RC India

Guys,

Just wanted to understand whether or not there is a weight ration for a foamy. I mean, let's say, if the airframe alone weighs 500gm, then is there a limit on the all up weight? Considering that the battery and motor are the two of the heaviest parts, how does one decide on the AUW?

Pankaj

I assume you are already familiar with our "mother post" when it comes to electric power consumption  :thumbsup:

http://www.rcindia.org/electric-power/amp-rating-and-motor-size/

Now when you need to pick a motor for your electric plane, you have to consider the weight of the airframe (including servos, receiver etc) + weight  of battery + weight of motor.  For the ESC, you can consider some standard value, as they do not vary wildly and are relatively light.  So your decisions come down to choice of battery and motor. 

As long as you have the weight of the airframe, you can look at various motor choices and look at their specs to see their weights.  Now add that and the weight of an appropriate battery to the total, and see if the motor you selected has enough "watts per pound" for the style of flying you are going to do for this total weight.  If not, you have to adjust somewhere, like going for a bigger motor, but you have to redo the weight again in that case. The whole process becomes even simpler if you fix (ie, make a confirmed choice) on the battery also (which is easy these days as most ESCs support both 2s and 3s), in which case the only variable you are working with is the weight of the motor.  You should leave some slack in your calculations.

So the short answer is, AUW = confirmed weight of items in your hand already (like airframe) + weight of the motor and battery that you are considering (obtained from the specs of these items).

If you have 1:1 power, your plane will go unlimited vertical. That means the model will go against gravity pull :)

So if you have a 500gm plane( including electrics) and a motor with 500gm thrust, we can call it as 1:1 power. The plane is flyable with a power of 2:1 and it will be underpowered if the ratio is 3:1.

So when you choose a motor look for the maximum thrust and mind that many motors are overrated. SO to be in the safe side subtract 50-75gms off the thrust number. The thrust is also depended on the quality of the propeller+size(diameter and pitch). If you use a bigger propeller you will get higher thrust but you might kill the motor if run for over 10 sec(over heating or short circuiting) which you shouldnt do :)

Anwar,

My post was from the view point that for a given wingspan and area (and hence the airframe weight) adding weight would result in change in flight characteristics.

Even if the power to weight ratio is maintained, the wing loading changes with different motor/battery combos. This would result in different takeoff, landing and stall speeds. So the question is where does one draw a line - if all one would like to?

Pankaj

The question is, does it really ? That is, considering you have enough compensatory thrust provided by the power system.

Now I am venturing into areas that are not my strengths, so other experienced members may pitch in.

I think Design from Scratch is required from Experts. The process may be like.
Speed = jshf
Weight = g
Motor  selection(Power & Weight) with prop (Thrust & Pitch). [or Arbitrary & crosscheck]
Check thrust & weight.
Crosscheck Aleron, elevator, rudder area, Speed, moment, With servo movement.
Resultant torque of rotation for aerobatics. [of course at rated speed].
The servo power & response [Drag] delay.
Complete weight with battery, Servo, Motor, Prop, ESC, Receiver & Extra attachment if required.
Crosscheck with wingarea, wingspan, elevator area etc & drag, lift.
result observed and design parameter.
Reselection of Motor, Prop if required.
Calculation of flight time with battery & again reselection of battery(Sep.) if required.
Minor adjustment with wing angle, elevator, rudder, twist, repositioning.

Fitting Radio, building model & test flight.

Final checking with design & deviation.

=============================================================
Please note that I am a newbie with minor idea. This are not followed for design & scratch building and trial error, experience & guidance can replace the all theory and make this hobby enjoyable. So, Please excuse me if I made the boring design process.
=============================================================
But anyone use the theoretical idea (or after making back calculation)?
=============================================================