Model Motors

[AndyR05]

Ladies and Gents,

I'm building a model RC tank in 1/6th scale (made in alloy), and I have been trying to work out the size of motors I need to drive this particular beast. I have spent a while researching the clacs to use but am slightly confused on using the correct methodology.

What I understand teh process to be:
1) Establish the weight of model - given in Inventor
2) Establish desired top speed, whichshall determin the gearing ratio.
3) Establish force required (and thus the torque) to move model from stationary on a horizontal plane to full speed.
4) Establish force required (and thus the torque) to move the model from stationary on an inclined plane to full speed.

Now is this the correct process? I think so?

Any advice on this would be great and if anybody is interested I can post some 3d pictures of teh model as it stands in Inventor?

Cheers in advance
AndyR

 

[MintJulep]

You've pretty much got the process. You are missing two key points however.

You have to know the total drag vs. speed curve for your vehicle.

You have to decide what acceleration characteristic you want to have on the level and up an incline.

Note that top speed is the point where drag and thrust are equal, and hence there is no more acceleration.

Note also that most real-world motors are not constant torque devices.

 

[AndyR05]

Mintjulep,

Thanks for your reply.

Total Drag Vs Speed curve, I'm not familiar with this expression. I understand that when drag is equal to thurst then velocity will remain constant.

If I were to do the clacs in MathCAD, would somebody on here be able to do a sense check on my calcs? Ie not just check the formulaes are applied correctly but checking my methodology?

Cheers
AndyR

 

[zeusfaber]

For your application, the "total drag" effect is likely to be dominated by friction in the drivetrain - which varies with speed.

Required torque may simply be the torque required to match this friction at the model's top speed - but you will need to check that throughout the speed range, motor torque at that speed minus friction at that speed (referred back to the shaft as torque) is a. positive and b. sufficient to give you realistic looking acceleration.

A.

 

[AndyR05]

ok thanks.

I think this is would be the most professional approach, but not sure if the additional work is solving the math is worth work.

I have created a more simplistic spreadsheet which analysis the model to determine the max force to accelerate up a given incline. I then use this data to calculate the torque required to propel the model.

I then compare this data with the known spec of various motors to determine if it will be suitable. I have what I think of as a neat solution, it's not perfect but I don't want the motor to closly spec'd, ie I do want it over powered to give some redundancy....

If you want you could have a look at the spreadsheet and give it an appraisal?

Cheers
AndyR

 

[JimCasey]

Dynamic friction proprtional to the square of velocity is not much of a factor in tanks. SO your power requirement is some constant times velocity times weight.

If you truly scale everything from the original full-sized tank the 1/6 scale model would weigh 1/216 ( 1/6^3) as much as the original. To appear to be going the same scale speed as the original it would need to go 1/6 as fast. So it would need 1/216 x 1/6 = 1/1296 the horsepower of the original full-sized tank.

 

[MiketheEngineer]

Electric Motor or gas??

In most cities there are a number of clubs where guys (and gals) build and race model cars and trucks. You might want to check them out.

Scaling - for many reasons doesn't always work.... I have never known anyone who wished they had put a smaller engine in a model.....

Good Luck and have fun.