# Best glide speed vs. best glide angle? Maximum glide range?

I have some open questions regarding best glide in a descent:

• I understood, that there is a best glide speed. This best glide airspeed occurs at the highest lift-to-drag ratio (L/D). This best glide speed is normally given in the operator's manual of the aircraft.

Is it true, that this best glide speed is given in terms of indicated airspeed IAS and hence is independent of the elevation?

• What about the sink rate when flying at best glide speed? I have seen hodographs where the sink rate is given over airspeed. (Diagram taken fom Taken from: Coombes, Reachability analysis of landing sites for forced landing of a UAS)

Is this again inidcated airspeed IAS?

• As we all know, the TAS decreases with decreasing elevation when maintaining a constant IAS. For navigation with respect to earth's surface I need ground speed GS which is directly related to TAS via the wind speed (which we can assume to be zero for the moment)

• To determine the glide path angle I suppose, that the ground speed and the sink rate is relevant.

Supposing, the best glide speed is IAS contrary to true airspeed TAS, how can I determine the glide range?

What is the resulting flight path angle when maintaining best glide speed? Does it change with decreasing elevation?

• My intuition tells me, that when flying at constant IAS, the flight path angle should get steeper with decreasing elevation.

Is this assumption true?

Every reference to clarify these questions is welcome.

Currently I try to implement an autopilot for an emergency landing assistant that maintains a constant flight path angle during gliding descent. This enables me to exactly determine the distance traveled when I know the starting and the end elevation which in turn allows to do a proper path planning. My gut feeling tells me, that in this case I don't make use of the maximum possible gliding range as I have to start with a too steep flight path angle at high elevation to maintain this constant till the ground.

Is there any good idea how to predict the distance traveled when only knowing the start and target elevation?

Thanks a lot and Cheers,

Felix

• Aug 17, 2018 at 10:32
• "As we all know, the TAS decreases with elevation when maintaining a constant IAS" - really? I didn't know, and am still in doubt. Aug 17, 2018 at 15:48
• @PeterKämpf is correct: True airspeed increases with altitude for constant indicated airspeed. Also it is better to define 'best glide'. You are referring to best L/D speed, which is the flattest glideslope. The other 'best' speed is minimum sink speed, which is slower. Min sink speed will keep you in the air for longer, but will not get you as far through the air as best L/D speed. Aug 17, 2018 at 19:09
• @a.out: You are both perfectly right and I formulated it in a mistakable way: it must be: "As we all know, the TAS decreases with decreasing elevation when maintaining a constant IAS." Aug 18, 2018 at 13:23

Is it true, that this best glide speed is given in terms of indicated airspeed IAS and hence is independent of the elevation?

Yes. The best glide speed needs you to fly at the optimum polar point, and with the same IAS you will fly at the same lift coefficient and polar point.

To determine the glide path angle I suppose, that the ground speed and the sink rate is relevant.

No, the angle will be the same regardless of altitude (if we neglect Reynolds number effects for now). What does change is the speed over ground - this goes up with height.

What is the resulting flight path angle when maintaining best glide speed? Does it change with decreasing elevation?

No, it doesn't. The glide path will be the same, only at higher altitude the airplane will fly faster and proportionally sink faster. What changes is the time scale, not the geometric scale.

EDIT: The added graph is either given in TAS on the x scale and for a specific altitude, or it is given in IAS and valid for sea level to maximum ceiling (if we neglect Reynolds number effects for the moment). Note that at higher altitude the airplane will fly faster and proportionally sink faster, so the indicated sink speed in the second case must be multiplied with the square root of the density ratios between ground and actual altitude to arrive at the true sink speed.

P.S.: Terminology:

Altitude is the vertical distance from sea level to where an airplane flies. See here for more.

Elevation is the vertical distance from sea level to the ground or to an obstacle.

Height is vertical distance between an aircraft and the local ground.

• Well, there's a tricky point here in that if x axis is given in IAS not TAS, then the vertical scale also needs to be thought of as "indicated" not "true". I.e., if you change the altitude for which the graph is intended to be valid, you would have to make a change to the vertical speed scale, if it is meant to be in actual real units of meters per second. Feb 18, 2023 at 20:52
• @quietflyer Right, I do mention that at higher altitude the airplane will fly faster and proportionally sink faster, and assume that people keep that in mind over two sentences. Maybe that is too optimistic. Feb 18, 2023 at 21:38