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question about a brushless motor.

#1
hi all, I am a aeronautical student and we have a project at school where we need to build an rc plane that can carry as much weight as possible.
my school suplies us with all the electronics. We have a D2836/11 750 KV brushless motor (http://rcsearch.info/hobbyking/i12922/) and we got 4 propellers (11 / 4,7) and we are curious if the engine is strong enough to power 2 propellers instead of just one. If so we want to connect those 2 by a simple drivetrain.

Do you guys think this would be viable idea?
 
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Hondo76251

Well-known member
#2
I would think you'd have better luck just running the prop normally. Adding a drivetrain will add parasitic loss and weight. The motor is only capable of so many watts, adding propellers wont really give it more power. Instead I'd make sure you have an esc and battery that can give the motor as much power as she can handle. A higher cell count battery will spin the motor faster, that's the easiest way to maximize power so long as you dont burn up the motor!
 

cranialrectosis

Well-Known Member
Mentor
#4
I think you have a deadline due to the school project and are looking at an ENORMOUS learning curve if you have never built/flown.

Not knowing your current skill level I can only assume based on your question that this is new for you and your classmates.

If I am correct, I suggest closely observing the KISS principle. I don't think you have the time to experiment up to a two prop system that works unless you already have a bunch of experience with single prop planes already under your belt.

One prop is WAY simpler than two. Direct drive is WAY simpler than a drivetrain. And when I say WAY simpler, I mean months or years of experience.

Now, on to your question. An electric motor has a wattage limitation. Wattage is determined by KV (750), prop length and pitch (11/4.7) and voltage. Per Hobby King your motor will handle one, 11/6 prop on 3s (12v).

On 4S(16v) you will probably fry the motor and ESC (FIRE!!). On 2S (7.4v) you can go up to a 12x6 prop.

To tinker without causing fire, you can run ecalc.

As a general rule, longer props at slower speeds (lower kv and voltage) are more efficient for flight time. Shorter props and higher voltage for thrust but at a reduction of flight time.

If you want to run 2 props, 11/7 is way too long for that motor on 2S or 3S. I think you would end up running 6" props on a 6S lipo and losing much of your thrust due to entropy.


Whatever you decide, I recommend you start a build thread with photos of your build as you build it and video of any flight attempts. This will document your build and allow the experienced members of our forum to assist you if things go south. It will also allow us to cheer you when you succeed!

Welcome to the FliteTest forums @Niels van de Wetering!
 
#6
I think you have a deadline due to the school project and are looking at an ENORMOUS learning curve if you have never built/flown.

Not knowing your current skill level I can only assume based on your question that this is new for you and your classmates.

If I am correct, I suggest closely observing the KISS principle. I don't think you have the time to experiment up to a two prop system that works unless you already have a bunch of experience with single prop planes already under your belt.

One prop is WAY simpler than two. Direct drive is WAY simpler than a drivetrain. And when I say WAY simpler, I mean months or years of experience.

Now, on to your question. An electric motor has a wattage limitation. Wattage is determined by KV (750), prop length and pitch (11/4.7) and voltage. Per Hobby King your motor will handle one, 11/6 prop on 3s (12v).

On 4S(16v) you will probably fry the motor and ESC (FIRE!!). On 2S (7.4v) you can go up to a 12x6 prop.

To tinker without causing fire, you can run ecalc.

As a general rule, longer props at slower speeds (lower kv and voltage) are more efficient for flight time. Shorter props and higher voltage for thrust but at a reduction of flight time.

If you want to run 2 props, 11/7 is way too long for that motor on 2S or 3S. I think you would end up running 6" props on a 6S lipo and losing much of your thrust due to entropy.


Whatever you decide, I recommend you start a build thread with photos of your build as you build it and video of any flight attempts. This will document your build and allow the experienced members of our forum to assist you if things go south. It will also allow us to cheer you when you succeed!

Welcome to the FliteTest forums @Niels van de Wetering!
Tnx for the reply! I we dont have that much experience, i myself have graduated as a aircraft mechanic, but dont have that much experience with electronics.

We are really in the beginning fase of the project and we are need to have the configuration locked down now, so therefore we were brainstorming to what might be possible and what not. We indeed dont have the time to put too much effort in it.
 

Piotrsko

Well-known member
#7
Traditional approach for weight lifting is as much square area wing as the motor will pull. The specs on the motor say 800 grams, so to accelerate the flying weight has to be less than that. Rules of thumb dictate 20% of the wing area should be horizontal stabilizer at least 3 wing chord lengths away from the wing. Another rule of thumb is if it looks pretty much like an airplane, it will fly pretty much like an airplane.
 

Hoomi

Well-known member
#8
As Hondo said, adding any kind of additional drive train is going to induce a certain amount of loss, in addition to extra weight. With a project like this, it's best to remember the old engineer's KISS rule - "Keep It Simple, Stupid." (Don't take that last word as directed at you - that's just how the old acronym adage went). The simpler you build it, the less things there are to go wrong, AND, the less power loss you're going to have in your propulsion.

There are pages out there showing the motor-propeller-ESC combos recommended for different applications, and those can give you a good idea of the best ESC and propeller combo for your motor for a lifting capacity. You don't need speed nearly as much as steady thrust to pull the weight along. Remember, too, that the more you accelerate your load, the more power it's going to require and drain from your battery. A slower moving plane with loads of lift is going to serve you better than a faster moving plane.

A scaled up Simple Scout design might serve you well. It's a proven flyer with decent wing area and stable flying characteristics, and a favorite of many of us here on the FT Forums. The build is overall fairly lightweight while still maintaining a decent structural strength. It would be fairly easy to leave off the "turtledeck," and incorporate an open area or openable hatch in front of the cockpit, right over the wing, for payload. Doing so would help to maintain CG while loading, which is one thing you're definitely going to have to watch carefully when adding payload. If the CG is shifted too much, you're looking at the makings of a crash.
 

Bricks

Well-known member
#9
In your original post you mention carry as much weight as possible if this is added weight then your platform has to be as light as possible with as much lift your power system and platform can achieve. Things to consider low drag, high lift airfoil, built as light as possible. I would not put any type of finish on as this adds weight, would do a three channel or even a V tail to cut down on weight from servos and wires definently not use hot glue..
 
#10
As Hondo said, adding any kind of additional drive train is going to induce a certain amount of loss, in addition to extra weight. With a project like this, it's best to remember the old engineer's KISS rule - "Keep It Simple, Stupid." (Don't take that last word as directed at you - that's just how the old acronym adage went). The simpler you build it, the less things there are to go wrong, AND, the less power loss you're going to have in your propulsion.

There are pages out there showing the motor-propeller-ESC combos recommended for different applications, and those can give you a good idea of the best ESC and propeller combo for your motor for a lifting capacity. You don't need speed nearly as much as steady thrust to pull the weight along. Remember, too, that the more you accelerate your load, the more power it's going to require and drain from your battery. A slower moving plane with loads of lift is going to serve you better than a faster moving plane.

A scaled up Simple Scout design might serve you well. It's a proven flyer with decent wing area and stable flying characteristics, and a favorite of many of us here on the FT Forums. The build is overall fairly lightweight while still maintaining a decent structural strength. It would be fairly easy to leave off the "turtledeck," and incorporate an open area or openable hatch in front of the cockpit, right over the wing, for payload. Doing so would help to maintain CG while loading, which is one thing you're definitely going to have to watch carefully when adding payload. If the CG is shifted too much, you're looking at the makings of a crash.
Hahaha i was already wondring what KISS meant XD, no worries i get that its just an adage.

We are defenitly making a high wing aircraft because of the stability. The extra payload we need are metal strips so we want to make sure there is space for them in the fuselage length wise so we can keep the CG as close tho the initial position. A nose heavy plane flies bad and a tail heavy one flies once.
 
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#11
In your original post you mention carry as much weight as possible if this is added weight then your platform has to be as light as possible with as much lift your power system and platform can achieve. Things to consider low drag, high lift airfoil, built as light as possible. I would not put any type of finish on as this adds weight, would do a three channel or even a V tail to cut down on weight from servos and wires definently not use hot glue..
Yes you are correct, we are gonna make a power required to power availabe diagram and next week we are gonna select an airfoil using xfoil (a progrma that gives data of airfoils)

The v-tail is a good idea and im defenetly consider it.
 

Wildthing

Well-known member
#14
If you can't change the prop, battery, esc or motor and are locked into that power setup weight is what you have to get rid of as @Bricks stated. Anything you can get rid of or things like trimming any excess wiring and shortening them as much as possible . Any grams saved anywhere building will help you in lifting more weight.
 
#16
Great choice. Buy design is a power lifter and built light. I thought 1 to 1 power/weight would fly pretty well and any extra weight would just decrease performance? And one thing he did not tell us was how far it had to fly. Does it have to fly the length of a football field, 50 yards 20 yards will you be hand launching or will it have landing gear? I could be wrong about the power to weight ratio but on RC groups one of the guys told me that a 1 to 1.5 would go vehicle out of sight. So I'm thinking a little differently from what I've read. Now I could be wrong and if I am please correct me so I'll know and have the right thinking myself. I love to fly and I'm great on the sticks but scratch building is all new to me as well.
 
#17
As Hondo said, adding any kind of additional drive train is going to induce a certain amount of loss, in addition to extra weight. With a project like this, it's best to remember the old engineer's KISS rule - "Keep It Simple, Stupid." (Don't take that last word as directed at you - that's just how the old acronym adage went). The simpler you build it, the less things there are to go wrong, AND, the less power loss you're going to have in your propulsion.

There are pages out there showing the motor-propeller-ESC combos recommended for different applications, and those can give you a good idea of the best ESC and propeller combo for your motor for a lifting capacity. You don't need speed nearly as much as steady thrust to pull the weight along. Remember, too, that the more you accelerate your load, the more power it's going to require and drain from your battery. A slower moving plane with loads of lift is going to serve you better than a faster moving plane.

A scaled up Simple Scout design might serve you well. It's a proven flyer with decent wing area and stable flying characteristics, and a favorite of many of us here on the FT Forums. The build is overall fairly lightweight while still maintaining a decent structural strength. It would be fairly easy to leave off the "turtledeck," and incorporate an open area or openable hatch in front of the cockpit, right over the wing, for payload. Doing so would help to maintain CG while loading, which is one thing you're definitely going to have to watch carefully when adding payload. If the CG is shifted too much, you're looking at the makings of a crash.
I use the simple scout to drop paratroopers, up to 4 at a time. I've made another, adding 1" to each wing (because: why not?) simplifyied by
leaving out the turtledeck and omitting the scalloping and other decorative features. It's a graceful and fairly easy flyer.

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synjin

Well-known member
#20
This is just anecdotal/observation, but with the Delanne tandem wing configuration I get equal performance in a larger airframe as a smaller traditional configuration. A 2836? You could probably fly a 48" wingspan tandem with that, with a 36" to 48" long fuselage, 4" or 5" high and wide. Make the fuselage lifting as well. It would probably be 600g-750g airplane. With a good runway I bet you could lift a kilo...maybe more. Now that I think of it, maybe a lot more.

Funny, this is about what I'm making right now, though I'll be using two 2212 930Kv motors (I have 8 of these sitting in a box. They didn't work out for that quad build.) with 8x6x3 props and a 3S 3600maH battery. My fuselage is a little bigger than a Mini Guinea. If it flies, I'll see how much it can lift.

Of course, Nials posted this in February, so I suspect the project is over.