RC India

Hey,
I used MotoCalc to get the electronic requirements of the Su-27
Su-27 spec:
1) wingspan - 24 inch.
2) area - 168.25 sq.inch.
3) weight - 8.5 oz (241gm)
4) Flat wing - using a foam board.
5) thickness of foam - 6mm

These are the results given by MotoCalc:-
1) Motor - Welgard C1827-31, 1800rpm/V, no-load current:0.2A, resistance:0.056ohm, weight:20g, brushless, outrunner.
2) ESC - Great Planes Electrifly Silver Series 25 ESC, resistance:0.015ohm, mac current: 25A, weight:26g, brushless.
3) Propeller - Generic 7x4.5in, Dia:7inch, pitch:4.5inch, P.const: 1.31, T.const:0.95.
4) Battery - Kokam 910SHD, Li-Po, 15C, cell capacity:910mAH@3.7V, impedance:0.0275ohm, 2S


MotOpinion - Su-27
152.4m above Sea Level, 101.3kPa, 14°C

Motor: Welgard C1827-31; 1800rpm/V; 0.2A no-load; 0.056 Ohms.
Battery: Kokam 910SHD (15C); 2 cells; 910mAh @ 3.7V; 0.0275 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 7x4.5in Prop; 7x4.5 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Su-27; 10.8dm²; 308g; 28.5g/dm²; Cd=0.044; Cl=0.17; Clopt=0.37; Clmax=0.69.
Stats: 247 W/kg in; 216 W/kg out; 8.5m/s stall; 11.7m/s opt @ 64% (27:40, 20°C); 17.2m/s level @ 91% (12:40, 25°C); 7.31m/s @ 38.7°; -1.93m/s @ -9.5°.

Possible Power System Problems:

The estimated steady-state still-air battery temperature at the hands-off cruise airspeed and throttle setting (approximately 57.0°C) is higher than the suggested maximum temperature for this cell type (50°C). This could result in battery pack damage unless adequate cooling airflow is provided and/or run times are kept short. A lower current would also decrease the battery temperature.
Current can be decreased by using fewer cells, a smaller diameter or lower pitched propeller, a higher gear ratio, or some combination of these methods.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (10.5A) falls approximately between the motor's maximum efficiency current (4.91A) and its current at theoretical maximum output (60.5A), thus making effective use of the motor.

Aerodynamic Notes:

The static pitch speed (20.4m/s) is within the range of approximately 2.5 to 3 times the model's stall speed (8.47m/s), which is considered ideal for good performance.
With a wing loading of 28.5g/dm², a model of this size will have trainer-like flying characteristics. It would make an ideal trainer, for use in calm to light wind conditions.
The static thrust (3.42N) to weight (3.02N) ratio is 1.13:1, which will result in extremely short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and vertical climb-outs. This model will probably be able to perform a hover or torque roll.
At the best lift-to-drag ratio airspeed, the excess-thrust (1.86N) to weight (3.02N) ratio is 0.615:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

General Notes:

This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.

To all the professional fliers & designers:
Please let me know whether these results are acceptable or not.

Thanks,
Vikalp Luthra.

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