V Speeds on the Drag Curve

[QJB]

hey blah blah blah,

The drag curve indicates how much thrust the engine has to produce to maintain level flight at various airspeeds. Because a constant amount of lift is required to overcome weight for level flight, at slower speeds the wing must be kept at a higher angle of attack. With this increase in angle of attack comes an increase in drag, this is induced drag, responsible for the back side of the curve. One might expect then that as the aircraft flies faster (with an ever decreasing angle of attack) that drag would reduce. This is true of induced drag, but air resistance increases with speed and this again increases drag. Termed parasite drag. If you plot the two curves together you can see the total drag curve for the aircraft, and thus the thrust required curve. The various speeds are as follows:

minimum drag speed: the lowest point on the total drag graph. Also corresponds to the point where induced drag and parasite drag are equal. Anything that requires an increase in angle of attack for a given speed (like an increase in weight) will create an increased amount of induced drag for a given airspeed and thus the airspeed for minimum drag will increase. On the other hand, anything that creates more parasite drag, such as an increase in the parasite area of the aircraft will move the speed lower. Minimum drag speed corresponds to the best glide speed, although for many aircraft this is a lower figure due to the increased parasite drag from a windmilling prop.

Vs: Theoretically the minimum speed for level flight would occur on the low end of the curve where thrust available meets thrust required, but in reality the angle of attack required at this speed is too great and the wing stalls first.

Check out the power required curve which is just the drag required x velocity. Minimum power required corresponds to best endurance, maximum level flight speed is where the power available meets power required at the top end of the speed range. Best rate of climb occurs at max excess power and best angle at max excess thrust.

Sorry for rambling, the best way to find speeds is to look in the flight manual for your aircraft. I'm sure they will be there. If you want to be really precise they are normally given for MTOW and could be reduced in proportion with the square root of weight decrease, but this is really unnecessary for light aircraft.

J