OK Tim,
I plugged you numbers into Motocalc. I tweeked on the prop P constant and the battery voltage under load to get approximately the same RPM at the prop and same current draw as you stated above. The wattage is down around 1600 however which is more in line with what I would have expected. I wonder if you have bum watt meter?
MotOpinion - Venus II
850ft above Sea Level, 29.92inHg, 56°F
Motor: Great Planes Rimfire 50-65-450 (#4770); 450rpm/V; 2.2A no-load; 0.024 Ohms.
Battery: Thunder Power TP3850 (G4 ProPower 30C) (30C); 6 cells; 3850mAh @ 3.7V; 0.0056 Ohms/cell.
Speed Control: Castle Creations Phoenix 80; 0.001 Ohms; High rate.
Drive System: APC 17x8 Electric; 17x8 (Pconst=1.21; Tconst=1) direct drive.
Airframe: Venus II; 866sq.in; 139.1oz RTF; 23.1oz/sq.ft; Cd=0.043; Cl=0.07; Clopt=0.46; Clmax=0.81.
Stats: 188 W/lb in; 164 W/lb out; 28mph stall; 37mph opt @ 76% (24:11); 98mph level; 2376ft/min @ 47.1°; -474ft/min @ -8.4°.
Warning:
MotoCalc was unable to determine a throttle setting for hands-off cruise airspeed, so the best lift-to-drag ratio airspeed and throttle setting will be used instead.
Inability to determine a throttle setting for an airspeed usually means the model is not capable of reaching the required speed with the given power system, or the airfoil information has not been specified correctly.
Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (55.7A) falls approximately between the motor's maximum efficiency current (43.1A) and its current at theoretical maximum output (423.5A), thus making effective use of the motor.
The voltage (19.3V) exceeds 12V. Be sure the speed control is rated for at least the number of cells specified above.
Possible Aerodynamic Problems:
The static pitch speed (58.7mph) is less than 2.5 times the stall speed (27.7mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane or other slow-flying model.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a higher cell count, or some combination of these methods.
The diameter (17.0in) to pitch (8.00in) ratio is greater than 2:1, which will result in reduced propeller efficiency at flying speeds. An appropriate smaller diameter, higher pitched propeller would improve this.
Aerodynamic Notes:
With a wing loading of 23.1oz/sq.ft, 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 (249.2oz) to weight (139.1oz) ratio is 1.79:1, 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 (101.6oz) to weight (139.1oz) ratio is 0.731:1, which will give very steep climbs and incredible acceleration. This model can easily do consecutive loops, and has sufficient in-flight thrust for any aerobatic maneuver.
General Notes:
This analysis is based on calculations that do NOT take motor heating effects into account. This also means that no calculations were done to determine if the motor might overheat.
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.