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Projects brushless motor

diego18901

New Member
hey guys, I'm kinda new to making my own rc planes but i decided to take my chance to build a plane from scratch.
but i have been having trouble with chooseing a motor, the rc plane is a scaled version of a 1952 c35 bonanza, the wingspan is 175 cm, and i have no idea what size motor i need,

i am trying to stay true to the dimension but on scale soo i use a 12-13 inch 2 blade propellor, it weighs between 1-2 kg, what motor should i need to rrecieve about 80-120 mph.

friendly greetings from the netherlands,
Diego
 

RandyDSok

Well-Known Member
Well... If nothing else, remember to have fun learning how to do it... and don't be hard on yourself if you make mistakes along the way.

There isn't much of a set of rules ... or not a set that doesn't require some aeronautical engineering skills. But there are some things you can do to help you get close to what you need. Look for existing models with similar characteristics as the one you are trying to outfit. So something with the same approximate weight, low wing, approx. wing span and size and see what is recommended for those models. I'd also look at more than one just to get a range of motors to select from.

Next... to get you close, take the size of the original airplane and see how that scales to make your estimates with. In your case, the full size C35 Bonanza has a wingspan of 32 ft 10 in... your model is around a 69" model... So it's approx. 17.5% the size. Top speed on the full size is said to be about 165 mph with a 152 mph cruise speed.... That means a scale cruise speed would be closer to 26.25 mph ... not the exaggerated 80-120 mph you mentioned. So I'd suggest not going for much over 60 mph at the most, perhaps a little more but not a lot.

Ok... on the scaling I just mentioned. That isn't exactly how it's done, it's just a place to start. The previous method of using an existing model and what they suggest is more accurate. Basically, while the physical scale is on the mark, it isn't taking into account that the air the model flies through isn't scaled at all. Because of that, just scaling off of size isn't exact when it comes to the actual aeronautical engineering aspects of motor selection.

I haven't researched other similar models... I'll leave that work to you to do. Still, I can make a guess on what motor may be close to fitting your needs. The motor I'm thinking of is the Motrolfly DM2820-750. This is a high quality motor so you may be able to find a lower priced equivalent. The link I have for a shop is for here in the USA which won't do you much good but I've heard of them being popular overseas as well, which is another reason I picked that brand. That motor is rated for 800w with a 4s lipo battery using a 12x6 to 13x6.5 prop for a model weighting up to 60 oz. which is within your 1-2kg range. Again, I'd recommend a little more than just going by my guestimate and do a bit of research to help ensure you get something that matches your needs closer.

Hope that helps point you in the right direction.
 

diego18901

New Member
Well... If nothing else, remember to have fun learning how to do it... and don't be hard on yourself if you make mistakes along the way.

There isn't much of a set of rules ... or not a set that doesn't require some aeronautical engineering skills. But there are some things you can do to help you get close to what you need. Look for existing models with similar characteristics as the one you are trying to outfit. So something with the same approximate weight, low wing, approx. wing span and size and see what is recommended for those models. I'd also look at more than one just to get a range of motors to select from.

Next... to get you close, take the size of the original airplane and see how that scales to make your estimates with. In your case, the full size C35 Bonanza has a wingspan of 32 ft 10 in... your model is around a 69" model... So it's approx. 17.5% the size. Top speed on the full size is said to be about 165 mph with a 152 mph cruise speed.... That means a scale cruise speed would be closer to 26.25 mph ... not the exaggerated 80-120 mph you mentioned. So I'd suggest not going for much over 60 mph at the most, perhaps a little more but not a lot.

Ok... on the scaling I just mentioned. That isn't exactly how it's done, it's just a place to start. The previous method of using an existing model and what they suggest is more accurate. Basically, while the physical scale is on the mark, it isn't taking into account that the air the model flies through isn't scaled at all. Because of that, just scaling off of size isn't exact when it comes to the actual aeronautical engineering aspects of motor selection.

I haven't researched other similar models... I'll leave that work to you to do. Still, I can make a guess on what motor may be close to fitting your needs. The motor I'm thinking of is the Motrolfly DM2820-750. This is a high quality motor so you may be able to find a lower priced equivalent. The link I have for a shop is for here in the USA which won't do you much good but I've heard of them being popular overseas as well, which is another reason I picked that brand. That motor is rated for 800w with a 4s lipo battery using a 12x6 to 13x6.5 prop for a model weighting up to 60 oz. which is within your 1-2kg range. Again, I'd recommend a little more than just going by my guestimate and do a bit of research to help ensure you get something that matches your needs closer.

Hope that helps point you in the right direction.
That is a lot of information too take in on a saturday morning, but i tried tto have a lit of research in it and everyone says something else, 1 person says all you need is 750 kv on a 4s, other says 1200 kv and others say look for 2000 kv minimum.

But the tip of looming at other models is really good and i will also look into the motor, thank you soo much
 

diego18901

New Member
That is a lot of information too take in on a saturday morning, but i tried tto have a lit of research in it and everyone says something else, 1 person says all you need is 750 kv on a 4s, other says 1200 kv and others say look for 2000 kv minimum.

But the tip of looming at other models is really good and i will also look into the motor, thank you soo much
Well... If nothing else, remember to have fun learning how to do it... and don't be hard on yourself if you make mistakes along the way.

There isn't much of a set of rules ... or not a set that doesn't require some aeronautical engineering skills. But there are some things you can do to help you get close to what you need. Look for existing models with similar characteristics as the one you are trying to outfit. So something with the same approximate weight, low wing, approx. wing span and size and see what is recommended for those models. I'd also look at more than one just to get a range of motors to select from.

Next... to get you close, take the size of the original airplane and see how that scales to make your estimates with. In your case, the full size C35 Bonanza has a wingspan of 32 ft 10 in... your model is around a 69" model... So it's approx. 17.5% the size. Top speed on the full size is said to be about 165 mph with a 152 mph cruise speed.... That means a scale cruise speed would be closer to 26.25 mph ... not the exaggerated 80-120 mph you mentioned. So I'd suggest not going for much over 60 mph at the most, perhaps a little more but not a lot.

Ok... on the scaling I just mentioned. That isn't exactly how it's done, it's just a place to start. The previous method of using an existing model and what they suggest is more accurate. Basically, while the physical scale is on the mark, it isn't taking into account that the air the model flies through isn't scaled at all. Because of that, just scaling off of size isn't exact when it comes to the actual aeronautical engineering aspects of motor selection.

I haven't researched other similar models... I'll leave that work to you to do. Still, I can make a guess on what motor may be close to fitting your needs. The motor I'm thinking of is the Motrolfly DM2820-750. This is a high quality motor so you may be able to find a lower priced equivalent. The link I have for a shop is for here in the USA which won't do you much good but I've heard of them being popular overseas as well, which is another reason I picked that brand. That motor is rated for 800w with a 4s lipo battery using a 12x6 to 13x6.5 prop for a model weighting up to 60 oz. which is within your 1-2kg range. Again, I'd recommend a little more than just going by my guestimate and do a bit of research to help ensure you get something that matches your needs closer.

Hope that helps point you in the right direction.
Before i forget, I also did some basic calculations(not 100% accurate but as close as someone who is not an aeroengineer and amateur to the hobby can get) the max wright of the plane is around 4 kg. I thought that is way too heavy thus the 1.5-2.5 kg.

I also looked into some planes on based on size the first thong i saw was the giant corsair on the flite test youtube channel
 

diego18901

New Member
Okay, soo i just got a few other questions, would it be beneficial to reinforce the bonanza and make it heavjer and sturdier because of the wind,

I know its possible to hook up 2 servo to 1 channel, but is it also possible to use 4 servo on 1 channel(using 3 y cables) or even an odd number like 3,

Last, if i where to up the weight of my bonanza, what would you recommend for electronic upgrades?

Love to hear from you guys
 

RandyDSok

Well-Known Member
Yeah, scratch building has a lot of details to consider and requires a lot of past experience to get right. I assumed that while you were new to scratch building, that you were not new to the RC airplane hobby and could draw more on your experience with how the planes you have owned were built and constructed. If you don't have that experience, then you picked a project that is like learning to run when you haven't yet learned to crawl ( as the saying goes ). I'll continue on with the assumption that you actually do have more experience because it is required if you are going to have a successful build ( and I just expect you just got a little overwhelmed with the info I posted ).

An assumption I made... when you said the build was going to be around 1kg-2kg ... I assumed you were well on your way to completing the airframe and could better estimate the final weight. I picked a motor to fit a model in the middle of that weight at around 1.7kg. You can't just pull numbers out of the air, you must base them on actual plans that you have created which already define the construction materials first. However, you can look at existing models to get an idea. I now suspect you need a motor with a higher wattage and probably a lower kv rating and if so, perhaps a little bigger prop than you listed and what I had originally based a motor as a starting place for.

Those models need to be of the same airframe TYPE and SIZE. A Corsair is not a general aviation aircraft like the Bonanza you mentioned. A Corsair is a warbird which will likely be heavier and has a gull wing not a standard flat low wing configuration with some dihedral in it. Most warbirds of that era are also nose heavy making landings a lot more tricky than what a general aviation airplane would be.

When selecting a motor... it is more about the wattage and amperage than the kv as some of the info you got stated. Typically, the larger diameter props need to spin slower ( kv ) in order to not pull too many amps and burn out the motor. The voltage and amount of amperage ( current ) will determine the total wattage ( power ) that is required. You will often see a 12" to 13" prop with a motor having a kv rating around the 650 to 750 kv rating but as I mentioned, it isn't about the kv ( rate of spin ) but how much power ( in my previous example, it was rated at 800w ) and therefore the amperage that a motor can handle. I now suspect you will need a larger motor with a lower kv rating and a higher power ( wattage ) rating to meet the needs of a heavier plane than you were originally stating and also so you can use a larger diameter prop or at the least, a larger pitch prop.

The number of servo's you will likely need will be determined by the size of the control surfaces. Larger planes will have larger control surfaces and often use two servos per surface. All of my own planes never exceeded around 50" wingspans so they only required having a single servo per surface. I'm uncertain at what size wingspan they start recommending 2 servo's per surface. I suspect that at close to 70" like you mentioned, you are getting close to 2 per but I don't know that specifically. Again, if you are looking at an existing model of the same type and size, you should be able to get an idea by seeing what they recommend for that model type/size.

You asked about increasing the weight of the model to handle more wind... It's more about what kind of stresses the airframe will see than just the wind. You'll need to pay attention to the strength in the motor mount area, the landing gear area and possibly across the wingspan at where it meets the saddle of the fuselage ( within the wing itself ). Hollow carbon fiber tubes work well at being able to spread out the stresses in this areas. Keeping the weight down and still providing enough strength across the airframe is going to be a juggle and you can only go by the experience you have with your other models and/or other plans of other similar aircraft you can find. Old school methods using stronger woods than balsa ( birch, basswood etc ) also work as does using an aluminum tube ( s ) for the wing spar area if you can't source carbon fiber equivalents.


So in general... you first need to do more research on airframes of the same type and size so you can then work on your own design. Then you will need to build up the airframe and actually weigh out the model before determining what motor you will require. It's at that point that you can determine the number of servos you will need per surface ( I suspect only 1 or 2 at the most based on what I've seen on other large models ).
 

diego18901

New Member
Yeah, scratch building has a lot of details to consider and requires a lot of past experience to get right. I assumed that while you were new to scratch building, that you were not new to the RC airplane hobby and could draw more on your experience with how the planes you have owned were built and constructed. If you don't have that experience, then you picked a project that is like learning to run when you haven't yet learned to crawl ( as the saying goes ). I'll continue on with the assumption that you actually do have more experience because it is required if you are going to have a successful build ( and I just expect you just got a little overwhelmed with the info I posted ).

An assumption I made... when you said the build was going to be around 1kg-2kg ... I assumed you were well on your way to completing the airframe and could better estimate the final weight. I picked a motor to fit a model in the middle of that weight at around 1.7kg. You can't just pull numbers out of the air, you must base them on actual plans that you have created which already define the construction materials first. However, you can look at existing models to get an idea. I now suspect you need a motor with a higher wattage and probably a lower kv rating and if so, perhaps a little bigger prop than you listed and what I had originally based a motor as a starting place for.

Those models need to be of the same airframe TYPE and SIZE. A Corsair is not a general aviation aircraft like the Bonanza you mentioned. A Corsair is a warbird which will likely be heavier and has a gull wing not a standard flat low wing configuration with some dihedral in it. Most warbirds of that era are also nose heavy making landings a lot more tricky than what a general aviation airplane would be.

When selecting a motor... it is more about the wattage and amperage than the kv as some of the info you got stated. Typically, the larger diameter props need to spin slower ( kv ) in order to not pull too many amps and burn out the motor. The voltage and amount of amperage ( current ) will determine the total wattage ( power ) that is required. You will often see a 12" to 13" prop with a motor having a kv rating around the 650 to 750 kv rating but as I mentioned, it isn't about the kv ( rate of spin ) but how much power ( in my previous example, it was rated at 800w ) and therefore the amperage that a motor can handle. I now suspect you will need a larger motor with a lower kv rating and a higher power ( wattage ) rating to meet the needs of a heavier plane than you were originally stating and also so you can use a larger diameter prop or at the least, a larger pitch prop.

The number of servo's you will likely need will be determined by the size of the control surfaces. Larger planes will have larger control surfaces and often use two servos per surface. All of my own planes never exceeded around 50" wingspans so they only required having a single servo per surface. I'm uncertain at what size wingspan they start recommending 2 servo's per surface. I suspect that at close to 70" like you mentioned, you are getting close to 2 per but I don't know that specifically. Again, if you are looking at an existing model of the same type and size, you should be able to get an idea by seeing what they recommend for that model type/size.

You asked about increasing the weight of the model to handle more wind... It's more about what kind of stresses the airframe will see than just the wind. You'll need to pay attention to the strength in the motor mount area, the landing gear area and possibly across the wingspan at where it meets the saddle of the fuselage ( within the wing itself ). Hollow carbon fiber tubes work well at being able to spread out the stresses in this areas. Keeping the weight down and still providing enough strength across the airframe is going to be a juggle and you can only go by the experience you have with your other models and/or other plans of other similar aircraft you can find. Old school methods using stronger woods than balsa ( birch, basswood etc ) also work as does using an aluminum tube ( s ) for the wing spar area if you can't source carbon fiber equivalents.


So in general... you first need to do more research on airframes of the same type and size so you can then work on your own design. Then you will need to build up the airframe and actually weigh out the model before determining what motor you will require. It's at that point that you can determine the number of servos you will need per surface ( I suspect only 1 or 2 at the most based on what I've seen on other large models ).
The reason i say too use 4 servo at the same time, is to time the retracts on a single channel, also everyone is saying somethong else about what you need to look for motor, soo I'm like really confused on that part, also you are right about the corsair being to much of a warbird. Soo i wil definitely do more research and i have been thonking maybe use the bonanza as a starter on my "monster" build as i call it, and goo on from there maybe a mustang corsair, maybe even going up to bombers like b52
 

diego18901

New Member
The reason i say too use 4 servo at the same time, is to time the retracts on a single channel, also everyone is saying somethong else about what you need to look for motor, soo I'm like really confused on that part, also you are right about the corsair being to much of a warbird. Soo i wil definitely do more research and i have been thonking maybe use the bonanza as a starter on my "monster" build as i call it, and goo on from there maybe a mustang corsair, maybe even going up to bombers like b52
Yeah, scratch building has a lot of details to consider and requires a lot of past experience to get right. I assumed that while you were new to scratch building, that you were not new to the RC airplane hobby and could draw more on your experience with how the planes you have owned were built and constructed. If you don't have that experience, then you picked a project that is like learning to run when you haven't yet learned to crawl ( as the saying goes ). I'll continue on with the assumption that you actually do have more experience because it is required if you are going to have a successful build ( and I just expect you just got a little overwhelmed with the info I posted ).

An assumption I made... when you said the build was going to be around 1kg-2kg ... I assumed you were well on your way to completing the airframe and could better estimate the final weight. I picked a motor to fit a model in the middle of that weight at around 1.7kg. You can't just pull numbers out of the air, you must base them on actual plans that you have created which already define the construction materials first. However, you can look at existing models to get an idea. I now suspect you need a motor with a higher wattage and probably a lower kv rating and if so, perhaps a little bigger prop than you listed and what I had originally based a motor as a starting place for.

Those models need to be of the same airframe TYPE and SIZE. A Corsair is not a general aviation aircraft like the Bonanza you mentioned. A Corsair is a warbird which will likely be heavier and has a gull wing not a standard flat low wing configuration with some dihedral in it. Most warbirds of that era are also nose heavy making landings a lot more tricky than what a general aviation airplane would be.

When selecting a motor... it is more about the wattage and amperage than the kv as some of the info you got stated. Typically, the larger diameter props need to spin slower ( kv ) in order to not pull too many amps and burn out the motor. The voltage and amount of amperage ( current ) will determine the total wattage ( power ) that is required. You will often see a 12" to 13" prop with a motor having a kv rating around the 650 to 750 kv rating but as I mentioned, it isn't about the kv ( rate of spin ) but how much power ( in my previous example, it was rated at 800w ) and therefore the amperage that a motor can handle. I now suspect you will need a larger motor with a lower kv rating and a higher power ( wattage ) rating to meet the needs of a heavier plane than you were originally stating and also so you can use a larger diameter prop or at the least, a larger pitch prop.

The number of servo's you will likely need will be determined by the size of the control surfaces. Larger planes will have larger control surfaces and often use two servos per surface. All of my own planes never exceeded around 50" wingspans so they only required having a single servo per surface. I'm uncertain at what size wingspan they start recommending 2 servo's per surface. I suspect that at close to 70" like you mentioned, you are getting close to 2 per but I don't know that specifically. Again, if you are looking at an existing model of the same type and size, you should be able to get an idea by seeing what they recommend for that model type/size.

You asked about increasing the weight of the model to handle more wind... It's more about what kind of stresses the airframe will see than just the wind. You'll need to pay attention to the strength in the motor mount area, the landing gear area and possibly across the wingspan at where it meets the saddle of the fuselage ( within the wing itself ). Hollow carbon fiber tubes work well at being able to spread out the stresses in this areas. Keeping the weight down and still providing enough strength across the airframe is going to be a juggle and you can only go by the experience you have with your other models and/or other plans of other similar aircraft you can find. Old school methods using stronger woods than balsa ( birch, basswood etc ) also work as does using an aluminum tube ( s ) for the wing spar area if you can't source carbon fiber equivalents.


So in general... you first need to do more research on airframes of the same type and size so you can then work on your own design. Then you will need to build up the airframe and actually weigh out the model before determining what motor you will require. It's at that point that you can determine the number of servos you will need per surface ( I suspect only 1 or 2 at the most based on what I've seen on other large models ).
Btw so thank you soo much for helping me out
 

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