Geared VS Direct Drive Glider Motor?

[Yotaman1985]

Hey everybody,

Looking at building a parts list for my E-Forza I will be getting soon. I have never built a powered glider like this before and see both geared and direct drive motor options.

Obviously direct drive seems simpler, less moving parts but Im curious what the benefits are of a geared motor are? Looking at building it as a 1kw power setup or there abouts.

Thanks in advance!

-Jeremy

 

[jure]

this is an interesting question !

one of the reasons is the diameter of the rotor , which affects the torque , as they are directly proportional.

power is the product of the angular velocity and torque.

available propellers are most optimal around 7,000 RPM ( and up to 10,000 RPM ) .

so , it is possible to wind a motor to have the right Kv, to turn at the optimal RPM , ( at a TBD certain voltage ) , to achieve some power.

in general , the issue boils down to the cross section of the motor ( diameter times length ) to achieve a given torque.
there are in the limit some fabrication issues, which may give an advantage to the gearbox ( even with its own liabilities ).

I prefer direct drives , without the weight and reliability penalty.

I can give you more details ...

 

[Wayne]

I like the sound of a gear drive, but prefer a direct drive for all other reasons.

For the Forza, you are nearly forced into a decent inrunner do to diameter options at 1k of power. If you like @Jarron can work up a killer setup for you.

 

[Yotaman1985]

I like the sound of a gear drive, but prefer a direct drive for all other reasons.

For the Forza, you are nearly forced into a decent inrunner do to diameter options at 1k of power. If you like @Jarron can work up a killer setup for you.

Thanks Wayne, I would be interested in what he would choose. Something in the 800-1kw range is what I was thinking. Since Ive never built something of this level before I am looking for any help. lol

Looking on hyperflight, they list the XPower F3426/7 Storm 1250 RPM and Tenshock GDM1520B Planetary Gear 4.75:1 as options which is what prompted my question.

​

 

[Jarron]

If you don't want absolutely ridiculous power that would cause the prop to make funny noises, that F3426/7 would be fine. A geared motor will have greater power density than a comparable sized direct drive motor to spin a bigger prop with lots of pitch usually. However with higher power levels (multi kw setups), you need to keep your duty cycle short for the sake of the geartrain, you can't do big wide open speed passes.
Another example would be if you had say, a somewhat heavy 3m powered glider that for some reason, they went with a 30mm nose, you would only get so much power out of the 28mm outrunner motor that would just BARELY fit.

 

[Yotaman1985]

If you don't want absolutely ridiculous power that would cause the prop to make funny noises, that F3426/7 would be fine. A geared motor will have greater power density than a comparable sized direct drive motor to spin a bigger prop with lots of pitch usually. However with higher power levels (multi kw setups), you need to keep your duty cycle short for the sake of the geartrain, you can't do big wide open speed passes.
Another example would be if you had say, a somewhat heavy 3m powered glider that for some reason, they went with a 30mm nose, you would only get so much power out of the 28mm outrunner motor that would just BARELY fit.

Thanks for the info Jarron. If I wanted a set up, not crazy powerful, but where I could cruise around/a few open field high speed passes per flight, what would your suggestion be? I have been watching Martin's E-Forza build in the forums here and after chatting with him, I think I want to go with a little lighter setup so as to not need as much weight to balance it and try to keep the overall weight down a little.

 

[Wayne]

Jeremy - How would you like to fly the E-Forza?

1- Impressive power bursts to climb to altitude, then shut down the motor, fold the prop and blast around the sky doing acro and speed passes until another massive blast of power is needed to repeat the cycle.

2- A little less power, but able to keep the motor on longer periods and fly more like an electric sport plane with the motor on most all of the time?

Due to the limited space, I'd lean towards an inrunner motor and an over-square prop (lots of pitch), but let's see what Jarron comes up with.

What is the AUW of your E-Forza right now?

 

[Yotaman1985]

Thanks Wayne,

I would probably lean more towards option 2 at this time. I would still like good/great vertical performance if possible but the ability to cruise around under lower power and not be limited to the motor being an on/off switch like a high performance hotliner setup.

Going back through my posts, it looks like the AUW for my Forza in slope form is 2145 grams. Trying to make the time to come pickup the E-fuse from you guys but havent figured out a good day yet.

 

[Tmcfarland]

I’ll certainly leave the specs to the experts at Aloft but once you figure out what you need if you can get in in an encapsulated motor with wires coming out the back, I think that is the way to go.

 

[Yotaman1985]

I’ll certainly leave the specs to the experts at Aloft but once you figure out what you need if you can get in in an encapsulated motor with wires coming out the back, I think that is the way to go.

Thanks Tmcfarland, that is definitely the setup I am looking for as the nose looks pretty snug

 

[Tmcfarland]

Thanks Tmcfarland, that is definitely the setup I am looking for as the nose looks pretty snug

I have this motor in a 3 meter glider and it has been great!

 

[MartinH]

Hi Jeremy,

For the Forza i think you need thrust more than pitch speed if you want to do big aeros, fast dives and passes in glide etc.. I think my numbers come out at around 5kg thrust at 75 mph pitch speed. I’ve used this formula on several aerobatic mouldies (Typhoon, Alpha 2.0, Forza, Orion) over the years and it’s perfect for me. If you are looking for crazy fast speed runs with the motor on, the Forza probably isn’t the best anyway.

For cruising around, climbing and diving, a lower power setup DD will probably serve you well.

The DD inrunner or outrunner in a can option is simple but will turn a much smaller prop, resulting in less efficiency. The main reason for going the DD route is that it’s cheaper. The geared hacker in mine is nearly 300 dollars now so it’s quite an investment at 3x the price of the Xpower. BTW even after blasting around for 10 mins, the geared motor is still barely warm.

The tenshock geared motor looks like a great option on paper - a low price geared inrunner capable of 1kw, turns a large prop on 4S but U need to choose the KV version as there are 4 to choose from.

I hope that this helps, Martin

 

[Yotaman1985]

Thanks Martin, that does help and is useful info.

 

[Wayne]

We also have some new Dualsky motors on the way, but will be a while before we see them.

 

[WimH]

Im curious what the benefits are of a geared motor are? Looking at building it as a 1kw power setup or there abouts.

Thanks in advance!

-Jeremy

The geared motor can swing a bigger propeller. A bigger propeller is (a lot!) more efficient than a smaller one. So the overall efficiency of a geared electric drive is higher, even though the gear introduces a small loss. This means longer (run) flight times and/or lighter weight (smaller motor, batteries,esc). In F5B for instance there is a maximum watt minute rule, so gears are mandatory here. For fun flying I'd favour direct drive (personally I always favour direct drive). Bear in mind that "encapsulated " drives again have lower efficiency than an equal weight "normal" outrunner.

 

[Wayne]

For sport flying I don't think many care about efficiency.