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Old Nov 30, 2011, 04:19 PM
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carbondale il
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I'm wondering if four 5" props have the same thrust as one 20" prop. If I can get 1/2" diameter sprockets I can enclose the whole thing in the fuse and wing.
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Old Nov 30, 2011, 04:46 PM
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Well for one thing you now have 8 blade tip vortices instead of 2... so you can see how the efficiency is impaired.

5" props on a 48" model seem too small to me - off the top of my head I'd say 6 or 7". Then there's the issue of whether you have the clearance.
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Old Nov 30, 2011, 05:13 PM
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Jon, I have the clearance for 6" props, and maybe 7". I have to wait for a 100% size copy of the plan to be sure. I don't want to let this go. Yesterday i was ready to give up. I believe I can make it work and thanks everyone for your interest and comments. The CG is about 30% back from the wing's LE but the propellers extend forward from the LE about 3". That might be enough forward weight, along with part of the rubber motor in the nose to keep the nose ballast to a minimum. I'm thinking the forward rubber motor needs to be just a little thicker than the aft motor... winding it up will have to meet some resistance as the propellers turn with the rubber.
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Old Dec 03, 2011, 11:14 PM
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Heard from WMBerg micro products. $200. for the drive train. Waiting to hear from Power Drive for a quote. The Berg system is a 1/8" wide timing belt with no stretch and .505" in diameter pulleys with a 1/8" bore hub hole and 20 teeth each. The timing belt has a wire extending beyond the bottom in the middle which fits into a small grove in the pulley. The pulleys are aluminum and I'm waiting for a quote for plastic. The guys I spoke with were very helpful. We talked about the RPM's, resistance and torque.
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Old Dec 14, 2011, 01:02 PM
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This has been discussed before but...the Liberator will have to have a 44" wing-span so the belts I had made will fit snugly but not stretched. I don't want to stretch them because the stress would be too much on the wing's LE and mostly at the nacelles. The largest propeller size is 5", but there will be four of them. I'm considering making four bladed props by making notches at the prop hubs and combining two together. What are the pros and cons?

Kev

another question - if I make the rubber motor in two sections: forward and aft of the drive train, where the forward motor is half as long as the aft, connected at the drive train just ahead of the wing, how will it unwind?
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Old Dec 14, 2011, 02:33 PM
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The other way to do the props is to make a fairly solid hub from aircraft plywood or even metal like aluminium and join separate blades to the hub. With the size of props for a 48 inch model this becomes more practical than for small stuff.

If you're talking about using simple commercial plastic props and notching them then a bit of a buildup of good epoxy around the joint would be advisable after bonding the props together using a solvent based plastic cement. The epoxy "hub" would aid by encapsulating the joint and stabilizing it.

You can't do a split motor setup like that unless both sides are the same length and have the same number of turns. Also note that the two motors need to be wound in opposite directions. If you do it the way you wrote then the rear motor will either have too few turns in it to provide full power or the front motor will run out of turns before the rear and the rear will wind up the front but backwards. Either way you would not get the full power out of the overall motor mass.

This whole winding of two motors is actually why it's far better to run one motor and have a set of transfer gears in the nose just behind the front bomb aimers bubble and gun turret "nose block". Then a shaft runs above the motor back to the wing root.
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Old Dec 14, 2011, 04:18 PM
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Thanks, Bruce. I considered the front transfer gears at the nose but thought I needed to abandon it due to the weight of the gears. But I probably will need the nose weight. That would give it a much longer motor.

Kev
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Old Dec 26, 2011, 02:46 PM
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Quote:
Originally Posted by kevin matthews View Post
I'm wondering if four 5" props have the same thrust as one 20" prop. If I can get 1/2" diameter sprockets I can enclose the whole thing in the fuse and wing.
Disk area ratio for four 5" props vs one 20" prop using the area of a circle formula of pi times radius squared is 4(pi(2.5)^)2/pi(10)^2.

The pi factors cancel and you have 4(6.25)/100=25/100 or one quarter the swept area. Seen from a disk loading perspective, there is no comparison between the two.
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Old Dec 26, 2011, 03:08 PM
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Quote:
Originally Posted by BMatthews View Post
You can't do a split motor setup like that unless both sides are the same length and have the same number of turns. Also note that the two motors need to be wound in opposite directions. If you do it the way you wrote then the rear motor will either have too few turns in it to provide full power or the front motor will run out of turns before the rear and the rear will wind up the front but backwards. Either way you would not get the full power out of the overall motor mass.
If the front and rear motors have different lengths, the two motors need a different cross sectional area. If a motor of length L takes 500 turns and is made of 4 strands of 1/4" rubber, you can make up a much shorter motor of 2 strands of 1/4" rubber that maxes out at 500 turns, or a longer motor of 6 strands of the same rubber that also maxes out at the same winds.

The torque of the two motors would be combined. Balanced motors would virtually negate any losses from a thrust bearing and an unbalanced motor would still reduce frictional losses, though to a lesser extent.
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Old Dec 30, 2011, 09:33 PM
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Freefright, I have to admit your math is way beyond my comprehension. Question: are counter rotating propellers absolutely necessary - the two on the right spinning clockwise and the two on the left counterclockwise ( or the other way around ) ?
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Old Dec 31, 2011, 05:39 AM
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I would say not neccessary Kev. Lots of people have done multi engined rubber successfully without...

Jon
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Old Dec 31, 2011, 11:53 AM
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Quote:
Originally Posted by kevin matthews View Post
Freefright, I have to admit your math is way beyond my comprehension. Question: are counter rotating propellers absolutely necessary - the two on the right spinning clockwise and the two on the left counterclockwise ( or the other way around ) ?
Never mind the math. I've inserted a graphic that shows the size of hole that would be punched in the sky by four 5" props compared to one 20" prop. The disk area of the larger prop is 4 times that of the 4 smaller props.

As to the direction of rotation of the props, the concern is primarily the torque effect on roll axis. Because the torque is applied at a distance from the center of mass, the net effect is less than it would be from a centrally mounted single propeller. There will be trimming differences between counter and non-counter rotating props but if you want the path of flight to vary as the props wind down, keeping them going the same direction would give a more familiar trimming experience
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Old Jan 10, 2012, 12:16 PM
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I've settled on building a 27" wing-span Liberator powered by four independent 3 1/2" diameter plastic propellers. I've taken four 5" propellers and have cut the tips off, rounded the new tips a little and balanced them. I'm going to use four motor sticks which will extend aft beyond the wing's trailing edge with hopefully a loop each of 1/8" rubber. I haven't settled on the motor stick's length yet. From what I understand that is allowed in the competition regulations. I have the plan copied to size. The next step is to convert the plan, which is for a much larger RC plane, to free flight. I am not, any time in the near future, spending $200. for a micro chain and sprocket single rubber motor drive train. This is still going to be fun.
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Old Jan 10, 2012, 12:51 PM
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Kevin, the issue seems to be that you went from trying to do it with simple home made solutions that were just not adequite to jumping right into the whole "buy it" theme. But in between there's room for making your own drive train items from scratch. Oh sure, they won't be the toothed belting like you were looking at. But with a bit of care and experimenting everyday inexpensive materials can be adapted to suit. The key is to figure out what sort of loads each part carries and pick your experimental ideas based on what you feel will work. Then if it doesn't you look at some other material and try that.

This manner of thinking was the basis of some early posts suggesting that you turn your own pulleys from laminated balsa "plywood" and finish the surface to create light but true running pulleys or drive drums. Such work is well within the ability of a home hobby shop. You just need to set up a small motor or use a Dremel rotary tool as a light duty part time wood lathe. From there music wire and brass tubing for the drives and bearings along with some selected low friction plastics would complete what would be a useable drive train.
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Old Jan 11, 2012, 06:27 AM
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Kev, sounds good - although the chain/pulley drive is still worth a go at some point I think. But mutli engine rubber models have been done successfully even without motor sticks - for some inspiration here's a pic of Chris Starleaf's B24.

Jon

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