Can any one suggest motors for my RC aircraft? - RC Groups
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Oct 25, 2017, 05:43 PM
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Can any one suggest motors for my RC aircraft?


Hi,

I'm working on a university project designing an electric drone.
It weighs in at 25kg and has a wing area of 0.42m^2.
At its cruising speed of 30m/s it will generate just over 30N of total drag.

I'm hoping to design it to have an endurance of over 210miles but I'm having trouble finding the best solution to generating thrust. I've been reading up about motors all day and and have come up with potentially having 3 electric motors all powering propellers generating a maximum 1500g of thrust each this means I should be able to run all 3 at about 2/3 power

This would require about 60amps of power, so with 180,000milliamps of batteries to fly for 3 hours (or 210 miles) this would be almost 14kg of batteries .

This doesn't really sound right to me because 14kg is a bit unrealistic! Could someone please help me understand a possible solution for my problem, thank you!

EDIT: I've been looking at some of the other projects on here and I think my weight estimation for my aircraft might be far too high? The aircraft will also be holding a payload which I'd ideally like to weigh atleast 3kg and this will be dropped halfway during the flight.
Last edited by macoroar; Oct 25, 2017 at 06:09 PM.
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Oct 25, 2017, 06:23 PM
just look at it smokin'
z-matrix's Avatar
Quote:
Originally Posted by macoroar
Hi,

I'm working on a university project designing an electric drone.
It weighs in at 25kg and has a wing area of 0.42m^2.
At its cruising speed of 30m/s it will generate just over 30N of total drag.

I'm hoping to design it to have an endurance of over 210miles but I'm having trouble finding the best solution to generating thrust. I've been reading up about motors all day and and have come up with potentially having 3 electric motors all powering propellers generating a maximum 1500g of thrust each this means I should be able to run all 3 at about 2/3 power

This would require about 60amps of power, so with 180,000milliamps of batteries to fly for 3 hours (or 210 miles) this would be almost 14kg of batteries .

This doesn't really sound right to me because 14kg is a bit unrealistic! Could someone please help me understand a possible solution for my problem, thank you!

EDIT: I've been looking at some of the other projects on here and I think my weight estimation for my aircraft might be far too high? The aircraft will also be holding a payload which I'd ideally like to weigh atleast 3kg and this will be dropped halfway during the flight.

Good luck with that dude!

You should have 1/2 weight as thrust that will fly comfortably, 12.5 kg thrust can be achieved with a 2.5 kW- 4 kW motor
Also take voltage into account, 3s pack is 3x heavier with same Ah rating than an 1s pack.
If you can get it flywith solar cells then your problem is solved, second option would be hydrogen fuel cells for best distance
If you give it up then use a balloon filled with hydrogen or helium, or ht air.

Let me know if you succeed...
(btw modelers use less than 3-5kg total weight aircrafts)
Oct 25, 2017, 06:47 PM
Registered User
Quote:
Originally Posted by z-matrix
Good luck with that dude!

You should have 1/2 weight as thrust that will fly comfortably, 12.5 kg thrust can be achieved with a 2.5 kW- 4 kW motor
Also take voltage into account, 3s pack is 3x heavier with same Ah rating than an 1s pack.
If you can get it flywith solar cells then your problem is solved, second option would be hydrogen fuel cells for best distance
If you give it up then use a balloon filled with hydrogen or helium, or ht air.

Let me know if you succeed...
(btw modelers use less than 3-5kg total weight aircrafts)
The project is currently theoretical and I've been working on the aerodynamics up until now but I feel like I defiantly may have overestimated the weight. If I reduced the weight to closer to 10Kg I would still need 50N of thrust to have a T/W of 0.5 but as all I need to do is get the aircraft to fly in a constant direction at a constant speed at a low altitude could I get away having the thrust just equal to the drag which could be just about 25N at 30m/s?

Would I be able to use two of these as they claim here to be able to produce about 1500g (or 15N?) of thrust each.
Oct 26, 2017, 12:24 AM
just look at it smokin'
z-matrix's Avatar
Quote:
Originally Posted by macoroar
The project is currently theoretical and I've been working on the aerodynamics up until now but I feel like I defiantly may have overestimated the weight. If I reduced the weight to closer to 10Kg I would still need 50N of thrust to have a T/W of 0.5 but as all I need to do is get the aircraft to fly in a constant direction at a constant speed at a low altitude could I get away having the thrust just equal to the drag which could be just about 25N at 30m/s?

Would I be able to use two of these as they claim here to be able to produce about 1500g (or 15N?) of thrust each.
Check out the Electric Sailplanes forum, those can even soar and get 50:1 glide ratio, but at this scale you will need a full scale/10 wingspan (est),
and the larger the plane the larger the battery, it will not be cheap.
The 200 km range is more like a dream with a li-po though. And your fpv will have difficulties transmitting and receiving signals at that distance.


Z
Oct 26, 2017, 06:15 AM
Registered User
Quote:
Originally Posted by z-matrix
Check out the Electric Sailplanes forum, those can even soar and get 50:1 glide ratio, but at this scale you will need a full scale/10 wingspan (est),
and the larger the plane the larger the battery, it will not be cheap.
The 200 km range is more like a dream with a li-po though. And your fpv will have difficulties transmitting and receiving signals at that distance.


Z
I'll take a look! Are my assumptions about the two 1500g motors having enough thrust to maintain a low speed correct though? If the aircraft was closer to 10kg?

To control the drone I'm going to look in to a small computer like a raspbery pi to locations can be pre-programmed on the ground and it flies on autopilot. It's a mostly theoretical project but I'll try to build some parts of it!

I've altered my solidworks model and if I made the aircraft out of 1.5mm carbon fibre this would reduce the empty weight without any components to maybe 6-7kg, with a few kg of equipment and finally the payload it could be just 13kg
Oct 27, 2017, 08:38 AM
Jack
jackerbes's Avatar
Read the attached rules of thumb to pick a motor.

By your description your plane has a little over 4.5 sq.ft. of wing area, a weight of 881 oz. If your numbers are right, it has a wing loading of 196 oz./sq. ft. So something is wrong with your numbers...

Gliders fly at 10 oz./sq.ft., war birds at up to 25 oz./sq. ft., nothing will fly at 196 oz./sq.ft. as that is like a bowling ball!

Jack
Oct 27, 2017, 09:48 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
E-flight calculators (compilation) (sticky).

Some well-structured reading and handy e-tools for rainy/windy days. Will save you, and us , a lot of questions. Notably the 'what went wrong?' kind of questions
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And please, please, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.


Mass is leading when choosing total amount of motorpower.
Motorcurrent is proportional to pitch¹, voltage squared, Kv³ and diameter⁴.
Power is proportional to pitch¹, voltage cubed, Kv³ and diameter⁴.

Prettig weekend Ron
Oct 27, 2017, 09:50 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
And my favourite quote
Quote:
Originally Posted by scirocco
Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Prettig weekend Ron
Oct 27, 2017, 10:14 AM
Registered User
Quote:
Originally Posted by jackerbes
Read the attached rules of thumb to pick a motor.

By your description your plane has a little over 4.5 sq.ft. of wing area, a weight of 881 oz. If your numbers are right, it has a wing loading of 196 oz./sq. ft. So something is wrong with your numbers...

Gliders fly at 10 oz./sq.ft., war birds at up to 25 oz./sq. ft., nothing will fly at 196 oz./sq.ft. as that is like a bowling ball!

Jack
It is definitely going to be heavy but I'm modelling it after the Zipline Drone so it will also need to be able to carry a few kg in payload and have a range of (hopefully) over 200 miles, which the zipline manages to do while weighing 13kg.

I've reestimated the aircraft weight as to be closer to 15kg (500oz) now if it was made from carbon fibre.

thats about 5kg for the fusealage, wings etc, a 3kg payload, 1kg in equipment and at this rate 6kg in batteries!
Oct 27, 2017, 10:19 AM
Jack
jackerbes's Avatar
Quote:
..Kv is not a figure of merit, in that higher or lower is better...
I think that tends to belittle the importance that the Kv has in matching the battery voltage to the RPM range the motor will operate over.

Maybe is would be more accurate to say:

Kv is not a figure of merit, in that higher or lower is better, until you have chosen the battery voltage and propeller. Those factors should be decided first and then it is the battery voltage and Kv that matches the motor's RPM range to the propeller.

Jack
Oct 27, 2017, 10:29 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by macoroar
It is definitely going to be heavy but I'm modelling it after the Zipline Drone so it will also need to be able to carry a few kg in payload and have a range of (hopefully) over 200 miles, which the zipline manages to do while weighing 13kg.

I've reestimated the aircraft weight as to be closer to 15kg (500oz) now if it was made from carbon fibre.

thats about 5kg for the fusealage, wings etc, a 3kg payload, 1kg in equipment and at this rate 6kg in batteries!
Are there some details on the zipline aircraft somewhere? That would be interesting, to know more about that plane and how much power they are using now.

The 500 oz. figure gets the loading down to 111 oz./sq.ft. but that is still a very heavily loaded wing. Adding wing area will greatly reduce that and it will also make the power needed to get the required flight duration and range a little easier to attain.

Jack
Oct 27, 2017, 12:16 PM
Registered User
Neil Stainton's Avatar
If you are doing a theoretical project you need to derive a modified version of the Breguet Range Equation, plug in suitable figures for LiPo power density, motor efficiency (80%), prop efficiency (80%), etc and see what is achievable.

But I would guess that using generally available (non-military) hardware a 4 hour endurance will be near the maximum possible, and will require over 50% of the weight to be batteries. Maximising L/D will also be required, which implies a high aspect ratio wing and a higher structure weight, and thus a lower battery weight proportion. So the airframe optimisation is a balancing act between these two factors.

A 70 mph cruise implies a stall speed near 40 mph which will be too high for hand launch, so a launch system will be required.

Neil.
Oct 27, 2017, 12:24 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Other thread in engines subforum
Is a gas engine the solution to my problem?
Oct 27, 2017, 12:26 PM
Registered User
Quote:
Originally Posted by Neil Stainton
If you are doing a theoretical project you need to derive a modified version of the Breguet Range Equation, plug in suitable figures for LiPo power density, motor efficiency (80%), prop efficiency (80%), etc and see what is achievable.

But I would guess that using generally available (non-military) hardware a 4 hour endurance will be near the maximum possible, and will require over 50% of the weight to be batteries. Maximising L/D will also be required, which implies a high aspect ratio wing and a higher structure weight, and thus a lower battery weight proportion. So the airframe optimisation is a balancing act between these two factors.

A 70 mph cruise implies a stall speed near 40 mph which will be too high for hand launch, so a launch system will be required.

Neil.
What is the highest energy density for Li-Po batteries? I've found this website which claims 250Wh/kg which would be very good.

It doesn't need to exist in the form of a commercial RC battery with correct wiring as long as the technology is possible to get 250Wh/kg and I can specify a source for the technology I could say these cells will be modified to work for the drone. In the link above however the voltage output is only 3.7V.

Are there any other examples of very high density li-po batteries, maybe even some which are ready to be used in RC aircraft?
Oct 27, 2017, 12:27 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by jackerbes
I think that tends to belittle the importance that the Kv has in matching the battery voltage to the RPM range the motor will operate over.
Maybe is would be more accurate to say:
Kv is not a figure of merit, in that higher or lower is better, until you have chosen the battery voltage and propeller. Those factors should be decided first and then it is the battery voltage and Kv that matches the motor's RPM range to the propeller. ...
The rest of Scirocco's quote takes care of that Jack

Prettig weekend Ron


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