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Old Aug 12, 2012, 09:09 AM
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carbondale il
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Bruce, you're right about the scale look concerning the props but I'm concerned about enough thrust. Four bladed props will give me 226", twice the 113" with just two blades for each. Honestly, I really appreciate your help, I just can't picture all of it in my mind.

Kev
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Old Aug 12, 2012, 09:30 AM
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United States, FL, Perry
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I have made these props several times and they work very well.
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Old Aug 12, 2012, 04:38 PM
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Been spending the day drafting the plan for the wing and wing ribs. I'm leaving out several ribs to save weight but not at the root or at the nacelles. Quite a few at the tips. The plan I'm going by is a design for a much larger and stronger RC model. I had to compensate for the 3/32" sheeting on the wing as the ribs outlines are shown without the sheeting, so on my plan I made the ribs just a little larger but keeping the LE and TE as per the RC plan. It worked out that a the size I'm building at, the sheeting shows at 1/16" thick. The RC plan shows ailerons and flaps and so my plan is much simpler with a 1/16" x 1/2" TE. The LE has a 1/16" stick which fits in a notch at the rib's LE and runs the length for each semi-span. On top of that there are two 1/16" x 1/2" "sticks" which taper down to the tips and are laminated. They will need to be sanded to a round LE. As well, the TE will need to be sanded to a taper. I'm not sure yet about the spars. So far I have designed in a single main spar 1/2" x 1/8" at the roots and tapers to 1/8" x 1/8" at the tips. I'll probably bolt the wing semi-spans to the fuse with small nylon bolts and nuts and glue as well. I'll need to reinforce that fuse area where the wing semi-spans attach by running a couple of large spars through.

Kev
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Old Aug 12, 2012, 04:47 PM
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Here's the plan

Kev
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Old Aug 12, 2012, 04:51 PM
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Here's the RC plan.

Kev
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Old Aug 12, 2012, 08:05 PM
Balsa Flies Better!
Stamford, CT
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Kev

A better starting point might be Mike Midkiff's 51" Liberator which was designed for 4 electrics- but FF. Try here: http://www.ozarkmodelaviation.com/products.php?cat=8 - but please note- I agree with Bruce- I think that direct drive rubber makes more sense- and electric a heck of a lot more. The airplane will be lighter and fly better as an electric.

Sam
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Old Aug 12, 2012, 08:12 PM
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The 'Wack, BC, Canada
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Quote:
Originally Posted by kevin matthews View Post
Bruce, you're right about the scale look concerning the props but I'm concerned about enough thrust. Four bladed props will give me 226", twice the 113" with just two blades for each. Honestly, I really appreciate your help, I just can't picture all of it in my mind.

Kev
Well, it simply doesn't work like that. The swept area is the swept area. Adding blades does increase solidity of the propellor disc area. But a 4 blade prop will simply not be twice as effective as the same diameter 2 blade prop.
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Old Aug 12, 2012, 09:05 PM
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Peck-Polymers offers a wide blade 7" plastic prop. I'm thinking of making 6" props from them and in 4 blades. I'm not going to give up on this. Sam does have a point about going electric but I see no need for a different plan. but I am going to try to figure a way for rubber until I am faced with impossible... I might be able to get another 1/2" added to the prop diameter.

Kev
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Old Aug 12, 2012, 09:37 PM
Balsa Flies Better!
Stamford, CT
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That 7" Sig wide blade prop is useless- as Joe Wagner pointed out- it makes a great ice cream scoop. The blade has too much camber.- it's draggy.

I just went through this same exercise with my B-26- I tried the 4 blader Superior props (I'll sell you two if you want) at a 6" diameter- but I found that the black 6" Testor (aka Tern Aero prop) is very hard to beat. The only multiblade (more than 2) setups that seem to work are long skinny blades- and even then, I think two bladers are probably better. It's the same reason people tried monoblades- you want the prop blade in undisturbed air- if you have more blades, there's more disturbed air.

FYI- A simple scale up of weight would suggest that you shoot for 170-200 grams of airplane. My B-26 has just hit 32 seconds and weighs around 85 grams with rubber- and I think I can get some more time out of it with a different trim setup. This of course, is with two 6" props.

I'll also throw out another wacko idea....

Don't put the motor in the fuselage- put the motors in the wing (one motor driving two props). Use an Estes rocket tube to contain the motor- that way the wing structure can be pretty light.

The other problem with the Liberator is that it's got a short fuse. If you put the motor in back of the wing- it'll be terribly tail heavy-your best bet is to put the motor in front of the prop drive shaft, but now you haven't got much more motor length than the nacelles.

Sam
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Old Aug 12, 2012, 10:23 PM
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Thanks, Sam, for the heads up on the props. Including the front and rear canopy / gun turret, the fuse is 30". My "motor stick" length is 22". It will run under the wing and into the nose area. I don't think it will be as heavy as you have stated. I'm using 4 -6 lb C grain balsa except for the four longerons and the other stressed sections. The ribs and formers will be 1/20" except for the eight nacelle ribs. I can't get my head around wing tubes and the rubber motor inside the wing, tell me more.

Kev
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Old Aug 12, 2012, 11:57 PM
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A thought is going with two 9 1/2" props on the outside nacelles only. Two 11" props will fit. If I extend the inside nacelle bearings another 1/2" I can use 6" props there. The 11" will overlap some.
Another idea is to go with four 8" props - they will overlap some but with the extended inside nacelle bearing it'll work if I make a cavity in the sides of the fuse to give the props room to spin.

Kev
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Old Aug 13, 2012, 02:39 AM
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Quote:
Originally Posted by Megowcoupe View Post
FYI- A simple scale up of weight would suggest that you shoot for 170-200 grams of airplane.
I'd agree with this target: 170-200g gives you a Cubic loading of 3.3 to 4. Should give you a nice performer.

Kev, the problem with the props is not that they can't make enough thrust, they can, but they will do it at significantly higher revs than the single big prop. Which means short motor runs. So as I see it the 1:1 gearing ratio is quite a challenge. Normally you'd get the higher revs (and more duration) from a ratio of around 1:2.

The other thing is that this is a high aspect ratio plane (A=11.56 from the full size specs) so that will give a good glide ratio and require less thrust than you might think from the 50" span. I think it could be quite successful with 6" props but you must keep the weight down and I'd be tempted to give up on the 1:1 - better to have a fun sport model than a poor competition model? Your call though of course...


Jon
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Old Aug 13, 2012, 10:57 AM
Balsa Flies Better!
Stamford, CT
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Hi Kev

Estes makes resin reinforced cardboard tubes which are pretty rugged. There are several problems putting the motors in the wing:
1) rotational mass
2) winding
3) Diameter of the tube in terms of wing height.

Wings can't be used to hold motors as they're normally built- they can't take the torsional loads. Hence all the loads of the motor have to be borne by the rocket tube that contains it- but these things are pretty strong, and I'll bet they could take it.

Requirements- good 90 degree gear drive. If you want all 4 props turning- then one option would be to use the gear drive to drive the inner prop- and then use some type of chain/sprocket arrangement (heck, could be rubber bands) to drive the outer props off the inner one.

Structurally, this is an easy solution- the whole airplane gets to be made very light because all the loads are being borne by the motor tubes -well- there will be some loads connecting the motor tube to the prop.

In terms of rotational mass- it's pretty ugly- depending on how far out you put the tubes in the wings, you'll have a lot of weight outboard and with long skinny wings, I predict cartwheel city. For winding- you're on you're own, but I could see winding through a wingtip.

In terms of airfoil thickness- the Lib had a thick wing, which sucks for rubber power- a thinner airfoil would work much better but is also trickier to build. However, if you're going with a fat airfoil, then accommodating the motor tube gets lots easier.

FYI- there was a Japanese toy model of a high wing twin executive type airplane that used some type of flex cable drive. The other similar drive that might be worth looking at was used in the Victor Stanzel airplanes-batteries/motor in a handle- flex drive to a prop on a lightweight airframe/helicopter- kind of fun for a kid.

HTH

Sam
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Old Aug 13, 2012, 11:47 AM
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carbondale il
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Jon, I don't know where the competition bug came from. I had a college teacher who disliked competition. Maybe it's "The thrill of victory" as the old TV show Wide World Of Sports used to open with. Or having my name in the records along with Applehoney. Competition is quite a motivator. Sam, I looked into using rubber bands as pulley belts but I discovered there could be quite a substantial power loss.
Got the last of the copies made today needed for the plan drafting.

Kev
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Old Aug 13, 2012, 02:51 PM
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I'm with you there in general Kev, just this one I think will be tricky under the 1:1 rule. But either way it's a very interesting project...
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