thrust vs weight

[ghost civic]

is there a way to determine how much weight a given motor/prop can move?

ive use the thrust calculator which in theory says my motor/prop can give 2 pounds of thrust. how much weight can that fly? is there a way to guestimate? I figure wing type would have a big role in this. lets assume you have a large, trainer type wing.

 

[c172ae]

instead of thrust to weight ratio, I use watt per pound.

Less than 50W/lb - very lightweight / low wing loading slow flyer.
50 to 80 W/lb - powered gliders, basic park flyers and trainers, classic biplanes and vintage ('Old Timer') type planes.
80 to 120 W/lb - general sport flying and basic/intermediate aerobatics. Many scale (eg warbirds) subjects suit this power band.
120 to 180W/lb - serious aerobatics, pattern flying, 3D and scale EDF jets
180 to 200+W/lb - faster jets and anything that requires a lot of muscle!

 

[ghost civic]

when you say watt, do you mean the watts (power) the motor pulls while spinning "X" prop?
I don't think the calc I found gives that info. it does give theoretical kW required to spin a certain size/pitch prop.

this is part of why I love the RC hobby. ive been in it for 7 years and still plenty more to learn! I have a good handle on ground based electric stuff, but planes present all new challenges and info to learn.

 

[Epitaph]

Another thing to have in account is the size of the prop being used when using W/lb, and is one of the reasons EDFs and jets need such high values (apart from they are fast, period). The thing with a wing is it produces lift due to the amount of airflow around it. Now a prop plane even at low speed has quite a lot of airflow on the wings because the prop is pushing it around, where as an EDF gets all the airflow around the wings purely from speed, as the internal thrust aspect of it "bypasses" the wing. This is why you can have a slow flying prop plane easily because some of the air is from the prop itself, also why they have lower take-off speeds. Thus you can have 2 planes with the same fuselage (take the Viggen for example), that needs much more pure thrust from the EDF version, and a lot more watts therefore, than the pusher prop version, and you can see this by testing the thrust given by each, the prop version giving better performance than the EDF even if it's giving less thrust.

 

[ghost civic]

yeh I had one EDF. got it used, crashed it right away (failed hand toss) and the nose took a fair bit of damage.
I much prefer a plane that can take off and land at low speeds, instead of needing a whole parking lot to take off and having to land at 20mph...lol.
they do sound really cool tho.

 

[c172ae]

when you say watt, do you mean the watts (power) the motor pulls while spinning "X" prop?
I don't think the calc I found gives that info. it does give theoretical kW required to spin a certain size/pitch prop.

this is part of why I love the RC hobby. ive been in it for 7 years and still plenty more to learn! I have a good handle on ground based electric stuff, but planes present all new challenges and info to learn.

just multiply the volts of the battery bu the amount of amps you are pulling, and you get watts. which calculator did you use?

 

[ghost civic]

just multiply the volts of the battery bu the amount of amps you are pulling, and you get watts. which calculator did you use?

this calc: http://personal.osi.hu/fuzesisz/strc_eng/
I don't actually have a way to measure amps my motor is pulling.

 

[c172ae]

that calculator is made to measure the thrust of engines you already know the RPM of. it doesn't actually say anything about what kind of thrust you will get if you don't know the RPM. try this one instead: http://www.ecalc.ch/motorcalc.php?ecalc&lang=en

 

[ghost civic]

that calculator is made to measure the thrust of engines you already know the RPM of. it doesn't actually say anything about what kind of thrust you will get if you don't know the RPM. try this one instead: http://www.ecalc.ch/motorcalc.php?ecalc&lang=en

I will try that one, but taking the kv of your motor times the volts you get a rpm without load.

 

[c172ae]

I will try that one, but taking the kv of your motor times the volts you get a rpm without load.

yes, but it doesn't tell you anything about how much that RPM is going to drop when you put a propeller on the engine.

 

[ghost civic]

I don't see my motor on the list for that calc. dynam A21. that's what I get for using the cheap stuff

 

[c172ae]

well, looks like you have to buy a wattmeter (everyone should have one). you may also try to see if you can find some info about the motor somewhere on the web.

 

[nxc]

I don't see my motor on the list for that calc. dynam A21. that's what I get for using the cheap stuff

If you pay the subscription you can put in whatever values you like. $0.99 gets you a month, which is well worth it for even 10 minutes of playing around with different battery/esc/motor/prop combinations.

 

[ghost civic]

I thought for sure I wouldn't find any data besides prop size, kv and input voltage....but on hobbyking it has test info for this motor:
Test Data;
Battery: 11.5v
Current: 8.55A
Propeller: 8x4
Thrust: 537g
Speed: 9950rpm

 

[ghost civic]

so that thrust calc I had used was pretty dang close according to that test data. it gave .50kg of thrust. just a hair shy of the 537g that hobbyking says it can give with a 8x4.

but how heavy of a plane can that fly?
I do know that the dynam Icanfly has a flight weight of 530g. but that was running the same motor with a 9x5 prop.
so does this mean flight weight can come within 70 or 80% of total thrust?

 

[rcspaceflight]

Don't you usually want about a 1:1 power to weight ratio? But for 3D planes you want a 2:1 or a 2.5:1 power to weight ratio? For a trainer you could get away with 1:1.5 or something? Maybe I'm completely wrong on that.

Theoretically a 1:1 ratio means the plane could hover at full throttle, but doesn't have unlimited vertical. Unlimited vertical is usually overkill. Except with a 3D plane.

 

[c172ae]

yes, somewhere along those lines. but it should be said that my favorite aircraft is a 120cm c182 that weights 1.3 kg with a 8000mah 3-cell - and is powered by a blue wonder. that way, I feel the plane instead of feeling the power.

 

[ghost civic]

Don't you usually want about a 1:1 power to weight ratio? But for 3D planes you want a 2:1 or a 2.5:1 power to weight ratio? For a trainer you could get away with 1:1.5 or something? Maybe I'm completely wrong on that.

Theoretically a 1:1 ratio means the plane could hover at full throttle, but doesn't have unlimited vertical. Unlimited vertical is usually overkill. Except with a 3D plane.

that all kinda makes sense to me. I wasn't sure if thrust could be thought of the same was as HP. I thought maybe a pound of thrust was needed to push .5 pound of plane...in other words, the thrust had to be greater just so it could move.

but it makes sense when looking at the data ive found for the dynam trainer I had. it weighed 530g (nearly 1.2 pounds) and the thrust with the 9x5 prop would have been 1.2 or 1.3 pounds. tho I think at some point I likely had a 8x4 prop on it, which would have been more like .90 pounds of thrust.

so if I use a 8x4 on my motors, and keep my planes no more than 1 pounds flying weight then they should do just fine.

I do wonder what the limit may be tho. can 1 pound of thrust fly a 2...or even 3 pound plane? I suppose it would come down to the wing and how much lift it could generate at low speeds.

 

[rcspaceflight]

I do wonder what the limit may be tho. can 1 pound of thrust fly a 2...or even 3 pound plane? I suppose it would come down to the wing and how much lift it could generate at low speeds.

I wonder that too. I know you don't need a 1:1 ratio. I usually aim for that because I know it's safe. Especially for RC planes that don't have an airfoil.

 

[c172ae]

what we do know, is that a 3.2 kg plane with a KF airfoil that wasn't sanded down was able to fly on 950 grams of thrust. however, it used almost a minute to get up to 10 meters...