First off...i'm not a noob when it comes to rc planes and flying,..... but the question might be
My skill level is <u>somewhere inbetween </u>sport flying and 3d flying.
I have been flying foamies for about two years and I have flown the stratos, super cub, cessna 1400mm, p-40 warhawk 1400mm, p-51d mustang1400mm, pitts python 1400mm .I bought myself a 1400mm foamy Dynam SU-26. it uses a outrunner 650kv motor with a 50 AMP ESC and using a 4 cell 2800 MAH batt.The trouble that im having is not taking off nor is it flying and doing stunts...it's the landing. With the plane weighing 2.85 LBS Iknow that (for me)it is to light with that motor.My problem is landing,more exactly getting it to come down to earth and not staying in the air. Dont get my wrong ...I know how to land.Ihave lost count how many times i have landed.
What i'm trying to say is that im going to balsa for my next plane. The new plane will be the SU-26 made from balsa with the same motor.

Now for the question. With that motor how do u calculate thrust to weight ratio. Is it a personal feeling like expo or is there a formula to it

Thrust values vary from one propeller to the next.. keep that in mind. Be prepared for the inrush of information, this sort of topic has come up before and there are alot of guys out there that know all kinds of good stuff.

You might also be interested in watts per pound along with the thrust because the more thrust, the more wattage you consume.

Take a look at this and see if it gives you some idea of what I mean (you might already know this, but it never hurts to offer additinoal information).

Read through the following and work it out.

Once done, you will know what wattage you are working with and what motor and esc to buy. Always go to the higher level when figuring out what motor and esc to buy.. if it works out to 17 amp ESC, go to a 20 amp ESC. Match the motor to the esc. If it doesn't match up with what you picked out, move to a motor/esc combination that does, get the weight (should not be that much difference) and work it out again and get the right one.

You should be fine.

Here are some guidelines:

1. Power can be measured in watts. For example: 1 horsepower = 746 watts

2. You determine watts by multiplying ‘volts’ times ‘amps’. Example: 10 volts x 10 amps = 100 watts

Volts x Amps = Watts

Therefore,

Amps = Watts / Volts ( / = divided by ). Simple elementary math.


3. You can determine the power requirements of a model based on the ‘Input Watts Per Pound’ guidelines found below, using the flying weight of the model (with battery):

50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
70-90 watts per pound; Trainer and slow flying scale models
90-110 watts per pound; Sport aerobatic and fast flying scale models
110-130 watts per pound; Advanced aerobatic and high-speed models
130-150 watts per pound; Lightly loaded 3D models and ducted fans
150-200+ watts per pound; Unlimited performance 3D and aerobatic models

4. Determine the Input Watts Per Pound required to achieve the desired level of performance:

Example:

Model: E-flite Brio 10 ARF
Estimated Flying Weight w/Battery: 2.1 lbs
Desired Level of Performance: 150-200+ watts per pound; Unlimited performance 3D and aerobatics
2.1 lbs x 150 watts per pound = 315 Input Watts of total power (minimum) required to achieve the desired performance

5. Determine a suitable motor based on the model’s power requirements. In this case, you will need a motor/esc/battery combination for 30 amps continuous... 3 cell LiPo at 11.1 volts (nominal) is 315/11.1=28.xxx amps, round out to the higher 30 amps.

Didn't realize one was above or below the other. I've known precise sport flyers who can fly rolling circles at walking speed and sloppy 3D fliers who couldn't handle dead sticks.

Sorry. I can't help you much with electric motors. I jumped right past them directly into glow and have graduated on up to gas now. <br type="_moz" />

Over in the wattflyer forum there are stickys and tutorials about this. I find it very hard to relate to foamies as planes even when talking electrics but they are very popular in the electric forum.