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Old Aug 23, 2012, 11:42 AM
flight999
England
Joined Dec 2002
2,344 Posts
Hi Kev. A very interesting topic. A couple of ideas you might like to
consider particularly in the mass saving area.
Thin ribs capped with carbon strip. You wouldn't even have to allow
for the thickness of balsa sheeting. .002" carbon wouldnt even notice
ompared to balsa sheet.
If you gear the propellors through a drive chain you could possibly
ofset the drive somewhat so that you could use (say) 2 x 7" props
overlapping?
Worth trying perhaps.
flight.
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Old Aug 23, 2012, 06:32 PM
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carbondale il
Joined Jan 2007
2,557 Posts
Flight, thanks for the carbon suggestion. There isn't enough room from the inside nacelles to the fuse side to fit in a 7"prop. 6" is the largest I can go. I've been debating whether to go with two 9 1/2" or four 6" props and I've decided to go with the 6" props. I believe the drive train I'm going with couldn't take the stress of the larger props, plus, I'd have to really beef up the wing, at least to the outside nacelles and I don't want the weight for that. The Turnaero props arrived but I made a mistake and got four 9 1/2" props and two 6" ers. I have to place another order. Also the teflon washers I bought are too small, they are for .032" prop shafts and I'm going with the 3/64". I did get the small black thrust buttons I need for the hubs and bearings of the sprockets. The nose gears will use the larger white thrust buttons. Guillow's sent the wrong clear plastic parts sheet so I had to call them and get them to send the right one, which is the canopy and the nose canopy.

BTW, I've been building a Guillow's Javelin. I had to buy so much so I added the kit to my order. It's got a 24" wing-span and a tapered polyhedral wing with a root chord of 2 1/2". I've built 1/8" washout in both wing tips beginning from the outside polyhedral breaks and I'm wondering if that's too much. I hope not, but if so, I'll build another wing.

Kev
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Old Aug 26, 2012, 05:59 PM
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carbondale il
Joined Jan 2007
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The clear plastic parts sheet arrived from Guillow's. They have a Liberator display model kit close to the same size as the one I'm building. My model size was based on this.

Kev
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Old Aug 27, 2012, 09:28 PM
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carbondale il
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I've been looking in to small coil compression springs. I'm planning on putting one on each prop shaft between the thrust button and prop, with the necessary washers. That way the props will be pushed forward about 1/4". I will be able to push them back and expose the 90 degree angle of the shafts which fit into the prop notches to change props if necessary. Very seriously wanting to go with four bladed props. That's what the Liberator has.

Kev
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Old Aug 28, 2012, 02:28 AM
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Yak 52's Avatar
UK
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Quote:
Originally Posted by kevin matthews View Post
Very seriously wanting to go with four bladed props. That's what the Liberator has.
I don't see why not Kev. There will be some loss of efficiency over two-bladers but the overall power 'absorbtion' will be better, a good thing with limited revs from 1:1 gearing. It may well kill the glide performance though: with the extra drag when the props are stopped. I believe Chris Starleaf used 4-bladers on the original Dash 8 but then swopped to 2 blades.

Jon
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Old Aug 28, 2012, 08:44 AM
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carbondale il
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Jon, the props won't be free wheeling when the motor runs out. I think that will be better and with less drag. I've been told by my Cousin, who is a commercial jet pilot, that free wheeling causes a LOT of drag.

Kev
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Old Aug 28, 2012, 09:30 AM
Registered User
Ajax, Ontario, Canada
Joined Oct 2004
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That may be so in fullsize when a prop can maybe feathered instead but hardly applicable at our level.

Take any rubber model, trim for a good glide with a f/w prop - and then 'stop' that prop. The result is quite obvious in the steep glide path thereafter. I once published a beginners model ('First Night') which used a fixed prop simply to bring it back down fast ready for a youngster to fly again.

If freewheels caused more drag .. why have they been in use on rubber models for close to a century? With respect to your jet pilot cousin ... fullsize and model aerodynamics are at very different levels.
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Old Aug 28, 2012, 11:33 AM
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Jim is right, but so is your cousin

The amount of drag caused by a free wheeling prop depends mostly on what it's driving. On a full sized piston aircraft the prop is turning over the engine which absorbs a lot of energy from the airstream as drag. With a free flight prop that disengages there is very little drag from freewheeling and especially so with the high pitch, wide blade props and slow airspeeds of rubber models. It's better to let them disengage and spin freely than to stop them, where they would present a large flattish area to the airflow and cause substantial parasite drag.

In nearly all other types of aviation stopping a prop is less drag, but we operate in a tiny pocket where the conditions provide an exception.

Four blades will have more drag in the glide either way, but the extra power absorbtion will mean a longer motor run.
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Old Aug 28, 2012, 12:00 PM
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carbondale il
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Thanks for the info, guys. There is no way I can devise to make them so they can free wheel. The prop shafts will be attached to the drive train sprockets so then the prop shafts won't be able to extend when the motor runs out. In other words, the props on typical models are pulled aft by the motor while wound but after the motor has run out the shafts are free to disengage the props. On my model, the prop shafts are fixed.

Kev
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Old Aug 28, 2012, 12:59 PM
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Ajax, Ontario, Canada
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I think it is feasible with something like a Garami clutch on each prop assembly but would be a tricky job getting all four engaged at one time. The small type made (still made?) by Superior Props would be easier to engage but think you'd have to have such specially made to a convenient size.
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Old Aug 28, 2012, 07:58 PM
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carbondale il
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Thanks, Applehoney, I'll look into it.

Kev
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Old Sep 01, 2012, 10:28 AM
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carbondale il
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I found a company who carries the small coil springs I need - DR Templeman Co. Their minimum order quantity is 25 springs. My plan is to put 1" free length coil compression springs between the thrust buttons and the propellers, around the prop shafts. I'm going to set the props on their shafts so the springs are compressed to 50%; 1/2". I'll be able to compress the props by 75%; 1/4" to change the propellers. The plastic parts - the nose canopy and the pilot's canopy are not spot on in size but I'll be able to adjust the fuse frame to accommodate.

Kev
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Old Sep 02, 2012, 08:34 PM
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carbondale il
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Picked up some Gorilla Glue brand super glue. It's supposed to be the best. I need to glue a few parts together in the drive train like two sprockets together x 3 and thrust buttons for hubs. Also thrust buttons for the gear hubs. Then eventually the 3/64" wire shafts.

Kev
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Old Sep 06, 2012, 10:14 AM
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Well, I was wrong...the Liberator's props are 3 bladed. I've been working with the Gorilla Glue with the wire which will stick out of the nose with a "hook" so I can wind it at the nose, as well as at the tail. The glue is in between the common gel, and "watery" glue consistency.

kev
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Old Sep 07, 2012, 06:13 PM
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The rest of the propellers, thrust buttons and washers arrived. I've made the 4 bladed props and am waiting for the glue to dry.

Kev
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