Being fond of aviation, I play with software simulators a lot.

I have noticed that while jets seem to be able to glide for a long distance even at zero throttle, propeller aircraft need to be throttled right up to the runway.

Is this accurate in real planes? If so, why?


5 Answers 5


"Glide performance" is measured by "Lift-to-drag ratio":

In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the amount of lift generated by a wing or vehicle, divided by the aerodynamic drag it creates by moving through air

When you look for examples you'll see that for the most part larger airliners do indeed have a more favourable L/D ratio

  • Boieng 747 = 17:1
  • Airbus A380 = 20:1
  • Cessna 172 = 11:1

As to why, that is the way they are designed. A good glide ratio is a favourable feature of an aircraft as less drag will imply better efficiency, and airlines are more concerned with efficiency than, for example, a training aircraft which might be more concerned with stability.

More information can be found here: How far can airplanes glide?

  • 2
    $\begingroup$ Is it just that expensive aircraft are more carefully designed? E.g. the engine type is irrelevant. $\endgroup$ Commented Sep 28, 2020 at 8:10
  • 6
    $\begingroup$ And for a commercial airliner fuel efficiency (and thus L/D) is much much more important than for a sports plane. A better comparison would be between commercial airliners with jets and props. $\endgroup$
    – ROIMaison
    Commented Sep 28, 2020 at 8:30
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    $\begingroup$ @AnonymousPhysicist: There are probably many reasons. For instance, retractable wheels add complexity but reduce drag. $\endgroup$
    – MSalters
    Commented Sep 28, 2020 at 11:17
  • $\begingroup$ @AnonymousPhysicist it's also a factor of production price, ie straight wing is much easier to manufacture than wing sweep (no torsion moment at wing root). But it improves L/D to have swept wings. $\endgroup$
    – paul23
    Commented Sep 29, 2020 at 13:00
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    $\begingroup$ It's a bit unfair to compare the glide ratio of a fixed-gear aircraft with that of an airliner. At least the gear-down configuration should be chosen, and then an airliner doesn't look so good any more. $\endgroup$ Commented Sep 29, 2020 at 15:39

A few reasons: if we're talking glide range from altitude, then jets certainly can glide longer! They fly MUCH higher than most props do, even turboprops! Second, propellers are draggy when they're at idle- you effectively have a big speedbrake on the nose of your plane. This is the reason a lot of planes have featherable props- they can be turned into the wind to minimize drag when the engine fails, increasing your glide range. I hope the answer helped!

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    $\begingroup$ Good point on propeller drag: while a jet will still produce quite a bit of thrust at idle, the propeller already creates drag at flight idle. $\endgroup$
    – Bianfable
    Commented Sep 28, 2020 at 14:30
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    $\begingroup$ Why would a jet generate quite a bit of thrust at idle? $\endgroup$
    – ROIMaison
    Commented Sep 28, 2020 at 15:47
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    $\begingroup$ @ROIMaison because they move SO MUCH AIR! It's insane how much air they move! $\endgroup$
    – MD88Fan
    Commented Sep 28, 2020 at 15:54
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    $\begingroup$ @RossPresser the amount of air moved is fairly irrelevant, compared with the fact unlike a propeller driven by an IC engine, you can turn the fan of a big high-powered jet engine quite easily with one finger. Unless there is something stopping the rotors from turning, they don't create much drag on the air flow through the engine even if it is shut down. $\endgroup$
    – alephzero
    Commented Sep 28, 2020 at 17:28
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    $\begingroup$ @RossPresser It's not that jet engines idle with less drag. It's that unlike piston engine that can idle very slow jet engines would stall if it idles any lower. Thus the "massive amount of thrust" that a jet engine generates at idle is merely the minimum amount of thrust it can generate before it shuts down due to starvation of oxygen. You can of course rig a piston engine to also idle at high revs - while most engineers and most common people would not consider such a setting as "idle" for a piston engine, "idle" is just what the lowest throttle setting make the engine run at $\endgroup$
    – slebetman
    Commented Sep 29, 2020 at 0:36
  1. There's absolutely no difference as such between the two. If you removed the engines from both, there's no inherent difference.

  2. Note that, naturally, modern airliners (which are all jet) are incredibly better and more sophisticated than old-fashioned historic prop aircraft in every way. So, totally unrelated to the engines, of course they have incredibly better flight characteristics, including gliding. This has nothing at all to do with the engines. It's exactly the same as that a 2020 model Lexus is staggeringly better in every way (including drag coefficient) than a 1960s muscle car. They have completely different engines (cats versus no cats) but that's irrelevant.

  3. Note that there are PLENTY of examples of propeller aircraft that have incredible gliding characteristics that completely crush the gliding characteristics of the best modern airliners. Two examples:

enter image description here enter image description here

  1. In the question OP you mention throttle settings. "Gliding" would normally mean with the engines totally OFF. I would guess that nobody here knows or can help with issues relating to what the throttle settings mean in your sim. (It could well be that in the prop planes the engines are totally off, but, in the jet planes they are still running - who knows?)

  2. Note that if a propellor is just sitting there not turning, yes, it makes a huge amount of drag. (For this reason you can feather many propellors.) BUT then again big jet engines (not running) are huge physical objects, and surely create drag. It would seem to be fairly meaningless to compare the two. (How would you "compare" them - equivalent power output, or? The tiny little prop on a small aircraft would surely have far less drag than the huge barrel engines on a jumbo jet.)

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    $\begingroup$ This has nothing at all to do with the engines – may I beg to differ? Flight idle of a jet is about 50% of max RPM, so there is still some positive thrust left. An idling prop, on the other hand, is windmilling the engine and has negative thrust, AKA drag. One of the hardest parts in flying the Me-262 was getting that thing to stop, especially for pilots who had only experienced recip engines with props before. $\endgroup$ Commented Sep 29, 2020 at 15:45
  • $\begingroup$ ah, @PeterKämpf , would you please note point "4" and also the general confusion about the question. $\endgroup$
    – Fattie
    Commented Sep 29, 2020 at 15:57
  • $\begingroup$ @PeterKämpf At the same time, thrust is exponential with N1, so idle thrust is like 3-4% of rated, even though it's running at 50% of its rated RPM. It's not a linear relationship. $\endgroup$
    – J...
    Commented Sep 29, 2020 at 19:07
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    $\begingroup$ @J Depends on speed. At low speed the idle thrust is positive and at high speed it is negative. And where did I say there would be a linear relationship between RPM and thrust? $\endgroup$ Commented Sep 29, 2020 at 19:26
  • $\begingroup$ @PeterKämpf You didn't - but you made an implicit comparison that suggested a relationship between RPM and thrust where the typical reader would reasonably make certain inferences about what it implied and, typically, that's something like a linear relationship. Your comment was just unclear and easily misinterpreted to suggest that flight idle produces much more thrust than it really does. Since the whole discussion circles around that, I felt it was important to clarify. And yes, at speed, even that thrust is not sufficient to overcome drag. $\endgroup$
    – J...
    Commented Sep 30, 2020 at 11:51

Jet-propelled planes are often aerodynamically 'cleaner' than propeller planes. Hence, drag is smaller and L/D is higher. So they glide much better...

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    $\begingroup$ used to be? so they don't any more? $\endgroup$
    – Michael
    Commented Sep 28, 2020 at 19:43
  • $\begingroup$ No... I wrote 'use to be', not 'used to be'... One never knows what the future may bring... $\endgroup$
    – xxavier
    Commented Sep 29, 2020 at 9:35
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    $\begingroup$ "use to be" is not making a clear point. Could you express that in another way? Is it "The intended usage of jet planes requires the most aerodynamic design possible" ? $\endgroup$
    – Criggie
    Commented Sep 29, 2020 at 12:22
  • $\begingroup$ @Michael actually yes, originally jet (in considering with turbofan) engines were inside the wing, though this gave problems when one engine failed (often causing a cascade of problem), thus regulations made jets kind of obsolete. $\endgroup$
    – paul23
    Commented Sep 29, 2020 at 13:01

To talk about maximizing glide ratio is the same as talking of maximizing Lift/Drag (derivation can be given, "exercise for the reader"). Since lift is a given when gliding (we don't typically lose weight when engines are inoperative - and we consider a steady symmetric flight). To maximize L/D we minimize the drag.

For a car, or other land based vehicle (car, train etc) this typically means we move as slow as possible.

An aircraft has a different drag profile. Due to the fact that the wing doesn't generate work, bending the airflow downwards also means it has lower horizontal relative velocity (airflow stays same total relative velocity). Thus the lift vector is slightly backwards.

This is the lift-induced-drag. And since at higher velocity we need to bend the air less (more volume of air is moving per unit time), this drag reduces with velocity.

This leads to an image like below:

enter image description here

Now the equation for the drag coefficient is written like:

$$C_D = C_{D_0} + \frac{C_L^2}{\pi AR e}$$

(With $C_L$ being the lift coefficient, $e$ efficiency span factor which counts for winglets etc and $AR$ the aspect ratio of the craft). And $C_{D_0}$ the sum of all parasitic drags.

We are typically quite good at reducing skin-parasite drag. So designing an aircraft for a higher velocity means a better design. Another thing to note is that a higher aspect ratio makes a more efficient gliding craft => hence gliders have long slender wings.

However this would be equal for both props and fan/jet craft. What makes a fundamental difference is another form of parasitic drag: wave drag. Wave drag is massive compared to the other forms of drag. And it happens where (locally) the relative velocity with air is above mach 1. (IE on top of the wing, where curvature is maximum you typically have highest velocity). A propeller is moving not only as fast as the aircraft, but is also adding another rotating component, so the tips of a propeller have a higher relative velocity than the rest of the aircraft. Where a fan engine typically has an inlet designed to slow down the incoming airflow, so that the relative speed isn't above mach 1.

For this reason a turbo-prop has to be designed to cruise at lower mach number. This that cruising is less efficient. So picking a propelor as your initial design strategy means you design an aircraft where aerodynamic efficiency is not a main design parameter. Maybe landing/takeof distance is more important, or cheap production/maintenance is a driving parameter.

Since aerodynamics isn't a main parameter it will show in the L/D curve.

If glide performance with paramount I'm sure one could design a very efficient propelor craft, props typically can rotate the blades anyways, so you could at "power off" easily design those to give minimal drag.

It's just not really something to consider.


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