Energy Management: Best Glide and Best L/D
When you fly you don’t really think about energy management so much. As long as the fan out front is turning, you don’t have to waste a lot of brain cells on the topic.
Much is said about the importance of maintaining the ‘best gliding speed’ but what is important is to maintain an optimum glide speed; a penetration speed which takes atmospheric conditions into account, for example sinking air or a headwind. The gliding community refers to this as the speed to fly. The normal recommendation for countering a headwind is to add half the estimated wind speed to Vbg which increases the rate of sink but also increases the ground speed. For a tailwind deduct half the estimated wind speed from Vbg which will reduce both the rate of sink and the groundspeed. Bear in mind that it is better to err towards higher rather than lower airspeeds.
The good news is the glide ratio is not affected by weight, but the bad news is the heavier you are it does occur at a higher speed. That’s why an AOA would be so much better than our airspeed indicators.
- Take a few lessons in a glider, this will teach you an immense amount energy management
- Do some tests in the airplane you fly. Pull the power to idle and test its glide performance. If you have an AOA, fly your max L/D index. If not, fly your best glide speed and in both cases measure your rate of descent. Get a feel for it noting that an idling engine won’t mimic that same behavior as dead one, especially with a windmilling prop, as mentioned above.
You can also go here and play with this simple tool to see different scenarios: http://www.csgnetwork.com/glideratiocalc.html
While this is not an exhaustive essay on the subject, I do hope it spurs some thought in your everyday flying.
If you are interested in learning more, might I recommend:
Aerodynamics for Engineering Students, Sixth Edition - this is for the folks who love math with lots of weird symbols that I don’t understand.
Aerodynamics for Naval Aviators: NAVWEPS 00-8OT-80 - This is read by many who want an above average level of understanding.
Illustrated Guide to Aerodynamics - my favorite of the three for folks who want to know more, but don’t like math.