13
$\begingroup$

I just saw a documentary on the design of the Boeing 787 "Dreamliner" and one of the designers said that the wings were inspired by gliders. Looking at the production version of the plane, it is obvious that the wings are indeed more glider-like than other airliners.

How does that affect the glide ratio? I can't find any data on this (is there any data on that at all?)

$\endgroup$
10
$\begingroup$

As discussed here, glide ratio is related to the L/D (lift-to-drag) ratio of the plane. When talking about commercial aircraft, "glide ratio" is typically used to mean the ratio with engines inoperative. If the jet engines aren't running, they add a lot of drag, which will decrease the L/D ratio from the typical max value.

This analysis estimates the max L/D of the 787 at 20.8, while this site estimates 21.

Other sources estimate L/D of the A330 and A340 at 20. The 747 is estimated at about 18. So based on these estimates, the 787 is slightly better than other large airliners.

Most of the efficiency gains of the 787 are from the lower weight of composite materials (which allow bigger wings) and more efficient engines.

$\endgroup$
  • 1
    $\begingroup$ Efficienciency is geometric, it has nothing to di with weight. Only the speed where you'll find that efficienciency is influences by weight, this is why gliders are often loaded with water. $\endgroup$ – Caterpillaraoz May 16 '18 at 11:08
  • $\begingroup$ @Caterpillaraoz is correct that L/D (and therefore glide ratio) is independent of weight. The final paragraph of the answer (efficiency gains) addresses a separate issue (weight of the airframe). A lighter airframe requires less lift for level flight, so there is less drag, resulting in lower thrust requirements. $\endgroup$ – a.out Mar 13 at 15:47
1
$\begingroup$

There are many factors to glide ratio of an airplane, but a good indication is the aspect ratio (see more here: Wikipedia ). The aspect ratio tells us how "slim" the wing is.

The slimmer a wing, the less induced drag it will create. The downside is that the longer a wing is, the more parasitic drag it creates. So it is a tradeoff for glider designers. For low speed flight, the longer the wing, the better. But for high speed flight, a short wing is better. This can be seen in open class gliders, where the trend the past 30 years have been to create greater wingspan gliders. The last few years, this has changed, and glider manufacturers are now producing new generations of open class gliders that have shorter wingspans than their predecessors(in the case of the new Quintus vs its predecessor, the Nimbus 4: 23m vs 26,5m)

In gliders, a high aspect ratio in nearly all cases (there are exceptions) translate to a high glide ratio compared to low aspect ratio gliders. Examples of gliders from the same generation (or close to the same) and their aspect ratios and (theoretical) glide ratios:

(AR: aspect ratio; GD: glide ratio)

LS4: AR: 21,7; GR: 40

Ventus 1: AR: 23,7; GR:44

Discus: AR 21,3; GR 42

Nimbus 3: AR 35,9; GR 58

ASH25: AR 39,8; GR 60

Now an extreme example that has been produced in very few numbers; the ETA has an aspect ratio of 51,3 and a glide ratio of 72.

Now, enough about gliders! If we take airliners, then the Gimli Glider (Boeing 767) had a reported glide ratio of 12:1 in an emergency situation.

A 767-200 has an aspect ratio of 8~, along with a glide ratio of (according to Wikipedia) 12:1.

An Airbus A340-300 has an aspect ratio of 10~, and a 737-NG with winglets has an aspect ratio also of 10~. On the other hand, a 787 has an aspect ratio of 11~.

But without factory numbers of any of the airliners, it would be hard to say. But in any case, going by the aspect ratio, the 787 should have a better glide ratio, compared to the other airliners i mentioned.

With regards to the 787 wing features, then the leading edge sweepback is very popular among glider manufacturers today. There is also a noticable increased dihedral(wing bending upwards) further out the wing, this is also seen on gliders.

It is hard to suggest if these factors will increase the glide ratio of the 787 compared to other planes, as the wing is certainly designed for optimal performance at cruise speed, and the speed for best glide ratio is likely alot lower.

The only conclusion i would guess at, is that because of an aspect ratio that is about 10% higher than comparable airliners, then the 787 gliders better (but i have no idea of how much). If i would have to ballpark it, i would guess somewhere around 16-18 in glide-ratio.

$\endgroup$
  • 2
    $\begingroup$ The part about aspect ratio is right on but you go off the rails with "conclusion I would guess at" $\endgroup$ – rbp Apr 10 '15 at 20:44
-6
$\begingroup$

Total guesswork here, but a typical airline will have a glide ratio of 15-20:1. Gliders attain 30-60:1.

According to Boeing, the wing has a "highly efficient lift-to-drag ratio", which to me means 20-25:1. So that's my estimate.

$\endgroup$
  • $\begingroup$ Please explain downvote. I answered the question using educated guesswork and sources, and no one else has answered with an official source. $\endgroup$ – Yos233 Feb 19 '14 at 2:12
  • 10
    $\begingroup$ I didn't downvote, but I can understand why it was downvoted. Guesswork is not really a valid answer. $\endgroup$ – Philippe Leybaert Feb 20 '14 at 14:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.