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

SCREAMIN' DEMON / JETT46

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Old 12-25-2009, 09:08 PM
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combatpigg
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Default SCREAMIN' DEMON / JETT46

.......so what's so special about this? I think it's a good "cheap speed" kind of combo I got here.

This plane originally had a OS.32 and was good for about 130 mph.
It's got 2 HS225s for the flaps and a Hobbico servo for the fuel shutoff.
The battery pack is soldered together AA cells right behind the spar.
The RX is a 555.
Hayes tank with Sig foam padding.
The Jett .46 was got off auction for $160 with very little [if no] signs of prior use.

The engine has run great, but with black gunk in the exhaust. The head button fits in the liner a little sloppy, so I cut my own and made it a little deeper. The exhaust now has a nice amber color. Thanks to Roger for bringing up a loose head button he had. It doesn't need to be very loose for it to start chattering away in there.
I made 2 shims to raise the liner, .005" each.The exhaust has about 170 degrees now, the head clearance is .020".
I'm running the engine with 25% nitro.
The stock carb has a 3/8" bore, I made a venturi with a 7/16" bore and the original remote needle fits where the draw bar used to go.
I haven't seen much if any power improvement over stock with rpm ground checks. Nothing that has caused me to open the needle from the original setting.
I suspect the muffler is acting like a governor........
the crankshaft bore looks like it might be holding up more gains. Not sure what the proper way is to open it up a little. In the past I've wrapped sandpaper around a dowel and used lots of patience.

Most of the passes today were in the mid 160s, there looks like there was a couple that hit 170!
The engine/plane is just a silky smooth combo. Launches are basically straight up with a 9x8 prop and the landings are very gentle with plenty of time to set up the approach.

To get the CG right, there is an oz of lead in the trunk next to the RX.
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Old 12-26-2009, 01:07 AM
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Mike Connor
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Default RE: SCREAMIN' DEMON / JETT46


ORIGINAL: combatpigg

...I suspect the muffler is acting like a governor........
That is good speed for your setup. Plus you don't have to mess with a finicky engine or other high dollar equipment. You may be right about the muffler acting as a governor. I had the SJ 46 and 50 and found the slightly shorter SS muffler brought the static rpm with a 9x8 to about 18,000 rpm. The black Q-500 muffler may work good with the right prop.
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Old 12-26-2009, 01:50 AM
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Default RE: SCREAMIN' DEMON / JETT46

Mike, I'll have to check what WOS says about rpm. I think this might be one of those cases where the actual speed exceeds the prop's speed.
Today was totally clear and calm, most of the passes were long ones...500 feet or more. Last year at this time we had 3 feet of snow.
This Jett needle has such a fine tooth, it takes too long for me to do a peak rpm test with it...so I don't bother trying to tach it any more.


I came up with 19,300 for the inflight rpm, which works out to 146 mph. Most of the passes measured hit in the 160s without question.
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Old 12-26-2009, 11:02 AM
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Default RE: SCREAMIN' DEMON / JETT46

You may have hit peak with your muffler. According to Bob B.:
Standard muffler is designed for between 15,000 and 18,000 rpm (minimum and max suggested ground peak rpm)
SS muffler is designed for 17,000 and 19,500 rpm
Black q-500 racing muffler 18,000 - 20,500
With the work you have done to the engine it may be trying to go faster. Have you tried open exhaust?
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Old 12-26-2009, 12:03 PM
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Default RE: SCREAMIN' DEMON / JETT46

I should try open exhaust next. I'll see if the rear cover is thick enough for a pressure tap.
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Old 12-26-2009, 02:20 PM
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Default RE: SCREAMIN' DEMON / JETT46

It is. I've put one in for a buddy.
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Old 12-26-2009, 03:54 PM
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Default RE: SCREAMIN' DEMON / JETT46

Looks like you are having a lot of fun

The muffler on there is the "regular" sport jett muffler. RPM range is from about 15,000 to about 18,000 rpm. Typically dials in around 16 - 16.5K ground rpm and expects to see the engine up in the 17.5k range in flight. It is designed for the heavier sport props, the APC 10x6 being a baseline around 16,500 rpm. So yes, it is working as a governor. The inlet work is good, but that is not the limitation until the rpm and mass flow go up as well.

If you were to switch to the black "race" muffler (18,500 +) or the SS/LX muffler (17K-20K), you would see the rpm jump with the smaller prop, and probably some good results from the larger venturi.

Also... worth noting, the head button/liner on the jett engines are matched fit. Usually when someone replaces a piston and liner, we recommend replacing the head along with the set. When Dub rebuilds/repairs he matches them up in the shop. So it might have been the case where someone (prior owner) repaired the engine along the way, and did not get a good match. One other item that comes up - sometimes engine owners remove the head shims believing they will get a power increase. Usually that is not the case, and in a worst case folks have come up with loose head bolts, a cylinder head leak, or a distorted head button due to resulting heat change (lean engine).

Bob
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Old 12-26-2009, 03:59 PM
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Default RE: SCREAMIN' DEMON / JETT46


ORIGINAL: combatpigg

I should try open exhaust next. I'll see if the rear cover is thick enough for a pressure tap.
You can try and replace the upper left backplate cover screw with a pressure fitting. It should be drilled through to the inside of the port. You might have to make one to do this. At one time I found some hollow 5-40 screws (vented) that i play with when testing some of the pumps.
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Old 12-26-2009, 09:05 PM
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Default RE: SCREAMIN' DEMON / JETT46

Thanks Vic and Bob!

I dove into the pressure fitting job before getting your advice and got a #4 threaded nipple to go through a mount lug at an angle. We'll see how this engine runs wide open...hopefully tomorrow. I don't know if I'll like the harsh sound...if there isn't a big improvement in speed the standard muffler works fine with the 9x8 prop, and it has a beautiful, mellow sound.
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Old 12-27-2009, 10:06 AM
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Default RE: SCREAMIN' DEMON / JETT46

chances are you will not see a big improvement, if at all. The tuned exhaust typically performs better than open exhaust.

What you have there in that configuration is basically a side exhaust (and front intake) version of a 1981 ST X-40 F-1 engine

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Old 12-27-2009, 05:57 PM
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Default RE: SCREAMIN' DEMON / JETT46

You might be right about open exhaust being a dead end, Bob. I haven't seen any improvement back at home, so I'm shelving that idea for now........

....because I finally got my old Webra .50 running.
I found a rear bearing at VXB BALLBEARINGS for $35. It has stainless races and ceramic balls, plastic retainer. I think it's rated for 75,000 rpm IIRC.
The best part of the deal is at checkout they ask you if you would like a digital caliper for $7.99!
I set the exhaust at 170 degrees and this engine has a 7/16" intake.
The open exhaust bseline was 19,600 with a 8x8.
After much trimming of the header and even some of the pipe it hits 20,000. I'm not sure what's holding up the show for more rpm, but the pipe can't physically be cut any further. Every 3/8" cut gave steady improvement. I notice that the stinger of this pipe is pretty large, I might try a reducer.
The engine ran great on the plane, but the majority of the passes were in the low 150s.
Inflight rpm was 20,700 so the 8x8 prop on this plane sucks...pitch speed is 168 mph.
It was another perfect day, about 50 degrees and no wind.
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Old 12-27-2009, 06:57 PM
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Default RE: SCREAMIN' DEMON / JETT46

Just wondering why you ran a 9x8 on the Jett .46 and dropped to an 8x8 on the Webra .50. Did you try the 8x8 on the Jett or visa versa?

Thanks
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Old 12-27-2009, 08:17 PM
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Default RE: SCREAMIN' DEMON / JETT46

I figure with the JETT I already have an engine pre-tuned for a 9x8 that works extremely well on this plane, so didn't see the need to duplicate it.
With the Webra, I had a fresh engine with a virgin pipe and wanted to see if it could be tuned well enough to justify running a 8x8.
At this point the results with the Webra are disappointing...I expected to see 22,000. It's still early yet.
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Old 12-27-2009, 11:56 PM
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Default RE: SCREAMIN' DEMON / JETT46

Just got done with some tinkering. Raised the exhaust port with a fine carbide bit and left in the liner shim. A quick check with a scribed mark on the prop driver shows about 175 degrees of exhaust.

8x8 20,700 an improvement of 700 rpm

7.2x8.6 21,700 nice, but if this prop is discontinued...who cares?

8x7 21,500 interesting to see it is more load than the 7.2x8.6

Onward through the fog. This sucker sure does pump a lot of fuel ASAP. [:-]
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Old 12-28-2009, 12:58 AM
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Default RE: SCREAMIN' DEMON / JETT46

APC sure is getting out of the speed props,
Within the last year or so, RIP:[&o]
7.2X8.6
7.4X8.25 C
8.8X9.75
11X14
13X13N
These are the ones I know about or love to use, so there may be more.
Product liability or speed freaks dying out?

Must have made too much money this year, even their on line ordering system is shut down for the holidays LOL.
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Old 12-28-2009, 01:02 AM
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Default RE: SCREAMIN' DEMON / JETT46


ORIGINAL: combatpigg



7.2x8.6 21,700 nice, but if this prop is discontinued...who cares?

8x7 21,500 interesting to see it is more load than the 7.2x8.6

The steeper prop may be partially stalled on the ground, betcha that'll change in the air and it'll be the other way around.
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Old 12-28-2009, 04:48 AM
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Default RE: SCREAMIN' DEMON / JETT46

I'd feel better about trying to launch the 7.2x8.6 if it was spinning a little faster...but I guess it deserves a look. It's my last one........[&o]

I put a degree wheel on the engine and found that my estimation of 170 degrees of exhaust is really only 160. This is with the timing already raised from what it was already. Without knowing more about engine voodoo, I'm a little bit leery about going for more. The intake closing was also pretty conservative at 50 degrees ATDC. I increased it to 55 and opened up the hole going through the shaft slightly with a dowel and sandpaper.

The net result of all this won't be seen until tommorow. The biggest difference between these 1/4" prop shaft engines and the 5/16" prop shaft engines is obviously the size of the hole going through the shaft can flow so much more. This shaft seems soft enough to drill, right now it's just a hair over 3/8".......there seems to be enough meat to go 7/16". I'd like to try it if I knew it wouldn't become a disaster.
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Old 12-28-2009, 11:55 PM
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Default RE: SCREAMIN' DEMON / JETT46

CP raising the exhaust timing was good move, but you will need to lower the deck height ot regain some of the lost compression, this will also help the fuel consumption as well..FWIW, good times! tomorrow i am going to R/C country to try and pillage some of the good APC prop stock, love being back in sacramento for the holidays.
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Old 12-29-2009, 12:53 AM
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Default RE: SCREAMIN' DEMON / JETT46

LF, I picked up a hunk of round stock today to address the head situation. I'm not sure if I'm beating a dead horse here. Right now the performance with a 8x8 is pretty good. The mods I did last night showed no ill effects, the runs today were very good. The camera work didn't turn out, so I don't know if there was any improvement. Some how I took great footage of me pitting the plane, then turning the camera off for the actual runs.....[:@]
I had a real hairy launch, too. I released the model and bumped it straight up, it hooked to the left, leveled out and cleared the roof line of my car by 5 feet as it gained flying speed. No more flights until I build a launcher, that was too close. If there is some wind, the launches are more predictable but the last several days have been dead calm.

Hopefully you score well in Sacramento!
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Old 12-29-2009, 11:51 AM
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Default RE: SCREAMIN' DEMON / JETT46

I've had a few scary left turns on launch.. even after taking out small amounts of unintentional left thrust though that sure helped. Last time I launched on lower rates by accident (stupidity actually) I had a left banking pancake touch-n-stop $4 APC moment - I try not to make the mistake of launching without high rate on aileron, as usually I end up with gobs of right stick to keep it level for a moment as it picks up a few mph. And some launches are a piece of cake - depends on the care and sobriety of the launcher.

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Old 12-29-2009, 03:19 PM
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Default RE: SCREAMIN' DEMON / JETT46

The problem with this delta is I didn't leave any space between the flaps to grab the TE and give the model a level push. I end up grabbing it at the LE and giving it a backhanded fling like a frisbee. I snagged one of the winglets as I let go...

The launcher should make launching the 8" and 7" props more of a sure thing. .
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Old 12-29-2009, 07:47 PM
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Default RE: SCREAMIN' DEMON / JETT46

Well, I didn't get the launcher built yet, but it's on the way. I flew this thing just once to get it clocked.
The results were so-so, but at least the 8x8 prop was pulling it's weight for the given rpm.
158 mph and 20,700 rpm.
This was the umpteenth run on a R5 plug, so the compression must be set pretty low.
I have to wait for a lathe drive belt to show up before a new head can be turned out.
Unless there is some kind of explosive rpm jump just lurking around the corner with more experimenting, it's beginning to look like the Webra 50 is a "9X8" type of engine.
For the sake of comparison I'd like to know how much exhaust timing the OS46DF has and how well it turns a 8x8.
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Old 12-29-2009, 07:54 PM
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Default RE: SCREAMIN' DEMON / JETT46

I'm curious, what kind of lathe do you have CP, I also have a bit of a machine Shop Equipment fettish.[sm=tongue_smile.gif]
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Old 12-29-2009, 08:25 PM
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Default RE: SCREAMIN' DEMON / JETT46

It's a Chinese 9x20 sold by Harbor Freight. I don't know anything about machine equipment or relative values...but I'm not thrilled with what I ended up getting for $900. Probably could have got the $400 machine and alot of attachments / accessories instead.
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Old 12-29-2009, 08:45 PM
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Default RE: SCREAMIN' DEMON / JETT46

I remember a $300 1/2" floor model drill press we bought at work, one of those King cheap import specials. I have never seen a quill so sloppy, even on my dad's beat up old antique downstairs. It wasn't even useable as a disposable drill press for meatheads, the reason we wanted it.

I have a 1958-ish Logan 9B-28 shop lathe, but it's in storage right now.

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