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Author Topic: Gearing up propellers on rubber powered models  (Read 822 times)
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sprogs
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« on: January 24, 2022, 01:19:15 PM »

Scale rubber power models look wrong with the correct size of prop for flight. Would it be possible to gear up the motor to give a higher prop speed with a smaller prop and what would be the design parameters ?
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billdennis747
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« Reply #1 on: January 24, 2022, 01:40:48 PM »

Scale rubber power models look wrong with the correct size of prop for flight. Would it be possible to gear up the motor to give a higher prop speed with a smaller prop and what would be the design parameters ?
Yes. Page 21
file:///C:/Users/bill/Downloads/small_model_aircraft_that_fly%20(1).pdf
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PB_guy
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« Reply #2 on: January 24, 2022, 02:17:51 PM »

Book is in the Builder's Plan Gallery - https://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=2826
ian
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Robmoff
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« Reply #3 on: January 24, 2022, 05:44:56 PM »

The Frog Interceptor (and the rest of the series) used a metal gearbox back in the 30s/40s. Bentom and Union used plastic gearboxes and so did the DPR Models HyperCub. I've got a few examples stashed away. .  .   .    .
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lincoln
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« Reply #4 on: January 24, 2022, 08:17:45 PM »

MRC used to sell some geared model kits. You might find one on eBay. I don't know if they were great or awful. I wish I remembered his name, but there was a guy, years ago, who used to sell small gears, cast in epoxy, sized for peanut scale.
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packardpursuit
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« Reply #5 on: February 11, 2022, 11:33:12 AM »

Bill Warner did a larger (don't think it was "jumbo") Leopard Moth, with a geared, small dia. prop.  It sure looked better, IMHO, but Bill told me it was not a great success. He used off the shelf slot car gears and just guessed at ratios, pitch, and diameter, etc. It was not adjustable and recall that he thought the biggest problem was loss of power via the gears, themselves. IIRC, his plan and article appeared in a late 60's M.A.N.
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Indoorflyer
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« Reply #6 on: February 11, 2022, 12:02:03 PM »

It actually had two rubber motors connected to the noseblock/gearbox mechanism.   (Nov 1967  MAN)

https://outerzone.co.uk/plan_details.asp?ID=9342
« Last Edit: February 11, 2022, 12:22:10 PM by Indoorflyer » Logged
lincoln
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« Reply #7 on: February 11, 2022, 06:32:31 PM »

I'd guess there are a bunch of articles on gears in old Aeromodellers. I think I read an article about doing off with more up to date materials, in another publication, maybe from the '80s. Maybe the easy way might be to start with geared electric motor parts, such as from the GWS IPS motors. There's an outfit called Homefly that sells little gears, bearings, carbon shafts, etc.

Somewhere, I have two very lightweight gadgets that allow a patient modeller to drive one prop with 6 (or maybe only 3?) skinny motors, allowing more turns in a given length fuselage. Very elegant, light, and simple, using cranks instead of gears. But winding so many motors would be annoying, I think, unless you could find a 6 rotored eggbeater to convert. (Twin pushers are sometimes wound with converted, regular eggbeaters.

(Auto-misspell disapproves of eggbeaters.)
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PB_guy
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« Reply #8 on: February 12, 2022, 12:44:59 PM »

Just a thought about gearing up motors for increased speed. Every time you add a gear, you are adding friction. Consider that friction losses will rob you of up to 10% of your power for each added gear. Another way of saying this is that the total amount of kinetic energy will still be expressed, but it will be expressed over a longer time period. In order to make up for this 'loss', you will have to add more power - that is - more rubber to compensate.

ian
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TimWescott
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« Reply #9 on: February 13, 2022, 05:39:28 PM »

Expect lower efficiency.

As mentioned, the gears will rob you of efficiency, although if you adjust them for higher backlash (by running them a bit loose) this will help.  Using gears made of low-friction materials, and experimenting with lubricants, would probably help too (graphite or the new teflon powder stuff).

Not yet mentioned, but propellers suffer from induced drag, too, and smaller props will have more induced drag.  So you'll lose efficiency there, too.

But -- if you can get it to fly, and it looks a lot better for the more realistic prop, maybe it's worth it!
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Hughs Aircraft
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« Reply #10 on: February 13, 2022, 10:14:53 PM »

I think gears could help; were there some Wakefield models in the past with gears? The FAC does not allow gears. I think the reason is that gears would not reflect the philosophy of old time and low budget modeling.
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Indoorflyer
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« Reply #11 on: February 13, 2022, 11:23:42 PM »

Not exactly true re FAC and gears  Roll Eyes Roll Eyes

ATTACHED:  What the FAC rulebook says,  and an example of a prop unit with gearing (Tom Hallman's Koolhoven).  Dave Livesay built a contra rotating system that several flyers have used.
Attached files Thumbnail(s):
Re: Gearing up propellers on rubber powered models
Re: Gearing up propellers on rubber powered models
« Last Edit: February 13, 2022, 11:36:42 PM by Indoorflyer » Logged
USch
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« Reply #12 on: February 14, 2022, 08:51:28 AM »

I think gears could help; were there some Wakefield models in the past with gears? The FAC does not allow gears....
From the rule book:
"ONLY gear arrangement with a 1:1ratio may be used."

That's it.
In the '50 with unlimited rubber amount/weight the Wakefields used to have 2 motors parallel in the fuselage. At the rear a gear with 2 identical gearwheels had them coupled for a longer motor run.

Urs
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calgoddard
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« Reply #13 on: February 14, 2022, 01:21:53 PM »

A recent edition of Free Flight Quarterly (FFQ) contained an excellent article on how to build gear drives for small and medium size rubber powered scale models. The author had considerable success with these models.  He gave extensive details on exactly how to build the gear drives and included excellent pictures.

While there are some losses of efficiency (e.g. 20%) the author wanted to build (and fly) more realistic looking rubber powered scale models.  The gear drives he built allowed him to use smaller diameter props.  To some modelers, a Peanut Piper Cub, for example, looks very unrealistic with a 6-inch diameter Prop.

FFQ is a subscription service, available via download on the Internet.  The articles in this publication are written by true experts in free flight from around the world and are absolutely first rate. The publisher relies on the subscription fees (very reasonable) for income. For this reason, and out of respect for copyright laws, those interested will need to contact the publisher to see if they can purchase the single edition containing the article about gear drives in rubber powered scale models.

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lincoln
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« Reply #14 on: February 14, 2022, 08:39:02 PM »

I think gears could help; were there some Wakefield models in the past with gears? The FAC does not allow gears. I think the reason is that gears would not reflect the philosophy of old time and low budget modeling.
Maybe so on low budget, though I think it's more like low time, and we know there's no rule against putting a hundred hours or more into a fancy model. "Old time" is just not so. Plenty of geared models from the 1930's. If I'm not mistaken, that 6 motor into one prop, also banned unless they've changed the rules, dates back to 1931.
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lincoln
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« Reply #15 on: February 14, 2022, 08:58:44 PM »

Sorry, that last message got posted before I was done with it, and now I'm too lazy to write the last part over again.
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Indoorflyer
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« Reply #16 on: February 14, 2022, 11:34:05 PM »

Too late to edit now(hate that forum restriction), but I meant to say Keith Sterner not Dave Livesay in Reply #11.
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