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prop lengths , faster or slower

Old 11-04-2003, 08:42 PM
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Default prop lengths , faster or slower

Hello guys I have a quick question , which only needs a simple answer , before the question Im going to give my best guess answer, the longer the props are the slower the rotation is , Im no Einstein but this equates to large tires vs small tires , its rotation no matter how you look at it. So I beleive the smaller prop. will create more thrust than the bigger one ...thus making the plane fly faster .

Question: I have two props one is 6 inches and 6/8 long the other is 6 inches long. the original prop I received with plane is the longer and it flys EXCELLENT.......(FREEDOM FLYER ) Then I ordered extras, so I can practice aerobatics and not worry about breaking props, and received the shorter ones , Im just worried about the performance of the plane changing when I use the smaller props . Thanks and all answers will be appreciated
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Old 11-04-2003, 10:46 PM
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Uncas
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Default RE: prop lengths , faster or slower

I think generally the longer the prop, the more powerfull and the more efficient the power system is within limits. I suggest you to look up those GWS tables for their power systems - but they also change gear ratios as the prop length increases.

For instance, my Tiger Moth (IPSD) would be most efficient with a 15 inch prop and most powerful with a 13 inch; however, I use a 9 inch prop because of my landing gear.

I do not think your tire analogy is quite correct. The speed at the tip of the longer prop is greater for a given RPM. In fact, the speed at any given point on the prop is increasing as you moves outward, therefore you have to twist the prop. I do not think the RPM will be slower with a longer prop unless you are stalling your motor. I may be wrong but I think DC motor speed is dependant on motor terminal voltage.
Old 11-04-2003, 10:59 PM
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Default RE: prop lengths , faster or slower

The speed at the tip of the longer prop is greater for a given RPM. In fact, the speed at any given point on the prop is increasing as you moves outward,
this has to be wrong , have you ever seen an ice skater do a spin with his or her hands held out , then they move them in they start turning faster ? well I beleive this is what happens as the prop gets shorter not longer, and what is GWS , forgive for my ignorance , but this is important because I just received 13 extra props that are smaller and definitly will send them if it sloes my plane down !
Old 11-04-2003, 11:31 PM
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Default RE: prop lengths , faster or slower

All other things being equal the motor will attempt to maintain a constant RPM unless it stalls, unlike the skater who is just conserving energy. As a motor loads up it will draw more current to maintain speed, if it completely stalls it draws a lot of current and burns up.

The outer portion of the blade will be travelling faster because it is covering more distance in a given revolution than an inner portion but the time to complete the revolution is the same for both portions.

GWS is Grand Wing Servo. They sell a lot of Park Flyers and Power Systems (Motor, with gears and props), and servos etc.

The real issue here is will your shorter prop work. It may have a greater pitch than the longer one and may be just fine ( I could be wrong here). I am not familiar with the freedom flyer. Is it direct drive, no gears? What does the props each say. Most have a number stamped on them which descibes the length first then the pitch.
Old 11-04-2003, 11:39 PM
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Default RE: prop lengths , faster or slower

with all due respect , is this situation really hard to fiqure out ? I dont know and I admit that if you dont please do the same ..... all we are talking about is the length , other than that the props look the same , so thanks for answering but no thanks for confusing me more [&o]

p.s. no numbers on props....
Old 11-05-2003, 08:01 AM
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Default RE: prop lengths , faster or slower

Let's try and simplify this a little.

With the same battery, motor and gearbox, both sizes propellers will spin at roughly the same RPM. The smaller propeller will produce LESS thrust because it is smaller. It won't work as well as the larger prop.

That's it in a nutshell.
Old 11-05-2003, 06:31 PM
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Default RE: prop lengths , faster or slower

so theoretically the smaller prop is better because it uses less power being that it produces less thrust, I understand the difference is not great but when hold plane in my hand i can feel the deference in the pull of either prop , I emailed the manufacturer to ask why I received these smaller props (electronics4u2) and still didn't get an answer , and this why , I made that poll because poor service is not acceptable in my book !

thanks MATT , I greatly appreciate you clearing this up !
Old 11-06-2003, 07:52 AM
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Default RE: prop lengths , faster or slower

If your goal is a longer flight, then yes, the smaller prop is better.

I generally expect a level of customer service commesurate with the amount of money I paid for the item. Bargain basement plane means this guy won't have the resources to provide satisfactory customer service. The Freedom Flyer is like $99.99 isn't it? Comparable planes like the aerobird and T-hawk are $150 or more. If I'm paying $150, I'd expect them to kiss my butt, but for $100, I'd not be surprised if it takes a few days to receive a reply, if you get one at all.
Old 11-06-2003, 08:13 AM
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Default RE: prop lengths , faster or slower

Matt you crack me up , money is money is money is money , If I by somthing for 6 bucks lets say vitimins, I expect the manufacturer to know as much about them as he possibly can , and I also expect not to get sick from taking them.....I think you are a little up set with the lower priced planes verses the great performances they give , the only reason the FF is priced so low is because it doesnt have a field charger , wheels , esc, throttle , must I go one , and my lord whats really the difference between $150 and $100 ? I bought the megatech skyliner and and paid $289.00 for a bunch of info that meant nothing because I couldnt get to stay in the air long enough to apply the info lol So no disrespect but lay off of the lower price planes every one isnt a forum moderator like you making MAJOR MONEY !
Old 11-09-2003, 12:31 AM
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Default RE: prop lengths , faster or slower

As others have mentioned there is no simple answer. There are a lot of variables to take into consideration. In general, larger diameter props offer the best efficency.

So no disrespect but lay off of the lower price planes every one isnt a forum moderator like you making MAJOR MONEY !
Humm..
I didn't realize that forum moderators were making money doing this. Where do I sign up.

Bill
Old 11-12-2003, 09:46 AM
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Default RE: prop lengths , faster or slower

Having been flying for only a year I understand your frustration. With glow engine it's very simple there are a limted number of props for each engine. With electrics there are at least five variables: Motor, Cell count, Cell type, Gear ratio, and Prop size. Go to Diversity Models and play with their free P-Calc. It has about a hundred motors, fifty different cell types, then you plug in the rest of the information: Cell count, Prop Diameter, Prop Pitch, Gear Ratio.

You should know going into this the maximum cell count, amperage, and RPM for your motor, as well as a idea of how much amperage your batteries can realistically supply. For example 720mAH AAAcells aren't going to dump 70A. Espirt Models has a nice break down of these numbers of their battery page.

Hope this helps.
Old 11-12-2003, 10:06 AM
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Default RE: prop lengths , faster or slower

RC fiend- your comparison of the skater with arms in/out -- has to do with "conservation of angular momentum"-- nothing to do with props --
anyway -as for best props - here is a short course.
first pick the airspeed , the prop will be required to work in.
find how much power you have to swing it.
Unless these two are known -all the rest is just hopeless.
If you are working at very low speeds - then you need to consider what helicopters do
If you are looking at higher speeds - look at racing plane setups
Finding a prop which provides instant low speed thrust PLUS high speed performance- can only be done in one way-- add all the power you can come up with.
The efficiency of a prop has many factors - some small dia props are very good at low speeds -if they ar spun fast enough (ducted fans) and tip losses are controlled .
on little models -cut and try - as the theory boys are typically completely out of their element on tiny fractional hp prop/motorsetups.
The GWS tables are about as good as it gets - and they are very good.
Old 11-12-2003, 05:49 PM
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Default RE: prop lengths , faster or slower

May I add a bit to all of this information. 50 years of gas experience does not help me as I pursue the wonderful world of electric flight. I am as lost as a babe in the woods. Although I do have a better working knowledge now than when I started last year. One thing I did was purchase some SR Electric Flight Techniques. Excellant information. Volume E11 has to do with Prop selction and theory. The relationship between prop diameter and pitch along with specific prop recommendations. This article and many others is available from SR Batteries. And should you ask. No I do not work for them. Have a great day.
Old 11-12-2003, 06:02 PM
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Default RE: prop lengths , faster or slower

thanks guys but is there a cut and dry answer to which size prop longer or shoter on the same set up is better for a plane/glider , guys im lazy , at least when it comes to finding out specifics like this , I have even done a search engine on props and it only confused me more , Im starting to think there is no answer....and guys if you recommend different material such a SR etc. please put the link in your post so I can click it and go to exactly what you are talking about ,, thanks again , oh! the maufacturer said the shorter prop will make the plane fly faster . I flew the plane with the shorter prop and notice the lift was alot better . So thanks again , good luck and practice hard .

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