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direct drive vs. gear drive- prop selection and application

Old 11-13-2007, 07:54 AM
  #1  
smitty
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Default direct drive vs. gear drive- prop selection and application

Hi all,

1. What's the difference in a direct drive and a gear drive set up using the same motor?

2. What would the prop ranges be for both?

3. What type of plane/flying style would each cover?

Just curious.

Gonna try to power up an Ultra Flyer (the cheapy foam tosser)

Any suggestions?

Fleet: SF Soarstar, Typhoon2 3D, Currently building Great Planes Turmoil


Thanks, folks.

Smitty
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Old 11-13-2007, 02:03 PM
  #2  
Gnascher
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Originally Posted by smitty View Post
Hi all,

1. What's the difference in a direct drive and a gear drive set up using the same motor?

2. What would the prop ranges be for both?

3. What type of plane/flying style would each cover?

Just curious.

Gonna try to power up an Ultra Flyer (the cheapy foam tosser)

Any suggestions?

Fleet: SF Soarstar, Typhoon2 3D, Currently building Great Planes Turmoil


Thanks, folks.

Smitty
1. In general, you wouldn't have the possibility of the choice for a given motor. Technically, you could use a REALLY SMALL prop in a direct drive application using an in-runner, or a REALLY BIG prop with a geared outrunner ... but it's probably not your best choice.

Think of it this way ... an inrunner is like a small-displacement high-revving engine like you might find in a high-performance motorcycle. It makes power by spinning VERY fast but produces very low torque numbers. In order to swing a large, efficient prop you need a gearbox to raise the torque available at the prop. Inrunners typically have KV values of around 5000, and it's not uncommon to gear them down something like 5 or 6:1. KV stands for revolutions per volt. So, a 5000KV motor on an 11.1V lipo should spin at ~55,500 rpm. Gear it down at 5:1 and you're spinning a prop at 11,100 rpm. If you use an 8 pitch prop, that's a theoretical pitch speed of ~84mph.

An outrunner is more like the engine in mack truck. It spins much slower at full throttle and creates a larger amount of torque at the output shaft. Outrunners typically have KV values somewhere around 1000. With an 11.1V Lipo, you're spinning at 11,100 rpm (just like the geared down inrunner).

Now ... why are they both there? In general, inrunners are more efficient than outrunners. More of your battery pack's electricity will go into spinning the motor, and less will go out the back as heat. However, with an inrunner you're carrying the additional weight of the gearbox. There's also the mechanical complexity ... gearboxes are prone to stripping gears and having pinion gears slip off. BUT in a crash ... the motor is less likely to take damage and you'll just have to fix or replace the damaged gearbox.

Outrunners are less efficient, but typically make up for that with a smaller, lighter package. Outrunners are simpler to mount, require less space and weight and have fewer parts to wear out or break down. They are more vulnerable to crash damage, but are usually reparable with replacement shafts and bearings.

2. Inrunners and outrunners can use the same props ... but inrunners need to be geared down. It's essentially impractical to direct-drive from an inrunner (you'd need a teeny little prop and probably need a high-start or something to get the plane up to speed before the prop could be effective) or to gear-drive an outrunner (you'd need a HUGE prop).

3. Flying style is irrelevant ... you could use either setup in just about any plane. It really comes down to personal preference, and what you've got room to mount behind the cowl. But ... there is one advantage that inrunners have over outrunners. A given outrunner has a relatively small range of props that it can spin. A geared inrunner can spin a very wider range of props because you can change the gearing in the gearbox. So, if you're the type of person who likes to continually fiddle with props and ratios ... you're an inrunner person! In general, I think you are more likely to see inrunners used in smaller planes (very common on slow-sticks, and really small micro scale stuff) and on EDF applications. The larger high-performance and 3D planes are more apt to be outrunners ... but there are certainly always exceptions.

My suggestion for your foamie? A direct-drive outrunner ... cheap and simple.
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Old 11-13-2007, 06:12 PM
  #3  
Ryan CSRC
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Originally Posted by smitty View Post
Hi all,

1. What's the difference in a direct drive and a gear drive set up using the same motor?

2. What would the prop ranges be for both?

3. What type of plane/flying style would each cover?

Just curious.

Gonna try to power up an Ultra Flyer (the cheapy foam tosser)

Any suggestions?

Fleet: SF Soarstar, Typhoon2 3D, Currently building Great Planes Turmoil


Thanks, folks.

Smitty
1. Most gear drives are used to provide more torque, so given the same motor they would swing a larger prop at slower RPM for the same power and amp draw. In theory though a gear drive can be made at any ratio so it could in face make the motor spin a smaller prop at higher rpm, but no one really uses them like that in the RC world.

2. Prop range specifics obviously depend on the motor and gear drive chosen, but again with the gear drive, prop size will be bigger.

3. They can really cover most of the same flying ranges. In general (I know there are exceptions but in general), for sport flying outrunners are run direct drive and inrunners are run geared. In this case they overlap eachother, both work for the same application.

If you need help choosing an outrunner that will work for your plane, click on the motor wizard on our website.

Ryan Lefevre
www.CommonSenseRC.com
The Go To Guys For Electric Power
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Old 11-13-2007, 07:58 PM
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smitty
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Gnascher & Ryan

Thanks for the info. Very helpful.

The reason I asked was that I saw an ad in Model Airplane News for Himax motors & gearboxes.

The HA 2015 is a 100W motor, the 2025 is 175W. Below that was the HG2015/25 gearbox drive (it had a motor in it & said 100W-175W).

I'm assuming that the gearbox was for the 2015 motor or the 2025 motor. If that's the case, that was what my question was about.

Anyway, thanks again. You cleared up some confusion.

Smitty
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Old 11-13-2007, 09:30 PM
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fr4nk1yn
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While not your question, flexibility comes into play.
An outrunner isn't as flexible as an inrunner in a gearbox.

An outrunner for highspeed will not be able to run a large prop, unless put in a gearbox but that's getting ahead.
A 2025 can use a 6.6:1 ratio and turn a 12x6 easily or a 3.75:1 ratio and run a 9x7.5 prop at much faster speeds.
I've used my 2025-4200 in both my AP SlowStick and Formosa.
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Old 11-14-2007, 02:10 AM
  #6  
Dr Kiwi
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Originally Posted by smitty View Post
Gnascher & Ryan

Thanks for the info. Very helpful.

The reason I asked was that I saw an ad in Model Airplane News for Himax motors & gearboxes.

The HA 2015 is a 100W motor, the 2025 is 175W. Below that was the HG2015/25 gearbox drive (it had a motor in it & said 100W-175W).

I'm assuming that the gearbox was for the 2015 motor or the 2025 motor. If that's the case, that was what my question was about.

Anyway, thanks again. You cleared up some confusion.

Smitty
I think you have figured this out anyway, but better to be sure. Just to clarify one more thing - yes, the 2015 is rated at 100-110W max, and yes, the 2025 is rated at 175W. Either motor would fit in that gearbox - hence the 100W-175W - for the gearbox. Just putting a 100W motor in the gearbox does not make it capable of coping with 175W - it is still limited to 100W but it CAN drive a far larger prop, when geared, than it can in direct drive.
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