Lift/Drag ratio chart
Best Glide Speed
Most aircraft accidents occur during the take-off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. On this part of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
A pilot uses best glide speed when he needs to fly the longest distance per unit of altitude lost. It is also used when the engine fails and a suitable landing place must be reached. Best glide speed is at that point where the lift/drag ratio is at its highest.
The effects of aircraft weight and the use of flaps on best glide speed and ratio will be reviewed too.
Descending without power
During this descend the throttle is fully closed and there is no thrust coming from the propeller. Actually the propeller is driven by the airflow (windmilling) and even this creates some drag. Setting the propeller full coarse might help a bit reducing this drag.
When the aircraft is at altitude the pilot uses this potential energy to convert that to kinetic energy, speed. Put differently: altitude is converted to speed in a glide without engine power. This is done by lowering the nose and trimming the aircraft to its best glide speed. During a power-on descend speed is also controlled by nose attitude and the rate of descent (glide angle) is controlled by the amount of power set.
Engine Failure
When the engine fails the aircraft is usually at altitude and at a speed greater than best glide speed. It is therefore advantageous to put the excess airspeed to good use and convert that to altitude while reducing and trimming for best glide speed. A higher altitude will also give you more options to land.
Best glide speed
Found at the point where the Lift/Drag ratio vs the Angle of Attack is at its maximum, see image to the right.
It is also found on the Thrust Required vs Airspeed diagram where total drag is minimum and found where the induced drag intersects with the parasite drag line. See image to the left.
At this speed the aircraft travels the greatest distance per unit altitude lost. At this point the aircraft also has the maximum amount of thrust remaining from the engine (not the total amount of thrust available) and this coincides with the best angle of climb speed, VX. If you would fly at this speed the fuel consumption would be the lowest for the distance travelled, a high NM/usg.
Power Required and Drag vs Airspeed
Maximum range flying (TAS or GS/fuel flow) is best done at this airspeed.
Weight
A higher aircraft weight will not affect glide angle but the best glide speed will be higher and the rate of descend will increase too. You will be on the ground sooner.
Flaps
They increase drag and ruin the best L/D ratio, glide angle will be steeper and glide range shorter. Use them only when a landing is assured during a real power failure.
Wind
Tailwind increases the glide distance, it pays to fly with the wind in case of engine failure.
