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On an another question, an answer said: "You don't need an engine to fly as airplanes are designed to glide without it."

I suspect this heavily depends on the type of the aircraft, so lets assume we are considering a small airplane.

How far could an airplane glide? What governs the 'glideability' of the plane? Is it possible, for an airplane with an engine/engines, to leverage this to save fuel while flying, or are they too heavy/otherwise unable to do this?

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glideslope and initial altitude will be the key words –  ratchet freak Apr 2 at 20:03
    
As a glider is also an airplane, if the weather is right, the upper limit is only limited by pilot endurance. –  vsz Apr 3 at 3:12
    
A Boeing 767 pilot was able to glide his plane down safely after total power loss, so even large planes can glide with sufficient initial airspeed: en.wikipedia.org/wiki/Gimli_Glider –  HorusKol Apr 3 at 6:09
    
@HorusKol all aircraft can glide some distance. What that distance is depends on aerodynamics, atmospheric conditions, initial speed, and weight. And of course pilot skill. –  jwenting Apr 3 at 9:13
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Another example of an airliner gliding is Air Transat 236 -- around 120km in an Airbus A330 in 2001. –  David Richerby Apr 3 at 11:25

5 Answers 5

up vote 20 down vote accepted

I suspect this heavy depends on the type of the aircraft, so lets assume we are considering a small airplane.

Absolutely right, it does vary widely. The parameter you're asking about is called glide ratio and it is directly related to another parameter called lift-to-drag ratio or L/D ratio. This is a fundamental characteristic of the aerodynamics of a particular aircraft. L/D ratio varies with airspeed; for determining best engine-out glide performance, the L/D ratio at "best glide airspeed" is used. "Best glide airspeed" is the speed that maximizes the L/D ratio, and this maximum value is known as L/Dmax.

The maximum L/D ratio (L/Dmax) of a Cessna 172 is about 9, so its glide ratio is about 9:1 - for every 9 units traveled forward it will lose 1 unit of altitude. So, it will glide about 9,000 feet for every 1,000 feet of altitude available. This is a fairly typical value for small planes.

To show you how widely variable this is, a modern glider can achieve ratios above 60:1, while the Space Shuttle ranged from about 1:1 at high speed, early in reentry, to 4.5:1 on final approach.

Notably, large transport aircraft tend to have significantly higher L/D ratios than small aircraft: a 747 can achieve an L/Dmax of about 17:1.

What governs the 'glideability' of the plane?

As above, its lift-to-drag ratio. Very casually speaking this is just a measure of how "aerodynamic" the airplane is, comparing its capability to generate lift with the drag it creates in the process. The better its lifting capabilities, or the less drag it generates, the higher the ratio.

For more information, I highly recommend a free online book on aerodynamics called See How It Flies by John Denker. It is written for pilots, not mathematicians or physicists, so it explains the concepts very intuitively without a lot of equations. It talks about L/D ratio and explains some of the factors that affect it. (I would recommend this book to any pilot anyway.)

Is it possible, for an airplane with an engine/engines, to leverage this to save fuel while flying, or are they too heavy/otherwise unable to do this?

Since glide ratio is directly related to (indeed it's the same thing as) L/D ratio, you could say that airplanes already do take advantage of it. The higher their L/D at cruise airspeed, the more fuel-efficient they will be (because less thrust will be required to counteract drag in steady-state flight).

If you're asking about shutting the engines off during approach/landing to save fuel, there is another question here on ASE which specifically addresses this. (The answer, in short, is no, it's not practical.)

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Great answer. Maybe include some examples of about how far some planes could glide in typical situations? –  fooot Apr 2 at 21:23
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Worth mentioning: as dvnrrs says, the L/Dmax airspeed is the "best glide" speed. Pilots memorize the best glide airspeed for their airplane, and if the engine craps out, we're trained to immediately go to that airspeed by raising or lowering the nose (usually raising) to maximize our glide. It also turns out that if you want to maximize the range you can fly on the fuel you have, you would also fly at that airspeed. In my plane, you can reduce the fuel consumption from 9.5 gph to 5.5 gph by flying at best glide speed, but it's a bear to maintain that speed, so you don't do it very often. –  Edward Falk Apr 2 at 21:55
    
2dvnrrs, the figure you post for the 747 is likely with engines idling? With four shutdown engines I would be really surprised if it could achieve such glide ratio. –  Martin Argerami Apr 2 at 22:14
    
@MartinArgerami You may be right; my source was the linked Wikipedia article. If someone has more official figures for a typical large transport I'll update accordingly... –  dvnrrs Apr 3 at 11:57
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At the moment the upper limit for glider L/D is around 65 to 68, see e.g. the EB 28 and EB29. 60s are not too uncommon for open class gliders. –  yankeekilo Apr 4 at 7:35

Glide is measured in what's called "glide ratio". For every foot of altitude the airplane loses, how far forward can it go? Sailplanes (gliders) have a glide ratio around 40 to 1. It can be much more or less, depending on the model. The glide ratio of the Cessna 172, the most popular single engine airplane, is about 10 to 1. The Boeing 767 that lost all power attained a glide ratio of about 12 to 1 in practice.

Airplanes generally do not use this to save fuel. It's much more efficient just to fly the airplane normally, or at low power if they are looking to conserve fuel. To glide would require turning the engine on and off. That is not efficient in jets, turboprops or piston engines. I believe some unmanned, high altitude aircraft may turn their engines off, though.

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For what it's worth - the particular 767 incident referenced in this answer was the Gimli Glider. –  dvnrrs Apr 2 at 20:23

Glide distance does indeed depend heavily on the airplane's characteristic glide ratio, which depends on airspeed, as the answers above describe. However, the answers above assume still air, and real air is never still.

If you glide with a steady tailwind, you'll glide further than you will in still air; a steady headwind will cut your glide. (You'll get the same sink rate and therefore the same time aloft, but you'll make a different amount of distance over the ground.) If you're at altitude, it can be difficult to tell which way the wind is blowing unless you already know or unless you have a GPS so that you (or it, automatically) can compare your speed across the ground with your speed through the air.

Also, downdrafts will cut your altitude and thereby your glide distance; but updrafts will do the opposite. As a matter of fact, in the right kind of weather pilots who are trained to find updrafts--glider pilots, for example--can stay aloft without power for as long as they like. Updrafts are in general powered by the sun, though, so after dark it gets difficult to find "the right kind of weather." Also, updrafts tend to be localized and fairly stationary, so flying circles to stay in one, if it's weaker than you expected, may use up more of your gliding time than it adds and so reduce the total distance over the ground that you can glide.

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To give some concrete examples:

A Cessna 172 might fly at 2000 ft for a short hop, or at 12500 ft for a longer cross-country flight. My rule of thumb is about 1.5 nautical miles glide per 1000 ft altitude above ground level, thus:

  • From 2000 ft, a Cessna would be able to glide about 3 nautical miles (3.5 statute miles, 5.5 km), and it would take about 3 minutes for that.
  • From 12500 ft, just in proportion, it would be about 18 nautical miles (21 statute miles, 33 km), giving the pilot an ample 18 minutes to troubleshoot, find a suitable landing spot, and communicate the predicament.

A big jet flies higher during cruise, say at 38000 ft, and has a better glide ratio.

  • From 38000 ft, a jet might glide for 75 to 100 nautical miles (140 to 180 km), and take about 20 minutes to do so.
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"Glideability" is flight. A fixed wing aircraft that is controllable under power, is controllable without it. Some aircraft are specifically designed to be unstable in flight, such as the F-16, and F-117. in those cases computers assist to provide artificial stability; as long as the computers are working, the aircraft is still controllable without a functioning powerplant.

Glide ratio varies significantly between fixed wing aircraft. Unpowered gliders are generally higher than 20/1, while many small planes see 17/1 or less. The space shuttle is 4.5/1. (Horizontal/Verticle)

Glide efficiency is mostly a function of drag, not weight. For example: gliders sometimes carry water ballast to increase weight, since higher weight results in higher glide speed. (The water is dumped to slow the aircraft before landing.)

The FAA publishes airmans manuals, and you can find a variety of other flight instruction manuals available online or at the local airport. Any Private Pilot or Sport Pilot manual will discuss the basic principles of flight in some detail, They usually make for very clear and easy reading.

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