Thread Tools
Dec 25, 2014, 11:56 PM
Registered User
Thread OP
Help!

Between speed & endurance


Hi Everyone,

I need help about 2 configuration.

The parameter :
The airplane flying weight spec is 700gr
The airplane is dolphin jet with pusher config.
The motor is rctimer bc 2212/6 342 watt 30A
Prop is 5.5x4.5

What will happen if :
1. Use 3s 2200mAh 25c which will make the AUW 860gr.
2. Use 2s 5000mAh 20c which will make the AUW 890gr.

I mean, what will happen to the speed and endurance performance with one if these configuration ? (Theoretically)

Hope to have input as much as i can.

Thanks before
Last edited by cc_devil; Dec 28, 2014 at 10:33 PM.
Sign up now
to remove ads between posts
Dec 26, 2014, 05:42 AM
An itch?. Scratch build.
eflightray's Avatar
Speed mostly comes from pitch speed, (rpm/power/watts). Endurance mostly comes from low watts, (and low drag/weight).

Using the same prop but having less power, (2s), plus adding weight, (heavier Lipo ?), could makes things worse.

You may find that for the same speed, flying with 3s and the lighter 2200mAh Lipo at reduced throttle, might give similar results to flying with the 2s and the heavier Lipo.
As you will need more throttle to compensate for the lack of power and extra weight using 2s.

Could be swings and roundabouts.

Endurance really needs the complete power train considering, and that includes a suitable prop.
Dec 26, 2014, 10:07 AM
Just here to have fun with RC
Rhea's Avatar
Plug some numbers into this calculator and see what it shows you.
Dec 26, 2014, 01:10 PM
Senile Member
Lnagel's Avatar
At full throttle with the same prop the 3s will provide the most thrust at the highest power level. The result is that speed will be maximum and endurance will be minimum.

At full throttle the 2s will provide about half as much thrust at about 1/3 the power as the 3s. The result is reduced speed and increased endurance. There are three things to consider here. The most important is will the airplane have enough thrust to fly? Another is that if the airplane does fly it will have about 3 times the endurance with a 2s 2200 mah battery as it will with a 2200 mah 3s battery so there would be no need to go to a heavier 2s 5000 mah battery. The last thing to consider is that with a 3s battery you can always throttle down to achieve about the same speed and endurance as the 2s battery will supply at full throttle. So if the power system won't burn up with a 3s battery then use it for speed and simply throttle down for endurance.

Larry
Dec 26, 2014, 01:12 PM
Registered User
Robert4613's Avatar
Quote:
Originally Posted by Rhea
Plug some numbers into this calculator and see what it shows you.
Thanks for the link! Saving for future use.
Dec 27, 2014, 12:24 PM
Drone offender FA377YHFNC
Wasn't going to say anything but then I got interested. That particular RC Timer motor isn't in the Drive Calculator database so I substituted the very similar Suppo 2212/6 motor. Relative numbers will be valid. If duration doubles for this motor it will double for yours. The actual thrust and wattage might be slightly different. It will be different between any two motors of the same type too! So relationships are pretty valid, actual parameters maybe less so.

Looks like your chosen prop will draw fewer watts than the APC 7x4 E that RC Timer recommends. Both props will draw comfortably under the power rating of the motor, an important thing to do with Chinese motors. Let's look at what happens in four instances.

First we'll take a look at the recommended 7x4 prop for 3 cells.Static thrust 1128g. Pitch velocity 111 kph, 69 mph. WOT flying time 4 min, 31 sec.

7x4 prop for 2 cells. Static thrust 605g. Pitch velocity 82 kph, 51 mph. WOT flying time 18 min, 41 sec.

5x4 prop for 3 cells. Static thrust 632 g. Pitch velocity 137 kph, 85 mph. WOT flying time 6 min, 6 sec.

5x4 prop for 2 cells. Static thrust 316 g. Pitch velocity 99 kph, 61.5 mph. WOT flying time 26 min, 17 sec.
Dec 28, 2014, 10:46 PM
Registered User
Thread OP
Quote:
Originally Posted by Rhea
Plug some numbers into this calculator and see what it shows you.
Thanks for the link...
Quote:
Originally Posted by eflightray
Speed mostly comes from pitch speed, (rpm/power/watts). Endurance mostly comes from low watts, (and low drag/weight).

Using the same prop but having less power, (2s), plus adding weight, (heavier Lipo ?), could makes things worse.

You may find that for the same speed, flying with 3s and the lighter 2200mAh Lipo at reduced throttle, might give similar results to flying with the 2s and the heavier Lipo.
As you will need more throttle to compensate for the lack of power and extra weight using 2s.

Could be swings and roundabouts.

Endurance really needs the complete power train considering, and that includes a suitable prop.
Quote:
Originally Posted by Lnagel
At full throttle with the same prop the 3s will provide the most thrust at the highest power level. The result is that speed will be maximum and endurance will be minimum.

At full throttle the 2s will provide about half as much thrust at about 1/3 the power as the 3s. The result is reduced speed and increased endurance. There are three things to consider here. The most important is will the airplane have enough thrust to fly? Another is that if the airplane does fly it will have about 3 times the endurance with a 2s 2200 mah battery as it will with a 2200 mah 3s battery so there would be no need to go to a heavier 2s 5000 mah battery. The last thing to consider is that with a 3s battery you can always throttle down to achieve about the same speed and endurance as the 2s battery will supply at full throttle. So if the power system won't burn up with a 3s battery then use it for speed and simply throttle down for endurance.

Larry
@eflightray & Lnagel : so, you guys suggest to use a bigger cells, to be able to achive the speed and endurance by playing with the throttle ?
Quote:
Originally Posted by Rockin Robbins
Wasn't going to say anything but then I got interested. That particular RC Timer motor isn't in the Drive Calculator database so I substituted the very similar Suppo 2212/6 motor. Relative numbers will be valid. If duration doubles for this motor it will double for yours. The actual thrust and wattage might be slightly different. It will be different between any two motors of the same type too! So relationships are pretty valid, actual parameters maybe less so.

Looks like your chosen prop will draw fewer watts than the APC 7x4 E that RC Timer recommends. Both props will draw comfortably under the power rating of the motor, an important thing to do with Chinese motors. Let's look at what happens in four instances.

First we'll take a look at the recommended 7x4 prop for 3 cells.Static thrust 1128g. Pitch velocity 111 kph, 69 mph. WOT flying time 4 min, 31 sec.

7x4 prop for 2 cells. Static thrust 605g. Pitch velocity 82 kph, 51 mph. WOT flying time 18 min, 41 sec.

5x4 prop for 3 cells. Static thrust 632 g. Pitch velocity 137 kph, 85 mph. WOT flying time 6 min, 6 sec.

5x4 prop for 2 cells. Static thrust 316 g. Pitch velocity 99 kph, 61.5 mph. WOT flying time 26 min, 17 sec.
@Rockin Robbins :
As we can see, the endurance with 5.5x4.5 prop and 2 will be about 26 min In WOT mode, which is WOW.
But, with almost 900gr AUW, the worst case is, it will go with everything divided by 3.
So, it will be around 12 minutes, with 30km/h in WOT.

@ Rockin Robbins, eflightray & Lnagel : Refer to this, do u think the 3s will be able to do/achieve the same thing by reducing the throttle ?

O yeah, isnt it make the watt greater when using a bigger mah battery ? Because the watt is discharge rating multiply by the mah ?

Thanks
Dec 29, 2014, 12:46 AM
http://www.sgvhumane.org/
cmdl's Avatar
Quote:
Originally Posted by cc_devil
O yeah, isnt it make the watt greater when using a bigger mah battery ? Because the watt is discharge rating multiply by the mah ?
watts are drawn by the prop based on current drawn. a larger capacity only implies greater discharge ability. the actual amps drawn depend on how fast the prop is spun.
Dec 29, 2014, 12:02 PM
Drone offender FA377YHFNC
Quote:
Originally Posted by cc_devil
Thanks for the link...



@eflightray & Lnagel : so, you guys suggest to use a bigger cells, to be able to achive the speed and endurance by playing with the throttle ?

@Rockin Robbins :
As we can see, the endurance with 5.5x4.5 prop and 2 will be about 26 min In WOT mode, which is WOW.
But, with almost 900gr AUW, the worst case is, it will go with everything divided by 3.
So, it will be around 12 minutes, with 30km/h in WOT.
Keep in mind that the 2S 5.5x4.5 configuration although you have wide open throttle time of 26 minutes, you only have 316g of thrust for an 890g plane. It will fly but it won't be spirited by any means. You'll have to actually fly it on the wing like a full sized non-military aircraft. If duration is top priority and others way down the list that's your route. If performance is a high #2 or higher it might not be the the way you want to go.

Remember I specified wide open throttle time so 26 minutes is 26 minutes, not 12.
Dec 29, 2014, 08:24 PM
Registered User
Thread OP
Quote:
Originally Posted by Rockin Robbins
Keep in mind that the 2S 5.5x4.5 configuration although you have wide open throttle time of 26 minutes, you only have 316g of thrust for an 890g plane. It will fly but it won't be spirited by any means. You'll have to actually fly it on the wing like a full sized non-military aircraft. If duration is top priority and others way down the list that's your route. If performance is a high #2 or higher it might not be the the way you want to go.

Remember I specified wide open throttle time so 26 minutes is 26 minutes, not 12.
O ic, i though it will be 12 minutes WOT because the thrust is only 316g to push 890g plane.
which means :
316g = 26 min
So
300g x 3 = 26/3
Dec 30, 2014, 12:24 PM
Drone offender FA377YHFNC
Quote:
Originally Posted by cc_devil
O ic, i though it will be 12 minutes WOT because the thrust is only 316g to push 890g plane.
which means :
316g = 26 min
So
300g x 3 = 26/3
Nope. 316b of thrust is all you're going to get wide open. Times three is not an option. That will be enough to fly the plane but not for very steep rates of climb. And that will last for 26 minutes wide open, longer with lesser throttle.
Dec 30, 2014, 08:23 PM
Registered User
Thread OP
Quote:
Originally Posted by Rockin Robbins
Nope. 316b of thrust is all you're going to get wide open. Times three is not an option. That will be enough to fly the plane but not for very steep rates of climb. And that will last for 26 minutes wide open, longer with lesser throttle.
Does it mean weight to thrust ratio is 3:1 ?
How can a motor pull/push 3 times heavier plane than it can do ?
Dec 31, 2014, 09:13 AM
Registered User
Robert4613's Avatar
Subscribing. Following thread to learn something here.
Dec 31, 2014, 09:58 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Quote:
Does it mean weight to thrust ratio is 3:1 ?
How can a motor pull/push 3 times heavier plane than it can do ?
Fill a shopping cart with bricks. You can still push it with a finger, even if it weighs 500 pounds. You can even go uphill as long as the slope is very gentle. If it gets too steep, you start crying and your finger breaks.

Minimum thrust is equal to the plane's weight divided by L/D max, plus parasitic drag. If your plane can glide five feet forward for every foot it descends, you only need thrust equal to (slightly above) 1/5 its total weight in order to maintain level flight. Climbing at any appreciable rate will require significantly more thrust, so a very low thrust-to-weight ratio will result in quite a slow climb rate.
Dec 31, 2014, 11:34 AM
Drone offender FA377YHFNC
Now from post #1 this plane is a Dolphin Jet, presumably the Hobby King Dolphin Jet. Now this is an EDF trainer. Being an EDF, a beginner EDF in fact, its thrust to weight ratio is going to be quite low anyway, so this plane is designed to accomodate a low thrust high pitch speed arrangement that your selected propeller would develop with a 2S battery.

Is it this plane?
Review: HK Dolphin Jet EDF RC plane (7 min 11 sec)
Last edited by Rockin Robbins; Dec 31, 2014 at 11:42 AM.


Quick Reply
Message:

Thread Tools