Energy Management: Best Glide and Best L/D

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best glide speed

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.

But what about folks who fly without engines, like glider pilots? They are constantly considering the relationship between the potential energy they have in the form of altitude and their kinetic energy, speed, and where they will get their next morsel of lift. It won’t be from a fossil-fuel-belching piston engine, but rather from the movement of our atmosphere, which gliders are designed to exploit.
In gliding flight, in a glider or powered aircraft, the Lift to Drag ratios are equal to the Glide Ratio (when flown at constant speed).
Drag_Curve
In the drag chart above, note how the drag goes up rapidly if you go too fast or too slow. This illustrates the need to resist the temptation of ‘stretching the glide’ by sneaking the nose up on an off airport landing. That’s a recipe for disaster.
Here are some glide ratios for a few common GA airplanes:

 

glide ratio

Glide ratios for some common GA aircraft

Note that many of these are around 10:1. So let’s do a little calculation:
Illustration of glide ratio
Vbg –  Best Glide speed –  the speed at which you can cover the most distance for altitude trade-off
Vmd – Minimum Descent speed – the speed that results in the lowest rate of sink in a power-off glide, thus providing the longest time in the air from the potential energy of height. Vmd is the airspeed used by gliders when utilising the atmospheric lift from thermals or waves. This is the airspeed to select if you are close to a favorable landing site with ample height and a few more seconds in the air to sort things out would be welcome.

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.

Both Vbg distance and Vmd time are adversely affected by the extra drag of a windmilling propeller, which creates much more drag than a stopped propeller following engine shut-down. If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling may be stopped by temporarily reducing airspeed so that the negative lift is decreased to the point where internal engine friction will stop rotation. Do not stall the aircraft trying to get the prop stopped.
prop_drag_contribution

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.

If you really want to have  a better understanding of this topic, I would encourage you to do two things:
  1. Take a few lessons in a glider, this will teach you an immense amount energy management
  2. 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.

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Written by
  • Brent Owens
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