Lift/Drag ratio chart
Best Glide Speed
Most aircraft accidents occur during the take-off or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing do occur every now and then. In this section 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.
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.
We have seen that some aircraft manufacturers specify a minimum sink speed too in addition to the best glide speed.
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 produced by the propeller. Actually the propeller is driven by the airflow (windmilling) and even this creates some drag. Should you have a controllable propeller: set it to full coarse (pull the blue knob) 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 raising the nose and trimming the aircraft to its best glide speed (if flying faster than VGLIDE). During a normal 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.
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.
Minimum Sink Speed
This speed is lower than the best glide speed and mainly used when the pilot wants to gain time in stead of distance. This might become helpful when at an reasonable high altitude and you need time to think about your options before gliding towards a suitable landing site at best glide speed (distance).
Best glide speed
You will find this at the point where the Lift/Drag ratio vs the Angle of Attack is at its maximum, see the image to the right.
It is also found on the Thrust Required versus Airspeed diagram where total drag is minimum and found where the induced drag intersects with the parasite drag line.
See the 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 low usg/NM or liter/km.
Power Required and Drag vs Airspeed
Maximum range flying (TAS or GS/fuel flow) is best done at this airspeed.
A higher aircraft weight will not affect the glide angle but in stead the best glide speed will be higher and the rate of descend will increase too. In short: you will be on the ground sooner when the aircraft is heavier.
As you are already know: flaps increase lift and drag and this drag increase will ruin the best L/D ratio. The glide angle will be steeper and glide range shorter. It is best to use them only when a landing is assured during an actual engine failure or when practicing these on a runway with enough length.
In situations like this it really pays to know where the wind is coming from. Tailwind increases the gliding distance. In case of engine failure the pilot can use this effect to reach a good landing spot. As winds usually increase with altitude, flying high will also help the pilot experiencing an engine failure in this regard.