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 Engine Prop Sizes

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Old 01-24-2015, 07:05 AM
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petec
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Default Ducted Fan Engine Prop Sizes

So I did some searches on RCU with varying results trying to get prop size suggestions for the K&B 7.5 DF engines I just picked up. So I figured instead of reinventing the wheel I thought if I put thread on here asking for help, for all DF eniges that you guys have run, it would be a good reference for everyone.

Sooooo.....can anyone give me some suggestions for my 45s and also post data on props you have used with other DF engines.
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Old 01-24-2015, 07:55 AM
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I think the engine would feel right at home turning an APC Q40 prop.
Something like a 7.4x8.0
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Old 01-24-2015, 09:10 AM
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I was needing to go through my prop drawer in the shop and see what I have and this will give me a reason to do so. Thanks for the suggestion.
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Old 01-24-2015, 01:52 PM
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I would stick with a CF prop though, I might have just stated the obvious, .
The Q40 props are a good choice, can you still get them?
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Old 01-24-2015, 01:57 PM
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I had some Q40 props, I've been away for a few years. Still need to go pick through my prop drawer.....There may even be some Bolly racing props in there.

If I can't get them my choices were either clip down a bigger prop to get the stronger hub or find a CF prop. Having a prop come apart at 20K+ isn't my idea of fun.
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Old 01-24-2015, 01:59 PM
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Yes you can still get them.

The APC Q40 props are pretty stout. Not like a continuous strand CF prop of course, but still, they stand up to some pretty heavy abuse.
On a modern Nelson Q40 LS engine, a 7.4x7.5 prop will probably be turning something like 27.000 or 28.000 rpm in the air. I doubt the old K&B will turn it up that high...
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Old 01-24-2015, 02:54 PM
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Yeah I agree chances of getting that high in the RPM range are slim, none, and forget it.
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Old 01-24-2015, 10:03 PM
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I never ran a 7.5 K&B, but I ran a number of 6.6 engines in Formula One. You would prop them for 21,000 to 21,500 on the ground and they would unload about 5,000 in the air. all on wood props, open exhaust, and 65% nitro. Props were typically 8 1/4 X 7 1/2.
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Old 01-25-2015, 09:44 AM
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I was told/suggested shoot for 25 in the air, and to look at props in the 7.5x7.5, 7.25 x 8 range. I bought some Eliminator P8 and P10 for my OPS .45's, I'd say similar shaft load would be appropriate - P8 (7-3/8 x 8.25, P9 7-3/8 x 8, SC-5 7.5 x 8 would be where I'd start. When it's warmer I'll probably bench the 7.5's on the P8 to get an idea where to go, I'd like to test one out on a Demon sometime, when I get to it.
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Old 01-25-2015, 09:10 PM
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APC never made any props that didn't have a pretty sound application for the times. I found that their 7.2 x 8.6 was the upper limit for my pipe timed .45 - .50 engines.
Too bad they don't make that prop any more, but it makes me wonder what the big demand ever was for it.
I'll wager they made it shortly after the Diamond Dust was in production..
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Old 01-26-2015, 06:21 AM
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Fred told me some time ago when I asked - how many props do you have to sell on average to make it worthwhile to design and make the tooling? Answer: 5000.

I'd believe they were aimed at the pylon circuit but maybe at the upper end of practical P/D ratio/shaft load to be used enough to sell enough. People who know the K&B metrics told to think along those sizes, the 7.2 x 8.xx, so I figure the three sizes I listed are the closest equivs from Eliminator. But it would be cheaper and more convenient to pull them out of an APC bag wouldn't it..?
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Old 01-26-2015, 09:33 AM
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5000 props? Makes sense.
Figure they make 1$ or less on a 3-4$ prop (handling, packing and shipping and other operational costs aside), and lets say 5000+$ for the tooling for a prop...it would take quite a few props to turn a profit.

Same reason you probably make 0$ profit on your Demons (or worse even ) , when you take everything into account...
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Old 01-26-2015, 10:47 AM
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I don't lose, I don't gain. Another 50 kits the tools are covered. Well, except the laser. It is for fun.
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Old 01-27-2015, 05:04 PM
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All good info.....cant wait to get the SD short kits and give a K&B a home.
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Old 01-27-2015, 06:57 PM
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I had a K&B .40 Formula One engine with open pipe on a Diamond Dust that was a real crowd pleaser.
The rear intake design was so efficient that it was able to suck in a #6 lock washer that was laying around in the area behind the engine. The previous owner of the engine was using lock washers under the header pipe bolt heads and even though I don't like to use lock washers like that, I let it slide. After a flight the plane landed with a header bolt missing...and so I figured the washer was also laying on the ground some where. After replacing the bolt I got several thrilling flights until one day the engine mysteriously seized up and landed missing the prop and spinner. It was really difficult to dismantle the jammed piston / liner and wedged neatly in the side of the piston was that lock washer from Hell.
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Old 01-27-2015, 07:40 PM
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Originally Posted by petec View Post
All good info.....cant wait to get the SD short kits and give a K&B a home.
I have two 7.5's needing homes too. I just finished drawing the new OS.46 & clone motor mount plates (try to cut them tonight but I am fading zzzz).. so why don't I work up a set of plates for the 7.5's too. With the long beak, I figure they should mount with the apex of the LE coming up behind the drive plate.. I'll draw it and show you.
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Old 01-29-2015, 05:44 PM
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Mine are front carb, but any help you want to offer in the way of the plates is very welcome.
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Old 02-03-2015, 09:36 AM
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it cost about 4,800-5,200 dollars to make a new prop at APC from what I was told. This is for props under 12 inch or so.
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