Maximum Distance v.s Maximum Endurance
星期六…下雨天…今日不做運動就又溫下書…每日一點點麻 …
Best distance speed (Maximum distance speed)
No matter we use distance and range to describe how far the aircraft can fly, we are literally talking about the horizontal distance that the aircraft can cover.
Best Glide Speed? (Best Range speed when losing engines)
When the aircraft lost its engine(s), the speed and configuration that will get you the most distance forward for each increment of altitude lost. That’s best glide speed.
In my own words, you trade vertical distance for horizontal distance, and the speed that you can fly most further, that’s the best glide speed.
Best glide speed will be roughly halfway between Vx (best angle of climb speed) and Vy (best rate of climb speed).
Best glide speed is proportional to weight!
best glide speed will be a little lower for lower aircraft weights.
Best Endurance speed? (Minimum Sink Speed when engines failure)
Best endurance speed is rarely found in Pilot Operating Handbooks, but it will be a little slower than maximum glide range speed.
Simply speaking, we tend to land the aircraft as soon as possible in the event of engine failure, so we usually emphasise on best glide speed instead of best endurance speed during engine failure.
https://youtu.be/pGd7wem46_s?si=1uz_YUjd-Brjrmg4
What about in engine running condition?
Best Range – tangent of engine output curve
Best Endurance – minimum engine output required (thrust curve or power curve)
Minimum drag speed is called Lift / Drag Max, or L/D max. It’s where induced drag equals parasite drag. It is the so called best glide speed.
However, when the engines are normally running, we look into the engine output curve (i.e the thrust curve or power curve, depending on the type of engine)! The tangent point is the so called maximum range speed.
Differences between propeller and turbine engine?
Thrust and power are not the same thing.
We must have this concept at the beginning!
Thrust is a pure force;
Power is the product of force and velocity.
(Power = Force * Velocity)
The turbine jet engines produce thrust ;
Reciprocating engines and turboprops do not directly create thrust. Instead, they spin a shaft. The more fuel they burn, the faster the shaft spins. So, the faster they spin the shaft, the more velocity it has, the more power they generate. Of course, the shaft connects to a propeller, and the propeller creates thrust.
It is also why we have throttle levers or power levers on the aircraft with reciprocating engine;
However, we called thrust levers for aircraft with turbine jet engines
From the diagrams, we can see that for propeller type aircraft (with reciprocating engines), the maximum range speed at L/Dmax. In contrast, the maximum range speed on aircraft with turbine engine is much higher than L/D max speed.
More references
https://www.faa.gov/sites/faa.gov/files/2022-01/Best%20Glide%20Speed%20and%20Distance.pdf
https://www.boldmethod.com/learn-to-fly/aerodynamics/best-range-speed-prop-vs-jet/
