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

prop help

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Old 04-26-2009, 06:30 PM
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JEREMYB
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Default prop help

Hey guys.. I need some advise on a prop for this plane. I've been using a 10/7 on it, and its been super fast. Its called a critter. The wing is about ten years old or so. when I used to fly it then the plane would take off from your hand real easy.. Now with this prop tho its fast it takes along time to get up to speed, and you have to give it a pretty good toss into the air...I don't reamember what motor I had on it before, but it was about the same size. Which prop would still give me some good top speed, but maybe give me some more tourque to help get it flying easier? The motor is a OS AX 46. I hate to give up any speed, but shes has to be easy to get in the air too.

THANKS FOR THE HELP.

JB
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Old 04-26-2009, 07:32 PM
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Default RE: prop help


ORIGINAL: JEREMYB

Hey guys.. I need some advise on a prop for this plane. I've been using a 10/7 on it, and its been super fast. Its called a critter. The wing is about ten years old or so. when I used to fly it then the plane would take off from your hand real easy.. Now with this prop tho its fast it takes along time to get up to speed, and you have to give it a pretty good toss into the air...I don't reamember what motor I had on it before, but it was about the same size. Which prop would still give me some good top speed, but maybe give me some more tourque to help get it flying easier? The motor is a OS AX 46. I hate to give up any speed, but shes has to be easy to get in the air too.

THANKS FOR THE HELP.

JB
How heavy is it..? If it is pokey getting on step with an FX .46 on a 10-7, I'm not sure what to suggest except a .61. As a prop choice that should have lots of pull out of your hand - if you were running a 9-8 then I'd have more ideas for example. The first thing I would do is put on a 10-6 and evaluate launch behaviour - improved, yes/no. Have you ever flown it on a 9-7 or 9-8 - these are not the direction I would first suggest, but I'm curious.

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Old 04-26-2009, 09:25 PM
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Default RE: prop help

Propping for "torque" [known as thrust] is opposite of propping for speed, but sometimes you find a prop that puts the engine into it's sweet spot. I think a 9x7 or a 9x8 is universally the best for .45-50 powered planes in this size range. Like MJD says, a lot depends on how much this sucker weighs.
The heavier the plane, the fewer the choices you will have for props.
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Old 04-26-2009, 10:15 PM
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ORIGINAL: combatpigg

Propping for "torque" [known as thrust] is opposite of propping for speed, but sometimes you find a prop that puts the engine into it's sweet spot. I think a 9x7 or a 9x8 is universally the best for .45-50 powered planes in this size range. Like MJD says, a lot depends on how much this sucker weighs.
The heavier the plane, the fewer the choices you will have for props.
A plain old 10-6 might do almost as well as the 10-7 on it now plus pull harder out of his hand. Few more rpm to make up for pitch, more static. Never hurts to try.
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Old 04-26-2009, 11:26 PM
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Default RE: prop help

Sometimes squirrelly launches are due to inadequate fin area. A "thrusty" prop is a bandaid solution.
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Old 04-26-2009, 11:48 PM
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ORIGINAL: combatpigg

Sometimes squirrelly launches are due to inadequate fin area. A "thrusty" prop is a bandaid solution.
I hadn't looked at the aircraft again since the first time I read this thread. Yeah, I can see how that could have a whole lot to do with it. Not much moment arm on those, and long wings.. a few weeble-wobbles until it gets moving perhaps?

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Old 04-27-2009, 12:08 AM
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Default RE: prop help

Those fins look marginal. Obviously they work, but I would make another pair that are swept to the rear. A pair of little tip fins will work wonders, too. You gotta figure that this plane should be able to do 100 or more with a .45.......guys with flying wings have been hitting 100 mph with 1 HP motors since before I was born.
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Old 04-27-2009, 09:03 PM
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Default RE: prop help

OK, thanks for everyones help. the plane right now moves well over 100mph.. I did weigh it tonight, and she is 4 1/2 lbs empty. The wing is 48" long. Yes she does the weeble wobbles until she gets going. I should also mention that Sunday was the 1st day for the plane and the motor. I ran about 1/2 gallon throught it. So she still is alittle rich. I'll lean her out next time I go, and see what she does then. some one suggested a 9x7 or a 9x8?? seams small, but I haven't had a speed plane in a long time... I'll try a 10/6, but what do you all think about the 9x7 or a 9x8? any thoughts
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Old 04-27-2009, 09:44 PM
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ORIGINAL: JEREMYB

OK, thanks for everyones help. the plane right now moves well over 100mph.. I did weigh it tonight, and she is 4 1/2 lbs empty. The wing is 48" long. Yes she does the weeble wobbles until she gets going. I should also mention that Sunday was the 1st day for the plane and the motor. I ran about 1/2 gallon throught it. So she still is alittle rich. I'll lean her out next time I go, and see what she does then. some one suggested a 9x7 or a 9x8?? seams small, but I haven't had a speed plane in a long time... I'll try a 10/6, but what do you all think about the 9x7 or a 9x8? any thoughts
I mentioned them with caution, not knowing the weight of that wing. As a rule, those props would have a little less grunt at hand launch speeds, but would unload to higher rpm in the air than their 10" counterparts if the airframe drag allows them to do so, meaning higher top end speed. 4.5 pounds sounds heavy for a 4 foot plank flying wing, but not heavy in terms of the area there and the power available, just I'd figure it to be lighter. A .46 is plenty of power, but I can see how it needs a sec to get itself straightened out and hooked up, if the wings are plowing a bit and without enough tail volume, in other words corrective force to get its tail straightened out quickly. If you could move the fins back or increase the area, i.e. increase the tail volume somehow, it should behave better if this is indeed the culprit.

There is the outside chance that a 9-7 for example may bring the engine up the horsepower curve a bit and help, but in general lowering diameter is counterintuitive to launch problems. And typically a decent Schneurle .46 on a 10-7 pulls pretty hard off the starting line. Shave 8-16 oz off that airframe and try a 9x7 or 9x8 and you could see quite a few more mph.

The long wings have higher inertial moments than equivalent stubbies - something like that anyway - meaning I think they make the fins work pretty hard at low airspeeds and esp. with the typical upsets after a hand launch. If the wings are an over-contributor to the all up weight, then this makes a lot of sense. If the wings are light, and there is a lot of mass in the fuselage pod instead, then it doesn't jive so well. Where is the CG?

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Old 04-28-2009, 07:57 AM
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Default RE: prop help

The cg is at 2". Most of the weight is in the wings. A good section of the middle is figerglassed.
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Old 04-28-2009, 08:04 AM
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ORIGINAL: JEREMYB

The cg is at 2". Most of the weight is in the wings. A good section of the middle is figerglassed.
That jives then. CG is at 2", but what is the chord? I am curious about the stability margin.

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Old 04-28-2009, 08:10 AM
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Default RE: prop help

the cord is 12". Once up to speed.. she is very stable.. flys like shes on rails.
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Old 04-28-2009, 08:53 AM
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ORIGINAL: JEREMYB

the cord is 12". Once up to speed.. she is very stable.. flys like shes on rails.
I believe it. CG sounds okay, just wondering where it was while thinking about your observations.

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