with the O-ring is intended to be connected to a tuned pipe via a short aluminum coupler, and a flexible coupler. The aluminum couplers were turned out years ago by racers with home machine shops. They are very thin, and have a small groove cut in them about 1/4" in from one end. That end would push over the O-ring until it dropped into the groove. The coupler stays in place once the pipe is tied down somewhere down the line.

I would try Mac's Products for both a rise adapter and a pipe for this. I think the pipe P/N is 1040. You would remove the existing coupler and bolt on the riser in its place.

On the 6.5 I use an APC 8.75x8. On the ground I set it a tad rich at 18.5-19K. It unloads in the air at what must be 20K+.
Hope that helps.
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Old 07-31-2003, 04:03 AM
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Default K&B 6.5 Picture

Windwalker,
Attached is a picture of the aluminum coupler and flexible coupler used to couple to a Nitro pipe on my Lazer Arrow delta.
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Old 07-31-2003, 06:33 AM
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Default Re: ducted fan motor

Originally posted by windwalker
What's the max rpm MA and APC props can handle safely?

I don't know about Master Airscrew props, but on the APC website it say that their sport props shouldn't be run faster than 190000/prop diameter in inches...

Example: take a 10" prop... 190000/10=19000 rpm max

The 10" prop won't explode at 19000 of course... I expect there will be a generous safety margin on that figure... but still, don't go too far...

The site says nothing about their pylon props, but I guess these are made to withstand very high rpm... so I wouldn't worry about them.
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Old 07-31-2003, 12:51 PM
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Default Re APC Props

Rudeboy,
The APC pylon props are a very different design than their sport props, though the material appears the same to me. I am far from an expert but I have observed many pylon races here where the Q500 racers were turning 23K 'on the pipe' using APC pylon props. For certain applications such as Q40 they offer a carbon fiber version. Their website states that the greatest danger is when adjusting for peak RPM on the ground. Having just read that recently I now approach with more apprehension and respect.
I'm sure people with experience will jump in here to enlighten us further.
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Old 07-31-2003, 03:46 PM
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Default K&B 7.5 DF

windwalker,
Hopefully attached is a picture of a modded Scat Cat with Modded K&B 7.5. Haven't flown it for 20 years but it was close to 150MPH when last flown, I used a Macs bolt on "Fuse Top Header" and a Macs 7.5 Muffled Tuned Pipe. Iused the largest Rev Up Series 400 Pylon Prop available, I believe it was a 8.75X9. It would turn 19500 RPM on the ground and about 21K in the air. The Pipe and header are still listed on Macs Website
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Old 08-01-2003, 12:11 AM
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Default ducted fan motor

Mac's has a website. MACSPRO.com
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